Different WAAS altitude readings

"Wyatt Emmerich" wrote in message
...
My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be functioning
properly other than this.

Your Echo Flight GPS is not functioning properly. My guess is that it is not
processing the WAAS signal. The difference between the two units should be
no more than 50 feet.

On Sat, 26 Jun 2004 22:27:34 GMT, "Wyatt Emmerich"
wrote:

My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be functioning
properly other than this.


Actually, neither one may be accurate.

Depending on where you are on the field, I see charted elevations of
306-342'.

Airport elevations and GPS elevations are determined differently.
Actually, there are over twenty different standards for determining
altitudes. And how accurately a particular WAAS enabled GPS receiver
translates it's GPS altitude to "airplane" altitude can vary depending on
the software algorithms being used.

Although I would expect that TSO 146 certified GPS receivers would perform
the conversion in the same way, I would not have that expectation with
regard to portable, non-certified GPS receivers.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Interesting. Could you elaborate?


"Ron Rosenfeld" wrote in message
...
On Sat, 26 Jun 2004 22:27:34 GMT, "Wyatt Emmerich"
wrote:

My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be
functioning
properly other than this.


Actually, neither one may be accurate.

Depending on where you are on the field, I see charted elevations of
306-342'.

Airport elevations and GPS elevations are determined differently.
Actually, there are over twenty different standards for determining
altitudes. And how accurately a particular WAAS enabled GPS receiver
translates it's GPS altitude to "airplane" altitude can vary depending on
the software algorithms being used.

Although I would expect that TSO 146 certified GPS receivers would perform
the conversion in the same way, I would not have that expectation with
regard to portable, non-certified GPS receivers.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Sun, 27 Jun 2004 04:49:52 GMT, "Wyatt Emmerich"
wrote:

Interesting. Could you elaborate?

On what?

If you look through the NOAA site (especially the ngs pages) and also
GOOGLE the topic, you'll find lots of information on altitude standards.

The FAA site has information on TSO146 and WAAS.

I know that here in the NE, a few hundred feet discrepancy between baro
altitude and GPS altitude is not unusual with a TSO146 box.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

But this was the difference between two GPS altitudes, both reporting that
they had WAAS receptivity.

"Ron Rosenfeld" wrote in message
news
On Sun, 27 Jun 2004 04:49:52 GMT, "Wyatt Emmerich"
wrote:

Interesting. Could you elaborate?

On what?

If you look through the NOAA site (especially the ngs pages) and also
GOOGLE the topic, you'll find lots of information on altitude standards.

The FAA site has information on TSO146 and WAAS.

I know that here in the NE, a few hundred feet discrepancy between baro
altitude and GPS altitude is not unusual with a TSO146 box.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Sun, 27 Jun 2004 13:49:48 GMT, "Wyatt Emmerich"
wrote:

But this was the difference between two GPS altitudes, both reporting that
they had WAAS receptivity.

And as I posted, you were using two different non-certified units.

What makes you believe they were using the same standards and algorithm to
translate the GPS signal into an equivalent for a barometric altitude.

You can obtain further information from the sites I mentioned, and by using
Google.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Ron Rosenfeld wrote in message . ..
On Sun, 27 Jun 2004 04:49:52 GMT, "Wyatt Emmerich"
wrote:

Interesting. Could you elaborate?

On what?

If you look through the NOAA site (especially the ngs pages) and also
GOOGLE the topic, you'll find lots of information on altitude standards.

The FAA site has information on TSO146 and WAAS.

I know that here in the NE, a few hundred feet discrepancy between baro
altitude and GPS altitude is not unusual with a TSO146 box.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

I believe there's only a couple of avionics units that were first out
of the gate to be 'partially' certified at this point (Chelton for
Capstone project and UPSAT). This may have changed as it's right on
the leading edge of happening, but my understanding was (at least as
of a few months ago) that *none* actually had been certified yet for
vertical guidance.

I found the answer to my own question. This is a must read for any pilot who
uses a GPS.

http://www.esri.com/news/arcuser/0703/geoid2of3.html

"Wyatt Emmerich" wrote in message
...
My Garmin 296 indicates 330 MSL at KHKS, which is correct. My Echo Flight
GPS indicates 140 MSL. Both are locked on to WAAS.

How can I have such a large discrepancy? Both units seem to be functioning
properly other than this.