Different WAAS altitude readings

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)

C J Campbell wrote:

"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.

The sections were rearranged but Bob Garder is right; the prohibition still
exists. What the language you cite refers to is LPV. If you have IFR
certified LPV on your aircraft (which I doubt), then you can use LPV from the
PFAF to Decision Altitude, only. There is no authorization to use LPV in any
other manner to determine altitude, nor is there any other provision to use
WAAS-augmented GPS in general to replace the barometric altitmeter for
determining altitude under IFR.

"John R. Copeland" wrote:

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---

Most of the modern iron has Baro VNAV which, in addition to being authorized in the final approach
segment as primary vertical guidance where VNAV minimums are charted, is used extensively for
flying constant angle descents in the segments of an IAP prior to the Baro VNAV P-FAF. That's
different that LPV, which can only be used in the final approach segment where LPV minimums are
charted, and you have LPV approval.

If the CNX-80 will be able to do both Baro VNAV and LPV, that will be very advanced, indeed. Is
that the plan?

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)

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.

On Sun, 27 Jun 2004 04:21:24 -0700, wrote:

If the CNX-80 will be able to do both Baro VNAV and LPV,

The CNX80 has a baro input -- I have an encoder with 10' resolution. I
believe the plan is for baro VNAV when a GPS signal is unavailable. That
may even be a part of the TSO 146 spec, but I'm not certain of that.


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)

wrote in message ...
=20
=20
"John R. Copeland" wrote:
=20
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---
=20
Most of the modern iron has Baro VNAV which, in addition to being =
authorized in the final approach
segment as primary vertical guidance where VNAV minimums are charted, =
is used extensively for
flying constant angle descents in the segments of an IAP prior to the =
Baro VNAV P-FAF. That's
different that LPV, which can only be used in the final approach =
segment where LPV minimums are
charted, and you have LPV approval.
=20
If the CNX-80 will be able to do both Baro VNAV and LPV, that will be =
very advanced, indeed. Is
that the plan?
=20

I've been curious about the plan for the CNX-80 myself.
It might be my own eagerness making me expect LPV capability.
I've been trying hard to be patient, waiting for FAA approval of v2.0,
whereupon I hope the plans will all become clear to the rest of us.
Or is that hope too optimistic?
---JRC---

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.