Mode S and Mode C

[Kempus]

Done a search on the forum and cant find out the difference between mode s and mode c transponders.

I understand how mode a will only give TA's and in order to get RA's both transponders need to be on mode c and it gives you an RA in the vertical plane only. So what is special about mode s and why does it need to come in soon?

the reason i ask is there is a question in the bank that says;

- What inputs are there to TCAS 2?

a/ Mode A transponder which gives TA and RA's
b/ Mode C transponder which co-ordinates avoidance manoeuvres
c/ Mode C and Mode S which co-ordinate avoidance manoeuvres
d/ Mode S transponder which co-ordinates avoidance manoeuvres

I understand that Mode S they both speak to each other to give co-ordinated avoidance manoeuvres but does Mode C not do that when it gives you an RA or have i been studying too much???

:confused:

[Capt Pit Bull]

Mode S is the basic equipment required for TCAS to function in YOUR aircraft.

To get an RA YOU need TCAS (inc mode S transponders), The other aircraft must have altitude reporting, i.e. mode C with altitude encoding.

Mode S operating principle is completely different to mode C, its to big to explain here in any detail. The very short version:

One of the design challenges for TCAS is to avoid completely clogging the SSR frequencies with dozens of aircraft each interrogating all of the others. The main tool for doing this is Selective Interrogation (thats what mode S stands for.)

A mode S transponder periodically announces its existence, along with a unique hardwired identifier.

Equipment using mode S maintains a roll call of aircraft in the area and then sends out a message "Selectively" inviting each aircraft on its roll call to pass its details. TCAS also emits a standard "mode C all call" which invites any mode C aircraft to reply (but NOT mode S).

The mode S interrogations have a number of nifty features that reduce the total amount of bandwidth required. e.g. aircraft are interrogated less often if they are further away. Aircraft are interrogated with the minimum possible power to make contact. These methods are essential to reduce the congestion on the SSR frequencies.

Hope that helps,

CPB

[Kempus]

yeah, i does a bit! thanks!