Multipath in an aeronautical satellite system

The multipath problem as encountered in a civil aeronautical satellite system is analyzed. A model for the multipath reflection is developed and the performance of the communications channels through the satellite is evaluated. The aeronautical satellite system will be used to control air traffic over the Atlantic and Pacific oceans. Aircraft will fly at a height of 10 to 20 km. Severe impairment of the communications performance is expected due to reflection of radiowaves from the surface of the sea and inadequate protection by the aircraft antenna against multipath. A simple theoretical model for reflections from the sea is developed. Under the assumption that only very simple coding schemes can be used to improve the bit error rate for digital transmissions the application of frequency diversity, space diversity and time diversity is discussed. It is shown that frequency diversity is more suitable for the link from the satellite to the aircraft while space diversity can be used for the link from the aircraft to the satellite. The application of time diversity does not reduce the bit error rate by a great amount because of correlation between consecutive bits. It is shown that large delays between messages are necessary to make time diversity attractive. Time diversity would therefore lead to coding by blocks and to storage requirements.