ADS-B Technical question...

[dans2992]

Ok, so I've got a Raspberry Pi monitoring local ADS-B transmissions on 978 and 1090.

I've noticed that of course standard transmissions are of the form "Axxxxx" where "x" are hexidecimal digits representing the ICAO Mode S code.

Rebroadcasts (typically on 978) are of the format "2xxxxx".

Is there anything at all that can be inferred from the 978 rebroadcast addresses? How are these picked? Is it totally random? Will a given aircraft retain it's "2xxxxx" address throughout a flight? Is it based on the squak code?

 

[John Collins]

Ok, so I've got a Raspberry Pi monitoring local ADS-B transmissions on 978 and 1090.

I've noticed that of course standard transmissions are of the form "Axxxxx" where "x" are hexidecimal digits representing the ICAO Mode S code.

Rebroadcasts (typically on 978) are of the format "2xxxxx".

Is there anything at all that can be inferred from the 978 rebroadcast addresses? How are these picked? Is it totally random? Will a given aircraft retain it's "2xxxxx" address throughout a flight? Is it based on the squak code?

You need to check the address qualifier code (three bits) to interpret the usage or meaning of the 24 bit address.

0 - ADSB ICAO 24 bit address
1 - ADSB Self assigned temporary address (anonymous mode)
2 - TISB/ADSR with ICAO 24 bit address
3 - TISB with file track identifier (assigned by ATC computer)
4 - Surface Vehicle
5 - Fixed Beacon
6 - ADSR with non ICAO address (anonymous mode)

The file track identifier is assigned for the flight, as is the self assigned address as long as the aircraft remains in anonymous mode.

 

[dans2992]

Interesting.

Is the "flight" considered over when the aircraft goes out of radar range? Could a TIS-B target potentially have the same hex code on a long flight all the way across the country?

Can a ground vehicle advertise "ADS-B in" capability and receive TIS-B traffic services from a ground station?


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[John Collins]

Interesting.

Is the "flight" considered over when the aircraft goes out of radar range? Could a TIS-B target potentially have the same hex code on a long flight all the way across the country?

Can a ground vehicle advertise "ADS-B in" capability and receive TIS-B traffic services from a ground station?


Sent from my iPhone using Tapatalk

It is from power on to power off for anonymous mode. For TISB it would be from the point a unique transponder mode C target was being tracked until the track was lost. I don't see why a ground vehicle at an airport with surface ADS-B couldn't be receiving surface tier information.