some notes on ADS-B
In earlier posts assumptions were made about ADS-B, some correct and some others slightly less. These notes should help to clarify.
ADS-B was implemented for the sole purpose of traffic information and collision avoidance provided to airborne and groundbased equipment. Tracking over time was and is not a main goal.
Although data may be interrogated, the transmitters mostly send it insollicited and once per second in a so called broadcast (to anybody listening) , hence the B after ADS. Many aircraft broadcasting once per second within the range of about 100 nautical miles require the following characteristics:
Messages must be short and well distributed over the 1 second period. As the data is repeated, there is no error correction required, but error recognition must be excellent When a message is corrupted, the system can wait for the next one in sequence.
Data is not broadcast in one long message but sliced into parts (registers) containing no more than 56 bits (7 bytes) and protected against wrong reading by a 24 bit parity which provides reliable error detection. It takes about 1/10000th of a second to send a slice, so the theoretical max number of well distributed broadcasts would be 10000 within the same range. The practical limit is of course far less and even if there are only 2 aircraft within range, they may occasionally block each other out.
Short messages mean data condensed to the minimum. Thus e.g a timestamp in not included as it is meaningless in real time navigation. Anything else than 3d-position, 3d-vector and aircraft id is not repeated in the standard broadcast. It may be interrogated though in special cases and TCAS advisories may be exchanged between aircraft. More data can be sent as a response to a radar scan by means of mode S transponders. The last thing one would do is synchronize all the broadcasts as they would effectively block each other out within the same range.
Data sources may be different. The most simple ADS-B device, the FLARM, derives all values from an internal GPS. In complex aircraft equipment, the data may be provided by computers or other complex sensors. Mode-S transponders send pressure altitude while FLARM send msl altitude. V-Speed may also be derived from different sensors or simply by differentiating altitude changes. Roll data is not required for collision avoidance (TCAS gives only up and down advisories, never left or right) and is therefore not part of the squitter messages. Same applies for other conditions. But there is a ground mode and an airbourne mode, ground mode not transmitting values like speed etc. If you want to know more, google “ads-b for dummies”.
Both airborne and ground equipment may now process these raw data and make assumptions about other flight parameters. But any follow-on calculation assumes a reasonable flight enveloppe. Thus, in a crash analysis, processed ADS-B data as provided by FR24 must be used cautiously.