Airplanes, that only have a 1090 MHz Mode-S transponder, without an ADS-B function do not send data about their position. Without extra effort these aircrafts are not visible for display in an ADS-B system or flight tracking network.For this, the continuous and automatically transmitted status messages of the Mode-S transponder of an aircraft can be used for mathematical calculations, the so-called multilateration method (MLAT) for position determination.
The position of these aircraft (without ADS-B) can be detected by the use of at least 3 receivers for a common reception area. For this purpose, the receivers transmit all received Mode-S telegrams via Internet/LAN to a central MLAT server, which very precisely calculates the position data from the transit times of the receive signals (TDOA method). The position data can be returned to each receiver via the data channel.
The positional accuracy depends on the number of receivers for the receiving area. The more data from different recipients is available, the more accurate the result. The update performance on the receivers is approximately one second. Our server typically calculates 1-5 locations per aircraft and second, of which one per second will be sent back to the receivers. The latency is arround 1.5 seconds.
The significant advantage of our Radarcape ADS-B receivers is a nanosecond accuracy of the timestamps due to GPS synchronisation. Due to this MLAT calculations within the Radarcape system do not need beacon transmitters or reference ADS-B aircraft for correct operation. In difference to other receivers in the price class, our receivers are equipped with a high precision GPS synchronized clock and the timestamps have an accuracy of app. 50nS. As a result, each individual calculation already has excellent accuracy, and can be used without averaging. Furthermore, it is also possible to make MLAT calculations for ADS-B aircraft if their data are doubtful or should be checked. The bandwidth requirement of the required data network is only a few kilobytes/sec upstream and is also scalable.
Independence from ADS-B and for verification purposes
Airfield noise measurements