ATC USE OF WAM

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.

By Bill Gunn
ATP/CFII. Compliance Mgr,
State of Texas Aviation Division


Wide Area Multilateration (WAM) dramatically increases ATC surveillance coverage for select eastern Rocky Mountain airports. A look at how this system works is shown in this depiction.

Pilots who have flown to or from any of the Rocky Mountain airports know the challenges. Even in good weather arrivals and departures require vigilance. Add poor weather conditions and the stakes rise dramatically. Winter ski season is a peak period for scheduled, on-demand charter and private aircraft operations—exactly when the weather can be the most problematic.

A unique partnership solution between the state of Colorado and FAA has created a new chapter in certified surveillance traffic separation in the National Airspace System (NAS).

Background

In 2004 the Colorado DOT Aeronautics Division addressed the impact that the lack of surveillance capability by ATC had on the growing winter sport industry in the near eastern range of the Rocky Mountains. Radar coverage by Denver Air Traffic Control Center (ZVD) goes no lower than 9000 to 15,000 ft MSL, yet the 12 busiest airports serving this area are all in the shadow of the mountains and well below Denver Center's radar service volume.

When the weather was VFR most of these airports could handle 15 to 25 operations per hour. When the peak season arrived, however, the weather was typically IMC and ATC's procedural capacity dropped the rate to 4 or 5 operations per hour for each airport.

Colorado's candidate airports for increased surveillance by ATC.

Frequent missed approaches with diversions, limited ramp space and weather delays combined to hamper air traffic movement exactly when the winter sports industry was in peak swing. West bound enroute traffic out of the Denver area would often have to be re-routed via New Mexico or Wyoming to avoid the saturated airspace immediately to the west.

Both of the larger 2 airports serving the ski areas, ASE (Aspen CO) and EGE (Eagle CO) have Air Traffic Control Towers (ATCT) with added radar that supports a dedicated approach control in the case of Aspen and a feed to Denver Center for Vail and Eagle County.

Along with ATCTs and approach control, these added radar systems do a good job of serving these busy airports. Other candidate airports for increased surveillance by ATC were not deemed justified by FAA for the expensive and manpower-intensive requirements of ATCTs and approach control. Another solution had to
be found.

Other solutions proposed—WAM looks best

Colorado was willing to help fund a solution as long as FAA could certify the system as a means to separate traffic in the NAS and only if the system could be deployed in a 2 to 4 year time frame. Several solutions were considered, such as added radar, ADS-B tracking and Wide Area Multilateration (WAM). WAM was selected for many reasons. But the biggest challenge still remained—WAM had never been certified as
a sole means to separate traffic in the NAS.

How multilateration works

Multilateration (Mlat) is based on the constant speed of radio waves. Mlat over a large area (WAM) can fix the position of an aircraft. Ground transceiver sites interrogate the aircraft's transponder and use the difference in time of reception of this interrogation by each of the individual ground sites.

These multiple ground sites are all synchronized to a common time signal accurate to billions of a second. This produces a hyperbolic shaped aircraft line of position from each site. The intersection of these lines of position (LOPs) provides a highly accurate aircraft fix. At least 3 properly positioned LOPs are required to get a fix but the WAM network for Denver Center requires at least 4 for redundancy.

The concept of time difference of reception for navigation is common to many older systems such as LORAN (Long Range Navigation) and the WW II British GEE system. One of the critical elements is that all of the Mlat sites are synchronized to a single common timing source. Wide Area Multilateration for Denver Center uses ground based timing sources to groups of ground sites that interrogate and receive transponder replies.

WAM fits current infrastructure and is not GPS dependent

It is significant that Mlat and WAM are not GPS dependent in any way.
FAA's Airport Surface Detection Equipment, Model X (ASDE X) was one consideration supporting the use of WAM. Combined with surface movement radar in place at several hub airports ASDE X gave the FAA's Air Traffic Organization confidence in the capability of WAM.

Although ASDE X is not a stand-alone traffic separation system, a WAM system at Innsbruck, Austria was certified for traffic separation and has been in use since November 2004. After much thought and study it was decided that Denver Center would be the controlling agency for the added capacity WAM would bring. One of the reasons for this decision was the fact that the 10 airports WAM would support are non-towered and there were no other terminal ATC facilities suitable.

Many technical issues had to be resolved to permit multilateration positioning to work in concert with surveillance radar. It was important that the system work with existing aircraft equipment so WAM interrogates both mode A/C and mode S transponders. Another important consideration was altitude determination. It was decided that WAM would be used to fix the aircraft's position in 2 dimensions while verified altitude readout would determine vertical separation of traffic, as is done with radar surveillance.

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