A Mode S transponder will 'squitter' whether or not it is seen by a radar. If it is seen by a radar it will also 'reply' to the radar interrogations.
is this correct?
I was under the impression that a transponder will only reply to an interrogation unless it is ADS-B.
Transponders reply to radar. That is what they are for.
Mode S was invented as a replacement for the somewhat limited Mode A/C. Operating on the same frequencies and with Mode A/C backward compatibility built into its design, Mode S was a means of transferring much more data between the air and ground over these interrogation/reply transactions. It also provided the additional benefit of selective interrogation based a unique address assigned to each aircraft. "Selective" is actually what the "S" in Mode S stands for. This eliminates many of the problems of ambiguity and message garbling of the old Mode A and Mode C.
However, in the quite early stages of Mode S development it was recognised that its use could be significantly extended. Transmissions which contained the unique aircraft address did not necessarily have to be triggered only in response to a radar interrogation. Instead, information could be spontaneously broadcast for the benefit of anyone who wanted to listed. Thus the concept of "squitter" was born.
Initially this was only a "short" or "ID" squitter. Containing little more than the address, this was intended to make the aircraft's existence known to the TCAS of nearby aircraft.
The concept was then extended - literally "Extended Squitter". These longer and more frequent transmissions are used to carry ADS-B (provided the transponder has a suitable on-board navigation source fed into it).
In summary, a Mode S transponder looks like:
- an ADS-B target to an ADS-B receiver;
- a Mode S target to a Mode S radar;
- a Mode A/C target to a Mode A/C radar
It does the whole job. And all at the same time.