Development of Algorithms for Space Based Automatic Identification System
Naval Research Board
In the recent times terrorism has become a major issue for the state to handle. Major cities along the coastline are highly prone to sea/waterborne terrorist attacks. A case in point is the terrorist attack on Mumbai, in November 2008. Supervising the ships, managing the traffic flow into harbors and preventing accidental collisions of ships over high seas is becoming a Herculean task. In the current scenario because of the increasing maritime traffic there is a need globally to improve the maritime safety and efficiency of navigation, safety of life at sea and protection of the maritime environment. Also knowing the position of every ship in every geographic position would be valuable for vessel traffic monitoring, border control and detection of illegal activity.
The Automatic Identification System (AIS), often called as Universal Ship borne AIS, is used on ships and vessel traffic services (VTS) for identifying and locating vessels by electronically exchanging navigation data with other nearby ships, VTS stations and shore based system. But, the coverage area of the ship borne AIS system is similar to other VHF applications, and is mainly dependent on the range to the horizon from the position of the VHF antenna. A typical value at sea is 40 nautical miles maximum.
A satellite based AIS system is a way of obtaining the above mentioned objectives with a global coverage thus allowing the localization of ships worldwide. However a satellite system calls for several technical issues and operational challenges.
The orbit inclination and the ship message reporting interval distributions have been shown to impact the system performance. This is due to the fact that both aspects modify the total number of ships seen under the satellite antenna FoV per TDMA slot, and thus the number and frequency with which messages collide at the receiver.
The major problem an AIS satellite system faces is the collision of messages coming from different vessels that are not in reciprocal terrestrial radio-frequency visibility but are within the satellite field of view. Indeed, the satellite antenna footprint is much, much larger than the self-organized area of a typical AIS cell that is around 40 nautical miles, and thus includes many vessels which could transmit messages on the same TDMA time-slot. This intra-system interference occurring among vessels that are not in terrestrial radiofrequency visibility is the foremost limiting factor in the detection capability of a satellite-based receiver.
The focus of the research is on the analysis & development of interference cancellation techniques to improve the detection probability. A satellite-based AIS communication system is feasible to detect terrestrial AIS messages from space with an innovative satellite AIS receiver, which takes advantage of the frequency Doppler shift. Frequency difference between two carriers being transmitted by each AIS transceiver, Time of arrival / Phase difference, Randomness of the time slots used for transmission, GPS time reference tagging parameters shall also be considered apart form Doppler frequency for resolving the message collision issue and identify the ships.
Exhaustive simulations have been carried out to study the feasibility of resolving the carriers based on Doppler frequency. Three ships in each time slot of the TDMA frame could be resolved without ambiguity. Further refinement and additional parameter effects are being studied.
Dr. V. Sambasiva Rao
Mr. Mahendra Nayak, Dr. Deepali Koppad, Mr. Sreeprasad.