Wide Area Multilateration (WAM) surveillance systems provide a robust and effective
means of surveilling aircraft; these systems can operate both actively (interrogating aircraft for
necessary messages) or passively, using available messages of opportunity, to determine
aircraft positions. WAM systems do not require any information from aircraft transmissions other
than a form of identification to group multiple received messages from the same transmission
together, resulting in a surveillance solution that is independent of any onboard aircraft systems,
such as Global Navigation Satellite System (GNSS). The methods and calculations required to
perform multilateration are standardized in the aviation industry and have been implemented by
many systems around the world.
