GPS navigation

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.

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 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.

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)

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

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

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"

"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...

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.

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.