From reading how the anonymous ICAO number is created, the UAT WAAS GPS source needs a lock. That is from where the position and timing comes that creates the string.
So, IF your ADSB UAT has the WAAS gps installed IN the unit, there is no way to get enough data to populate the self generated anonymous ICAO inputs immediately. If a WAAS source exterior to and feeding position to the UAT has a good position before powering the UAT, then Anonymous mode should be available sooner. Immediately? 5 seconds? Don't know.
So, not just on, but good gps lock at the UAT position source.
This would be the transmit side of anonymity. On the receiver side, an In device nearby can see you right away, but ATC does not see you until an ADSB GBT receives your Out.
My PAPRs say I had a valid flight as low as 200'. Not sure if that correlates to Mode C during a Valid flight, or actual UAT valid reception at the FAA GBT. I am not at a surface airport in the PAPR decoder link.
https://www.google.com/url?sa=t&sou...AegQIAhAB&usg=AOvVaw0aL_PlXuSIwz8GbtBlGyDyThe self-assigned temporary address shall be generated as follows:
Let: ADDRP = the ICAO 24-bit address that has been assigned to the aircraft;
ADDRT = the temporary address that is to be generated;
M(1) = the 12 least significant bits (LSBs) of the ownship “LATITUDE”
field (per §3.2.1.5.2.1) at the time the temporary address option is
selected;
M(2) = the 12 least significant bits (LSBs) of the ownship “LONGITUDE”
field (per §3.2.1.5.2.1) at the time the temporary address option is
selected;
M(3) = 4096 × M(1) + M(2); and
TIME = the number of seconds that have elapsed since UTC midnight at the
time the temporary address option is selected, represented as a 24-bit
number."