Wide Area Multilateration in the CONUS:
An alternate ATC surveillance system

WAM allows controllers to see aircraft beyond the reach of radar coverage, preventing delays, cancellations and improving airport capacity.

A simplified diagram of the WAM system at Denver Center. Key to moving ahead with the system was FAA's ability to certify WAM as a means to separate traffic.

WAM was installed in 2 phases

To get the system in place and certified to the aggressive schedule the state wanted, WAM was installed in 2 phases. Phase I would be for Craig (CAG), Steamboat Springs (SBS), Hayden (HDN) and Rifle (RIL). The project started in 2006 and became operational in 2010. Key components are the transceiver sites, timing sources and target processors.

All aspects of the system including uninterrupted power, multipath communications, and physical security had to be met. To be part of the NAS, an availability level of 99.996% was required. This means that any system component can be down only 12 minutes in a year for unscheduled maintenance.

As many sites as possible are co-located with existing state or federally owned properties for security reasons. Remote sites have levels of physical security to meet FAA's rigid standards. There is sufficient site overlap to ensure uninterrupted surveillance even if 1 or more sites are down.

WAM is arranged in overlapping 5 nm service area circles topping out at the base of radar coverage and located along the routes to and from the target airports. For Phase I, there were 2 distinct areas created called the Hayden Constellation and the Rifle Constellation, totaling 20 ground sensors. A "virtual" radar site was selected between the 2 constellations as a point of reference to feed track information to Denver Center.

Phase I WAM siting

Both constellations feed the virtual radar, which permits Denver Center to integrate WAM positioning seamlessly with SSR. Note enroute fixes are established within the WAM coverage area.

Much thought was put into the protocol the Denver Center controllers would use to transition from radar to WAM. Ideally, the process needed to be so seamless that the controllers could just separate traffic without any added workload. WAM emulates Denver Center's ATCBI-6 radar.

The virtual radar site updates to Denver Center every 12 seconds, which is the rate for radar sweeps. The WAM update capability is much greater than 12 seconds. However, for Phase I the idea was to get the technology working and certified as a stand-alone surveillance system.

Phase II

As soon as Phase I gained full operation, Phase II added Durango (DRO), Gunnison (GUC), Montrose (MTJ), and Telluride (TEX). A much larger number of ground sites were required to ensure coverage. Phase II became operational for all 4 airports at the end of July 2013.

Pilot/controller perspective and benefits

On arrival, as your aircraft passes below radar coverage, the Denver Center sector controller sees you all the way to the airport via a seamless transition to WAM. Several of the airports with WAM coverage permit surveillance by ATC all the way to touchdown. For departure all of these airports have either published departure or special/obstacle departure procedures that will keep the aircraft within WAM coverage until transition to the floor of radar coverage.

As your aircraft passes from WAM coverage to the base of radar coverage, the controller's presentation switches back over to radar. The idea here is that both the controller and crew have no added tasks to ensure coverage. The capacity to move aircraft to and from these airports greatly increased when ATC could assure positive separation using surveillance vs procedural separation.

From the controller's perspective, it is as if a light has been turned on increasing what can be seen. To date the system has worked well and has been welcomed by the controllers and crews alike.

Phasing in of ADS-B with WAM

Bill Payne of William Payne and Associates Engineering is the state's program manager for this unique state/federal program. He says the goal was clear but the path was full of challenges and many of the problems to be solved had never been addressed before.

Some aspects fell right into place while other issues seemed elusive and took a lot of hard teamwork to solve. For example, how to ensure year around access to the remote sites for servicing when it was clear that winter weather would make getting to and from some of the sites extremely difficult.

There was discussion about adding ADS-B capability to the Phase I WAM network but to get the system up and running more quickly it was decided that Phase I would be Mlat capable only.

Phase I network does receive 1090 Extended Squitter ADS-B messages but these are not currently used. Phase II network is fully ADS-B capable for both 1090ES and 978 MHz Universal Access Transceiver ADS-B protocols. Controllers can display ADS-B positioning within the Phase II WAM network for those aircraft that have a certified ADS-B Out function.

A program to fully add ADS-B capability to the Phase I network in the future will be implemented before 2020 as these remote Mlat sites can also serve as ADS-B ground transceiver sites.

Multiple benefits with WAM, especially if GPS is lost

This alternate technology, showing that Mlat and WAM can meet ATC's standards for traffic separation, is significant in that FAA is working on a backup plan to separate traffic in case there is a loss of GPS once ADS-B becomes the primary system for surveillance (see Sept 2013 PP; "FAA's plan for the retention of legacy navaids in the future"). Mlat and WAM may very likely be used as a backup for GPS to separate traffic in the terminal areas for the busiest airports in the NAS.

Juneau Alaska has a WAM network that in principle provides the same service for traffic surveillance in an environment where radar just cannot work. Interestingly, Austria is working on an Mlat WAM network for their entire country.

How widely Mlat and WAM may be used in future for surveillance is yet unknown. Still, the concept works well and may play an even wider role in the future in the US NAS.

Bill Gunn is the compliance manager for the State of Texas Aviation Division. He lectures nationally for a private aviation advocacy group and flies for work and pleasure.


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