Overview

Radar is an acronym for radio detection and ranging. Even though most of us assume radar to be an echoing system, radio detection and ranging can be achieved by another means. A radar can also be an answer-back system, in which an interrogator, transmitting a challenge, causes a transponder to receive and transmit a reply code back to the interrogator. To differentiate between the echo-type radar of preceding chapters, and the answer-back systems, the echo-type radars are called primary radars, and these answer-back types are called secondary radars. Secondary radar systems have been built both with ground interrogators and transponders, and with both aircraft interrogators and transponders. The predominant method is the current air traffic control radar beacon system (ATCRBS), in which the interrogator is located at a ground station, usually (but not always) located at a primary radar facility. The terms, secondary radar, radar beacon, and beacon radar, are synonymous.

A characteristic of the secondary radar system is that echoes from the transmitted interrogation will not enter the receiver, because the interrogator s receiver is tuned to 1090 MHz, but the transmitter radiates at 1030 MHz. A tuned circuit called, a diplexer, is used to steer the 1030 MHz transmitted energy to the antenna, and to steer the received replies to the receiver. The configuration limits the secondary radar system to being only a data-exchange device, unable to detect echoes from hazardous weather, ground obstructions, and those aircraft...