Instrument Overview
===================

Radio science investigations utilizeelements both on a spacecraft and at ground stations.
The spacecraft part of the radio science instrument is the Radio Frequency subsystem (RF), which is described immediately below and in more detail in [SRINIVASANETAL 2007]. The spacecraft description is followed by a description of the ground element, the NASA Deep Space Network (DSN). For more informationabout MESSENGER, see [SOLOMONETAL2007].


Instrument Specifications - Spacecraft
======================================

Instrument Id : RSS Instrument Host Id : MESS 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

Instrument Overview - Spacecraft
================================
The MESSENGER RF/telecommunications system receives and transmits at X-Band channel #27 (7.177 GHz uplink and8.432 GHz downlink).

There are four Low-Gain Antennas (LGAs) and two high-gain phased-array antennas (HGAs). The HGAs are diametrically opposite-facing. Each phased-array antenna is capable of electronically steering +/- 60 degrees in a single plane, the spacecraft Z=0 plane. Spacecraft rotation in conjunction with the electronic pointing of the antenna beam provides high-gain coverage of the Earth directionin every phase of the mission.

When not in HGA downlink operations, the spacecraft will be oriented such that the main suite of instruments will be pointed at the planet. The sunshade must always face toward the sun (+/-12 degrees), so the instrument pointing is accomplished by rotating about the sun-line. This forces the XY plane away from the ecliptic, away from the Earth, and forces the spacecraft-to-Earth vector to lie outside of the phased-array antenna patterns. When HGAs are not available, Doppler data can be acquired using the LGAs. Thisconfiguration lowers the signal-to-noise ratio and the Doppler quality.
During HGA-downlink passes and most use of the LGA, the RF power levels will be sufficiently high to acquire good quality Doppler tracking data when noise from the solar plasma is low. When the MESSENGER-Sun-Earth angle is greater than 100 degrees, the solar plasma will usuallyincrease Doppler noise above the requirement of 0.1 mm/s.

The largest sources of error are thermal noise at the DSN receivers and solar-plasma noise due to the densityof the solar wind near the Sun.

The exact frequency transmitted from the spacecraft are controlled by the signal received from a ground station ('two-way' or 'coherent' mode) or by an on-board oscillator ('one-way' or 'non-coherent' mode). In some circumstances an uplink signal is transmitted from one ground station while two ground stations participate in reception; this is known as the 'three-way' mode. In the absence of an uplink signal, the spacecraft system switches automatically to the one-way mode. The on-board frequency reference could beeither of two redundant 'auxiliary' crystal oscillators.

Each transponder includes a receiver, command detector, exciter, and low-power amplifier. The transponders provide the usual uplink command and downlink data transmission capabilities. The following modulation states can be commanded: telemetry alone, ranging alone, telemetry andranging, or carrier only.

Science Objectives
==================
The radio tracking data--primarily Doppler--will be used to improve knowledge of the Mercury gravity field, both the static and the time-variable components, and to improve knowledge of the state of the planet's core. To fully achieve this goal requires Doppler and other tracking data for extended periods while MESSENGER is in orbit around Mercury. Without the orbit data, the flyby data are only marginally useful toward this goal. RF data from both phases of the mission (flybys and orbit phase) will be used to derive a spherical harmonic representation of the gravitational potential of theplanet.

The RF data also will be used to constrain the planet shape by monitoring the signal intensity at the ingress and egress of an occultation. The spacecraft will be occulted from Earth during a majority of orbits. The time of the occultation is used to estimate the radius of the planet at the grazing-ray location. On ingress, RF contact will be coherent (2-way), and timing data from the DSN phased-lockedreceiver are available at the resolution of the raw data, 0.1 seconds.
For egress measurements, the RF signal will not be coherent (MESSENGER does not have an ultra-stable oscillator, and the transponder will not be locked to the uplink at egress), so the best time resolution will be achieved using the DSN Radio Science Receiver (RSR). Because the RSR has the potential for better time resolution than the phased-locked receiver, RSR data are collected, also, at ingress. The RSR time resolution depends on the RF power at ingress. The gravity science team will analyze the Doppler, range, and DDOR data using GEODYN. The data will be analyzed to produce gravity coefficients to at least degree and order 16. Prior to MESSENGER, the only information on Mercury's gravity field is from the three Mariner 10 flybys in 1974. Analyses ofMariner 10 data produced estimates of GM, J2, and C22 coefficients.
Uncertainties for J2 and C22 were about 50%. The first two MESSENGER flybys refined GM and C22, but J2 is not well constrained by equatorialflybys.

The analysis of the tracking data (both range data and VLBI) can be used to improve the modelling of the orbit of the planet in future versions of the solar system planetary ephemerides. Solomon 2001 and2007 contain details of science objects for all of the investigations.
Srinivasan 2007 contains details of the RS investigation.

Operational Considerations - Spacecraft
=======================================


Calibration Description - Spacecraft
====================================
The RF/telecom system was fully tested prior to launch
During cruise, Doppler data are used by the navigation team. They assess performance of the RF system, whichmeets RS requirements.


Platform Mounting Descriptions - Spacecraft
===========================================

Investigators
=============
M. Zuber, F. Lemoine, D. Smith
Instrument Section / Operating Mode Descriptions - Spacecraft
=============================================================

Instrument Overview - DSN
=========================
Three Deep Space Communications Complexes (DSCCs) (near Barstow, CA; Canberra, Australia; and Madrid, Spain) comprise the DSN tracking network. Each complex is equipped with several antennas [including at least one each 70-m, 34-m High Efficiency (HEF), and 34-m Beam WaveGuide (BWG)], associated electronics, and operational systems. Primary activity at each complex is radiation of commands to and reception of telemetry data from active spacecraft. Transmission and reception is 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 of up to 400 kW areavailable.

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.

Principal investigators vary from experiment to experiment.
See the corresponding section of the spacecraft instrumentdescription or the data set description for specifics.

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 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 see reports by [ASMAR&RENZETTI1993] and [ASMARETAL1995]. For designspecifications on DSN subsystems see [DSN810-5].

Subsystems - DSN
================
The Deep Space Communications Complexes (DSCCs) are an integral part of Radio Science instrumentation, along with 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 describe the functions performed by the individual subsystems of a DSCC. This material has been adapted from [ASMARETAL1995] and [JPLD-16765]; for additionalinformation, consult [DSN810-5].

Each DSCC includes a set of antennas, a Signal Processing Center (SPC), 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 25 | | DSS 27 | | DSS 14 | | DSS 15 | | DSS 16 |
|34-m BWG| |34-m HSB| | 70-m | |34-m HEF| | 26-m |-------- -------- -------- -------- --------
| | | | |
| v v | v| --------- | ---------
--------->|GOLDSTONE|<---------- |EARTH/ORB|
| SPC 10 |<-------------->| LINK |
--------- ---------
| SPC |<-------------->| 26-M |
| COMM | ------>| COMM |
--------- | ---------
| | |
v | v------ --------- | ---------
| NOCC |<--->| JPL |<------- | |
------ | CENTRAL | | GSFC |
------ | COMM | | NASCOMM |
| MCCC |<--->| TERMINAL|<-------------->| |
------ --------- ---------
^ ^
| |
CANBERRA (SPC 40) <---------------- |
|
MADRID (SPC 60) <----------------------

GOLDSTONE CANBERRA MADRID
Antenna SPC 10 SPC 40 SPC 60-------- --------- -------- --------
26-m DSS 16 DSS 46 DSS 66 34-m HEF DSS 15 DSS 45 DSS 65 34-m BWG DSS 24 DSS 34 DSS 54DSS 25
DSS 26
34-m HSB DSS 27DSS 28
70-m DSS 14 DSS 43 DSS 63 Developmental DSS 13

Subsystem interconnections at each DSCC are shown in the diagram below, and they are described in the sections that follow. The Monitor and Control Subsystem is connected to allother subsystems; the Test Support Subsystem can be.

----------- ------------------ --------- ---------
|TRANSMITTER| | | | TRACKING| | COMMAND |
| SUBSYSTEM |-| RECEIVER/EXCITER |-|SUBSYSTEM|-|SUBSYSTEM|-
----------- | | --------- --------- |
| | SUBSYSTEM | | | |
----------- | | --------------------- |
| MICROWAVE | | | | TELEMETRY | |
| SUBSYSTEM |-| |-| SUBSYSTEM |-
----------- ------------------ --------------------- |
| |
----------- ----------- --------- -------------- |
| ANTENNA | | MONITOR | | TEST | | DIGITAL | |
| SUBSYSTEM | |AND CONTROL| | SUPPORT | |COMMUNICATIONS|-
----------- | SUBSYSTEM | |SUBSYSTEM| | SUBSYSTEM |
----------- --------- --------------


DSCC Monitor and Control Subsystem----------------------------------
The DSCC Monitor and Control Subsystem (DMC) is part of the Monitor and Control System (MON) which also includes the ground communications Central Communications Terminal and the Network Operations Control Center (NOCC) Monitor and Control Subsystem. The DMC is the center of activity at a DSCC. The DMC receives and archives most of the information from the NOCC needed by the various DSCC subsystems during their operation. Control of most of the DSCC subsystems, as well as the handling and displaying of any responses to control directives and configuration and status information received from each of the subsystems, is done through the DMC. The effect of this is to centralize the control, display, andarchiving functions necessary to operate a DSCC.
Communication among the various subsystems is done using a Local Area Network (LAN) hooked up to each subsystem via anetwork interface unit (NIU).


DSCC Antenna Mechanical Subsystem---------------------------------
Multi-mission Radio Science activities require support from the 70-m, 34-m HEF, and 34-m BWG antenna subnets. The antennas at each DSCC function as large-aperture collectors which, by double reflection, cause the incoming radio frequency (RF) energy to enter the feed horns. The large collecting surface of the antenna focuses the incoming energy onto a subreflector, which is adjustable in both axial and angular position. These adjustments are made to correct for gravitational deformation of the antenna as it moves between zenith and the horizon; the deformation can be as large as 5 cm. The subreflector adjustments optimize the channeling of energy from the primary reflector to the subreflector and then to the feed horns. The 70-m and 34-m HEF antennas have 'shaped' primary and secondary reflectors, with forms that are modified paraboloids. This customization allows more uniform illumination of one reflector by another. TheBWG reflector shape is ellipsoidal.

On the 70-m antennas, the subreflector directs received energy from the antenna onto a dichroic plate, a device which reflects S-band energy to the S-band feed horn and passes X-band energy through to the X-band feed horn. In the 34-m HEF, there is one 'common aperture feed,' whichaccepts both frequencies without requiring a dichroic plate.
In the 34-m BWG, a series of small mirrors (approximately 2.5 meters in diameter) directs microwave energy from the subreflector region to a collection area at the base of the antenna -- typically in a pedestal room. A retractable dichroic reflector separates S- and X-band on some BWG antennas or X- and Ka-band on others. RF energy to be transmitted into space by the horns is focused by the reflectors into narrow cylindrical beams, pointed with high precision (either to the dichroic plate or directly to the subreflector) by a series of drive motors and gear trains that can rotate the movable components and their supportstructures.

The different antennas can be pointed by several means. Two pointing modes commonly used during tracking passes are CONSCAN and 'blind pointing.' With CONSCAN enabled and a closed loop receiver locked to a spacecraft signal, the system tracks the radio source by conically scanning around its position in the sky. Pointing angle adjustments are computed from signal strength information (feedback) supplied by the receiver. In this mode the Antenna Pointing Assembly (APA) generates a circular scan pattern which is sent to the Antenna Control System (ACS). The ACS adds the scan pattern to the corrected pointing angle predicts. Software in the receiver-exciter controller computes the received signal level and sends it to the APA. The correlation of scan position with the received signal level variations allows theAPA to compute offset changes which are sent to the ACS.
Thus, within the capability of the closed-loop control system, the scan center is pointed precisely at the apparent direction of the spacecraft signal source. An additional function of the APA is to provide antenna position angles and residuals, antenna control mode/status information, and predict-correction parameters to the Area Routing Assembly (ARA) via the LAN, which then sends this information to JPL via the Ground Communications Facility (GCF) for antennastatus monitoring.

During periods when excessive signal level dynamics or low received signal levels are expected (e.g., during an occultation experiment), CONSCAN should not be used. Under these conditions, blind pointing (CONSCAN OFF) is used, and pointing angle adjustments are based on a predeterminedSystematic Error Correction (SEC) model.

Independent of CONSCAN state, subreflector motion in at least the z-axis may introduce phase variations into the received Radio Science data. For that reason, during certain experiments, the subreflector in the 70-m and 34-m HEFs may be frozen in the z-axis at a position (often based on elevation angle) selected to minimize phase change and signal degradation. This can be done via Operator Control Inputs (OCIs) from the LMC to the Subreflector Controller (SRC) which resides in the alidade room of the antennas. The SRC passes the commands to motors that drive the subreflector tothe desired position.

Pointing angles for all antenna types are computed by the NOCC Support System (NSS) from an ephemeris provided by the flight project. These predicts are received and archived by the CMC. Before each track, they are transferred to the APA, which transforms the direction cosines of the predicts into AZ-EL coordinates. The LMC operator then downloads the antenna predict points to the antenna-mounted ACS computer along with a selected SEC model. The pointing predicts consist of time-tagged AZ-EL points at selected time intervals along with polynomial coefficients forinterpolation between points.

The ACS automatically interpolates the predict points, corrects the pointing predicts for refraction and subreflector position, and adds the proper systematic error correction and any manually entered antenna offsets. The ACS then sends angular position commands for each axis at the rate of one per second. In the 70-m and 34-m HEF, rate commands are generated from the position commands at the servo controller and are subsequently used to steer theantenna.

When not using binary predicts (the routine mode for spacecraft tracking), the antennas can be pointed using 'planetary mode' -- a simpler mode which uses right ascension (RA) and declination (DEC) values. These change very slowly with respect to the celestial frame. Values are provided to the station in text form for manual entry. The ACS quadratically interpolates among three RA and DEC pointswhich are on one-day centers.

A third pointing mode -- sidereal -- is available for tracking radio sources fixed with respect to the celestialframe.

Regardless of the pointing mode being used, a 70-m antenna has a special high-accuracy pointing capability called 'precision' mode. A pointing control loop derives the main AZ-EL pointing servo drive error signals from a two- axis autocollimator mounted on the Intermediate Reference Structure. The autocollimator projects a light beam to a precision mirror mounted on the Master Equatorial drive system, a much smaller structure, independent of the main antenna, which is exactly positioned in HA and DEC with shaft encoders. The autocollimator detects elevation/cross- elevation errors between the two reference surfaces by measuring the angular displacement of the reflected light beam. This error is compensated for in the antenna servo bymoving the antenna in the appropriate AZ-EL direction.
Pointing accuracies of 0.004 degrees (15 arc seconds) are possible in 'precision' mode. The 'precision' mode is not available on 34-m antennas -- nor is it needed, since theirbeamwidths are twice as large as on the 70-m antennas.


DSCC Antenna Microwave Subsystem--------------------------------
70-m Antennas: Each 70-m antenna has three feed conesinstalled in a structure at the center of the main reflector.
The feeds are positioned 120 degrees apart on a circle.
Selection of the feed is made by rotation of the subreflector. A dichroic mirror assembly, half on the S-band cone and half on the X-band cone, permits simultaneous use of the S- and X-band frequencies. The third cone is devoted toR&D and more specialized work.

The Antenna Microwave Subsystem (AMS) accepts the received S- and X-band signals at the feed horn and transmits them through polarizer plates to an orthomode transducer. The polarizer plates are adjusted so that the signals are directed to a pair of redundant amplifiers for each frequency, thus allowing simultaneous reception of signals in two orthogonal polarizations. For S-band these are two Block IVA S-band Traveling Wave Masers (TWMs); for X-band theamplifiers are Block IIA TWMs.

34-m HEF Antennas: The 34-m HEF uses a single feed for both S- and X-band. Simultaneous S- and X-band receive as well as X-band transmit is possible thanks to the presence of an S/X 'combiner' which acts as a diplexer. For S-band, RCP or LCP is user selected through a switch so neither a polarizer nor an orthomode transducer is needed. X-band amplification options include two Block II TWMs or an HEMT Low Noise Amplifier (LNA). S-band amplification is provided by an FETLNA.

34-m BWG Antennas: These antennas use feeds and low-noise amplifiers (LNA) in the pedestal room, which can be switched in and out as needed. Typically the following modes are available: 1. downlink non-diplexed path (RCP or LCP) to LNA-1, with uplink in the opposite circular polarization; 2. downlink non-diplexed path (RCP or LCP) to LNA-2, with uplink in the opposite circular polarization 3. downlink diplexed path (RCP or LCP) to LNA-1, with uplink in the same circular polarization 4. downlink diplexed path (RCP or LCP) to LNA-2, with uplink in the same circular polarization For BWG antennas with dual-band capabilities (e.g., DSS 25) and dual LNAs, each of the above four modes can be used in a single-frequency or dual-frequency configuration. Thus, for antennas with the most complete capabilities, there are sixteen possible ways to receive at a single frequency (2 polarizations, 2 waveguide path choices, 2 LNAs, and 2bands).


DSCC Receiver-Exciter Subsystem-------------------------------
The Receiver-Exciter Subsystem is composed of two groups of equipment: the closed-loop receiver group and the open-loop receiver group. This subsystem is controlled by the Receiver-Exciter Controller (REC) which communicates directly with the DMC for predicts and OCI reception andstatus reporting.

The exciter generates the S-band signal (or X-band for the 34-m HEF only) which is provided to the Transmitter Subsystem for the spacecraft uplink signal. It is tunable under command of the Digitally Controlled Oscillator (DCO) whichreceives predicts from the Metric Data Assembly (MDA).

The diplexer in the signal path between the transmitter and the feed horn for all three antennas (used for simultaneous transmission and reception) may be configured such that it is out of the received signal path (in listen-only or bypass mode) in order to improve the signal-to-noise ratio in thereceiver system.

Closed Loop Receivers: The Block V receiver-exciter at the 70-m stations allows for two receiver channels, each capable of L-Band (e.g., 1668 MHz frequency or 18 cm wavelength), S-band, or X-band reception, and an S-band exciter for generation of uplink signals through the low-power orhigh-power transmitter.

The closed-loop receivers provide the capability for rapid acquisition of a spacecraft signal and telemetry lockup. In order to accomplish acquisition within a short time, the receivers are predict driven to search for, acquire, and track the downlink automatically. Rapid acquisition precludes manual tuning though that remains as a backup capability. The subsystem utilizes FFT analyzers for rapid acquisition. The predicts are NSS generated, transmitted to the CMC which sends them to the Receiver-Exciter Subsystem where two sets can be stored. The receiver starts acquisition at uplink time plus one round-trip-light-time or at operator specified times. The receivers may also be operated from the LMC without a local operator attending them. The receivers send performance and status data,displays, and event messages to the LMC.

Either the exciter synthesizer signal or the simulation (SIM) synthesizer signal is used as the reference for the Doppler extractor in the closed-loop receiver systems, depending on the spacecraft being tracked (and Project guidelines). The SIM synthesizer is not ramped; instead it uses one constant frequency, the Track Synthesizer Frequency(TSF), which is an average frequency for the entire pass.

The closed-loop receiver AGC loop can be configured to one of three settings: narrow, medium, or wide. It will be configured such that the expected amplitude changes are accommodated with minimum distortion. The loop bandwidth (2BLo) will be configured such that the expected phase changes can be accommodated while maintaining the bestpossible loop SNR.

Open-Loop Receivers (OLR): The OLR utilized a fixed first Local Oscillator (LO) frequency and a tunable second LO frequency to minimize phase noise and improve frequency stability. The OLR consisted of an RF-to-IF downconverter located at the feed , an IF selection switch (IFS), and a Radio Science Receiver (RSR). The RF-IF downconverters in the 70-m antennas were equipped for four IF channels: S-RCP, S-LCP, X-RCP, and X-LCP. The 34-m HEF stationswere equipped with a two-channel RF-IF: S-band and X-band.
The IFS switched the IF input among the antennas.


DSCC Transmitter Subsystem--------------------------
The Transmitter Subsystem accepts the S-band frequency exciter signal from the Receiver-Exciter Subsystem exciter and amplifies it to the required transmit output level. The amplified signal is routed via the diplexer through the feed horn to the antenna and then focused and beamed to thespacecraft.

The Transmitter Subsystem power capabilities range from 18 kW to 400 kW. Power levels above 18 kW are available only at70-m stations.


DSCC Tracking Subsystem-----------------------
The Tracking Subsystem primary functions are to acquire and maintain communications with the spacecraft and to generateand format radiometric data containing Doppler and range.

The DSCC Tracking Subsystem (DTK) receives the carrier signals and ranging spectra from the Receiver-Exciter Subsystem. The Doppler cycle counts are counted, formatted, and transmitted to JPL in real time. Ranging data are also transmitted to JPL in real time. Also contained in these blocks is the AGC information from the Receiver-Exciter Subsystem. The Radio Metric Data Conditioning Team (RMDCT) at JPL produces a Tracking and Navigation Service File (TNF),which contains Doppler and ranging data.

In addition, the Tracking Subsystem receives from the CMC frequency predicts (used to compute frequency residuals and noise estimates), receiver tuning predicts (used to tune the closed-loop receivers), and uplink tuning predicts (used to tune the exciter). From the LMC, it receives configuration and control directives as well as configuration and status information on the transmitter, microwave, and frequency andtiming subsystems.

The Metric Data Assembly (MDA) controls all of the DTK functions supporting the uplink and downlink activities. The MDA receives uplink predicts and controls the uplink tuning by commanding the DCO. The MDA also controls the Sequential Ranging Assembly (SRA). It formats the Doppler and range measurements and provides them to the GCF for transmission toNOCC.

The Sequential Ranging Assembly (SRA) measures the round trip light time (RTLT) of a radio signal traveling from a ground tracking station to a spacecraft and back. From the RTLT, phase, and Doppler data, the spacecraft range can be determined. A coded signal is modulated on an uplink carrier and transmitted to the spacecraft where it is detected and transponded back to the ground station. As a result, the signal received at the tracking station is delayed by its round trip through space and shifted in frequency by the Doppler effect due to the relative motion between thespacecraft and the tracking station on Earth.


DSCC Frequency and Timing Subsystem-----------------------------------
The Frequency and Timing Subsystem (FTS) provides all frequency and timing references required by the other DSCC subsystems. It contains four frequency standards of which one is prime and the other three are backups. Selection of the prime standard is done via the CMC. Of these four standards, two are hydrogen masers followed by clean-up loops (CUL) and two are cesium standards. These four standards all feed the Coherent Reference Generator (CRG) which provides the frequency references used by the rest of the complex. It also provides the frequency reference to the Master Clock Assembly (MCA) which in turn provides time to the Time Insertion and Distribution Assembly (TID) which provides UTCand SIM-time to the complex.

JPL's ability to monitor the FTS at each DSCC is limited to the MDA calculated Doppler pseudo-residuals, the Doppler noise, the SSI, and to a system which uses the Global Positioning System (GPS). GPS receivers at each DSCC receive a one-pulse-per-second pulse from the station's (hydrogen maser referenced) FTS and a pulse from a GPS satellite at scheduled times. After compensating for the satellite signal delay, the timing offset is reported to JPL where a database is kept. The clock offsets stored in the JPL database are given in microseconds; each entry is a mean reading of measurements from several GPS satellites and a time tag associated with the mean reading. The clock offsets provided include those of SPC 10 relative to UTC (NIST), SPC 40relative to SPC 10, etc.


Radio Science Receiver (RSR)----------------------------
A radio frequency (RF) spacecraft signal at S-band, X-band, or Ka-band is captured by a receiving antenna on Earth, down converted to an intermediate frequency (IF) near 300 MHz and then fed via a distribution network to one input of an IF Selector Switch (IFS). The IFS allows each RSR to select anyof the available input signals for its RSR Digitizer (DIG).
Within the RSR the digitized signal is then passed to the Digitial Down Converter (DDC), VME Data Processor (VDP), andData Processor (DP) [JPLD-16765].

\ ----------- ------ ----- ----- -----
\ | RF TO IF | | |----| | | | | |
|----| DOWN |----| |----| |----| DIG | | DP |
/ | CONVERTER | | |----| | | | | |
/ ----------- | IF |----| IFS | ----- -----
ANTENNA --| DIST |----| | | |
300 MHz IF --| | .. | | ----- -----FROM OTHER --| |----| | | | | |
ANTENNAS --| | ----- | DDC | | VDP |
------ | | | |
----- -----
| |
-------

In the DIG the IF signal is passed through a programmable attenuator, adjusted to provide the proper level to the Analog to Digital Converter (ADC). The attenuated signal is then passed through a Band Pass Filter (BPF) which selects a frequency band in the range 265-375 MHz. The filtered output from the BPF is then mixed with a 256 MHz Local Oscillator (LO), low pass filtered (LPF), and sampled by the ADC. The output of the ADC is a stream of 8-bit real samples at 256 Msamples/second (Msps). DIG timing is derived from the station FTS 5 MHz clock and 1 pulse per second (1PPS) reference; the DIG generates a 256 MHz clock signal for later processing. The 1PPS signal marks the data sample taken atthe start of each second.

The DDC selects one 16 MHz subchannel from the possible 128 MHz bandwidth available from the DIG by using Finite Impulse Response (FIR) filters with revolving banks of filter coefficients. The sample stream from the DIG is separated into eight decimated streams, each of which is fed into two sets of FIR filters. One set of filters produces in-phase (I) 8-bit data while the other produces quadrature-phase (Q) 8-bit data. The center frequency of the desired 16 MHz channel is adjustable in 1 MHz steps and is usually chosen to be near the spacecraft carrier frequency. After combining the I and Q sample streams, the DDC feeds the samples to the VDP. The DDC also converts the 256 MHz data clock and 1PPS signals into a msec time code, which is also passed to theVDP.

The VDP contains a quadruply-redundant set of custom boardswhich are controlled by a real-time control computer (RT).
Each set of boards comprises a numerically controlled oscillator (NCO), a complex multiplier, a decimating FIR filter, and a data packer. The 16 Msps complex samples from the DDC are digitally mixed with the NCO signal in the complex multiplier. The NCO phase and frequency are updated every millisecond by the RT and are selected so that the center frequency of the desired portion of the 16 MHz channel is down-converted to 0 Hz. The RT uses polynomials derived from frequency predictions. The output of the complex multiplier is sent to the decimating FIR filter where its bandwidth and sample rate are reduced (see table below). The decimating FIR filter also allows adjustment of the sub-channel gain to take full advantage of the dynamic range available in the hardware. The data packer truncates samples to 1, 2, 4, 8, or 16 bits by dropping the least significant bits and packs them into 32-bit data words. Q-samples are packed into the first 16 bits of the word, and I-samples into the least significant 16 bits (see below). In 'narrow band' operation all four sets of sets of custom boards can be supported simultaneously. In 'medium band' operation no more than two channels can be supported simultaneously. In'wide band' operation, only one sub-channel can be recorded.

|============================================================|
| RSR Sample Rates and Sample Sizes Supported ||================+=======+======+=================+==========|
| Category | Rate | Size | Data Rate |Rec Length| | | (ksps)|(bits)|(bytes/s) (rec/s)| (bytes) ||================+=======+======+=========+=======+==========|
|Narrow Band (NB)| 1 | 8 | 2000 | 1 | 2000 || | 2 | 8 | 4000 | 1 | 4000 |
| | 4 | 8 | 8000 | 1 | 8000 |
| | 8 | 8 | 16000 | 1 | 16000 |
| | 16 | 8 | 32000 | 2 | 16000 |
| | 25 | 8 | 50000 | 2 | 25000 |
| | 50 | 8 | 100000 | 4 | 25000 |
| | 100 | 8 | 200000 | 10 | 20000 |
| | 1 | 16 | 4000 | 1 | 4000 |
| | 2 | 16 | 8000 | 1 | 8000 |
| | 4 | 16 | 16000 | 1 | 16000 |
| | 8 | 16 | 32000 | 2 | 16000 |
| | 16 | 16 | 64000 | 4 | 16000 |
| | 25 | 16 | 100000 | 4 | 25000 |
| | 50 | 16 | 200000 | 10 | 20000 |
| | 100 | 16 | 400000 | 20 | 20000 |
|Medium Band (MB)| 250 | 1 | 62500 | 5 | 12500 || | 500 | 1 | 125000 | 5 | 25000 |
| | 1000 | 1 | 250000 | 10 | 25000 |
| | 2000 | 1 | 500000 | 20 | 25000 |
| | 4000 | 1 | 1000000 | 40 | 25000 |
| | 250 | 2 | 125000 | 5 | 25000 |
| | 500 | 2 | 250000 | 10 | 25000 |
| | 1000 | 2 | 500000 | 20 | 25000 |
| | 2000 | 2 | 1000000 | 40 | 25000 |
| | 4000 | 2 | 2000000 | 100 | 20000 |
| | 250 | 4 | 250000 | 10 | 25000 |
| | 500 | 4 | 500000 | 20 | 25000 |
| | 1000 | 4 | 1000000 | 40 | 25000 |
| | 2000 | 4 | 2000000 | 100 | 20000 |
| | 250 | 8 | 500000 | 20 | 25000 |
| | 500 | 8 | 1000000 | 40 | 25000 |
| | 1000 | 8 | 2000000 | 100 | 20000 |
|Wide Band (WB) | 8000 | 1 | 2000000 | 100 | 20000 || | 16000 | 1 | 4000000 | 200 | 20000 |
| | 8000 | 2 | 4000000 | 200 | 20000 |
|============================================================|

|============================================================|
| Sample Packing ||=================+==========================================|
| Bits per Sample | Contents of 32-bit Packed Data Register ||=================+==========================================|
| 16 | (Q1),(I1) |
| 8 | (Q2,Q1),(I2,I1) |
| 4 | (Q4,Q3,Q2,Q1),(I4,I3,I2,I1) |
| 2 | (Q8,Q7,...Q1),(I8,I7,...I1) |
| 1 | (Q16,Q15,...Q1),(I16,I15,...I1) |
|============================================================|

Once per second the RT sends the accumulated data records from each sub-channel to the Data Processor (DP) over a 100 Mbit/s ethernet connection. In addition to the samples, each data record includes header information such as time tags and NCO frequency and phase that are necessary for analysis. The DP processes the data records to provide monitor data, such as power spectra. If recording has been enabled, the recordsare stored by the DP.

NCO Phase and Frequency-----------------------
At the start of each DSN pass, the RSR is provided with a file containing a list of predicted frequencies. Using these points, the RT computes expected sky frequencies at the beginning, middle, and end of each one second time interval. Based on the local oscillator frequencies selected and any offsets entered, the RT computes the coefficients of a frequency polynomial fitted to the DDC channel frequencies at these three times. The RT also computes a phase polynomial by integrating the frequency polynomial and matching phases at the one secondboundaries.

The phase and frequency of the VDP NCO's are computed every millisecond (000-999) from the polynomial coefficients as follows:
nco_phase(msec) = phase_coef_1 + phase_coef_2 * (msec/1000) + phase_coef_3 * (msec/1000)**2 + phase_coef_4 * (msec/1000)**3
nco_freq(msec) = freq_coef_1 + freq_coef_2 * ((msec + 0.5)/1000) + freq_coef_3 * ((msec + 0.5)/1000)**2
The sky frequency may be reconstructed using
sky_freq = RF_to_IF_LO +DDC_LO -
nco_freq + reside_freq
where RF_to_IF_LO is the down conversion from the microwave frequency to IF (bytes 42-43 in the data record header) DDC_LO is the down-conversion applied in the DIG and DDC (bytes 40-41 in the data record header) Resid_Freq is the frequency of the signal in the VDP output

Detectors - DSN
===============
Nominal carrier tracking loop threshold noise bandwidth at X-band is 10 Hz. Coherent (two-way) closed-loop system stability is shown in the table below:
integration time Doppler uncertainty (secs) (one sigma, microns/sec)------ ------------------------
10 50
60 20
1000 4

For the open-loop subsystem, signal detection is done insoftware.


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.

The nominal procedure for initializing open-loop receiver attenuator settings is described below. In cases where widely varying signal levels are expected, the procedure may be modified in advance or real-time adjustments may be made toattenuator settings.


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
=======================

DSCC Antenna Mechanical Subsystem---------------------------------
Pointing of DSCC antennas may be carried out in several ways.
For details see the subsection 'DSCC Antenna Mechanical Subsystem' in the 'Subsystem' section. Binary pointing is the preferred mode for tracking spacecraft; pointingpredicts are provided, and the antenna simply follows those.
With CONSCAN, the antenna scans conically about the optimum pointing direction, using closed-loop receiver signal strength estimates as feedback. In planetary mode, the system interpolates from three (slowly changing) RA-DEC target coordinates; this is 'blind' pointing since there is no feedback from a detected signal. In sidereal mode, the antenna tracks a fixed point on the celestial sphere. In 'precision' mode, the antenna pointing is adjusted using an optical feedback system. It is possible on most antennas to freeze z-axis motion of the subreflector to minimize phasechanges in the received signal.


DSCC Receiver-Exciter Subsystem-------------------------------
The diplexer in the signal path between the transmitter and the feed horns on all antennas may be configured so that it is out of the received signal path in order toimprove the signal-to-noise ratio in the receiver system.
This is known as the 'listen-only' or 'bypass' mode.


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 880/749. 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.


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 13 (34-m R&D) 6372.125125 35.0660185 243.2055430 DSS 14 (70-m) 6371.993286 35.2443527 243.1104638 DSS 15 (34-m HEF) 6371.966540 35.2403133 243.1128069 DSS 24 (34-m BWG) 6371.973553 35.1585349 243.1252079 DSS 25 (34-m BWG) 6371.983060 35.1562594 243.1246384 DSS 26 (34-m BWG) 6371.993032 35.1543411 243.1269849
Canberra DSS 34 (34-m BWG) 6371.693561 -35.2169868 148.9819620 DSS 43 (70-m) 6371.689033 -35.2209234 148.9812650 DSS 45 (34-m HEF) 6371.675906 -35.2169652 148.9776833
Madrid DSS 45 (34-m BWG) 6370.025429 40.2357708 355.7459008 DSS 63 (70-m) 6370.051221 40.2413537 355.7519890 DSS 65 (34-m HEF) (see next paragraph)
The coordinates for DSS 65 until 1 February 2005 were6370.021697 40.2373325 355.7485795
In cartesian coordinates (x, y, z) this was(+4849336.6176, -0360488.6349, +4114748.9218)
Between February and September 2005, the antenna was physically moved to(+4849339.6448, -0360427.6560, +4114750.7428)


Measurement Parameters - DSN
============================
Closed-loop data are recorded in Tracking and Navigation Service Files (TNFs), as well as certain secondary products such as the Orbit Data File (ODF). The TNFs are comprised of SFDUs that have variable-length, variable-format records with mixed typing (i.e., can contain ASCII, integer, and floating-point items in a single record). These files all contain entries that include measurements of Doppler, Range, and signal strength, along withstatus and uplink frequency information.


ACRONYMS AND ABBREVIATIONS - DSN

================================
ACS Antenna Control System ADC Analog-to-Digital Converter AGC Automatic Gain Control AMS Antenna Microwave System APA Antenna Pointing Assembly ARA Area Routing Assembly AUX Auxiliary AZ Azimuth BPF Band Pass Filter bps bits per second BWG Beam WaveGuide (antenna) CDU Command Detector Unit 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 DDC Digital Down Converter DEC Declination deg degree DIG RSR Digitizer DMC DSCC Monitor and Control Subsystem DOR Differential One-way Ranging DP Data Processor DSCC Deep Space Communications Complex DSN Deep Space Network DSP DSCC Spectrum Processing Subsystem DSS Deep Space Station DTK DSCC Tracking Subsystem E east EIRP Effective Isotropic Radiated Power EL Elevation FET Field Effect Transistor FFT Fast Fourier Transform FIR Finite impulse Response FTS Frequency and Timing Subsystem GCF Ground Communications Facility GHz Gigahertz GPS Global Positioning System HA Hour Angle HEF High-Efficiency (as in 34-m HEF antennas) HEMT High Electron Mobility Transistor (amplifier) HGA High-Gain Antenna HSB High-Speed BWG IF Intermediate Frequency IFS IF Selector Switch IVC IF Selection Switch JPL Jet Propulsion Laboratory K Kelvin Ka-Band approximately 32 GHz KaBLE Ka-Band Link Experiment kbps kilobits per second kHz kilohertz km kilometer kW kilowatt LAN Local Area Network LCP Left-Circularly Polarized LGR Low-Gain Receive (antenna) LGT Low-Gain Transmit (antenna) LMC Link Monitor and Control LNA Low-Noise Amplifier LO Local Oscillator LPF Low Pass Filter m meters MCA Master Clock Assembly MCCC Mission Control and Computing Center MDA Metric Data Assembly MHz Megahertz MON Monitor and Control System MSA Mission Support Area N north NAR Noise Adding Radiometer NBOC Narrow-Band Occultation Converter NCO Numerically Controlled Oscillator NIST SPC 10 time relative to UTC NIU Network Interface Unit NOCC Network Operations and Control System NRV NOCC Radio Science/VLBI Display Subsystem NSS NOCC Support System OCI Operator Control Input ODF Orbit Data File OLR Open Loop Receiver OSC Oscillator PDS Planetary Data System POCA Programmable Oscillator Control Assembly PPM Precision Power Monitor RA Right Ascension REC Receiver-Exciter Controller RCP Right-Circularly Polarized RF Radio Frequency RMDCT Radio Metric Data Conditioning Team RMS Root Mean Square RSR Radio Science Receiver RSS Radio Science Subsystem RT Real-Time (control computer) 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 sps samples per second SRA Sequential Ranging Assembly SRC Sub-Reflector Controller SSI Spectral Signal Indicator TID Time Insertion and Distribution Assembly TLM Telemetry TSF Tracking Synthesizer Frequency TWM Traveling Wave Maser TWNC Two-Way Non-Coherent TWTA Traveling Wave Tube Amplifier UNK unknown UTC Universal Coordinated Time VCO Voltage-Controlled Oscillator VDP VME Data Processor X-band approximately 7800-8500 MHz