Space-Based Detection of Spoofing AIS Signals Using Doppler Frequency

The Automatic Identification System (AIS) is a self-reporting system based on VHF radio to transmit a vessel's identity, position, speed, heading and other parameters to improve maritime domain awareness. However, AIS information can be programmatically spoofed by terrorists or other criminals, who often choose to masquerade as innocent civilians and exploit the vulnerabilities of military and civilian infrastructures for their purposes. Therefore, detecting and localizing a spoofing AIS ship become a critical and challenging issue for maritime security. This paper presents an algorithm to detect and geolocalize a spoofing AIS emitter using space-based AIS signals with its Doppler frequency. With an AIS signal sensor on a fast orbiting satellite, the measured AIS Doppler frequency of an AIS emitter can be used to define a double-napped cone of which the satellite is at its vertex and satellite velocity coincides with its axis, such that the theoretical Doppler frequency derived from the radial velocity to the AIS emitter matches the measured Doppler frequency. All such matches can only lie on either cone extending out from the satellite, which cuts the Earth's surface in two curves, so we know that the AIS emitter must lie somewhere on these curves. Two such AIS Doppler frequency measurements for the same stationary AIS emitter produce two valid curves which intersect at the position of the AIS emitter. Multiple Doppler frequency measurements can be used to better estimate the position fix of an AIS emitter, hence determine the spoofing AIS ship if the estimated position fix unreasonably differs from the position carried in its AIS message. A set of formulas are derived which relate an AIS emitter position to its Doppler frequency measurements.