Multilateration: a new eye in the sky and on the ground

 - January 22, 2008, 6:04 AM

Although few pilots may know the word, multilateration is quickly becoming a household term among air traffic controllers and airport authorities. But pilots will ultimately be one of its major beneficiaries.

Multilateration is today’s version of triangulation, where the location of an object is determined by taking its bearing from several different places. It is analogous to what pilots did in those far-off days of D/F, way before GPS, loran and VOR. Except that multilateration uses several small, unmanned, radio receivers spread around an airport that continuously monitor transponder, ADS-B and IFF transmissions from aircraft.

Computer processing of these signals yields accurate aircraft positions equal to, and often better than, GPS, and significantly better than airport surveillance radar. It is also much faster than radar, with around 12 responses from each target for every one from radar. Its benefits are therefore much tighter control over air traffic, more efficient track spacing and maneuvering and potential delay reductions in the terminal area. But it also provides airport ground controllers with clear identification
of individual aircraft moving on the airport surface, where current airport radar scopes merely display a series of shapeless blobs.

For this reason, the FAA’s next-generation airport surface surveillance radar, called ASDE-X, will combine multilateration systems with conventional radar. Sensis of DeWitt, N.Y., has been awarded a $100 million ASDE-X contract, the initial part of which will see installations at 22 of the nation’s busiest airports. It is expected that all of the FAA’s earlier ASDE installations will eventually be upgraded to incorporate multilateration capabilities. Sensis has been particularly successful in Europe, with an installation at London Heathrow now operational and a dozen other large European airports scheduled to follow suit this year.

Recognizing that vehicle/aircraft collisions are a continuing threat during surface operations, the company this year introduced a small, low-power mode-S unit, called VeeLo, designed for airport vehicles, which would present a clearly identifiable target in a multilateration environment. Sensis also has a strong military involvement, with several DOD multilateration system contracts, including installations in Humvees and other tactical vehicles for battlefield command and control support.

Sensis is one of just two companies in the world offering airport multilateration systems, although its European successes will undoubtedly attract more competitors. The second firm is Rannoch, Alexandria, Va. While both organizations use similar technology, Rannoch aims its products at different aviation applications, although it has sold ATC systems to Nav Canada and others. Primarily, Rannoch specializes in unusual configurations that demonstrate the system’s flexibility. Later this year at the non-tower Frederick Airport, Md., multilateration tracking will add automatic, real-time, synthesized voice traffic alerts, such as “traffic two miles on final,” via the airport’s Unicom. Similarly, under a joint NASA contract with Rockwell Collins, Rannoch developed an automatic, TCAS-like runway incursion alert for general aviation aircraft.

But the company’s main market is in airport management, where noise monitoring, flight-path adherence and landing-fee allocation and billing are key concerns. This is well demonstrated at, where clicking on aircraft inbound to San Francisco reveals, from their individual mode-S signals, their owner, tail number, type and model, in addition to mtow and calculated landing fee, which is then automatically printed as a bill and mailed to the operator.

Somehow, it seems unlikely that the original inventors of the mode-S concept anticipated that they were helping develop an automated landing-fee collection system. But that’s the way technology goes in the 21st century.