mitter (Ts) and radiated to another earth station; this assembly of equipment in the satellite is called a transponder. At that second earth station the down-link Geostationary Satellite carrier is received and amplified (Re), down-converted (D/C) and demodulated (D) so that the signal can be passed to its destination. In practical systems the communication path is often duplex, the return signal channel usually being transmitted through the same Figure 51.6 Some typical communication satellite orbits Transponder in satellite (not strictly to scale) Most non-geostationary satellites periodically disappear below the horizon; if unbroken operation is required, several or many satellites must be deployed in suitable configured orbits and the communication links between earth stations must be transferred Up-link Down-link from one satellite to another at intervals ranging from several times per day down to several times per hour. Furthermore the motion of these satellites, relative to the earth stations, causes the earth station to earth station transmission time to vary with time, leading to synchronisation problems with high speed digital systems and dop- pler shift of signal frequencies in analogue systems. However, the GSO has two serious disadvantages, which it shares Signal Signal with some elliptical orbits. The free space transmission loss be- tween a satellite and an earth station and the transmission delay, source destination earth station to earth station, are both very large. Figure 51.7 shows Earth station A Earth station B how these quantities vary with the altitude of the satellite above the Earth's surface (the transmission loss also varies with carrier fre- quency and the figure shows values for 10GHz). Figure 51.5 The basic satellite communication chain Lower transmission loss for low altitude offers big advantages for some applications. The communication chain 51/11 25 mm/h 1-way transmission delay (mSec) 100 mm/h 5 mm/h Maximum Oxygen Minimum 1.25 mm/h Water vapour Satellite altitude (thousands of