Theory of operation Air traffic control radar beacon system

1 theory of operation

1.1 interrogation
1.2 reply

1.2.1 side lobe suppression

theory of operation

the steps involved in performing atcrbs interrogation follows: first, atcrbs interrogator periodically interrogates aircraft on frequency of 1030 mhz. done through rotating or scanning antenna @ radar s assigned pulse repetition frequency (prf). interrogations typically performed @ 450 - 500 interrogations/second. once interrogation has been transmitted, travels through space (at speed of light) in direction antenna pointing until aircraft reached.

when aircraft receives interrogation, aircraft transponder send reply on 1090 mhz after 3.0 μs delay indicating requested information. interrogator s processor decode reply , identify aircraft. range of aircraft determined delay between reply , interrogation. azimuth of aircraft determined direction antenna pointing when first reply received, until last reply received. window of azimuth values divided 2 give calculated centroid azimuth. errors in algorithm cause aircraft jitter across controllers scope, , referred track jitter. jitter problem makes software tracking algorithms problematic, , reason why monopulse implemented.

the interrogation

interrogations consist of 3 pulses, 0.8 μs in duration, referred p1, p2 , p3. timing between pulses p1 , p3 determines mode (or question) of interrogation, , nature of reply should be. p2 used in side-lobe suppression, explained later.

mode 3/a uses p1 p3 spacing of 8.0 μs, , used request beacon code, assigned aircraft controller identify it. mode c uses spacing of 21 μs, , requests aircraft s pressure altitude, provided altitude encoder. mode 2 uses spacing of 5 μs , requests aircraft transmit military identification code. latter assigned military aircraft , small percentage of aircraft reply mode 2 interrogation.

the reply

replies interrogations consist of 15 time slots, each 1.45 μs in width. reply encoded presence or absence of 0.45 μs pulse in each slot. these labeled follows:

f1 c1 a1 c2 a2 c4 a4 x b1 d1 b2 d2 b4 d4 f2 spi

the f1 , f2 pulses framing pulses, , transmitted aircraft transponder. used interrogator identify legitimate replies. these spaced 20.3 μs apart.

the a4, a2, a1, b4, b2, b1, c4, c2, c1, d4, d2, d1 pulses constitute information contained in reply. these bits used in different ways each interrogation mode.

for mode a, each digit in transponder code (a, b, c, or d) may number 0 seven. these octal digits transmitted groups of 3 pulses each, slots reserved first digit, b second, , on.

in mode c reply, altitude encoded gillham interface, gillham code, uses gray code. gillham interface capable of representing wide range of altitudes, in 100-foot (30 m) increments. altitude transmitted pressure altitude, , corrected altimeter setting @ atc facility. if no encoder attached, transponder may optionally transmit framing pulses (most modern transponders do).

in mode 3 reply, information same mode reply in there 4 digits transmitted between 0 , 7. term mode 3 utilized military, whereas mode civilian term.

the x bit used test targets. bit transmitted bomarc missiles used air-launched test targets. bit may used drone aircraft.

the spi pulse positioned 4.35μs past f2 pulse (3 time slots) , used special identification pulse . spi pulse turned on identity control on transponder in aircraft cockpit when requested air traffic control. air traffic controller can request pilot ident, , when identity control activated, spi bit added reply 20 seconds (two 4 rotations of interrogator antenna) thereby highlighting track on controllers display.

side lobe suppression

the ssr s directional antenna never perfect; inevitably leak lower levels of rf energy in off-axis directions. these known side lobes. when aircraft close ground station, side lobe signals strong enough elicit reply transponders when antenna not pointing @ them. can cause ghosting, aircraft s target may appear in more 1 location on radar scope. in extreme cases, effect known ring-around occurs, transponder replies excess resulting in arc or circle of replies centered on radar site.

to combat these effects, side lobe suppression (sls) used. sls employs third pulse, p2, spaced 2μs after p1. pulse transmitted omnidirectional antenna (or antenna difference channel) ground station, rather directional antenna (or sum channel). power output omnidirectional antenna calibrated that, when received aircraft, p2 pulse stronger either p1 or p3, except when directional antenna pointing directly @ aircraft. comparing relative strengths of p2 , p1, airborne transponders can determine whether or not antenna pointing @ aircraft when interrogation received. power difference antenna pattern (for systems equipped) not adjusted of p1 , p3 pulses. algorithms used in ground receivers delete replies on edge of 2 beam patterns.

to combat these effects more recently, side lobe suppression (sls) still used, differently. new , improved sls employs third pulse, spaced 2μs either before p3 (a new p2 position) or after p3 (which should called p4 , appears in mode s radar , tcas specifications). pulse transmitted directional antenna @ ground station, , power output of pulse same strength p1 , p3 pulses. action taken specified in new , improved c74c as:

2.6 decoding performance.

c. side-lobe suppression. transponder must suppressed period of 35 ±10 microseconds following receipt of pulse pair of proper spacing , suppression action must capable of being reinitiated full duration within 2 microseconds after end of suppression period. transponder must suppressed 99 percent efficiency on received signal amplitude range between 3 db above minimum triggering level , 50 db above level , upon receipt of spaced interrogations when received amplitude of p2 equal or in excess of received amplitude of p1 , spaced 2.0 ±0.15 microsecond p3.

any requirement @ transponder detect , act upon p2 pulse 2μs after p1 has been removed new , improved tso c74c specification.

most modern transponders (manufactured since 1973) have sls circuit suppresses reply on receipt of 2 pulses in interrogation spaced 2.0 microseconds apart above mtl minimum triggering level threshold of receiver amplitude descriminator (p1->p2 or p2->p3 or p3->p4). approach used comply original c74c , complies provisions of new , improved c74c.

the faa refers non-responsiveness of new , improved tso c74c compliant transponders mode s compatible radars , tcas terra problem , , has issued airworthiness directives (ads) against various transponder manufacturers, on years, @ various times on no predictable schedule. ghosting , ring around problems have recurred on more modern radars.

to combat these effects recently, great emphasis placed upon software solutions. highly 1 of software algorithms proximate cause of mid-air collision recently, 1 airplane reported @ showing altitude pre-flight paper filed flight plan, , not altitude assigned atc controller (see reports , observations contained in below reference atc controlled airplane passenger study of how radar worked).

see reference section below errors in performance standards atcrbs transponders in us.

see reference section below faa technician study of in-situ transponders.