Spacecraft transponder noise

4.6 Spacecraft transponder noise

Transponders accept an input carrier signal and produce an output signal at a different frequency. The process is phase-coherent; that is, for every N integer cycles of the input there are M integer cycles of the output (with M/N being the transponding ratio). When this condition is achieved, the transponder is operating normally and produces an output that is “locked” to the input signal. The Cassini spacecraft has two transponders⁶ . The standard flight transponder (“KEX”) accepts the X-band uplink and produces two phase-coherent outputs, one at f_X $⋅$ 880/749 (= X-band downlink frequency) and another at f_X $⋅$ 3344/749 (= Ka1 downlink frequency), where f_X is the frequency of the X-band uplink signal observed at the spacecraft. These signals are amplified, routed to the spacecraft high-gain antenna, and transmitted to the earth. Another flight unit, the Ka-band Translator (“KaT”), accepts a Ka-band uplink signal and produces a phase coherent signal with frequency f_k $⋅$ 14/15 (= Ka2 downlink frequency), where f_k is the Ka-band signal frequency observed at the spacecraft.

Pre-launch test data of transponders similar in design to Cassini’s KEX showed negligible (i.e. $<$ 10^–15) frequency noise. Prelaunch tests of the KaT similarly showed negligible frequency noise ( $≃$ 10^–16 at $τ$ = 1000 s), provided the received Ka-band uplink signal was relatively strong (greater than about –127 dBm, see [1 , 19 , 74 , 22 ]) at the input to the KaT.

Appropriate linear combinations of the frequency time series of the three downlinks can be used to estimate and remove (at the Fourier frequencies of interest) downlink and round-trip plasma noise [63 , 62 , 64 , 59 ] in GW observations. For example, the downlink plasma noise time series can be determined by forming f_X-(880/3344) f_Ka1, which is independent of FTS noise, antenna mechanical noise, spacecraft buffeting, and GWs (since these are all nondispersive.) These plasma corrections were also used with good success by Bertotti, Iess, and Tortora [30 ] in a precision test of relativistic gravity involving Cassini tracking very close to the sun.