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 usedfor 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; unless noted otherwise, 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 follows. Because the DSN was continually changing, that description has been tailored to each Voyagerencounter.


Instrument Specifications - Spacecraft
======================================
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. Except for hardware failures, noted below, theVoyager 1 and Voyager 2 spacecraft subsystems were identical.

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 - Spacecraft
================================
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 transmittersat both S-band (2295 MHz nominal) and X-band (8415 MHz nominal).
Compared with S-band, X-band is 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 thequality 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' of240/221 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 conventionalcrystal 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 [ALLAN1966]. 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 spacecraftpassed through the radiation belts of the outer planets.
Equivalent X-band microwave frequencies for the Voyager 1 USO during key events were (multiplying by 3/11 yields the S-band frequency):
8,414,995,272.530 Hz (Titan occultation) 8,414,995,272.376 Hz (Saturn occultation)
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 22watts.

The signals were radiated via a 3.66 m diameter parabolic high gain antenna (HGA). The HGA 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 theHGA 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 2000K, the carrier tracking loop bandwidth was 18 Hz, and the ranging channel noisebandwidth was 1.5 MHz.

More information can be found in [ESHLEMANETAL1977].


Science Objectives
==================
Science objectives fell into two broad areas of investigation -- those that could be met using high-precision radiometric data (sometimes known as 'tracking' data) and those that could be met from studying characteristics of the radio signal after its interaction with an atmosphere, plasma, ring particles, or other intervening medium. The tracking data were fundamental to inferring the gravitational forces on the spacecraft and relativistic effects along the radio path; both the measured time delay during a two-way transmission and the Doppler shift were used. Investigators seeking knowledge of atmospheric structure, spatial and size distributions of ring particles, and velocity of the solar wind measured amplitude, frequency (and phase), and polarization of the radio signals which were captured by Earth receiving systems. There are, of course,investigations which use both types of data.


Gravity Measurements--------------------
The frequency of the downlink carrier signal was precisely measured to determine the magnitude of the Doppler shift caused by acceleration of the spacecraft as it passed near either a single body or a system of bodies. Since the magnitude of the Doppler shift is related to the gravitational field strength, the mass of the body (or bodies) can be determined. If the radius of the body is known (as fromcalibrated images), the density can be calculated.

Doppler and range tracking measurements yield accurate spacecraft trajectory solutions. Simultaneously with reconstruction of the spacecraft orbit, observation equations for the central mass, low order coefficients for the field,and a small number of ancillary parameters can be solved.
Measurements of the gravity field provide significant constraints on inferences about the interior structure oftarget bodies.

The Pioneer 10 and 11 spacecraft came closer to Jupiter than Voyager, so there was no net improvement in the Jupiter mass estimate from Voyager. But Voyager probed the Galilean satellites at closer range, and better mass estimates were obtained. The Voyager encounters with Saturn, in conjunction with the close flyby of Pioneer 11, yielded a mass estimate comparable to that of Jupiter along with several low-order zonal harmonic coefficients. Voyager 2 was targeted for a close encounter with Miranda, an inner satellite of Uranus; that, combined with long tracking arcs through the Uranian system, yielded the first good estimates of masses for the five largest satellites and an improved mass estimate for Uranus itself. The Voyager 2 very close near-polar flyby with Neptune yielded estimates for the zonal harmonic coefficients J2 and J4 in addition to estimates for the massof both Neptune and Triton.


Atmospheric and Ionospheric Radio Occultation Measurements----------------------------------------------------------
Atmospheric measurements by the method of radio occultation contribute to an improved understanding of structure, circulation, dynamics, and transport in atmospheres of remote planetary bodies. These results are based on detailed analysis of the radio signal received from the spacecraft as it enters and exits occultation by the planet. Three phases of an atmospheric investigation may be defined. The first is to obtain vertical profiles of atmospheric structure (temperature and pressure in the neutral atmosphere and electron density in the ionosphere) with emphasis on large- scale phenomena. During this stage, it is necessary to know the mean molecular weight of the atmosphere; for Voyager the hydrogen-helium mixing ratio could be determined for each planet using the radio data in conjunction with Voyager IRIS data. Second is to investigate absorption at various levels in the atmosphere -- such as by methane. Third is to study details of the structure, such as result from propagation of buoyancy waves within a neutral atmosphere or from alignment of charged particles along magnetic field linesin an ionosphere.

Retrieval of atmospheric profiles requires coherent samples of the radio signal that has propagated through the atmosphere, plus accurate knowledge of the antenna pointing and the spacecraft trajectory. The spatial and temporal coverage in radio occultation experiments are determined by the observing geometry, including the spacecraft trajectory. For deep atmospheres, changes in antenna pointing may be required to compensate for refractive bending by the atmosphere. At Jupiter and Saturn both diametric and grazing occultations were obtained using the two Voyager spacecraft; measurements were obtained at both equatorial and polarlatitudes. Voyager 1 also obtained profiles for Titan.
Voyager 2 continued to Uranus and Neptune, and also obtainedoccultation profiles at Triton.


Radio Measurements on Planetary Rings-------------------------------------
Radio occultation measurements of planetary rings are carried out using procedures similar to those employed for atmospheric occultations. Although absorption by ring particles must be considered, the dominant effect on strength of the directly propagating signal is believed to be conservative scattering-- that is, scattering which disperses the signal in direction without significant absorption. Profiles of received signal strength can be inverted to yield the radial distribution of ring material. Doppler spreading of the signal scattered in the near-forward direction can be used to infer the particle size distribution, especially when measurements at the twoVoyager radio wavelengths are combined.

Ring occultations were planned and observed using Voyager 1 at Saturn and Voyager 2 at Uranus. Measurements were carried out at Neptune using Voyager 2, but no rings or arcs were detected using the radio system. A post-encounter search fora radio ring occultation at Jupiter was unsuccessful.

Voyager 2 also carried out an oblique forward scattering experiment during its Saturn encounter. The spacecraft high- gain antenna was deflected from the Earth direction so that it illuminated the ring system; but no scattered signal wasdetected.


Solar Conjunction Experiments-----------------------------
Solar conjunction experiments were conducted to improve understanding of the structure and dynamics of the solar corona and wind, to improve understanding of relativistic effects when radio waves propagate near the Sun, and to testthe different elements of the radio science subsystem.
Approximately once per year, each Voyager spacecraft appeared to pass behind the solar disk, as seen from Earth. Radio waves propagating between Voyager and Earth stations were refracted and scattered (scintillation) by the solar plasma [WOO1993]. Intensity fluctuations can be related to fluctuations in electron density along the path, while Doppler or phase scintillations can be related to both electrondensity fluctuations and also the speed of the solar wind.
Many plasma effects decrease as the square of the radio frequency; plasma effects are about an order of magnitudestronger at S-band than X-band.


Experimental Relativity-----------------------
The gravitational field of the Sun causes a time delay on signals that propagate near the Sun of approximately 300 microseconds. Although previous tests had verified the effect to an accuracy of a few percent, Voyager measurements could be conducted annually and at two frequencies, allowingseparation of plasma effects.

Gravitational fields of the gas giant planets also affected radio signals by causing them to have apparent frequencies lower than predicted. The change in frequency is related to the mass of the planet. By measuring the change in frequency as the spacecraft approached the planet, a value for the mass could be calculated. This value could then be compared with the mass derived from two-way tracking data. The spacecraft Ultra-Stable Oscillator was used for these measurements; two-way transmissions have nearly canceling frequency shifts as the signal travels to the spacecraft and then returns. The dual frequencies available from Voyager allowed correction for plasma effects along the radio path, but calibration for radiation damage to the USO during encounters was moredifficult.


Operational Considerations - Spacecraft
=======================================
Descriptions given here are for nominal performance. The spacecraft transponder system comprised redundant units, each with slightly different characteristics. Astransponder units age, their performance changes 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 alwaysunder 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. Unfortunately, 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 atconstant frequency (to an accuracy of 100 Hz).

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 in 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 wasaccurate 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 notbe used for the radio transmitters.


Calibration Description - Spacecraft
====================================
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,approximately the error magnitude and direction.

Prior to and after encounters, the spacecraft frequency reference was switched to the USO for several hours and thecarrier 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 damagedthe crystal and changed its characteristics slightly.


Platform Mounting Descriptions - Spacecraft
===========================================
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 theHGA pointed.


Principal Investigators
=======================
The Radio Science Team Leader through the Jupiter encounters was Von R. Eshleman. The Team Leader for the Saturn, Uranus,and Neptune encounters was G. Leonard Tyler.


Instrument Section / Operating Mode Descriptions - Spacecraft
=============================================================
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 (no information available) 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 240/221 880/221One-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

Instrument Overview - DSN
=========================
Three Deep Space Communications Complexes (DSCCs) (near Barstow, CA; Canberra, Australia; and Madrid, Spain) comprise the DSN tracking network. During the Voyager-Saturn era each complex was equipped with several antennas (including at least one 64-m and and one 26-m antenna), associated electronics, and operational systems. Primary activity at each complex was radiation of commands to and reception of telemetry data from active spacecraft. Transmission and reception was possible in several radio-frequency bands, the most common being S-band (nominally a frequency of 2100-2300 MHz or a wavelength of 14.2-13.0 cm) and X-band (7100-8500 MHz or 4.2- 3.5 cm). Transmitter output powers up to 400 kw wereavailable.

Ground stations have the ability to transmit coded and uncoded waveforms which can be echoed by distant spacecraft. Analysis of the received coding allows navigators to determine the distance to the spacecraft; analysis of Doppler shift on the carrier signal allows estimation of the line-of-sight spacecraft velocity. Range and Doppler measurements are used to calculate the spacecraft trajectory and to infer gravityfields of objects near the spacecraft.

Ground stations can record spacecraft signals that havepropagated through or been scattered from target media.
Measurements of signal parameters after wave interactions with surfaces, atmospheres, rings, and plasmas are used to inferphysical and electrical properties of the target.

The Deep Space Network is managed by the Jet Propulsion Laboratory of the California Institute of Technology for theU.S. National Aeronautics and Space Administration.
Specifications include:
Instrument Id : RSS-VG1S Instrument Host Id : DSN Pi Pds User Id : N/A Instrument Name : RADIO SCIENCE SUBSYSTEM Instrument Type : RADIO SCIENCE Build Date : N/A Instrument Mass : N/A Instrument Length : N/A Instrument Width : N/A Instrument Height : N/A Instrument Manufacturer Name : N/A
For more information on the Deep Space Network and its use in radio science investigations see the reports by[ASMAR&RENZETTI1993], [ASMAR&HERRERA1993], and [ASMARETAL1995].
For design specifications on DSN subsystems see [DSN810-5].
For an example of use of the DSN for Radio Science see[TYLERETAL1992].


Subsystems - DSN
================
The Deep Space Communications Complexes (DSCCs) are an integral part of the Radio Science instrument, along with other receiving stations and the spacecraft Radio Frequency Subsystem. Their system performance directly determines the degree of success of Radio Science investigations, and their system calibration determines the degree of accuracy in the results of the experiments. The following paragraphs describethe functions performed by the individual subsystems of a DSCC.
This material has been adapted from [ASMAR&HERRERA1993]; foradditional information, consult [DSN810-5].

Each DSCC includes a set of antennas, signal processing equipment, and communication links to the Jet Propulsion Laboratory (JPL). The general configuration is illustrated below; antennas (Deep Space Stations, or DSS -- a term carried over from earlier times when antennas were individuallyinstrumented) are listed in the table.

-------- -------- --------
| DSS 11 | | DSS 12 | | DSS 14 |
| 26-m | | 26-m | | 64-m |-------- -------- --------
| | |
| v v| --------------
------->|COMMUNICATIONS|
| CENTER |
--------------
|
v---------- ---------
| NETWORK | | JPL |
|OPERATIONS| | CENTRAL |
| AND |<-->| COMM |
| CONTROL | | TERMINAL|
| CENTER | ---------
----------

GOLDSTONE CANBERRA MADRID
Antenna CALIFORNIA AUSTRALIA SPAIN-------- ---------- --------- --------
26-m DSS 11 DSS 44 DSS 62 26-m DSS 12 DSS 42 DSS 61 64-m DSS 14 DSS 43 DSS 63 Developmental DSS 13


DSCC Transmitter Subsystem--------------------------
Two transmitters were available at 64-m antennas; output power of the first could be adjusted over the range 0.2-20 kW, while the second could be adjusted over 10-100 kW. Nominal tuning range was 2100-2120 MHz with the -1 dB points at 2110and 2118 MHz.

Only the 0.2-20 kW transmitter was available at 26-m antennas.
Tuning range was the same.



Multi-Mission Receiver (MMR)----------------------------
The Multi-Mission Receiver provided four channels of data foroccultations studies during the Voyager encounters at Saturn.
A programmable local oscillator/synthesizer was used to keep the signal as close to the center of a 10 MHz IF filter as predictions would permit. The output was sent to the Radio Science Subsystem for sampling and recording. Filter bandwidths for S-RCP and S-LCP ring occultations and scattering observations were 50 kHz; the correspondingbandwidths for X-RCP and X-LCP were 150 kHz.

For signals with narrower spectral ranges, the 10 MHz IF output was mixed to 100 kHz where filters as narrow as 100 Hzcould be applied.


DSS Radio Science (DRS) Subsystem---------------------------------
The Radio Science Subsystem sampled output from the MMR and recorded it on high-speed analog video tape for later conversion to computer compatible tape (CCT) formats. Sample rates for the Voyager 1 Titan and Saturn encounters were 300ksps on all receiver outputs.

Narrower filters and lower sampling rates could be selectedfor special purposes.


DSS Frequency and Timing Subsystem----------------------------------
Frequency and timing were provided by three references: a rubidium standard, a hydrogen maser, and a cesium beam standard. Precisions are shown in the tables below:
Reference Frequency Stability Integration Time----------------- ------------------- ----------------
Rubidium Standard 5 parts in 10^12 1 second 5 parts in 10^13 100 seconds 5 parts in 10^13 1000 seconds 5 parts in 10^13 12 hours 1 part in 10^11 1 year Hydrogen Maser 3 parts in 10^13 1 second 2 parts in 10^14 100 seconds 2 parts in 10^14 12 hours 2 parts in 10^13 1 year Cesium Beam Standard 5 parts in 10^12 1 second 8 parts in 10^13 100 seconds 2.5 parts in 10^13 1000 seconds 8 parts in 10^14 10000 seconds
Station Time relative to the DSN master clock was accurate to 20 microseconds based on rubidium standard synchronization and to 3 milliseconds based on calibration by HF radio. The DSN master clock was accurate to 50 microseconds relative to the National Bureau of Standards, based on calibration using a portable cesium clock. The DSS frequency offset relative to the DSN master reference frequency was accurate to 1 part in 10^11 based on rubidium standard or cesium beam standard synchronization and to 2 parts in 10^13 based on a hydrogenmaser.


Optics - DSN
============
Performance of DSN ground stations depends primarily on size of the antenna and capabilities of electronics. These are summarized in the following set of tables. Note that 64-mantennas were upgraded to 70-m between 1986 and 1989.
Beamwidth is half-power full angular width. Polarization is circular; L denotes left circular polarization (LCP), and Rdenotes right circular polarization (RCP).

DSS Antenna Characteristics
Transmit Receive--------------- --------------------
Quantity 64-m 26-m 64-m 26-m-------- ----- ----- ------------ -----
Frequency (MHz) 2110- 2110- 2270- 8400- 2270-2120 2120 2300 8440 2300
Wavelength (m) 0.142 0.142 0.131 0.036 0.131 Gain (dBi) 60.7 51.8 61.7 71.3 53.2 Beamwidth (deg) 0.15 0.36 0.14 0.038 0.33 Polarization RCP RCP RCP RCP RCPLCP LCP LCP LCP LCP
LIN LIN LIN
CP Ellipticity (dB) 2.2 1.0 0.28 1.0 0.4 SNT-TWM1-unspec (K) 25 33 -diplex (K) 22 -orthog (K) 18 -TWM2-unspec (K) 41 -diplex (K) 26 -orthog (K) 23
Notes: (1) DSS 14 receive gain was 71.3 dB at X-band; but gain at DSS 43 and DSS 63 was 71.8 dB (2) Polarizations available at 64-m antennas wereRCP and LCP (simultaneously) or rotatable linear.
Polarizations available at 26-m antennas wereRCP or LCP or fixed linear.


Electronics - DSN
=================

DSCC Open-Loop Receiver (RIV)-----------------------------
The open loop receiver block diagrams below show the Modified Block III Open-Loop Receiver (DSS 14 and 43) and the Narrowband Multi-Mission Receiver (MMR) (DSS 63) used during early Voyager encounters. Only the S-band block diagrams are shown; expressions for reconstructing both S- and X-band signal frequencies (Fs and Fx, respectively) from the observed output frequencies (Folr) are given below thediagrams.

DSS 14 and 43 DSS 63
S-Band S-Band 2295 MHz 2295 MHz Input Input| |
v v--- --- --- ---
| X |<--|x48|<-- ~46 MHz ~41 MHz-->|x48|-->| X |--- --- --- ---
| |
50| |300
MHz| |MHz v v--- ---
| X |<-- 60 MHz 290 MHz -->| X |--- ---
| |
10| |10
MHz| |MHz v v--- ---
| X |<-- 10 MHz 10 MHz -->| X |--- ---
| |
v v Output Output

Reconstruction of the antenna frequency from the frequency of the signal in the recorded data can be achieved through use of one of the following formulas. Frequency of the Programmable Oscillator Control Assembly (Fpoca) is approximately 46 MHz at DSS 14 and 43 and approximately41 MHz at DSS 63.

DSS 14 and 43
Fs = 48*Fpoca + 50*10^6 - Folr Fx = (11/3)*(48*Fpoca + 50*10^6) - Folr
DSS 63

Fs = 48*Fpoca + 300*10^6 + Folr Fx = (11/3)*(48*Fpoca + 300*10^6) + Folr

Filters - DSN
=============

DSCC Open-Loop Receiver (RIV)-----------------------------
Filters (usually at the 10 MHz intermediate frequency) could be selected by the user to match expected width of the signal or uncertainty in its location. Filters and sampling rates used during the Voyager Saturn encounters were:
DSS 43 DSS 63
------------------ -------------------
3 dB Sample 3 dB Sample Bandwidth Rate Bandwidth Rate--------- ------- --------- --------
S-band 4.1 kHz 10 ksps 50. kHz 300 ksps X-band 15.0 kHz 30 ksps 150. kHz 300 ksps



Detectors - DSN
===============

DSCC Open-Loop Receivers------------------------
Open-loop receiver output is detected in software by theradio science investigator.


DSCC Closed-Loop Receivers--------------------------
Nominal carrier tracking loop threshold noise bandwidths at S- and X-band were 10-12 and 30 Hz, respectively. Samplerates for Doppler were 1-10 per second.


Calibration - DSN
=================
Calibrations of hardware systems are carried out periodically by DSN personnel; these ensure that systems operate at required performance levels -- for example, that antenna patterns, receiver gain, propagation delays, and Doppler uncertainties meet specifications. No information on specific calibration activities is available. Nominal performance specifications are shown in the tables above. Additional information may beavailable in [DSN810-5].

Prior to each tracking pass, station operators perform a series of calibrations to ensure that systems meet specifications for that operational period. Included in these calibrations is measurement of receiver system temperature in the configuration to be employed during the pass. Results of these calibrationsare recorded in (hard copy) Controller's Logs for each pass.

Filters for the Open-Loop Receivers were checked during the Test and Calibration period after the Titan and Saturn observations concluded. A test signal was injected at a constant frequency, then stepped across the passband to measurefilter gain at discrete frequencies.


Operational Considerations - DSN
================================
The DSN is a complex and dynamic 'instrument.' Its performance for Radio Science depends on a number of factors from equipment configuration to meteorological conditions. No specificinformation on 'operational considerations' can be given here.


Operational Modes - DSN
=======================

Closed-Loop vs. Open-Loop Reception-----------------------------------
Radio Science data can be collected in two modes: closed- loop, in which a phase-locked loop receiver tracks the spacecraft signal, or open-loop, in which a receiver samples and records a band within which the desired signal presumably resides. Closed-loop data are collected using Closed-Loop Receivers, and open-loop data are collected using Open-Loop Receivers in conjunction with the DSCC Spectrum Processing Subsystem (DSP). See the Subsystems section for furtherinformation.


Closed-Loop Receiver AGC Loop-----------------------------
The closed-loop receiver AGC loop can be configured to one of three settings: narrow, medium, or wide. Ordinarily it is configured so that expected signal amplitude changes are accommodated with minimum distortion. The loop bandwidth is ordinarily configured so that expected phase changes can beaccommodated while maintaining the best possible loop SNR.


Coherent vs. Non-Coherent Operation-----------------------------------
The frequency of the signal transmitted from the spacecraft can generally be controlled in two ways -- by locking to a signal received from a ground station or by locking to an on-board oscillator. These are known as the coherent (or'two-way') and non-coherent ('one-way') modes, respectively.
Mode selection is made at the spacecraft, based on commands received from the ground. When operating in the coherent mode, the transponder carrier frequency is derived from the received uplink carrier frequency with a 'turn-around ratio' typically of 240/221. In the non-coherent mode, the downlink carrier frequency is derived from the spacecraft on-board crystal-controlled oscillator. Either closed-loop or open-loop receivers (or both) can be used with either spacecraft frequency reference mode. Closed-loop receptionin two-way mode is usually preferred for routine tracking.
Occasionally the spacecraft operates coherently while two ground stations receive the 'downlink' signal; this issometimes known as the 'three-way' mode.


Open-Loop Sampling------------------
The Open-Loop Receiver sampling system can operate in four sampling modes with from 1 to 4 input signals. Inputchannels are assigned to ADC inputs during configuration.
Modes are summarized in the tables below:
Mode Analog-to-Digital Operation---- ----------------------------
1 4 signals, each sampled by a single ADC 2 1 signal, sampled sequentially by 4 ADCs 3 2 signals, each sampled sequentially by 2 ADCs 4 2 signals, the first sampled by ADC #1 and the second sampled sequentially at 3 times the rate by ADCs #2-4

Location - DSN
==============
Station locations are documented in [GEO-10REVD]. Geocentriccoordinates are summarized here.

Geocentric Geocentric Geocentric Station Radius (km) Latitude (N) Longitude (E)--------- ----------- ------------ -------------
Goldstone DSS 12 (26-m STD) 6371.997815 35.1186672 243.1945048 DSS 13 (develop) 6372.117062 35.0665485 243.2051077 DSS 14 (64-m) 6371.992867 35.2443514 243.1104584
Canberra DSS 42 (26-m STD) 6371.675607 -35.2191850 148.9812546 DSS 43 (64-m) 6371.688953 -35.2209308 148.9812540
Madrid DSS 61 (26-m STD) 6370.027734 40.2388805 355.7509634 DSS 63 (64-m) 6370.051015 40.2413495 355.7519776

Measurement Parameters - DSN
============================

Open-Loop System----------------
Sampled output from the Open-Loop Receivers (OLRs) is a stream of 8-bit quantized voltage samples. The nominal input to the Analog-to-Digital Converters (ADCs) is +/-10 volts, but the precise scaling between input voltages and output digitized samples is usually irrelevant for analysis; the digital data are generally referenced to a known noise or signal level within the data stream itself -- for example, the thermal noise output of the radio receivers which has a known system noise temperature (SNT). Raw samples comprise the data block in each data record; a header record contains ancillary information such as time tag for thefirst sample in the data block.


Closed-Loop System------------------
Closed-loop data are recorded in Archival Tracking Data Files (ATDFs), as well as certain secondary products such as the Orbit Data File (ODF). The ATDF Tracking Logical Record contains entries including status information andmeasurements of ranging, Doppler, and signal strength.


ACRONYMS AND ABBREVIATIONS - DSN

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ACS Antenna Control System ADC Analog-to-Digital Converter AMS Antenna Microwave System APA Antenna Pointing Assembly ARA Area Routing Assembly ATDF Archival Tracking Data File AZ Azimuth CMC Complex Monitor and Control CONSCAN Conical Scanning (antenna pointing mode) CRG Coherent Reference Generator CUL Clean-up Loop DANA a type of frequency synthesizer dB decibel dBi dB relative to isotropic dBm dB relative to one milliwatt DCO Digitally Controlled Oscillator DEC Declination deg degree DMC DSCC Monitor and Control Subsystem DSCC Deep Space Communications Complex DSN Deep Space Network DSP DSCC Spectrum Processing Subsystem DSS Deep Space Station DTK DSCC Tracking Subsystem E east EL Elevation FTS Frequency and Timing Subsystem GCF Ground Communications Facility GPS Global Positioning System HA Hour Angle HEF High-Efficiency (as in 34-m HEF antennas) IF Intermediate Frequency IVC IF Selection Switch JPL Jet Propulsion Laboratory K Kelvin kHz kilohertz km kilometer ksps kilosamples per second kW kilowatt L-band approximately 1668 MHz LAN Local Area Network LCP Left-Circularly Polarized LMC Link Monitor and Control LNA Low-Noise Amplifier LO Local Oscillator m meters MCA Master Clock Assembly MCCC Mission Control and Computing Center MDA Metric Data Assembly MHz Megahertz MMR Multi-Mission Receiver MON Monitor and Control System MSA Mission Support Area N north NAR Noise Adding Radiometer NBOC Narrow-Band Occultation Converter NIST SPC 10 time relative to UTC NIU Network Interface Unit NOCC Network Operations and Control System NSS NOCC Support System OCI Operator Control Input ODF Orbit Data File ODR Original Data Record ODS Original Data Stream OLR Open Loop Receiver POCA Programmable Oscillator Control Assembly PPM Precision Power Monitor RA Right Ascension REC Receiver-Exciter Controller RCP Right-Circularly Polarized RF Radio Frequency RIC RIV Controller RIV Radio Science IF-VF Converter Assembly RMDCT Radio Metric Data Conditioning Team RTLT Round-Trip Light Time S-band approximately 2100-2300 MHz sec second SEC System Error Correction SIM Simulation SLE Signal Level Estimator SNR Signal-to-Noise Ratio SNT System Noise Temperature SOE Sequence of Events SPA Spectrum Processing Assembly SPC Signal Processing Center SRA Sequential Ranging Assembly SRC Sub-Reflector Controller SSI Spectral Signal Indicator STD Standard (as in 34-m STD antennas) TID Time Insertion and Distribution Assembly TSF Tracking Synthesizer Frequency TWM Traveling Wave Maser UNK unknown UTC Universal Coordinated Time VF Video Frequency X-band approximately 7800-8500 MHz