The Study On Network Performance Of Shipborne Automatic Identification System (AIS)

AIS is a kind of ship surveillance system, belonging to the wireless mobile communication network category, and the network performance is a key problem in the research and application of AIS. Based on the project "Study on The Basic Theory AIS Based on STDMA Technology" organized by the Ministry of Communications of the P.R.C., a broad and deep research on the network throughput, monitoring capacity and monitoring radius is developed in this paper and some useful results are obtained.Firstly in this paper, the development course of AIS and its basic operation principles are introduced briefly, and also the communication principle of STDMA is analyzed. Then, based on these principles, regard AIS related international standards set up by IMO, IEC and ITU as basis, the network performance of AIS, such as throughput, monitoring capacity and monitoring radius were profoundLy studied. By the analysis and study of the network performance of AIS, several conclusions were drawn as follows:1) When got fully organized and the network load is lower than 60.5%, the throughput of AiS is almost 100%. While not fully organized and the network load may not lower than 60.5%, then according to the distance to AIS stations, the monitoring area of AIS is divided into 3 zones. From close-by to far-off, the 3 zones are: protected zone, discrimination zone and garbled zone. When load is relatively high, even 500%, very good performance of throughput can still be achieved in the protected zone, thus guaranteeing that the close quarter things can be caught. So, the performance of AIS's network throughput can satisfy navigation demand.2) Under general situations, the monitoring capacity of AIS is 814 ships in the VTS waters and 246 ships in non-VTS waters. So, the application of AIS can satisfy request for the quantity of shipping in each waters at present. But in the future or in the waters with very heavy traffic it is necessary to adopt corresponding measures.3) The monitoring range of AIS is mainly limited to line-sight characteristic of the radiowave of VHF frequency. So, it is mainly related to the height of AIS's antenna. Generally, the ship AIS's monitoring range is 25 nautical miles and the VTS AIS's monitoring range is 45 nautical miles. In the initial stage of ships' meeting, this range may be impacted a certain one, but it would be less than 1 nautical mile. In addition, were the synchronization time source lost, the monitoring range might aIso be impacted. But within 3 hours after the loss of time source, it could still maintain its normal values. So, under the normal situation, the monitoring range of AIS can meet the demands of navigation.