Novel demodulator for spaceborne AIS reception with increased robustness against noise

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Novel demodulator for spaceborne AIS reception with increased robustness against noise

Jurado Gallardo, Maria Angeles / Ruy, Ghislain

The Automatic Identification System (AIS) is a traffic monitoring system used on ships to identify and localize vessels in a short range for collision avoidance. Gaussian Minimum Shift Keying (GMSK) is used for modulation of AIS signals. The interest of receiving AIS messages at long distances from the shore has grown recently, for example, to improve security or detect ships carrying illegal operations. This can be accomplished by satellite AIS detection, since the Very High Frequency (VHF) terrestrial communications have a short range. This presents mainly two challenges because the system was not originally designed for this kind of channel. The first one is the large Doppler range of the received signals due to the satellite speed. The second one is the reception of signals from two or more ships at the same time due to the broad satellite coverage. In this paper, an advanced GMSK demodulator based on several FM discriminators in parallel is presented to handle the large Doppler shift problem and with a robust performance against noise. Synthesized and real signals are used to study the performance of the proposed receiver.