Automatic Dependent Surveillance -^sBroadcast (ADS-B) is a new standard adopted by many aviation authorities worldwide and offers a great leap forward in aircraft surveillance capabilities. More information is made available through ADS-B than with conventional primary and secondary radar technologies and, as ADS-B does not require expensive radar infrastructure, the cost of implementation is far lower. Whether ADS-B will allow substantial decommissioning of conventional radar is the subject of ongoing debate; however, most air traffic management service providers see the benefits in the implementation of a relatively low cost flight tracking technology. Countries with vast tracts of land or mountainous terrain not viable for conventional radar see the new technology as highly cost beneficial. Other countries, with significant investment in conventional radar, such as the United States, see major benefits and savings in operations and maintenance costs in upgrading to ADS-B technology. Many countries, including Australia, are fully committed to ADS-B technology. Several working groups, consisting of members of the international aviation community, are participating in the development of aircraft avionics and ground systems standards, such as RTCA Special Committee 186, which developed the ADS-B Minimum Aviation System Performance Standards (MASPS). In the United States, the FAA has successfully used ADS-B in Alaska as part of the Capstone program, and is now planning to introduce the system into the Continental United States (CONUS). Other aviation authorities have also embraced the use of wide area multilateration coupled with ADS-B, including Taiwan, New Zealand, Mongolia, and Austria. A major part of the business case for ADS-B is the savings generated through the decommissioning or reduced reliance on conventional radar systems. In order to maintain some form of redundancy, a networked, system wide back-up surveillance system is required. One possible method is to use a Wide Area Multilateration (WAMLAT) network that independently calculates aircraft position. Multilateration can also provide surveillance augmentation during the transition to 100% ADS-B aircraft equipage and can be used for the monitoring the quality of ADS-B reports. This paper presents analyses of the performance of a WAMLAT network as a back-up for ADS-B. Included is an assessment of the overall independence and availability of WAMLAT in the event of ADS-B and GNSS satellite failure scenarios.
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