Mode S Interrogations

[Golden Rivet]

Currently studying for more exams and have come up against conflicting explanations as to the duration of the P4 pulse.

One book states the mode S transponder will reply with a Mode S reply on receipt of a 0.8 microsecond P4 pulse, and a second book says it will remain silent and only reply with a Mode S reply on receipt of a 1.6 microsecond P4 pulse.

Any transponder Gurus out there ?

Thanks in advance

GR

[RaRadar]

Not sure if I qualify as a guru but this is the info you want!

There are two types of Mode S Intermode All Call:

Mode A/C Only All Call (short 0.8us P4)
A Mode S transponder will NOT reply to such an interrogation.
Mode A/C/S All Call (long 1.6us P4)
A Mode S transponder will reply to this interrogation.

An SSR transponder will of course reply to both as it does not know about the P4. (There have been some problems with SSR transponders though, look up 'Terra Mode' in relation to FAA Mode S operation.)

The long P4 interrogations are unlikely to be used operationally as it is not possible to include the interrogator identity code. The normal method of acquiring Mode S aircraft is by the
Mode S Only All Call.

RR (good luck in exams)

[Golden Rivet]

Many thanks - in that case my new and expensive avionics book has it ar*e about face.

Cheers

GR