European GNSS Independent ATM Surveillance Satellite Constellation

The present invention provides a method for space-based air traffic surveillance, completely independent of Global Navigation Satellite Systems (GNSS) and the aircraft self-position reporting. It uses MODE-S squitter radio signals that are recurrently transmitted by the aircraft and does not require any additional equipment on board of the aircraft or any change in the operation procedures.

The GNSS independence distinguishes the proposed solution from the Automatic Dependent Surveillance-Broadcast (ADS-B) which is reporting the aircraft position based on the GNSS information. It uses uses MODE-S squitter radio signals that are recurrently transmitted by the aircraft and which contain a static 24-bit aircraft unique identifier (known as ICAO aircraft address) and other known bits (such as parity bits for the static address). This static content of the MODE-S signals is particularly important as it allows (satellite) receivers to significantly improve the signal detection as well as timing and carrier frequency estimation of the incoming signal, hence improving the accuracy of post-processing algorithms for geo-locating the aircraft.

The aircraft position is computed in real-time by applying known techniques such as multi-lateration techniques relying on Time difference of arrival (TDOA) or Frequency difference of Arrival (FDOA). Multiple satellites will receive each aircraft MODES-S squitter (broadcasting) signals either with some time delays or with relevant frequency difference due to Doppler, proportional to the distance of aircraft from each satellite, making it possible to calculate aircraft position geometrically, simply based on the difference in the time of arrival or the frequency Doppler shift of the signal received by multiple satellites.

In order to determine the position of the aircraft, at least four independent observations of the same MODE-S signal is required. However, a higher number of observations (if available) can be used to improve the accuracy of the position and resolve the dilution of precision caused by the geometrical location of satellites with respect to the target aircraft.

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