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A Wide Area Multilateration (WAM) system derives an aircraft's position by calculating the Time Difference of Arrival (TDOA) of aircraft signals from 3 or more ground antennas. WAM systems can display aircraft with conventional Mode-C or Mode-S transponders. Although WAM is not considered to be a dedicated ADS-B system, it can be modified to use the 1090ES equipment on some aircraft.
WAM's greatest strength is its ability to provide very accurate aircraft surveillance without requiring additional aircraft avionics. Its principle disadvantage is that it requires a minimum of 3 to 4 times as many ground stations to cover the same area as 1090ES or 978MHz UAT ADS-B.
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