I posted earlier about seeing differences of up to 500' between the
altitude shown on my Garmin GPS 196 (even while receiving WAAS) and my
altimeter. The GPS would consistently indicate higher than the
altimeter. Dan Luke suggested the cause might be non-standard
temperatures. After some more experimenting, I think Dan was correct.

On a flight last week, the GPS altitude and altimeter matched within 20
feet. The temperature was close to standard, which made me think that
temperature might be in fact be the culprit as Dan suspected.

I did another trip over the weekend and found the GPS reading high for
altitude again, and the temperatures were above standard. Here's what I
got....

SATURDAY SUNDAY
Indicated Altitude 7500 8500
GPS Altitude 7700 8677
Pressure Altitude 7350 8310
True Altitude 7700 8700
Density Altitude 8200 9100
Altimeter setting 30.07 30.11
Temp (C) +8 +5
Std Temp (C) 0 -2

So it looks like the GPS is showing True Altitude (which makes a lot of
sense). True altitude is the actual height above MSL, and that will
differ from indicated altitude when the temperature is not standard, or
you forget to change your altimeter setting. The true altitude numbers
above were calculated using my good old-fashioned E6B.

So the GPS is showing the right value. However, that value is different
than what you altimeter shows when the temperature is not standard. When
temperatures are above standard, the GPS altitude will be higher than
indicated and when termperatures are below standard the GPS altitude
will show lower than indicated. Does this make sense?

I wonder how this will affect the upcoming WAAS LPV approaches. They are
going to have a decision altitude based on indicated altitude, not true
altitude. How will the difference between true altitude and indicated
altitude affect the approach, since the GPS will be giving vertical
guidance based on true altitude?

Phil
www.pfactor.com