Theory of Operation
The steps involved in performing an ATCRBS interrogation are as follows: First, the ATCRBS interrogator periodically interrogates aircraft on a frequency of 1030 MHz. This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF). Interrogations are typically performed at 450 - 500 interrogations/second. Once an interrogation has been transmitted, it travels through space in the direction the antenna is pointing at the speed of light until an aircraft is reached. When the aircraft receives the interrogation, the aircraft transponder will send a reply on 1090 MHz after a 3.0 μs delay indicating the requested information. The interrogator's processor will then decode the reply and identify the aircraft. The range of the aircraft is determined from the delay between the reply and the interrogation. The azimuth of the aircraft is determined from the direction the antenna is pointing when the first reply was received, until the last reply is received. This window of azimuth values is then divided by two to give the calculated "centroid" azimuth. The errors in this algorithm cause the aircraft to jitter across the controllers scope, and is referred to as "track jitter." The jitter problem makes software tracking algorithms problematic, and is the reason why monopulse was implemented.
Read more about this topic: Air Traffic Control Radar Beacon System
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