GPS altitude again is close to actual

There was a story a while back (don't know if it's true or not, but sounded
legit) that some guy was demonstrating his latest, greatest GPS by using it
to taxi into his hangar. It wasn't quite that accurate and the repair bill
wasn't cheap.

mike

"Mxsmanic" wrote in message
...

It worries me that I see a lot of ignorance of GPS in the aviation
community. It is not surprising given the newness of the technology,
but it is worrisome because people often rush to embrace a new
technology because of the gee-whiz factor, long before they understand
the technology and its limitations. It's like people who drive off a
pier into a river because they don't realize that GPS can be
dramatically incorrect in urban environments.

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mike regish writes:

There was a story a while back (don't know if it's true or not, but sounded
legit) that some guy was demonstrating his latest, greatest GPS by using it
to taxi into his hangar. It wasn't quite that accurate and the repair bill
wasn't cheap.

One problem with GPS is that accuracy can be rapidly and significantly
degraded by the presence of buildings or mountains or other obstacles
that reflect or block signals. This is why GPS isn't likely to be
very accurate in the streets of Manhattan. The system itself provides
good accuracy, but in order to obtain that accuracy, you have to be
able to receive the signals without interference. On the ocean, in
the countryside, or in the open sky, you can receive signals very well
indeed, but once you are on the ground, the situation changes.

Another problem, not actually part of GPS per se, is moving maps.
Your GPS position may be accurate, but that doesn't guarantee that the
map is accurate. If the mountain on the map is in the wrong place in
relation to its real-world position, having high accuracy from GPS
won't help you. Very often map errors are more of a problem than
errors in the GPS itself.

Note that WAAS and LAAS will _not_ compensate for either of the above
types of error. Differential GPS systems like this work best when you
are at exactly the spot used as a reference for the corrections. If
you are anywhere else, the corrections may not be right for your
position. The further away you are from the surveyed reference
position used to generate the corrections, the more likely it is that
your position will be incorrect.

Some of these systems also correct for atmospheric and other effects,
but here again, the corrections are most useful when you are in the
exact position for which they are generated. If the reference point
is in Cheyenne and you are in Denver, the corrections may be well off
the mark.

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Mxsmanic wrote:
[...]
Note that WAAS and LAAS will _not_ compensate for either of the above
types of error. Differential GPS systems like this work best when you
are at exactly the spot used as a reference for the corrections. If
you are anywhere else, the corrections may not be right for your
position. The further away you are from the surveyed reference
position used to generate the corrections, the more likely it is that
your position will be incorrect.

While correct for the case of LAAS and DGPS, this is not correct in the
case of WAAS.

Hint: W != L

Regards,
Jon

writes:

While correct for the case of LAAS and DGPS, this is not correct in the
case of WAAS.

It's true for WAAS, too. WAAS has only a few fixed reference points
and extrapolates for all other points.

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Mxsmanic wrote:
writes:

While correct for the case of LAAS and DGPS, this is not correct in the
case of WAAS.

It's true for WAAS, too. WAAS has only a few fixed reference points
and extrapolates for all other points

You don't understand how WAAS works.

You stated:

" The further away you are from the surveyed reference
position used to generate the corrections, the more likely it is that
your position will be incorrect."

This is false. Being inside the reference network is all that matters,
proxitimity to a WRS does not matter.

Regards,
Jon

writes:

You don't understand how WAAS works.

Yes, I do. There are only 29 reference points in WAAS, including
Alaska. The corrections are completely accurate for these surveyed
reference points. For all other points, the corrections are
extrapolations.

This is different from LAAS and many DGPS systems, which use local
reference points to develop corrections for local receivers. No
significant extrapolation is required, so potential accuracy is
higher.

This is false. Being inside the reference network is all that matters,
proxitimity to a WRS does not matter.

No. The exact conditions of atmospheric disturbances and other
sources of inaccuracy cannot be fully predicted on the basis of
non-local references. The only truly accurate way to get this
information is to measure it at the point where it will be used.
However, this is very expensive, which is why WAAS was developed. It
trades a slight loss of accuracy for much lower cost.

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Mxsmanic wrote:
writes:

You don't understand how WAAS works.

Yes, I do. There are only 29 reference points in WAAS, including
Alaska. The corrections are completely accurate for these surveyed
reference points. For all other points, the corrections are
extrapolations.

You don't understand how WAAS works.

The corrections are to grid points based on observables from multiple
reference stations.

This is different from LAAS and many DGPS systems, which use local
reference points to develop corrections for local receivers. No
significant extrapolation is required, so potential accuracy is
higher.

This is false. Being inside the reference network is all that matters,
proxitimity to a WRS does not matter.

No. The exact conditions of atmospheric disturbances and other
sources of inaccuracy cannot be fully predicted on the basis of
non-local references. The only truly accurate way to get this
information is to measure it at the point where it will be used.
However, this is very expensive, which is why WAAS was developed. It
trades a slight loss of accuracy for much lower cost.

You don't understand how WAAS works.

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Mxsmanic wrote:

writes:

You don't understand how WAAS works.

Yes, I do. There are only 29 reference points in WAAS, including
Alaska. The corrections are completely accurate for these surveyed
reference points. For all other points, the corrections are
extrapolations.

I suspect that Jon is far smarter on WAAS that either one of us Mx.

Ron Lee

Mxsmanic wrote:


Another problem, not actually part of GPS per se, is moving maps.
Your GPS position may be accurate, but that doesn't guarantee that the
map is accurate. If the mountain on the map is in the wrong place in
relation to its real-world position, having high accuracy from GPS
won't help you. Very often map errors are more of a problem than
errors in the GPS itself.


Once again you don't know what you're talking about. Even if the map
were out of spec a little it doesn't matter as you aren't flying that
close to the mountains if you are IFR. An actual pilot would know that.




Note that WAAS and LAAS will _not_ compensate for either of the above
types of error.

Doesn't matter.





Differential GPS systems like this work best when you
are at exactly the spot used as a reference for the corrections. If
you are anywhere else, the corrections may not be right for your
position. The further away you are from the surveyed reference
position used to generate the corrections, the more likely it is that
your position will be incorrect.

Completely irrelavant for aviation.