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The MSSR MODE S Radar uses different protocols to interrogate A/C or Mode S transponders:

Mode S ALL CALL:

              # Mode A/C.   
              # Mode A/C/S (Intermodes short)

Mode S Only ALL CALL:

              # Mode-S only All-Call (UF/DF 11)

My question is why do we need to send the intermode protocol if we can interrogate A/C transponders with Mode A/C?

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Mode S transponders are designed to be backwards compatible. If you send a mode A/C interrogation like an older radar would, then mode S transponders will respond just like A/C transponders would, since they assume that the interrogation is from a radar without mode S capabilities.

The intermode A/C all-call interrogation is just like the normal A/C interrogation, but with an extra pulse. This extra pulse tells mode S transponders not to respond and is ignored by mode A/C transponders.

In other words, this is the only way to selectively interrogate A/C transponders without getting a bunch of extra replies from mode S transponders. In congested airspace this is a big benefit.

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