This paper addresses the problem of demodulating messages received by a low-orbit satellite (altitude between 700 and 800 km) and transmitted by vessels using the Automatic Identification System (AIS). AIS is a Self-Organized Time Division Multiple Access (SO-TDMA) system, in which vessels periodically transmit information (mainly including MMSI – identification code of the ship– and its GPS position). The main application of the actual AIS system is collision avoidance between ships but a satellite reception would lead to a global supervision of maritime traffic, which could be of great interest for a lot of applications (military but also civil applications as fleet surveillance and monitoring).

This paper introduces a new error correction strategy using cyclic redundancy checks (CRC) for a trellis coded system in the presence of bit stuffing. The proposed receiver is designed to simultaneously demodulate, decode and correct the received message in the presence of bit stuffing. It is based on a Viterbi algorithm exploiting the conditional transitions of an appropriate extended trellis. The receiver is evaluated with automatic identification system (AIS) messages constructed with a 16 bit CRC and a Gaussian Minimum Shift Keying (GMSK) modulation. The stuffed bits are inserted after any sequence of five consecutive bits 1 as requested by the AIS recommendation. Simulation results illustrate the algorithm performance in terms of packet error rate. A gain of more than 2,5 dB is obtained when compared to the conventional GMSK receiver.

This paper introduces a new error correction strategy using cyclic redundancy checks (CRC) for a trellis coded system in the presence of bit stuffing. The proposed receiver is designed to simultaneously demodulate, decode and correct the received message in the presence of bit stuffing. It is based on a Viterbi algorithm exploiting the conditional transitions of an appropriate extended trellis. The receiver is evaluated with automatic identification system (AIS) messages constructed with a 16 bit CRC and a Gaussian Minimum Shift Keying (GMSK) modulation. The stuffed bits are inserted after any sequence of five consecutive bits 1 as requested by the AIS recommendation. Simulation results illustrate the algorithm performance in terms of packet error rate. A gain of more than 2,5 dB is obtained when compared to the conventional GMSK receiver.

The automatic identification system (AIS) is a system allowing ships and coast stations to exchange some information by VHF radio. This information includes the identifier, status, location, direction and speed of the emitter. The aim of this thesis is to allow the reception of AIS messages by low Earth orbit satellites without modifying the existing ship equipments. With this system, it becomes possible to know the position of all ships over the Earth. As a consequence, several new services become available, such as global traffic monitoring or determining boat location (for ship-owners). Satellite reception of AIS signals is subjected to a higher noise level when compared to ground level reception. This noise makes classical demodulation and decoding methods unusable. A first contribution of this thesis is to develop new demodulators using error correction methods. These demodulators take advantage of the presence of a cyclic redundancy check (CRC) block in the messages as well as known information about the structure of messages and data. Generalizations of the proposed receiver have also been studied in order to take into account the phase noise of the received signals and the possible collision of messages sent simultaneously by several vessels. The last part of this thesis is devoted to the study of localization methods for ships that do not transmit their location in AIS messages. This localization takes advantage of information contained in the received messages such as the propagation delay and the carrier frequency shift due to the Doppler effect, and a ship movement model.

The automatic identification system (AIS) is a system allowing ships and coast stations to exchange some information by VHF radio. This information includes the identifier, status, location, direction and speed of the emitter. The aim of this thesis is to allow the reception of AIS messages by low Earth orbit satellites without modifying the existing ship equipments. With this system, it becomes possible to know the position of all ships over the Earth. As a consequence, several new services become available, such as global traffic monitoring or determining boat location (for ship-owners). Satellite reception of AIS signals is subjected to a higher noise level when compared to ground level reception. This noise makes classical demodulation and decoding methods unusable. A first contribution of this thesis is to develop new demodulators using error correction methods. These demodulators take advantage of the presence of a cyclic redundancy check (CRC) block in the messages as well as known information about the structure of messages and data. Generalizations of the proposed receiver have also been studied in order to take into account the phase noise of the received signals and the possible collision of messages sent simultaneously by several vessels. The last part of this thesis is devoted to the study of localization methods for ships that do not transmit their location in AIS messages. This localization takes advantage of information contained in the received messages such as the propagation delay and the carrier frequency shift due to the Doppler effect, and a ship movement model.

Le système d’identification automatique (AIS) est un système permettant aux navires de s’échanger certaines informations, dont leur position, par radio. L’objectif du travail réalisé est de permettre la réception des messages AIS par un satellite en orbite basse sans modifier le matériel existant équipant les navires. Il sera ainsi possible de connaître la position de tous les navires à travers le monde. Plusieurs services sont possibles, comme le contrôle maritime ou, pour les armateurs, la connaissance permanente de la position de leurs bateaux. Ce séminaire présentera le correcteur d’erreurs élaboré afin de s’affranchir du niveau élevé de bruit et les méthodes permettant de prendre en compte les propriétés des signaux AIS.