Abstract
The demand for the mobile communications is growing, as is the trend for the introduction of new services and for these to be available over wide geographical areas. Satellites, due to their inherent ability to support long distance reliable communication, are believed to play a pivotal role in realising such systems at national and international levels. Until the recent past, satellites were used to provide a communication facility to mobile platforms that could afford to carry relatively heavy and complex terminals such as those in maritime use. However, many mobile services for aeronautical and land users have also been introduced since 1990. Land Mobile Satellite (LMS) communications have until now been restricted to low data rates and geostationary orbit (GEO) satellites. Increasingly the demand is for integrated speech and data services. Signal propagation in land mobile conditions is one of the most important design considerations limiting the performance of a satellite based mobile communication system. In order to provide communication services through a viable satellite network, a proper knowledge of link degradation due to the mobile channel conditions is essential. In the land mobile environment, shadowing of the line-of-sight (LOS) signal and multipath propagation can limit the link performance for considerable percentages of time. Satellites in orbits other than the geostationary orbit have, therefore, been proposed for mid-to-high latitude regions in order to provide high elevation angles at the mobiles, thus reducing losses due to blockage, and as a result improve link margin requirements. In addition, to achieve world-wide mobile communications, constellations of low earth orbit (LEO) satellites have been proposed. However, sufficient information on link performance, under reduced shadowing conditions at high elevation angles, does not exist and this is critically important in the planning or implementation of future HEO and LEO based satellite systems. The research work reported here was carried out in an attempt to extend the existing propagation database applicable to high elevation angle land mobile satellite (LMS) channel at the L-band. However, other frequencies corresponding to the S and Ku bands, were also included to generate data at frequencies for likely future applications and enable a direct comparison of propagation characteristics to be made between the bands. In the absence of appropriate satellite source(s), the multiband propagation measurement campaign was undertaken using a helicopter. The helicopter provided a platform which simulated the high elevation angle satellite signal. Different environments such as suburban, wooded and open were considered, and mobile measurements were made at various high path elevation angles in each environment. The statistical analysis of the measurement data provided useful information on LMS channel behaviour and its dependency on the signal frequency, elevation angle and the mobile channel environment. It has been observed that smaller attenuations are experienced as the elevation angle increases or the radio frequency decreases. Significant effect on the channel fading is observed due to variable amounts of shadowing encountered under varying channel environments. Using the results of the measurement campaign described in this thesis, it has been possible to propose a fading model for use in future planning for high elevation paths that will be encountered in both HEO and LEO mobile satellite systems.