ICAO has defined that element of its CNS /ATM plan, Surveillance, as the use of Automatic Dependent Surveillance (ADS), utilising satellite technology for the determination of aircraft position.

This paper explains a further development of the concept currently being evaluated by the US FAA and utilising GPS, TCAS, Modes S and Squitter technologies, to provide surveillance.

In Mode S the aircraft transponder transmits its unique Mode S address omnidirectionally once per second. These transmissions (squitters) contain 112 bits of data and have a duration of about 60 microseconds. They are broadcast on the transponder reply frequency of 1090 Mhz. 56 bits are used for the unique address and 56 bits for other information used in ADS-B.

There are three types of ADS-B messages;

1)  Identification squitters – Mode S identity ( broadcast every 5 seconds);

2)  Position/Velocity squitters – GPS horizontal position and barometric altitude ( broadcast every 0.5 seconds, with GPS velocity (ground track) broadcast every second)

3)  Supplementary squitters – still to be determined but could be pilot reports etc.

The positional information of the aircraft is obtained from the GPS. The accuracy of which is 100m for 95% of the time which is acceptable for enroute or non precision use , however , for precision use, differential GPS is required.

Operating on Mode S is TCAS, a cockpit display of traffic information (CDTI) which will therefore display to the pilot transmissions of other aircraft (enhancements currently underway on TCAS will provide full utilisation of the squitter information, allowing for location and identification of nearby Mode S aircraft).

The ground station is a modified TCAS Unit and Commercial Antenna.

With omnidirectional antenna the range is predicted to be 50-100nm. With 6 sector and 12 sector antenna, the range will be increased to 200nm.

Air-Ground, Surveillance of airborne aircraft from a terminal area or enroute ground station.

Air-Air, Surveillance of airborne aircraft, from an aircraft, for improved TCAS (Permitting TCAS to minimise its interrogations and eliminate the range restriction it currently has in high traffic airspace, suppressing unnecessary alerts through improved miss distance filtering and permitting horizontal manoeuvres for conflict resolution.)

Ground-Ground, Surveillance of aircraft and vehicles on the ground. All aircraft and vehicles must be equipped.

In February 1994 a field evaluation was conducted at Boston Logan Airport, utilising aircraft and vehicles. Coverage measurements revealed that vehicles with a 1 second position update covered the area for 99.9% of the time and airborne coverage was provided out to a range of 20nm.

In November 1994 a trial was conducted in the Gulf of Mexico, utilising GPS-squitter surveillance. Aircraft involved were, a Bell 206, Cessna 421 and PHI Bell 206. These tests confirmed the viability of the system to provide positional data on the aircraft within and beyond the radar horizon.

This technology has the potential of providing an equal or better service than ground based radar’s at a fraction of their costs.

It will provide TCAS utilisation to eliminate current range limitations, reduce traffic alerts and permit horizontal as well as vertical manoeuvres for collision avoidance.

GPS squitter may be an enabling technology for Free Flight permitting aircraft to see and avoid other nearby aircraft , independent of ground radar coverage.