Jim Bob,
I received a response from skyBeacon .
“The skyBeacon does not report AMSL. it reports height above ellipsoid per FAA requirements for ADS-B. ”
The rest of this post is purely for academics.
As you can see it’s definitely different than MSL. GPS altitude is based on the World Geodetic System 1984 WGS 84.
Excerpt below from this link.
(Actually even coordinates are time sensitive due to continental drift. But let’s stick elevations).
https://www.esri.com/news/arcuser/0703/geoid1of3.html
Differing Measurements
GPS has transformed how altitude at any spot is measured. GPS uses an ellipsoid coordinate system for both its horizontal and vertical datums. An ellipsoid—or flattened sphere—is used to represent the geometric model of the earth.
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The surface of global undulations was calculated based on altimetric observations and very precise (up to two centimeters) measurements taken from the TOPEX/POSEIDON satellite. This data was represented in the Earth Geodetic Model (EGM96), which is also referred to as the spherical harmonic model of the earth's gravitational potential.
Conceptually, this precisely calculated ellipsoid, called an oblate ellipsoid of revolution, was intended to replicate the MSL as the main geodetic reference or vertical datum. If this ellipsoid vertical datum is used, height above the ellipsoid will not be the same as MSL and direct elevation readings for most locations will be embarrassingly off. This is caused, in part, because the GPS definition of altitude does not refer to MSL, but rather to a gravitational surface called the reference ellipsoid. Because the reference ellipsoid was intended to closely approximate the MSL, it was surprising when the two figures differed greatly.
The TOPEX/POSEIDON satellite, launched in 1992, was specifically designed to perform very precise altimetric observations. These measurements have demonstrated that neither human error nor GPS inaccuracies are responsible for the sometimes substantial discrepancies between ellipsoid and MSL measurements. In fact, the three-dimensional surface created by the earth's sea level is not geometrically correct, and its significant irregularities could not be mathematically calculated; this explains the difference between the ellipsoid-based GPS elevation readings and elevations shown on accurate topographic maps.
A brief examination of elevation readings for Esri headquarters in Redlands, California, demonstrates these differences. The campus elevation is shown on topographic quadrangle maps and high-resolution digital elevation models (DEMs) for the area as approximately 400 meters above MSL. However, a precise, nonadjusted GPS reading for the same location typically shows the elevation as 368 meters.
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The map shows the areas of the globe that would have a sea level below the theoretical surface of the WGS84 ellipsoid, or the theoretical and geometrically correct sea level (shown in blue). The sharp contrast between the blue and green indicates where the ellipsoid and geoid intersect. With the continents displayed as opaque, the remaining area covered by water reveals where sea level is actually at zero elevation relative to the WGS84 ellipsoid.
Why is there a 32-meter difference? The GPS receiver uses a theoretical sea level estimated by a World Geodetic System (WGS84) ellipsoid, which does not perfectly follow the theoretical MSL. The MSL, approximated by an ellipsoid, is related to gravity or the center of mass of the earth. Discrepancies between a WGS84 ellipsoid, and the geoid vary with location. To continue with this example, elevation readings for Yucaipa, a city located less than 10 miles east of Redlands, differ by 31.5 meters.
Conti
