Gonzo,
The transponders have 8 digits, 0-7, the corresponds to the following in binary
0-000
1-001
2-010
3-011
....
7-111
ie say when you set 7700, you are transmitting 111111000000 to the radar site.
TCAS I provides traffic advisories (TA) and proximity warning of nearby traffic to assist the pilot in the visual acquisition of intruder aircraft. TCAS I is mandated for use in the by a number of regulatory authorities for turbine-powered, passenger- carrying aircraft having more than 10 and less than 31 seats. TCAS I is also used by a number of general aviation fixed and rotary wing aircraft.
TCAS II provides traffic advisories and resolution advisories (RA), i.e., recommended escape maneuvers, in the vertical dimension to either increase or maintain the existing vertical separation between aircraft. Airline aircraft, including regional airline aircraft with more than 30 seats, and general aviation turbine-powered aircraft use TCAS II equipment.
The TCAS concept uses the same radar beacon transponders installed on aircraft to operate with ATC ground-based radars. The level of protection provided by TCAS equipment depends on the type of transponder the target aircraft is carrying. i.e if a 747-400 with TCAS II (with mode S transponder) comes near a general aviation aircraft with a mode A transponder (no altitude return), the 747-400 will only be able to get the range, and bearing of the general aviation aircraft, but no altitude return, and no vertical seperation.
The latest changes to TCAS II, collectively known as TCAS II Version 7,were certified in early 2000 and are now being implemented by the industry. TCAS II Version 7 is the only version of TCAS II that complies with the ICAO Standards and Recommended Practices (SARPs) for ACAS II. As such, Version 7 is currently being mandated for carriage in certain countries or regions, e.g., Europe, Australia, and India, and has been mandated for carriage in 2003 by the International Civil Aviation Organization (ICAO).
A Mode S transponder is required to be installed and operational for TCAS II to be operational. If the Mode S transponder fails, the TCAS Performance Monitor will detect this failure and automatically place TCAS into Standby. The Mode S transponder performs the normal functions to support the ground-based ATC system and can work with either an ATCRBS or a Mode S ground sensor. The Mode S transponder is also used to provide air-to-air data exchange between TCAS-equipped aircraft so that coordinated, complementary RAs can be issued when required.
Because of the selective address feature of the Mode S system, TCAS surveillance of Mode S equipped aircraft is relatively straightforward. TCAS listens for the spontaneous transmissions, or squitters, that are generated once per second by the Mode S transponder. Among other information, the squitter contains the unique Mode S address of the sending aircraft.
Following the receipt and decoding of a squitter message, TCAS sends a Mode S interrogation to the Mode S address contained in the squitter. The Mode S transponder replies to this interrogation and the reply information is used by TCAS to determine the range, bearing, and altitude of the Mode S aircraft.
