secondary surveillance radar
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secondary surveillance radar[′sek·ən‚der·ē sər′vā·ləns ‚rā‚där]
The secondary radar that operates in conjunction with the airborne transponder of the air-traffic control radar beacon system (ATCRBS).
secondary surveillance radar (SSR)
A radar system that requires complementary aircraft equipment (transponder). The transponder generates a coded reply signal in response to transmissions from the ground station (interrogator). Since this system relies on transponder-generated signals rather than signals reflected from the aircraft, as in primary radar, it offers significant operational advantages, such as increased range and positive identification. The interrogator has a fixed interval of two microseconds between pulses P1 and P2, but the position of P3 is variable. There are four modes, or types, of reply allocated for civil; they are designated A, B, C, and D. All secondary surveillance radars operate on the same frequencies; the interrogation is at 1030 MHz and the transponder reply is at 1090 MHz. It also provides automatic altitude reporting facilities as well as other types of information to the ATC (air traffic control) controller. The three main components of the system are the interrogator, a transponder (fitted in the aircraft), and a radarscope. Also called an air traffic control radar beacon system (ATCRBS).