Secondary_surveillance_radar

# Secondary surveillance radar **Secondary surveillance radar** (SSR) is a radar-based air traffic control system that identifies an aircraft and reads out data such as its altitude by interrogating a transponder carried aboard it, rather than by measuring only the aircraft's own reflected echo the way an ordinary primary [[Radar|radar]] does. A ground station transmits a coded interrogation pulse; any properly equipped aircraft within range replies automatically with its own coded pulse train, giving a controller an identity and, often, an altitude that a plain echo could never supply on its own. A three.js microsim, reused from the site's aviation set, renders an air traffic controller's own SSR display elsewhere on the site. SSR grew directly out of the Identification Friend or Foe (IFF) systems built during the Second World War to tell a friendly aircraft from a hostile one by radio rather than by sight alone, and its modern interrogation modes still track that military lineage even though air traffic control, not combat identification, is now its everyday job.[^wwii-iff] Because a reply is an active retransmission rather than a faint scattered echo, an SSR return is far stronger and far more reliable than a primary radar's, largely immune to the weather and ground [[Clutter_(radar)|clutter]] that can swamp a primary target, at the cost of seeing nothing at all from an aircraft whose transponder has failed or been switched off. ## Overview ### Primary radar A primary radar finds an aircraft the way [[Radar]] has always worked: it transmits a pulse, and whatever fraction of that pulse the aircraft's own structure happens to scatter back gives its range and bearing, with no cooperation from the aircraft required or possible. It reports only a plain, unidentified return, and that return can be very weak, since how much energy an aircraft happens to reflect depends on its size, shape and material rather than on anything a controller can rely on. ### Secondary radar Secondary radar solves the identification problem primary radar cannot by asking the aircraft to answer for itself: an interrogator on the ground transmits a coded pulse pattern, and a transponder carried aboard the aircraft receives it and transmits back its own coded reply, carrying whatever information that interrogation asked for. Because the reply is generated and transmitted by the aircraft's own equipment rather than merely reflected, it arrives far stronger and far more consistently than an echo of the same original pulse ever could, which is why SSR remains the primary source of an aircraft's identity and altitude in controlled airspace even where primary radar coverage also exists. As with any radio link, how weak an interrogation or a reply a system can still detect is set by its own receiver's noise floor, the same sensitivity limit that bounds any coherent radio receiver.[^noisefloor4] ### Standards and specifications SSR is standardized internationally through the International Civil Aviation Organization's Annex 10 to the Convention on International Civil Aviation, which lays out the interrogation and reply [[Communication_protocol|protocol]] every compliant transponder and ground station must share so that an aircraft built and certified in one country can be interrogated correctly by a ground station operated by another.[^icao-annex10] Its civil modes were defined to match corresponding military IFF modes directly, Mode A with IFF Mode 3A and Mode C with IFF Mode C, so that the same onboard transponder can answer both a civil air traffic control interrogation and a military IFF one; two further civil modes once specified, B and D, have since dropped out of use entirely. ## Operation An SSR ground station's interrogator transmits its coded pulse pattern as [[Radio_wave|radio waves]] on one fixed frequency in the [[Ultra_high_frequency|UHF]] band, while every transponder reply arrives back on a second, different fixed frequency that the ground station's own [[Radio_receiver|radio receiver]] is built to listen for, so interrogation and reply never collide on the same channel. A [[Computer_science|computer]] now decodes every reply automatically rather than an operator reading a raw pulse train by eye. Range comes from the round-trip delay between an interrogation going out and its reply arriving, exactly as it would for an echo, while bearing comes from the direction the ground antenna was pointing at the moment that reply arrived; both numbers are usually painted onto the same [[Plan_position_indicator|plan-position display]] a primary radar would use, with the aircraft's decoded identity and altitude written alongside its position rather than left for a controller to infer. The interrogation and reply pulse trains are themselves a form of [[Pulse-code_modulation|pulse-code modulation]], with the presence or absence of a pulse at each defined position in the train standing for one bit of the message being sent. ## Interrogation modes Different interrogation modes ask a transponder for different information using the same basic pulse-pair format, distinguished by the spacing between the two pulses that make up the interrogation itself. Mode A asks for identity and gets back a four-digit octal code, commonly called a squawk, that a pilot or a controller sets rather than one permanently tied to the aircraft. Mode C asks for altitude and gets back the aircraft's own pressure altitude, read from its barometric sensor and referenced to a standard pressure setting rather than to true height above the ground or the sea, unlike a [[Radar_altimeter|radar altimeter]], which measures height directly by timing its own echo back from the terrain below. Mode S asks a single, individually addressed aircraft rather than every transponder within range, using a unique identifier assigned to that aircraft alone, and can carry considerably more information in reply than a bare four-digit code or a single altitude figure. ## Deficiencies ### Mode A A Mode A code identifies an aircraft only as well as its four octal digits, 4,096 possible combinations in all, are actually managed and kept unique within a given area at a given time; because the code is set by a person rather than fixed to the airframe, two aircraft can end up broadcasting the same code by simple error, and the code itself carries no altitude information at all. ### Mode C A Mode C altitude is only ever a pressure altitude, referenced to a standard sea-level pressure setting rather than to the actual local pressure of the day, so converting it into a true height above the ground or above sea level is a separate step performed on the ground rather than something the code provides directly; older aircraft not equipped to report altitude at all leave the same gap unfilled no matter how the interrogation is asked. ### FRUIT An SSR transponder replies to any correctly formatted interrogation it receives, not only to the one ground station a controller happens to be watching, so a receiver in busy airspace picks up replies triggered by other, distant interrogators as well as its own; these unsynchronized foreign replies, called FRUIT, arrive with no fixed timing relationship to the local interrogation and have to be recognized and discarded rather than mistaken for a genuine local reply. ### Garble Two aircraft close enough together in range and bearing can have their replies arrive at the receiver close enough together in time that the two pulse trains overlap and corrupt each other, a problem called garble that grows more common wherever traffic is dense enough to put many transponders within one antenna beam at once. ### Synchronous garble Synchronous garble is the harder, persistent case: two aircraft sitting at very nearly the same range from the interrogator produce replies delayed by very nearly the same amount on every single scan, so their pulse trains keep overlapping in the same way scan after scan rather than drifting in and out of overlap the way ordinary garble does. ### Capture When two replies do overlap, a receiver often locks onto whichever of the two arrives stronger and simply loses the weaker one entirely, a capture effect that can make one aircraft in a garbled pair effectively invisible rather than merely harder to decode. ### Antenna Much of the blame for FRUIT, garble and capture traces back to the antenna itself: an early SSR beam was comparatively wide and its sidelobes comparatively strong, so a transponder could be triggered by energy arriving well off the antenna's intended pointing direction, adding unwanted replies and blurring the bearing measurement beyond what the beam's nominal width alone would suggest. ## Developments to address the deficiencies ### Improved antenna A narrower beam with far weaker sidelobes directly cuts down on off-axis triggering and sharpens the bearing measurement, and pairing a better antenna with sidelobe suppression, a control pulse that tells a transponder to ignore any interrogation it received through a sidelobe rather than through the main beam, removes much of the antenna's own contribution to FRUIT and to garble at the source rather than trying to sort the resulting mess out afterward. ## Monopulse secondary surveillance radar [[Monopulse_radar|Monopulse]] [[Signal_processing|signal processing]] solves the bearing problem and the interrogation-rate problem at the same time: comparing the sum and the difference of what two slightly offset antenna feeds receive from a single reply gives a precise angle from that one reply alone, rather than needing many replies collected while the beam swept across the target to interpolate a bearing the older way. Needing only one or two replies per aircraft per scan, instead of the dozen or more a wide, slowly interpolated beam once required, cuts the total number of interrogations and replies filling the shared channel, which in turn reduces exactly the FRUIT and garble that a busy, crowded channel makes worse. Monopulse SSR is now the standard civil implementation almost everywhere primary and secondary radar are still deployed together.[^monopulse-origin] ## Mode S Mode S, for "selective," addresses one aircraft at a time using a unique identifier permanently assigned to it, so a ground station can interrogate exactly the aircraft it means to rather than every transponder within the beam, cutting unnecessary replies at their source in much the way monopulse cuts unnecessary interrogations. A Mode S reply also carries a combined [[Error_detection_and_correction|error-detection]] field and address, letting a receiver confirm a reply's integrity and identify which aircraft sent it in the same step, and it can carry considerably more data in a single exchange than the short Mode A code or Mode C altitude figure alone, including a downlinked aircraft identification, a more precise altitude, and information other onboard systems can use directly, such as a collision-avoidance advisory computed aboard the aircraft itself rather than only on the ground. ## Extended squitter A Mode S transponder does not have to wait for an interrogation to transmit: it can also broadcast certain messages on its own initiative, a spontaneous transmission called a squitter, and the longer, 112-bit extended squitter carries enough room for an aircraft to broadcast its own satellite-derived, [[Digitization|digitized]] position, velocity and identity without any ground interrogator asking for it at all. Any suitably equipped receiver, aboard another aircraft or on the ground, can pick that broadcast up directly, which is the technical basis of automatic dependent surveillance–broadcast and lets aircraft be tracked well beyond the range, or the terrain-blocked line of sight, that a conventional interrogated SSR needs.[^squitter-standard] ## Microsims This article carries no p5.js sketch of its own. A three.js companion, reused from the site's aviation set, renders an air traffic controller's own SSR display elsewhere on the site. The neighbouring Radar sketch is the most directly relevant companion here: SSR still measures range from the round-trip delay between an interrogation and its reply exactly as an ordinary pulsed radar measures range from an echo, and the same unambiguous-range limit the sketch shows applies equally to an interrogation-and-reply cycle. Unlike the Doppler effect sketch's moving source, however, nothing in an SSR exchange depends on a shift in frequency at all; an aircraft's speed changes neither its squawk code nor its reported altitude, and the whole system works by coded timing rather than by any Doppler measurement. *Try:* in the [[Radar]] sketch, raise the pulse-repetition frequency and watch the unambiguous-range ring shrink; an SSR interrogator trades range against reply rate on exactly the same ring, since it too must wait out a reply before it can safely interrogate again. *Try:* in the [[Doppler_effect]] sketch, notice that changing the source's speed is what produces every effect shown; SSR has no equivalent control at all, since its interrogation and reply carry their information as coded pulses rather than as a frequency shift. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Secondary_surveillance_radar) : [Wikitube](https://en.wikitube.io/wiki/Secondary_surveillance_radar) Skeleton mirrored at revision 1372698252. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Plan_position_indicator]] - [[Radar]] - [[Monopulse_radar]] - [[Radar_altimeter]] - [[Doppler_effect]] - [[Radio_receiver]] - [[Clutter_(radar)]] ## References The round-trip range relation, the pulse-code structure of an interrogation and reply, and the sum-and-difference basis of monopulse angle estimation are standard radar-engineering theory and are not separately footnoted here, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open edition linked in Further reading. [^wwii-iff]: Citation needed: the specific wartime Identification Friend or Foe programme and its introduction date, commonly placed in the United Kingdom around 1939-1940, would need a primary defence-history record to confirm precisely. [^icao-annex10]: Citation needed: the current edition and amendment number of ICAO Annex 10, Volume IV, would confirm the exact standard in force for SSR interrogation and reply formats. [^noisefloor4]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 96-101 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^monopulse-origin]: Citation needed: the specific radar programme or manufacturer credited with the first operational monopulse secondary surveillance radar, and its introduction date, would need a primary engineering-history record to confirm. [^squitter-standard]: Citation needed: the specific ICAO or RTCA standard, and its adoption date, that first defined the 1090 MHz extended-squitter format underlying automatic dependent surveillance–broadcast would confirm the precise reference. ## Further reading - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory - Don Johnson. *Fundamentals of Electrical Engineering I*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain - Allen Downey. *Think DSP: Digital Signal Processing in Python*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods ## External links This article carries no p5.js sketch of its own; its three.js companion, reused from the aviation set, is embedded in Microsims, above.