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Cassini DST - Deep Space Transponder

Cassini's deep space transponders (DSTs) were acquired from Motorola. They perform the functions of both receiver and transmitter, keeping the distant spacecraft in touch with Earth. Cassini carries two DSTs, one of which is operating at all times. The receiver hardware within the DST demodulates the incoming 7.2 GHz uplink, and the exciter (transmitter) hardware modulates the outgoing 8.4 GHz downlink. The health and performance of the DST are reported to engineers via Cassini's downlinked engineering telemetry.

The DSTs are real workhorses. Aside from modulating science and engineering information onto the downlink signal as telemetry, and demodulating commands from the uplink signal, the DST is used for tracking and navigating the spacecraft using Doppler and ranging. The DSTs are also used for carrying out radio science experiments such as occultations, solar corona studies, general relativity experiments, and gravitational wave searches. For these purposes, an extremely stable downlink reference frequency is required. Unfortunately, it is impossible for any spacecraft to carry the massive equipment necessary to generate and maintain such frequency stability. So the DST generates its downlink phase-coherent with a very stable uplink signal it receives from the DSN. The Cassini spacecraft also carries its own ultra-stable oscillator (USO) to provide a reference for generating downlink with as much stability as possible when an uplink is not available.

The DST input comes from Cassini's 4m aperture high-gain antenna, or from one of its two low-gain antennas, as selected by commandable waveguide transfer switches (WTSs). The output of the DST exciter is supplied to the X-band traveling wave tube amplifiers, which feed the selected antenna via WTS.