Radio science experiments use the spacecraft radio and ground antennas (such as those of NASA's Deep Space Network) as the science instrument. These experiments measure the refractions, Doppler shifts, and other modifications to radio signals that occur when the spacecraft is "occulted" by (i.e., passes behind) planets, moons, atmospheres, and physical features such as planetary rings. From these measurements, scientists can derive information about the structures and compositions of the occulting bodies, atmospheres, and rings.
The Radio Science Subsystem (RSS) will use the spacecraft X-band communication link as well as the S-band downlink and the Ka-band uplink and downlink to study the compositions, pressures, and temperatures of the atmospheres and ionospheres of Saturn and Titan; the radial structure of Saturn's rings and the particle size distribution within the rings; and body and system masses within the Saturnian system. It will also be used to search for gravitational waves coming from beyond our solar system. The RSS consists of a Ka-band traveling wave tube amplifier, a translator, an exciter; an S-band transmitter; and various microwave components. For more information on these components, click on their names. (NOTE: Before proceeding, you may wish to review the description of the Radio Frequency Subsystem (RFS).
The purpose of the Ka-band traveling wave tube amplifier (K-TWTA) subassembly is to amplify the signals going to the high-gain antenna to the power level necessary for them to be received by the Deep Space Network. The K-TWTA subassembly consists of the a traveling wave tube (TWT) and an electronic power conditioner (EPC). The nonredundant TWT is the signal amplifier. It can be commanded into a standby mode for low d.c. power consumption. The EPC converts d.c. power from the Power and Pyrotechnic Subsystem (PPS) to the voltages required to operate the TWT. It can power the TWT in the standby mode or power down the TWT in case of TWT or EPC fault detection. The EPC also supplies engineering telemetry to the RFS and provides direct-access signals.
The Ka-band translator (KAT) subassembly receives the 34-GHz uplink carrier from the high-gain antenna and translates it by a factor of 14/15 for retransmission back to the DSN. The phase and phase-shift of the signal are used for the actual science observations and measurements. The KAT contains a power converter that allows it to operate from the 30-volt d.c. power bus. It also supplies engineering telemetry data to the RFS and provides for direct access.
The Ka-band exciter (KEX) generates a stable 32-GHz signal and provides an RF power combiner to combine the RF signal generated by the KAT with its own signal. It is powered by the 30-volt spacecraft bus, and it supplies telemetry data to the RFS and provides direct access.
The S-band transmitter (SBT) receives a 115-MHz signal from the RFS, multiplies it by 20, amplifies it to 10 watts, and supplies the resultant signal at approximately 2290 MHz to the high-gain antenna. This carrier signal is used for radio science experiments. The transmitter contains a power converter to allow operation from the 30-volt power bus, and it supplies telemetry data to the RFS and provides direct access.
The microwave components consist of two band pass filters (BPFs) and waveguide components. BPFs are filters that allow only certain wavelengths of microwave energy to pass, with all other wavelengths being blocked. In this case, the BPFs permit reception and transmission of the Ka-band signals using different antenna feed polarizations and provide isolation between the transmit and receive frequencies. Waveguide is essentially tubing of precise dimensions that provides a path for microwave energy of a certain wavelength. In this subsystem it is used for all Ka-band microwave component interconnections.