Telemetry, Tracking, and Command Subsystem


This diagram shows more detail of the spacecraft Transponders (Tx1 and Tx2). The diagram includes the other relevant units in the TTC subsystem so that you can still see the Transponders' connectivity with other units.

The transponders provide the sole source of communication between the spacecraft and the Earth. Each transponder consists of a Receiver which receives commands and ranging signals, a modulator, and both an X-band and S-band transmitter which produce the radio frequencies needed for transmission or telemetry and ranging signals to the Earth.



The spacecraft receivers provide acquisition and tracking of the uplink signal, demodulation of telecommands and ranging signals, provide a reference frequency to the transmitters to allow coherent operation, and provide a Conscan signal derived from the coherent Automatic Gain Control (AGC), to the spacecraft Attitude and Orbit Control Subsystem (AOCS). They also deliver lock status signals to the AOCS, the transmitters, and command decoders. The reciver best-lock frequency is 2111.607254 MHz.


S-band Transmitter

Ulysses data is normally transmitted via the X-band downlink; the S-band capability was only used during the early part of the mission when the spacecraft was relatively close to the Earth. However, S-band is occasionally used as well, in order to gather Radio Science data. The spacecraft nominal S-band frequency is 2293.148148 MHz (±35kHz). The S-band transmitter is currently turned off, but was used during Jupiter Flyby to investigate the structure of the Io Plasma Torus, and used for Solar Coronal Sounding during oppositions and conjunctions. The S-band transmitter has also been used as part of the Gravity Wave Experiment.


X-band Transmitter

The X-band transmitter receives telemetry inputs from the Data Handling Subsystem (DHS). The DHS provides NRZL convolutionally coded telemetry data which is PSK modulated onto a squarewave subcarrier. The X-band exciter nominal frequency is 8408.209876 MHz (±128kHz). The X-band exciter output in routed to one of the two Travelling Wave Tube Amplifiers (TWTAs) for amplification before transmission via the High Gain Antenna.