Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:mo.rss::1.1
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| NAME |
RADIO SCIENCE SUBSYSTEM
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| TYPE |
ATMOSPHERIC SCIENCES
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| DESCRIPTION |
Instrument Overview =================== The Mars Observer spacecraft telecommunications subsystem was to have served as part of a Radio Science instrument for investigations of Mars. The remainder of the 'instrument' was located at ground stations of the NASA Deep Space Network (DSN). Much of the equipment at both ends was shared, being used for routine telecommunications as well as for Radio Science. Radio data were themselves shared; Doppler and range measurements were used to calculate the spacecraft trajectory and to search for gravity waves. Measurements of signal parameters after waves had propagated through the interplanetary medium were used to infer properties of the medium, characterize small perturbations in spacecraft attitude, and search for gravity waves. Data included in this archive data set are from routine tracking and special tests conducted while Mars Observer was in its Cruise Phase between Earth and Mars. Cruise operations were used to improve operator proficiency in carrying out activities related to radio science observations and to measure the performance of various parts of the radio science and ground data systems. Instrument Overview - Spacecraft ================================ Spacecraft Radio System: The spacecraft radio system was constructed around a redundant pair of Mars Observer Transponders (MOTs) which received at 7.2 GHz and transmitted at 8.4 GHz, both of which are called 'X-band' frequencies. The exact transmitted frequency was controlled by the signal received from a ground station or by an on-board oscillator. The transponders provided the usual uplink command and downlink data transmission capabilities. The MOT also included a ranging 'channel' which could be driven either by signals from the ground for measurement of absolute range or by tones generated on-board to enable measurement of differential range between two ground stations. An MOT generated a downlink signal in either a 'coherent' or a 'non-coherent' mode, also known as the 'two-way' and 'one-way' modes, respectively. When operating in the coherent mode, the MOT behaved as a conventional transponder, its transmitted carrier frequency being derived from the received uplink carrier frequency with a 'turn-around ratio' of 880/749. In the non-coherent mode, the downlink carrier frequency was derived from one of the spacecraft's on-board oscillators. One of the on-board oscillators, the 'ultra-stable oscillator' (USO), had been specially provided to serve as a precision frequency reference for radio occultation experiments. Built by the Applied Physics Laboratory of John Hopkins University, the USO was positioned within the spacecraft so as to minimize its exposure to thermal variations and mechanical vibration. The USO consisted of a quartz crystal resonator contained in a temperature-controlled titanium dewar. Its output frequency was near 19.14 MHz. The X-band carrier for non-coherent downlink transmission was produced by multiplying the USO output frequency by 440. The Mars Observer USO was from a newer generation than those flown on the Voyager and Galileo spacecraft. Its frequency stability, as characterized by the Allan deviation and after accounting for linear drift, was measured to be better than 2E-13 for integration times in the range one to several hundred seconds. This represents more than an order of magnitude improvement in performance over the Voyager and Galileo devices. After a 25 day warm-up period, the USO was expected to have a daily aging rate of 6E-11 and a long term frequency change after five years of significantly less than 1E-6. Laboratory measurements also showed state-of-the-art phase noise and spectral purity as well as amplitude stability performance for a device of this type. During Cruise, the USO drifted 0.2 Hz/day at its X-band output frequency; Allan Deviation for the end-to-end non-coherent system was consistent with the 2E-13 measurements before launch. Each MOT also contained a separate, 'auxiliary' oscillator which could be used to generate the non-coherent downlink carrier. However, the frequency stability of the auxiliary oscillators was about six to seven orders of magnitude poorer than the USO. They were, therefore, not applicable to these radio science investigations. The strength of a spacecraft carrier signal, and thus the quality of the radio occultation data, depends on its modulation state. Mars Observer telemetry data were Manchester encoded and then used to modulate a 1 MHz square wave subcarrier which, in turn, modulated the X-band carrier. Telemetry modulation typically resulted in carrier suppression of either 15.2 dB (simultaneous transmission of science and engineering telemetry) or 9.3 dB (engineering telemetry only). The two-way ranging channel, when activated, suppressed the carrier by about 0.2 dB; the differential range modulation reduced the carrier by another 0.2 dB when in use. Each modulation type -- telemetry, range, and differential range modulation -- could be switched on or off independently. Traveling wave tube amplifiers, driven at saturation, amplified the MOT output before the signals were radiated via (nominally) a steerable 1.5 m diameter parabolic high gain antenna (HGA). Effective isotropic radiated power was about 83 dBm. In contrast to past practice for planetary missions, the HGA was mounted at the end of a 5.6 meter boom so that the phase center of the antenna was about 6 meters from the spacecraft center of mass. Pertinent details of the subsystem are shown below: Instrument Id : RSS Instrument Host Id : MO Pi Pds User Id : LTYLER 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 : MOTOROLA Principal Investigator ---------------------- The Team Leader for the Radio Science Team was G. Leonard Tyler of Stanford University. Scientific Objectives ===================== Two different types of radio science experiments were to be conducted with Mars Observer: radio tracking experiments in which the magnitude and direction of the planet's gravity field were derived from the Doppler and ranging measurements, and radio propagation experiments in which modulation on the signal received on Earth could be attributed to properties of an occulting medium, such as the atmosphere of Mars. Gravity Measurements -------------------- Measurement of the gravity field provides significant constraints on inferences about interior structure of Mars. Precise, detailed study of the spacecraft motion in Mars orbit would have yielded the mass distribution of the planet. Topographic data obtained by the Mars Observer Laser Altimeter (MOLA) would have formed a critical adjunct to the measurements of the gravity field since only after the gravitational effects are adjusted for topography can the gravity anomalies be interpreted geophysically. For example, our present understanding of the geometrical flattening is sufficiently uncertain that discussions as to whether or not Mars is in hydrostatic equilibrium cannot be conclusive until altimeter data are available; similarly, the large uncertainty in the precession constant precludes knowledge of individual moments of inertia. Values for these basic quantities derived from Mars Observer would have been useful in studies of Mars' internal stress, radial density, and internal temperature. Studies of the gravity field would have emphasized both the global field and local characteristics of the field. The first task was to determine the global field. Doppler and range tracking measurements spanning periods of several days would have yielded accurate spacecraft trajectory solutions. Simultaneously with reconstruction of the spacecraft orbit, observation equations for approximately 2500 field coefficients and a small number of ancillary parameters would have been derived. This type of gravity field solution is essential for characterizing tectonic phenomena such as the Tharsis bulge and can also be used to study localized features. Results were expected to be available some months after the data were collected, although the final global gravity field would not have been available for one or two years after the end of the Mission. 'Short-arc,' line-of-sight Doppler tracking measurements obtained when the Earth-to-spacecraft line-of-sight was within a few degrees of the orbit plane were expected to provide the highest resolution of local features. The results from this type of observation typically are presented as contoured acceleration profiles of specific features (e.g., craters, volcanoes, etc.) or line-of-sight acceleration maps of specific regions. The high spatial resolution of these products makes them especially useful to geophysicists for study of features in the size range of 300 to 1,000 km. Because of the relative simplicity of the data analysis, results can be available within a few weeks after the data are collected. Radio Occultation Measurements ------------------------------ Atmospheric measurements by the method of radio occultation were expected to contribute to an improved understanding of the circulation, dynamics, and the transport of volatiles and dust in the atmosphere of Mars. These results would have been based on detailed analysis of the radio signal received from Mars Observer as it entered and exited occultation by the planet. Two coordinated investigations of the atmosphere were planned within the Radio Science Team. The first was to focus on obtaining vertical profiles of atmospheric structure with emphasis on investigation of large-scale phenomena, while the second was to concentrate on studies of the small-scale atmospheric structure that causes scintillations in the radio signal. Retrieval of atmospheric profiles requires coherent samples of the radio signal that has propagated through Mars' atmosphere, plus accurate knowledge of the spacecraft trajectory. The latter was to be obtained first from the Mars Observer Navigation Team and later, internally, from Radio Science Team gravity investigators. Initial solutions were expected to provide atmospheric structure -- temperature and pressure vs. absolute radius -- to altitudes of at least 50 km with about 240 m vertical resolution; these 'standard' profiles were expected within 2-4 weeks of data acquisition. After correction for diffractive propagation effects, vertical resolutions of 10-20 m might have been obtained. These 'high-resolution' profiles require an increase in the complexity of retrieval algorithms, which were being revised when the spacecraft was lost. Measures of total columnar content, surface pressure, planet radius at the occultation point, and properties of the ionosphere, if detectable, would have been obtained for each occultation. The spatial and temporal coverage in the radio occultation experiments would have been determined by the geometry of the spacecraft orbit. In the mapping phase of the mission, the Mars Observer spacecraft was to circle Mars in a low-altitude, sun-synchronous, near-polar orbit with a period of about 117 min. As most orbits of the spacecraft included an occultation by Mars, this trajectory provided frequent opportunities for radio occultation measurements of the neutral atmosphere and ionosphere. The occultations generally occurred at high latitudes in both hemispheres. The rotation of the planet between successive orbits would have allowed systematic measurements at regular intervals of about 29 degrees in longitude. Although most occultations were expected to be in polar regions, the view from Earth of the sun-synchronous spacecraft orbit would have evolved slowly in response to the orbital motions of Earth and Mars about the Sun, yielding opportunities for occultation measurements at a variety of latitudes and local times over the course of the 687-day mission. Operational Considerations ========================== Descriptions given here are for nominal performance. The spacecraft transponder system comprised redundant units, each with slightly different characteristics. As transponder units age, their performance changes slightly. More importantly, the performance for Radio Science depended on operational factors such as the modulation state for the transmitters, which cannot be predicted in advance. The performance also depended on factors which were not always under the control of the Mars Observer Project. During radio occultation periods, telemetry modulation would have been turned off; during many of the tests included in the Cruise data set telemetry was also suspended. No special modulation requirements were required during routine tracking for trajectory determination. The steerable 1.5 m diameter parabolic high gain antenna (HGA) was mounted at the end of a 5.6 meter boom so that the phase center of the antenna was about 6 meters from the spacecraft center of mass. An area of concern to the Mars Observer Radio Science and Navigation Teams was the set of 'pseudo-Doppler' effects introduced into the radio tracking and occultation data by motion of the HGA relative to the spacecraft center of mass; it would have been necessary to remove these during data reduction before deriving the gravity field or occultation profiles. Six sources of HGA motion were identified: (i) the spacecraft rotation needed to maintain nadir pointing while in orbit, (ii) articulation of the HGA to maintain Earth point, (iii) HGA boom flexure, (iv) short term migration of the spacecraft center of mass associated with effects such as 'sloshing' of propellants and motions of articulating booms, (v) spacecraft attitude instability, and (vi) spacecraft response to cycling of momentum wheels. The first two error sources are deterministic to the extent that their effects can be taken into account accurately with rather straightforward modeling. Boom flexure was believed to be insignificant. Other motions of the spacecraft center of mass were studied; there were reasons to believe that they would not have proven to be major problems. Item (v) remained a concern at the time the spacecraft was lost. When first analyzed in 1987, tracking error from this source alone was calculated to be nearly an order-of-magnitude greater than that from the next largest source. For the normal 'mapping' configuration, nadir pointing of the spacecraft would have been controlled by referencing spacecraft attitude to the Mars 'horizon' as defined by carbon dioxide emissions from the atmosphere. Atmospheric disturbances can affect determination of the horizon, however, causing the spacecraft attitude to deviate from the true nadir orientation. Doppler error from HGA motion, then, can be coupled to atmospheric features, some of which are stable and associated with surface features (e.g., the semi-permanent waves associated with the Tharsis volcanoes, and the winter polar hood). This coupling would have made such signatures difficult or impossible to distinguish in the data from Doppler effects caused by the spacecraft response to gravity features. After the 1987 analysis, attitude control design engineers at the spacecraft contractor (General Electric, AstroSpace Division) and at the Jet Propulsion Laboratory (JPL) implemented new control algorithms to reduce spacecraft attitude response to local atmospheric features. By mid-1993, analytically predicted Doppler effects resulting from HGA motion relative to the spacecraft center of mass no longer represented the dominant error source for gravity field or atmospheric occultation measurements. Predicted errors associated with the attitude control system remained significant, however, and study of this question was continuing. Item (vi) was identified as a potential source of error only after launch of Mars Observer. Normal rotation of the spacecraft resulted in transfer of angular momentum among the on board momentum wheels. At regular intervals, spaced by 90 degrees of rotation, one set of wheels or another would pass through zero angular momentum and physically stop. Static friction prevented smooth resumption of wheel rotation as the spacecraft continued turning; an attitude transient resulted, which could last for several minutes. Modeling of these 'sticksion' events based on reconstruction of attitude allowed most Doppler effects to be removed from non-coherent Cruise data. But signal-to-noise ratios would have been considerably higher during Mapping, and the accuracy to which this could have been carried out under the best radio science observing conditions remained a concern. Calibration =========== No information is available on calibration of the radio system. Operational Modes ================= The Mars Observer telecommunications system could be viewed as having two sections, which could be operated in the following modes: Section Mode ------------------------------------------- Oscillator two-way (coherent) one-way (non-coherent) RF output low-gain antenna high-gain antenna Selected parameters describing transponder performance are listed below: Oscillator Parameters: X-Band Two-Way Transponder Turnaround Ratio 880/749 One-Way Transmit Frequency (MHz) 8423.200 RF Output parameters: X-Band Power at HGA output 46.5 dBm High-Gain Antenna: Gain (dBi) 39.95 Polarization Right Circular 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 standard (STD)], 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 : LTYLER Instrument Name : RADIO SCIENCE SUBSYSTEM Instrument Type : RADIO SCIENCE Build Date : N/A Instrument Mass : N/A Instrument Length : N/A Instrument Width : N/A Instrument Height : N/A Instrument Manufacturer Name : N/A For more information on the Deep Space Network and its use in radio science investigations see the reports by [ASMAR&RENZETTI1993] and [ASMAR&HERRERA1993]. For design specifications on DSN subsystems see [DSN810-5]. For an example of use of the DSN for Radio Science see [TYLERETAL1992]. Subsystems - DSN ================ The Deep Space Communications Complexes (DSCCs) are an integral part of the Radio Science instrument, along with other receiving stations and the spacecraft Radio Frequency Subsystem. Their system performance directly determines the degree of success of Radio Science investigations, and their system calibration determines the degree of accuracy in the results of the experiments. The following paragraphs describe the functions performed by the individual subsystems of a DSCC. This material has been adapted from [ASMAR&HERRERA1993]; for additional information, 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 individually instrumented) are listed in the table. -------- -------- -------- -------- -------- | DSS 12 | | DSS 18 | | DSS 14 | | DSS 15 | | DSS 16 | |34-m STD| |34-m STD| | 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 STD DSS 12 DSS 42 DSS 61 DSS 18 DSS 48 DSS 68 34-m HEF DSS 15 DSS 45 DSS 65 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 between 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 STD 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 34-m STD primary reflectors are classical paraboloids, while the subreflectors are standard hyperboloids. On the 70-m and 34-m STD 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. 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. Unlike the 70-m and 34-m HEFs which have azimuth-elevation (AZ-EL) drives, the 34-m STD antennas use (hour angle-declination) HA-DEC drives. The same positioning of the subreflector on the 34-m STD does not create the same effect as on the 70-m and 34-m HEFs. Pointing angles for all three 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 for the 70-m and 34-m HEFs or into HA-DEC coordinates for the 34-m STD antennas. The LMC operator then downloads the antenna AZ-EL or HA-DEC predict points to the antenna-mounted ACS computer along with a selected SEC model. The pointing predicts consist of time-tagged AZ-EL or HA-DEC 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. In the 34-m STD antennas motors, rather than servos, are used to steer the antenna; there is no feedback once the 34-m STD has been told where to point. 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 STD Antennas: These antennas have two feed horns, one for S-band signals and one for X-band. The horns are mounted on a cone which is fixed in relation to the subreflector. A dichroic plate mounted above the horns directs energy from the subreflector into the proper horn. The AMS directs the received S- and X-band signals through polarizer plates and on to amplification. There are two Block III S-band TWMs and two Block I X-band TWMs. 34-m HEF Antennas: Unlike the other 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. 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 IV 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 Block III receiver-exciter at the 34-m STD stations allows for two receiver channels, each capable of S-band or X-band reception and an exciter used to generate an uplink signal through the low-power transmitter. The receiver-exciter at the 34-m HEF stations allows for one channel only. 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). 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 in the antenna, 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 IVC switches the IF input between the 70-m and 34-m HEF 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 Block III or Block IV 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) tape 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 Spectrum Processing Subsystem (DSP) ---------------------------------------- The DSCC Spectrum Processing Subsystem (DSP) located at the SPC digitizes and records on magnetic tapes the narrowband output data from the RIV. It consists of a Narrow Band Occultation Converter (NBOC) containing four Analog-to- Digital Converters (ADCs), a ModComp CLASSIC computer processor called the Spectrum Processing Assembly (SPA), and two to six magnetic tape drives. Magnetic tapes are known as Original Data Records (ODRs). Electronic near real-time transmission of data to JPL (an Original Data Stream, or ODS) may be possible in certain circumstances; The DSP is operated through the LMC. Using the SPA-R software, the DSP allows for real-time frequency and time offsets (while in RUN mode) and, if necessary, snap tuning between the two frequency ranges transmitted by the spacecraft: coherent and non-coherent. The DSP receives Radio Science frequency predicts from the CMC, allows for multiple predict set archiving (up to 60 sets) at the SPA, and allows for manual predict generation and editing. It accepts configuration and control data from the LMC, provides display data to the LMC, and transmits the signal spectra from the SSI as well as status information to NOCC and the Project Mission Support Area (MSA) via the GCF data lines. The DSP records the digitized narrowband samples and the supporting header information (i.e., time tags, POCA frequencies, etc.) on 9-track magnetic tapes in 6250 or 1600 bpi GCR format. Through the DSP-RIC interface the DSP controls the RIV filter selection and attenuation levels. It also receives RIV performance monitoring via the RIC. In case of failure of the DSP-RIC interface, the RIV can be controlled manually from the front panel. All the RIV and DSP control parameters and configuration directives are stored in the SPA in a macro-like file called an 'experiment directive' table. A number of default directives exist in the DSP for the major Radio Science experiments. Operators can create their own table entries. Items such as verification of the configuration of the prime open-loop recording subsystem, the selection of the required predict sets, and proper system performance prior to the recording periods will be checked in real-time at JPL via the NOCC displays using primarily the remote SSI display at NOCC and the NRV displays. Because of this, transmission of the DSP/SSI monitor information is enabled prior to the start of recording. The specific run time and tape recording times will be identified in the Sequence of Events (SOE) and/or DSN Keyword File. The DSP can be used to duplicate ODRs. It also has the capability to play back a certain section of the recorded data after conclusion of the recording periods. 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 64-m 70-m 34-m 34-m Transmit STD HEF -------- ----- ----- ----- ----- Frequency (MHz) 2110- 2110- 2025- N/A 2120 2120 2120 Wavelength (m) 0.142 0.142 0.142 N/A Ant Gain (dBi) 62.7 55.2 N/A Beamwidth (deg) 0.119 0.31 N/A Polarization L or R L or R N/A Tx Power (kW) 20-400 20 N/A Receive ------- Frequency (MHz) 2270- 2270- 2270- 2200- 2300 2300 2300 2300 Wavelength (m) 0.131 0.131 0.131 0.131 Ant Gain (dBi) 61.6 63.3 56.2 56.0 Beamwidth (deg) 0.108 0.27 0.24 Polarization L & R L & R L or R L or R System Temp (K) 22 20 22 38 DSS X-Band Characteristics 64-m 70-m 34-m 34-m Transmit STD HEF -------- ----- ----- ----- ----- Frequency (MHz) 8495 8495 N/A 7145- 7190 Wavelength (m) 0.035 0.035 N/A 0.042 Ant Gain (dBi) 74.2 N/A 67 Beamwidth (deg) N/A 0.074 Polarization L or R L or R N/A L or R Tx Power (kW) 360 360 N/A 20 Receive ------- Frequency (MHz) 8400- 8400- 8400- 8400- 8500 8500 8500 8500 Wavelength (m) 0.036 0.036 0.036 0.036 Ant Gain (dBi) 71.7 74.2 66.2 68.3 Beamwidth (deg) 0.031 0.075 0.063 Polarization L & R L & R L & R L & R System Temp (K) 27 20 25 20 Electronics - DSN ================= DSCC Open-Loop Receiver ----------------------- The open loop receiver block diagram shown below is for 70-m and 34-m High-Efficiency (HEF) antenna sites. Based on a tuning prediction file, the POCA controls the DANA synthesizer the output of which (after multiplication) mixes input signals at both S- and X-band to fixed intermediate frequencies for amplification. These signals in turn are down converted and passed through additional filters until they yield baseband output of up to 25 kHz in width. The baseband output is digitally sampled by the DSP and either written to magnetic tape or electronically transferred for further analysis. S-Band X-Band 2295 MHz 8415 MHz Input Input | | v v --- --- --- --- | X |<--|x20|<--100 MHz 100 MHz-->|x81|-->| X | --- --- --- --- | | 295| |315 MHz| |MHz v v --- -- --- --- | X |<--|x3|<------ ------>|x11|-->| X | --- -- |115 33| --- --- | |MHz MHz| | | | | | 50| --- --- |50 MHz| 72 MHz->| X | | X | |MHz v --- --- v --- ^ ^ --- | X |<--60 MHz | 43 43| 60 MHz-->| X | --- |MHz MHz| --- | 9.9 | | 9.9 | | MHz ------------- MHz | | | ^ | | 10| v | v |10 MHz| --- ---------- --- |MHz |------>| X | | DANA | | X |<------| | --- |Synthesizr| --- | | | ---------- | | v v ^ v v ------- ------- | ------- ------- |Filters| |Filters| ---------- |Filters| |Filters| |3,4,5,6| | 1,2 | | POCA | | 1,2 | |3,4,5,6| ------- ------- |Controller| ------- ------- | | ---------- | | 10| |0.1 0.1| |10 MHz| |MHz MHz| |MHz v v v v --- --- --- --- | X |- -| X | | X |- -| X | --- | | --- --- | | --- ^ | | ^ ^ | | ^ | | | | | | | | 10 | | 0.1 0.1 | | 10 MHz | | MHz MHz | | MHz | | | | v v v v Baseband Baseband Output Output Reconstruction of the antenna frequency from the frequency of the signal in the recorded data can be achieved through use of one of the following formulas. Radio Science IF-VF (RIV) Converter Assembly at 70-m and 34-m High-Efficiency (HEF) antennas: FSant=3*[POCA+(79/11)*10^6] + 1.95*10^9 - Fsamp - Frec FXant=11*[POCA-10^7] + 8.050*10^9 - 3*Fsamp + Frec Multi-Mission Receivers at 34-m Standard antennas: FSant=48*POCA + 3*10^8 - 0.75*Fsamp + Frec FXant = (11/3)*[48*POCA + 3*10^8 - 0.75*Fsamp] + Frec where FSant = S-band antenna frequency FXant = X-band antenna frequency POCA = POCA frequency Fsamp = sampling frequency Frec = frequency of recorded signal Filters - DSN ============= DSCC Open-Loop Receiver ----------------------- Nominal filter center frequencies and bandwidths for the Open-Loop Receivers are shown in the table below. Filter Center Frequency 3 dB Bandwidth ------ ---------------- -------------- 1 0.1 MHz 90 Hz 2 0.1 MHz 450 Hz 3 10.0 MHz 2000 Hz 4 10.0 MHz 1700 Hz (S-band) 6250 Hz (X-band) 5 10.0 MHz 45000 Hz 6 10.0 MHz 21000 Hz Detectors - DSN =============== DSCC Open-Loop Receivers ------------------------ Open-loop receiver output is detected in software by the radio science investigator. 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 Doppler uncertainty (secs) (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. Open-Loop Receiver Attenuation Calibration ------------------------------------------ The open-loop receiver attenuator calibrations are performed to establish the output of the open-loop receivers at a level that will not saturate the analog-to-digital converters. To achieve this, the calibration is done using a test signal generated by the exciter/translator that is set to the peak predicted signal level for the upcoming pass. Then the output level of the receiver's video band spectrum envelope is adjusted to the level determined by equation (3) below (to five-sigma). Note that the SNR in the equation (2) is in dB while the SNR in equation (3) is linear. Pn = -198.6 + 10*log(SNT) + 10*log(1.2*Fbw) (1) SNR = Ps - Pn (SNR in dB) (2) Vrms = sqrt(SNR + 1)/[1 + 0.283*sqrt(SNR)] (SNR linear) (3) where Fbw = receiver filter bandwidth (Hz) Pn = receiver noise power (dBm) Ps = signal power (dBm) SNT = system noise temperature (K) SNR = predicted signal-to-noise ratio 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. DSCC Receiver-Exciter Subsystem ------------------------------- The diplexer in the signal path between the transmitter and the feed horns on all three antennas may be configured so that it is out of the received signal path in order to improve the signal-to-noise ratio in the receiver system. This is known as the 'listen-only' or 'bypass' mode. Closed-Loop vs. Open-Loop Reception ----------------------------------- Radio Science data can be collected in two modes: closed- loop, in which a phase-locked loop receiver tracks the spacecraft signal, or open-loop, in which a receiver samples and records a band within which the desired signal presumably resides. Closed-loop data are collected using Closed-Loop Receivers, and open-loop data are collected using Open-Loop Receivers in conjunction with the DSCC Spectrum Processing Subsystem (DSP). See the Subsystems section for further information. 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. DSCC Spectrum Processing Subsystem (DSP) ---------------------------------------- The DSP can operate in four sampling modes with from 1 to 4 input signals. Input channels are assigned to ADC inputs during DSP configuration. Modes and sampling rates are summarized in the tables below: Mode Analog-to-Digital Operation ---- ---------------------------- 1 4 signals, each sampled by a single ADC 2 1 signal, sampled sequentially by 4 ADCs 3 2 signals, each sampled sequentially by 2 ADCs 4 1 signal, sampled sequentially by 3 ADCs 8-bit Samples 12-bit Samples Sampling Rates Sampling Rates (samples/sec per ADC) (samples/sec per ADC) --------------------- --------------------- 50000 31250 25000 15625 12500 10000 10000 6250 5000 5000 4000 3125 2500 2000 1250 1000 1000 500 400 250 200 200 Location - DSN ============== Station locations are documented in [GEO-10REVD]. Geocentric coordinates are summarized here. Geocentric Geocentric Geocentric Station Radius (km) Latitude (N) Longitude (E) --------- ----------- ------------ ------------- Goldstone DSS 12 (34-m STD) 6371.997815 35.1186672 243.1945048 DSS 13 (develop) 6372.117062 35.0665485 243.2051077 DSS 14 (70-m) 6371.992867 35.2443514. 243.1104584 DSS 15 (34-m HEF) 6371.9463 35.2402863 243.1128186 DSS 16 (26-m) 6371.9608 35.1601436 243.1264200 DSS 18 (34-m STD) UNK UNK UNK Canberra DSS 42 (34-m STD) 6371.675607 -35.2191850 148.9812546 DSS 43 (70-m) 6371.688953 -35.2209308 148.9812540 DSS 45 (34-m HEF) 6371.692 -35.21709 148.97757 DSS 46 (26-m) 6371/675 -35.22360 148.98297 DSS 48 (34-m STD) UNK UNK UNK Madrid DSS 61 (34-m STD) 6370.027734 40.2388805 355.7509634 DSS 63 (70-m) 6370.051015 40.2413495 355.7519776 DSS 65 (34-m HEF) 6370.021370 40.2372843 355.7485968 DSS 66 (26-m) 6370.036 40.2400714 355.7485976 DSS 48 (34-m STD) UNK UNK UNK Measurement Parameters - DSN ============================ Open-Loop System ---------------- Output from the Open-Loop Receivers (OLRs), as sampled and recorded by the DSCC Spectrum Processing Subsystem (DSP), is a stream of 8- or 12-bit quantized voltage samples. The nominal input to the Analog-to-Digital Converters (ADCs) is +/-10 volts, but the precise scaling between input voltages and output digitized samples is usually irrelevant for analysis; the digital data are generally referenced to a known noise or signal level within the data stream itself -- for example, the thermal noise output of the radio receivers which has a known system noise temperature (SNT). Raw samples comprise the data block in each DSP record; a header record (presently 83 16-bit words) contains ancillary information such as: time tag for the first sample in the data block RMS values of receiver signal levels and ADC outputs POCA frequency and drift rate 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 117 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 AMS Antenna Microwave System APA Antenna Pointing Assembly ARA Area Routing Assembly ATDF Archival Tracking Data File AZ Azimuth CMC Complex Monitor and Control CONSCAN Conical Scanning (antenna pointing mode) CRG Coherent Reference Generator CUL Clean-up Loop DANA a type of frequency synthesizer dB deciBel dBi dB relative to isotropic DCO Digitally Controlled Oscillator DEC Declination deg degree DMC DSCC Monitor and Control Subsystem DSCC Deep Space Communications Complex DSN Deep Space Network DSP DSCC Spectrum Processing Subsystem DSS Deep Space Station DTK DSCC Tracking Subsystem E east EL Elevation FTS Frequency and Timing Subsystem GCF Ground Communications Facility GPS Global Positioning System HA Hour Angle HEF High-Efficiency (as in 34-m HEF antennas) IF Intermediate Frequency IVC IF Selection Switch JPL Jet Propulsion Laboratory K Kelvin km kilometer kW kilowatt L-band approximately 1668 MHz LAN Local Area Network LCP Left-Circularly Polarized LMC Link Monitor and Control LNA Low-Noise Amplifier LO Local Oscillator m meters MCA Master Clock Assembly MCCC Mission Control and Computing Center MDA Metric Data Assembly MHz Megahertz MON Monitor and Control System MSA Mission Support Area N north NAR Noise Adding Radiometer NBOC Narrow-Band Occultation Converter NIST SPC 10 time relative to UTC NIU Network Interface Unit NOCC Network Operations and Control System NSS NOCC Support System OCI Operator Control Input ODF Orbit Data File ODR Original Data Record ODS Original Data Stream OLR Open Loop Receiver POCA Programmable Oscillator Control Assembly PPM Precision Power Monitor RA Right Ascension REC Receiver-Exciter Controller RCP Right-Circularly Polarized RF Radio Frequency RIC RIV Controller RIV Radio Science IF-VF Converter Assembly RMDCT Radio Metric Data Conditioning Team RTLT Round-Trip Light Time S-band approximately 2100-2300 MHz sec second SEC System Error Correction SIM Simulation SLE Signal Level Estimator SNR Signal-to-Noise Ratio SNT System Noise Temperature SOE Sequence of Events SPA Spectrum Processing Assembly SPC Signal Processing Center SRA Sequential Ranging Assembly SRC Sub-Reflector Controller SSI Spectral Signal Indicator STD Standard (as in 34-m STD antennas) TID Time Insertion and Distribution Assembly TSF Tracking Synthesizer Frequency TWM Traveling Wave Maser UNK unknown UTC Universal Coordinated Time VF Video Frequency X-band approximately 7800-8500 MHz
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| REFERENCES |
Asmar, S.W., and R.G. Herrera, Radio Science Handbook, JPL D-7938, Volume 4, Jet Propulsion Laboratory, Pasadena, CA, 22 January 1993.
Asmar, S. W., N. A. Renzetti, The Deep Space Network as an instrument for radio science research, NASA Technical Reports Server, 1993STIN...9521456A, 1993.
DSN Geometry and Spacecraft Visibility, Document 810-5, Rev. D, Vol. 1, DSN/Flight Project Interface Design, Jet Propulsion Laboratory, Pasadena, CA, 1987.
Tyler, G.L., G. Balmino, D.P. Hinson, W.L. Sjogren, D.E. Smith, R. Woo, S.W. Asmar, M.J. Connally, C.L. Hamilton, and R.A. Simpson, Radio Science Investigations with Mars Observer, Journal of Geophysical Research, 97, 7759-7779, 1992.
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