The Namibia Wam system was supplied by Era, of the Czech Republic, and employs 36 widely separated and unmanned ground stations that listen for aircraft transponder replies to radar interrogations and then retransmit those replies to a central processing station. In Namibia, which has no radar, selected listening posts transmit pseudo, but otherwise identical, radar interrogations.
At the processing station, the slightly different times of the replies’ arrival at a number of listening posts, plus their individually coded aircraft identification, are used to calculate the aircraft’s position. The process is virtually instantaneous, and both the listening and processing stations can handle a large number of replies without saturation. Depending on the geometry of the station layouts, accuracy is usually better than 200 meters (656 feet), and in smaller local configurations it can be less than 50 meters (164 feet).
The Wam system can also process incoming ADS-B transmissions from overflying aircraft, and while in those cases the aircraft’s ADS-B message includes its GPS position, the processing station still calculates the aircraft’s position as a routine integrity check of the satellite system.
Tobias Günzel, deputy director for aviation, administration and navigation for the Namibian Civil Aviation Directorate, said, “As a result of the Wam system performance we have already begun to deploy an extension of the Wam system around Walvis Bay and in Caprivi, with additional extensions being contemplated to provide Wam redundancy for Hosea Kutako International Airport at Windhoek, as well as Eros Airport. This would increase the number of ground stations to fifty-four.”
Neighboring countries may also link up with the existing Wam system of Namibia to get Wam coverage reaching across national borders. This is an already proven Wam practice, and allows seamless and economical expansion of national systems into regional configurations, free of the gaps or unnecessary overlaps that often occur in closely spaced radar installations. Following Namibia’s successful initiative, Wam could be expected to expand over Africa’s airspace, and thereby reduce procedural ATC while increasing capacity and throughput.
Wide area multilateration might well be called one of air traffic management’s best kept secrets. While the FAA has not made significant investments in the technology, other than the smaller multilateration systems employed with ASDE-X airport radar, other nations have readily adopted local- and wide-area systems, with an advanced configuration serving the RNP approach to Queenstown Airport in the mountainous terrain of New Zealand’s South Island.