How High Are We Now?
We all fly at erroneous altitudes—even when accompanied with a GPS. Here’s how to determine and understand the best way to get the most precise reading.
If you have a GPS and a blind encoder in your panel, you may have three independent ways to determine your altitude. But which one is most accurate? We all grew up on baro altitude, so after a short review, we’ll plunge into the GPS world of the WGS84 datum, your height above ellipsoid (HAE) and mean sea level (MSL) altitudes.
According to the barometric law, the pressure drop is approximately exponential with altitude. But to compute altitude from that pressure using that law, you need the pressure at the surface below you and the temperature profile between you and that surface. We get the surface pressure because it’s first converted to MSL pressure before given to you. But the temperature data isn’t available, so there’s a standard model of the atmosphere: the MSL temperature is 15 degrees C and drops about 2 degrees C per 1,000 feet. We fly at these erroneous altitudes so that we can avoid each other. But we can’t avoid rocks at known altitudes if we have incorrect altitudes.
An altimeter or an encoder helps determine baro altitude. After entering the altimeter setting, the altimeter hands wind up as the outside pressure is reduced, giving you the correct reading for standard atmospheres. Your altimeter basically is a mechanical calculator to perform the calculations indicated in the barometric law.
A blind encoder also measures pressure, but that pressure (p), relative to standard MSL pressure (29.92 inches of Hg) is digitized into “gray code” and sent by your transponder to ATC computers that determine your altitude. If you’re below 18,000 feet, they first change the 29.92 inches to the actual setting near you (say, 30.16), and take that ratio p divided by 30.16 and compute your altitude based on a standard atmospheric model in the barometric law. Your GPS navigator may be able to do that same calculation if your encoder data is sent to the device.
The Chelton and Garmin 480 both ask for the baro correction (local altimeter setting), and they can tell you your baro altitude. In the GNS480, you can put data fields on maps 2 to 4, and among the choices are both baro and GPS altitude, so putting them side by side provides an interesting comparison.