VUSAT ( HAMSAT ) Example Part 1

 Concerning the up and down frequencies of VUSat, HAMSAT. HAMSAT : downlink centrally : 145.90 MHz, uplink centrally : 435.25 MHz. Bandwidth 0.060 MHz. If the values are correct, we find these two passbands : 435 MHz : 435.220.000 - 435.280.000 ( uplink band ) 145 MHz : 145.870.000 - 145.930.000 ( downlink band ) and a mode B calc factor T ( f,T osc ) : T (1) = 581.15 INV transponder T (2) = 289.35 NON INV transponder Some simple mathematical manipulations deliver the 'doppler shift compensated translation equations' ( INV transponder ) A1 and B1 ..... A1 RX=T-[TX+DS70]+DS02 B1 TX=T-[RX-DS02]-DS70 DS02 is 2m doppler and DS70 70cm doppler value. DS02=DS70/3 and DS70=DS02•3 ..... An example : suppose there is a transmission to the satellite on 435.25 MHz ( centrally, the middle of the band ). The LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ). The coming back signal ( downlink ) on the ( ground station ) antenna has now a frequency : formula A1 RX=T-[TX+DS70]+DS02=581.15-[435.25+(-0.008)]+(-0.008/3)=145.90533etc. MHz. Doppler values concerning LEO satellites are changing less if AOS or LOS situations exist ( concerning overhead passes, also see http://www.qsl.net/vk3jed/doppler.html ). Therefore it could be convenient to make such a calculation as above. It is possible to construct a SATELLITE MODE B FREQUENCY TRANSLATION CHART ( included doppler shift compensation ) if you want. See the small formulas I worked out. Some very good results ( examples ) : see THE SATELLITE handbook ( ARRL edition ISBN 0-87259-658-3 4-3fig4.2 & 4-4fig4.3 ..... ). Do not think that an error in the result ( example above, the downlink frequency ) exists. Perhaps you expected a much lower value because the satellite was moving away. Because of the combination concerning this transponder type ( mode B INV ) and the doppler effects the current result exists ! Concerning a NON INV transponder, T ( f,T osc ) = 289.35 : A2 RX=[TX+DS70]-T+DS02 B2 TX=[RX-DS02]+T-DS70 An example : suppose someone transmits on 435.25 MHz ( centrally, the middle of the band ) and the LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ). The coming back signal ( downlink ) on the ( ground station ) antenna has now a frequency : formula A2 RX=[TX+DS70]-T+DS02=[435.25+(-0.008)]-289.35+(-0.008/3)=145.88933etc. MHz. Remark : both uplink frequencies are the same ( in example 1 and 2 ), centrally, in the middle of the band, but the downlink signals are different. The signal, arriving at the transponder antenna, is not centrally anymore ( because of the doppler influences ! ). The signal is not anymore existing in the middle of the band. Both transponder types give exclusively the same result if Fup=Fcenter. If, after the launch, now the frequencies are not quite correct ( because of 'drift' of the oscillatorfrequency ), is it easy to find a new T-factor. Perhaps you work with special equipments with very good tolerances. After measuring and analysing you can find the new T-factor. Do not forget the doppler values which act during the measurements !

 VUSAT ( HAMSAT ) Example Part 2