What is the purpose of wide area multilateration?
Wide Area Multilateration (WAM) is used as an ATC surveillance system. Like Secondary Surveillance Radar (SSR) it is classified as a cooperative independent surveillance system; the aircraft has to cooperate (i.e. an active transponder) but position is determined independent from data sent from the aircraft (unlike e.g. ADS-B). 

A WAM system relies on Time Difference of Arrival (TDoA) measurements to determine the position of aircraft. At least 4 receivers with synchronised timing are needed to calculate the position of a transmitted signal. In practice WAM systems have a redundancy built in to them and are operated with more than 4 receivers. It can operate purely passively but usually the system will interrogate aircraft transponders to ensure a sufficient update rate can be maintained. This is a downside of the system; with its omnidirectional interrogators it is often considered as using much capacity on 1030 MHz, causing an increased transponder load (every incoming interrogation needs to be processed by a transponder, even if it is intended for another aircraft and no reply is to be given).

The system is used as an alternative to secondary radar. The installation of a receiver is much cheaper than a radar, but the life cycle cost of the surveillance system depends also on site renting costs, communication costs and maintenance cost. Maintenance tends to be cheaper (no rotating radar head), but communication is more expensive (more lines). Overall, WAM is a cost effective alternative to SSR.

WAM is able to achieve very high accuracy and high update rates when a sufficient number of sensors are installed. For example, German Air Traffic control DFS operates the Precise Approach Monitoring system in the Frankfurt TMA with 36 receivers and achieves and accuracy better than 25 meters with an update rate of 1 second. 

In mountainous terrain, valleys can be covered by installing receivers on mountain peaks, which is much cheaper than covering it all by radars. Austrian Air Traffic Control AUSTROCONTROL operates a nationwide system consisting of 60 receivers.

In terrain that is difficult to access for maintenance or where there is no infrastructure to install radars WAM may also provide a solution. On the North Sea both Dutch (LVNL) and British (NATS) Air Traffic Control operate WAM systems to provide air taffic surveillance services to helicopters operating on the oil platforms. Receivers are installed on oil platforms.

WAM is used around the world, other deployments include for example Afganistan, Australia, Denmark, Ghana, Namibia, USA and many more. 

The system can at the same time provide an ADS-B data stream from received ADS-B messages and can add extra security checks by comparing measurements of the data sources.