Aviation: Minnesota Department of Transportation

Wide Area Multilateration (WAM)

Summary
The Minnesota Office of Aeronautics wants to fill a large radar coverage gap in Minnesota with a new technology called Wide Area Multilateration (WAM). The Office of Aeronautics and the FAA are working together to find a way to bring WAM to Minnesota.

The Surveillance Gap Radar Coverage Gap

There is a large area of Minnesota where radar coverage is not available. Aircraft operating in this area have to be above 4,000 feet and sometimes as high as 10,000 feet to be in radar contact. There are 21 airports in this area including two with commercial service: Bemidji and Brainerd. The area also contains Camp Ripley: an Army Airfield with significant training operations and a restricted area northwest of the airfield. In this airspace Instrument Flight (IFR) operations are limited to one in/one out access. Air traffic services to pilots like traffic advisories, flight following and weather advisories are simply not available. We recently had an aircraft go down in this area and it took 3 days to locate the wreckage. There was no chance of rescue, only recovery. If radar coverage was available, the pilot might have been talking with air traffic control and received assistance to prevent the crash. At the very least, the radar tapes would have narrowed the search area significantly.

Background

The Office of Aeronautics has been working with the FAA trying to establish radar coverage to the initial approach fix altitudes in this area for more than a decade. We set state money aside to buy a radar facility in 2001, but the FAA determined that solution was not cost effective. In 2008, FAA offered to perform an alternative analysis to select a technology to address the surveillance gap. Three alternatives were evaluated: radar, ADS-B (Automatic Dependent Surveillance - Broadcast), and WAM (Wide Area Multilateration). All three of these technologies are ground based, line-of-sight technologies, requiring the aircraft to be in sight of the ground station, so the farther away an aircraft is from the ground station, the higher the aircraft has to be to receive service.

Radar

Radar is an old technology. Moreover, the alternatives analysis found that three radar stations would be required to fill the surveillance gap. Radars are expensive to install and maintain and the benefit/cost numbers hadn't improved since 2001.

ADS-B

ADS-B is FAA's chosen technology for replacing radar. The system is made of two parts: a network of ground stations and avionics in the aircraft. The avionics use the WAAS (Wide Area Augmentation System) GPS signals to determine and then broadcast the aircraft's location. The network on the ground receives this signal and processes it into a target for air traffic controllers.

ADS-B supports many of the technologies in the FAA's NextGen air traffic management plan. FAA has installed ADS-B stations in the Gulf of Mexico to support helicopter operations to oil rigs, along the eastern seaboard of the United States and in Alaska. They have a plan to install ADS-B antennas throughout the continental United States by 2014.

The FAA has entered into a contract with ITT to install and operate the network of ADS-B ground stations. This contract mirrors the existing radar coverage so unfortunately, it does not include the surveillance gap in Minnesota. ADS-B Graphic

ADS-B is dependent upon GPS for aircraft position and altitude. The system accepts the aircraft's position based upon a transmission from the aircraft. Since this is a new technology it requires dedicated equipment to be installed in the aircraft to send this signal to the ground. In order for air traffic control (ATC) to use ADS-B for stand-alone air traffic vectoring all aircraft in the area must be equipped with ADS-B. In areas without current radar coverage, installing ADS-B would provide some services to ADS-B equipped aircraft. If a non-ADS-B aircraft had been cleared for an approach, ATC would still have to keep the ADS-B aircraft out of that airspace since they could not see both aircraft and therefore not provide separation between them. The airspace would still be one-in, one-out for IFR operations.

The FAA has just published a new rule that requires ADS-B equipment to be installed in all aircraft operating in Class A, Class B or Class C airspace by 2020. In Minnesota, that will require all aircraft which fly above 18,000 feet or within 30 nautical miles of the Minneapolis-St. Paul Airport (MSP) to be equipped with ADS-B transmitters. While we expect the majority of aircraft to equip for ADS-B, we expect that most operators will delay installing the avionics until close to the deadline. The FAA has predicted that the equipment will come down in price as more units are installed and many owners will want to wait until the bugs are worked out of the initial designs.

The FAA feels that aircraft owners will want to equip to receive the benefits of ADS-B. Flight Information Service - Broadcast (FIS-B) and Traffic Information Service - Broadcast (TIS-B) are two services that will be available as a part of ADS-B. FIS-B provides weather data to the cockpit like NEXRAD weather radar and terminal forecasts. TIS-B provides traffic information that can be displayed in the cockpit. Both of these services require additional avionics to be installed known as ADS-B "In" equipment. These avionics are not required and many operators have already paid to install private weather and traffic systems. It is unknown how many aircraft will equip with ADS-B In or the cost of equipping.

Though GPS outages are expected to be rare, reliance upon ADS-B as the stand alone technology for air traffic control also means reliance upon GPS.

WAM

WAM uses a network of ground based transceivers to determine aircraft positions. How WAM Works Graphic The ground stations interrogate aircraft transponders, and measure the time before a response arrives. This allows each ground station to know its distance from an aircraft. Distances from three ground stations are used to triangulate the aircraft's location. Since the aircraft has to be in sight of three ground stations, the WAM system requires more ground stations than radar or ADS-B. Transponders are currently used to identify and track aircraft in the radar environment of today's air traffic control system. No new equipment needs to be installed in the aircraft for WAM to work. So while the cost to the government is higher for the installation of WAM, there is no cost to aircraft owners.

WAM has been successfully deployed in Colorado. The FAA partnered with the Colorado Department of Transportation to develop and install a WAM system in the mountains west of Denver where radar coverage was poor. Controllers can now see aircraft almost to the ground where before they lost them as soon as they dropped below the mountain peaks. In addition to improvements in service, cutting time and costs for aircraft operators, this system has sped up the search and rescue operations in at least two cases where ATC was able to direct searchers to almost the exact location of the crash.

WAM is an interim technology that can be used now, and which also supports the transition to ADS-B. Where WAM and ADS-B are both available, ATC can see all the aircraft (ADS-B equipped and non-equipped), and therefore provide all the advantages of ADS-B. In the Western Minnesota radar gap, aircraft owners may hesitate to equip early with ADS-B. WAM would encourage early equipage. Once the majority of the fleet has equipped with ADS-B equipment WAM could be turned off or maintained as a back-up to ADS-B.

There are two very good reasons to build WAM. First, WAM works with the existing fleet, no new avionics are required for aircraft, and second, it supports the transition to ADS-B .

Costs

The cost for equipment and installation of the WAM is approximately $4.7M. Recurring costs for maintenance and communications are estimated at approximately $200,000 annually. The pricing information for equipment was obtained from the manufacturer. The Office of Aeronautics has installed, and now owns and maintains 19 ILS, 12 VOR/DME, 3 RCO, and 78 AWOS. The cost of installation, communication, utilities, and maintenance were all estimated by using actual costs incurred by the office for similar services.

Benefits

The Office of Aeronautics conducted benefit/cost analysis with all the resources available to us. These included FAA documents, ASDI (Aircraft Situation Display to Industry) data, MN State Aviation System Plan, Minneapolis Air Traffic Control Center staff, NTSB accident reports, Civil Air Patrol, Camp Ripley personnel, air ambulance operators, and a survey of all Minnesota aircraft owners. Benefits Graphic The benefit cost ratio was greater than 2. Most of the benefits fit into one of two categories: safety or efficiency.

WAM will make the airspace over and around 21 Minnesota airports more efficient. It will reduce delays in getting instrument clearances both for aircraft waiting to takeoff and those needing an instrument approach for landing. Aircraft flying instrument approaches to these airports could receive vectors from Minneapolis Center. Rather than flying the entire instrument approach procedure the aircraft could be vectored to a point in the approach that was more in line with the direction they were coming from. Since instrument approaches are needed when the weather is at its worst, speeding access to airports under these conditions not only saves time and fuel, but also gives an aircraft the best chance of getting safely on the ground when the weather takes an unexpected turn for the worse.

WAM will improve safety and efficiency, allowing ATC to provide services to aircraft that fly below the current radar coverage. Currently ATC requires aircraft below their radar flying on instrument flight plans to fly airways from point to point. The aircraft basically get in line and follow each other along the same route. Where ATC can see the aircraft's position, they can provide separation without having to utilize the airway system and allow aircraft transiting the area direct to their destinations. While these services are available at higher altitudes, many aircraft cannot currently climb to an altitude where radar coverage exists either because of aircraft performance limitations or the hazards of icing at higher altitudes.

Visual flight rules (VFR) traffic will also benefit from WAM. VFR aircraft operating below the current radar coverage are not able to obtain ATC services for flight following. With WAM, aircraft transiting this area under VFR could get flight following from ATC that would aid in traffic avoidance. They could also get assistance with avoiding restricted airspace or steer toward nearby airports if they encounter trouble. In case an aircraft does go down, WAM can speed up the rescue effort by providing a more accurate starting point for the search and may keep a rescue from becoming a recovery.

Benefit at Camp Ripley

Camp Ripley is utilized for a significant amount of training which includes instrument proficiency work. Air Force regulations do not allow aircraft to practice instrument approaches under VFR without terminal radar service available. IFR instruction is performed under a waiver. Increasing the radar coverage in that region would relieve them of their requirement to maintain that waiver. Camp Ripley Photo

One of the missions of the 133rd includes Adverse Weather Aerial Delivery System (AWADS), it's essentially an all weather airdrop capability that allows them to conduct airdrops without any visual references to the ground. They practice this using the drop zone that resides within restricted area, R4301. In order to do this, the guard must maintain a waiver from both the Air Force and FAA. With increased radar coverage in the Brainerd / Camp Ripley area it's possible that these waivers would no longer be required. More importantly the improved coverage would be a significant increase in safety during these operations. As it stands now, the guard descends below radar coverage in the general vicinity Southeast of Little Falls and Minneapolis Center does not see them on radar again until they're climbing on the Northwest side of R4301. If Center could see them, as well as other traffic, during these operations and provide traffic advisories everyone would benefit. One of the biggest fears in conducting AWADS operations on a day where the weather is just good enough to allow for VFR traffic is the potential for a mid-air collision.

Miller Range Control sequences a significant amount of traffic, fixed-wing, rotor-wing, and Unmanned Aerial Systems (UAS) through R4301 yet has no ability to "see" that traffic. Giving Camp Ripley/Miller Range Control a feed off of an increased coverage could potentially give them this ability to "see" all traffic down range, again improving the overall safety of operations within R4301. In addition to R4301 operations, improving the radar coverage in and around Ripley would allow Army helicopter traffic to identify the Final Approach Fix for the ILS RWY 31 approach into Miller, something they cannot do at this time limiting their IFR capabilities.

Benefit to Air Ambulance

North Air Care is a major provider of air medical transport in the Northwestern part of Minnesota. Air Ambulance Photo North Air Care has a total of 8 EMS (Emergency Medical Service) helicopters; 5 are based at airports in Lakeville, Princeton, Redwood Falls, Brainerd and Bemidji. These helicopters are on standby 24 hours a day, 365 days a year. The other 3 helicopters are backup for scheduled and unscheduled maintenance, and also training. In other words 5 helicopters at 5 bases are on active standby at nearly all times.

North Memorial Air Care does approximately 2800 patient transfers annually. By lowering the radar altitude to 3000' North Memorial estimates a 5% to 10% increase in transfers. Most of this gain would be in winter months when they need to fly at lower altitudes to avoid the icing at the higher altitudes.

Questions:

For more information contact Bob Milton at 651-234-7250 or bob.milton@state.mn.us.

WAM Report in PDF Format:

Click here to view the WAM report in PDF format.

Aeronautics & Aviation

Wide Area Multilateration (WAM)