The June 2016 presentation was on aviation electronics – High Tech Radio – the use of Multilateration (MLAT) and Automatic Dependent Surveillance Broadcast (ADS–B) surveillance technology. This presentation was given by Rob Williams, KJ4LWN (left, below), with AMRAD president Terry McCarty. Photo courtesy of Jerry Whelan, AB3SX.
There is apparently a Raspberry Pi project that will allow us to build a receiver processor of these radio signals. See: http://flightaware.com/adsb/piaware/about
I ) MLAT:
- MLAT-enabled receivers keep the FlightAware server up to date on which Mode S aircraft are being received. When a particular aircraft’s in-flight data is being received from three or more MLAT receivers, the onboard server requests sufficient Mode S data from the receiver to sync the time and multilaterate the position of an aircraft. An MLAT-enabled feeder will use approximately 50 Kbps of upstream bandwidth while actively contributing to MLAT flight tracking.
- Multilateration is a proven technology that has been in use for many decades. It was developed for military purposes to accurately locate aircraft — many of which did not wish to be “seen” — by using a method known as Time Difference of Arrival (TDOA).
- Multilateration employs a number of ground stations, which are placed in strategic locations around an airport, its local terminal area or a wider area that covers the larger surrounding airspace.
- These units listen for “replies,” typically to interrogation signals transmitted from a local Secondary Surveillance Radar (SSR) transponder or a multilateration station. Since individual aircraft will be at different distances from each of the ground stations, their replies will be received by each station at fractionally different times. Using advanced computer processing techniques, these individual time differences allow an aircraft’s position to be precisely calculated.
II ) ADS–B:
- That is the new system that allows aircraft and ground stations determines aircraft position via satellite navigation using radio to periodically broadcast real-time data which can be received by air traffic control ground stations as a replacement for currently used radar. ADS-B data bursts can also be received by other aircraft to provide situational awareness and allow directed self separation. ADS-B is “automatic” in that it requires no pilot or external input and it is “dependent” in that it depends on data from the aircraft’s navigation system.
- The system relies on two avionics components—a high integrity GPS navigation source and a data link (ADS-B unit). ADS-B data links operate at 1090 MHz (essentially a modified Mode S transponder) or at 978 MHz. The plan is to fly below 18,000 feet (5,500 m) using the 978 MHz link and to use the 1090 MHz frequency above that altitude.. The 978 MHz transceivers transmit data in a single, short duration transmission whereas the 1090 MHz transceiver breaks the ADS-B position report message into 5 packets ADS-B is seen as a valuable technology to enhance airborne collision avoidance systems (ACAS).