Satgen261 Satellite Spotter Part 9 by GM4IHJ 26 March 94 BID of this msg is SGEN261 Please use this BID if you retransmit this msg Multi Frequency satellites. There are several good reasons for simultaneous transmission on several frequencies from a satellite . Satellites which do this can be divided into two classes. (i) Those which transmit on frequencies which are harmonic multiples of of a common base frequency. (ii) Those which use several adjacent channels at regular frequency spacing. Class (i) sats are almost all LEO Low Earth Orbiters. Class (ii) sats include most commercial and military geosats but exclude almost all LEO Low Earth Orbiters. The reasoning behind these two distinct classes of satellite , has class (i) being used to overcome some of the propagation probles of LEO sats, most of which use VHF, often over the Arctic or Antartic where propagation from space can be very irregular. By contrast class (ii) frequency channelisation is the standard terrestrial separation of multiple users into exclusive channels. Table 1 List of Typical Multi Frequency Satellite Types Type Typical Frequencies Class No. of sats Reason for multifreq Elint 153.42 204.56 i 6 or 7 3 x 51.14 and 4 x 51.14Mhz S+F 153.6 204.8 i 2 or 3 3 x 51.2 and 4 x 51.2 MHz Navsat 149.982 399.952 i About 25 3 x49.994 and 8 x49.994MHz Experimental 137.676 i 2 or 3 Polar Bear + Hilat Arctic " 413.028 435.974 PropTest multiples x 22.946 NOAA 136.77 137.62 1707.2 i 4 or 5 Better reception on S Band Fleetsats 230.4 to 269.95 ii 5 or 6 Mostly 50Khz channel spacing Intelsats 3704 to 4198 ii Many Usually 36 channel 36 MHz BW Marisat 1535 to 1542 ii 3 or 4 About 200 KHz channel spacing GPS 1227.6 mil 1575.42 civil 24 Mil chan is more accurate Glosnav 1250 mil 1603.5 civil ? Mil chan is more accurate Note... One of the signals on the class (i) multifrequency sats is sometimes simple unmodulated carrier. This permits maximum recovery of propagation data, and/or , very stable frequency diversity reception whereby the two signals can be frequency changed in separate mixers fed by a common harmonic oscillator to get the same base frequency Eg Elint 51.14 Electronic comparison of the two signals at the same frequency then gives a marked improvement in overall reception and recovery of much better phase data in the modulation. Prior to 1990 several class (i) type multifrequency sats used the second (higher) frequency signal which was unmodulated carrier, to demodulate the other signal. Unfortunately the higher signal did not prove to be totally free of fading or scintillation, as had been anticipated , so this practice has now been dropped. A closely similar practice has been retained with the Navsats and the Elints because each of these type do give better results with two signals, albeit never perfect. 73 GM4IHJ @ GB7SAN