Instrument Information
| INSTRUMENT_TYPE | "RADIO SCIENCE" |
| INSTRUMENT_DESC |
Instrument Overview =================== The Radio Science investigations on Cassini were unique in that they utilized instrumentation with elements both on the spacecraft and on the ground. The spacecraft element was further distinguished in being distributed among several subsystems on the Cassini Orbiter. Cassini Radio Science can be regarded as a solar-system-sized instrument observing at microwave frequencies, with one end of the radio path on the spacecraft and the other end at the NASA Deep Space Network (DSN) stations on the ground. The Radio Science 'instrument' operated in two fundamental modes, depending on whether the microwave optical path had one or two legs. For 'two-way' measurements, the 'uplink' signal from the ground could be a single carrier at either X-band (7.2 GHz) or Ka-band (34 GHz); or both carriers could be transmitted at the same time. The spacecraft radio equipment then acted as a repeater, collecting the carrier signal with the spacecraft High Gain Antenna (HGA), transforming it to one or more 'downlink' frequencies (2.3 GHz, 8.4 GHz, or 32 GHz), amplifying and re-collimating it, and sending it back to Earth. The returned signal was detected using DSN ground receiving equipment, amplified and downconverted, and recorded for later analysis. Uplink signals were generated by the DSN exciter, using the local frequency and timing system as a reference. At Launch and Cruise, this reference was a Hydrogen Maser. Note: in the future, these masers could be combined with a Compensated Sapphire Oscillator (CSO) to meet Radio Science requirements for increased stability. The uplink signals were amplified, radiated through feed horns, and collimated by a large parabolic ground antenna, which was continuously aimed at the Cassini spacecraft. The actual transmission frequencies could be adjusted to allow the spacecraft receivers to lock to the uplink signals and to compensate, in finite steps, for the main part of the Doppler effect between the Earth and the Cassini Orbiter. For one-way measurements, the signal source was on board the Cassini Orbiter. The output from an extremely stable on-board reference oscillator (the Ultrastable Oscillator, or USO) was transformed to downlinks at S-band (2.3 GHz), X-band (8.4 GHz), or Ka-band (32 GHz) by elements in the Radio Frequency Subsystem (RFS) and Radio Frequency Instrument Subsystem (RFIS). These signals were amplified and radiated through the HGA toward Earth. After passing through the medium of interest (plasma, rings, a neutral atmosphere, or gravitationally curved space), the perturbed signal was collected by a DSN antenna, amplified and downconverted, and recorded for later analysis. The spacecraft part of the Cassini Radio Science instrument is described immediately below; that is followed by a description of the DSN (ground) part of the instrument. Instrument Overview - Spacecraft ================================ On the Cassini Orbiter, the Radio Science instrument was encompassed in the Radio Science Subsystem (RSS). RSS was really a virtual subsystem comprising elements from three physical spacecraft subsystems, two of which had other functions to perform. The subsystems that participated in RSS were the RFIS, the RFS, and the Antenna Subsystem. Specifications included: Instrument Id : RSS Instrument Host Id : CAS Pi Pds User Id : UNK Instrument Name : RADIO SCIENCE SUBSYSTEM Instrument Type : RADIO SCIENCE Build Date : UNK Instrument Mass : UNK Instrument Length : UNK Instrument Width : UNK Instrument Height : UNK Instrument Manufacturer Name : UNK ----- F2 -------- F2 ----- | |<----------------------------------| | | ---------------- | X-BAND | | LGA1| | | F3 | X-BAND | X-BAND| F3 |DIPLEXER| F3 | LGA2| | |--------->| HYBRID | TWTA |------>| |--------->| | | | ---------------- -------- ----- | | F4 ---------------- F5 ------ F5 ----- F5 ----- | |--------->| | |--->| Ka- |--->| |--->| | | | | | | | BAND | | BPF | | | | |--------->| Ka- | Ka- |--->| TWTA |--->| |--->| | | DST | F3 | BAND | BAND | F7 ------ F7 ----- F7 | | | | |EXCITER| HYBRID | | | | | | | | F7 ---------- F8 | HGA | | | | | |<----| Ka-BAND |<---------| | | | ---------------- |TRANSLATOR| | | | | F4 ------------- ---------- F6 | | | |-------------->| S-BAND |------------- | | | TRANSMITTER | | | | | ----- ------------- ----- | | | USO | ----- ----- (a) ----- -------- ----- | | | | | HGA | | | ---------------- | X-BAND | | LGA1| | | F3 | X-BAND | X-BAND| F3 |DIPLEXER| F3 | LGA2| | |--------->| HYBRID | TWTA |------>| |--------->| | | | ---------------- -------- ----- | | F4 ---------------- F5 ------ F5 ----- F5 ----- | |--------->| | |--->| Ka- |--->| |--->| | | | | | | | BAND | | BPF | | | | |--------->| Ka- | Ka- | | TWTA | | | | | | DST | F3 | BAND | BAND | ------ ----- | | | | |EXCITER| HYBRID | | | | | | | | ---------- | HGA | | | | | | | Ka-BAND | | | | | ---------------- |TRANSLATOR| | | | | F4 ------------- ---------- F6 | | | |-------------->| S-BAND |------------- | | | TRANSMITTER | | | | | F4 ----- ------------- ----- | |<----| USO | ----- ----- (b) Fig. 1: Configuration of the Cassini Orbiter Radio Science Subsystem for (a) two-way operation and (b) one-way operation. The RFS comprised the USO, DST, X-Band TWTA, and X-Band Diplexer. The RFIS comprised the Ka-Band Exciter, Hybrid, Ka-Band TWTA, BPF (Band Pass Filter), Ka-Band Translator, and S-Band Transmitter. The Antenna comprised the HGA, LGA1, and LGA2. In (a) F1 is the DST receiver channel frequency and Fk is the KAT VCO frequency; in (b) F1 is the DST exciter channel frequency. Then the other frequencies are as follows: F2 = 749*F1 (~7.2 GHz; X-band up) F3 = 880*F1 (~8.4 GHz; X-band down) F4 = 12*F1 (~115 MHz; internal reference) F5 = 3344*F1 (~32 GHz; Ka-band down) F6 = 240*F1 (~2.3 GHz; S-band down) F7 = 294*Fk (~32 GHz; Ka-band down) F8 = 315*Fk (~34 GHz; Ka-band up) Radio Frequency Subsystem |
