GPS Altitude with WAAS

Phil Verghese writes:

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.

In Canada, learning about temperature errors in the altimeter is a
standard part of the PPL curriculum, but I've noticed that it's not so
familiar to U.S. pilots (at least not private pilots). We have tables
in our AIP and other publications showing what errors to expect, and
when flying IFR, we are required to add those errors to all instrument
approach altitudes (MDA, DH, etc.) in very cold temperatures.

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?

Yes. A rough rule of thumb is that your altimeter will be off by 4
feet, per degree Celsius difference from ISA, per thousand feet above
the station reporting the altimeter setting. This works, of course,
only with the standard lapse rate -- if there's an inversion or any
other non-standard lapse rate between you and the field reporting the
altimeter setting, this formula won't work.

Let's assume that your field is at 1000 ft MSL with 20 degC above
standard temperature and a standard lapse rate all the way up. When
you're sitting on the field, your altimeter should show your actual
elevation:

4 * 20 * 0 = 0 ft error

So you'll see 1000 ft on your altimeter when you dial in the field's
altimeter setting.

At 200 ft AGL (typical ILS DH), your error will will be

4 * 20 * 0.2 = 16

so that when your altimeter reads 1200 ft on short final, you're
really around 1216 ft MSL, or 216 ft AGL.

At 9000 ft MSL (8000 ft AGL), the error will be much larger:

4 * 20 * 8 = 640 ft

So when your altimeter says 9000 ft, you'll really be up at 9640 ft.
That's no big deal, but it could be dangerous in cold temperatures, if
you were trying to clear (say) an 8000 ft ridge with 1000 ft
clearance. If the temperature was 20 degC below standard, and your
altimeter setting was from a field at 1000 ft MSL, you'd actually be
flying at 8360 ft when your altimeter read 9000 ft, just barely
clearing the ridge.

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?

The approach will be a safer than approaches based on a barometric
altimeter, all things considered. Barometric altimeters are fine for
cruise, since everyone sees the same error and flies at the same
altitude anyway (just not the one they think they're flying at).


All the best,


David