Gpn, AIUI, a transponder squawks every time it is interrogated. A mode A or C does so every time a radar head anywhere in ground-air range sweeps it, which is most of the time, anywhere. A Zaon etc. “hears” that squawk every time, and assesses proximity by its signal strength or something.
A mode S only squawks when it is told to. The “S” means that ATC can select when they want it to or not. For GA aircraft with S outside CAS, that is much less frequent – say once per second instead of umpteen times per second (hence requiring less battery capacity for a particular flight length – one of the much-vaunted claims by the CAA to justify it for gliders etc.).
TCAS generates its own interrogations. See below, as others have posted in the past:
[quote] TCAS Function:
TCAS works its way through the Roll Call, sending Selective Interrogations to those aircraft. This gives it a picture of all Mode S aircraft in the vicinity. As aircraft move out of range, their announcement signal eventually gets weak enough to be ignored so they are dropped from the roll call.
It then does a Mode C "All call" sweep, to catch non mode S aircraft. Note, it only cares about altitude, so its sends Mode C interrogations only - However, . . . [snip] - non altitude encoding aircraft still reply to Mode C calls, just with an empty frame, so the existence and range to the target can still be derived.
Other Mode S / TCAS tricks for interference reduction.
Whisper / Shout.
TCAS sends its mode S interrogations at variable power levels. In a nutshell, it 'Whispers' at low power to the target, if no reply is received it transmits progressively more power, eventually 'Shouting'. Its uses a successful transmission as a benchmark for its next attempt, and progressively adjusts power to the minimum required to maintain contact.
TCAS interference Limiting Mode.
Lengthy topic, more pilot oriented, will post if anyone interested (Its already written for an email query I received, so no trouble).
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[snip] . . . just to clarify a couple of points.
At present TCAS and Mode S ground stations operate completely independently. A ground station would see TCAS squitter or air to air surveillance as interference and visa versa.
Mode S ground stations do not use the squitter data but transmit an All Call interrogation to find out who's out there which will also include its interrogator identity code. This then gets included in the transponder reply. All Mode S interrogations and replies are parity encoded so that the data is only used if it contains the expected aircraft address or interrogator code.
The way that Mode A/C systems reject interference is to interrogate a number of times and only believe the reply if it gets the same information more than once (There's also some clever decoders that correlate replies on amplitude and angle of arrival.)
At the moment the only link between TCAS and ground based Mode S is that the transponder will generate a message to the ground station that contains the RA information if one is generated. This is of limited use to co-ordinate actions, it wouldn't arrive in time, but could be used to clearly identify the aircraft concerned on the display. At present there's not many operational Mode S stations outside the US and even these don't use this feature. However, most of Europe will be updating to Mode S in the next couple of years.
Lots more proposed for Mode S in the future, see the Eurocontrol web-site for info on 'Enhanced Surveillance' and ADS-B using 1090 extended squitter.
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As before, no doubt an expert will be along to correct me if any part of this is wrong.
Chris N.
PS – where I wrote squawk, that should probably be “squitter". And of course, neither TCAS nor Zaon etc. can detect a non-transponder-equipped flying device of any sort - glider, paraglider, LAA non-electric type, or whatever. Nor falling parachutists. Nor birds. Nor radiosonde balloons.
Last edited by chrisN; 21st Jul 2008 at 13:31.