Say Altitude.

Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?

Many times. Glitches can occur with their software. My most common
experience was having one controller tell me to turn off the altitude
squawk, being handed off to another sector, asking the new controller if I
should try Mode C again and having no further problems. Don't sweat it. If
they don't like what they are seeing they will ask for verbal altitude
reports.

Bob Gardner

"Hankal" wrote in message
...
Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show
300
feet lower and gives me the altimer reading. I confirm. He says "turn off
you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in
my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my
mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is
happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?

In article , Hankal
wrote:

I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.

GPS altitude is only accurate to +/-150 feet.
It is a "rough" reference, not to be considered "precise".

GPS is not nearly as accurate in the z dimension as in the horizontal
dimension. I often see mine (even with WAAS corrections) differ as
much as 3-400 feet from my altimeter.

On 05 Apr 2004 00:29:34 GMT, (Hankal) wrote:

Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?

Nathan,

GPS is not nearly as accurate in the z dimension as in the horizontal
dimension. I often see mine (even with WAAS corrections) differ as
much as 3-400 feet from my altimeter.

Reference for your altimeter is a pressure level (hence it's called altitude
and not height), reference for GPS height is an approximation of the earth
called the "geoid", usually in form of WGS-84. I'm not sure if WAAS takes
this into account. If not the difference is system immanent.

Regards

Kai Glaesner

You didn't specify if your GPS was connected to the altitude encoder or not.
If it's an IFR GPS, it'll be connected to the encoder, which only outputs the altitude
to the nearest 100 feet. Since you say it stayed at 7060 after minor altitude
changes, I'm guessing that it was connected... and the altimeter setting in the GPS
made the nice, round 100' increments "round" to 7060. Remember, the encoder only
outputs pressure altitude, which is then corrected to actual altitude on the ground
(or in an IFR GPS) by the equipment given the current altimeter setting. If it's
unusually differing temperatures where you are than where the altimeter is, the
plane's altimeter could be a bit different than the encoder's corrected output.

-Cory

Hankal wrote:
: Long CX IFR at 7000
: Controller comes on "say Altitude" My response "7000" He says you show 300
: feet lower and gives me the altimer reading. I confirm. He says "turn off you
: mod C"
: I comply.Look at my GPS which is at 7060.
: I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
: 7060.
: The controller comes back and says" I think this is due to a cold front in my
: flight area.
: 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
: C. He concurs my altitude at 7000.
: Request lower for decend for landing. The gps works fine now, ATC is happy.

: HAS ANYONE ELSE EVER EXPERINCED THIS?




--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

You didn't specify if your GPS was connected to the altitude encoder or not.

If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
altitude

My GPS is not connected.
Under most conditions it agrees with my
altimeter.
It was just strange that the GPS and ATC
did not not reflect the same as my altimeter.
Hank

OK... even still, though, the GPS isn't too accurate, and is based on absolute
altitude, not barometric altitude (mentioned previously). Also, the temperature
affects the altimeter... how high AGL were you? I read an interesting article (avweb,
I think) regarding temperature correction and obstacle clearance. When the altimeter
reading is down low and temps are cold, the higher you go the more error the altimeter
tells you. So, if the altimeter setting is down in the valley at 1000' MSL, but
you're shooting the approach with MEA at 10K', there can be issues with terrain
avoidance. Supposedly Canadian and AK folks get more indoctrination to this than
those in the lower '48.

-Cory

Hankal wrote:
:You didn't specify if your GPS was connected to the altitude encoder or not.
:
:If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
:altitude

: My GPS is not connected.
: Under most conditions it agrees with my
: altimeter.
: It was just strange that the GPS and ATC
: did not not reflect the same as my altimeter.
: Hank

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
When WAAS is in common use this will probably change.

Bob Gardner

"EDR" wrote in message
...
In article , Hankal
wrote:

I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays
at
7060.

GPS altitude is only accurate to +/-150 feet.
It is a "rough" reference, not to be considered "precise".

"Bob Gardner" wrote in message
news:R0hcc.78998$K91.171081@attbi_s02...
You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
altitude.
When WAAS is in common use this will probably change.

The altitude error is part of the WGS-84 model and is not a WAAS issue any
longer. The CNX-80 provides VNAV capability through the use of pressure
altitude from a transponder source and the pilot's baro correction input;
married to a partial TAWS data base. GPS/WAAS is not likely to become an
acceptable substitute for pressure altitude. Larger airplanes use air data
derived pressure altitude in conjunction with GPS for improved RNP
capability.