GPS navigation

[David W]

Hello to all,

Do most modern aircraft rely on the GPS for navigation, and to what
extent do they rely on it?

And is it true that such aircraft's navigation systems use maps
'optimised' for GPS?

And finally, if I may, what level of positional accuracy must aircraft
relying on GPS for navigation work with at typical altitudes
(presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
over a hundred metres?

Thanks very much in advance.


Regards,

David,
England.

[Dan Luke]

"David W" wrote:

> Hello to all,
>
> Do most modern aircraft rely on the GPS for navigation, and to what
> extent do they rely on it?

What kind of modern aircraft are you talking
about--bizjets...airliners...military?

> And is it true that such aircraft's navigation systems use maps
> 'optimised' for GPS?

What does 'optimised' for GPS mean?

> And finally, if I may, what level of positional accuracy must aircraft
> relying on GPS for navigation work with at typical altitudes
> (presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
> over a hundred metres?

Tens of meters or less.

What's behind your questions; what are you trying to get at? If you posted
that, you might get more suitable answers.

--
Dan
C172RG at BFM

[Bob Noel]

In article . com>,
"David W" > wrote:

> Do most modern aircraft rely on the GPS for navigation, and to what
> extent do they rely on it?

define "modern"

> And is it true that such aircraft's navigation systems use maps
> 'optimised' for GPS?

No.

> And finally, if I may, what level of positional accuracy must aircraft
> relying on GPS for navigation work with at typical altitudes
> (presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
> over a hundred metres?

Currently, the accuracies required are over hundreds of meters.
The tightest restrictions on aircraft operations are currently those
in RNP-4 RNAV airspace. The aircraft must be laterally within
4 nmi of intended course 95% of the time, and have an unannunciated
loss of containment (i.e., break 8 nmi) with a probability of 10-5
per flight hour.

GPS accuracies in the tens of meters are only currently required for
approaches.

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

[Jay Honeck]

> Do most modern aircraft rely on the GPS for navigation, and to what
> extent do they rely on it?

If you mean "do most modern pilots rely on GPS for navigation?", the answer
is probably "yes". Modern aircraft do still come with a variety of
navigational equipment, but everything in the new panels is centered around
GPS.

> And is it true that such aircraft's navigation systems use maps
> 'optimised' for GPS?

Not sure what you mean -- but most GPS' have built-in moving maps that
display your position relative to the "real" world, as depicted on the map.

> And finally, if I may, what level of positional accuracy must aircraft
> relying on GPS for navigation work with at typical altitudes
> (presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
> over a hundred metres?

GPS positioning is accurate to within 10 meters, often less. That's way
more accurate than any aircraft operating in the flight levels needs.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Michael Ware]

"David W" > wrote in message
oups.com...
> Hello to all,
>
> Do most modern aircraft rely on the GPS for navigation, and to what
> extent do they rely on it?
>
> And is it true that such aircraft's navigation systems use maps
> 'optimised' for GPS?
>
> And finally, if I may, what level of positional accuracy must aircraft
> relying on GPS for navigation work with at typical altitudes
> (presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
> over a hundred metres?
>
> Thanks very much in advance.
>
>
> Regards,
>
> David,
> England.
>
As far as what lateral positional accuracy is AVAILABLE using GPS, about
thirty meters, about 3 meters utilizing WAAS.

--
Hello, my name is Mike, and I am an airplane addict...

[David W]

Hi Dan,

Sorry, my questions could have been better posed.


> > Do most modern aircraft rely on the GPS for navigation, and to what
> > extent do they rely on it?
>
> What kind of modern aircraft are you talking
> about--bizjets...airliners...military?

I had primarily airliners and military aircraft in mind, but I didn't
state it (as would have been helpful).



> > And is it true that such aircraft's navigation systems use maps
> > 'optimised' for GPS?
>
> What does 'optimised' for GPS mean?

I'm not sure. Somebody with whom I am having a 'debate' has asserted
that there is an altitude-dependent error component (if I may call it
that) on positions determined by GPS (and I presume that this alleged
error component affects the horizontal component of a 3D position, as
well as vertical (altitude) component). In his own words:

"GPS is optimised for sea level, Blanchefort [a mountaintop ruined
castle] is 467 metres above sea level, couple this with a slant range
to a satellite of several thousand miles and the curvature of the earth
and you have error. At least up to 100 metres..."

I personally can find no evidence which supports his claim that a) GPS
is optimised for sea level, or b) GPS coordinates obtained at a few
hundred metres above (mean)
sea level are affected by his alleged altitude-induced error.

I replied (verbatim quote - please excuse the sarcastic tone!):

"This is really bad news. Modern aircraft - many of which rely heavily
on GPS for navigation - are in big trouble then, aren't they? I mean,
if the error at just 467 metres above MSL is ''at least up to 100
metres'', then surely it must be several kilometres by the time we get
up to altitudes like 30,000 ft., right?"

His reply (with some non-essentials removed):

"Aircraft ... are using a map optimised to the GPS system and this is
the key point which seems to be passing you by."


> What's behind your questions; what are you trying to get at? If you posted
> that, you might get more suitable answers.

I hope that that is sufficiently answered above. I didn't want to
burden this group with extensive background information and endless
quotes from this slightly silly debate!


Regards,

David,
England.

[Peter Duniho]

"David W" > wrote in message
ups.com...
>> > And is it true that such aircraft's navigation systems use maps
>> > 'optimised' for GPS?
>>
>> What does 'optimised' for GPS mean?
>
> I'm not sure. Somebody with whom I am having a 'debate' has asserted
> that there is an altitude-dependent error component (if I may call it
> that) on positions determined by GPS (and I presume that this alleged
> error component affects the horizontal component of a 3D position, as
> well as vertical (altitude) component).

It is theoretically true that altitude can affect the geometry of the GPS
receiver and satellites being received in such a way as to increase error.
However, the orbital altitude (10,988 nautical miles) is so much higher than
flight altitudes (usually up to 9 or 10 nautical miles at the very most, and
for commercial airliners 6 nautical miles give or take is more typical),
it's hard to imagine any significant error being caused by that.

Even in the most extreme case, an airplane is only going to climb high
enough to change the distance to the satellite by 0.05-0.10%. And this
assumes the satellite is directly overhead; when it's not, the distances are
even greater and altitude even less significant.

It's important to note: the GPS system doesn't actually care about
elevation. It cares about distances and calculates a 3D position based on
measured distances (simplistically stated, anyway). If altitude was a
significant problem, then so would any variation in distance from the
receiver to the satellite, and the change in that distance due to the
relative angle in the sky of the satellite is MUCH greater than any change
in elevation possible by an airplane. If altitude caused a problem, the GPS
would be incredibly unreliable all the time, since the satellites are rarely
directly overhead.

Not that your debate partner will see this. He'll just say something about
the system being "optimised" for the effects of satellite orbits on the
distance between receiver and satellite. But it's true, nonetheless.

> In his own words:
>
> "GPS is optimised for sea level, Blanchefort [a mountaintop ruined
> castle] is 467 metres above sea level, couple this with a slant range
> to a satellite of several thousand miles and the curvature of the earth
> and you have error. At least up to 100 metres..."

I still don't know what it means for GPS to be "optimised for sea level".
It would be entertaining (though probably not educational) to learn what the
guy believes was done to "optimise" GPS for sea level.

Certainly is boggles the mind to think that elevation gain results in a
1-to-5 error ratio (that is, 1 unit of error for every 5 units of elevation
gain). That's a remarkably fragile system he's describing, and it certainly
doesn't apply to GPS as it exists today.

> [...]
> His reply (with some non-essentials removed):
>
> "Aircraft ... are using a map optimised to the GPS system and this is
> the key point which seems to be passing you by."

More BS. "A map optimised to the GPS system"? Again, even pondering what
this could mean is bewildering. Is he saying that, while there's some large
error, it's always a known error and so can be compensated for with the map
system? Why wouldn't a GPS receiver simply be designed instead to use this
known error and correct the calculated position based on that?

Of course, that's all theoretical. In reality, there's no "known error"
(not in the sense that one knows exactly the magnitude AND direction of the
error), and no reason to "optimise" the map "to the GPS system". You get
the same map on the ground as you do aloft, and the GPS simply plots your
position on the map.

It's true that there are different mapping systems in use, but they have to
do with how one projects the geometrically perfect information from GPS onto
the geometrically imperfect planet we live on.

> I hope that that is sufficiently answered above. I didn't want to
> burden this group with extensive background information and endless
> quotes from this slightly silly debate!

Yes, I'd agree it's at least "slightly silly". :) I usually just put
people who write stuff that ridiculous into my killfile.

Pete

[Dan Luke]

"David W" wrote:

[snip]

>> > And is it true that such aircraft's navigation systems use maps
>> > 'optimised' for GPS?
>>
>> What does 'optimised' for GPS mean?
>
> I'm not sure. Somebody with whom I am having a 'debate' has asserted
> that there is an altitude-dependent error component (if I may call it
> that) on positions determined by GPS (and I presume that this alleged
> error component affects the horizontal component of a 3D position, as
> well as vertical (altitude) component). In his own words:
>
> "GPS is optimised for sea level, Blanchefort [a mountaintop ruined
> castle] is 467 metres above sea level, couple this with a slant range
> to a satellite of several thousand miles and the curvature of the earth
> and you have error. At least up to 100 metres..."

Nonsense. Your opponent in the debate is a ninny.

> I personally can find no evidence which supports his claim that a) GPS
> is optimised for sea level, or b) GPS coordinates obtained at a few
> hundred metres above (mean)
> sea level are affected by his alleged altitude-induced error.
>
[snip]
>
> His reply (with some non-essentials removed):
>
> "Aircraft ... are using a map optimised to the GPS system and this is
> the key point which seems to be passing you by."

Utter twaddle. There are no such maps.

>> What's behind your questions; what are you trying to get at? If you
>> posted
>> that, you might get more suitable answers.
>
> I hope that that is sufficiently answered above. I didn't want to
> burden this group with extensive background information and endless
> quotes from this slightly silly debate!

No problem. You came to the right place after your opponent introduced his
bogus aviation arguments.

--
Dan
C172RG at BFM

[Roy Smith]

"David W" > wrote:
> "Aircraft ... are using a map optimised to the GPS system and this is
> the key point which seems to be passing you by."

Nonesense. Aircraft that are equipped with GPS use the same charts that
non-GPS aircraft use.

The FAA has a pretty good introduction to GPS as use in aviation in the
Airman's Information Manual. The section you want is
http://www.faa.gov/ATpubs/AIM/Chap1/aim0101.html#1-1-19. If that doesn't
get you direct to the right paragraph, search for "1-1-19. Global
Positioning System (GPS)".

[Mark Manes]

There is really no slant range in GPS Navagation. It is Triangulation &
Timing.
You need at least 3 satellites in view to get an accurate 3D position.
Distance is
determined by accurate timing of how long it takes an encoded signal to
reach
the GPS reciever. Thus with known distances from 3 satellites you can
calculate or triangulate (within the tolerance) the exact position of the
GPS reciever. This position
includes altitude, although altitude in my experience seems to be less
accurate. But for all I know the GPS altitude may be more accurate than
altitude read from my altimeter, which is
corrected for non standard pressure. Hope this short explanation helps

Mark



"David W" > wrote in message
ups.com...
>> What does 'optimised' for GPS mean?
>
> I'm not sure. Somebody with whom I am having a 'debate' has asserted
> that there is an altitude-dependent error component (if I may call it
> that) on positions determined by GPS (and I presume that this alleged
> error component affects the horizontal component of a 3D position, as
> well as vertical (altitude) component). In his own words:
>
> "GPS is optimised for sea level, Blanchefort [a mountaintop ruined
> castle] is 467 metres above sea level, couple this with a slant range
> to a satellite of several thousand miles and the curvature of the earth
> and you have error. At least up to 100 metres..."
>
> I personally can find no evidence which supports his claim that a) GPS
> is optimised for sea level, or b) GPS coordinates obtained at a few
> hundred metres above (mean)
> sea level are affected by his alleged altitude-induced error.
>
> I replied (verbatim quote - please excuse the sarcastic tone!):
>
> "This is really bad news. Modern aircraft - many of which rely heavily
> on GPS for navigation - are in big trouble then, aren't they? I mean,
> if the error at just 467 metres above MSL is ''at least up to 100
> metres'', then surely it must be several kilometres by the time we get
> up to altitudes like 30,000 ft., right?"
>
> His reply (with some non-essentials removed):
>
> "Aircraft ... are using a map optimised to the GPS system and this is
> the key point which seems to be passing you by."
>
>
>> What's behind your questions; what are you trying to get at? If you
>> posted
>> that, you might get more suitable answers.
>
> I hope that that is sufficiently answered above. I didn't want to
> burden this group with extensive background information and endless
> quotes from this slightly silly debate!
>
>
> Regards,
>
> David,
> England.
>

[Bob Gardner]

I've been watching this discussion in the sci.geo.satellit-nav newsgroup,
and my reaction to your friend's ideas is "What an idiot."

Bob Gardner

"David W" > wrote in message
oups.com...
> Hello to all,
>
> Do most modern aircraft rely on the GPS for navigation, and to what
> extent do they rely on it?
>
> And is it true that such aircraft's navigation systems use maps
> 'optimised' for GPS?
>
> And finally, if I may, what level of positional accuracy must aircraft
> relying on GPS for navigation work with at typical altitudes
> (presumably 20,000 to 40,000 ft)? Are we talking tens of metres, or
> over a hundred metres?
>
> Thanks very much in advance.
>
>
> Regards,
>
> David,
> England.
>

[David W]

Bob Gardner wrote:
> I've been watching this discussion in the sci.geo.satellit-nav newsgroup,
> and my reaction to your friend's ideas is "What an idiot."
>

My post to that group requesting advice and comment was way too long; I
imagine it made for quite tedious reading, which I don't think helped
my case as much as it might have done.

But I'm happy with the replies (in that group and certainly here),
overall :-)

My opponent, even though he evidently knows next-to-nothing about the
GPS, or GPS survey, is trying to discredit a set of GPS coordinates
that has cost me time, energy and lots of money to obtain (obtained
with diligence and after a sufficient number of hours of research (in
my opinion)). And he thinks I should use theodolites and trig points
instead of GPS.


Thanks to all that have replied.


Regards,

David,
England.

[Bob Gardner]

To address the question of optimized maps for aviation use, if that indeed
is relevant to this discussion, he should go to an outlet that sells
aviation maps and ask for a GPS-optimized map. I think that he will be
surprised.

Bob Gardner

"David W" > wrote in message
oups.com...
> Bob Gardner wrote:
>> I've been watching this discussion in the sci.geo.satellit-nav newsgroup,
>> and my reaction to your friend's ideas is "What an idiot."
>>
>
> My post to that group requesting advice and comment was way too long; I
> imagine it made for quite tedious reading, which I don't think helped
> my case as much as it might have done.
>
> But I'm happy with the replies (in that group and certainly here),
> overall :-)
>
> My opponent, even though he evidently knows next-to-nothing about the
> GPS, or GPS survey, is trying to discredit a set of GPS coordinates
> that has cost me time, energy and lots of money to obtain (obtained
> with diligence and after a sufficient number of hours of research (in
> my opinion)). And he thinks I should use theodolites and trig points
> instead of GPS.
>
>
> Thanks to all that have replied.
>
>
> Regards,
>
> David,
> England.
>

[Morgans]

"David W" > wrote
>
> My opponent, even though he evidently knows next-to-nothing about the
> GPS, or GPS survey, is trying to discredit a set of GPS coordinates
> that has cost me time, energy and lots of money to obtain (obtained
> with diligence and after a sufficient number of hours of research (in
> my opinion)). And he thinks I should use theodolites and trig points
> instead of GPS.

Ask him how modern surveyors locate points on property.

Hint. GPS.
--
Jim in NC

[Bob Noel]

In article <0o_ig.19660$8q.17433@dukeread08>, "Mark Manes" >
wrote:

> You need at least 3 satellites in view to get an accurate 3D position.

4 satellites unless you already know GPS time or know your altitude.

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

[Jose]

> Somebody with whom I am having a 'debate' has asserted
> that there is an altitude-dependent error component

The distance to a station using DME is "slant range", the distance to
that same station using GPS is a projection, so some slight error is
introduced there. It is possible that your friend is mis-interpreting
or mis-applying this tidbit.

> "Aircraft ... are using a map optimised to the GPS system and this is
> the key point which seems to be passing you by."

There are several geoids in use, and the choice of geoid may introduce
position errors. A geoid is a surface of equal gravitational potential
(a "mean sea level") and is an idealized surface - the real earth has
bumps and wiggles which are modeled imperfectly (or not at all) in some
models. Your friend may also be mis-applying this tidbit.

Jose
--
The price of freedom is... well... freedom.
for Email, make the obvious change in the address.

[Thomas Borchert]

David,

> GPS is optimised for sea level, Blanchefort [a mountaintop ruined
> castle] is 467 metres above sea level, couple this with a slant range
> to a satellite of several thousand miles and the curvature of the earth
> and you have error. At least up to 100 metres..."
>

That's just plain BS. Just one example: The space shuttle uses GPS for
navigation. That should be plenty high to convince the poster otherwise.

--
Thomas Borchert (EDDH)

[Thomas Borchert]

Mark,

> You need at least 3 satellites in view to get an accurate 3D position.
>

4. You need to correct for receiver clock error.

--
Thomas Borchert (EDDH)

[Roy Smith]

Thomas Borchert > wrote:
> That's just plain BS. Just one example: The space shuttle uses GPS for
> navigation. That should be plenty high to convince the poster otherwise.

Would an ordinary GPS receiver (say, my $99 eTrex) work on the shuttle? I
can see how the basic principles are the same, but I'm guessing my eTrex
wouldn't be happy with the speeds the shuttle is doing.

[Thomas Borchert]

Roy,

> I
> can see how the basic principles are the same, but I'm guessing my eTrex
> wouldn't be happy with the speeds the shuttle is doing.
>

Many cheaper GPS receiver do indeed have a speed limit. Is it given in the
specs for the eTrex?

--
Thomas Borchert (EDDH)

[David W]

Hi all,

I am writing my conclusion and summary to this so-called 'debate' I'm
having (the one which prompted me to come here and alt.satellite.gps to
seek expert advice and opinion).

Would it be considered discourteous or bad etiquette if I were to quote
some of the opinions expressed in this thread into my summary?

Or shall I just refer my opponent (and effectively our readers as it's
an open debate) to this usenet group?


Regards,

David.

[Morgans]

"David W" > wrote

> Would it be considered discourteous or bad etiquette if I were to quote
> some of the opinions expressed in this thread into my summary?
>
> Or shall I just refer my opponent (and effectively our readers as it's
> an open debate) to this usenet group?

Quote away, as far as I am concerned.

It is bad to quote private E-mails on Usenet, but I don't think it is bad to
quote something already in public view, into private communications.
--
Jim in NC

[Blanche Cohen]

To the OP:

Your "friend" may be thinking of the "datum plane", which is the
almost spherical object that is the mathmatical model of the
earth.

And there are other navigational systems for aircraft. Military
aircraft (IIRC) are inertial guidance systems rather than GPS.
Or at least they used to be.

[Dylan Smith]

On 2006-06-11, David W > wrote:
> "GPS is optimised for sea level, Blanchefort [a mountaintop ruined
> castle] is 467 metres above sea level, couple this with a slant range
> to a satellite of several thousand miles and the curvature of the earth
> and you have error. At least up to 100 metres..."

Well, in a light plane cruising between 4,000 and 8,000 feet, my
handheld Garmin 195 reports an estimated position error of 13 feet -
about 4 metres. Your friend doesn't know what he's talking about.

The map is only optimized for aviation in that it depicts airspace - the
receiver itself and the way it generates the map is no different from a
GPS for use by sailors or drivers or hikers.

--
Yes, the Reply-To email address is valid.
Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de

[peter]

Thomas Borchert wrote:

> > I can see how the basic principles are the same, but I'm guessing my eTrex
> > wouldn't be happy with the speeds the shuttle is doing.
> >
> Many cheaper GPS receiver do indeed have a speed limit. Is it given in the
> specs for the eTrex?

There are technology export regulations that limit GPS receivers to
operation below altitudes of 60 kft or below speeds of 1000 knots.
Most consumer models, including the eTrex, are specified to work up to
those limits. However, some units have lower limits - for example,
most Magellan models specify a maximum speed of 951mph.