Setting altimeters with no radio [Archive]

Neil Gould

November 12th 06, 11:27 AM

Recently, Mxsmanic > posted:

> "Jim Macklin" > writes:
>
>> GPS, even a $100 hikers model will solve the problem. But I
>> just say, look at the ground, you can judge 1,000 feet
>> pretty well and you only need to apply the hemisphere rule
>> above 3,000 AGL.
>
> GPS is far less accurate than an altimeter, and I don't think the
> regulations say "if you have no radio, use GPS."
>
I'm not sure why you think that "GPS is far less accurate than an
altimiter...", as an altimeter only need be accurate to 75' to be legal.
GPS can do much better than that, and are unaffected by barometric
pressure; the result is a potential source of problems that require pilots
to fly by the altimeter, not the GPS. None of this has anything to do
with regulations, of course.

To answer your original question, the prudent pilot will take the
barometric pressure of their destination into consideration during
preflight planning, and adjust the altimeter accordingly. Most of the
time, the pressure won't change all that drastically at the destination in
the time it takes to fly 100 miles, and non-radio VFR pilots aren't likely
to fly in weather where the pressure is changing too rapidly.

Neil

Neil Gould

November 12th 06, 11:32 AM

Recently, Mxsmanic > posted:

> Jim Logajan writes:
>
>> You can find the answer in FAR 91.121(a)(iii).
>
> Found it, thanks. I'm surprised that just setting it at the airport
> would suffice--one could conceivably fly for hundreds of miles VFR,
> and the altimeter could change significantly along the way.
>
That really isn't as likely as you think, as the flights you're referring
to only takes a couple of hours. If the pressure is changing significantly
faster than that, it's likely to look pretty ugly outside either before
you leave or en-route.

Neil

mike regish

November 12th 06, 11:43 AM

I think we can judge 1000' from the air better than somebody on the ground
because we spend a lot of time in the pattern at 1000' agl. We gt used to
the perspective and can verify it with our altimeters.

mike

"Super Dave" > wrote in message
ink.net...
>
> "Jim Macklin" > wrote in message
> ...
>> GPS, even a $100 hikers model will solve the problem. But I
>> just say, look at the ground, you can judge 1,000 feet
>> pretty well and you only need to apply the hemisphere rule
>> above 3,000 AGL.
>>
>
> On the other hand, we always tell airport noise complainers that it is
> very
> hard to accurately judge aircraft distances, such as whether the plane is
> 500 feet or 1000 feet high. :)
>
>

Jim Macklin

November 12th 06, 12:07 PM

they give altitude, accurate to within a few feet. just set
the altimeter to read the same. then you know adjusted
pressure. considering the legal requirements, it is
perfectly adequate.

see http://mtp.jpl.nasa.gov/notes/altitude/altitude.html

"Peter Duniho" > wrote in
message ...
| "Jim Macklin" > wrote
in message
| ...
| > GPS, even a $100 hikers model will solve the problem.
|
| No, it won't.
|
|

Jim Macklin

November 12th 06, 12:14 PM

Looking down you're eye sees details of known sizes, roads,
cars and houses. It is also able to see angles and
relations on the surface. Judging altitude from the air is
fairly easy. I always require my pre-solo students to fly
the traffic pattern [dual] with all the instruments covered.
I have them tell me their altitude and speed and then
uncover the instruments. They are always within 100 feet of
the correct altitude.

On the other hand, looking up, there are no references to
judge height. Is it a full size B52 or a model, is it 2
miles away or 25, the human eye /brain needs reference
points. But a trained observer is taught how to get those
references, such as the size of the airplane, the relation
of their thumb at arms length and the object, etc.

"Super Dave" > wrote in message
ink.net...
|
| "Jim Macklin" > wrote
in message
| ...
| > GPS, even a $100 hikers model will solve the problem.
But I
| > just say, look at the ground, you can judge 1,000 feet
| > pretty well and you only need to apply the hemisphere
rule
| > above 3,000 AGL.
| >
|
| On the other hand, we always tell airport noise
complainers that it is very
| hard to accurately judge aircraft distances, such as
whether the plane is
| 500 feet or 1000 feet high. :)
|
|

Jim Macklin

November 12th 06, 12:14 PM

Jim Macklin

November 12th 06, 12:15 PM

agreed
"Neil Gould" > wrote in message
m...
| Recently, Mxsmanic > posted:
|
| > "Jim Macklin" >
writes:
| >
| >> GPS, even a $100 hikers model will solve the problem.
But I
| >> just say, look at the ground, you can judge 1,000 feet
| >> pretty well and you only need to apply the hemisphere
rule
| >> above 3,000 AGL.
| >
| > GPS is far less accurate than an altimeter, and I don't
think the
| > regulations say "if you have no radio, use GPS."
| >
| I'm not sure why you think that "GPS is far less accurate
than an
| altimiter...", as an altimeter only need be accurate to
75' to be legal.
| GPS can do much better than that, and are unaffected by
barometric
| pressure; the result is a potential source of problems
that require pilots
| to fly by the altimeter, not the GPS. None of this has
anything to do
| with regulations, of course.
|
| To answer your original question, the prudent pilot will
take the
| barometric pressure of their destination into
consideration during
| preflight planning, and adjust the altimeter accordingly.
Most of the
| time, the pressure won't change all that drastically at
the destination in
| the time it takes to fly 100 miles, and non-radio VFR
pilots aren't likely
| to fly in weather where the pressure is changing too
rapidly.
|
| Neil
|
|
|

Stefan

November 12th 06, 12:38 PM

Jim Macklin schrieb:
> they give altitude, accurate to within a few feet. just set
> the altimeter to read the same.

NO, dont' do this. The whole aviation system (airspace, separation...)
is based on pressure altitude, not geometric altitude.

Stefan

mike regish

November 12th 06, 12:59 PM

Well, kind of. Most, if not all altimeters now, compensate for pressure
variations to show geometric (if that's the right term for it) altitude.

With SA disables in the GPS system, altitude is much more accurate than it
was. How accurate, I don't know. When I compare my altimeter with my GPS's,
they're usually about 100' apart. I'm not sure which is more accurate.

mike

"Stefan" > wrote in message
. ..
> Jim Macklin schrieb:
>> they give altitude, accurate to within a few feet. just set the
>> altimeter to read the same.
>
> NO, dont' do this. The whole aviation system (airspace, separation...) is
> based on pressure altitude, not geometric altitude.
>
> Stefan

Jim Macklin

November 12th 06, 01:20 PM

The question was IF no radio available to get a corrected
altimeter setting... if a GPS is available the GPS altitude
is better than no correction at all.

If I take-off in a airplane with no radio and fly through a
cold front in VFR, the altimeter setting is likely to change
enough to have a 500 foot or greater error with a 100 mile
flight. A GPS can be used to get closer than that.

Probably, most NORDO flights are at or below 3,000 AGL under
VFR. Terrain clearance is more critical to such a flight
based on the altimeter than is the mid-air collision problem
since the "rule" only applies above 3,000 AGL.

Someday, the rules may change, but within today's rules,
using GPS "as the best available" or only source is better
than no source at all.

"mike regish" > wrote in message
. ..
| Well, kind of. Most, if not all altimeters now, compensate
for pressure
| variations to show geometric (if that's the right term for
it) altitude.
|
| With SA disables in the GPS system, altitude is much more
accurate than it
| was. How accurate, I don't know. When I compare my
altimeter with my GPS's,
| they're usually about 100' apart. I'm not sure which is
more accurate.
|
| mike
|
| "Stefan" > wrote in message
| . ..
| > Jim Macklin schrieb:
| >> they give altitude, accurate to within a few feet.
just set the
| >> altimeter to read the same.
| >
| > NO, dont' do this. The whole aviation system (airspace,
separation...) is
| > based on pressure altitude, not geometric altitude.
| >
| > Stefan
|
|

TxSrv

November 12th 06, 01:20 PM

Mxsmanic wrote:
>> You can find the answer in FAR 91.121(a)(iii).
>
> Found it, thanks. I'm surprised that just setting it at the airport
> would suffice--one could conceivably fly for hundreds of miles VFR,
> and the altimeter could change significantly along the way.

That Reg does not apply to flight within 3,000 above the surface.
You have to look at two other Regs to conclude that.
Therefore, under 3,000, an adjustable altimeter is not required
per 91.205. FARs don't apply to you; stay away.

F--

Stefan

November 12th 06, 01:23 PM

mike regish schrieb:

> When I compare my altimeter with my GPS's,
> they're usually about 100' apart. I'm not sure which is more accurate.

The point is not which one is more accurate. The point is that everybody
uses the same frame of reference.

Stefan

Stefan

November 12th 06, 01:25 PM

Jim Macklin schrieb:
> The question was IF no radio available to get a corrected
> altimeter setting... if a GPS is available the GPS altitude
> is better than no correction at all.

Agreed.

> Probably, most NORDO flights are at or below 3,000 AGL under
> VFR. Terrain clearance is more critical to such a flight

In such a flight you usually just look out of the window for terrain
clearance.

Stefan

Jim Macklin

November 12th 06, 01:38 PM

Agreed also, but if at night, the ground can be hard to see
if there is no moon or ground lights. The question is some
what academic, even aircraft without radios often have a
handheld and altimeter settings can be had from ATIS, etc.

The point about GPS, it is accurate in relation to terrain.
It is not good for IFR separation or ATC boundaries.

"Stefan" > wrote in message
...
| Jim Macklin schrieb:
| > The question was IF no radio available to get a
corrected
| > altimeter setting... if a GPS is available the GPS
altitude
| > is better than no correction at all.
|
| Agreed.
|
| > Probably, most NORDO flights are at or below 3,000 AGL
under
| > VFR. Terrain clearance is more critical to such a
flight
|
| In such a flight you usually just look out of the window
for terrain
| clearance.
|
| Stefan

mike regish

November 12th 06, 02:10 PM

My point is that they both are, basically, the same frame of
reference-height above sea level.

mike

"Stefan" > wrote in message
...
> mike regish schrieb:
>
>> When I compare my altimeter with my GPS's, they're usually about 100'
>> apart. I'm not sure which is more accurate.
>
> The point is not which one is more accurate. The point is that everybody
> uses the same frame of reference.
>
> Stefan

Stefan

November 12th 06, 02:16 PM

mike regish schrieb:
> My point is that they both are, basically, the same frame of
> reference-height above sea level.

Which is wrong.

Stefan

mike regish

November 12th 06, 02:43 PM

Well, what exactly are their reference datum(s?).

mike

"Stefan" > wrote in message
...
> mike regish schrieb:
>> My point is that they both are, basically, the same frame of
>> reference-height above sea level.
>
> Which is wrong.
>
> Stefan

Mxsmanic

November 12th 06, 02:52 PM

"Jim Macklin" > writes:

> they give altitude, accurate to within a few feet.

Unfortunately, no, they do not. GPS is accurate for lateral
navigation, not vertical navigation. GPS altitudes can easily be off
by as much as 200 feet at ground level in comparison to a correctly
set altimeter, and at altitude the disparity can reach 500 feet.

The reason for this is that the angles used for triangulation of
lateral positions are large and permit a high level of precision, but
the angles for triangulation of altitude are very small and it's very
easy to be off by a wide margin. GPS was designed to measure lateral
positions accurately, but it performs poorly for altitude. It is
typically much less accurate than an altimeter for altitude, and the
computed altitude constantly changes (significantly) as the satellites
move, in a way that is, for aviation purposes, practically random.

> ... just set the altimeter to read the same.

This is a good way to fly into a mountain, or another aircraft, or the
runway.

> ... then you know adjusted
> pressure. considering the legal requirements, it is
> perfectly adequate.

No, it is not. It's especially dangerous for RVSM flight, but it's so
inaccurate that it should never be used for anything, except as a last
resort (if the altimeters disintegrate, or whatever).

--
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Mxsmanic

November 12th 06, 02:54 PM

mike regish writes:

> Well, kind of. Most, if not all altimeters now, compensate for pressure
> variations to show geometric (if that's the right term for it) altitude.

Not true. Altimeters show substantial differences with geometric
altitude as altitude increases. Air traffic control is based on
pressure altitudes, not geometric altitudes.

> With SA disables in the GPS system, altitude is much more accurate than it
> was. How accurate, I don't know.

GPS altitude can be from 200 to 500 feet off easily.

> When I compare my altimeter with my GPS's,
> they're usually about 100' apart. I'm not sure which is more accurate.

The altimeter is much more accurate.

--
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Mxsmanic

November 12th 06, 02:56 PM

"Jim Macklin" > writes:

> If I take-off in a airplane with no radio and fly through a
> cold front in VFR, the altimeter setting is likely to change
> enough to have a 500 foot or greater error with a 100 mile
> flight. A GPS can be used to get closer than that.

Unfortunately, no, it cannot.

> Probably, most NORDO flights are at or below 3,000 AGL under
> VFR. Terrain clearance is more critical to such a flight
> based on the altimeter than is the mid-air collision problem
> since the "rule" only applies above 3,000 AGL.

GPS altitudes are especially dangerous to use near the ground, becuase
they are so inaccurate.

> Someday, the rules may change, but within today's rules,
> using GPS "as the best available" or only source is better
> than no source at all.

True, if you are far from the ground and you cannot see the ground.
Don't trust it to keep you clear of terrain at low altitudes.

--
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Mxsmanic

November 12th 06, 02:57 PM

"Jim Macklin" > writes:

> The point about GPS, it is accurate in relation to terrain.

It is dangerously inaccurate with respect to terrain.

--
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Mxsmanic

November 12th 06, 02:58 PM

Stefan writes:

> The point is not which one is more accurate. The point is that everybody
> uses the same frame of reference.

Both are valid points. GPS is too inaccurate to use for measuring
altitude in aviation, _and_ it uses a different frame of reference,
which leads to increasingly large disparities between GPS and pressure
altitude at higher altitudes.

--
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Mxsmanic

November 12th 06, 02:58 PM

mike regish writes:

> My point is that they both are, basically, the same frame of
> reference-height above sea level.

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

--
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Mxsmanic

November 12th 06, 03:01 PM

Neil Gould writes:

> I'm not sure why you think that "GPS is far less accurate than an
> altimiter..." ...

Because I studied GPS for a long time and was using it before anyone
else knew what it was, and I know how badly it measures altitude. GPS
was designed to accurately measure longitude and latitude; at best, it
gives only a rough estimate of altitude (ships and troops, the
original users of GPS, don't need to know their altitudes).

> ... as an altimeter only need be accurate to 75' to be legal.

GPS is routinely off by hundreds of feet.

> GPS can do much better than that ...

No, it cannot ... not vertically. It's actually much worse.

--
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Mxsmanic

November 12th 06, 03:11 PM

mike regish writes:

> Well, what exactly are their reference datum(s?).

GPS uses a theoretical model of the earth's surface--a geoid. It may
or may not coincide with sea level.

However, that's not the main source of inaccuracy in GPS altitudes.
The main source is the difficulty of measuring altitude when the
position to be measured involves so many very oblique angles from the
reference satellites. It's easier to measure lateral position looking
straight down than it is to measure vertical position looking at a
shallow angle from 1000 miles away.

--
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Stefan

November 12th 06, 03:16 PM

Mxsmanic schrieb:

>> Well, what exactly are their reference datum(s?).

> GPS uses a theoretical model of the earth's surface--a geoid. It may
> or may not coincide with sea level.

Which is as theoretical a value.

Stefan

Stefan

November 12th 06, 03:35 PM

mike regish schrieb:
> Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

GPS altitude gives you a geometrically derived value above some
reference surface. Mostly it's the WGS84 geoid. Of course this can be
converted to MSL altitude if you wish. But this isn't the point.

The point is that the pressure altimeter measures, well, a pressure, not
an altitude. It displays a value in feet, but actually, this is wrong.
It may be true in very specific conditions, but the real atmosphere very
seldom matches the theoretical ISA atmosphere model on which the
altimeter is based. And, most important, there is no way to calculate
the real (geometric) altitude from the displayed pressure altitude. (Of
course it could, if you knew the complete atmosphere profile, but you
usually don't.)

So the point is: GPS gives you the true altitude in reference to the
earth, but this doesn't help you, because the whole aviatic system
(airspace boundaries, ATC clearances, traffic separation) is based on
pressure altitude, and there is no practical way to convert one into the
other. If you are given an ATC clearance for a certain pressure altitude
but fly GPS altitude instead, then you act exactly like that bozo who
drives on the wrong side of the road.

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

But I'm sure you knew all this before, because after all, you've passed
a written.

Stefan

mike regish

November 12th 06, 03:41 PM

Above 18K feet, everybody is on pressure altitude. Down in the sewers, where
I fly, we're all on local pressure.

mike

"Mxsmanic" > wrote in message
...
>
> Not true. Altimeters show substantial differences with geometric
> altitude as altitude increases. Air traffic control is based on
> pressure altitudes, not geometric altitudes.
>
> The altimeter is much more accurate.

That's what I would assume with my particular GPS.

mike regish

November 12th 06, 03:45 PM

Do those amounts vary with location? How large a difference is there between
the 2? I remember reading about the 2 standards, but forget how the mean
geoid is determined.

But you're right. If that's true, and I don't doubt it is, GPS would be
better suited to terrain avoidance and less so to aircraft separation.

mike

"Mxsmanic" > wrote in message
...
> mike regish writes:
>
>> My point is that they both are, basically, the same frame of
>> reference-height above sea level.
>
> Nope. GPS is height above the mean surface of the geoid, altimeter is
> height above mean sea level. They can be hundreds of feet apart.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

mike regish

November 12th 06, 03:52 PM

Gee, that was actually a well thought out, well explained, and in my case
anyway, very informative post.

It's been 9 years since my written, but I don't remember a whole lot about
geoid reference in it. I do remember the WSG84 reference in my handheld
(which I never use for altitude, BTW) though, so it must have been there.

mike

P.S. I got a 96

"Stefan" > wrote in message
...
> mike regish schrieb:
>> Well, what exactly are their reference datum(s?).
>
> GPS altitude and pressure altitude are completely different and, most
> important, *incompatible* systems.

> <very knowledgeable post snipped>

> But I'm sure you knew all this before, because after all, you've passed a
> written.
>
> Stefan

Gary Drescher

November 12th 06, 04:01 PM

"TxSrv" > wrote in message
. ..
> Mxsmanic wrote:
>>> You can find the answer in FAR 91.121(a)(iii).
>>
>> Found it, thanks. I'm surprised that just setting it at the airport
>> would suffice--one could conceivably fly for hundreds of miles VFR,
>> and the altimeter could change significantly along the way.
>
> That Reg does not apply to flight within 3,000 above the surface. You have
> to look at two other Regs to conclude that.

The requirement stated in 91.121a1iii does not include any exception for
flights under 3000' AGL. If you have an adjustable altimeter, you have to
set it according to 91.121, regardless of your altitude.

> Therefore, under 3,000, an adjustable altimeter is not required per
> 91.205.

According to 91.205, an adjustable altimeter is only required for IFR
(regardless of altitude).

--Gary

Denny

November 12th 06, 04:18 PM

My filter failed and this message popped up... Not an issue of any
importance in itself and fixed again with two mouse clicks...
However, seeing the list of you tripping over yourselves to answer his
trolling have me howling with laughter...

denny

Mxsmanic wrote:
> Since it is possible to fly without voice radio equipment, and given
> that (if I understand correctly) pilots are supposed to have their
> altimeters set correctly to a reference located not more than 100
> miles from their position, how does an aircraft without a radio keep
> its altimeter properly set as it travels?
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

Steven P. McNicoll[_1_]

November 12th 06, 04:20 PM

"TxSrv" > wrote in message
. ..
>
> For VFR, an adjustable altimeter is not required. Therefore, such a 100
> mile rule doesn't exist.
>

What aircraft altimeter is not adjustable? IFR flight requires a sensitive
altimeter adjustable for barometric pressure, VFR flight simply requires an
altimeter. But even altimeters that cannot be adjusted for barometric
pressure are adjustable. My Champ' came with a single-needle altimeter,
once around the dial was 10,000 feet. I hated it, the needle never seemed
to move. But there was an adjustment knob that rotated the dial so the
needle would point to field elevation.

The regulation requires aircraft without radios to set the altimeter to the
elevation of the departure airport or an appropriate altimeter setting
available before departure.

Cirrus

November 12th 06, 04:21 PM

Mxsmanic wrote:
> Since it is possible to fly without voice radio equipment, and given
> that (if I understand correctly) pilots are supposed to have their
> altimeters set correctly to a reference located not more than 100
> miles from their position, how does an aircraft without a radio keep
> its altimeter properly set as it travels?
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

So MXsmanic, you strike again.

For those who wonder why people give you the answers you get, there's a
good reason.
Now it's my turn to tell people about you. I also did some searching on
your past postings too. You show up here pretending to be a pilot, and
a lot of people who haven't fallen for the act try and give you helpful
advice, to which you aren't thankful. What the heck is this game you
are playing with everyone? I'm VERY grateful that I was tipped off to
your game by some honest posters last week.

If you were more grateful, I might feel differently. Or if you told
people in your posts that you aren't a pilot, and don't ever intend to
be one, that would be fine too. But all of your questions, and there
are a lot of them, are worded so that people are misled. Could you
please start including in your posts that you are looking for info for
your MS flight simulator? Don't get me wrong, I like flight simulators,
and there's nothing wrong with non-pilots asking questions- they
deserve the same respect. But the way you decieve people here is
disrespectful. Don't take advantage of the fact that pilots are
inclined to help eachother.

Gary Drescher

November 12th 06, 04:33 PM

"Cirrus" > wrote in message
oups.com...
> If you were more grateful, I might feel differently. Or if you told
> people in your posts that you aren't a pilot, and don't ever intend to
> be one, that would be fine too.

Sorry, but I think you're overreacting a bit. It's true that in a few of his
posts, Mx misleadingly describes his simulator exploits as though they were
real. But the vast majority of his posts don't do that, and many of them
make it explicit that he's not a pilot. In any case, everyone participating
in this thread so far is aware that Mx isn't a pilot.

I think your impression of Mx's posts is probably skewed by the fact that
your introduction to Mx happened to occur with one of the few posts of his
that was phrased misleadingly with regard to his flight status.

--Gary

mike regish

November 12th 06, 04:34 PM

Dang. I actually learned something from this post.

An' ah already gots one o them pilot licencse thingys.

mike

"Cirrus" > wrote in message
oups.com...
>
> So MXsmanic, you strike again.
>
> For those who wonder why people give you the answers you get, there's a
> good reason.
>

Gary Drescher

November 12th 06, 04:36 PM

"Denny" > wrote in message
ups.com...
> My filter failed and this message popped up... Not an issue of any
> importance in itself and fixed again with two mouse clicks...
> However, seeing the list of you tripping over yourselves to answer his
> trolling have me howling with laughter...

If pilots don't know the answer, then he's asking a good question.

I've objected to some of his posts in the past, but there's no point in
objecting when his posts are reasonable.

--Gary

Steven P. McNicoll[_1_]

November 12th 06, 04:42 PM

"TxSrv" > wrote in message
. ..
>
> That Reg does not apply to flight within 3,000 above the surface. You have
> to look at two other Regs to conclude that. Therefore, under 3,000, an
> adjustable altimeter is not required per 91.205. FARs don't apply to you;
> stay away.
>

An altimeter adjustable for barometric pressure is not required for VFR
flight at any altitude.

If you're operating at any altitude below 18,000 feet and you're equipped
with a radio and you have an altimeter adjustable for barometric pressure
you must set it to the current reported altimeter setting of an appropriate
available station, VFR and IFR.

Ron Lee

November 12th 06, 05:32 PM

Stefan > wrote:

>The only use for GPS altitude in aviation is to calculate the final
>glide of a glider. (Or an IFR approach, but I don't know enough about
>IFR flight to comment this.)

Most of what you said is correct except this one. My belief is that
GPS altitude is fine for terrain separation. But not for aircraft
separation which you correctly state is based upon pressure altitude.

Ron Lee

Ron Lee

November 12th 06, 05:34 PM

Mxsmanic > wrote:

>GPS is routinely off by hundreds of feet.

With SA off the vertical error is probably better that altimeter
accuracy requirements. At least close enough for most users and if
you can get WAAS corrections it is very good.

Ron Lee

karl gruber[_1_]

November 12th 06, 06:06 PM

"Ron Lee" > wrote in message
...
> Stefan > wrote:
>
>>The only use for GPS altitude in aviation is to calculate the final
>>glide of a glider. (Or an IFR approach, but I don't know enough about
>>IFR flight to comment this.)
>
> Most of what you said is correct except this one. My belief is that
> GPS altitude is fine for terrain separation. But not for aircraft
> separation which you correctly state is based upon pressure altitude.
>
> Ron Lee

Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he can
give you an education on this subject.

Karl

new_CFI

November 12th 06, 06:15 PM

Mxsmanic > wrote in
:

> Since it is possible to fly without voice radio equipment, and given
> that (if I understand correctly) pilots are supposed to have their
> altimeters set correctly to a reference located not more than 100
> miles from their position, how does an aircraft without a radio keep
> its altimeter properly set as it travels?
>

Set it to field elevation of the departure airport before you take off. Or
if you want you can do a time distance formula, just point the nose
straight down and when your airspeed stabalizes note indicated altitude and
start time. The clock should stop on inpact, just multiply the time it
took by your airspeed, convert the distance to feet. Now find the
differance between your figure and the altitude you started time at and you
have your correction. Unfortunatly you dont die in a sim.

Ron Lee

November 12th 06, 06:29 PM

"karl gruber" > wrote:

>
>"Ron Lee" > wrote in message
...
>> Stefan > wrote:
>>
>>>The only use for GPS altitude in aviation is to calculate the final
>>>glide of a glider. (Or an IFR approach, but I don't know enough about
>>>IFR flight to comment this.)
>>
>> Most of what you said is correct except this one. My belief is that
>> GPS altitude is fine for terrain separation. But not for aircraft
>> separation which you correctly state is based upon pressure altitude.
>>
>> Ron Lee
>
>Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he can
>give you an education on this subject.

Not sure what he can help on since I understand GPS quite well. I
also fly in mountainous terrain which I doubt that he does.

Ron Lee

Mxsmanic

November 12th 06, 06:42 PM

mike regish writes:

> Do those amounts vary with location?

Yes, but unfortunately that is only one of the variables. They also
vary dramatically with the changing positions of the satellites above
(and the satellites move significantly from one minute to the next).
Atmospheric conditions and other factors also come into play.

Overall, though, the problem is that the satellites are positioned to
optimize lateral navigation--but that also positions them in a way
that is unfavorable to vertical navigation. The vertical accuracy can
never be as high as the lateral accuracy; it can't even come close.
That's the way the system is designed.

Perhaps one day GPS will provide altitudes accurate to within a few
feet; but if it does, then by that time the lateral positions will be
accurate to within millimeters.

> How large a difference is there between the 2?

It varies by location and conditions. I'm not sure of the exact
differences. One problem is that the geoid altitude is fixed, whereas
pressure altitudes depend on atmospheric conditions.

If everyone used GPS for altitude, it would probably work out okay in
some cases, particularly at high altitudes. But as long as anyone is
using a conventional altimeter, the differences are large enough to be
dangerous.

> But you're right. If that's true, and I don't doubt it is, GPS would be
> better suited to terrain avoidance and less so to aircraft separation.

I prefer to reserve GPS for lateral navigation only. For that it
works quite well, on a par with other navaids (depending on various
factors). For altitude it's a waste of time. Better to have a
standard altimeter and a radar altimeter for terrain.

--
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Mxsmanic

November 12th 06, 06:44 PM

Ron Lee writes:

> With SA off the vertical error is probably better that altimeter
> accuracy requirements.

No. The figure of hundreds of feet is with SA OFF. It's even worse
with SA on.

> At least close enough for most users and if
> you can get WAAS corrections it is very good.

If you have external corrections, you can do far better. But you're
still at least as well off with a standard altimeter. At least right
now, you don't really gain anything for altitude measurement with GPS.

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Mxsmanic

November 12th 06, 06:46 PM

mike regish writes:

> Dang. I actually learned something from this post.

You've also learned about GPS.

--
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Mxsmanic

November 12th 06, 06:49 PM

Ron Lee writes:

> Not sure what he can help on since I understand GPS quite well.

If you think you can use GPS safely for altitude in aviation, you need
to understand it better.

> I also fly in mountainous terrain which I doubt that he does.

In that case, you need to understand GPS better very soon.

--
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karl gruber[_1_]

November 12th 06, 06:50 PM

You CLEARLY don't understand GPS at all!

Karl

"Ron Lee" > wrote in message
...
> "karl gruber" > wrote:
>
>>
>>"Ron Lee" > wrote in message
...
>>> Stefan > wrote:
>>>
>>>>The only use for GPS altitude in aviation is to calculate the final
>>>>glide of a glider. (Or an IFR approach, but I don't know enough about
>>>>IFR flight to comment this.)
>>>
>>> Most of what you said is correct except this one. My belief is that
>>> GPS altitude is fine for terrain separation. But not for aircraft
>>> separation which you correctly state is based upon pressure altitude.
>>>
>>> Ron Lee
>>
>>Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he
>>can
>>give you an education on this subject.
>
> Not sure what he can help on since I understand GPS quite well. I
> also fly in mountainous terrain which I doubt that he does.
>
> Ron Lee

Mxsmanic

November 12th 06, 06:51 PM

new_CFI writes:

> Unfortunatly you dont die in a sim.

Why is that unfortunate? That's one of the advantages to simulation.
Indeed, when you clip a mountaintop east of Telluride because you
trusted the GPS altitude in your simulator, you learn a valuable
lesson that may keep you from getting killed in real life.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Roy Smith

November 12th 06, 06:54 PM

In article >,
Mxsmanic > wrote:

> new_CFI writes:
>
> > Unfortunatly you dont die in a sim.
>
> Why is that unfortunate? That's one of the advantages to simulation.
> Indeed, when you clip a mountaintop east of Telluride because you
> trusted the GPS altitude in your simulator, you learn a valuable
> lesson that may keep you from getting killed in real life.

One of the advantages of masturbation is that you can't get AIDS from it.
That hardly makes it a complete substitute for a real person of the
opposite gender.

Mxsmanic

November 12th 06, 07:15 PM

Roy Smith writes:

> One of the advantages of masturbation is that you can't get AIDS from it.
> That hardly makes it a complete substitute for a real person of the
> opposite gender.

In surveys I've seen, many people have said that they masturbate
precisely because it doesn't carry the risk of STDs that sexual
intercourse has. And supposedly masturbation produces more intense
orgasms than intercourse as well. While these parallels are not
completely relevant to flight simulation, they do illustrate why
simulation is sometimes preferable to reality. Certainly this is the
case when it comes to dire emergencies and accidents--whence the
popularity of simulators in flight training.

--
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mike regish

November 12th 06, 07:16 PM

Nope. Knew that.

mike

"Mxsmanic" > wrote in message
...
> mike regish writes:
>
>> Dang. I actually learned something from this post.
>
> You've also learned about GPS.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

Jim Logajan

November 12th 06, 07:20 PM

"BT" > wrote:
> he's flying a friggin sim..

This forum is open to everyone, clued and clueless, sane and insane, so
it's impossible to tell by the absurdity of a post whether it is being
posted in jest or is sincerely believed by the poster. Hence my question.

>
> "Jim Logajan" > wrote in message
> .. .
>> "BT" > wrote:
>>> you land every 100 miles or less
>>> reset your altimeter to field elevation
>>> and then fly another 100 miles
>>
>> I presume you're trying to pull his leg?
>
>

Peter Duniho

November 12th 06, 07:56 PM

"Jim Macklin" > wrote in message
...
> they give altitude, accurate to within a few feet. just set
> the altimeter to read the same. then you know adjusted
> pressure. considering the legal requirements, it is
> perfectly adequate.

First of all, no they don't give altitude accurate to within a few feet.
Secondly, they give a completely different kind of altitude measurement than
the altimeter provides. Even if the GPS were accurate to within feet for
altitude, setting your altimeter to the GPS displayed altitude would not be
the same as having the current, local altimeter setting.

Pete

Doug[_1_]

November 12th 06, 08:08 PM

My regular altimeter broke, and while it was being fixed I flew using
the altimeter in my IFR GPS -- which is coupled to the altimeter in my
transponder--and which BTW has an adjustment for barometer reading. I
am not sure exactly how it works, I think it uses both GPS and the
transponder altimeter reading. It was King KLN90B and a King
transponder, if that matters.

Grumman-581[_1_]

November 12th 06, 08:23 PM

On Sun, 12 Nov 2006 06:43:31 -0500, mike regish wrote:
> I think we can judge 1000' from the air better than somebody on the
> ground because we spend a lot of time in the pattern at 1000' agl. We gt
> used to the perspective and can verify it with our altimeters.

Plus, there is also the issue of we have more to took at to get a
reference on... People on the ground are trying to judge distance by
looking at a single object that is relatively rather small... Nothing to
really compare it to... On the other hand, if we are above water with no
surrounding terrain or boats, I would suspect that we might have a bit
more difficulty judging altitude...

Doug[_1_]

November 12th 06, 09:09 PM

My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.

karl gruber[_1_]

November 12th 06, 09:16 PM

No it isn't coupled to your transponder..............at all. Your encoding
altimeter is inputed to the transponder.

GPS altitude isn't accurate enough.

Karl
"Doug" > wrote in message
oups.com...
> My regular altimeter broke, and while it was being fixed I flew using
> the altimeter in my IFR GPS -- which is coupled to the altimeter in my
> transponder--and which BTW has an adjustment for barometer reading. I
> am not sure exactly how it works, I think it uses both GPS and the
> transponder altimeter reading. It was King KLN90B and a King
> transponder, if that matters.
>

karl gruber[_1_]

November 12th 06, 09:16 PM

No. It isn't!

Karl
"Doug" > wrote in message
oups.com...
> My IFR GPS, a King KLN90B is connected to the altimeter in my
> transponder. It is also adjustable to the barometric setting.
>

Ron Lee

November 12th 06, 09:20 PM

"Doug" > wrote:

>My regular altimeter broke, and while it was being fixed I flew using
>the altimeter in my IFR GPS -- which is coupled to the altimeter in my
>transponder--and which BTW has an adjustment for barometer reading. I
>am not sure exactly how it works, I think it uses both GPS and the
>transponder altimeter reading. It was King KLN90B and a King
>transponder, if that matters.
>

If your GPS unit has integrity (RAIM) then it can use the transponder
altitude as a pseudo-satellite thus increasing RAIM availability.

Ron (Who apparently knows nothing about GPS" Lee

Neil Gould

November 12th 06, 09:37 PM

Recently, Mxsmanic > posted:

> Neil Gould writes:
>
>> I'm not sure why you think that "GPS is far less accurate than an
>> altimiter..." ...
>
> Because I studied GPS for a long time and was using it before anyone
> else knew what it was, and I know how badly it measures altitude. GPS
> was designed to accurately measure longitude and latitude; at best, it
> gives only a rough estimate of altitude (ships and troops, the
> original users of GPS, don't need to know their altitudes).
>
Ever hear of WAAS?

From the Garmin site: "A WAAS-capable receiver can give you a position
accuracy of better than three meters 95 percent of the time. "

My experience with my 295 bears this out.

Neil

Ron Lee

November 12th 06, 10:07 PM

(Ron Lee) wrote:

>"Doug" > wrote:
>
>>My regular altimeter broke, and while it was being fixed I flew using
>>the altimeter in my IFR GPS -- which is coupled to the altimeter in my
>>transponder--and which BTW has an adjustment for barometer reading. I
>>am not sure exactly how it works, I think it uses both GPS and the
>>transponder altimeter reading. It was King KLN90B and a King
>>transponder, if that matters.
>>
>
>If your GPS unit has integrity (RAIM) then it can use the transponder
>altitude as a pseudo-satellite thus increasing RAIM availability.
>
>Ron (Who apparently knows nothing about GPS" Lee

Correction... Encoding altimeter input is the additional source for
RAIM determination.

Ron Lee

.Blueskies.

November 12th 06, 10:07 PM

"Peter Duniho" > wrote in message ...
: "Jim Macklin" > wrote in message
: ...
: > they give altitude, accurate to within a few feet. just set
: > the altimeter to read the same. then you know adjusted
: > pressure. considering the legal requirements, it is
: > perfectly adequate.
:
: First of all, no they don't give altitude accurate to within a few feet.
: Secondly, they give a completely different kind of altitude measurement than
: the altimeter provides. Even if the GPS were accurate to within feet for
: altitude, setting your altimeter to the GPS displayed altitude would not be
: the same as having the current, local altimeter setting.
:
: Pete
:
:

What about GPS precision approaches...where does the glideslope information come from?

WAAS?

peter

November 12th 06, 10:09 PM

Mxsmanic wrote:
> mike regish writes:
>
> > My point is that they both are, basically, the same frame of
> > reference-height above sea level.
>
> Nope. GPS is height above the mean surface of the geoid, altimeter is
> height above mean sea level. They can be hundreds of feet apart.

You're thinking of height above the ellipsoid, which can be hundreds of
feet different from height above the geoid. But the geoid does
represent the mean sea level height - including in places that are far
from the sea. Internally GPS receivers generally initially calculate
height relative to the ellipsoid model of the earth's shape (using the
WGS-84 model parameters). However, recent models with which I'm
familiar then apply a correction term based on an internal lookup table
to convert the ellipsoid height to the geoid height (equivalent to
height above MSL) at that particular location. See:
http://www.esri.com/news/arcuser/0703/geoid1of3.html
The altitudes that end up being displayed by the GPS after its internal
correction are therefore based on elevation above MSL with some
measurement uncertainty that's dependent on the current satellite
geometry.

Robert Chambers

November 12th 06, 10:11 PM

How about doing a touch and go at an airport with a known altitude,
while you are retracting flaps, quick twist of the dial to set field
elevation and away you go.

(this was a joke by the way, much as the original question)

Neil Gould wrote:
> Recently, Mxsmanic > posted:
>
>
>>"Jim Macklin" > writes:
>>
>>
>>>GPS, even a $100 hikers model will solve the problem. But I
>>>just say, look at the ground, you can judge 1,000 feet
>>>pretty well and you only need to apply the hemisphere rule
>>>above 3,000 AGL.
>>
>>GPS is far less accurate than an altimeter, and I don't think the
>>regulations say "if you have no radio, use GPS."
>>
>
> I'm not sure why you think that "GPS is far less accurate than an
> altimiter...", as an altimeter only need be accurate to 75' to be legal.
> GPS can do much better than that, and are unaffected by barometric
> pressure; the result is a potential source of problems that require pilots
> to fly by the altimeter, not the GPS. None of this has anything to do
> with regulations, of course.
>
> To answer your original question, the prudent pilot will take the
> barometric pressure of their destination into consideration during
> preflight planning, and adjust the altimeter accordingly. Most of the
> time, the pressure won't change all that drastically at the destination in
> the time it takes to fly 100 miles, and non-radio VFR pilots aren't likely
> to fly in weather where the pressure is changing too rapidly.
>
> Neil
>
>
>

Morgans[_2_]

November 12th 06, 10:11 PM

"Ron Lee" > wrote

> Not sure what he can help on since I understand GPS quite well. I
> also fly in mountainous terrain which I doubt that he does.

I think you missed seeing Karl's tongue, planted firmly in his cheek.
--
Jim in NC

Morgans[_2_]

November 12th 06, 10:14 PM

"Neil Gould" > wrote

> From the Garmin site: "A WAAS-capable receiver can give you a position
> accuracy of better than three meters 95 percent of the time. "
>
> My experience with my 295 bears this out.

So you are trying to convince the town idiot, now?

You know better than that.
--
Jim in NC

Mxsmanic

November 12th 06, 10:27 PM

Doug writes:

> My IFR GPS, a King KLN90B is connected to the altimeter in my
> transponder. It is also adjustable to the barometric setting.

It's the altimeter that provides the accuracy for measurement of
altitude in that case, not the GPS.

--
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Mxsmanic

November 12th 06, 10:30 PM

Neil Gould writes:

> Ever hear of WAAS?

Yes. And it's not part of GPS.

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peter

November 12th 06, 10:36 PM

Mxsmanic wrote:
> "Jim Macklin" > writes:
>
> > they give altitude, accurate to within a few feet.
>
> Unfortunately, no, they do not. GPS is accurate for lateral
> navigation, not vertical navigation. GPS altitudes can easily be off
> by as much as 200 feet at ground level in comparison to a correctly
> set altimeter, and at altitude the disparity can reach 500 feet.
>
> The reason for this is that the angles used for triangulation of
> lateral positions are large and permit a high level of precision, but
> the angles for triangulation of altitude are very small and it's very
> easy to be off by a wide margin.

This indicates a basic lack of understanding of GPS technology. The
GPS receiver never deals with measurement of any angles nor with
triangulation. What is measured are the precise times of arrival of
the signals from the satellites. Since the satellites encode the
signals with timing information from their sychronized atomic clocks
and also send detailed orbital data to define their own positions, the
receiver is able to determine the relative distances to the various
satellites based on the speed of light/radio and the observed relative
signal delays. Using this distance information together with the known
positions of the satellites then allows for a determination of the
position of the receiver. Note that this never involves a measurement
of any angles.

It is true that altitude measurements are generally somewhat less
accurate than horizontal position measurements due to the basic
geometry of receiving satellite signals from only the satellites that
are above you. Ideal measurement of altitude would also involve some
satellites below you but of course their signals are blocked by the
earth. Similarly, east-west positions are a bit better accuracy than
north-south since the satellites are equally likely to be east and west
of you but there's a greater likelihood of them being to the south
rather than the north (at least from the northern hemisphere).

My long-term evaluation of GPS altitude accuracy has shown that I get
values within 35' of accurately surveyed altitudes at least 95% of the
time ever since Selective Availability was turned off. Using the WAAS
correction data improves this to get the accuracy down to 20' with 95%
confidence. Both of these are based on having a reasonably
unobstructed view of the sky (which generally isn't hard in an aircraft
unless the antenna is poorly positioned).

So from a technical standpoint GPS altitudes these days are pretty good
although some care should be taken to check the actual satellite
geometry and reception at the time of any critical measurements.
However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.

Jim Macklin

November 12th 06, 11:35 PM

You rarely get AIDS from a "real person of the opposite
gender" which is why GAY means "got aids yet."

This does not include the use of IV drugs, the other big
vector.

"Roy Smith" > wrote in message
...
| In article >,
| Mxsmanic > wrote:
|
| > new_CFI writes:
| >
| > > Unfortunatly you dont die in a sim.
| >
| > Why is that unfortunate? That's one of the advantages
to simulation.
| > Indeed, when you clip a mountaintop east of Telluride
because you
| > trusted the GPS altitude in your simulator, you learn a
valuable
| > lesson that may keep you from getting killed in real
life.
|
| One of the advantages of masturbation is that you can't
get AIDS from it.
| That hardly makes it a complete substitute for a real
person of the
| opposite gender.

Jim Macklin

November 12th 06, 11:47 PM

never said it would, said it was better than nothing.

"Peter Duniho" > wrote in
message ...
| "Jim Macklin" > wrote
in message
| ...
| > they give altitude, accurate to within a few feet. just
set
| > the altimeter to read the same. then you know adjusted
| > pressure. considering the legal requirements, it is
| > perfectly adequate.
|
| First of all, no they don't give altitude accurate to
within a few feet.
| Secondly, they give a completely different kind of
altitude measurement than
| the altimeter provides. Even if the GPS were accurate to
within feet for
| altitude, setting your altimeter to the GPS displayed
altitude would not be
| the same as having the current, local altimeter setting.
|
| Pete
|
|

Bob Noel

November 12th 06, 11:58 PM

In article >,
(Ron Lee) wrote:

> Correction... Encoding altimeter input is the additional source for
> RAIM determination.

Nit: encoding altimeter CAN be an additional input. It is not required.

--
Bob Noel
Looking for a sig the
lawyers will hate

Peter Duniho

November 13th 06, 12:14 AM

".Blueskies." > wrote in message
t...
> What about GPS precision approaches...where does the glideslope
> information come from?
>
>
> WAAS?

WAAS is insufficient for a precision GPS approach. GPS precision approaches
use "LAAS", which is basically the same as WAAS except that the differential
station is much closer to the airport (I suppose in some or many cases it
may even be colocated...not sure). It's "local area" instead of "wide
area".

WAAS and LAAS are both a form of differential GPS, and it's true that both
increase the accuracy of GPS significantly, LAAS more so than WAAS.
However, a) Jim never restricted his claim to GPS using WAAS, and b) even
with WAAS (or LAAS, for that matter), the GPS does not indicate the same
altitude that a properly set altimeter would.

Pete

Jessica Taylor

November 13th 06, 12:34 AM

Peter Duniho wrote:

> ".Blueskies." > wrote in message
> t...
> > What about GPS precision approaches...where does the glideslope
> > information come from?
> >
> >
> > WAAS?
>
> WAAS is insufficient for a precision GPS approach.

Nonsense. A fully deployed WAAS can provide precision GPS appoaches with
performance comparable to ILS (Cat 1).

> GPS precision approaches
> use "LAAS",

Not necessarily. In addition to WAAS, JPALS may be used in the future.

> which is basically the same as WAAS except that the differential
> station is much closer to the airport

LAAS transmits signals on the UHF band. WAAS does not. LAAS can eventually
provide more accuracy.

[email protected]

November 13th 06, 01:25 AM

Jim Macklin wrote:
> You rarely get AIDS from a "real person of the opposite
> gender" which is why GAY means "got aids yet."
>
> This does not include the use of IV drugs, the other big
> vector.

Good lord, how freakin off topic can this get. Politics, religion and
miscellaneous hatred shouldn't be a part of aviation, can't we all just
get along.

Jim Macklin

November 13th 06, 01:44 AM

If you look at the MovieTone news reels from the 1950's, the
UN WHO was in Africa inoculating entire villages with one
needle and a gallon bottle of serum. The stopped many
deaths and diseases and also probably spread AIDS to whole
populations.

For those of us born before 1950, we remember that disease
kills.

Many diseases are spread by airplanes, traveling faster than
an incubation period to all corners of the world.

> wrote in message
ups.com...
| Jim Macklin wrote:
| > You rarely get AIDS from a "real person of the opposite
| > gender" which is why GAY means "got aids yet."
| >
| > This does not include the use of IV drugs, the other big
| > vector.
|
| Good lord, how freakin off topic can this get. Politics,
religion and
| miscellaneous hatred shouldn't be a part of aviation,
can't we all just
| get along.
|

Peter Duniho

November 13th 06, 02:06 AM

"Jessica Taylor" > wrote in message
...
>> WAAS is insufficient for a precision GPS approach.
>
> Nonsense. A fully deployed WAAS can provide precision GPS appoaches with
> performance comparable to ILS (Cat 1).

Yes, congratulations. I was misinformed (behind the times, actually), and
for once you actually have the right answer.

Thank you for the correction.

Ron Lee

November 13th 06, 02:17 AM

Jessica Taylor > wrote:

>> WAAS is insufficient for a precision GPS approach.
>
>Nonsense. A fully deployed WAAS can provide precision GPS appoaches with
>performance comparable to ILS (Cat 1).
>
>> GPS precision approaches use "LAAS",
>
>Not necessarily. In addition to WAAS, JPALS may be used in the future.
>
>> which is basically the same as WAAS except that the differential
>> station is much closer to the airport
>
>LAAS transmits signals on the UHF band. WAAS does not. LAAS can eventually
>provide more accuracy.
>
OK, who is this Jessica dudette? Seems to be uncannily knowledgeable.

It may depend on the meaning of "precision." If you assume any
approach with vertical guidance then WAAS does. If you mean CAT I or
better that depends on where WAAS is/gets to performance wise. It was
supposed to provide CAT I but has had problems.

I have not kept up to date on LAAS but last I heard it may or may not
be developed.

Ron Lee

Jessica Taylor

November 13th 06, 02:50 AM

Peter Duniho wrote:

> "Jessica Taylor" > wrote in message
> ...
> >> WAAS is insufficient for a precision GPS approach.
> >
> > Nonsense. A fully deployed WAAS can provide precision GPS appoaches with
> > performance comparable to ILS (Cat 1).
>
> Yes, congratulations. I was misinformed (behind the times, actually), and
> for once you actually have the right answer.
>
> Thank you for the correction.

Always a pleasure to help get you up to the times, Peter.

Bob Noel

November 13th 06, 02:54 AM

In article >,
(Ron Lee) wrote:

> It may depend on the meaning of "precision."

Historically, "precision" wrt precision approach means only that it provides
vertical guidance.

--
Bob Noel
Looking for a sig the
lawyers will hate

Don Poitras

November 13th 06, 04:37 AM

In rec.aviation.piloting karl gruber > wrote:
> No. It isn't!

Don't know about the 90B, but my 89B has an input from the transponder/encoder.
You can use it for advisory VNAV. Since the input is pressure altitude, you
need to keep the pressure setting current. Mode C is only accurate to 100
feet though, so you will see it jump up and down by that amount. There is
also a page that shows the GPS calculated altitude. I've never seen it closer
than a couple of hundred feet to the true altitude. (on the ground; it could
be more acurate in the air, but I'd only be comparing it to the transponder
or the regular altimeter and maybe they go crazy in the air, but they always
agree within 100 feet of each other.) Sometimes I fly 50 feet higher or lower
to make the GPS presure-supplied altitude read my assigned altitude so that
my flight-aware track (and ATC) believe that I can maintain an altitude as
well as an auto-pilot. :)

> Karl
> "Doug" > wrote in message
> oups.com...
> > My IFR GPS, a King KLN90B is connected to the altimeter in my
> > transponder. It is also adjustable to the barometric setting.
> >

--
Don Poitras

Peter Duniho

November 13th 06, 04:54 AM

"Ron Lee" > wrote in message
...
> It may depend on the meaning of "precision." If you assume any
> approach with vertical guidance then WAAS does. If you mean CAT I or
> better that depends on where WAAS is/gets to performance wise. It was
> supposed to provide CAT I but has had problems.

As Bob says, the word "precision" simply implies vertical guidance.

As for whether one can actually get Cat I minima with WAAS, I'm taking the
FAA's word for it. I didn't bother to look for any GPS LPV approaches
specifically to see that they actually exist. I just know that the FAA web
site says (via press release) that as of Oct 2004, they were starting to
roll out Cat I-equivalent GPS approaches.

Travis Marlatte

November 13th 06, 04:59 AM

"Stefan" > wrote in message
...
> mike regish schrieb:
> The point is that the pressure altimeter measures, well, a pressure, not
> an altitude. It displays a value in feet, but actually, this is wrong.

Certainly true.

> ... the whole aviatic system (airspace boundaries, ATC clearances, traffic
> separation) is based on pressure altitude

I agree with that statement too.

> .. If you are given an ATC clearance for a certain pressure altitude but
> fly GPS altitude instead, then you act exactly like that bozo who drives
> on the wrong side of the road.

The only ATC clearances for a pressure altitude would be in the flight
levels. Since the question was about setting a pressure altimeter, I would
say that the flight levels are irrelevant. Below the flight levels, ATC
clearances are for pressure compensated altitude above MSL, so yes it is
based on pressure and not true altitude but close enough. My GPS gives me a
calculated altitude above a theoretical sea level that's also close enough.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 13th 06, 05:06 AM

"Mxsmanic" > wrote in message
...

> No, it is not. It's especially dangerous for RVSM flight, but it's so
> inaccurate that it should never be used for anything, except as a last
> resort (if the altimeters disintegrate, or whatever).

It was posed as a possible means to adjust a pressure altimeter with no
radio. I'd say that it is better than nothing, in that case. Since you can't
be NORDO in the IFR system, separation is the responsibility of the pilot's
eyes anyway.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 13th 06, 05:12 AM

"Mxsmanic" > wrote in message
...
> "Jim Macklin" > writes:
>
>> GPS, even a $100 hikers model will solve the problem. But I
>> just say, look at the ground, you can judge 1,000 feet
>> pretty well and you only need to apply the hemisphere rule
>> above 3,000 AGL.
>
> GPS is far less accurate than an altimeter, and I don't think the
> regulations say "if you have no radio, use GPS."
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

The regs don't say a lot of things. You asked the question. Besides the OT
debate, you got two basic responses 1) It doesn't matter 'cause you're VFR
anyway and 2) any old GPS could give you an altitude to use a reference in
the case where the local pressure was significantly different from the
departure.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 13th 06, 05:15 AM

VFR, it doesn't really matter, does it?

--
-------------------------------
Travis
Lake N3094P
PWK
"Mxsmanic" > wrote in message
...
> Jim Logajan writes:
>
>> You can find the answer in FAR 91.121(a)(iii).
>
> Found it, thanks. I'm surprised that just setting it at the airport
> would suffice--one could conceivably fly for hundreds of miles VFR,
> and the altimeter could change significantly along the way.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

A Lieberma

November 13th 06, 05:15 AM

"Travis Marlatte" > wrote in
t:

> Since
> you can't be NORDO in the IFR system, separation is the responsibility
> of the pilot's eyes anyway.

Maybe you should say you cannot be INTENTIONALLY NORDO (squawking 7600).

It would be pretty hard to maintian visual separation in the clag should
the radios go belly up.

Allen

Grumman-581[_1_]

November 13th 06, 05:18 AM

On Sun, 12 Nov 2006 17:35:40 -0600, Jim Macklin wrote:
> You rarely get AIDS from a "real person of the opposite
> gender" which is why GAY means "got aids yet."

But do you want to stake your life on it? Because, well, that's *exactly*
what you're doing...

Travis Marlatte

November 13th 06, 05:21 AM

Gotta agree. Roy and Jim, what they heck are you doing? Don't let MX get to
you like this. Just ignore him, if he drives you that mad!!

--
-------------------------------
Travis
Lake N3094P
PWK
> wrote in message
ups.com...
> Jim Macklin wrote:
>> You rarely get AIDS from a "real person of the opposite
>> gender" which is why GAY means "got aids yet."
>>
>> This does not include the use of IV drugs, the other big
>> vector.
>
> Good lord, how freakin off topic can this get. Politics, religion and
> miscellaneous hatred shouldn't be a part of aviation, can't we all just
> get along.
>

Mxsmanic

November 13th 06, 05:28 AM

Travis Marlatte writes:

> The only ATC clearances for a pressure altitude would be in the flight
> levels. Since the question was about setting a pressure altimeter, I would
> say that the flight levels are irrelevant. Below the flight levels, ATC
> clearances are for pressure compensated altitude above MSL, so yes it is
> based on pressure and not true altitude but close enough. My GPS gives me a
> calculated altitude above a theoretical sea level that's also close enough.

"Close enough": famous last words.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic

November 13th 06, 05:36 AM

peter writes:

> This indicates a basic lack of understanding of GPS technology.

It reveals exactly the opposite. That's how GPS determines position.

> The GPS receiver never deals with measurement of any angles nor with
> triangulation. What is measured are the precise times of arrival of
> the signals from the satellites.

Surprising though it may be, those "precise times of arrival" are the
sides of a triangle.

> Since the satellites encode the
> signals with timing information from their sychronized atomic clocks
> and also send detailed orbital data to define their own positions, the
> receiver is able to determine the relative distances to the various
> satellites based on the speed of light/radio and the observed relative
> signal delays. Using this distance information together with the known
> positions of the satellites then allows for a determination of the
> position of the receiver. Note that this never involves a measurement
> of any angles.

Actually it does. The arrival times define spheres in 3D space around
the satellites (the geoid can also be used as a reference sphere).
The intersections of these spheres effectively isolate the position of
the receiver. It's just a fancy version of good old triangulation,
and it works very well.

Unfortunately, however, it is optimized for lateral positioning, not
vertical positioning. To achieve the same vertical accuracy as
lateral accuracy, a much higher measurement precision is required.
For this reason, vertical measurement accuracy is very poor.

> It is true that altitude measurements are generally somewhat less
> accurate than horizontal position measurements due to the basic
> geometry of receiving satellite signals from only the satellites that
> are above you.

More than "somewhat" less accurate: they are usually unusable,
certainly for aviation.

> My long-term evaluation of GPS altitude accuracy has shown that I get
> values within 35' of accurately surveyed altitudes at least 95% of the
> time ever since Selective Availability was turned off.

How were you able to accurately survey your altitude in the air?

> So from a technical standpoint GPS altitudes these days are pretty good
> although some care should be taken to check the actual satellite
> geometry and reception at the time of any critical measurements.

It's hard to do that in the air.

> However, there are good reasons why barometric measurements are used
> instead for aviation to ensure consistency and uniform procedures.

The main reason is that it's more accurate.

GPS altitude data is so poor and so variable that I've given up using
it even on the ground. It's almost never anywhere near surveyed
altitudes, and it drifts all over the place. Indeed, you can watch it
change as you stand still on the ground, and that's with SA turned
off. I definitely would not want to depend on that in the air.

--
Transpose mxsmanic and gmail to reach me by e-mail.

peter

November 13th 06, 07:21 AM

Mxsmanic wrote:
> peter writes:
>
> > This indicates a basic lack of understanding of GPS technology.
>
> It reveals exactly the opposite. That's how GPS determines position.

No, your statement before was that it required measurement of angles
and "triangulation" whereas the actual procedure does not measure any
angles at all and is closer to "trilateration" or determining the
distances to the satellites at known positions
>
> > The GPS receiver never deals with measurement of any angles nor with
> > triangulation. What is measured are the precise times of arrival of
> > the signals from the satellites.
>
> Surprising though it may be, those "precise times of arrival" are the
> sides of a triangle.

Not sure what you mean by times being equal to sides (the units don't
match for one thing), but again, no angles are measured by the GPS
receiver.
>
> > Since the satellites encode the
> > signals with timing information from their sychronized atomic clocks
> > and also send detailed orbital data to define their own positions, the
> > receiver is able to determine the relative distances to the various
> > satellites based on the speed of light/radio and the observed relative
> > signal delays. Using this distance information together with the known
> > positions of the satellites then allows for a determination of the
> > position of the receiver. Note that this never involves a measurement
> > of any angles.
>
> Actually it does. The arrival times define spheres in 3D space around
> the satellites (the geoid can also be used as a reference sphere).

The geoid is not a sphere but rather a complex empirically determined
surface that closely approximates MSL on the earth (i.e. it is
certainly not anything like a sphere around the satellites as you state
above). It is not used by the GPS in the initial position
determination but may later be used in converting the calculated height
above the WGS-84 ellipsoid to an equivalent height above MSL.

> The intersections of these spheres effectively isolate the position of
> the receiver. It's just a fancy version of good old triangulation,
> and it works very well.
>
I agree it works well, but it doesn't involve measuring angles and is
therefore not "triangulation." I suggest you read the GPS tutorial at
Trimble's website.

> Unfortunately, however, it is optimized for lateral positioning, not
> vertical positioning.

No, the somewhat better horizontal vs. vertical accuracy is an inherent
consequence of not being able to receive signals from satellites that
are below us (and therefore blocked by the earth). That's not a
deliberate engineering optimization decision but just the way things
are.

> To achieve the same vertical accuracy as
> lateral accuracy, a much higher measurement precision is required.

No again. As the accuracy of GPS continues to improve, both the
horizontal and vertical accuracy gets better, but horizontal will
always be somewhat better so we won't achieve "the same vertical
accuracy." However, we can continue to improve both accuracies so that
they are good enough for most applications.

> > It is true that altitude measurements are generally somewhat less
> > accurate than horizontal position measurements due to the basic
> > geometry of receiving satellite signals from only the satellites that
> > are above you.
>
> More than "somewhat" less accurate: they are usually unusable,
> certainly for aviation.

The FAA doesn't seem to think so since Garmin recently indicated that
600 GPS LPV approaches have been approved by the FAA providing for
certified GPS with WAAS to be used down to 200' (same as Cat 1 ILS).
See
http://gps.faa.gov/programs/waasnews.htm
>
> > My long-term evaluation of GPS altitude accuracy has shown that I get
> > values within 35' of accurately surveyed altitudes at least 95% of the
> > time ever since Selective Availability was turned off.
>
> How were you able to accurately survey your altitude in the air?

I do my surveying on terra firma, but it is frequently also reasonably
high "in the air" i.e. on top of mountains. (Neither a GPS nor a
barometric altimeter cares if the 10000' below is occupied by a
mountain or by empty air.)
>
> > So from a technical standpoint GPS altitudes these days are pretty good
> > although some care should be taken to check the actual satellite
> > geometry and reception at the time of any critical measurements.
>
> It's hard to do that in the air.

Really? I find it very easy to do since the GPS receiver itself
indicates the satellite geometry and reception conditions.
>
> > However, there are good reasons why barometric measurements are used
> > instead for aviation to ensure consistency and uniform procedures.
>
> The main reason is that it's more accurate.

You might want to check what instruments are used by surveyors to get
accurate altitudes. E.g. the altitude of Mt. Everest was revised
fairly recently based on use of GPS. A barometric altimeter would have
been useless for that task.
>
> GPS altitude data is so poor and so variable that I've given up using
> it even on the ground. It's almost never anywhere near surveyed
> altitudes, and it drifts all over the place. Indeed, you can watch it
> change as you stand still on the ground, and that's with SA turned
> off. I definitely would not want to depend on that in the air.

Either your receiver is broken or you are using it incorrectly. (The
lack of knowledge about the fundamentals and ability to check on
satellite geometry suggests the latter possibility.). Of course
locations with poor GPS reception due to obstructions are far more
likely to be found on the ground than in the air.

Jim Macklin

November 13th 06, 08:30 AM

You can't begin an IFR flight as NORDO, but you certainly
can complete the flight that way.

"Travis Marlatte" > wrote in
message
t...
| "Mxsmanic" > wrote in message
| ...
|
| > No, it is not. It's especially dangerous for RVSM
flight, but it's so
| > inaccurate that it should never be used for anything,
except as a last
| > resort (if the altimeters disintegrate, or whatever).
|
|
| It was posed as a possible means to adjust a pressure
altimeter with no
| radio. I'd say that it is better than nothing, in that
case. Since you can't
| be NORDO in the IFR system, separation is the
responsibility of the pilot's
| eyes anyway.
|
| -------------------------------
| Travis
| Lake N3094P
| PWK
|
|

Jim Macklin

November 13th 06, 08:31 AM

Not below 3000 AGL, it does above that.

"Travis Marlatte" > wrote in
message
et...
| VFR, it doesn't really matter, does it?
|
| --
| -------------------------------
| Travis
| Lake N3094P
| PWK
| "Mxsmanic" > wrote in message
| ...
| > Jim Logajan writes:
| >
| >> You can find the answer in FAR 91.121(a)(iii).
| >
| > Found it, thanks. I'm surprised that just setting it at
the airport
| > would suffice--one could conceivably fly for hundreds of
miles VFR,
| > and the altimeter could change significantly along the
way.
| >
| > --
| > Transpose mxsmanic and gmail to reach me by e-mail.
|
|

Jim Macklin

November 13th 06, 08:33 AM

I have zero actual experience with AIDS, having left the
"open market" a long long time ago.

"Grumman-581" > wrote
in message
...
| On Sun, 12 Nov 2006 17:35:40 -0600, Jim Macklin wrote:
| > You rarely get AIDS from a "real person of the opposite
| > gender" which is why GAY means "got aids yet."
|
| But do you want to stake your life on it? Because, well,
that's *exactly*
| what you're doing...
|
|
|

Grumman-581[_1_]

November 13th 06, 10:08 AM

On Mon, 13 Nov 2006 02:33:10 -0600, Jim Macklin wrote:
> I have zero actual experience with AIDS, having left the "open market" a
> long long time ago.

When I was in the Navy, it was the "good ol' days" (i.e. anything that you
could possibly catch was curable by penicillin)... Well, that's not
completely true since herpes had come around before I left, but it didn't
kill you, it just made you *wish* you were dead... <sick-grin>

These days, I doubt that it would be as fun to be single and in the
Navy... Something about having to wear steel-belted radial body condoms
just seems like it would take the fun out of meeting strange women waking
up in bed with even *stranger* women... I alway thought it was interesting
how you could go to bed with a really nice looking woman that you
encounter at the bar and wake up with something that albeit female, you
had to wonder if it walked upright...

Neil Gould

November 13th 06, 11:45 AM

Recently, Mxsmanic > posted:

> Neil Gould writes:
>
>> Ever hear of WAAS?
>
> Yes. And it's not part of GPS.
>
It's a part of my 295, and every Garmin aviation GPS newer than that.
Nothing else is relevant to me as a pilot.

Neil

Jim Macklin

November 13th 06, 01:33 PM

The formula is ...
good looking= blood alcohol level times the number of hours
since your last full nights' sleep times the number of days
since you were last laid, divided by the months since your
last VD movie.

"Grumman-581" > wrote
in message
...
| On Mon, 13 Nov 2006 02:33:10 -0600, Jim Macklin wrote:
| > I have zero actual experience with AIDS, having left the
"open market" a
| > long long time ago.
|
| When I was in the Navy, it was the "good ol' days" (i.e.
anything that you
| could possibly catch was curable by penicillin)... Well,
that's not
| completely true since herpes had come around before I
left, but it didn't
| kill you, it just made you *wish* you were dead...
<sick-grin>
|
| These days, I doubt that it would be as fun to be single
and in the
| Navy... Something about having to wear steel-belted radial
body condoms
| just seems like it would take the fun out of meeting
strange women waking
| up in bed with even *stranger* women... I alway thought it
was interesting
| how you could go to bed with a really nice looking woman
that you
| encounter at the bar and wake up with something that
albeit female, you
| had to wonder if it walked upright...

Travis Marlatte

November 13th 06, 01:45 PM

"Jim Macklin" > wrote in message
...
> Not below 3000 AGL, it does above that.
>
> "Travis Marlatte" > wrote in
> message
> et...
> | VFR, it doesn't really matter, does it?
> |

You seem to be suggesting that when flying VFR above 3000', we have an
obligation to fly specific altitudes within some tolerence and, therefore,
need to have an accurate means of determining our altitude. That is
certainly not the case. Prudent, yes. Required, no.

-------------------------------
Travis
Lake N3094P
PWK

Roy Smith

November 13th 06, 03:08 PM

"Travis Marlatte" > wrote:
> You seem to be suggesting that when flying VFR above 3000', we have an
> obligation to fly specific altitudes within some tolerence and, therefore,
> need to have an accurate means of determining our altitude. That is
> certainly not the case. Prudent, yes. Required, no.

I would consider the following to be an "obligation":

*91.159***VFR cruising altitude or flight level.

Except while holding in a holding pattern of 2 minutes or less, or while
turning, each person operating an aircraft under VFR in level cruising
flight more than 3,000 feet above the surface shall maintain the
appropriate altitude or flight level prescribed below, unless otherwise
authorized by ATC:

(a) When operating below 18,000 feet MSL and‹

(1) On a magnetic course of zero degrees through 179 degrees, any odd
thousand foot MSL altitude +500 feet (such as 3,500, 5,500, or 7,500); or

(2) On a magnetic course of 180 degrees through 359 degrees, any even
thousand foot MSL altitude +500 feet (such as 4,500, 6,500, or 8,500).

(b) When operating above 18,000 feet MSL, maintain the altitude or flight
level assigned by ATC.

Jim Macklin

November 13th 06, 03:17 PM

The word SHALL has a legal meaning...
§ 91.159 VFR cruising altitude or flight level.
Except while holding in a holding pattern of 2 minutes or
less, or while turning, each person operating an aircraft
under VFR in level cruising flight more than 3,000 feet
above the surface shall maintain the appropriate altitude or
flight level prescribed below, unless otherwise authorized
by ATC:

(a) When operating below 18,000 feet MSL and-

(1) On a magnetic course of zero degrees through 179
degrees, any odd thousand foot MSL altitude +500 feet (such
as 3,500, 5,500, or 7,500); or

(2) On a magnetic course of 180 degrees through 359 degrees,
any even thousand foot MSL altitude +500 feet (such as
4,500, 6,500, or 8,500).

(b) When operating above 18,000 feet MSL, maintain the
altitude or flight level assigned by ATC.

Last updated: August 7, 2006

"Travis Marlatte" > wrote in
message
...
| "Jim Macklin" > wrote
in message
| ...
| > Not below 3000 AGL, it does above that.
| >
| > "Travis Marlatte" > wrote
in
| > message
| > et...
| > | VFR, it doesn't really matter, does it?
| > |
|
| You seem to be suggesting that when flying VFR above
3000', we have an
| obligation to fly specific altitudes within some tolerence
and, therefore,
| need to have an accurate means of determining our
altitude. That is
| certainly not the case. Prudent, yes. Required, no.
|
| -------------------------------
| Travis
| Lake N3094P
| PWK
|
|

Mxsmanic

November 13th 06, 04:48 PM

peter writes:

> No, your statement before was that it required measurement of angles
> and "triangulation" whereas the actual procedure does not measure any
> angles at all and is closer to "trilateration" or determining the
> distances to the satellites at known positions

It does indeed measure angles, rest assured. Of course, it does a lot
more than that.

> Not sure what you mean by times being equal to sides (the units don't
> match for one thing) ...

Then let's just leave it at that. This is not a sandbox, and I don't
have time to play.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic

November 13th 06, 04:48 PM

Neil Gould writes:

> It's a part of my 295, and every Garmin aviation GPS newer than that.

Those are receivers, not the GPS. WAAS is completely independent of
GPS.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Neil Gould

November 13th 06, 05:06 PM

Recently, Mxsmanic > posted:

> Neil Gould writes:
>
>> It's a part of my 295, and every Garmin aviation GPS newer than that.
>
> Those are receivers, not the GPS. WAAS is completely independent of
> GPS.
>
When we speak of GPS _usage_ in aviation, we are specifically and almost
exclusively talking about receivers. Those are the devices that we either
have in our planes or take with us to the plane. All that is relevant is
that we understand the devices that we use. Therefore, it is easy to
establish that equipment that we don't have and don't use is of lesser
importance. That is one difference between a real life and a sim life.

Neil

Stefan

November 13th 06, 05:20 PM

Travis Marlatte schrieb:
> VFR, it doesn't really matter, does it?

I've always thought that airspace boundaries are to be respected by VFR
traffic, too.

Stefan

Mxsmanic

November 13th 06, 05:30 PM

Neil Gould writes:

> When we speak of GPS _usage_ in aviation, we are specifically and almost
> exclusively talking about receivers.

I'm not sure who "we" are, but GPS is a navigation system, not just a
receiver.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Neil Gould

November 13th 06, 06:18 PM

Recently, Mxsmanic > posted:

> Neil Gould writes:
>
>> When we speak of GPS _usage_ in aviation, we are specifically and
>> almost exclusively talking about receivers.
>
> I'm not sure who "we" are, but GPS is a navigation system, not just a
> receiver.
>
Considering where you are posting this, it should be clear that "we"
refers to "pilots of real aircraft". Even if you couldn't have figured
that one out on your own from the snippet that you responded to, it should
have been obvious from the rest of the paragraph that you chose to snip.
To make it plainly clear, VORs, NDBs, Loran, et al are also "navigation
systems", but the only components that pilots typically work with are the
receivers, just as for GPS.

Neil

Steve Foley

November 13th 06, 06:48 PM

"karl gruber" > wrote in message
...

> Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he
> can give you an education on this subject.
>
> Karl

It's entirely possible he could give me an education on this subject, but
I'd rather go to a reliable source.

Barney Rubble

November 13th 06, 07:16 PM

But it would be kind of weird to be above 10K where a Mode-C transponder is
required, but not have a radio or altimeter...

"Steven P. McNicoll" > wrote in message
nk.net...
>
> "TxSrv" > wrote in message
> . ..
>>
>> That Reg does not apply to flight within 3,000 above the surface. You
>> have to look at two other Regs to conclude that. Therefore, under 3,000,
>> an adjustable altimeter is not required per 91.205. FARs don't apply to
>> you; stay away.
>>
>
> An altimeter adjustable for barometric pressure is not required for VFR
> flight at any altitude.
>
> If you're operating at any altitude below 18,000 feet and you're equipped
> with a radio and you have an altimeter adjustable for barometric pressure
> you must set it to the current reported altimeter setting of an
> appropriate available station, VFR and IFR.
>

Newps

November 13th 06, 07:30 PM

> "Mxsmanic" > wrote in message
> ...
>
>
>>No, it is not. It's especially dangerous for RVSM flight, but it's so
>>inaccurate that it should never be used for anything,

Your complete misunderstanding of the GPS system is showing. I have two
GPS's in the plane, one panel mount and one portable. It is rare that
either one is ever off by more than 100 feet in altitude. Using GPS for
your altitude for VFR flight would be perfectly fine.

Newps

November 13th 06, 07:35 PM

Mxsmanic wrote:

> Neil Gould writes:
>
>
>>It's a part of my 295, and every Garmin aviation GPS newer than that.
>
>
> Those are receivers, not the GPS. WAAS is completely independent of
> GPS.

It's integral to the system. You receive WAAS from a satellite.

Newps

November 13th 06, 07:36 PM

Mxsmanic wrote:

> Neil Gould writes:
>
>
>>When we speak of GPS _usage_ in aviation, we are specifically and almost
>>exclusively talking about receivers.
>
>
> I'm not sure who "we" are, but GPS is a navigation system, not just a
> receiver.

Exactly, a system. WAAS is part of that system.

Mxsmanic

November 13th 06, 07:55 PM

Steve Foley writes:

> It's entirely possible he could give me an education on this subject, but
> I'd rather go to a reliable source.

Better still, just don't depend on GPS for altitude.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic

November 13th 06, 07:56 PM

Newps writes:

> Your complete misunderstanding of the GPS system is showing. I have two
> GPS's in the plane, one panel mount and one portable. It is rare that
> either one is ever off by more than 100 feet in altitude. Using GPS for
> your altitude for VFR flight would be perfectly fine.

I can see the NTSB report now: "Pilot failed to understand limitations
of GPS navigation." But as long as I'm not flying with you, I don't
care. Do what you want.

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Mxsmanic

November 13th 06, 07:56 PM

Newps writes:

> Exactly, a system. WAAS is part of that system.

No, WAAS is a separate system.

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Mxsmanic

November 13th 06, 07:57 PM

Newps writes:

> It's integral to the system.

It's completely separate.

> You receive WAAS from a satellite.

Not a GPS satellite.

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Scott Post

November 13th 06, 08:16 PM

In article >,
Mxsmanic > wrote:
>
> This is not a sandbox, and I don't
> have time to play.
>

You've got to be kidding. Count your posts.

--
Scott Post

Denny

November 13th 06, 08:17 PM

I recently experimented with flying a GPS approach using the GPS
reported altitude as the "altimeter"... First, I flew the GPS approach
track making notes as to the altimeter reading versus the GPS reported
altitude on two GPS units in the cockpit...

Now, knowing the average local offset between the GPS datum circle and
the local ground altitude as reported by the local barometric pressure
altimeter <the official pair of altimeters in the pilots lounge>... I
wound up with 79 feet as my averaged offset between the GPS datum plane
and the local altimeter... Notice I am emphasizing the word 'local'
here... Your findings will differ with your area and with what they are
doing down inside that Colorado mountain at any given instant...... A
local WAAS transmitter and a WAAS receiver in my plane would, of
course, narrow that altitude difference... Also note the altitude
delta I found is almost within allowable altimeter error...

Anyway, the second trip around I concentrated on flying the altimeter
and IVSI on the
Garmins instead of the altimeter - jeez, are they twitchy; according to
them I never did establish level flight, they always showed either
climb or descent, albeit usually at a rate of ~10 or 20 fpm... Trying
to fly for real with an altitude instrument this twitchy would be
exhausting.. The descent phase on the various legs of the approach was
interesting but doable...

So, my little experiment shows that one can do a GPS approach - after a
fashion - using the GPS altitude reporting... Knowing in advance what
the local differential between the GPS and the cold hard ground would
be nice...

I intend to next fool around with using the GPS IVSI for flying an ILS
in place of the glide slope... I have done this with the barometric
IVSI and had the radar guys watch me... Two times out of three they
said they could not see any divergence... The one time they did see it
I was definitely off the glide slope as I got sloppy on speed control
and wound up too fast...

Fun stuff to do as opposed to droning around in circles...

denny

Newps wrote:
> > "Mxsmanic" > wrote in message
> > ...
> >
> >
> >>No, it is not. It's especially dangerous for RVSM flight, but it's so
> >>inaccurate that it should never be used for anything,
>
> Your complete misunderstanding of the GPS system is showing. I have two
> GPS's in the plane, one panel mount and one portable. It is rare that
> either one is ever off by more than 100 feet in altitude. Using GPS for
> your altitude for VFR flight would be perfectly fine.

Newps

November 13th 06, 08:44 PM

Mxsmanic wrote:

> Newps writes:
>
>
>>Exactly, a system. WAAS is part of that system.
>
>
> No, WAAS is a separate system.

Wrong, as usual. It's all part of the same system.

Newps

November 13th 06, 08:45 PM

Mxsmanic wrote:

> Newps writes:
>
>
>>It's integral to the system.
>
>
> It's completely separate.
>
>
>>You receive WAAS from a satellite.
>
>
> Not a GPS satellite.
>

A satellite is a satellite. The information that spews forth from that
satellite is just information that my receiver makes use of.

Doug[_1_]

November 13th 06, 08:46 PM

It's not something you need to know, in order to use the box.

Steven P. McNicoll[_1_]

November 13th 06, 08:50 PM

"Barney Rubble" > wrote in message
...
>
> But it would be kind of weird to be above 10K where a Mode-C transponder
> is required, but not have a radio or altimeter...
>

I've been above 10K without a transponder or radio, it didn't seem weird to
me at all.

mike regish

November 13th 06, 09:34 PM

Thanks.

mike

"peter" > wrote in message >

> You're thinking of height above the ellipsoid, which can be hundreds of
> feet different from height above the geoid. But the geoid does
> represent the mean sea level height - including in places that are far
> from the sea. Internally GPS receivers generally initially calculate
> height relative to the ellipsoid model of the earth's shape (using the
> WGS-84 model parameters). However, recent models with which I'm
> familiar then apply a correction term based on an internal lookup table
> to convert the ellipsoid height to the geoid height (equivalent to
> height above MSL) at that particular location. See:
> http://www.esri.com/news/arcuser/0703/geoid1of3.html
> The altitudes that end up being displayed by the GPS after its internal
> correction are therefore based on elevation above MSL with some
> measurement uncertainty that's dependent on the current satellite
> geometry.
>

mike regish

November 13th 06, 09:36 PM

Wow. That's news. What the hell are all these GPS companies trying to sell
us?

mike

"Mxsmanic" > wrote in message
...
> Neil Gould writes:
>
>> Ever hear of WAAS?
>
> Yes. And it's not part of GPS.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

mike regish

November 13th 06, 09:41 PM

No. Famous last words are usually "Oh ****."

mike

"Mxsmanic" > wrote in message
...
> Travis Marlatte writes:
>
>> The only ATC clearances for a pressure altitude would be in the flight
>> levels. Since the question was about setting a pressure altimeter, I
>> would
>> say that the flight levels are irrelevant. Below the flight levels, ATC
>> clearances are for pressure compensated altitude above MSL, so yes it is
>> based on pressure and not true altitude but close enough. My GPS gives me
>> a
>> calculated altitude above a theoretical sea level that's also close
>> enough.
>
> "Close enough": famous last words.
>
> --
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Robert M. Gary

November 13th 06, 10:18 PM

Ron Lee

November 13th 06, 10:48 PM

Mxsmanic > wrote:

>Newps writes:
>
>> Exactly, a system. WAAS is part of that system.
>
>No, WAAS is a separate system.

GPS consists of three segments: Ground (control system), Space (the
satellites) and User (Receivers typically). WAAS is an augmentation
system to GPS. It is operated by the FAA whereas the GPS is operated
by the USAF.

Ron Lee

Ron Lee

November 13th 06, 10:51 PM

Mxsmanic > wrote:

>Steve Foley writes:
>
>> It's entirely possible he could give me an education on this subject, but
>> I'd rather go to a reliable source.
>
>Better still, just don't depend on GPS for altitude.
>

Today I flew a real airplane and compared the GPS altitude from my GNS
430 with my altimeter. GPS was usually within 40' of the altimeter
which may not be the worst.

On the ground the GPS altitude was about 40' less than the GNS 430
indicated elevation of my airport but that number may be off since the
runway does slope.

Regardless, in this case GPS derived altitude was quite good.

Ron Lee

LWG

November 13th 06, 11:07 PM

I have always found good agreement between GPS altitude and my altimeter.
And I've read all about the derivation of GPS altitude, and I wouldn't think
it would be so close.

"Ron Lee" > wrote in message
...
> Mxsmanic > wrote:
>
>>Steve Foley writes:
>>
>>> It's entirely possible he could give me an education on this subject,
>>> but
>>> I'd rather go to a reliable source.
>>
>>Better still, just don't depend on GPS for altitude.
>>
>
> Today I flew a real airplane and compared the GPS altitude from my GNS
> 430 with my altimeter. GPS was usually within 40' of the altimeter
> which may not be the worst.
>
> On the ground the GPS altitude was about 40' less than the GNS 430
> indicated elevation of my airport but that number may be off since the
> runway does slope.
>
> Regardless, in this case GPS derived altitude was quite good.
>
> Ron Lee

Grumman-581[_1_]

November 13th 06, 11:44 PM

On Mon, 13 Nov 2006 18:07:31 -0500, LWG wrote:
> I have always found good agreement between GPS altitude and my
> altimeter. And I've read all about the derivation of GPS altitude, and I
> wouldn't think it would be so close.

Part of the problem of comparing the GPS receiver output to the altimeter
output is that the GPS receiver output is digital and you end up seeing
the digits constantly changing... If you displayed the output of the GPS
receiver on an analog gauge, it might be a bit better... If you averaged
the altitude over multiple data point, it might even be even more
stable... The non-aviation GPS receivers usually update their position
every second... The Garmin GPSMAP 496 updates 5 times per second... From
what I understand, that 5 Hz update rate is considered the minimum for an
accurate aviation guidance system... I've seen others that operated at 10
Hz... The more messages that you can get per second, the more you can
average out the calculations to keep the altitude from radically jumping
around...

November 14th 06, 12:16 AM

peter wrote:
> Mxsmanic wrote:
>
>>"Jim Macklin" > writes:
>>
>>
>>>they give altitude, accurate to within a few feet.
>>
>>Unfortunately, no, they do not. GPS is accurate for lateral
>>navigation, not vertical navigation. GPS altitudes can easily be off
>>by as much as 200 feet at ground level in comparison to a correctly
>>set altimeter, and at altitude the disparity can reach 500 feet.
>>
>>The reason for this is that the angles used for triangulation of
>>lateral positions are large and permit a high level of precision, but
>>the angles for triangulation of altitude are very small and it's very
>>easy to be off by a wide margin.
>
>
> This indicates a basic lack of understanding of GPS technology. The
> GPS receiver never deals with measurement of any angles nor with
> triangulation. What is measured are the precise times of arrival of
> the signals from the satellites. Since the satellites encode the
> signals with timing information from their sychronized atomic clocks
> and also send detailed orbital data to define their own positions, the
> receiver is able to determine the relative distances to the various
> satellites based on the speed of light/radio and the observed relative
> signal delays. Using this distance information together with the known
> positions of the satellites then allows for a determination of the
> position of the receiver. Note that this never involves a measurement
> of any angles.
>
> It is true that altitude measurements are generally somewhat less
> accurate than horizontal position measurements due to the basic
> geometry of receiving satellite signals from only the satellites that
> are above you. Ideal measurement of altitude would also involve some
> satellites below you but of course their signals are blocked by the
> earth. Similarly, east-west positions are a bit better accuracy than
> north-south since the satellites are equally likely to be east and west
> of you but there's a greater likelihood of them being to the south
> rather than the north (at least from the northern hemisphere).
>
> My long-term evaluation of GPS altitude accuracy has shown that I get
> values within 35' of accurately surveyed altitudes at least 95% of the
> time ever since Selective Availability was turned off. Using the WAAS
> correction data improves this to get the accuracy down to 20' with 95%
> confidence. Both of these are based on having a reasonably
> unobstructed view of the sky (which generally isn't hard in an aircraft
> unless the antenna is poorly positioned).
>
Your impression is supported by the published nominal accuracy of GPS:
+/- 10m horizontal, +/- 20m
vertical. The reduced vertical accuracy comes from the fact that
although the sats are at ~20,000 km
their horizontal spacing can be much larger than that. As you say, the
fix is 3D and it always is a 3D
solution once the minimum 4 sats are acquired for the solution. In
addition, if your receiver can hold more than 4 sats it may be able to
average the data to improve the fix. On my 12 channel boat GPS I see a
HDOP of ~1m these days. More important perhaps is that the aviation nav.
chart I use is not referenced to WGS84 which is strange as all my
sailing charts are. I understant that the "powers that be" are trying to
settle which geoid to use but I would imagine that the height
corrections could be quite large when it all gets sorted out. Is that
why the peak obstacle height over the sea is never marked as 0' (i.e.
MSL is not conforming to the geoid of reference)?

Cheers MC(student pilot)

Bob Noel

November 14th 06, 12:25 AM

In article >,
"mike regish" > wrote:

> No. Famous last words are usually "Oh ****."

and sometimes "Watch this"

--
Bob Noel
Looking for a sig the
lawyers will hate

November 14th 06, 12:26 AM

Mxsmanic wrote:

> Ron Lee writes:
>
>
>>With SA off the vertical error is probably better that altimeter
>>accuracy requirements.
>
>
> No. The figure of hundreds of feet is with SA OFF. It's even worse
> with SA on.
>
>

The published accuracy of the vertical fix with 4 sats is +/- 20m. With
more sats and an averaging reciever I've seen the HD0P closer to 1m
which would imply a VDOP of just a few meters -without WAAS. I've read
somewhere that with new advances in data processing accuracies of the
order of 10cm should be possible without WAAS. I believe these methods
use propagation models to more accurately determine the GPS timings.

Cheers MC (student pilot)

November 14th 06, 12:27 AM

Mxsmanic wrote:

> Neil Gould writes:
>
>
>>Ever hear of WAAS?
>
>
> Yes. And it's not part of GPS.
>

What do you think the WAAS is "augmenting"?

Cheers

Mxsmanic

November 14th 06, 12:28 AM

Newps writes:

> Wrong, as usual. It's all part of the same system.

That's not the way the DoD looks at it, and they built GPS.

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Mxsmanic

November 14th 06, 12:29 AM

Newps writes:

> A satellite is a satellite. The information that spews forth from that
> satellite is just information that my receiver makes use of.

Hardly. GPS satellites are very special, as are the transmissions
they produce. WAAS satellites are just commercial satellites
transmitting fairly ordinary information in fairly ordinary ways.

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Mxsmanic

November 14th 06, 12:30 AM

mike regish writes:

> Wow. That's news. What the hell are all these GPS companies trying to sell
> us?

Receivers that combine several different technologies, one of which is
GPS. Moving maps aren't part of GPS, either, and neither are
autopilots.

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Mxsmanic

November 14th 06, 12:37 AM

DR writes:

> What do you think the WAAS is "augmenting"?

If it were part of GPS, it couldn't augment itself. QED.

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Ron Lee

November 14th 06, 12:41 AM

DR > wrote:
>On my 12 channel boat GPS I see a
>HDOP of ~1m these days.

HDOP is unitless.

Ron Lee

Travis Marlatte

November 14th 06, 04:05 AM

"Jim Macklin" > wrote in message
...
> The word SHALL has a legal meaning...
> (1) On a magnetic course of zero degrees through 179
> degrees, any odd thousand foot MSL altitude +500 feet (such
> as 3,500, 5,500, or 7,500); or

You got me. I'll confess. I forgot that those were actually regulations. I
fly them. I just forgot they were regs.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 14th 06, 04:11 AM

"Mxsmanic" > wrote in message
...
> It does indeed measure angles, rest assured. Of course, it does a lot
> more than that.

It is complex. But it's not triangulation. If it were, you would only need
two well-placed satellites to fix a position. You need three satellites to
fix a 2-D position and four to fix a 3-D.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 14th 06, 04:19 AM

"Mxsmanic" > wrote in message
...
> Newps writes:
>
>> Wrong, as usual. It's all part of the same system.
>
> That's not the way the DoD looks at it, and they built GPS.

Excuse me? The DoD didn't build all of the GPS receivers out there either.
Are they not part of the GPS system by your definition? The only thing the
DoD built (or more accurately, bought) is the license to the technology and
to the satellites. The also bought military spec receivers. The rest is
commercial, free market use of the broadcast signals. WAAS included.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 14th 06, 04:26 AM

"Stefan" > wrote in message
. ..
> Travis Marlatte schrieb:
>> VFR, it doesn't really matter, does it?
>
> I've always thought that airspace boundaries are to be respected by VFR
> traffic, too.
>
> Stefan

I agree that there are many reasons why I need to know my pressure
compensated altitude to avoid busting airspace or illegally flying to close
to person or property.

The discussion was about the life-saving necessity of an accurate altitude
instrument. MX was claiming that GPS is inaccurate enough to kill you. I
presume that that can only happen in one of two ways 1) I fly into the
ground believing my inaccurate intrument or 2) I fly at an altitude
different than the regulated VFR cruise altitudes and run into another
plane.

I think both are highly unlikely in VFR conditions no matter how inaccurate
the instrument.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte

November 14th 06, 04:31 AM

>>
>> Regardless, in this case GPS derived altitude was quite good.
>>
>> Ron Lee
>
>

One of the challenges with GPS fixes (both horizontal and vertical) is that
there is a statistical distribution of the error. Most receivers advertise
being within 15' laterally, 90% of the time. It's that other 10% that you
gotta worry about.

Surveyors improve their accuracy by leaving the GPS receiver stationary to
get a statistical sampling that can reveal the true position.

--
-------------------------------
Travis
Lake N3094P
PWK

Morgans[_2_]

November 14th 06, 04:45 AM

>> Those are receivers, not the GPS. WAAS is completely independent of
>> GPS.
>
>
>
> It's integral to the system. You receive WAAS from a satellite.

Typical of the troll's pattern. (not you newps - I have the real troll blocked)
Argue with what we know to be true.

I don't understand why people continue with him.
--
Jim in NC

Ron Lee

November 14th 06, 05:20 AM

"Travis Marlatte" > wrote:
>>>
>>> Regardless, in this case GPS derived altitude was quite good.
>>>
>>> Ron Lee
>
>One of the challenges with GPS fixes (both horizontal and vertical) is that
>there is a statistical distribution of the error. Most receivers advertise
>being within 15' laterally, 90% of the time. It's that other 10% that you
>gotta worry about.
>
>Surveyors improve their accuracy by leaving the GPS receiver stationary to
>get a statistical sampling that can reveal the true position.
>
>Travis

Correct point Travis but since I fly VFR I use other references to
develop a "blended" solution. Plus my primary GPS receiver has
integrity to catch the really bad errors (rare).

Ron Lee

Jim Macklin

November 14th 06, 07:06 AM

WE all need to review, perhaps that is the best thing about
these newsgroups.

"Travis Marlatte" > wrote in
message
m...
| "Jim Macklin" > wrote
in message
| ...
| > The word SHALL has a legal meaning...
| > (1) On a magnetic course of zero degrees through 179
| > degrees, any odd thousand foot MSL altitude +500 feet
(such
| > as 3,500, 5,500, or 7,500); or
|
| You got me. I'll confess. I forgot that those were
actually regulations. I
| fly them. I just forgot they were regs.
|
| -------------------------------
| Travis
| Lake N3094P
| PWK
|
|

gpsman

November 14th 06, 07:41 AM

Stefan

November 14th 06, 05:46 PM

Travis Marlatte schrieb:

> The discussion was about the life-saving necessity of an accurate altitude
> instrument.

Sorry to nitpick, but the discussion was about how to set the altimeter
if you can't get the lastest pressure data with the radio. Some guy
proposed to use the GPS altitude as reference, and then some (other?)
guy stated that you better always fly on GPS altitudes, because it's
more accurate anyway. This was the point when I jumped in and said no,
don't do this.

Only then:

> MX was claiming that GPS is inaccurate enough to kill you.

Actually, it can. It's accurate within a few meters (depending on how
"enhanced" it is), but only in some 95% or so of the time and in some
90% or so places (the correct numbers don't matter). Which means that if
you happen to be in the wrong place at the wrong time, believing in the
GPS can actually kill you, at least theoretically. Of course I'm rather
looking out of the window when I fly in VMC, so this isn't an issue in
real life for me.

Stefan

Mxsmanic

November 14th 06, 06:04 PM

Travis Marlatte writes:

> Excuse me? The DoD didn't build all of the GPS receivers out there either.

I wasn't talking about the receivers.

--
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Neil Gould

November 14th 06, 06:27 PM

Recently, Mxsmanic > posted:

> Travis Marlatte writes:
>
>> Excuse me? The DoD didn't build all of the GPS receivers out there
>> either.
>
> I wasn't talking about the receivers.
>
You certainly were, given that your claim was that their altitude
reporting was dangerously inaccurate. No other component in the system
reports a particular altitude. Please try to keep up with your side of a
discussion.

Neil

Ron Lee

November 14th 06, 07:14 PM

Stefan > wrote:

>Actually, it can. It's accurate within a few meters (depending on how
>"enhanced" it is), but only in some 95% or so of the time and in some
>90% or so places (the correct numbers don't matter). Which means that if
>you happen to be in the wrong place at the wrong time, believing in the
>GPS can actually kill you, at least theoretically.

>Stefan

Actually the 95% number often associated with an accuracy is a
confidence. It is not an availability value. Thus under the
identical conditions 95 times out of a hundred you would be at that
accuracy or better. No value of the other 5% is given and may not be
very much worse. At least in this case not enough to be
life-threatening. Of course if VFR use your eyes and look outside.

Ron Lee

Doug[_1_]

November 14th 06, 07:34 PM

No one has mentioned using the manifold pressure guage as an altimeter.

November 14th 06, 08:34 PM

Mxsmanic wrote:
> DR writes:
>
>
>>What do you think the WAAS is "augmenting"?
>
>
> If it were part of GPS, it couldn't augment itself. QED.
>

You can wriggle but you haven't answered my question. It's not QED. If
you can't think about it let me help you. The WAAS is NOT a positioning
system is it? What does it *actually* augment?

Cheers

November 14th 06, 08:41 PM

Ron Lee wrote:

> DR > wrote:
>
>>On my 12 channel boat GPS I see a
>>HDOP of ~1m these days.
>
>
> HDOP is unitless.
>

Good point. I'm sorry that I was being so loose. But you could descibe
the 1 sigma dilutuion of precision in terms of distance at your position
and time -right? I was actually amazed to see that my Ryatheon GPS put
my boat right in the center of the correct dock -implying 2m accuracy
(or better)!

Cheers MC

Doug[_1_]

November 14th 06, 08:48 PM

Notice how IFR training doesn't include procedures if your altimeter
fails. Reason?

Jim Stewart wrote:
> Doug wrote:
>
> > No one has mentioned using the manifold pressure guage as an altimeter.
>
> Or how many times your ears popped (:

gpsman

November 14th 06, 09:11 PM

Mxsmanic wrote:
> Newps writes:
>
> > A satellite is a satellite. The information that spews forth from that
> > satellite is just information that my receiver makes use of.
>
> Hardly. GPS satellites are very special, as are the transmissions
> they produce. WAAS satellites are just commercial satellites
> transmitting fairly ordinary information in fairly ordinary ways.

WAAS consists of approximately 25 ground reference stations positioned
across the United States that monitor GPS satellite data. Two master
stations, located on either coast, collect data from the reference
stations and create a GPS correction message.

This correction accounts for GPS satellite orbit and clock drift plus
signal delays caused by the atmosphere and ionosphere. The corrected
differential message is then broadcast through one of two geostationary
satellites, or satellites with a fixed position over the equator. The
information is compatible with the basic GPS signal structure, which
means any WAAS-enabled GPS receiver can read the signal.

A WAAS-capable receiver can give you a position accuracy of better than
three meters 95 percent of the time.
http://www.garmin.com/aboutGPS/waas.html
-----

- gpsman

Ron Lee

November 14th 06, 10:38 PM

DR > wrote:

>>>On my 12 channel boat GPS I see a
>>>HDOP of ~1m these days.
>>
>> HDOP is unitless.
>>
>Good point. I'm sorry that I was being so loose. But you could descibe
>the 1 sigma dilutuion of precision in terms of distance at your position
>and time -right? I was actually amazed to see that my Ryatheon GPS put
>my boat right in the center of the correct dock -implying 2m accuracy
>(or better)!
>
>Cheers MC
>
I have to think about it. Position accuracy is given by this generic
equation:

DOP x UERE x 2 = position accuracy (2 sigma or 95%) where UERE is the
combination of the space and user equipment error sources.

Ron Lee

gpsman

November 14th 06, 11:36 PM

Travis Marlatte wrote:
> "Mxsmanic" > wrote in message
> ...
> > It does indeed measure angles, rest assured. Of course, it does a lot
> > more than that.
>
> It is complex. But it's not triangulation.

"GPS receivers use triangulation of the GPS satellites' navigational
signals to determine their location."
http://msl.jpl.nasa.gov/Programs/gps.html

A GPSr does not measure angles, it measures timing and ranging, as in
"NAVSTAR" (NAVigation Signal Timing And Ranging).

"In addition to knowing the distance to a satellite, a receiver needs
to know the satellite's exact position in space; this is known as its
ephemeris. Each satellite's signal transmits ephemeris information
about its exact orbital location. The GPS receiver uses this
information to precisely establish the position of the satellite."
http://www.lerc.nasa.gov/WWW/MAEL/ag/gps.htm
-----

- gpsman

Newps

November 15th 06, 03:57 AM

Doug wrote:
> No one has mentioned using the manifold pressure guage as an altimeter.
>

Because you can't do it in the air, unless you stop the engine first.

Travis Marlatte

November 15th 06, 04:24 AM

"gpsman" > wrote in message
ups.com...
> Travis Marlatte wrote:
>> "Mxsmanic" > wrote in message
>> ...
>> > It does indeed measure angles, rest assured. Of course, it does a lot
>> > more than that.
>>
>> It is complex. But it's not triangulation.
>
> "GPS receivers use triangulation of the GPS satellites' navigational
> signals to determine their location."
> http://msl.jpl.nasa.gov/Programs/gps.html
>
> A GPSr does not measure angles, it measures timing and ranging, as in
> "NAVSTAR" (NAVigation Signal Timing And Ranging).
>
> "In addition to knowing the distance to a satellite, a receiver needs
> to know the satellite's exact position in space; this is known as its
> ephemeris. Each satellite's signal transmits ephemeris information
> about its exact orbital location. The GPS receiver uses this
> information to precisely establish the position of the satellite."
> http://www.lerc.nasa.gov/WWW/MAEL/ag/gps.htm
> -----
>
> - gpsman
>

I had to look up the definition of triangulation. I did not realize that it
included range finding as a technique. I stand corrected. GPS receivers
don't measure angles.

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Travis
Lake N3094P
PWK