Altimeter vs GPS Altitude
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Recently while flying a VFR aircraft with an EFIS, I updated the altimeter setting and received a message from the system warning that the altimeter reading was 900+ feet different from the GPS derived altitude (OMG!). I commented about it afterward and was told, “oh the altimeter must need to be calibrated”.

The discrepancy has led me to ask several questions:

  1. How accurate is the GPS derived altitude? Is it preferred? (When I land in my steam gauge plane, ForeFlight with Stratus usually puts me somewhat underground).
  2. Is ADSB-out sending 29.92 altimeter data, or is it sending GPS altitude?
  3. When my EFIS reports a target above me, below me, or at my altitude, which data is it using for my ship and for the other ship when it does the math?
  4. If ADSB relies on GPS derived altitude, what happens in areas of GPS interference (I fly a lot near Alamogordo).
  5. If ADSB relies on 29.92 altimeter data, what happens when a VFR altimeter is out of calibration?
3 Replies
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@Kenneth Miess
How accurate will depend upon the GPS source. More and more aircraft are WAAS-capable which can give you a position accuracy of better than 3 meters, 95 percent of the time. 

WAAS derived altitude is defined as a geometric altitude relative to MSL and is not affected by local barometric pressures. Corrected barometric altitude and geometric altitude relative to MSL represent heights above MSL, but they differ by how they’re derived. GPS systems will use the undisturbed MSL mathematically extended continuously through the continents as MSL which will not necessarily be the same as a barometric derived MSL. Therefore, pilots reading geometric altitude relative to MSL could reduce their vertical separation from other aircraft, because geometric altitude sometimes differs from the barometrically derived altitudes that other aircraft are using. 

The altitude information that is transmitted comes from your altitude encoder, typically in your transponder and is referenced to 29.92, otherwise known as Pressure Altitude. This is the altitude used for vertical separation.

I have attached an AC on using and displaying geometric altitude relative to MSL.AC20-163.pdf

Attached files
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I am not sure were you got the information on MSL and GPS but I question that. I was part of the original GPS team in the Navy back in the 80s. First, MSL is different all over the the earth. Sea level is 80 feet lower in the ocean near Everest and higher than what you would expect in other parts. GPS assumes 4 active birds, it measures the time it takes to acquire the signal and the distance from the bird. Based on the 4 it gives you location from the center of the earth. There is also data from each bird for corrective bird positions. Your GPS unit may have software with local updates to interpret sea level, but I would be very cautious of it. Be sure to verify at your locations.
If you mount a GPS antenna on a helicopter under the blades, you will see your altitude a couple hundred feet underground at start up due to reflections under the blades. Another area that will bite you is flying at low level from sea to land.  After doing navigation for many years in the air and sea, it got real exciting when I had to do navigation in space.
I hope you find this helpful.
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1431 Posts

@Stephen Simonin
First, MSL is different all over the the earth. Sea level is 80 feet lower in the ocean near Everest and higher than what you would expect in other parts.

Mean Sea Level, which is the average sea level across all seas everywhere, is not the same as local sea level, which can vary as Stephen said.  As such, those local sea level variations have no effect on the difference between GPS altitude and local altimeter-based MSL altitude.  Where you could see those variations have an effect would be the difference between radar altitude over the ocean and local pressure-based altimeter readings at altitudes low enough that variations in local pressure lapse rate aren't significant.