There most certainly is a datalink in the mode-S transponders... that's what they're there for.
The basis of mode-S is that the radar doesn't require all the transponders in a certain direction to respond, like with mode-A/C, where two transponders may interfere in one direction, but can selectively ask for a response from individual airplanes.
First there is an socalled all-call signal, to which all transponders (A/C/S) will respond, the mode-S transponder then sends his specific adress. Hereafter this target will be selectively interrogated.
For communication there are several communicationprotocols available, which use 112 bit datablocks. 56 bits are used for identification, the other 56 can be used to transfer a message.
There is on top of that the possibility of extended length messages, which use max. 1280 bits, built out of max. 16 segments of 80 bits, via a special burst protocol. These 80-bits segments are embedded in the 112bit datablocks mentioned before.. .These 1280 bits are divided in several phases (annunciation, reservation, msg, close-out).
The mode-S-system has a parity bit to detect errors during transmission.
Offcourse mode-S is used as a data-transfer to exchange information for TCAS.
Several possible future applications of mode-S:. .- flight identification. .- flight information (heading, speed). .- meteorological info (spot wind, spot temp). .- ATC-last minute info (rwy closure)
So mode-S is indeed capable of handling much more than it does now. The only real application at this moment seems to be TCAS. It could be used air-ground but ground-air as well.
. .Not really much, but I hope it helps!
[ 21 February 2002: Message edited by: Pegasus77 ]</p>