Multilateration & MAGS L. Gonzales 2 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Outline 1. Principle of multilateration 2. Aircraft signal 3. Multilateration on airports 4. Conclusion 3 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission 1. Principle of multilateration 4 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Basic Principle of Multilateration Multilateration (MLAT) is a technique initially developed for military applications, which allows to passively locate co-operative targets by multistatic measurements. Passive: no interrogation from the surveillance system are required (i.e. receive only), provided the aircraft transmits a signal Co-operative: the principle requires appropriate onboard equipment (e.g. a transponder) Multistatic: The same signal needs to be received simultaneously by several ground stations 5 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Comparison with other Surveillance Principles Data measured by surveillance system? Interrog. required? Onboard equipment Surveillance Principle required? No dependent surveillance No passive Yes ADS-B co-operative Yes (partly) partly independent surveillance No passive Yes Mode S Multilateration co-operative Yes (partly) partly independent surveillance Yes active Yes Mode A/C Multilateration co-operative Yes (partly) partly independent surveillance Yes active Yes Secondary Surveillance Radar co-operative Yes independent surveillance Yes active No Primary Radar non-co-operative 6 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Principle of Multilateration Systems (1) Time of Arrival in A: TOA1 Time of Arrival in B: TOA2 A and B are a pair of Ground Stations receiving both a signal from an aircraft. The Time of Arrival TOA of the signal is measured by each Ground Station. The time difference TOA1-TOA2 corresponds to the distance difference X2 - X1 = c • (TOA2 – TOA1) X1 = c • TOA1 X2 = c • TOA2 7 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Principle of Multilateration Systems (2) Time of Arrival in A: TOA1 Time of Arrival in B: TOA2 At a given time, the Aircraft is on the locus of points having the distance X2 - X1 constant: X2 - X1 = c • (TOA2 - TOA1) This is a hyperbola (curve in blue) => Two ground stations allow to determine one hyperbola where the aircraft is located X1 = c • TOA1 X2 = c • TOA2 X2 - X1 = c • (TOA2 - TOA1) 8 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Principle of Multilateration Systems (3) Time of Arrival in A: TOA1 Time of Arrival in B: TOA2 A third station in C gives two more differences X2 - X1 = c • (TOA2 - TOA1) X2 - X3 = c • (TOA2 - TOA3) X1 - X3 = c • (TOA1 - TOA3) and thus allows to determine two more hyperbolas => The aircraft is located at the intersection of the three hyperbolas X3 = c • TOA3 Time of Arrival in C: TOA3 X2 = c • TOA2 X1 = c • TOA1 9 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Multilateration Principle Summary Ground stations determine the precise time of arrival (TOA) of received signals Intersection of several hyperbolas is the target position Knowing the speed of wave propagation, a hyperbolic line of position results TOA difference is calculated for each pair of ground stations Signal transmitted by aircraft transponder is received by several ground stations (a minimum of 3 for 2D position) in the vicinity This principle can be extended to measure 3D positions : a 4th ground station is then required 10 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Generic System Architecture To implement the principle of multilateration system, the generic system architecture consists of: A sufficient Number of Ground Stations (GS) capable of: receiving the signal(s) from aircraft located in the service area, measuring the time of arrival and forwarding the TOA to a central station, being synced to the same timebase A Central Processing Station (CPS): to receive the TOAs from the Ground Stations and to compute the aircraft position from the set of measurement. In addition the CS has to manage the fact that several aircraft can be located in the service area, A communication network to link all the GS to the CS 11 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Constraints related to the principle of multilateration systems The measurement of time of arrival must be very accurate As an inaccurate measurement will degrade the accuracy of the position calculation This can be achieved by high frequency sampling of incoming signals The clocks of the ground stations must be very well synchronised As a bias between GS clocks will imply a measurement error This can be achieved by several means : transmission of a calibration signal use of an universal common time reference signal (regional time signal transmitter, GPS) 12 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission 2. Aircraft Signal 13 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Cooperative target Unequipped aircraft will not be seen by the MLAT system. Only cooperating targets will be detected. For civil aviation, the signal transmitted by aircraft can be: either a Mode A/C or Mode S reply to any interrogator in the neighbourhood (e.g. Radar, ACAS) the Short Squitter (acquisition squitter for ACAS) transmitted once per second for aircraft equipped with a Mode S Transponder In the next future, the Extended Squitter transmitted twice per second for ADS-B equipped aircraft. In case the aircraft are not equipped with Mode S transponders, and no MSSR are available in the neighbourhood, a specific interrogator must be implemented to trigger Mode A/C replies. 14 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Aircraft Signal which can be used by multilateration Implementation is just starting 24bit aircraft address and the rest is variable Various rates up ADS-B ADS-B to 2 per second Extended Squitter Widespread due to ACAS mandate 24bit aircraft address and transponder capability Once per second ACAS Acquisition squitter « short squitter » Expanding (few ground Mode S interrogation) Mode S replies widespread due to ACAS mandate 24bit aircraft address. The rest depends on interrogation Ground ATC surveillance and ACAS Sent in response to interrogation Mode S reply Mode A or Mode Very widespread C code depending on interrogation Ground ATC surveillance and ACAS Sent in response to interrogation Mode A/C reply Transponder When sent Original purpose Data contents Use today transmission 15 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Difference between processing Mode S and Mode A/C aircraft With Mode S signals, each ground station receives a signal which is uniquely identified by the ICAO 24 bits address; this allows the MLAT system to unambiguously associate the various messages as belonging to the same aircraft For Mode A/C signals, the association is easy if the signal is a Mode A signal, but if it is a Mode C signal, the MLAT system must maintain a table of all aircraft in the service area before being sure to associate the replies received by ground stations as belonging to the same aircraft. 16 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Identification of aircraft (1) In civil applications, identity of the aircraft is required MLAT extracts aircraft identity information from the transmitted signal (also used to measure aircraft position) This is obtained by Mode A information when the signal is a reply to MSSR or Mode S interrogation 17 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Identification of aircraft (2) The identification is not straightforward in case no Mode S or MSSR radars are implemented in the neighbourhood. If the aircraft is equipped with a Mode S transponder it transmits the short squitter, including the 24 bits ICAO address of the aircraft The 24 bits ICAO address is currently not included in Flight Plans => does not allow to correlate the signal with aircraft ID The MLAT must then interrogate the aircraft to obtain Mode A information This will no longer be true with Extended Squitter as the Call Sign is transmitted by Extended Squitter If the aircraft is equipped with a MSSR transponder, the MLAT system must interrogate the aircraft to obtain a Mode A reply. 18 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Aircraft Altitude In the same manner as for identity, aircraft barometric altitude will be obtained by using Mode C. In the case of 3D MLAT system, only the geometric altitude of the aircraft is measured : Not used in “normal” surveillance operation Used in monitoring of the performance of aircraft altimeters, for example in the case of RVSM implementation. In this case a modelling of the variation of atmospheric pressure with altitude must be established ADS-B provides barometic and geometric Altitude 19 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission 3 - Conclusion 20 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Airport Multilateration Summary Strengths High performance For airports, it exceeds present SMR performance No additional aircraft equipage required Aircraft widely equipped with SSR transponders More and more aircraft are equipped with Mode S Transponders Lifecycle cost lower than Radar no rotating machinery, essentially maintenance-free Weaknesses erformance affected by ground effects (multipath, shadowing, etc) Change in installations and procedures may be required So transponder is not disabled on the ground 21 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Key Features Automatic aircraft labelling/ Identification assive aircraft location Growth potential to receive and forward ADS-B reporting Easily adaptable to airport layout Integration into STREAMS (Thales ATM’s SMGCS) Co-operative Multilateration System 22 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission MAGS Target Location Methods Passive multilateration Using all Mode S downlink formats received Active multilateration Includes a low power interrogator (100 W) for less covered areas and approach Capable of growth towards reception and processing of ADS-B / extended squitter 23 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission MAGS Adaptability Scaleable number of Ground Stations Adjustable antenna coverage Processing algorithms individually adaptable for each airport area Full local and remote control Easy integration into STREAMS Wide range of Commercial Off The Shelf (COTS) network equipment Industry standard interfaces and protocols 24 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission Coping with Multipath Antenna patterns and special MAGS signal processing design allow to reduce multipath influence It is essential to optimise ground stations placement A trade-off must be carried out between station geometry , multipath avoidance, and operational constraints 25 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission MAGS Technical Data TOA resolution: 128 MHz (7 ns / 2.4 m) Mean accuracy: < 7 m Detection probability: >95% >99% in restricted areas (e.g. runways) Mean update rate: 1/s Localisation capacity: 300 plots/s max. Interrogation capacity: 200/s 26 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission MAGS-GSR (outdoor Version) Off-the-Shelf Cabinet Heat Exchanger between Twin Walls 300 W Heater To be mounted to a wall, to a mast or standing on Ground (together with plinth as shown) 27 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission MAGS at Köln/Bonn Airport • Focus on one area (Apron, TWY A/B/E, RWY14L) • Five Ground Stations (1 GST 4 GSR) • Uses all valid Mode S downlink formats • Raw data shown, i.e. no tracking or filtering • Remote control, diagnosis and configuration over ISDN/SNMP 28 L. Gonzales 29/07/05 Thales ATM Do not reproduce without permission ACRONYMS ADS-B : Automatic Dependence Surveillance Broadcast COTS : Commercial Off The Shelf CS : Central Station DPX : Duplexer GSR : Receive only Ground Station GST : Transmit only Ground Station HDOP : Horizontal Dilution of Precision ISDN : Integrated Service Digital Network MAGS : Mode S Airport Ground Sensor NTA : Network Terminal Adapter RWY : Runways RXU : Receiver Unit SNMP : Simple Network Management Protocol SPB : System Processing Board SPC : System Processing Computer SSR :Secondary Surveillance Radar TOA : Time Of Arrival TWY : Taxiway TXU : Transceiver Unit作者: desperado 时间: 2010-9-9 12:27:27
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