Wide area multilateration

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Wide area multilateration (WAM) is a cooperative aircraft surveillance technology based on the same time difference of arrival principle that is used on an airport surface. WAM is a technique where several ground receiving stations listen to signals transmitted from an aircraft; then the aircraft's location is mathematically calculated -- typically in two dimensions, with the aircraft providing its altitude. [1] Aircraft position, altitude and other data are ultimately transmitted, through an Air Traffic Control automation system, to screens viewed by air traffic controllers for separation of aircraft. [2] It can and has been interfaced to terminal or en-route automation systems.

Multilateration is a navigation and surveillance technique based on the measurement of the times of arrival (TOAs) of energy waves having a known propagation speed. The time origin for the TOAs is arbitrary. For surveillance, a subject of interest – in cooperative surveillance, often a vehicle – transmits to multiple receiving stations having synchronized 'clocks'. For navigation, multiple synchronized stations transmit to a user receiver. To find the coordinates of a user in n dimensions, at least n + 1 TOAs must be measured. Multilateration systems are also called hyperbolic systems, for reasons discussed below.

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System performance

WAM provides performance that is comparable to secondary surveillance radar (SSR) in terms of accuracy, probability of detection, update rate and availability/ reliability. Performance varies as a function of the location of aircraft in relation to the ground sensors. WAM is adaptable to interrogation rates, output modes and output periods. Update rates and probability of detection can be tailored to various applications such as precision runway monitoring (PRM), terminal maneuvering area (TMA) and En-route surveillance. Interrogation rates can be reduced by passively processing replies to SSR or traffic collision avoidance system (TCAS) interrogations.

Secondary surveillance radar Radar system used in air traffic control

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that not only detects and measures the position of aircraft, i.e. bearing and distance, but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, SSR relies on targets equipped with a radar transponder, that replies to each interrogation signal by transmitting a response containing encoded data. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II, therefore the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.

Traffic collision avoidance system aircraft collision avoidance system which actively interrogates other aircraft transponders

A traffic collision avoidance system or traffic alert and collision avoidance system is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC). It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers. CFR 14, Ch I, part 135 requires that TCAS I be installed for aircraft with 10-30 passengers and TCAS II for aircraft with more than 30 passengers.

Avionics

WAM operates with SSR Mode A/C, Mode S, and Mode S ES messages; no aircraft equipage change or mandate is necessary. For ADS-B equipped aircraft, WAM provides an ADS-B target report as well as a multilateration target report. WAM can complement ADS-B by providing transitional surveillance for non ADS-B equipped targets, and can be used for ADS-B validation.

Integration into automation systems

WAM incorporates new ground station output formats specifically designed for WAM and ADS-B:

ASTERIX is a standard for the exchange of air traffic services (ATS) information. It is developed and maintained by the European ATS organization Eurocontrol. ASTERIX not only stands for All-Purpose Structured Eurocontrol Surveillance Information Exchange but also represents a state-of-the-art surveillance data format which is nearly being adopted by the world users community as the universal standard in this domain today.

Implementation considerations

The primary advantage of WAM is that it can be installed in mountainous terrain, where the line-of-sight propagation paths required for SSRs would be blocked. A second advantage is that, in many situations, its cost is lower than that of SSRs. Operational implementations include the U.S. Western Colorado and Juneau, Alaska areas and the Innsbruck, Austria region. It is reported that a WAM system has been installed in the Czech Republic. WAM systems are also used to verify aircraft altimeter accuracy in the U.S. and Europe.

Siting and installation

The design of a WAM system is dependent upon proper site selections. Below are some issues to consider when selecting sites:

  • Accessibility (limited by terrain, weather, availability of power and communications, etc.)
  • Availability of power/backup power, communications
  • Site ownership: customer or local/state government owned sites may be preferred to commercial sites
  • Environmental impact
  • Available space
  • Interference with other site equipment
  • Site acquisition and preparation: leasing, permits, required construction, etc.
  • Installation season (extreme weather, high snows, high seas)
  • Accessibility during installation
  • Special training (survival training for oil platforms, tower climbing)

Communications

Availability of communications is an important factor in site selection. Bandwidth, latency and reliability all need to be considered. In many cases, a dedicated network is not available. The system needs to rely on third party commercial communications such as local microwave networks, telecommunications provider, or satellite communications.

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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 and to distinguish those returns from ground clutter.

VERA passive sensor

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Airport surveillance radar

An airport surveillance radar (ASR) is a radar system used at airports to detect and display the presence and position of aircraft in the terminal area, the airspace around airports. It is the main air traffic control system for the airspace around airports. At large airports it typically controls traffic within a radius of 60 miles (96 km) of the airport below an elevation of 25,000 feet. The sophisticated systems at large airports consist of two different radar systems, the primary and secondary surveillance radar. The primary radar typically consists of a large rotating parabolic antenna dish that sweeps a vertical fan-shaped beam of microwaves around the airspace surrounding the airport. It detects the position and range of aircraft by microwaves reflected back to the antenna from the aircraft's surface. In the US the primary radar operates at a frequency of 2.7 - 2.9 GHz in the S band with a peak radiated power of 25 kW and an average power of 2.1 kW. The secondary surveillance radar consists of a second rotating antenna, often mounted on the primary antenna, which interrogates the transponders of aircraft, which transmits a radio signal back containing the aircraft's identification, barometric altitude, and an emergency status code, which is displayed on the radar screen next to the return from the primary radar. It operates at a frequency of 1.03 - 1.09 GHz in the L band with peak power of 160 - 1500 W.

Transponder (aeronautics) airborne radio transponder used to transmit specific aircraft information in response to interrogation

A transponder is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.

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Automatic dependent surveillance – broadcast

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Anchorage Air Route Traffic Control Center

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This is a list of the acronyms and abbreviations used in avionics.

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Flightradar24 is a Swedish internet-based service that shows real-time commercial aircraft flight information on a map. It includes flight tracks, origins and destinations, flight numbers, aircraft types, positions, altitudes, headings and speeds. It can also show time-lapse replays of previous tracks and historical flight data by airline, aircraft, aircraft type, area or airport. It aggregates data from multiple sources but, outside of the United States, mostly from crowdsourced information gathering by volunteers with ADS-B receivers. The service was founded as a hobby in 2006 by two Swedish aviation enthusiasts. It is available via a web page or mobile device apps. The Guardian considers the site to be "authoritative".

Airport Surface Surveillance Capability (ASSC) is a runway-safety tool that displays aircraft and ground vehicles on the airport surface, as well as aircraft on approach and departure paths within a few miles of the airport. The tool allows air traffic controllers and air crew in cockpits equipped with Automatic Dependent Surveillance-Broadcast (ADS-B) to detect potential runway conflicts by providing detailed coverage of movement on runways and taxiways. By collecting and fusing data from a variety of sources, ASSC is able to track vehicles and aircraft on airport surfaces and obtain identification information from aircraft ADS-B transponders.

References

  1. ↑ "Eurocontrol Promoting Wide Area Multilateration Surveillance", Flt Tech Online, January 07, 2007.
  2. ↑ "FAA Fact Sheet, Wide Area Multilateration in Colorado," www.faa.gov.
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