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I could not understand what is the difference between ADS-R with anonymous address (CF=6, IMF=1) and TIS-B CF=5, which on ICAO Doc 9871 is described,

for Version 1 as:

Fine TIS-B Message AA field contains a non-ICAO 24-bit address

and for Version 2 as:

TIS-B messages that relay ADS-B Messages using anonymous 24-bit addresses

Any specific example when CF=5 is used?

Thanks!

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My question was due to the confusing mention of ADS-B in what was supposed to be either TIS-B or ADS-R.

I've now confirmed that the text on ICAO Doc 9871 Version 2 I quoted was due to a error.

In fact DO-260B, and even Doc 9871 Version 2 at table C-37 specify that TIS-B CF=4 is "Fine TIS-B message, AA field contains a 24-bit anonymous aircraft address".

ICAO e-mailed me confirming that this has been fixed into their working document.

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  • $\begingroup$ it might be useful to quote a source for your discovery. Thanks! $\endgroup$ – bclarkreston Jul 20 '19 at 13:23
  • $\begingroup$ @bclarkreston thanks. I added further details. $\endgroup$ – nunojpg Jul 20 '19 at 14:29
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TIS-B is a service that broadcasts traffic information. This traffic information is gathered by using various surveillance sensors such as ADS-B and/or radar.

In case the surveillance source is a Mode A/C radar, and no ADS-B track can be correlated to the radar track, no 24 bit address is known for the aircraft. Now the TIS-B message will be filled with a 24 bit address that is created ad-hoc and temporarily assigned to that radar track. This is not a standard ICAO address (which is assigned through official procedures of the ICAO member states) but instead a (more or less random) number is used.

To clearly distinguish between the ICAO 24 bit addresses and the ad-hoc addresses (which may be accidentally the same) a different message indicator is used in TIS-B.

A similar scheme is used for ADS-B and ADS-R. In case an ADS-B transmitter cannot be assigned an ICAO 24 bit address (e.g. because the country only assigns addresses to aircraft and the transmitter is on a car or a fixed obstacle) an "anonymous address" can be uses. In that case a special flag is used to avoid confusion between ICAO addresses and anonymous addresses.

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  • $\begingroup$ I think your answer explains what is the use for non-ICAO/anonymous address, which is very obvious, but the question is instead what is the difference in both the frames I mentioned (ADS-R with anonymous address and TIS-B with non-ICAO/anonymous address). $\endgroup$ – nunojpg Feb 20 '19 at 9:34
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ADS-R is only used for relaying ADS-B messages from one datalink to another, e.g. 978UAT to 1090ES. This is currently only applicable to the US since no other country has adopted 978UAT.

TIS-B is used when a ground radar station transmits ADS-B messages on behalf of a non-ADS-B contact. If the contact has Mode S, the ground station will use the contact's Mode S address, which may be either assigned or anonymous. If the contact does not have Mode S, the ground station will generate a trackfile address, which is the 12-bit Mode A code plus 12 locally-assigned bits.

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