Section 6, Chapter 2 - Non-compliant / Faulty Mode S Transponders

Question 1

What does the radar determine during the first interrogation of any aircraft?

Answer 1

During the first interrogation, the radar determines from the transponder response whether the transponder is Mode S Compliant.

Question 2

What happens if a Mode S transponder is non-compliant?

Answer 2

Non-compliant Mode S transponders cannot be interrogated by the Mode S functions of either the Crowhill or THALES radar sources. Both radar sources are designed to be ‘backwards compliant’ and will continue to provide both MODE A and C data in this situation.

Question 3

What issues can arise when a faulty Mode S compliant transponder is detected?

Answer 3

If a faulty Mode S compliant transponder is detected, the radar will not continue to search for Mode A & C data and will rely on Mode S for data. This may result in:

• Incorrect or garbled data being present; or
• No SSR data being presented after the first interrogation.

The operational impact will vary depending on the surveillance method in use.

Question 4

How does the Thales Combined PSR/SSR or PSR with Crowhill ‘Overlay’ mode handle a faulty Mode S transponder?

Answer 4

In this mode, if a faulty Mode S transponder is present:

• PSR will continue to provide situational awareness for the controller.
• Operations can continue using PSR techniques to provide service to the aircraft.
• Garbled SSR data must be considered spurious, and the aircrew should deselect this function where possible.

Question 5

What is the procedure during SSR only operations if a faulty Mode S transponder is present?

Answer 5

If a faulty Mode S transponder is present during SSR only operations:

• The system will not provide any subsequent track or data block information other than from the first interrogation (data block and track may appear ‘frozen’).
• Controllers should manage the aircraft as if all aircraft were operating in a non-radar environment.

Note: If a data block is present, it should not be relied upon for providing separation information but should be retained for situational awareness.

Question 6

What should be done when incorrect interrogation or no interrogation of a Mode S transponder is possible?

Answer 6

All instances where incorrect interrogation or no interrogation is possible should be logged in the ATC watch log and a STAR report should be entered.