# How to convert a pressure height (ADS-B) to a geometric height?

As far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?

• Jan 10 '17 at 20:49
• In MSG format (after decoding) there is only one kind of altitude, which one? Jan 10 '17 at 21:19
• If you need height above the ground you need to know the ground altitude and subtract it from the altitude of the aircraft. ADS-B doesn't broadcast height.
– mins
Jan 10 '17 at 21:32
• ok, but my question is about the altitude of the aircraft.. where is the zero point? Jan 10 '17 at 21:33
• @delkov See this page, Barometric altitude uses a TC of 9 to 18, where GNSS (geometric) uses a position report with TC of 20 to 22. You need to know what the type code of the position report is that you are decoding. Jan 10 '17 at 21:37

ADS-B gives both pressure altitude and geometric altitude.

In the position messages you will find the pressure altitude which is essentially a pressure converted to an altitude. For this conversion the ICAO Standard Atmosphere is used.

In the velocity message you will find the difference between the geometric altitude and the pressure altitude. Since the difference between them is fairly limited a few bits could be saved by encoding the difference instead of the geometric altitude.

If you don't have the geometric altitude and you want to obtain it from pressure altitude (regardless where it cones from) you will have to know the relation between pressure and geometric altitude in the atmosphere.

You can model it according to the hydrostatic equations of the ICAO Standard Atmosphere and correct for actual temperature, lapse rate and local pressure. This gives fairly good results.

For higher altitudes data from meteorological models can be of help to improve accuracy.

• Do you know which altitude used in MSG Kinetic SBS-3 Format?! It looks like MSG,1, 0, 0,4006B3, 0,2009/06/19,06:15:17.421,2009/06/19,06:20:17.421,BAW4J,1775‌​0,381,097,51.4854,-1‌​.9028,-2496,4244,0,0‌​,0 Jan 12 '17 at 1:25
• As far as I know the SBS3 only gives you pressure altitude Jan 12 '17 at 5:37

This doesn't really make sense because you're asking for something with no baseline reference. What are you wanting a "geometric altitude" with respect to? Mt. Everest? Death Valley? Sea Level? A standard radio altimeter will give you "geometric altitude" from your position in the air to the ground immediately below you; up to 2500 ft.

That is, your barometric pressure altitude could be 30,029 feet. If you're flying over the top of Mt. Everest, your "geometric altitude," as sensed by the radio altimeter, would be approx. 1000 ft. If you're flying greater than 2500 ft. above Mt. Everest, you're radio altimeter won't have a reading, and you'll be reading a barometric height of approx. 32,500 ft.

There is no way to derive your "geometric altitude" from a barometric reading because the earth isn't perfectly flat.

• I need the altitude above the ground. please look upd* Jan 10 '17 at 21:20
• Got it. Mins is correct then. I was a little confused as to the wording of your question. Jan 10 '17 at 21:24
• "Geometric altitude" means height above the datum geoid, not above terrain. Usually the datum is WGS84, which I think is standard. Jan 10 '17 at 21:38

ADS-B uses standard atmosphere characteristics defined in International Standard Atmosphere (ISA). Decrease of pressure with altitude is part of this standard which assume a temperature of 15°C at sea level. A correction is necessary if the temperature is different. This correction is also standard.

In ISA, mean sea level pressure is 1013.25 hPa, and at low altitude each 30 ft the pressure decreases by 1 hPa.

You may use an online calculator or a table.

• I don't believe this is what is being asked for. The reader wants to know their "geometric altitude," which is not based on barometric pressure, but rather on radio altitude - which is only good for about 2500 ft. above the ground. Jan 10 '17 at 21:18
• please look upd* Jan 10 '17 at 21:23
• Fair enough. The wording of the question had me thrown a little bit. I'm pretty sure this is the way GPS altitude is measured, is it not? Jan 10 '17 at 21:25
• Are you sure, that ADS-B broadcast exactly GPS, not pressure altitude?! Jan 10 '17 at 21:35
• @Frank: Altiude in GPS is not above ground (topographic surface), nor even above the approximation of mean sea level, which is named a geoid (an equipotential of gravity, initially the EGM84, and today the EGM96 -- this height is named orthometric height), but actually above the WGS84 ellipsoid (ellipsoidal height). Altitude and local vertical are complex matters.
– mins
Jan 10 '17 at 23:35