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Once an air traffic control computer identifies an aircraft by its address, that aircraft goes into a “roll call.” Subsequent interrogations are transmitted on a schedule. As a result, to track a target, Mode S needs far fewer interrogations than earlier radars, which translates into more accurate position reporting.
A Mode S transponder doesn’t have to wait until it’s prompted from the ground to send out its address. It does so continually, and the unsolicited signals, or “squitters,” can also include readings from the aircraft’s altimeter, plus other flight information. This capability enables new kinds of air-to-air communication, such as the automatic signals of TCAS, the Traffic Alert and Collision Avoidance System, which helps prevent midair
collisions.
ADS-B goes further. At least once per second, the aircraft broadcasts not just ID and altitude but also the other essentials of target tracking, azimuth and range—all without interrogation from the ground.
It does this via an “extended squitter,” using a signal that is longer than most Mode S signals. With the extended squitter, the aircraft can downlink flight details such as airspeed, climb or descent rate, and magnetic heading. Other aircraft and ground stations within about 150 miles receive the information in their cockpits or consoles.
An ADS-B-equipped aircraft includes a Cockpit Display of Traffic Information, showing the pilot a view of neighboring traffic similar to what the controller sees on the ground. Other information is uplinked from the ground, including positions of nearby aircraft not equipped with ADS-B, weather data, and other updates.
Installing ADS-B will usually involve modifying the Flight Management System software and making new hard-wire connections between the FMS and the transponder.
Instead of using extended-squitter transponders broadcasting at 1090 MHz, general aviation aircraft will be equipped with simpler “universal access transceivers,” which broadcast at 978 MHz.
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