Ok, I think i've got it...
'Mode S' is a transonder (when interrogated), sends out
your squark,
your PA, (derived from your altimeter)
your callsign
your IACO ID
other information
ADS-b (i'm still not quite sure) is like (or is it just an add-on to?)a mode S transponder but
gives the information out without being interrogated,
information is 'digitalised' and is encoded in a specific standardised way,
information is partially GPS derived,
is compatable with mode S transopnders,
may be used to replace RADAR?
Does the information from incoming signals get displayed on a screen?
mode s can work without ADSb but ADSb can not work without modes?
TCAS sends an interrogation signal (that only other TCAS equiped aircraft can reply to? or does it send an interrogation to mode S transonders? or both?) and uses the information (that mode S sends?) to display nearby aircraft on a screen.
PCAS is the same as TCAS but does not send an interrogation signal (just waits for another TCAS to send a signal) and displays the information on a screen.
FLARM is constantly sending gps derived information over short distances to other FLARM equiped aircraft (usually gliders) Power FLARM does the same but also combines the informaion with PCAS. Information is displayed
on a screen.
Tell me if i'm wrong!!!