Multilateration: Radar Is Out

By by James Callan | February 1, 2014

MLat dual mode sensor transceiver receives transponder signals, can act as system interrogator.
Courtesy of ERA.

In August 2013 the U.K.’s Newcastle International Airport set a new world standard in Air Traffic Control (ATC) when it replaced its traditional Secondary Surveillance Radar (SSR) with a Wide Area Multilateration system (WAM), built by ERA, of Pardubice, Czech Republic.

Avionics readers have probably noticed the typical long, rectangular SSR antenna rotating slowly — at about 8 rpm — atop one of an airport’s high buildings. As the antenna’s beam sweeps through the surrounding airspace, it transmits coded interrogation messages to ATC transponder-equipped aircraft within 60 miles or more. Transponders reply individually by transmitting a coded response, including its aircraft’s unique ID and a short string of flight path data, such as altitude, speed, heading and other information. Separately, the SSR’s data processor determines the aircraft’s position by calculating its range from the radar (half the product of the interrogate/response roundtrip time, multiplied by the speed of radio waves) plus its bearing from the antenna. The ID, position and flight path data are then presented on the ATC’s screen as a data tagged symbol that follows the aircraft’s progress.

With multilateration (MLat) controllers see the same data on their screens, but the aircraft positioning and data processes that precede it are very different. At Newcastle, the system uses nine suitcase-size signal sensor receivers (Figure 1, opposite page) placed at accurately surveyed locations at different distances — some several miles away — around the airport. The number of sensors is not critical and at large metroplex airports with multiple runways there can be 20 or more. Each sensor also has a built in transmit capability, and one of them will be set to transmit transponder interrogation signals identical to those from the much larger SSR but via a small, omnidirectional antenna.

When the first train of transponder responses is received by an individual MLat sensor, its Time of Arrival (TOA) is accurately logged in, as are the responses that follow from more distant sensors, thereby producing a set of TOA differences that can be converted into a set of distances from each sensor that converge on the location of the aircraft transponder. Note also that every sensor has a unique code, thus making every discrete set of TOA differences unique and non-ambiguous. Each transponder’s position and its response information then goes to the MLat data processor, where the aircraft’s position is calculated, and it and its associated flight data are then forwarded to the controllers’ screens in identical formats as the SSR data. The MLat sensors can also receive and process aircraft GPS position from their Automatic Dependent Surveillance-Broadcast Out (ADS-B Out) transmissions.

Why MLat at Newcastle?

Figure 2: A typical remote MLat “listening post” sensor antenna with a reference antenna above. Courtesy of ERA.

According to Richard Knight, operations director at Newcastle Airport, “WAM Newcastle is an important project which ensures that Newcastle International Airport has the best available technology. This, alongside significant investments, such as a new Instrument Landing System (ILS) and on-going investment in the Air Traffic Control Tower, demonstrates the airport’s commitment to safety and progress.”The Newcastle installation is described as an Active MLat system since, in the absence of an SSR, it employs one of its sensors as a transponder interrogator, transmitting signal formats identical to those of an SSR. This is a standard MLat process where there are no SSRs, which allows MLat to be used by ATC to monitor critical airport runway approaches at Innsbruck, Austria, at Queenstown, New Zealand, and at Ostrava in the Czech Republic — all of which are in mountainous areas where conventional radar would be prohibitively costly — and in several other locations, although Newcastle is the first airport to replace an existing SSR with MLat.

However, a far greater number of MLat installations to date use MLat sensor configurations in conjunction with an already existing SSR. These are called passive MLat systems, since they do not transmit transponder interrogations. By far, the largest number of passive MLat installations at airports worldwide are integrated with airport Advanced Surface Movement Guidance and Control Systems (A-SMGCS, or ASDE-X in the U.S.), where passive sensors on and around the airport are interrogated by an airport surface surveillance radar to provide controllers with visibility on taxiing aircraft and vehicles in areas where hangars and other obstructions block the radar signals. At Amsterdam’s Schiphol airport, more than 400 surface vehicles are equipped with small, purpose-built transponders transmitting a unique vehicle code for instant controller recognition.

Figure 3: Proposed MLat/Ground-Air data link configuration to cover loss of GPS events.
Courtesy of FAA/Mitre APNT Study Group.

Offshore helicopter operators are also MLat users. At their low altitudes their radar returns can often be masked by sea clutter, while at the other extreme vertically-oriented MLat sensors are installed below the high altitude flight paths of intercontinental jets as remote checks of their onboard altimeter systems’ compliance with strict vertical separation rules.

MLat’s application flexibility is therefore an important system benefit. Typically, SSR’s are positioned at optimum locations to cover the major traffic flows, but once installed they are costly to relocate. However, increasing air traffic demands can call for route realignments into areas where SSR coverage may be less than optimum and in some cases a desirable realignment may be hampered by marginal coverage, possibly requiring an additional SSR. MLat’s flexibility, on the other hand, allows coverage to be adjusted by reconfiguration of the sensor locations, at relatively low expense. This flexibility, coupled with MLat’s significantly lower cost versus SSR, suggests an increasing demand for this new technology.

Looking ahead, one future demand that is gaining increased attention from the Federal Aviation Administration (FAA) and other national and regional aviation authorities is the need for an international backup system for GPS and the several complementary foreign satellite navigation systems that are expected to be operational after 2020. Like GPS, all transmit very low-powered signals that are vulnerable to accidental or deliberate interference, and FAA is studying candidate Alternative Positioning, Navigation and Timing (APNT) solutions to handle serious interference events. Underlining this need was a period of several months in 2011 and 2012 where a GPS-based prototype landing guidance system at the Newark Liberty International Airport suffered irregular and unpredictable bursts of interference that prevented its use. The culprit was eventually traced to a low power $30 GPS jammer in a private car that occasionally drove past the test site on a nearby highway. The landing system was subsequently moved to the center of the airport, far from the highway, the jammer was confiscated and the interference has — so far — not returned.

Three approaches have been shortlisted for more detailed FAA assessment: scanning Distance Measuring Equipment (DME) plus a reduced Very High Frequency Omnidirectional Range (VOR) network; large numbers of ground based, higher powered GPS “pseudolites” to overcome interference; and MLat. Each poses unique challenges. In the scanning DME case, such units are currently exclusive to airline and high-end corporate jets and are rarely installed due to high cost in the tens of thousands of general aviation and smaller commercial aircraft. Pseudolites were only built as prototypes in the earlier days of GPS, and production costs and potential quantities are uncertain. While MLat is a proven system, it does not transmit aircraft position information, other than to ATC. However, the study group has proposed that this could be achieved over ADS-B’s common Traffic Information Service-Broadcast (TIS-B) channel that all aircraft will carry; the group suggested an MLat/TIS-B configuration (figure 3). A decision on the final selection is, however, not expected until much more testing of the alternates is performed.

Nevertheless, MLat’s future looks extremely promising. Until now, MLat has been ATC’s hidden asset. Just as its data on controllers’ screens is indistinguishable from those of the earlier SSRs, so has it also been along the world’s air routes, airports and related activities, even though its services have become indispensable to wide segments of the aviation industry. MLat’s replacement of the traditional SSR at Newcastle may be the starting point of wider future recognition.

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