At the Air Traffic Control Association’s annual convention this fall, Syracuse, N.Y.-based Sensis announced that its multilateration system will replace the legacy precision runway monitor (PRM) radar used at the Sydney, Australia airport, to monitor aircraft flying simultaneous approaches to its closely spaced parallel runways, 16L and 16R.

This is an important breakthrough because, while the PRM radar increases runway throughput and reduces delays by as much as 20 percent, its high cost–said to be around $15 million per unit, uninstalled–has limited its deployment to five U.S. airports (Minneapolis/St. Paul, Philadelphia, St. Louis, San Francisco and Cleveland). The system is also deployed at Hong Kong, Frankfurt and Sydney.

The multilateration, or MLat, technique is much less costly, and can therefore be installed at many more airports. The 1980s design of the PRM is also limited to handling Mode A/C transponder returns, and cannot use mode-S or the coming ADS-B signals, while MLat handles them all.

MLat uses a number of small, unmanned “listening post” ground stations arrayed around an area of interest–in this case, the runways– that continuously monitor aircraft transponder signals and compute their precise position every second, rather than the 4-7 second interrogation rate of today’s secondary surveillance radars. The system then sends the data to the air traffic controllers’ screens, where it appears in formats identical to those of the conventional radar targets.

Yet while MLat is being installed more frequently in the U.S.–for example, with the FAA’s ASDE-X airport surface surveillance system–and at many other locations around the world for surface, terminal area and en route airspace surveillance, it had not yet proved itself capable of meeting the extremely demanding PRM role.

While MLat is used today for monitoring approaches to widely spaced parallel runways, until now only the PRM radar has been able to monitor aircraft flying simultaneous dependent approaches to parallel runways spaced between 4,300 and 3,400 feet apart, reducing to 3,000 feet with an offset localizer on one of the runways. Closely spaced parallel runways without precision monitoring call for independent, or staggered, approach streams, with significant in-trail spacing.

Extensive FAA testing at St. Louis and Detroit has established that MLat meets the required PRM performance standards, with the first U.S. installation planned for Detroit’s Runways 22L/R and 04L/R, and with certification scheduled for late next fall.

“Globally, airport congestion is an increasing concern–both to the flying public and to [air navigation service] providers,” said Tony Lo Brutto, Sensis vice president and general manager of air traffic systems. “By leveraging existing multilateration surface deployments, airports can further maximize runway usage as well as increase airport efficiency in a cost-effective manner.”

Environmental pressures, coupled with more restrictive zoning legislation, are already making it extremely difficult to obtain planning approval for the construction of a new runway outside an airport’s boundaries. As a result, it’s likely that at some point the only remaining option for most airports will be to build new runways inside the airport, closely parallel to an existing runway.