Mohammed Al Ahmed

Hi guys,

I have a question regarding transponder modes

Are the following statements correct?
Mode A: Transmits squawk code and speed only.
Mode C: Transmits squawk code, speed, and pressure altitude only.
Mode S: Transmits squawk code, speed, pressure altitude, and callsign. Plus multiple information formats.
Or all modes provides callsign?

Thanks in advance!

aterpster

A bit of history: The first civil transponder was Mode A only and had only 64 codes (8 x 8). 4,096 Mode A codes came along in the mid or late 1960s. (8 x 8 x 8 x 8) Mode C was added separately; I believe after Mode A was expanded to 4,096 codes.

The Mode A transponder was an outgrowth of the military IFF developed during World War II.

Mode S came along much later and is much more sophisticated.

Mode A provides only position. ATC computers uses the change of position to determine ground speed. Tags added to the Mode A position tag, such a flight or registration number, etc, are generated by the ATC computer from flight plan data.

Mode C provides pressure altitude only.

Mohammed Al Ahmed

Thank you guys.
I understand GS and callsign start shown on radar screen with mode S only. No GS/Callsign with modes A or C, That is correct?

chevvron

Mode A is the same as IFF Mode 3. At the first radar unit I trained at, the RAF controllers refered to 'Mode 3 Alpha'.
Don't know what Modes 1 and 2 were, but an 'ident' squawk is the same as IFF/SIF (Selective Identification Feature)

MarkerInbound

Mode A and C transponders says "Hi, I'm 1234." The ATC system has been told 1234 is FastJet 100 and so the controller sees FastJet 100 with the data block. With a Mode S Transponder the flight enters FastJet 100 in FMS as part of the setup and the transponder says "Hi, I'm FastJet 100."

Mohammed Al Ahmed

I got it now! Thank you guys for help!

aterpster

This is a fun review.

Do we have a participant who can explain the interrogator/reply details of Mode A and the similarities to DME interrogators?

chevvron

In the UK, it doesn't matter what transponder the aircraft carries, individual radar displays can 'convert' even a Mode A label to the aircraft callsign if it's entered manually and this will automatically be displayed on all of the radar unit's displays.

chevvron

Main difference is with SSR Mode A, the interrogator is on the ground and the aircraft's transponder replies to its interrogation. With DME (trying to remember my 'radar for controllers' course some 45 years ago) the ground based equipment transmits 'squitter' pulses and the aircraft equipment picks up this signal and locks on to pairs of these pulses to which the ground equipment replies, each DME being capable of handling about 200 aircraft. NB: I may have got this wrong!! I seem to remember the squitter pulses are transmitted in a rotating field called a 'romecon' or 'limecon' shape?; would that sound right?
Don't ask me how TACAN works though!!

chevvron

Mode C info is transmitted from the aircraft based on 1013.2; it is then converted to altitude by the processor in the radar display which must have the correct local QNH input.