Originally Posted by Fly-by-Wife
My understanding is that Primary Radar provides information on range and bearing. SSR provides information on identity and altitude.
No, as already said above.
Primary radar works by sending out radio wave pulses, and receiving the resulting reflections (echos). Those echoes can be from anything.... birds don't carry SSR but can still show up on primary radar!
The snag is, that those reflections are extremely weak... only a tiny bit of the energy of the radar strikes the "target" (plane in our case), and that tiny bit again gets spread out on the way back. It's amazing it works at all....
So people came up with the idea of a 'transponder' (which then became SSR), a small transmitter that would send a 'beep' back whenever it was 'hit' by a radar beam. So now you could rely on the plane 'shouting back' (the transponder reply) instead of the primary radar having to 'listen' for the faint echo.
But the principle stayed the same, the transponder, substituting itself for the echo, would show up at the same range and bearing as the echo.
That should explain why SSR provides information on range and bearing.
It didn't take long for people to figure out that more data could be added to the reply from the aircraft. From the few dashes added in WWII, it went to the various "squawks", set on a control unit in the aircraft, to the full ident, and now altitude. But the original range and bearing are always there.
Just thought I'd scribble this down, in case you wanted a halfway non-tech answer
Cheers
CJ