Originally Posted by
Old Akro
VH OWN did not have ADS-B and yet it showed up on both Flightaware & Flight radar.
Then they - FR24 & FA - must have been using MLAT, given they don't get a feed of RADAR data from Airservices.
From FR24:
MLAT
In some regions with coverage from several
FR24-receivers we also calculate positions of non-ADS-B equipped aircraft with the help of Multilateration
(MLAT), by using a method known as Time Difference of Arrival (TDOA). By measuring the the time it takes to receive the signal from aircraft with an older ModeS-transponder, it's possible to calculate the position of these aircraft. Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work. MLAT coverage can only be achieved above about 3,000-10,000 feet as the probability that four or more receivers can receive the transponder signal increases with increased altitude.