Aireon Space Based Aircraft Position Validation And Multilateration Solution

In April 2019 Aireon's space-based Automatic Dependent Surveillance-Broadcast (ADS-B) system went operational and began providing global air traffic surveillance data to customers around the world. This system is driven by the Aireon Hosted Payload (AHP) on each of the Iridium NEXT satellites, whose constellation allows for constant global ADS-B coverage. Leveraging the nature of the satellites' polar orbits and the size of the payloads' footprints has allowed Aireon to develop an Independent Position Validation (IPV) algorithm which can verify the fidelity of the ADS-B reported positions independently of aircraft's GPS position. This solution relies on the regions of overlapping coverage between adjacent payloads which allows for the ADS-B transmissions to be detected by more than one payload. From these simultaneous detections a Time Difference of Arrival (TDOA) calculation can be done which is core to the position validation solution. Part of the Independent Position Validation algorithm creates an independent reference track of an aircraft's position that is driven by these TDOA measurements. Given the size of the AHP's coverage footprint, regions of single satellite coverage only exist near the equator, in fact above 43 degrees and below -43 degrees latitude all aircraft are always covered by at least two satellites. But even at the equator where the satellite coverage is the most spread out, an aircraft has an 80% chance of being covered by more than one satellite. Using this coverage, the reference track is continually updated and compared to the reported positions by the aircraft to determine a validation state that is reported to the downstream users of the data. The user can then decide to record, flag, or remove any reports that are deemed suspect, so they do not interfere with air traffic control operations. As currently designed the Independent Position Validation algorithm only reports the validity of the reported ADS-B position although it has knowledge of its own derived position. Given the significant overlap of satellites and the high probability of detection in some airspaces a clear opportunity has been identified. An extension of this algorithm would be to fully calculate the horizontal position of the aircraft using a Satellite Wide Area Multilateration (SWAM) solution. A typical multilateration system requires highly accurate position and timing information for the receivers, generally done via site surveys and highly accurate clocks for the ground stations. Working with Iridium, the Aireon system receives Precision Timing and Position (PTP) messages which provide the timing and position accuracy required to perform the TDOA calculations; leveraging this information, a multilateration solution could be developed to determine the position of an aircraft using any received data without prior knowledge of the aircraft position. Given the spatial distribution of the satellites, and their long range, this guarantees two significant advantages to such a multilateration solution. First is that the Geometric Dilution of Precision (GDOP) will always be low because the receivers will always be far outside the target location, allowing for improved accuracies. Second is that one could augment a sparse ground network of ADS-B receivers, which already exist in many locations around the world, with any number of the payloads to provide a significant benefit to the Vertical Dilution of Precision (VDOP), a leading lack of uncertainty in terrestrial based multilateration solutions attempting to calculate an aircraft's altitude. This augmentation approach would allow for a terrestrial system to reduce the number of required ground stations, and significant cost, to provide a multilateration solution which could also calculate an accurate full three-dimensional position rather than just a horizontal position.