How Things Work: Aircraft Identification

A digital communications system could put the control tower in the cockpit.

Global Postioning System satellites provide lcoations while ADS-B-equipped aircraft share flight information. Communications satellites (not shown) can link air traffic control stations. (Illustration by Harry Whitver)
Air & Space Magazine

Air traffic control without radar? Those familiar towers, often 40 feet high and topped with 20-by- 10-foot rotating antennas, may gradually give way to ground units the size of dorm room refrigerators.

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These devices will be part of a system known as Automatic Dependent Surveillance-Broadcast (ADS-B), and they’re far cheaper than radars to build and maintain. System advocates promise other benefits, such as an increase in the U.S. airspace’s traffic capacity.

Those fridge-size ADS-B ground facilities are virtually devoid of moving parts because, unlike traditional radar units, they have no need to sweep the sky to transmit radio waves and then gauge their return. ADS-B units can be mounted on cell phone towers and many other kinds of structures, and in remote sites that couldn’t logistically accommodate big radars. Test programs are already placing them in the interior of Alaska and on oil platforms in the Gulf of Mexico. Airplanes equipped with ADS-B can also exchange data, conferring to pilots awareness of airspace now available only in the control tower.

ADS-B gets its position information from navigation systems on the aircraft, primarily the satellite-based Global Positioning System, or GPS. The typical airliner’s Flight Management System (FMS) also includes other navigation aids that can back up the GPS, such as the Inertial Reference Unit, which relies on ring-laser or fiber-optic gyros.

To understand the significance of ADS-B, it helps to know a little about what it’s replacing. With the most basic, or primary, radar, a rotating transmitter sends out high-power radio waves, which bounce off the target and return. The system notes where in the 360-degree sweep the target registered, which translates into the target’s azimuth. The time it takes the radio waves to reach the target and return indicates its distance from the transmitter, or its range. With those two coordinates, the target is pinpointed in two-dimensional space.

The primary system is enhanced by Secondary Surveillance Radar (SSR). With each radar sweep, a second, high-frequency signal is transmitted along with the primary. When an aircraft equipped with a transponder receives that signal, the transponder sends out a signal of its own, which registers at the ground station.

The SSR uses that return signal to determine aircraft location much more accurately than the primary system could alone, and it eliminates radar returns from spurious sources, such as birds and terrain. Responses from a Mode A transponder include a four-digit identification code assigned by a ground controller via radio, which pilots update manually during flight. Mode C transponders also transmit altitude information, obtained from the aircraft’s barometric altimeter.

An improved surveillance radar technology is Mode S, for Mode Select. Each Mode S-equipped aircraft has a unique, permanent identification number that remains during the life of the aircraft. It enables the air traffic control computer to tailor its interrogations, addressing only specified targets.

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

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