4, 6 and 8 link performance can be accepted. had optional access to spot beam antenna number 2. 51/12 Communication satellites and systems Channel Input Output multiplexer multiplexer 2225 MHz Transmit Global aerials receive aerials Delay 8 Rec No 1 -5 and amp pos'n TWT A equaliser atten Global Rec No 2 TWT B hybrid Rec No 3 Spot 1 Channels 2-12 Rec No 4 same as 1-11 Spot 2 Global Figure 51.9 INTELSAT IV transponder The principal characteristics of the transponder and antenna sub- 4GHz. There is a trend away from transponder power to bandwidth systems were as follows: ratios appropriate to high performance earth stations and towards more powerful transponders suitable for small antenna earth station- 1. Receive system gain to noise temperature ratio (G/T) equal to S. Most radically, transponders and on board signal path switching -17.6dB/K. systems are becoming optimised for digital transmission systems. 2. Up-link power flux density at receive antenna for output TWT On multi-beam satellites, in addition to long term flexibility swit- saturation equal to -73.7dBW/m2 to -55.7dBW/m². ches which enable signal paths to be set up on a semi-permanent 3. Transmit e.i.r.p. at single-carrier saturation: global beam equal basis between a specified up-link beam and a specified down-link to 22dBW per channel; spot beam equal to 33.7dBW channel. beam within a given frequency band, switch matrixes capable of 4. The receive antennas were LH circularly polarised and the reconfiguring beam to beam connections from millisecond to milli- transmit antennas were RH circularly polarised. second are now being taken into use for on-board switched time division multiple access systems (Watt, 1986). Development work By way of contrast, the EUTELSAT II design is very new, the first is also in progress on on-board signal processing equipment which being launched in 1991. The 14GHz band is used for up-links and can demodulate up-linked digital signals, regenerate the digital the 11GHz and 12GHz band for down-links. The system employs wavefor