Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:vg1.rss::2.1
NAME RADIO SCIENCE SUBSYSTEM
TYPE
DESCRIPTION Instrument Overview =================== Voyager Radio Science investigations at the giant planets utilized instrumentation with elements both on the spacecraft and at the DSN. Much of this was shared equipment, being used for routine telecommunications as well as for Radio Science. The performance and calibration of both the spacecraft and tracking stations directly affected the radio science data accuracy, and they played a major role in determining the quality of the results. The spacecraft part of the radio science instrument is described immediately below; in most cases, the description applies equally well to both Voyager 1 and Voyager 2 and it applies throughout the Voyager mission. The description of the DSN (ground) part of the instrument may be found in DSN context products. Instrument Specifications ========================= The Voyager spacecraft telecommunications subsystem served as part of a radio science subsystem for investigations of the giant planets. Many details of the subsystem are unknown; its 'build date' is taken to be 1977-09-05, the launch date for Voyager 1. Instrument Id : RSS-VG1S Instrument Host Id : VG1 Pi Pds User Id : UNK Instrument Name : RADIO SCIENCE SUBSYSTEM Instrument Type : RADIO SCIENCE Build Date : 1977-09-05 Instrument Mass : UNK Instrument Length : UNK Instrument Width : UNK Instrument Height : UNK Instrument Manufacturer Name : UNK Instrument Overview =================== The spacecraft radio system was constructed around a redundant pair of transponders. Each transponder was equipped with an S-band receiver (2115 MHz nominal frequency) and transmitters at both S-band (2295 MHz nominal) and X-band (8415 MHz nominal). Compared with S-band, X-band was less sensitive to plasma effects by a factor of about 10; use of both frequencies coherently on the 'downlink' allowed estimation of plasma content along the radio path. Use of X-band also significantly improved the quality of radio tracking data for gravity investigations. The transponder generated downlink signals in either 'coherent' or 'non-coherent' modes, also known as 'two-way' and 'one-way', respectively. When operating in the coherent mode, the transmitted carrier frequency was derived coherently from the received uplink carrier frequency with a 'turn-around ratio' of 240/221 at S-band and (11/3)*240/221 at X-band. In non-coherent mode the transmitted frequency was controlled by an on-board oscillator; the X- and S-band remained coherent in the ratio 11/3. A single Ultra-Stable Oscillator (USO) was used during radio occultations; it provided stabilities several orders of magnitude better than the conventional crystal oscillators, which were part of each transponder. Stability of the Voyager USO was specified in terms of its Allan Deviation -- the fractional frequency deviation from linear drift. Over 10 minute periods, the Allan Deviation ranged from 10**-12 to 4 10**-12 for integrations of 1-10 sec. Long-term fractional drift of the oscillator was about 5 10**-11 per day. Although the oscillator was hardened, there were discontinuities in the drift when the spacecraft passed through the radiation belts of the outer planets. Equivalent X-band microwave frequencies for the Voyager 1 USO during key events were: 8,414,995,272.530 Hz (Titan occultation) 8,414,995,272.376 Hz (Saturn occultation) Multiplying by 3/11 yields the S-band frequency. Traveling wave tube or solid state amplifiers boosted the transponder output. Output powers of 9 and 26 watts could be selected at S-band; the choices at X-band were 12 and 22 watts. The signals were radiated via a 3.66 m diameter parabolic high gain antenna (HGA). The transmit boresight gain of the HGA was 36 dB at S-band and 47 dB at X-band. The half-power half-width of the antenna beam was 0.32 degrees at X-band and 1.1 degrees at S-band. Transmit polarization was right-hand circular at S-band and either right- or left-hand circular at X-band. A Low-Gain Antenna (LGA) was mounted on the feed structure of the HGA and radiated approximately uniformly over the hemisphere into which the HGA pointed. It was used during maneuvers, spacecraft anomalies, and at other times when the HGA was not appropriate. For receiving, the S-band HGA gain was 35 dB at 2115 MHz and the polarization was right-hand circular. The receiving system noise temperature was approximately 2000 K, the carrier tracking loop bandwidth was 18 Hz, and the ranging channel noise bandwidth was 1.5 MHz. Operational Considerations ========================== Descriptions given here are for nominal performance. The spacecraft transponder system comprised redundant units, each with slightly different characteristics. As the transponder units ages, their performance changed slightly. More importantly, the performance for radio science depended on operational factors such as the modulation state for the transmitters, which cannot be predicted in advance. The performance also depended on factors which were not always under the control of the Voyager Project. Spacecraft receivers were designed to lock to the uplink signal. Without locking, Doppler effects -- resulting from relative motion of the spacecraft and ground station -- could result in loss of the radio link as the frequency of the received signal drifted. A series of failures in the Voyager 2 receivers left that transponder unable to track the uplink signal. Beginning in April 1978, Doppler shifts were predicted and the uplink carrier was tuned so that Voyager 2 would see what appeared to be a signal at constant frequency (to an accuracy of 100 Hz. There were no such problems with Voyager 1. During deep occultations by the giant planets, the bending angle resulting from refraction exceeded 10 degrees in some cases -- well beyond the half power beamwidth of the spacecraft antenna. In those cases, the pointing of the HGA was adjusted so that it followed a 'virtual' Earth and maximum signal strength could be sustained. These 'limb-track' maneuvers were critically dependent on accurate timing during the encounter. To protect against Voyager 1 timing errors at Titan (primarily from uncertainties in the radius and position of the satellite), no limb-track was attempted during ingress, and a fixed antenna offset was used during egress. Fortunately, timing was accurate enough that useful data were obtained from each event. Although the spacecraft radioisotope thermoelectric generators were not dependent on solar flux for power, their output decayed as the Voyager spacecraft moved outward through the solar system. During encounters with the outer planets, caution was required in budgeting power and the high-power mode could not be used for the radio transmitters. Calibration Description ======================= Prior to and during some encounter sequences, the spacecraft was commanded to execute a 'mini-ASCAL' maneuver. The HGA was moved slightly above the Earth line then slightly below the Earth line. The procedure was repeated to the left and right of the Earth line so that a 'cross-hair' pattern was mapped out. During the maneuver, the amplitude of the carrier signal was measured carefully. Analysis of the results showed whether the HGA was pointed accurately and, if not, allowed estimation of the error magnitude and direction. Prior to and after encounters, the spacecraft frequency reference was switched to the USO for several hours and the carrier signal was monitored using equipment at the DSN. These 'USO Tests' were used to calibrate the frequency and frequency drift of the USO. USO tests were particularly important before and after the spacecraft entered a severe radiation environment since the radiation typically damaged the crystal and changed its characteristics slightly. Platform Mounting Descriptions ============================== The centerline of the bus was the roll axis of the spacecraft; it also served as the z-axis of the spacecraft coordinate system with the high-gain antenna (HGA) boresight defining the negative z-direction. The HGA boresight was also defined as cone angle 0 degrees and as azimuth 180 degrees, elevation 7 degrees. The Low-Gain Antenna (LGA) was mounted on the feed structure of the HGA and radiated approximately uniformly over the hemisphere into which the HGA pointed. Instrument Section / Operating Mode Descriptions ================================================ The Voyager radio system consisted of two sections, which could be operated in the following modes: Section Mode ------------------------------------------- Oscillator two-way (coherent) one-way (non-coherent) RF output low-gain antenna high-gain antenna Selected parameters describing NASA Standard Transponder (NST) performance are listed below: Oscillator Parameters: S-Band X-Band Two-Way Transponder Turnaround Ratio 240/221 880/221 One-Way Transmit Frequency (MHz) 2296. 8415. Nominal Wavelength (cm) 13.06 3.56 RF Output parameters: S-Band X-Band RF Power Output (w) 9 or 26 12 or 22 Low-Gain Antenna: Half-Power Half Beamwidth (deg) UNK Gain (dBi) UNK EIRP (dBm) UNK Polarization Circular High-Gain Antenna: Half-Power Half-Beamwidth (deg) 1.1 0.32 Gain (dBi) 36 47 Polarization RCP RCP or LCP ACRONYMS AND ABBREVIATIONS ========================== dB decibel dBi dB relative to isotropic dBm dB relative to one milliwatt deg degree DSN Deep Space Network JPL Jet Propulsion Laboratory K Kelvin LCP Left-Circularly Polarized MHz Megahertz RCP Right-Circularly Polarized RF Radio Frequency S-band approximately 2100-2300 MHz sec second SLE Signal Level Estimator UNK unknown X-band approximately 7800-8500 MHz
MODEL IDENTIFIER
NAIF INSTRUMENT IDENTIFIER
SERIAL NUMBER not applicable
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Woo, R., Spacecraft Radio Scintillation and Solar System Exploration, Wave Propagation in Random Media (Scintillation), Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 1993.