Different WAAS altitude readings

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.

Can't address your problem, but AIM 1-1-20(a)(8) tells us not to use GPS
altitudes. Doesn't say anything about WAAS...yet.

Bob Gardner

"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.

Given this discrepancy, I understand why.

"Bob Gardner" wrote in message
...
Can't address your problem, but AIM 1-1-20(a)(8) tells us not to use GPS
altitudes. Doesn't say anything about WAAS...yet.

Bob Gardner

"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.

"Bob Gardner" wrote in message
...
Can't address your problem, but AIM 1-1-20(a)(8) tells us not to use GPS
altitudes. Doesn't say anything about WAAS...yet.

1-1-20's title is "Wide Area Augmentation System (WAAS)." There is no
1-1-20(a)(8). 1-1-20(b) gives guidance and procedures for flying approaches
with vertical guidance from WAAS and says that some installations of WAAS
may be certified for precision approaches as having greater accuracy than
barometric altimeters.

As a follow-up to the previous post: the new instrument PTS that goes into
effect in October requires students to be familiar with precision,
non-precision, and approaches with vertical guidance. We need you to update
your book! At the very least, turn what is written in 1-1-20 into plain
English.

"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)

I'm using the 2004 FAR-AIM as a convenience...guess I should have used my
Summit CD-ROM which is updated quarterly.

Bob

"C J Campbell" wrote in message
...

"Bob Gardner" wrote in message
...
Can't address your problem, but AIM 1-1-20(a)(8) tells us not to use GPS
altitudes. Doesn't say anything about WAAS...yet.

1-1-20's title is "Wide Area Augmentation System (WAAS)." There is no
1-1-20(a)(8). 1-1-20(b) gives guidance and procedures for flying
approaches
with vertical guidance from WAAS and says that some installations of WAAS
may be certified for precision approaches as having greater accuracy than
barometric altimeters.

I see that my reference should have been AIM 1-1-19(a)(8). Other than that,
the guidance remains the same.

I note the phrase "will be" sprinkled through the discussion of WAAS. Has
WAAS been approved for operational use? How many boxes have the
functionality described in 1-1-20?...I think the CX-80 does.

Bob Gardner

"C J Campbell" wrote in message
...

"Bob Gardner" wrote in message
...
Can't address your problem, but AIM 1-1-20(a)(8) tells us not to use GPS
altitudes. Doesn't say anything about WAAS...yet.

1-1-20's title is "Wide Area Augmentation System (WAAS)." There is no
1-1-20(a)(8). 1-1-20(b) gives guidance and procedures for flying
approaches
with vertical guidance from WAAS and says that some installations of WAAS
may be certified for precision approaches as having greater accuracy than
barometric altimeters.

Right now, the CNX-80 does not give vertical guidance, but that is
promised in software version 2.0, hopefully to be approved by Q3/2004.
That software v2.0 has been in flight test since Q4/2003.
I'm unaware of any box nearer than the CNX-80 to LPV capability.
---JRC---

"Bob Gardner" wrote in message =
...
I see that my reference should have been AIM 1-1-19(a)(8). Other than =
that,
the guidance remains the same.
=20
I note the phrase "will be" sprinkled through the discussion of WAAS. =
Has
WAAS been approved for operational use? How many boxes have the
functionality described in 1-1-20?...I think the CX-80 does.
=20
Bob Gardner
=20
"C J Campbell" wrote in =
message
...

"Bob Gardner" wrote in message
...
Can't address your problem, but AIM 1-1-20(a)(8) tells us not to =
use GPS
altitudes. Doesn't say anything about WAAS...yet.

1-1-20's title is "Wide Area Augmentation System (WAAS)." There is =
no
1-1-20(a)(8). 1-1-20(b) gives guidance and procedures for flying
approaches
with vertical guidance from WAAS and says that some installations of =
WAAS
may be certified for precision approaches as having greater accuracy =
than
barometric altimeters.