Parts of the system Air traffic control radar beacon system

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ground interrogation equipment

an atc ground station consists of 2 radar systems , associated support components. prominent component psr. referred skin paint radar because shows not synthetic or alpha-numeric target symbols, bright (or colored) blips or areas on radar screen produced rf energy reflections target s skin. non-cooperative process, no additional avionic devices needed. radar detects , displays reflective objects within radar s operating range. weather radar data displayed in skin paint mode. primary surveillance radar subject radar equation says signal strength drops off fourth power of distance target. objects detected using psr known primary targets.


the second system secondary surveillance radar, or ssr, depends on cooperating transponder installed on aircraft being tracked. transponder emits signal when interrogated secondary radar. in transponder based system signals drop off inverse square of distance target, instead of fourth power in primary radars. result, effective range increased given power level. transponder can send encoded information aircraft, such identity , altitude.


the ssr equipped main antenna, , omnidirectional omni antenna @ many older sites. newer antennas (as in adjacent picture), grouped left , right antenna, , each side connects hybrid device combines signals sum , difference channels. still other sites have both sum , difference antenna, , omni antenna. surveillance aircraft, e.g. awacs, have sum , difference antennas, can space stabilized phase shifting beam down or when pitched or rolled. ssr antenna typically fitted psr antenna, point in same direction antennas rotate. omnidirectional antenna mounted near , high, on top of radome if equipped. mode-s interrogators require sum , difference channels provide monopulse capability measure off-boresight angle of transponder reply.


the ssr repetitively transmits interrogations rotating radar antenna scans sky. interrogation specifies type of information replying transponder should send using system of modes. there have been number of modes used historically, 4 in common use today: mode 1, mode 2, mode 3/a, , mode c. mode 1 used sort military targets during phases of mission. mode 2 used identify military aircraft missions. mode 3/a used identify each aircraft in radar s coverage area. mode c used request/report aircraft s altitude.


two other modes, mode 4 , mode s, not considered part of atcrbs system, use same transmit , receive hardware. mode 4 used military aircraft identification friend or foe (iff) system. mode s discrete selective interrogation, rather general broadcast, facilitates tcas civilian aircraft. mode s transponders ignore interrogations not addressed unique identity code, reducing channel congestion. @ typical ssr radar installation, atcrbs, iff, , mode s interrogations transmitted in interlaced fashion. military facilities and/or aircraft utilize mode s.


returns both radars @ ground station transmitted atc facility using microwave link, coaxial link, or (with newer radars) digitizer , modem. once received @ atc facility, computer system known radar data processor associates reply information proper primary target , displays next target on radar scope.


airborne transponder equipment

the equipment installed in aircraft considerably simpler, consisting of transponder itself, mounted in instrument panel or avionics rack, , small l band uhf antenna, mounted on bottom of fuselage. many commercial aircraft have antenna on top of fuselage, , either or both antennas can selected flight crew.


typical installations include altitude encoder, small device connected both transponder , aircraft s static system. provides aircraft s pressure altitude transponder, may relay information atc facility. encoder uses 11 wires pass altitude information transponder in form of gillham code, modified binary gray code.



a light aircraft transponder


the transponder has small required set of controls , simple operate. has method enter four-digit transponder code, known beacon code or squawk code, , control transmit ident, done @ controller s request (see spi pulse below). transponders typically have 4 operating modes: off, standby, on (mode-a), , alt (mode-c). on , alt mode differ in on mode inhibits transmitting altitude information. standby mode allows unit remain powered , warmed inhibits replies, since radar used searching aircraft , exact location of aircraft.







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