Effect of Physical environment on Satellite System

In a mobile satellite system, the physical environment around a mobile terminal exerts a crucial influence. The environment has two effects it disturbs the stability of radio signals, setting a boundary on throughput, and limits the physical size of a terminal.

From a radio propagation viewpoint, maritime and aeronautical environments are more benign than land, where levels of fade and multipath fluctuations are significantly higher. Propagation conditions degrade progressively as the environment changes from highways to urban and generally gets worse as antenna size is reduced. Throughput and type of service are directly related to fading characteristics. Intermittent deep fades can break radio links, causing discontinuity to real time services, while multipath influences modulation and coding schemes. Providing a higher link margin mitigates such impairments, but increases demands on satellite power. The Doppler Effect, introduced by relative motion between satellite and mobile, is yet another impairment and in this respect, aeronautical terminals exhibit the worst degradation.

The physical space available on a mobile limits the weight and size of the antenna and transceiver. A terminal mounted on a ship can be larger than a terminal on an aircraft; a vehicle mounted terminal is smaller than either, whereas a hand held terminal must be small enough to fit in a user's pocket or purse. Demands on spacecraft power and sensitivity increase as terminal size is reduced and throughput increased.