I'm reading the FAA's Advisory Circular AC No:20-165 appendix 1 point 28, which talks about GNSS Antenna Offset and Position Offset.

How would I receive this information from an aircraft? Is it transmitted?

  • $\begingroup$ If the aircraft is equipped with ADS-B, then yes, this is transmitted in an Airborne Position Message, type code 20 to 22. Are you asking how you physically receive it (ADS-B receiver) or how to decode it? $\endgroup$ – Ron Beyer Jan 10 '17 at 21:40
  • $\begingroup$ What I ultimately need to know is where relevant to the fuselage the ADS-B GPS position is (GNSS Antenna). I'm hoping that this information tells where the reading is taken from so I can correctly position the aircraft. $\endgroup$ – ronMilne Jan 10 '17 at 21:45
  • $\begingroup$ The antenna is typically in-line with the fuselage, but may be placed anywhere along its length. If you are trying to get the offset of the GPS antenna from some fixed point on the aircraft then it isn't possible, antenna positions are not standardized with respect to a fixed point on the aircraft. I think most software assumes the GPS position is at the center (0,0,0) of the aircraft. How accurate are you trying to be? $\endgroup$ – Ron Beyer Jan 10 '17 at 21:49
  • $\begingroup$ I'm building 3d models of aircraft that I place on a map, now if the aircraft is at a gate and I receive a ground position (I'm also interested in airborne position too) I need to position the airplane with some degree of accuracy as it may end up in the wrong place as I'm unable to determine where the GPS position is relevant to (let's say) the sky bridge. $\endgroup$ – ronMilne Jan 10 '17 at 21:53
  • $\begingroup$ You may have a difficult problem there. You may need to assume that if the aircraft is within 1 length of the gate (and stopped) that it is at the gate. $\endgroup$ – Ron Beyer Jan 10 '17 at 22:01

The fields are transmitted when the aircraft has ADS-B protocol version 2 (assuming you are looking at 1090MHz) and is on the ground.

They are specifically added to the ADS-B MOPS (minimum operational performance standards) to allow for accurately depicting aircraft on the airport surface.

I don't know by heart which message type it is in, which bits are used and what the encoding is.

Best is to obtain a copy of the MOPS; RTCA DO260B or EUROCAE ED102A (they are essentially the same, get the cheapest).

  • $\begingroup$ For ground surveillance would I also need ADS-B Out for Airport Surface Surveillance (ADS-B APT, ED-163 / DO-321)? What information is different in both documents? $\endgroup$ – ronMilne Jan 11 '17 at 1:19
  • $\begingroup$ The ADS-B MOPS explains the messsage structure and defines tests for ADS-B transmitters and receivers. The ADS-B APT contain the requirements for applying ADS-B for airport surveillance. If you build something for air traffic control or airport operations you will need to get ED-163 as well. $\endgroup$ – DeltaLima Jan 11 '17 at 7:49

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