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.
How does GPS altitude differ from baro altitude? Your GPS solution from the satellites in view gives both horizontal and vertical positions. The lateral position from any kind of triangulation scheme always is the most accurate when there are stations on both sides of you. For your vertical position, you need satellites above and below, but you can’t see the satellites that are down under, so vertical errors are much larger than horizontal errors. The altitude solution is improved dramatically with WAAS, allowing vertical guidance on GPS approaches.
WAAS uses a network of U.S. ground stations that each know exactly where they are, so their three-dimensional GPS solution always is in error, an error they can broadcast to geosynchronous satellites in the East and West. Those errors are transmitted to your GPS as well as WAAS receiver, and by interpolation, the error at your location is first subtracted from your raw solution to give a more precise one. The claimed accuracy is one meter horizontal and two meters vertical.
Where is that position relative to the surface of the Earth below us, or to MSL? GPS solutions are altitudes above a mathematical surface model of the Earth, an ellipsoid known as the WGS84 datum.
The Earth is flatter at the poles than at the equator by about 21 kilometers (radius) and nearly ellipsoidal. If you slice the Earth through the poles on any longitudinal line, the cross section is an ellipse. If you slice the Earth at the equator, or any latitude, it’s circular. To create the WGS84 surface, take the ellipsis that’s flatter at the poles and spin it around on the polar axis to create the WGS84 ellipsoid. The semi-major axis is 6378.1 kilometers, and the semi-minor axis is 6356.8 kilometers. The centrifugal force of the spinning Earth made it bulge most at the equator.
How well does WGS84 represent the surface of the Earth? Clearly, it misses all the mountains and valleys, but then we really only want to know how well it compares with the MSLs (plural!) of the Earth. And here’s where it gets real interesting. What’s the MSL surface?