The Colorado Department of Transportation has launched a new technology initiative that will provide enhanced surveillance, and thereby improve access, to its many mountain airports. Currently, IFR flight operations at those locations are limited because surrounding mountains block the line-of-sight signals of the FAA’s ATC secondary (transponder interrogating) radars, preventing the monitoring of lower-level arriving and departing traffic. As a result, arrival rates in instrument weather at some airports using non-radar procedures can be reduced to three or four per hour.

Under a two-phase, $15 million program, the state will install networks of multilateration (MLat) stations in the vicinity of these airports, allowing Denver ARTCC controllers to track aircraft at much lower altitudes. State DOT officials point out that to obtain equivalent surveillance by using a large number of “legacy” radars over the project’s area would be “prohibitively expensive,” since a single radar installation would cost more than $7 million.

Supplied by Sensis of Syracuse, N.Y., the unmanned stations within the MLat networks will automatically triangulate transponder signals of each aircraft in the vicinity to a higher accuracy, and at a higher update rate, than conventional secondary radar, and their data will be transmitted continuously to ARTCC controllers’ screens, where they appear as normal targets. Consequently, it is expected that the 30-mile non-radar IFR procedural separation standards currently applied at the state’s mountain airports can be reduced to five miles, similar to those at the mountain-ringed airport at Innsbruck, Austria, where MLat was introduced some years ago.

The first phase of the Colorado program is forecast to cover Hayden, Rifle, Steamboat Springs and Craig in time for the 2008 ski season. State DOT officials are finalizing plans for the second phase, which is expected to cover Montrose, Telluride, Gunnison, Cortez, Durango and Alamosa, but an operational date has not yet been announced.

When both phases are complete, the FAA will assume ownership of the stations, including their operations and maintenance costs over their 15- to 20-year life and will also certify them as components of the National Airspace System.

Importantly for operators, while the Sensis stations can operate in either MLat or ADS-B modes, their Colorado configuration will not require aircraft to be ADS-B equipped. It has been unofficially suggested that when FAA ADS-B ground stations are installed in the future to serve Colorado airspace, they will be regarded as separate from the MLat networks, which will continue to provide a dedicated service to the mountain airports.

Multilateration Gains Traction Abroad
On the other side of the Atlantic, the UK’s National Air Traffic Service (NATS) has a similar monitoring requirement, but in this case over the North Sea. There, helicopters flying to offshore oil and gas platforms disappear from ATC controllers’ screens as they proceed outbound and go below the horizon of secondary surveillance radars on the Scottish coast.

The solution again is multilateration, with Sensis-supplied stations planned on several of the platforms. These will track the transponders of arriving and departing helicopters and continuously relay their data to NATS’ ATC Centre, where more than 25,000 offshore helicopter movements are handled each year. There, too, the Sensis stations will operate in non-ADS-B mode.

Over the last few years, multilateration has gathered significant momentum as an ATC surveillance tool, one that some feel will become a widely adopted complement to the future ADS-B. The appeal of the system is that it provides equal or better service than current surveillance radars at significantly lower cost, and requires no additional avionics beyond existing ATC transponders.

At a recent European MLat workshop, representatives of air navigation service providers, member nations, airlines, military forces and industry requested that Eurocontrol intensify its efforts “to promote the implementation of multilateration systems throughout Europe and to expedite the development of Eurocontrol surveillance products to process their data.”