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Doppler shift during S-band test
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Doppler shift during S-band test

November 11, 2009
The principle of the test is to send from the Goldstone DSN station a radio signal at either of the two S-band frequencies of the Huygens relay link (nominally 2040 and 2098 MegaHertz). Frequently the transmitted signal had to be adjusted such that the frequency of the received signal would match the one that will be received by the Orbiter's HGA during the Probe's descent in Titan's atmosphere. As everything moves with respect to each other (the spacecraft, Titan, the Earth) there are a number of different contributions to the total Doppler shift.

During the Probe mission, the relative distance between the Probe and the Orbiter will be decreasing at the rate of 5.65 kms-1 (the Orbiter velocity with respect to Titan). This will induce a positive (received frequency increase) Doppler shift of about 38 kHz on the 2 gigahertz signal.

During the S-band tests, the Cassini/Huygens spacecraft moves away from the Earth at about 40 kms-1 and the Earth rotation induces an additional differential velocity between the spacecraft and the ground station.

It all amounts to an average negative (received frequency decrease) of about 260 kilohertz on chain A at 2040 Megahertz and 268 kilohertz on chain B at 2098 megahertz. During the whole duration of the test, the Doppler shift varies by about ± 2 kilohertz.


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