Air traffic control radar beacon system

The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as the flight number designation and altitude of the aircraft. ATCRBS assists air traffic control (ATC) surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft (known as targets) and to distinguish those returns from ground clutter. The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as the flight number designation and altitude of the aircraft. ATCRBS assists air traffic control (ATC) surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft (known as targets) and to distinguish those returns from ground clutter. The system consists of transponders, installed in aircraft, and secondary surveillance radars (SSRs), installed at air traffic control facilities. The SSR is sometimes co-located with the primary surveillance radar, or PSR. These two radar systems work in conjunction to produce a synchronized surveillance picture. The SSR transmits interrogations and listens for any replies. Transponders that receive an interrogation decode it, decide whether to reply, and then respond with the requested information when appropriate. Note that in common informal usage, the term 'SSR' is sometimes used to refer to the entire ATCRBS system, however this term (as found in technical publications) properly refers only to the ground radar itself. An ATC ground station consists of two radar systems and their associated support components. The most prominent component is the PSR. It is also referred to as skin paint radar because it shows not synthetic or alpha-numeric target symbols, but bright (or colored) blips or areas on the radar screen produced by the RF energy reflections from the target's 'skin.' This is a non-cooperative process, no additional avionic devices are needed. The radar detects and displays reflective objects within the radar's operating range. Weather radar data is displayed in skin paint mode. The primary surveillance radar is subject to the radar equation that says signal strength drops off as the fourth power of distance to the target. Objects detected using the PSR are known as primary targets. The second system is the secondary surveillance radar, or SSR, which depends on a cooperating transponder installed on the aircraft being tracked. The transponder emits a signal when it is interrogated by the secondary radar. In a transponder based system signals drop off as the inverse square of the distance to the target, instead of the fourth power in primary radars. As a result, effective range is greatly increased for a given power level. The transponder can also send encoded information about the aircraft, such as identity and altitude. The SSR is equipped with a main antenna, and an omnidirectional 'Omni' antenna at many older sites. Newer antennas (as in the adjacent picture), are grouped as a left and right antenna, and each side connects to a hybrid device which combines the signals into sum and difference channels. Still other sites have both the sum and difference antenna, and an Omni antenna. Surveillance aircraft, e.g. AWACS, have only the sum and difference antennas, but can also be space stabilized by phase shifting the beam down or up when pitched or rolled. The SSR antenna is typically fitted to the PSR antenna, so they point in the same direction as the antennas rotate. The omnidirectional antenna is mounted near and high, usually on top of the radome if equipped. Mode-S interrogators require the sum and difference channels to provide the monopulse capability to measure the off-boresight angle of the transponder reply. The SSR repetitively transmits interrogations as the rotating radar antenna scans the sky. The interrogation specifies what type of information a replying transponder should send by using a system of modes. There have been a number of modes used historically, but four are in common use today: mode 1, mode 2, mode 3/A, and mode C. Mode 1 is used to sort military targets during phases of a mission. Mode 2 is used to identify military aircraft missions. Mode 3/A is used to identify each aircraft in the radar's coverage area. Mode C is used to request/report an aircraft's altitude. Two other modes, mode 4 and mode S, are not considered part of the ATCRBS system, but they use the same transmit and receive hardware. Mode 4 is used by military aircraft for the Identification Friend or Foe (IFF) system. Mode S is a discrete selective interrogation, rather than a general broadcast, that facilitates TCAS for civilian aircraft. Mode S transponders ignore interrogations not addressed with their unique identity code, reducing channel congestion. At a typical SSR radar installation, ATCRBS, IFF, and mode S interrogations will all be transmitted in an interlaced fashion. Some military facilities and/or aircraft will also utilize Mode S. Returns from both radars at the ground station are transmitted to the ATC facility using a microwave link, a coaxial link, or (with newer radars) a digitizer and a modem. Once received at the ATC facility, a computer system known as a radar data processor associates the reply information with the proper primary target and displays it next to the target on the radar scope. The equipment installed in the aircraft is considerably simpler, consisting of the transponder itself, usually mounted in the instrument panel or avionics rack, and a small L band UHF antenna, mounted on the bottom of the fuselage. Many commercial aircraft also have an antenna on the top of the fuselage, and either or both antennas can be selected by the flight crew.