Cuz Frank knows radars.

Multilateration, or MLat, is the latest thing coming down the pike in air traffic control technology, and itÆs picking up speed. According to its advocates, MLat promises to eventually replace most, if not all, of todayÆs transponder tracking ATC radars. WhatÆs more, they claim, MLat is cheaper, requires much less maintenance, is more accurate and works in situations where radar has problems.

Judging by the rate at which MLat systems are being adopted, those are no idle claims. Aviation authorities from Mongolia to Tasmania, from Vancouver to Taipei, from Beijing to Stockholm and at numerous other locations have adopted multilateration solutions to problems that were previously felt could only be resolved by radar, or couldnÆt be resolved at all, other than at great expense.

So what is MLat, how does it work, and what is it used for? ItÆs based on the old principle of triangulation, which you probably did do at school. That is, if you take a bearing of an object from three known, but widely separated, positions and plot those on a map, their intersection will show you the objectÆs location.

But MLat doesnÆt take bearings. The system employs a number of small, unmanned "listening post" stations which can be spread around an airport to monitor local traffic, or located much farther apart, in the systemÆs wide area configuration, to cover larger areas of airspace.

The stations simply listen for transponder returns, triggered by interrogations from ATC secondary surveillance radars (SSRs) or from TCAS-equipped aircraft, out to line-of-sight distances, which can be up to 300 miles for high-flying aircraft. And to cover all traffic combinations, MLat can monitor Mode-A/C, Mode-S, ADS-B and military IFF transponders.

Every second, each station sends the data from the received signals to a centrally located data processor ù a filing cabinet sized unit ù where fairly sophisticated triangulation and time-of-arrival computations derive the precise positions of all aircraft, which are then sent to the ATC center.

At the center, controllers see no difference between conventional radar and MLat targets on their screens, because the MLatÆs processor unit provides identical information in the identical format to the centerÆs display computers.

The only difference is that MLat data is refreshed at a much higher update rate than the typical 4.8-second, 360-degree sweep rate of the radarÆs rotating antenna.

In turn, that higher data rate provides controllers with a much smoother and more accurate progression of the targets across their screens, compared with the periodic "jumps" of those swept by radar.

Another plus of MLatÆs higher update rate is that while positive target confirmation and identification by conventional radar usually requires at least three consecutive antenna sweeps, or roughly 15 to 20 seconds, MLat can achieve it in just three or four seconds.