How different is aviation GPS?

[[email protected]]

As part of my work (civil engineer), I routinely use GPS equipment in
surveying. And construction specifications usually calls for this
equipment to be held stationary for as much as three hours where
crucial transition points are to be located, and for up to twenty
minutes at less important locations.

I guess since you folk use GPS to navigate all across the globe and
requires to be both very precise and instantaneous, my equipment is
very much inferior to what's used in aviation.

Has anyone here used the type of equipment I'm mentioning? You should
see the time the thing I use takes to stabilize itself to show the
elevation... you'd comfortably CFIT if you had that in your airplane
:))

Ramapriya

[Bob Noel]

In article . com>,
wrote:

[snip]
> Has anyone here used the type of equipment I'm mentioning? You should
> see the time the thing I use takes to stabilize itself to show the
> elevation... you'd comfortably CFIT if you had that in your airplane

yeah, but what is the accuracy of your GPS?
I suspect it is much more accurate than needed for aviation.

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

[Robert M. Gary]

You don't need much acuracy to find an airport. They tend to be the
large things with the long areas of pavement. As of today, the use of
GPS as sole naviation for preceision approachs in low visibility (fog)
is limited to "non precision" approaches, just good enough to get you
down to about 500 feet and near the end of the runway. At some point it
should be good enough to actually touch down on the runway with (LAAS).

-Robert


wrote:
> As part of my work (civil engineer), I routinely use GPS equipment in
> surveying. And construction specifications usually calls for this
> equipment to be held stationary for as much as three hours where
> crucial transition points are to be located, and for up to twenty
> minutes at less important locations.
>
> I guess since you folk use GPS to navigate all across the globe and
> requires to be both very precise and instantaneous, my equipment is
> very much inferior to what's used in aviation.
>
> Has anyone here used the type of equipment I'm mentioning? You should
> see the time the thing I use takes to stabilize itself to show the
> elevation... you'd comfortably CFIT if you had that in your airplane
> :))
>
> Ramapriya

[Travis Marlatte]

No real difference. The stabilization that you refer to is getting to an
accuracy that is not needed for ground-based or aviation-based navigation.
Typical near-instanstaneous accuracy for even cheap GPS receivers is a few
meters. Not good enough for surveying but certainly good enough to find a
60-foot wide (or better) runway.


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

> wrote in message
oups.com...
> As part of my work (civil engineer), I routinely use GPS equipment in
> surveying. And construction specifications usually calls for this
> equipment to be held stationary for as much as three hours where
> crucial transition points are to be located, and for up to twenty
> minutes at less important locations.
>
> I guess since you folk use GPS to navigate all across the globe and
> requires to be both very precise and instantaneous, my equipment is
> very much inferior to what's used in aviation.
>
> Has anyone here used the type of equipment I'm mentioning? You should
> see the time the thing I use takes to stabilize itself to show the
> elevation... you'd comfortably CFIT if you had that in your airplane
> :))
>
> Ramapriya
>

[[email protected]]

Bob Noel wrote:
>
> [snip]
> > Has anyone here used the type of equipment I'm mentioning? You should
> > see the time the thing I use takes to stabilize itself to show the
> > elevation... you'd comfortably CFIT if you had that in your airplane
>
> yeah, but what is the accuracy of your GPS?
> I suspect it is much more accurate than needed for aviation.


Not really Bob. Most building, refinery and pipeline surveys accept a
+/- 0.8 centimeter variation, so it isn't pinpoint like in aircraft, I
think.

Btw, just out of interest - does TCAS use GPS data in some ways too?

Ramapriya

[Bob Noel]

In article . com>,
wrote:

> Not really Bob. Most building, refinery and pipeline surveys accept a
> +/- 0.8 centimeter variation, so it isn't pinpoint like in aircraft, I
> think.

+/- 50+ meters is plenty accurate enough for aviation use in determining
lateral position. +/- 10 meters vertically would be wonderful but not always
obtained.

>
> Btw, just out of interest - does TCAS use GPS data in some ways too?

No.

ADS-B does, but not TCAS.

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

[[email protected]]

Bob Noel wrote:
>
> +/- 50+ meters is plenty accurate enough for aviation use in determining lateral position. +/- 10 meters vertically would be wonderful but not always obtained.


I confess I couldn't have guessed that :))


> > Btw, just out of interest - does TCAS use GPS data in some ways too?
>
> No.
>
> ADS-B does, but not TCAS.


Thanks, but how exactly does the TCAS get the coordinates of the
transponder it talks to, in determining whether or not an evasive
mechanism needs setting in? Anything like a +/- 50 meter accuracy there
would be disastrous.

Ramapriya

[Peter Duniho]

> wrote in message
ups.com...
> Thanks, but how exactly does the TCAS get the coordinates of the
> transponder it talks to, in determining whether or not an evasive
> mechanism needs setting in? Anything like a +/- 50 meter accuracy there
> would be disastrous.

TCAS typically uses the transponder output to determine altitude of the
other traffic, and basic radio direction and range-finding processing to
determine direction and distance.

For the purpose of TCAS, +/- 50 meters is perfectly sufficient. Alerts are
provided at distances much greater than that, and an error of 50 meters
would affect the timing of the alert by a second or less. When in a
congested area, where TCAS is most typically important, airplanes are
traveling on the order of 50-100 meters per second.

You seem to be under the impression that flying involves a very high degree
of accuracy with respect to position information. That's simply not true.
Even in a differential GPS precision approach or a Cat III ILS, accuracy is
only to within a meter or so, and for anything else tens, hundreds, or even
thousands of meters is sufficient (depending on the exact situation).

In addition, while you are relying on your GPS receiver for altitude,
airplanes almost never do. They have barometric altimeters that provide a
different reference (not necessarily more accurate...just different) that is
more appropriate for the operation of an airplane (generally speaking). So
the GPS data is used only in two dimensions, in which the instantaneous
accuracy is generally much better, as compared to 3D accuracy including
altitude.

Your demand for accuracy in construction is FAR greater than any need
aviation has. As far as the difference in the GPS receiver goes, it is
likely that the basic operation of the GPS receiver you are using is
identical to that of an aviation handheld.

That said, I'm surprised you are using sample averaging to gain the accuracy
you need. Surveyors do use differential GPS with great success, and it
provides similar accuracy to what you're getting, only in a matter of
seconds rather than hours. (DGPS used for surveys has better accuracy than
that used for aviation because the reference point is so close to the
measured location in the case of surveying, whereas it may be quite a
distance away for aviation).

Pete

[Thomas Borchert]

> I guess since you folk use GPS to navigate all across the globe and
> requires to be both very precise and instantaneous, my equipment is
> very much inferior to what's used in aviation.
>

Well, several wrong assumptions here.

1. Most airline aircraft do not or not regularly use GPS to "navigate
across the globe". They use inertial navigation. Some use inertial
navigation with position updates from GPS. GPS is typically used in
general aviation aircraft - because it is cheap.

2. For enroute navigation, GPS accuracy of the standard signal (50
meters or so) is plenty accurate, altitude is measured with
barometrics, not GPS. For approaches to airports, the prevalent method
of navigation is not GPS, but other means (google ILS and VOR). GPS
approaches can make do with standard GPS signals, however, in the US
more and more approaches using WAAS (a method of differential GPS) as
an enhanced signal. However, the approaches down to an automatic
landing are never done with GPS, they use ILS.

2. The key problem in aviation with GPS is immediate feedback to the
airplane in case the signal goes bad, aka signal integrity monitoring.
Certified aviation GPS receivers have higher standards in that regard.

--
Thomas Borchert (EDDH)

[Paul Tomblin]

In a previous article, said:
>I guess since you folk use GPS to navigate all across the globe and
>requires to be both very precise and instantaneous, my equipment is
>very much inferior to what's used in aviation.

Years and years ago, I was told how to access some diagnostic modes on my
Garmin GPSMAP 195. The diagnostic mode showed the state of all sorts of
internal stuff, and from that it was apparent that one major difference
between the aviation 195 and the marine 175 is that the 195 had
temperature and pressure sensors. I suspect they put that in there so
that they could correct the results from the GPS radio for those factors.
The other major difference was that the 195 had WAY more flash memory to
store all those aviation waypoints.

But the fact of the matter is that aviation or not, GPS doesn't do
altitude as precisely as position because of the basic geometry. There
are numerous explanations of why on-line. So for aviation use, we either
need WAAS (which might or might not be available) or use a barometric
altimeter for altitude, and for surveying you let it sit so it can
accumulate a lot of data and integrate it.

As a former surveyor, I can tell you that the altitude requirements for
surveying are a lot more precise than for aviation - if my bridge abutment
is built 2 feet high, I'm getting fired. If my plane is 2 feet high,
nobody is going to notice.

--
Paul Tomblin > http://xcski.com/blogs/pt/
"All life is transitory. A dream. We all come together in the same place at
the end of time. If I don't see you again here, I will see you in a little
while in the place where no shadows fall." - Delenn

[Jay Masino]

Paul Tomblin > wrote:
> Years and years ago, I was told how to access some diagnostic modes on my
> Garmin GPSMAP 195. The diagnostic mode showed the state of all sorts of
> internal stuff, and from that it was apparent that one major difference
> between the aviation 195 and the marine 175 is that the 195 had
> temperature and pressure sensors. I suspect they put that in there so
> that they could correct the results from the GPS radio for those factors.
> The other major difference was that the 195 had WAY more flash memory to
> store all those aviation waypoints.

Paul,
Do you happen to remember what the keystrokes were to get to the 195's
diagnostic mode? I'd be curious to see what mine is saying.

--- Jay


--

Jay Masino "Home is where the critters are"
http://www.JayMasino.com
http://www.OceanCityAirport.com
http://www.oc-Adolfos.com

[B A R R Y]

Paul Tomblin wrote:
>
> Years and years ago, I was told how to access some diagnostic modes on my
> Garmin GPSMAP 195. The diagnostic mode showed the state of all sorts of
> internal stuff, and from that it was apparent that one major difference
> between the aviation 195 and the marine 175 is that the 195 had
> temperature and pressure sensors.

I've had several handheld, general purpose / outdoor GPS units that gave
temperature and possibly (I don't clearly remember) pressure in
diagnostic mode.

Many newer marine units have external water temperature, depth sensor,
and water speed sensor capabilities, as well as the same memory
capabilites as aviation units. While airplanes need aviation waypoints
and related information, boaters need serious waterway chart storage, so
memory is just as important. The better marine units have the
waterborne equivalent of our VFR charts stored and displayed by the
unit. I really think price differences have more to do with our $70
inner tubes and $50 carriage bolts than technology differences.

Here's one link to diagnostic modes, use at your own risk:
<http://www.gpsinformation.org/dale/secret.htm>

The rest of the site:
<http://www.gpsinformation.org> is got enough info to spin your head! <G>

We've flown with a Garmin 196 next to an eTrex Vista and a 60CS (with an
included magnetic compass and barometric altimeter), the 196 was no more
accurate than the $3-400 handheld units. My co-owner has the marine
clone of the 296, loaded with "BlueCharts", and the unit is just as
accurate, feature laden, and powerful as the aviation model.

[Matt Barrow]

> wrote in message
oups.com...
> As part of my work (civil engineer), I routinely use GPS equipment in
> surveying. And construction specifications usually calls for this
> equipment to be held stationary for as much as three hours where
> crucial transition points are to be located, and for up to twenty
> minutes at less important locations.
>
> I guess since you folk use GPS to navigate all across the globe and
> requires to be both very precise and instantaneous, my equipment is
> very much inferior to what's used in aviation.
>
> Has anyone here used the type of equipment I'm mentioning? You should
> see the time the thing I use takes to stabilize itself to show the
> elevation... you'd comfortably CFIT if you had that in your airplane
> :))
>
Aircraft GPS systems are accurate to about 30 feet; how well do you think
that would work if your building a bridge, or more importantly, a tunnel?

[B A R R Y]

Matt Barrow wrote:
>
> Aircraft GPS systems are accurate to about 30 feet; how well do you think
> that would work if your building a bridge, or more importantly, a tunnel?
>

Some, maybe even many, aviation units can get far better accuracy with
differential and WAAS corrections.

[Roy Smith]

In article >,
B A R R Y > wrote:

> Many newer marine units have external water temperature, depth sensor,
> and water speed sensor capabilities, as well as the same memory
> capabilites as aviation units. While airplanes need aviation waypoints
> and related information, boaters need serious waterway chart storage, so
> memory is just as important. The better marine units have the
> waterborne equivalent of our VFR charts stored and displayed by the
> unit. I really think price differences have more to do with our $70
> inner tubes and $50 carriage bolts than technology differences.

Marine units (at least the high-end ones) often have tide charts built into
them too. Unlike winds aloft, tide heights and tidal currents are very
predictable and can be calculated years in advance. A good GPS unit will
have these tables built in, and can superimpose on the displayed chart a
field of current vectors, and tidal water depths.

Marine units also have a feature which is very handy on a boat, but more or
less useless on an airplane (unless you're hauling skydivers). Hit the MOB
(Man Over Board) button, and the unit will immediately store your current
position as a user waypoint and start navigating to it.

They also often have an anchor watch feature, which sounds an alarm if you
ever get more than N feet away from your current position. You do then
after you've anchored to alert you to the possibility that your anchor is
dragging (you set N to however many feet of rode you've put out).

Oh, yeah, and marine units tend to be waterproof. I don't know of many
aviation units that are designed to 1) float, and 2) survive be submersed
in salt water.

[Stubby]

wrote:
> Bob Noel wrote:
>> [snip]
>>> Has anyone here used the type of equipment I'm mentioning? You should
>>> see the time the thing I use takes to stabilize itself to show the
>>> elevation... you'd comfortably CFIT if you had that in your airplane
>> yeah, but what is the accuracy of your GPS?
>> I suspect it is much more accurate than needed for aviation.
>
>
> Not really Bob. Most building, refinery and pipeline surveys accept a
> +/- 0.8 centimeter variation, so it isn't pinpoint like in aircraft, I
> think.

I had my property surveyed and asked what the accuracy was. The
surveyor replied that 0.1 foot is the standard for most applications and
0.01 foot is required for commercial, high-precision applications.
They use GPS (DGPS??) but I don't know how.

[Jim Macklin]

Aircraft systems are designed to rather wide standards,
measured in meters and not the centimeter accuracy of a
surveyor's system. An aviation GPS will lock on to all
visible satellites and begin giving navigation solutions
within just a few minutes. The aviation and
automotive-marine-hiker GPS include a navigation computer
and will to some degree do all the calculations and display
results. Your impression about aircraft accuracy is 100%
backwards.


You can find the specifications for GPS systems on the
Garmin or other makers web sites.


> wrote in message
oups.com...
| Bob Noel wrote:
| >
| > [snip]
| > > Has anyone here used the type of equipment I'm
mentioning? You should
| > > see the time the thing I use takes to stabilize itself
to show the
| > > elevation... you'd comfortably CFIT if you had that in
your airplane
| >
| > yeah, but what is the accuracy of your GPS?
| > I suspect it is much more accurate than needed for
aviation.
|
|
| Not really Bob. Most building, refinery and pipeline
surveys accept a
| +/- 0.8 centimeter variation, so it isn't pinpoint like in
aircraft, I
| think.
|
| Btw, just out of interest - does TCAS use GPS data in some
ways too?
|
| Ramapriya
|

[Stubby]

Roy Smith wrote:
> In article >,
> B A R R Y > wrote:
>
> .....
> Hit the MOB
> (Man Over Board) button, and the unit will immediately store your current
> position as a user waypoint and start navigating to it.
>
We use the MOB feature to memorize where we parked the car while
Geocaching. Also, MARK saves a point on a route list. We "drop a mark"
every time a decision (to turn etc) is made. The route can be reversed
to get back to the car.

[Andrew Sarangan]

wrote:
> As part of my work (civil engineer), I routinely use GPS equipment in
> surveying. And construction specifications usually calls for this
> equipment to be held stationary for as much as three hours where
> crucial transition points are to be located, and for up to twenty
> minutes at less important locations.
>
> I guess since you folk use GPS to navigate all across the globe and
> requires to be both very precise and instantaneous, my equipment is
> very much inferior to what's used in aviation.
>
> Has anyone here used the type of equipment I'm mentioning? You should
> see the time the thing I use takes to stabilize itself to show the
> elevation... you'd comfortably CFIT if you had that in your airplane
> :))
>
> Ramapriya


In addition to what every one else mentioned, aviation GPS also has a
feature known as RAIM which stands for Receiver Autonomous Integrity
Monitoring. It warns you of any problems with the satellite signals.
Some have RAIM prediction as well.

[Matt Barrow]

"B A R R Y" > wrote in message
. net...
> Matt Barrow wrote:
>>
>> Aircraft GPS systems are accurate to about 30 feet; how well do you think
>> that would work if your building a bridge, or more importantly, a tunnel?
>
> Some, maybe even many, aviation units can get far better accuracy with
> differential and WAAS corrections.
>

WAAS will provide about 10 feet, but that only for approaches with WAAS
augmentation.

See nice graphic at http://www.garmin.com/aboutGPS/waas.html

[B A R R Y]

Matt Barrow wrote:
>
> WAAS will provide about 10 feet, but that only for approaches with WAAS
> augmentation.
>

WAAS actually works anytime the satellite correction signal is
available, you don't have to be flying an approach.

My Garmin 60CS, as well the eTrex Vista I previously owned, both use
WAAS corrections. I can't remember the last time I didn't get the
corrections.

[[email protected]]

Paul Tomblin wrote:
>
> As a former surveyor, I can tell you that the altitude requirements for
> surveying are a lot more precise than for aviation - if my bridge abutment
> is built 2 feet high, I'm getting fired. If my plane is 2 feet high,
> nobody is going to notice.


Looks as if I'm missing something central here, as usual :)

If there is a 4-hour flight that passes over some 10 waypoints and if
the FMGS keeps getting data that's off by 50 meters or so, am I to
understand that the aircraft will still make heading changes, etc.
that'd be in accordance with the programmed flight plan and that none
of the waypoints will be missed or indeed the final destination
precisely arrived at?

And someone mentioned an acceptable accuracy of 0.1 foot in property
surveying. If surveyors in my industry had that much latitude, there'd
be a lot of equipment skids that'd get installed quite inappropriately,
with lots of patched modifications thereon :)

Ramapriya

[Stubby]

B A R R Y wrote:
> Matt Barrow wrote:
>>
>> WAAS will provide about 10 feet, but that only for approaches with
>> WAAS augmentation.
>>
>
> WAAS actually works anytime the satellite correction signal is
> available, you don't have to be flying an approach.
>
> My Garmin 60CS, as well the eTrex Vista I previously owned, both use
> WAAS corrections. I can't remember the last time I didn't get the
> corrections.

A friend bought a 60CS recently and we noticed that my old Garmin-12 got
better reception and seemed more accurate. We called Garmin and the
fellow we talk with was surprised because both units use the same chips
inside!

[real_name]

In article . com>,
wrote:

> Looks as if I'm missing something central here, as usual :)
>
> If there is a 4-hour flight that passes over some 10 waypoints and if
> the FMGS keeps getting data that's off by 50 meters or so,

the errors don't accumulate.

[Montblack]

("Andrew Sarangan" wrote)
> In addition to what every one else mentioned, aviation GPS also has a
> feature known as RAIM which stands for Receiver Autonomous Integrity
> Monitoring. It warns you of any problems with the satellite signals. Some
> have RAIM prediction as well.


How does RAIM prediction work, I wonder?

The GPS unit knows where you are, knows where you're heading, and knows
where x number of signals will be - based on the satellites' "predictable"
orbits.... <g>

Ok. No clue.


Montblack

[Paul Tomblin]

In a previous article, (Jay Masino) said:
>Paul Tomblin > wrote:
>> Years and years ago, I was told how to access some diagnostic modes on my
>> Garmin GPSMAP 195. The diagnostic mode showed the state of all sorts of
>Paul,
> Do you happen to remember what the keystrokes were to get to the 195's
>diagnostic mode? I'd be curious to see what mine is saying.

Don't remember - it was probably 10 years ago, and probably on this
newsgroup. You might want to see if Google Groups has it. There were two
combinations of three buttons you could hold down as you started it - one
wiped all your routes, tracks and user waypoints, and the other started a
diagnostic mode. Don't get them confused.


--
Paul Tomblin > http://xcski.com/blogs/pt/
"All life is transitory. A dream. We all come together in the same place at
the end of time. If I don't see you again here, I will see you in a little
while in the place where no shadows fall." - Delenn

[Paul Tomblin]

In a previous article, Stubby > said:
>I had my property surveyed and asked what the accuracy was. The
>surveyor replied that 0.1 foot is the standard for most applications and
>0.01 foot is required for commercial, high-precision applications.
>They use GPS (DGPS??) but I don't know how.

You need a better surveyor. When I was doing road construction layout, we
were expected to get the marks within 5-7 millimeters. And when they
actually did the construction, they were allowed to be within 2-3
centimeters. (You've heard the expression: measure with a micrometer,
mark with chalk, cut with an axe.) Legal surveyors were supposed to be
*far* more accurate than us. For instance, we just held the chain (that's
the "measuring tape" to you) or laid it down on the ground. Legal
surveyors had a special device to make sure they were holding exactly the
right amount of tension on the chain because that's what it was calibrated
for.

--
Paul Tomblin > http://xcski.com/blogs/pt/
ALL programs are poems, it's just that not all programmers are poets.
-- Jonathan Guthrie in the scary.devil.monastery

[[email protected]]

Matt Barrow wrote:
> >
> Aircraft GPS systems are accurate to about 30 feet; how well do you think
> that would work if your building a bridge, or more importantly, a tunnel?


If I was owner, I'd end up with two tunnels for the price of one ;)

Ramapriya

[Paul Tomblin]

In a previous article, B A R R Y > said:
>My Garmin 60CS, as well the eTrex Vista I previously owned, both use
>WAAS corrections. I can't remember the last time I didn't get the
>corrections.

I have a Garmin 296 with WAAS, and I get WAAS nearly every time when I'm
flying (but not always), but I only seem to get it about half the time
when I'm driving. But I live, fly and drive up near the Canadian border,
so maybe the coverage isn't as good up here.

--
Paul Tomblin > http://xcski.com/blogs/pt/
Real Time, adj.:
Here and now, as opposed to fake time, which only occurs there and then.

[Peter Duniho]

> wrote in message
oups.com...
> Looks as if I'm missing something central here, as usual :)

Could be. :)

> If there is a 4-hour flight that passes over some 10 waypoints and if
> the FMGS keeps getting data that's off by 50 meters or so, am I to
> understand that the aircraft will still make heading changes, etc.
> that'd be in accordance with the programmed flight plan and that none
> of the waypoints will be missed or indeed the final destination
> precisely arrived at?

That is correct. The presumed 50 meter accuracy is constant throughout the
flight. It's not as though it's additive for each waypoint (or worse, as a
continuous function along the flight). Though frankly, even if it were,
you'd only be off by 500 meters after 10 waypoints which is still "no big
deal".

That's one of the many nice things about GPS. It is a continuous readout of
one's current position and any forward-looking navigation solution can be
derived from the instantaneous position information, without any previous
position information affecting the future calculations.

And of course, again...being 50 meters off in aviation just isn't that big
of a problem. Heck, being a mile off in aviation isn't that big of a
problem most of the time.

> And someone mentioned an acceptable accuracy of 0.1 foot in property
> surveying. If surveyors in my industry had that much latitude, there'd
> be a lot of equipment skids that'd get installed quite inappropriately,
> with lots of patched modifications thereon :)

Well, as that poster pointed out, it depends on the situation. But property
surveying for non-commercial purposes isn't likely to be used for any sort
of actual construction (except possibly locating a building, and for sure no
one is going to care if a building is off one inch one direction or another,
especially in a non-commercial situation).

As for equipment skids and such, since I don't know the details of your
industry I can't really comment on that. But it seems to me that if you
require that level of detail and are using GPS to accomplish it, you must be
dealing with positioning these skids at a significant distance from wherever
they are referenced to. Otherwise, I'd think one would use more
"conventional" surveying techniques to determine position, orientation, etc.

The only surveys I've ever hired were strictly property surveys, in which
property boundaries are determined, locations of roads, trees, terrain
contours, etc. Getting the results to within an inch is perfectly
sufficient for that type of survey. It's not hard to imagine a wide variety
of surveys for which the same holds true.

Just because some situations demand higher precision, that doesn't mean all
situations do.

Pete

[Peter Duniho]

"Stubby" > wrote in message
...
> A friend bought a 60CS recently and we noticed that my old Garmin-12 got
> better reception and seemed more accurate. We called Garmin and the
> fellow we talk with was surprised because both units use the same chips
> inside!

What antenna types do they both use?

When I bought a GPS (basic hiking handheld), I discovered that the smaller
GPS units used a flat square plate antenna that worked okay when one had a
perfectly clear view of the sky, but otherwise was noticeably inferior to
the "helix" antenna (the thumb-sized antenna that usually sticks out from
the top of a GPS unit).

Either should be fine in an airplane, where one gets a great view of the sky
all the time, but on the ground one would notice significant differences.

It's too bad, because I really liked the form-factor of the smaller GPS.
But it just didn't perform as well as the larger ones that can use the
better antenna.

Pete

[[email protected]]

Peter Duniho wrote:
>
> That is correct. The presumed 50 meter accuracy is constant throughout the
> flight. It's not as though it's additive for each waypoint (or worse, as a
> continuous function along the flight). Though frankly, even if it were,
> you'd only be off by 500 meters after 10 waypoints which is still "no big deal".

Now I know why planes occasionally land on the wrong runway in IFR
conditions ;))

> As for equipment skids and such, since I don't know the details of your
> industry I can't really comment on that. But it seems to me that if you
> require that level of detail and are using GPS to accomplish it, you must be
> dealing with positioning these skids at a significant distance from wherever
> they are referenced to. Otherwise, I'd think one would use more
> "conventional" surveying techniques to determine position, orientation, etc.

When I began my career, there were only conventional surveying
equipment. These days, however, rarely do you get a client in the oil
and gas industry who'll accept anything but a GPS survey. Btw, we
engineers are barred too from all surveying, although we're responsible
for supervision and copping attendant liabilities. The actual task
itself is carried out by qualified surveyors who do nothing else :)

Ramapriya

[Montblack]

("Peter Duniho" wrote)
> It's too bad, because I really liked the form-factor of the smaller GPS.


I live in a cave.

I have not seen the term "form-factor" used before.


Montblack

[B A R R Y]

Stubby wrote:
>>
> A friend bought a 60CS recently and we noticed that my old Garmin-12 got
> better reception and seemed more accurate. We called Garmin and the
> fellow we talk with was surprised because both units use the same chips
> inside!

I'm totally surprised, and a tad skeptical for a few reasons.

The 12 used a patch antenna, similar to the eMap and eTrex series. The
60 series uses a quadrafilar, which usually works much better. The 12
was very similar to the eMap in processing ability, which in my personal
hands-on experience is far better on the 60. The published accuracy of
the two units is different, notice that the 60 has a velocity spec much
better than the 12 in the following Garmin specs. Please note that the
velocity spec is so different, they don't even bother to use the same
unit of measurement.

60CS:
Update rate: 1/second, continuous
GPS accuracy:

Position: < 15 meters, 95% typical*

Velocity: 0.05 meter/sec steady state

WAAS accuracy:

Position: < 3 meters, 95% typical

Velocity: 0.05 meter/sec steady state

12CX:
Update Rate: 1 second, continuous

* Accuracy:
o Position: 15 meters (49 feet) RMS*
o 1-5 meters (3-15 feet) RMS with Garmin® GBR 21 DGPS
receiver (optional)
* Velocity: 0.1 knot RMS steady state


Furthermore, the 12 series didn't use WAAS. Is something wrong with
your 60CS? <G>

[B A R R Y]

Paul Tomblin wrote:
> In a previous article, B A R R Y > said:
>> My Garmin 60CS, as well the eTrex Vista I previously owned, both use
>> WAAS corrections. I can't remember the last time I didn't get the
>> corrections.
>
> I have a Garmin 296 with WAAS, and I get WAAS nearly every time when I'm
> flying (but not always), but I only seem to get it about half the time
> when I'm driving. But I live, fly and drive up near the Canadian border,
> so maybe the coverage isn't as good up here.
>

I'm in CT, so being right on the east coast, and not right against the
Canadian border might have a positive effect. AFAIK, the two special
satellites are over the Atlantic and Pacific.

[Paul Tomblin]

In a previous article, B A R R Y > said:
>Paul Tomblin wrote:
>> In a previous article, B A R R Y > said:
>>> My Garmin 60CS, as well the eTrex Vista I previously owned, both use
>>> WAAS corrections. I can't remember the last time I didn't get the
>>> corrections.
>>
>> I have a Garmin 296 with WAAS, and I get WAAS nearly every time when I'm
>> flying (but not always), but I only seem to get it about half the time
>> when I'm driving. But I live, fly and drive up near the Canadian border,
>> so maybe the coverage isn't as good up here.
>I'm in CT, so being right on the east coast, and not right against the
>Canadian border might have a positive effect. AFAIK, the two special
>satellites are over the Atlantic and Pacific.

They're also on the equator, so the further north you go the more likely
they're going to be occluded by buildings and terrain when you're on the
ground.

--
Paul Tomblin > http://xcski.com/blogs/pt/
Usenet is a co-operative venture, backed by nasty people -
follow the standards.
-- Chris Rovers

[Jim Macklin]

You also attach a thermometer to the tape because there is a
correction for expansion. If the tape was supported by the
ground, a tension of 10 pounds was required if I remember
correctly.


"Paul Tomblin" > wrote in message
...
| In a previous article, Stubby
> said:
| >I had my property surveyed and asked what the accuracy
was. The
| >surveyor replied that 0.1 foot is the standard for most
applications and
| >0.01 foot is required for commercial, high-precision
applications.
| >They use GPS (DGPS??) but I don't know how.
|
| You need a better surveyor. When I was doing road
construction layout, we
| were expected to get the marks within 5-7 millimeters.
And when they
| actually did the construction, they were allowed to be
within 2-3
| centimeters. (You've heard the expression: measure with a
micrometer,
| mark with chalk, cut with an axe.) Legal surveyors were
supposed to be
| *far* more accurate than us. For instance, we just held
the chain (that's
| the "measuring tape" to you) or laid it down on the
ground. Legal
| surveyors had a special device to make sure they were
holding exactly the
| right amount of tension on the chain because that's what
it was calibrated
| for.
|
| --
| Paul Tomblin >
http://xcski.com/blogs/pt/
| ALL programs are poems, it's just that not all programmers
are poets.
| -- Jonathan Guthrie in the scary.devil.monastery

[Peter Duniho]

> wrote in message
oups.com...
> Now I know why planes occasionally land on the wrong runway in IFR
> conditions ;))

Well, for what it's worth, wrong-runway (or even wrong airport) landings are
much more common in visual conditions, when the pilot is trusted to find the
runway himself.

Flying an ILS involves tuning a radio to a frequency that is specific to the
runway. If an airplane winds up in a position to land on the wrong runway,
then the ILS indication will be so far off that the pilot should be flying a
"missed approach" (that is, aborting the approach to try again).

In visual conditions, simple human error can result in landing in the wrong
place. In instrument conditions, there needs to be a series of poor
judgment decisions on the part of the pilot (or the simple error of tuning
the wrong frequency into the ILS receiver, of course :)...but that's not an
"accuracy of guidance" issue, so it doesn't seem relevant in this
discussion).

Pete

[Peter Duniho]

"Montblack" > wrote in message
...
> ("Peter Duniho" wrote)
>> It's too bad, because I really liked the form-factor of the smaller GPS.
>
> I live in a cave.
>
> I have not seen the term "form-factor" used before.

Sorry...I've seen the term in a variety of industries, but I admit it's
probably not yet part of the common vernacular. :) It simply means how the
device is packaged, and how that packaging affects the user-interface and
utility of the device.

Basically, I just mean that the smaller GPSs seem "handier". But they don't
perform as well, so I wound up with a larger one anyway.

Pete

[Peter Duniho]

"Paul Tomblin" > wrote in message
...
> I have a Garmin 296 with WAAS, and I get WAAS nearly every time when I'm
> flying (but not always), but I only seem to get it about half the time
> when I'm driving. But I live, fly and drive up near the Canadian border,
> so maybe the coverage isn't as good up here.

Yup. Even within the coverage area of WAAS, if you are not directly below
the satellite (or nearly so) you start running into signal blockage due to
the terrain. Here in the Pacific Northwest, where we are both far to the
north of the satellites and also on the eastern/western boundaries of the
coverage), WAAS is essentially unusable on the ground (though it works quite
well in the airplane).

For the curious, here are some web pages with information about where the
satellites are, and what the theoretical coverage is:
http://gpsinformation.net/exe/waas.html
http://gps.faa.gov/programs/waas/for_pilots.htm

I note that they are in the process of adding a new WAAS satellite, and have
for the moment moved the North America coverage somewhat to the west,
leaving the northeasternmost area of the US without WAAS coverage. They say
that by the Fall, the new satellite will be up, restoring coverage.
Presumably everyone will have somewhat better performance after that (and
some of us should probably be getting better performance now).

Pete

[Jose]

> I have not seen the term "form-factor" used before.

It's marketspeak for "shape".

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

[Paul Tomblin]

In a previous article, B A R R Y > said:
>I'm in CT, so being right on the east coast, and not right against the
>Canadian border might have a positive effect. AFAIK, the two special
>satellites are over the Atlantic and Pacific.

According to
http://gps.faa.gov/programs/waas/for_pilots.htm
both of them are in the middle of the Pacific now. And when they get a
third one this fall, it's still going to be over the Pacific as well.

--
Paul Tomblin > http://xcski.com/blogs/pt/
A male pilot is a confused soul who talks about women when he's flying,
and about flying when he's with a woman.

[Michael Ware]

"Montblack" > wrote in message
...
> based on the satellites' "predictable" orbits.... <g>
>
> Ok. No clue.
>
>
> Montblack
>
This is know as the 'ephemeris'.

[Stubby]

The term has been around in the engineering world for 40+ years that I
know of. It really has to do with the shape of the package and its
dimensions.


Peter Duniho wrote:
> "Montblack" > wrote in message
> ...
>> ("Peter Duniho" wrote)
>>> It's too bad, because I really liked the form-factor of the smaller GPS.
>> I live in a cave.
>>
>> I have not seen the term "form-factor" used before.
>
> Sorry...I've seen the term in a variety of industries, but I admit it's
> probably not yet part of the common vernacular. :) It simply means how the
> device is packaged, and how that packaging affects the user-interface and
> utility of the device.
>
> Basically, I just mean that the smaller GPSs seem "handier". But they don't
> perform as well, so I wound up with a larger one anyway.
>
> Pete
>
>

[Stubby]

B A R R Y wrote:
> Stubby wrote:
>>>
>> A friend bought a 60CS recently and we noticed that my old Garmin-12
>> got better reception and seemed more accurate. We called Garmin and
>> the fellow we talk with was surprised because both units use the same
>> chips inside!
>
> I'm totally surprised, and a tad skeptical for a few reasons.
>
> The 12 used a patch antenna, similar to the eMap and eTrex series.
My -12 does not have an external antenna.

> Furthermore, the 12 series didn't use WAAS. Is something wrong with
> your 60CS? <G>
Actually, it belongs to a geocaching friend. The reason we call Garmin
is because it appeared that something was wrong with the -60CS. If -12
and the -60CS actually have the same chip inside, they both would have
WAAS. In practice, I think they have the same accuracy.

Also, the old -12 is a lot easier to enter data into. But I do like the
whizzy color on the -60CS.

[Stubby]

OK. Just how does a standard surveryor's equipment measure distance.
Does it count interference fringes?


Jim Macklin wrote:
> You also attach a thermometer to the tape because there is a
> correction for expansion. If the tape was supported by the
> ground, a tension of 10 pounds was required if I remember
> correctly.
>
>
> "Paul Tomblin" > wrote in message
> ...
> | In a previous article, Stubby
> > said:
> | >I had my property surveyed and asked what the accuracy
> was. The
> | >surveyor replied that 0.1 foot is the standard for most
> applications and
> | >0.01 foot is required for commercial, high-precision
> applications.
> | >They use GPS (DGPS??) but I don't know how.
> |
> | You need a better surveyor. When I was doing road
> construction layout, we
> | were expected to get the marks within 5-7 millimeters.
> And when they
> | actually did the construction, they were allowed to be
> within 2-3
> | centimeters. (You've heard the expression: measure with a
> micrometer,
> | mark with chalk, cut with an axe.) Legal surveyors were
> supposed to be
> | *far* more accurate than us. For instance, we just held
> the chain (that's
> | the "measuring tape" to you) or laid it down on the
> ground. Legal
> | surveyors had a special device to make sure they were
> holding exactly the
> | right amount of tension on the chain because that's what
> it was calibrated
> | for.
> |
> | --
> | Paul Tomblin >
> http://xcski.com/blogs/pt/
> | ALL programs are poems, it's just that not all programmers
> are poets.
> | -- Jonathan Guthrie in the scary.devil.monastery
>
>

[Matt Barrow]

> wrote in message
oups.com...
> Matt Barrow wrote:
>> >
>> Aircraft GPS systems are accurate to about 30 feet; how well do you think
>> that would work if your building a bridge, or more importantly, a tunnel?
>
>
> If I was owner, I'd end up with two tunnels for the price of one ;)
>
One tunnel, two lanes. :~&

[Matt Barrow]

> wrote in message
oups.com...
> Peter Duniho wrote:
>>
>> That is correct. The presumed 50 meter accuracy is constant throughout
>> the
>> flight. It's not as though it's additive for each waypoint (or worse, as
>> a
>> continuous function along the flight). Though frankly, even if it were,
>> you'd only be off by 500 meters after 10 waypoints which is still "no big
>> deal".
>
> Now I know why planes occasionally land on the wrong runway in IFR
> conditions ;))

As someone already mentioned, the error is not cumulative. Then, too,
approach GPS is a different animal from enroute.

[Thomas Borchert]

> Now I know why planes occasionally land on the wrong runway in IFR
> conditions
>

You do? I don't. In fact, I don't think they do. That happens in visual
conditions.

--
Thomas Borchert (EDDH)

[B A R R Y]

Paul Tomblin wrote:

> According to
> http://gps.faa.gov/programs/waas/for_pilots.htm
> both of them are in the middle of the Pacific now. And when they get a
> third one this fall, it's still going to be over the Pacific as well.

Look at the page you cited one more time.

[Paul Tomblin]

In a previous article, B A R R Y > said:
>Paul Tomblin wrote:
>> According to
>> http://gps.faa.gov/programs/waas/for_pilots.htm
>> both of them are in the middle of the Pacific now. And when they get a
>> third one this fall, it's still going to be over the Pacific as well.
>
>Look at the page you cited one more time.

Ok, I see one satellite at 178E (middle of the Pacific), another at 142W
(middle of the Pacific) and another at 133W (middle of the Pacific). How
am I wrong? Van Nuys Airport in California is further east (118W) than
the furthest east of those satellites.

--
Paul Tomblin > http://xcski.com/blogs/pt/
"By the time they had diminished from 50 to 8, the other dwarves began
to suspect 'Hungry' ..."
-- Gary Larson, "The Far Side"

[B A R R Y]

Paul Tomblin wrote:
> In a previous article, B A R R Y > said:
>> Paul Tomblin wrote:
>>> According to
>>> http://gps.faa.gov/programs/waas/for_pilots.htm
>>> both of them are in the middle of the Pacific now. And when they get a
>>> third one this fall, it's still going to be over the Pacific as well.
>> Look at the page you cited one more time.
>
> Ok, I see one satellite at 178E (middle of the Pacific), another at 142W
> (middle of the Pacific) and another at 133W (middle of the Pacific). How
> am I wrong?

Nope! My mistake.

I was thinking of the pre-Feb. picture from another source. I only
quickly glanced at the second photo on an 11" notebook screen, and saw
the sat icon, not the 142W location notation of it's actual location.

Thanks!