Originally Posted by
silverstrata
i have had some dealings with transponders, and the rule of thumb is that.
Mode-c gives 1013 baro altitude.
Mode-s gives 1013 baro altitude.
Ads-b also gives 1013 baro altitude, to be compatible with the above.
Flarm and paw give gps altitude.
As far as i know, fr24 is simply picking up ads-b 1013 pressure altitudes, so you will need to know the qnh of the day, and the altitude of the airport, to calculate the true height of the aircraft. Transponders were designed for seperation on airways, not for separation with terrain, so the older units all used 1013 baro, and ads-b follows suit.
(if ads-b used gps alt, then atc would not be comparing like with like. However, newer systems like flarm and paw can happily use gps alt, because they all use gps, so they are comparing like with like.)
silver
metar HAAB 100500Z 06008KT 9999 FEW025 16/10 Q1029
Airfield elevation 2,334m / 7,625 ft