As far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?
e.g in MSG format: http://adsb-decode-guide.readthedocs.io/en/latest/position.html which kind of altitude is there?
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Sign up to join this communityAs far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?
e.g in MSG format: http://adsb-decode-guide.readthedocs.io/en/latest/position.html which kind of altitude is there?
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.
MSG,1, 0, 0,4006B3, 0,2009/06/19,06:15:17.421,2009/06/19,06:20:17.421,BAW4J,17750,381,097,51.4854,-1.9028,-2496,4244,0,0,0
$\endgroup$
– delkov
Jan 12 '17 at 1:25
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.
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.
http://adsb-decode-guide.readthedocs.io/en/latest/position.html which kind of altitude is there?
It's pressure altitude. For purpose of separation it does not matter what kind of altitude is used, but everybody has to use the same definition and pressure altitude is easiest to measure, so that is the standard.
Geometric altitude (note: “altitude” always means above sea level in aviation) is only used in EGPWS and as far as I can tell, never displayed to pilots.
Geometric height (note: “height” means above terrain) is measured with radio altimeter and used also for (E)GPWS, but also displayed to pilots and even read out during approach. It is not, however, transmitted in ADS-B or Mode-S, because it is not needed for the controllers or for collision avoidance.
As far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?
Only to certain accuracy. If you know pressure and temperature in the area where the plane is flying, you can correct some difference, but the lapse rate also depends on humidity and you won't know the complete profile, so there will be some unknown error that will increase with altitude.