Coherent receiver for AIS satellite detection

AIS is a universal shipborne Automatic Identification System recently introduced for traffic monitoring and safety at sea. Ships exchange navigational information to identify and localize vessels in a short range to avoid collisions. AIS signals use Gaussian Minimum Shift Keying (GMSK) for transmission. There is a growing need to develop a system which provides global maritime surveillance in order to better handle hazardous cargo transports, to improve safety, and to counteract illegal operations and terrorism. The possibility of extending the system in space to detect ships at long distances from the shore is considered in this paper. Since AIS was not originally designed for space detection, two challenges arise: first, received signals have a large Doppler due to the speed of the satellite, and second, two or more signals from different ships can be received at the same time due to the large coverage of the satellite. In this paper, we proposed an innovative coherent demodulator based on the Maximum Likelihood Estimation (MLE) and a Viterbi decoder to receive AIS signals from space and handle the large Doppler frequency shift. The MLE is used efficiently to estimate the frequency and the channel response.