ASTERIX CAT 21 Geometric Altitude

[17roger]

The ADS-B ASTERIX CAT 21 message contains Geometric Altitude in addition to Pressure Altitude (also called Flight Level).  Is it possible to get both altitudes from the RadarBox?

[ChrisGR]

You try from File-Preferences-RadarBox-Transition Altitude menu to change the value?

Chris

[17roger]

Thanks for your reply.  I'm pretty sure the transition altitude is just the altitude at which the software changes the altitude displayed from feet to FL (100's of feet).  I want both barometric (pressure) altitude and geometric (GPS) altitude.

[AirNav Support]

RadarBox will give you the geometric (GPS) altitude. You can then work out the barometric alitude by checking the local pressure at that time. RadarBox can do that for you however the data output only has the geometric (GPS) altitude.

[17roger]

Thanks for your reply, but it won't quite do it unless I can alternate back and forth quickly.  I need the pressure altitude reported by the altimeter on the aircraft and the GPS altitude reported by the GPS receiver on the aircraft.

[AirNav Support]

Contact us directly through support/sales and we will see what we can do for you.

[palmar]

What Radarbox shows is the barometric altitude at an altimeter setting of 1013 mb or 29.92 in. Radarbox can calculate an altitude over mean sea level at standard atmosphere with the help of the transition altitude and QNH entry on the Preferences dialog. But the raw data remain.

GPS or geometric altitude is not part of the squittered ADS-B messages. (Actually what it shows if it could is the difference between barometric and geometric altitude.)

It needs to be interrogated by a Mode-S radar head. These messages cannot be decoded by a surveillance receiver, because the interrogating information is required to understand the reply (there is NO such header as "GPS" for the reply string).

The same is true for IAS (Radarbox IAS is TAS), Mach, Heading (Radarbox Heading is Magnetic Track), Bank angle, roll rate, QNH setting and FMC selected altitude