Radio Science Subsystem (RSS)

mission specific

MER

Instrument Overview

The Mars Exploration Rover (MER) mission includes two spacecraft, Spirit (MER-2) and Opportunity (MER-1). The MER Radio Science data consist of measurements of the Doppler shift of the rover radio signal as measured by the NASA Deep Space Network (DSN) and by the Mars Odyssey orbiter (ODY). The primary purpose of all equipment was collection of telemetry from the rovers, with Doppler measurements made for rover position determination as required and on a best efforts basis to support 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 MER part of the radio science instrument is described immediately below; that is followed by a description of the relevant ODY relay radio system and a description of the DSN ( ground) part of the instrument.

Instrument Overview - MER Spacecraft (Rover X-band)

For communications with Earth, the rovers include an X-band radio system, which receives signals sent from Earth tracking stations at a frequency near 7.2 GHz, and transmits to Earth at a frequency near 8.4 GHz. The X-band radio system includes several antennas on various parts of the spacecraft to support different parts of the mission. For surface operations, the primary antenna is a High Gain Antenna (HGA) which must be accurately pointed at Earth to communicate with Earth tracking stations. A low-gain antenna (LGA) was also available in case of problems pointing the HGA. All useful Radio Science data were taken with the HGA. [JPL-D21239].

The Earth tracking stations measure the frequency of the radio signal received from the rovers as a measure of the relative velocity of the rovers with respect to Earth. In a simple one-way Doppler measurement, where the rover transmits and the Earth tracking station receives, the Doppler measurement accuracy is limited by the frequency stability of the reference oscillator on the rover, which is usually rather poor compared with the atomic frequency standards used in Earth tracking stations. In order to achieve high-quality Doppler measurements without requiring an atomic frequency standard on the rovers, the rover radio system is capable of operating in a mode (two-way or coherent) where the frequency of the signal transmitted by the rover is made to be a fixed multiple of the frequency it receives from the Earth tracking station. In this case the measurement of the frequency shift made by the Earth tracking station is proportional to twice the line-of-sight velocity, with a Doppler shift occurring on both the Earth-to-rover signal and on the rover-to-Earth signal. The accuracy of such measurements is generally limited by dispersion introduced by electrons in the interplanetary medium (also referred to as solar plasma). A typical frequency measurement accuracy is ~ 5 mHz, corresponding to a line-of-sight velocity accuracy of about 0.1 mm/s, over a measurement integration time of 60 seconds.

The electronics for generating the signals for transmission and for detection and lock to a received signal is designated the Radio Frequency Subsystem (RFS). The RFS includes a Solid-State Power Amplifier (SSPA), a Small-Deep-Space-Transponder (SDST), and coupler and switches for connections to the various antennas.

Instrument Specifications - Spacecraft (Rover X-band)

  • Instrument Id: RSSX
  • Instrument Host Id: MER2
  • Instrument Name: RADIO SCIENCE SUBSYSTEM
  • Instrument Type: RADIO SCIENCE
  • Build Date: 2003
  • Instrument Mass: UNK
  • Instrument Diameter, HGA: 0.28 m
  • Antenna type, HGA: printed dipole array
  • Transmit frequency: 8439.444446 MHz
  • Receive frequency: 7183.118057 MHz
  • Transmit gain: 24.8 dB
  • Receive gain: 20.5 dB
  • Polarization: Right-Hand Circular (RHCP)
  • Transmit beamwidth: 4.16 degree (2-sided 3 dB)
  • Receive beamwidth: 5.0 degree (2-sided 3-dB)
  • Transmit power (SSPA): 5 W
  • Instrument Length: UNK
  • Instrument Width: UNK
  • Instrument Height: UNK
  • Instrument Manufacturer Name: General Dynamics/JPL

Instrument Overview - Spacecraft (Rover UHF)

Because of the large distance from Mars to Earth and the limited antenna size and power available to the rovers, the rovers include a radio system for transmitting data to spacecraft orbiting Mars. Because the orbiters generally have larger antennas and higher available power, they can relay a much larger amount of data to Earth than the rover could by transmitting directly to Earth. The local radio signal is at Ultra-High Frequency (UHF), with the rovers transmitting to the orbiters near 401 MHz, and the orbiters transmitting to the rovers near 437 MHz. As with the X-band radio system, the rovers are capable of operating in a mode (two-way or coherent) with the transmitted signal referenced to the signal received by the orbiter. The Mars Odyssey orbiter uses a simple system to measure the frequency of the signal received from the rovers compared with the transmitted signal. The accuracy of the measurement is expected to be about 20 mHz, corresponding to a line-of-sight velocity accuracy of about 10 mm/s.

The UHF radio subsystem consists of a low-gain antenna for transmission during entry, descent, and landing (EDL); a moderate gain antenna for use during rover surface operations; an antenna selection switch; a diplexer for separation of transmitted and received signals; and a transceiver for generation of transmission signals and detection of received signals. Because of the relatively long wavelength of UHF signals, the antenna used was a simple monopole with nominal nearly omni-directional pattern but with large deviations due to interactions with the rest of the rover structure. [JPL-D21240].

The UHF radio receiver covers a fairly broad band around 437.1 MHz to be able to accept signals from a number of potential Mars-orbiting spacecraft; The MER 2 UHF transmit frequency is limited to a narrow channel to avoid potential interference with possible future nearby surface spacecraft.

Instrument Specifications - Spacecraft (Rover UHF)

  • Instrument Id: RUHF
  • Instrument Host Id: MER2
  • Instrument Name: RADIO SCIENCE SUBSYSTEM
  • Instrument Type: RADIO SCIENCE
  • Build Date: 2003
  • Instrument Mass: UNK
  • Antenna type, RUHF: monopole
  • Transmit frequency: 401.585625 MHz (to Odyssey)
  • Receive frequency: 437.1 MHz
  • Transmit gain: 0 dB (nominal)
  • Receive gain: 0 dB (Nominal) dB
  • Polarization: Right-Hand Circular (RHCP)
  • Transmit power (SSPA): 10 W
  • Instrument Length: UNK
  • Instrument Width: UNK
  • Instrument Height: UNK
  • Instrument Manufacturer Name: CMC Electronics/JPL

Instrument Overview- Relay (Odyssey UHF)

The Mars Odyssey orbiter includes a UHF radio system for relay of commands from Earth to Mars surface vehicles and for relay of telemetry from surface vehicles to Earth. The Odyssey radio system is similar to the MER UHF and includes a nearly identical transceiver, with the transmission and reception frequencies reversed from those of the MER vehicles.

The Odyssey UHF radio system transmits a signal near 437.1 MHz to vehicles on the surface of Mars, such as MER 1 and MER 2, and receives signals back near 401 MHz. The ODY UHF radio measures the difference in frequency between its on-board reference and received carrier frequencies which provides a (Doppler) measure of the relative velocity of the surface vehicle.

The measurement is made by counting the (integer) number of zero- crossings of the difference frequency plus a the remaining fraction of a cycle over a 5 second time interval. The count cannot distinguish whether the received frequency is higher or lower than the reference frequency. Thus the measurement is the absolute value of the difference of the received frequency minus the reference frequency.

For Mars Odyssey, the only available reference frequency is 401.584625 MHz. The frequency of the signal transmitted by ODY is a fixed multiple, 160/147, of the reference frequency. For the MER radio operating in coherent mode, the received frequency is multiplied by a fixed multiple, 147/160, to generate the signal transmitted back to the orbiter. For example, when ODY is directly overhead the orbiter, the ODY reference frequency and the signal frequency received from MER are the same and the measurement is zero, indicating zero velocity of the rover relative to ODY along the line of sight to the orbiter. In general, the absolute value of the relative velocity along the line of sight is (to first order in v/c) v = c/2 *F(measurement)/F(reference) where c is the speed of light.

Instrument Specifications - Relay (Odyssey UHF)

  • Instrument Id: UHFR
  • Instrument Host Id: ODY
  • Instrument Name: RADIO SCIENCE SUBSYSTEM
  • Instrument Type: RADIO SCIENCE
  • Build Date: 2001
  • Instrument Mass: UNK
  • Antenna type, RUHF: UNK
  • Transmit frequency: 437.1 MHz
  • Receive frequency: 401.6 MHz
  • Transmit gain: 0 dB (nominal)
  • Receive gain: 0 dB (Nominal) dB
  • Polarization: Right-Hand Circular (RHCP)
  • Transmit power (SSPA): 10 W
  • Instrument Length: UNK
  • Instrument Width: UNK
  • Instrument Height: UNK
  • Instrument Manufacturer Name: CMC Electronics/JPL

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 are available.

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 gravity fields of objects near the spacecraft.

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

Principal investigators vary from experiment to experiment. See the corresponding section of the spacecraft instrument description 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 the U. 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], [ASMAR&HERRERA1993], and [ASMARETAL1995]. For design specifications 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 [ASMAR& HERRERA1993]; for additional information, see [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 individually instrumented) 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) <------------------------
Antenna

GOLDSTONE

SPC 10

CANBERRA

SPC 40

MADRID

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 25
DSS 26

 DSS 34 DSS 54
34-m HSB

DSS 27
DSS 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 all other 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, and archiving 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 a network interface unit (NIU).

DMC operations are divided into two separate areas: the Complex Monitor and Control (CMC) and the Link Monitor and Control (LMC). The primary purpose of the CMC processor for Radio Science support is to receive and store all predict sets transmitted from NOCC such as Radio Science, antenna pointing, tracking, receiver, and uplink predict sets and then, at a later time, to distribute them to the appropriate subsystems via the LAN. Those predict sets can be stored in the CMC for a maximum of three days under normal conditions. The CMC also receives, processes, and displays event/alarm messages; maintains an operator log; and produces tape labels for the DSP. Assignment and configuration of the LMCs is done through the CMC; to a limited degree the CMC can perform some of the functions performed by the LMC. There are two CMCs (one on-line and one backup) and three LMCs at each DSCC The backup CMC can function as an additional LMC if necessary.

The LMC processor provides the operator interface for monitor and control of a link -- a group of equipment required to support a spacecraft pass. For Radio Science, a link might include the DSCC Spectrum Processing Subsystem (DSP) (which, in turn, can control the SSI), or the Tracking Subsystem. The LMC also maintains an operator log which includes operator directives and subsystem responses. One important Radio Science specific function that the LMC performs is receipt and transmission of the system temperature and signal level data from the PPM for display at the LMC console and for inclusion in Monitor blocks. These blocks are recorded on magnetic tape as well as appearing in the Mission Control and Computing Center (MCCC) displays. The LMC is required to operate without interruption for the duration of the Radio Science data acquisition period.

The Area Routing Assembly (ARA), which is part of the Digital Communications Subsystem, controls all data communication between the stations and JPL. The ARA receives all required data and status messages from the LMC/CMC and can record them to tape as well as transmit them to JPL via data lines. The ARA also receives predicts and other data from JPL and passes them on to the CMC.

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. The BWG 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,' which accepts 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 support structures.

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 the APA 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 antenna status 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 predetermined Systematic 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 to the 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 for interpolation 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 the antenna.

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 points which are on one-day centers.

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

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 by moving 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 their beamwidths are twice as large as on the 70-m antennas.

DSCC Antenna Microwave Subsystem

70-m Antennas: Each 70-m antenna has three feed cones installed 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 to R&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 the amplifiers 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 FET LNA.

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 2 bands).

DSCC Receiver-Exciter Subsystem

The Receiver-Exciter Subsystem is composed of three groups of equipment: the closed-loop receiver group, the open-loop receiver group, and the RF monitor group. This subsystem is controlled by the Receiver-Exciter Controller (REC) which communicates directly with the DMC for predicts and OCI reception and status 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) which receives 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 the receiver 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 or high-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 best possible loop SNR.

Open-Loop Receivers: The Radio Science Open-Loop Receiver (OLR) is a dedicated four channel, narrow-band receiver which provides amplified and downconverted video band signals to the DSCC Spectrum Processing Subsystem (DSP); it sometimes goes by the designation 'RIV'.

The OLR utilizes a fixed first Local Oscillator (LO) frequency and a tunable second LO frequency to minimize phase noise and improve frequency stability. The OLR consists of an RF-to-IF downconverter located at the feed , an IF selection switch (IVC), and a Radio Science IF-VF downconverter (RIV) located in the SPC. The RF-IF downconverters in the 70-m antennas are equipped for four IF channels: S-RCP, S-LCP, X-RCP, and X-LCP. The 34-m HEF stations are equipped with a two-channel RF-IF: S-band and X-band. The 34-m BWG stations vary in their capabilities. The IVC switches the IF input among the antennas.

The RIV contains the tunable second LO, a set of video bandpass filters, IF attenuators, and a controller (RIC). The LO tuning is done via DSP control of the POCA/PLO combination based on a predict set. The POCA is a Programmable Oscillator Control Assembly and the PLO is a Programmable Local Oscillator (commonly called the DANA synthesizer). The bandpass filters are selectable via the DSP. The RIC provides an interface between the DSP and the RIV. It is controlled from the LMC via the DSP. The RIC selects the filter and attenuator settings and provides monitor data to the DSP. The RIC could also be manually controlled from the front panel in case the electronic interface to the DSP is lost.

RF Monitor -- SSI and PPM: The RF monitor group of the Receiver-Exciter Subsystem provides spectral measurements using the Spectral Signal Indicator (SSI) and measurements of the received channel system temperature and spacecraft signal level using the Precision Power Monitor (PPM).

The SSI provides a local display of the received signal spectrum at a dedicated terminal at the DSCC and routes these same data to the DSP which routes them to NOCC for remote display at JPL for real-time monitoring and RIV/DSP configuration verification. These displays are used to validate Radio Science Subsystem data at the DSS, NOCC, and Mission Support Areas. The SSI configuration is controlled by the DSP and a duplicate of the SSI spectrum appears on the LMC via the DSP. During real-time operations the SSI data also serve as a quick-look science data type for Radio Science experiments.

The PPM measures system noise temperatures (SNT) using a Noise Adding Radiometer (NAR) and downlink signal levels using the Signal Level Estimator (SLE). The PPM accepts its input from the closed-loop receiver. The SNT is measured by injecting known amounts of noise power into the signal path and comparing the total power with the noise injection 'on' against the total power with the noise injection 'off.' That operation is based on the fact that receiver noise power is directly proportional to temperature; thus measuring the relative increase in noise power due to the presence of a calibrated thermal noise source allows direct calculation of SNT. Signal level is measured by calculating an FFT to estimate the SNR between the signal level and the receiver noise floor where the power is known from the SNT measurements.

There is one PPM controller at the SPC which is used to control all SNT measurements. The SNT integration time can be selected to represent the time required for a measurement of 30K to have a one-sigma uncertainty of 0.3K or 1%.

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 the spacecraft.

The Transmitter Subsystem power capabilities range from 18 kw to 400 kw. Power levels above 18 kw are available only at 70-m stations.

DSCC Tracking Subsystem

The Tracking Subsystem primary functions are to acquire and maintain communications with the spacecraft and to generate and 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 an Archival Tracking Data File (ATDF) 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 and timing 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 to NOCC.

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 the spacecraft 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 UTC and 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 40 relative to SPC 10, etc.

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-m antennas 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 R denotes right circular polarization (RCP).

DSS S-Band Characteristics

  70-m 34-m BWG 34-m HEF
TRANSMIT
Frequency (MHz) 2110-2120 2025-2120 N/A
Wavelength (m) 0.142 0.142 N/A
Ant Gain (dBi) 62.7 56.1 N/A
Beamwidth (deg) 0.119 N/A N/A
Polarization L or R L or R N/A
Tx Power (kW) 20-400 20 N/A
RECEIVE
Frequency (MHz) 2270-2300 2270-2300 2200-2300
Wavelength (m) 0.131 0.131 0.131
Ant Gain (dBi) 63.3 56.7 56.0
Beamwidth (deg) 0.108 N/A 0.24
Polarization L or R L or R L or R
System Temp (K) 20 31 38

DSS X-Band Characteristics

  70-m 34-m BWG 34-m HEF
TRANSMIT      
Frequency (MHz) 8495 7145-7190 7145-7190
Wavelength (m) 0.035 0.042 0.042
Ant Gain (dBi) 74.2 66.9 67
Beamwidth (deg) N/A N/A 0.074
Polarization L or R L or R L or R
Tx Power (kW) 360 20 20
RECEIVE
Frequency (MHz) 8400-8500 8400-8500 8400-8500
Wavelength (m) 0.036- 0.036- 0.036
Ant Gain (dBi) 74.2 68.1 68.3
Beamwidth (deg) 0.031 N/A 0.063
Polarization L or R L or R L or R
System Temp (K) 20 30 20

NB: X-band 70-m transmitting parameters are given at 8495 MHz, the frequency used by the Goldstone planetary radar system. For telecommunications, the transmitting frequency would be in the range 7145-7190 MHz, the power would typically be 20 kW, and the gain would be about 72.6 dB (70-m antenna). When ground transmitters are used in spacecraft radio science experiments, the details of transmitter and antenna performance rarely impact the results.

Detectors-DSN

DSCC Closed-Loop Receivers

Nominal carrier tracking loop threshold noise bandwidth at both S- and X-band is 10 Hz. Coherent (two-way) closed-loop system stability is shown in the table below:

Integration time (sec) Doppler uncertainty (one sigma, microns/sec)
10 50
60 20
1000 4

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 be available 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 calibrations are 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 to attenuator 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 specific information 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; pointing predicts 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 phase changes in the received signal.

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 be accommodated 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 reception in two-way mode is usually preferred for routine tracking. Occasionally the spacecraft operates coherently while two ground stations receive the 'downlink' signal; this is sometimes known as the 'three-way' mode.

DSN Station Locations

Station Geocentric Geocentric Geocentric
X(m) Y (km) Z (km)
Goldstone
DSS 13 (34-m R&D) -2351112.491 -4655530.714 +3660912.787
DSS 14 (70-m) -2353621.251 -4641341.542 +3677052.370
DSS 15 (34-m HEF) -2353538.790 -4641649.507 +3676670.043
DSS 24 (34-m BWG) -2354906.495 -4646840.128 +3669242.317
DSS 25 (34-m BWG) -2355022.066 -4646953.636 +3669040.895
DSS 26 (34-m BWG) -2354890.967 -4647166.925 +3668872.212
Canberra
DSS 34 (34-m BWG) -4461146.756 +2682439.293 -3674393.542
DSS 43 (70-m) -4460894.585 +2682361.554 -3674748.580
DSS 45 (34-m HEF) -4460935.250 +2682765.710 -3674381.402
Madrid
DSS 54 (34-m BWG) +4849434.555 -0360724.108 +4114618.643
DSS 63 (70-m) +4849092.647 -0360180.569 +4115109.113
DSS 65 (34-m HEF) +4849336.730 -0360488.859 +4114748.775

Measurement Parameters - DSN

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 150 entries including status information and measurements of ranging, Doppler, and signal strength.

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
  • ATDF: Archival Tracking Data File
  • AUX: Auxiliary
  • AZ: Azimuth
  • 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
  • DEC: Declination
  • deg: degree
  • DMC: DSCC Monitor and Control Subsystem
  • DOR: Differential One-way Ranging
  • 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
  • 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
  • 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)
  • LMA: Lockheed Martin Astronautics
  • 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
  • MPF: Mars Pathfinder
  • MHz: Megahertz
  • MON: Monitor and Control System
  • MOT: Mars Observer Transponder
  • 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
  • NRV : NOCC Radio Science/VLBI Display Subsystem
  • NSS : NOCC Support System
  • OCI : Operator Control Input
  • ODF : Orbit Data File
  • ODR : Original Data Record
  • ODS : Original Data Stream
  • ODY : Mars Odyssey Orbiter
  • 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
  • RIC: RIV Controller
  • RIV: Radio Science IF-VF Converter Assembly
  • RMDCT: Radio Metric Data Conditioning Team
  • RMS : Root Mean Square
  • RSS: Radio Science Subsystem
  • 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
  • UHF : Ultra High Frequency
  • UNK : unknown
  • USO : UltraStable Oscillator
  • UTC : Universal Coordinated Time
  • VCO : Voltage-Controlled Oscillator
  • VF :Video Frequency
  • X-band: approximately 7800-8500 MHz

see also