Characteristics of sea reflection fading in maritime satellite communications

The Japanese domestic experimental mobile satellite system (EMSS) has been studied to establish high quality channels for small vessels with compact and low-gain antennas. The present international maritime satellite communication system (INMARSAT) has also been studied for the next generation to extend services for small ships, for which the present ship terminals including the antenna systems are too large and too heavy. By using a low-gain antenna, however, multipath fading caused by sea reflections will become a serious problem in designing the new system. To take effective countermeasures to the fading effect, it is very important to get a better knowledge of the fading phenomena caused by sea reflections. From a standpoint of maritime satellite communications, experiments measuring the amount of fading were performed under various sea conditions and in two frequency bands. The wave height, which is considered the most important experimental parameter, was measured objectively by a wave rider buoy. The experimental results show that the amount of fading depends on a wave height and radio frequency and, as the most interesting result, as a wave increases above a certain level, the amount of fading decreases. These results are also discussed theoretically.