GSO satellites orbit the Earth in the equatorial plane with the same angular velocity as the Earth at a height of about 36 000 km above the equator. Geostationary satellites therefore appear stationary to an earth-bound observer and a single satellite can provide continuous service to roughly one third of the Earth's surface (but excludingpolar regions above ± 75 degrees of latitude). The maximum distance the satellite can "see" on the Earth's surface is about 42 000 km and means the propagation delay for a single hop via the satellite (once up and down) can be up to 280 ms. Geostationary satellites also move about their nominal positions causing a small but noticeable Doppler shift on both the feeder and mobile links.For personal and vehicle terminals, handover during a call between GSO satellites is unnecessary because the coverage is static and wide. However handover might be contemplated for aircraft terminals between different spot beams of the same satellite. In the latter case there is practically no difference in path length to consider. Within Europe, GSO satellites appear at low elevation angles. For the geographical latitude of 50°North (e.g.Luxembourg), the satellites reach approximately 31° elevation as a maximum when the satellite is due South: either East or West of this position the elevation slowly reduces. Frequent blocking of the line-of-sight signal therefore occurs from trees, buildings and hills. GSO satellites can work in such a shadowed environment but the satellite Equivalent Isotropic Radiated Power (EIRP) would have to be increased by 15 dB to 20 dB or more depending on the coverage required.This could be achieved but has a serious impact on the size and cost of the satellite. In addition, assuming that the mobile EIRP is limited, the satellite receive sensitivity also has to increase and this can only be done with very large spacecraft dish antennas. For this reason, only very low bit rate services (i.e. paging, alerting, etc.) might be viable under such circumstances until the user moves to a more favourable position to receive a voice call.
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