Say Altitude.

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$) *
************************************************** ***********************