urn:esa:psa:context:instrument:vex.rss 1.0 1.11.0.0 Product_Context urn:nasa:pds:context:instrument:vex.rss 2021-02-24 1.0 Changed inst LIDs from u:n:p:c:i:instID.scID to u:n:p:c:i:scID.instID Changed LIDs from urn:nasa:pds: to urn:esa:psa: And per "Guide toPDS4 Context Products" v1.7, changed all lidvid_reference to lid_reference urn:esa:psa:context:instrument_host:spacecraft.vex instrument_to_instrument_host Accomazzo, A., Venus Express mission operations report (MOR #24), VEX-ESC-RP-5000/24, reporting period 14-Apr-06 to 20-Apr-06, ESA European Space Operations Centre (ESOC), 21 April 2006. reference.ACCOMAZZO2006 Asmar, S.W., and R.G. Herrera, Radio Science Handbook, JPL D-7938, Volume 4, Jet Propulsion Laboratory, Pasadena, CA, 22 January 1993. reference.ASMAR-HERRERA1993 Asmar, S. W., N. A. Renzetti, The Deep Space Network as an instrument for radio science research, NASA Technical Reports Server, 1993STIN...9521456A, 1993. reference.ASMAR-RENZETTI1993 Asmar, S.W., R.G. Herrera, and T. Priest, Radio Science Handbook, JPL D-7938, Volume 6, Jet Propulsion Laboratory, Pasadena, CA, 1995. reference.ASMARETAL1995 Deep Space Network / Flight Project Interface Design Book, Document 810-5, Jet Propulsion Laboratory, Pasadena, CA. reference.DSN810-5 Deep Space Mission Systems, Tracking and Navigation Service, Requirements and Design, DSMS No. 821-104, Rev. B, JPL D-17235, Jet Propulsion Laboratory, Pasadena, CA, 2003. reference.DSN821-104 Deep Space Mission Systems, Radio Science Service, Requirements and Design, DSMS No. 821-110, Rev. A, JPL D-17241, Jet Propulsion Laboratory, Pasadena, CA, 2001. reference.DSN821-110 Deep Space Mission Systems, Mars Express (MEX) Project, Network Operations Plan, DSMS No. 871-049-041, JPL D-25808, Jet Propulsion Laboratory, Pasadena, CA, 2003. reference.DSN871-049-041 Haeusler, B., M. Paetzold, R. Mattei, and S. Remus, Venus Express radio science experiment VERA calibration report, VEX-VERA-UBW-AN-3005 I1.0, Institut fuer Raumfahrttechnik, Universitaet der Bundeswehr, Munich, 17 June 2005. reference.HAEUSLERETAL2005 Haeusler, B., M. Paetzold, G.L. Tyler, R.A. Simpson, M.K. Bird, V. Dehant, J.-P. Barriot, W. Eidel, R. Mattei, S. Remus, J. Selle, S. Tellmann, and T. Imamura, Radio science investigations by VeRa onboard the Venus Express spacecraft, Planetary and Space Science, 54, 1315-1335, 2006. reference.HAEUSLERETAL2006 Hinson, D.P., and J.M. Jenkins, Magellan Radio Occultation Measurements of Atmospheric Waves on Venus, Icarus, 114, 310-327, 1995. reference.HINSON-JENKINS1995 Jenkins, J.M., and P.G. Steffes, Results for 13-cm absorptivity and H2SO4 abundance profiles from the season 10 (1986) Pioneer Venus Orbiter radio occultation experiment, Icarus, 90, 129-138, 1991. reference.JENKINS-STEFFES1991 Jenkins, J.M., P.G. Steffes, D.P. Hinson, J.D. Twicken, and G.L. Tyler, Radio Occultation Studies of the Venus Atmosphere with the Magellan Spacecraft: 2. Results from the October 1991 Experiments, Icarus, 110, 79-94, 1994. reference.JENKINSETAL1994 Mars Global Surveyor Project, Telecommunications System Operations Reference Handbook, Version 2.1 (MGS 542-257), JPL Document D-14027, Jet Propulsion Laboratory, Pasadena, CA, 1996. reference.JPLD-14027 Kliore, A.J., and I.R. Patel, Thermal structure of the atmosphere of Venus from Pioneer Venus radio occultations, Icarus, 52, 320-334, 1982. reference.KLIORE-PATEL1982 Konopliv, A.S., W.B. Banerdt, W.L. Sjogren, Venus Gravity: 180th degree and order model, Icarus, 139, 3-18, 1999. reference.KONOPLIVETAL1999 Mattei, R., B. Haeusler, M. Paetzold, S. Remus, W. Eidel, S. Tellmann, T. Andert, J. Selle, M. K. Bird, R. A. Simpson, G. L. Tyler, V. Dehant, S. Asmar, J.-P. Barriot, and T. Imamura, The radio science experiment 'VeRa' onboard ESA's Venus Express spacecraft, 1st CEAS, European Air and Space Conference, Publication CEAS-2007-121, 2055-2064, 10-13 September 2007. reference.MATTEIETAL2007 Newman, M., G. Schubert, A.J. Kliore, and I.R. Patel, Zonal winds in the middle atmosphere of Venus from Pioneer Venus radio occultation data, Journal of Atmospheric Science, 41, 1901-1913, 1984. reference.NEWMANETAL1984 Paetzold, M., B. Haeusler, M.K. Bird, S. Tellmann, R. Mattei, S.W. Asmar, V. Dehant, W. Eidel, T. Imamura, R.A. Simpson, and G.L. Tyler, The structure of Venus' middle atmosphere and ionosphere, Nature, 450, 657-660, doi:10.1038/nature06239, 2007. reference.PAETZOLDETAL2007B Pettengill, G.H., P.G. Ford, and R.A. Simpson, Electrical properties of the Venus surface from bistatic radar observations, Science, 272, 1628-1631, 1996. reference.PETTENGILLETAL1996 Tracadis, P.W., M.T. Zuber, D.E. Smith, and F.G. Lemoine, Density Structure of the Upper Thermosphere of Mars from Measurements of Air Drag on the Mars Global Surveyor Spacecraft, J. Geophys. Res., 106, 23349-23357, 2001. reference.TRACADISETAL2001 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. reference.TYLERETAL1992 Tyler, G.L., G. Balmino, D.P. Hinson, W.L. Sjogren, D.E. Smith, R.A. Simpson, S.W. Asmar, P. Priest, and J.D. Twicken, Radio science observations with Mars Global Surveyor: Orbit insertion through one Mars year in mapping orbit, Journal of Geophysical Research, 106, 23327-23348, 2001. reference.TYLERETAL2001 Woo, R., Spacecraft Radio Scintillation and Solar System Exploration, Wave Propagation in Random Media (Scintillation), Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 1993. reference.WOO1993 RADIO SCIENCE SUBSYSTEM Atmospheric Sciences not applicable not applicable Instrument Overview =================== Venus Express (VEX) Radio Science investigations utilized instrumentation with elements on both the spacecraft and ground (Earth). Much of this was shared equipment, being used for routine telecommunications as well as for Radio Science. Ground systems were provided by the European Space Agency (ESA) at New Norcia, Australia, and Cebreros, Spain. Ground support was also provided by the U.S. National Aeronautics and Space Administration (NASA) Deep Space Network (DSN) at sites in Australia, Spain, and the United States. Because the VEX orbit period was 24 hours and pericenter was synchronized with viewing from Australia, all radio science activities were conducted using the ESA facility at New Norcia and the DSN complex at Canberra. Performance and calibration of both the spacecraft and ground systems directly affected the radio science data accuracy and played a major role in determining the quality of the results. The spacecraft was able to receive and transmit signals at both S-band (approximately 13 cm wavelength) and X-band (approximately 3.5 cm). Two low-gain and two high-gain antennas were available; but not all combinations of antenna and band could be selected. The spacecraft transmissions could use either an onboard oscillator for the frequency reference ('one-way' mode) or a signal transmitted from the ground ('two-way' mode). In the former case, an ultra-stable oscillator (USO) could be selected; in the latter case, either an S- or X-band signal from the ground could be used as the reference, depending on the antenna selected. This description covers only the spacecraft and DSN components of the instrument and only those portions of the VEX Radio Science investigations supported by DSN data. For more information see [HAEUSLERETAL2006] and [MATTEIETAL2007]. Science Objectives ================== Two different types of radio science measurements were carried out with Venus Express: Radiometric Measurements: 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. NB: Doppler measurements can be made in the one-way mode, but are usually more accurate if carried out in the two-way mode. Radio Propagation Measurements: 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. Radio propagation measurements can be conducted in either one-way or two-way mode. These measurements were applied - separately and together - to Venus science objectives such as inference of local gravity field anomalies, atmospheric drag, temperature and pressure of the atmosphere, electron density in the ionosphere, scattering properties of the surface, and structure of the solar wind. Gravity Measurements -------------------- Measurement of the gravity field provides significant constraints on inferences about the interior structure of Venus. Precise, detailed study of the spacecraft motion in Venus orbit can yield the mass distribution of the planet. Topographic data, such as those obtained by the MAGELLAN radar altimeter, form a critical adjunct to these measurements since only after the gravitational effects are adjusted for topography can the gravity anomalies be interpreted geophysically. Because of the thorough investigation of the Venus gravity field by MAGELLAN [KONOPLIVETAL1999], Venus Express studies focused on the characteristics of the field at 140-220 degrees east longitude and 40-80 degrees north latitude, where gaps in the MAGELLAN tracking were most common. The main area, known as Atalanta Plantia, is a low altitude basin of diameter of about 1500 km and depth of about 2 km. From its pericenter altitude of about 250 km, VEX was expected to provide improvements to the field in this area; those, in turn, would lead to better understanding of the lithosphere. A possible by-product of the gravity field analysis is information on the density structure of the upper atmosphere [TRACADISETAL2001]. The first campaign designed to detect and measure drag was conducted during the summer of 2008. Radio Occultation Measurements ------------------------------ Atmospheric measurements by the method of radio occultation contribute to an improved understanding of structure, circulation, dynamics, and transport in the atmosphere. These results are based on detailed analysis of the radio signal phase as the ray path enters and exits occultation by the planet, leading to profiles of temperature and pressure in the neutral atmosphere and profiles of electron density in the ionosphere. Once the refractivity profile of the neutral atmosphere has been determined, refractivity can be used to estimate defocusing loss and the remaining loss in signal intensity can be attributed to absorption. Venus radio occultation data have previously been collected and analyzed from Pioneer Venus Orbiter [KLIORE&PATEL1982] [NEWMANETAL1984] [JENKINS&STEFFES1991] and MAGELLAN [JENKINSETAL1994] [HINSON&JENKINS1995]. Retrieval of atmospheric profiles requires coherent samples (samples retaining both amplitude and frequency information) of the radio signal that has propagated through the atmosphere, plus accurate knowledge of the spacecraft trajectory. The latter was obtained from the VEX Flight Dynamics Team. Solutions from VEX occultation data provided neutral atmospheric structure from about 37 km altitude (limited by critical refraction) to about 100 km and electron density profiles from 100 to 400 km [PAETZOLDETAL2007B]. Spatial and temporal coverage in radio occultation experiments are determined by the geometry of the spacecraft orbit and the dates and times at which occultation data are acquired. Since VEX radio occultation experiments were conducted on a regular basis using a polar orbit, there was extensive occultation coverage at high northern and southern latitudes (e.g., beyond 60 degrees). As the orbit appeared to drift from edge-on to nearly broadside (as viewed from Earth), occultation points moved toward the equator and the entry/exit angle approached grazing. During the first 1000 orbits/days of VEX operations, there were six occultation 'seasons' of typically 60-70 orbits each interleaved with intervals of approximately the same duration when there were no occultations. Bistatic Surface Scattering Measurements ------------------------------------------ One of the spacecraft's high-gain antennas (HGA1 or HGA2) could also be pointed toward the surface of the planet. The strength of the signal scattered from the illuminated area could be measured and the results interpreted in terms of the dielectric constant of the surface material. The model for interpretation assumes Fresnel reflection at the specular angle. The cross-spectrum between the right- and left-circularly polarized echo channels (RCP and LCP, respectively) contains information about the argument of the dielectric constant -- that is, whether it is insulating or conducting. A conducting material was inferred from MAGELLAN bistatic radar observations using linear polarization near Cleopatra Patera in Maxwell Montes [PETTENGILLETAL1996]. One goal of the VEX bistatic radar experiments was to confirm the MAGELLAN results using circular polarization and to investigate whether similar signatures could be found at other high-altitude targets. Under certain circumstances, dispersion of the echo (its spectral broadening) may be interpreted in terms of the rms surface roughness on scales comparable to the wavelength. Solar Scintillation and Faraday Rotation Experiments ---------------------------------------------------- Solar scintillation observations were conducted to improve understanding of the structure and dynamics of the solar corona and wind. Because Venus orbits the Sun, spacecraft like VEX are transported behind the solar disk, as seen from Earth. Radio waves propagating between VEX and Earth are refracted and scattered by the solar plasma [WOO1993]. Intensity fluctuations can be related to fluctuations in electron density along the path, while Doppler or phase scintillations can be related to both electron density fluctuations and also the speed of the solar wind. Many plasma effects decrease as the square of the radio frequency; scintillations are about an order of magnitude stronger at S-band than X-band. The first solar conjunction observations with VEX were conducted during late 2006. Investigators and Other Key Personnel ===================================== Bernd Hausler, Principal Investigator Universitaet der Bundeswehr, Munich Riccardo Mattei, Planning and Analysis Universitaet der Bundeswehr, Munich Martin Paetzold, Deputy Principal Investigator Rheinisches Institut fuer Umweltforschung, Cologne Silvia Tellmann, radio occultation Rheinisches Institut fuer Umweltforschung, Cologne Richard Simpson, bistatic radar Stanford University, California G. Leonard Tyler, radio propagation Stanford University, California Veronique Dehant, gravity Observatoire Royale, Brussels Pascal Rosenblatt, gravity Observatoire Royale, Brussels Paul Withers, atmospheres Boston University, Massachusetts Instrument Specification - Spacecraft ===================================== The Venus Express spacecraft telecommunications subsystem served as part of a radio science subsystem for investigations of Venus. It, like the spacecraft itself, was modeled on the radio system used on Mars Express. Many details of the subsystem are unknown; but they are not important for understanding the science. The spacecraft 'build date' is taken to be 2005-11-01, shortly before launch. Instrument Id : VRA Instrument Host Id : VEX Pi Pds User Id : BHAEUSLER Instrument Name : VENUS EXPRESS ORBITER RADIO SCIENCE EXPERIMENT Instrument Type : RADIO SCIENCE Build Date : 2005-11-01 Instrument Mass : UNK Instrument Length : UNK Instrument Width : UNK Instrument Height : UNK Instrument Manufacturer Name : UNK Subsystems ---------- SWITCH TRANSPONDER 1 -------- ------- ----- -------------------- | |---| TWTA1 |--|\ /|<---------| X-Band Transmitter | HGA1 >--| | ------- | \ / | | | | | | X | -------| S-Band Transmitter | HGA2 >--| | ------- | / \ | | | | | RFDU |---| TWTA2 |--|/ \| | ---->| X-band Receiver | LGA1 >--| /WIU | ------- ----- | | | | | |<---------------------- | -->| S-Band Receiver | LGA2 >--| | | | -------------------- | |------------------------- | | |--------------------------- TRANSPONDER 2 | | -------------------- -------- <---| X-Band Transmitter | | | <---| S-Band Transmitter | TRANSPONDERS 1 and 2 were | | connected to provide --->| X-band Receiver | redundant, switchable | | functions. --->| S-band Receiver | -------------------- The Venus Express radio subsystem comprised several components (shown in the simplified diagram above), configured to provide redundant functions should any single component fail. The primary high-gain antenna (HGA1) was a body-fixed 1.30 m diameter parabolic dish which allowed transmission and reception at both S- and X-band. The HGA1 boresight was in the +X direction of the spacecraft coordinate system, offset 5 degrees in the +Z direction. Its gain was 25.7 dBi and 37 dBi at S- and X-band, respectively. The auxilliary high-gain antenna (HGA2) was mounted oppositely to HGA1 so that it could be used for communication with Earth when Venus was between the quadrature points of its orbit and HGA1 was being used as a Sun shield. HGA2 could only be used for communication at X-Band; its diameter was 0.2 m and its X-band gain was 25 dB. Two low-gain antennas (LGA1 and LGA2) were mounted on the top and bottom panels of the spacecraft; they operated only at S-band. The boresight for LGA1 was nominally in the +Z direction; the boresight of LGA2 was offset from the -Z spacecraft axis by 17.5 degrees in the +X direction. Both LGAs had very broad antenna patterns. HGA1 was the primary antenna for receiving telecommands from and transmitting telemetry signals to the ground when VEX was on the far side of the Sun. In that geometry HGA1 also provided thermal protection to the spacecraft. When Venus was between its quadrature points -- on the near side of the Sun -- HGA2 was the prime antenna and HGA1 served solely as a sun shield. The LGAs were used during the commissioning phase after launch and for emergency operations. The Radio Frequency Distribution Unit (RFDU) switched S-Band signals and the Waveguide Interface Unit (WIU) switched X-Band signals on the spacecraft. Switchable Traveling Wave Tube Amplifiers (TWTA1 and TWTA2) provided 65 watts of X-band transmitter power to the RFDU; their inputs could come from either Transponder 1 or Transponder 2. The S-band transmitter power was 5 watts, which was generated within the transponder units. The S-band uplink was received via LGA1, LGA2, or HGA1. In the coherent two-way mode the received frequency was used to derive the downlink frequencies by using the constant transponder ratios 880/221 and 240/221 for X-band and S-band downlink, respectively. The X-band uplink was received via either HGA. In the coherent two-way mode the received frequency was used to derive the downlink frequencies by using the constant transponder ratios 880/749 and 240/749 for X-band and S-band downlink, respectively. An X-band uplink generally enhanced the performance of the radio link because X-band is less sensitive to the interplanetary plasma along the propagation path. The X-band and S-band frequencies were related by a factor of 11/3. If an uplink existed and the 'one-way' mode had not been selected, the downlinks were coherent with the uplink by their respective transponding ratios. The dual-frequency downlink allowed separation of the classical Doppler shift, due to relative motion of the spacecraft and the ground station, from the dispersive media effects, due to the propagation of the radio waves through the ionosphere and interplanetary medium. In one-way mode, the downlink transponder 1 frequency was derived from a Temperature Controlled Crystal Oscillator (TCXO) and the transponder 2 frequency was derived from an onboard ultrastable oscillator (USO). The one-way mode could be selected by command from the ground. If the spacecraft receiver could not detect an uplink signal from the ground, the TCXO or USO was selected by default. TCXO stability was several orders of magnitude less than the uplink reference; the USO stability was 1 part in 10^13 over time scales of 10-100 seconds; this was an order of magnitude less stable than frequency references on the gorund. However, for long-duration, deep radio occultations, the USO was preferred because perturbations on the downlink signals could be attributed unambiguously to a single passage through the atmosphere. Although either USO or TCXO could have served for bistatic radar, the USO was preferred for its better performance. For other observations, the two-way coherent mode was preferred because of the higher stability provided by the ground frequency reference. The redundant transponders each consisted of an S-band and X-band receiver and transmitter. The spacecraft was capable of receiving one uplink signal at S-band (2100 MHz) via the LGAs, at either X-band (7100 MHz) or S-band via HGA1, or at X-band only via HGA2. The spacecraft could transmit a downlink signal at S-band (2300 MHz) and (simultaneously) a downlink signal at X-band (8400 MHz) using HGA1; or it could transmit one downlink signal at S-band via the LGAs or one X-band signal using HGA2. Operational Considerations -------------------------- Radio science observations often required operation of the spacecraft in orientations and configurations that were not compatible with spacecraft constraints, telecommunications, and requirements for other instruments. There were also limitations within the Radio Science Team, which resulted in a prioritization of radio science observations. The following list is representative but not complete. Thermal constraints were severe. Spacecraft cooling panels could not be exposed to direct sunlight nor could other panels hosting payload components be exposed for more than a few minutes. The -X spacecraft axis could never be closer than 90 degrees to the Sun direction, The minimum incidence angle on the +Z panel was 10 degrees to protect optical instruments. The +Y and -Y panels could be exposed to the Sun at incidence angles 80-90 degrees, but for no longer than one hour. Bistatic radar experiments could be conducted with HGA1 only if that antenna's boresight was within 90 degrees of the Sun direction. When Venus was between inferior conjunction and quadrature, all radio tracking and occultations were conducted using HGA2 (at X-Band only). Gravity observations were most interesting when two-way dual-frequency data could be collected as the spacecraft passed through pericenter. But pericenter time was highly contested with several other instruments, which also sought those opportunities to acquire data with the highest resolution. Bistatic radar probing of anomalously reflecting, high altitude regions was most revealing when conducted near the Brewster angle. For dielectric constants of 3 (typical of sand), the Brewster angle is 60 degrees, measured from the local vertical. For a dielectric constant of 10, the Brewster angle is 72 degrees. MAGELLAN bistatic radar had suggested dielectric constants with magnitudes as large as 100 in Maxwell Montes. X-Band signal absorption in the neutral atmosphere made detection of surface echoes at incidence angles larger than 70 degrees nearly impossible; so experiments seeking information about the high altitude anomalies were preferentially scheduled for S-Band at incidence angle larger than 60 degrees. In the latter half of 2006, perhaps as early as 4 August and definitely by November, the performance of the on-board S-Band radio system deteriorated. Tests showed that both the transmitted power from the spacecraft and signals reaching the on-board receiver were reduced by approximately 15 dB. A switch between the transponders and HGA1 was suspected, but neither a specific fault nor a correction was identified. Bistatic radar experiments, which depended on S-Band for surface echo detection, were suspended. In other investigations, S-Band was de-emphasized since the noise accompanying the weak S-Band signal sometimes degraded the results compared with analysis of X-Band data alone. Calibration ----------- For many experiments, calibration data were collected in conjunction with the scientific observations. For example, carrier power and frequency could be determined before and/or after bistatic radar and radio occultation experiments when the antenna was pointed toward Earth. The beamwidth and pointing accuracy of HGA1 were calibrated during post-launch tests [ACCOMAZZO2006]. The half-power points were about 3.3 and 0.86 degrees from the boresight at S- and X-band, respectively (compared with 3.5 and 0.96 degrees predicted). The boresight itself was -0.32 deg toward the spacecraft +Y axis and 0.02 degrees toward the +Z axis at S-Band, compared with prediction; and it was -0.14 degrees toward +Y and +0.04 degrees toward +Z at X-Band compared with prediction. For radio tracking data, error sources in two-way mode were estimated at 1.7 AU Earth-Venus distance using the New Norcia ground station, a 1 Hz tracking loop bandwidth, and 10 seconds integration were estimated to yield a thermal contribution to the Doppler velocity of 0.004 mm/s [HAEUSLERETAL2005]. Platform Mounting ----------------- The high gain antenna HGA1 was rigidly attached to the +X wall of the spacecraft bus. Therefore, the VEX HGA frame (VEX_HGA) was defined as a fixed offset frame with its orientation given relative to the VEX_SPACECRAFT frame: +Z axis of the HGA frame was in the antenna boresight direction (nominally -5 degrees off the spacecraft +X axis toward the spacecraft +Z axis); +Y axis of the HGA frame was in the direction of the spacecraft +Y axis; +X completed the right hand frame; The origin of the HGA frame was located at the geometric center of the HGA dish outer rim circle. ^+Zhga | | | +Xhga | +Yhga _____o-------> \ / .________________. .__`._____.'__. .________________. | \ | | / | | \ | ___ | / | | | | .' ` +Ysc | | | |o=| | x------->o| | | | | `_|+Zsc | | | | / | | | \ | ._________________/ .______|______. \_________________. -Y Solar Array | +Y Solar Array V -Xsc Nominally a single rotation of -85 degrees about the +Y axis was needed to align the spacecraft frame with the HGA frame. The auxilliary high-gain antenna HGA2 was rigidly mounted to the +Z side of the spacecraft bus. Its pointing direction was symmetric to HGA1 with respect to the +Z axis of the spacecraft frame. The HGA2 frame was defined relative to the VEX_SPACECRAFT frame: +Z axis of the HGA2 frame was in the antenna boresight direction, +5 degrees off the spacecraft -X axis toward the spacecraft +Z axis. +Y axis was in the direction of the spacecraft +Y axis +X axis completes the right hand frame. The origin of the frame was located at the geometric center of the HGA2 dish outer rim circle. Operating Modes --------------- A two-way dual-frequency radio link was used for atmospheric drag campaigns, gravity observations, and solar corona investigations. Such a radio link benefited from the superior frequency stability of the ground station. A one-way dual-frequency radio link was used for radio occultation experiments because the uplink was a poor frequency reference during deep occultations. In the experiments above the dual-frequency downlink at X-band and S-band was used to separate classical and dispersive Doppler shifts, allowing correction of the observed frequency shift by any plasma contribution. Note, however, that the S-Band link was compromised starting in late 2006; it was ineffective for estimating plasma effects and was discontinued for some observations (see Operational Considerations, above). A one-way dual-frequency radio link at S- and X-band was used for bistatic radar experiments. In these experiments, HGA1 was pointed toward Venus and could not be used to capture an uplink signal, receive commands, or transmit telemetry. After the S-Band link was compromised in August 2006, bistatic radar experiments were suspended (see Operational Considerations, above). In the above experiments, operation was preferred with full power in the carrier (no telemetry or other modulation on the downlink) to maximize signal-to-noise ratio. Instrument Specification - DSN ============================== Three Deep Space Communications Complexes (DSCCs) (near Barstow, CA; Canberra, Australia; and Madrid, Spain) comprised the DSN tracking network. Each complex was 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 was radiation of commands to and reception of telemetry from active spacecraft. Transmission and reception was possible in several radio-frequency bands, the most common being S-band (nominally a frequency of 2100-2300 MHz or a wavelength of 14.2-13.0 cm) and X-band (7100-8500 MHz or 4.2-3.5 cm). Transmitter output powers of up to 400 kW were available. The Deep Space Network was managed by the Jet Propulsion Laboratory of the California Institute of Technology for the U.S. National Aeronautics and Space Administration. Specifications included: Instrument Id : RSS Instrument Host Id : DSN Pi Pds User Id : BHAEUSLER Instrument Name : RADIO SCIENCE SUBSYSTEM Instrument Type : RADIO SCIENCE Build Date : UNK Instrument Mass : UNK Instrument Length : UNK Instrument Width : UNK Instrument Height : UNK Instrument Manufacturer Name : UNK So far as radio science was concerned, the DSN was an evolving 'instrument;' the paragraphs which follow describe its capabilities during the first year of Venus Express orbital operations. 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]. For DSN use with MGS Radio Science see [TYLERETAL1992], [TYLERETAL2001], and [JPLD-14027]. Subsystems - DSN ---------------- The Deep Space Communications Complexes (DSCCs) were an integral part of Radio Science instrumentation. Their system performance directly determined the degree of success of Radio Science investigations, and their system calibration determined 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] and [DSN871-049-041]; for additional information, consult [DSN810-5], [DSN821-110], and [DSN821-104]. Each DSCC included a set of antennas, a Signal Processing Center (SPC), and communication links to the Jet Propulsion Laboratory (JPL). The general configuration is illustrated below; not all antennas are shown. -------- -------- -------- -------- -------- | 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 | |AMMOS |<--->| TERMINAL|<-------------->| | ------ --------- --------- ^ ^ | | CANBERRA (SPC 40) <---------------- | | MADRID (SPC 60) <---------------------- The following table lists the DSN antennas (Deep Space Stations, or DSS's -- a term carried over from earlier times when antennas were individually instrumented) available for Venus Express. Not all antennas were actually used for VEX; their capabilities varied and some were more suitable for VEX Radio Science than others. GOLDSTONE CANBERRA MADRID Antenna SPC 10 SPC 40 SPC 60 -------- --------- -------- -------- 26-m DSS 16 DSS 46 DSS 66 34-m HEF DSS 15 DSS 45 DSS 65 34-m BWG DSS 24 DSS 34 DSS 54 DSS 25 DSS 55 DSS 26 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 are described in the sections that follow. The Monitor and Control Subsystem was connected to all other subsystems; and the Test Support Subsystem could have been. ----------- ------------------ --------------------- |TRANSMITTER|_| UPLINK |_| COMMAND |_ | SUBSYSTEM | | SUBSYSTEM | | SUBSYSTEM | | ----------- ------------------ --------------------- | | | ----------- ------------------ --------------------- | | MICROWAVE |_| DOWNLINK |_| TELEMETRY |_| | SUBSYSTEM | | 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) was part of the Monitor and Control System (MON) which also included the ground communications Central Communications Terminal (CCT) and the Network Operations Control Center (NOCC) Monitor and Control Subsystem. The DMC was the center of activity at a DSCC. The DMC received and archived 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, was done through the DMC. The effect of this was to centralize the control, display, and short-term archiving functions necessary to operate a DSCC. Communication among the various subsystems was done using a Local Area Network (LAN) hooked up to each subsystem via a network interface unit (NIU). DMC operations were divided into two separate areas: the Complex Monitor and Control (CMC) and the Network Monitor and Control (NMC). The primary purpose of the CMC processor for Radio Science support was to receive and store all predict sets transmitted from NOCC -- such as 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 could be stored in the CMC for a maximum of three days under normal conditions. The CMC also received, processed, and displayed event/alarm messages, and maintained an operator log. Assignment and configuration of the NMCs was done through the CMC; to a limited degree the CMC could perform some of the functions performed by the NMC. There were two CMCs (one on-line and one backup) and three NMCs at each DSCC. The backup CMC could function as an additional NMC if necessary. The NMC processor provided 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 one or more Radio Science Receivers (RSRs), the DSCC Uplink Subsystem (UPL), and one or more DSCC Downlink Tracking and Telemetry Subsystems (DTTs). The NMC also maintained an operator log which included all operator directives and subsystem responses. One important Radio Science-specific function that the NMC performed was receipt and transmission of the system temperature and signal level data from the PPM, for display at the NMC console, and for inclusion in Monitor blocks. These blocks were recorded on magnetic tape as well as appearing in the NOCC displays. The NMC was required to operate without interruption for the duration of the Radio Science data acquisition period. The Area Routing Assembly (ARA), which was part of the Digital Communications Subsystem, controlled all data communication between the stations and JPL. The ARA received all required data and status messages from the NMC/CMC, and could record them to tape as well as transmit them to JPL via data lines. The ARA also received predicts and other data from JPL, and passed them on to the CMC. DSCC Antenna Mechanical Subsystem --------------------------------- Radio Science activities generally required support from the 70-m, 34-m HEF, and 34-m BWG antenna subnets. The antennas at each DSCC functioned as large-aperture collectors which, by double reflection, caused the incoming radio frequency (RF) energy to enter the feed horns. The large collecting surface of the antenna focused the incoming energy onto a subreflector, which was adjustable in both axial and angular position. These adjustments were made to correct for gravitational deformation of the antenna as it moved between zenith and the horizon; the deformation could be as large as 7 cm. The subreflector adjustments optimizde 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 had 'shaped' primary and secondary reflectors, with forms that were modified paraboloids. This customization allowed more uniform illumination of one reflector by another. The BWG reflector shape was ellipsoidal. On the 70-m antennas, the subreflector directed received energy from the antenna onto a dichroic plate, a device which reflected S-band energy to the S-band feed horn and passed X-band energy through to the X-band feed horn. In the 34-m HEF, there was one 'common aperture feed,' which accepted both frequencies without requiring a dichroic plate. In the 34-m BWG, a series of small mirrors (approximately 2.5 meters in diameter) directed 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 separated the S and X bands on some BWG antennas, or the X and Ka bands on others. RF energy to be transmitted into space by the horns was 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 could rotate the movable components and their support structures. The different antennas could be pointed by several means. Two pointing modes commonly used during tracking passes were CONSCAN and 'blind pointing.' With CONSCAN enabled and a closed-loop receiver locked to a spacecraft signal, the system tracked the radio source by conically scanning around its position in the sky. Pointing angle adjustments were computed from signal strength information (feedback) supplied by the receiver. In this mode the Antenna Pointing Assembly (APA) generated a circular scan pattern which was sent to the Antenna Control System (ACS). The ACS added the scan pattern to the corrected pointing angle predicts. Software in the receiver-exciter controller computed the received signal level and sent it to the APA. The correlation of scan position with the received signal level variations allowed the APA to compute offset changes which were sent to the ACS. Thus, within the capability of the closed-loop control system, the scan center was pointed precisely at the apparent direction of the spacecraft signal source. An additional function of the APA was 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 sent 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 were expected (e.g., during an occultation experiment), CONSCAN could not be used. Under these conditions, blind pointing (CONSCAN OFF) was used, and pointing angle adjustments were based on a predetermined Systematic Error Correction (SEC) model. Independent of CONSCAN state, subreflector motion in at least the z-axis could 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 was frozen in the z-axis at a position (often based on elevation angle) selected to minimize phase change and signal degradation. This could be done via Operator Control Inputs (OCIs) from the NMC to the Subreflector Controller (SRC) which resided in the alidade room of the antennas. The SRC passed the commands to motors that drove the subreflector to the desired position. Pointing angles for all antenna types were computed by the NOCC Support System (NSS) from an ephemeris provided by the flight project. These predicts were received and archived by the CMC. Before each track, they were transferred to the APA, which transformed the direction cosines of the predicts into AZ-EL coordinates. The LMC operator then downloaded the antenna predict points to the antenna-mounted ACS computer along with a selected SEC model. The pointing predicts consisted of time-tagged AZ-EL points at selected time intervals along with polynomial coefficients for interpolation between points. The ACS automatically interpolated the predict points, corrected the pointing predicts for refraction and subreflector position, and added the proper systematic error correction and any manually entered antenna offsets. The ACS then sent angular position commands for each axis at the rate of one per second. In the 70-m and 34-m HEF, rate commands were generated from the position commands at the servo controller and were subsequently used to steer the antenna. When not using binary predicts (the routine mode for spacecraft tracking), the antennas could be pointed using 'planetary' mode -- a simpler mode which used right ascension (RA) and declination (DEC) values. These changed very slowly with respect to the celestial frame. Values were provided to the station in text form for manual entry. The ACS quadratically interpolated among three RA and DEC points which were on one-day centers. A third pointing mode -- sidereal -- was available for tracking radio sources fixed with respect to the celestial frame. Regardless of the pointing mode being used, a 70-m antenna had a special high-accuracy pointing capability called 'precision' mode. A pointing control loop derived the main AZ-EL pointing servo drive error signals from a two- axis autocollimator mounted on the Intermediate Reference Structure. The autocollimator projected a light beam to a precision mirror mounted on the Master Equatorial drive system, a much smaller structure, independent of the main antenna, which was exactly positioned in HA and DEC with shaft encoders. The autocollimator detected elevation/cross- elevation errors between the two reference surfaces by measuring the angular displacement of the reflected light beam. This error was 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) were possible in 'precision' mode. The 'precision' mode was not available on 34-m antennas -- nor was it needed, since their beamwidths were twice as large as on the 70-m antennas. DSCC Antenna Microwave Subsystem -------------------------------- 70-m Antennas: Each 70-m antenna had three feed cones installed in a structure at the center of the main reflector. The feeds were positioned 120 degrees apart on a circle. Selection of the feed was made by rotation of the subreflector. A dichroic mirror assembly, half on the S-band cone and half on the X-band cone, permitted simultaneous use of the S- and X-band frequencies. The third cone was devoted to R&D and more specialized work. The Antenna Microwave Subsystem (AMS) accepted the received S- and X-band signals at the feed horn and transmitted them through polarizer plates to an orthomode transducer. The polarizer plates were adjusted so that the signals were directed to a pair of redundant amplifiers for each frequency, thus facilitating the simultaneous reception of signals in two orthogonal polarizations. For S-band these were two Block IVA S-band Traveling Wave Masers (TWMs); for X-band the amplifiers were Block IIA TWMs. 34-m HEF Antennas: The 34-m HEF used a single feed for both S- and X-band. Simultaneous S- and X-band receive as well as X-band transmit was possible thanks to the presence of an S/X 'combiner' which acted as a diplexer. For S-band, RCP or LCP was user selected through a switch, so neither a polarizer nor an orthomode transducer was needed. The X-band amplification options included two Block II TWMs or a High Electron Mobility Transistor (HEMT) Low Noise Amplifier (LNA), while the S-band amplification was provided by a Field Effect Transistor (FET) LNA. 34-m BWG Antennas: These antennas used feeds and low-noise amplifiers (LNA) in the pedestal room, which could be switched in and out as needed. Typically the following modes were 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 and dual LNAs, each of the above four modes could be used in a single- or dual-frequency configuration. Thus, for antennas with the most complete capabilities, there were sixteen possible ways to receive (2 polarizations, 2 waveguide path choices, 2 LNAs, and 2 bands). DSCC Uplink Subsystem --------------------- The Uplink Subsystem (UPL) comprised the Exciter, the Command Modulation, Uplink Controller, and Uplink Ranging assemblies. The UPL was based around the Block V Exciter (BVE) equipment. The BVEs generated uplink carrier and uplink range phase data, and delivered these data directly to the Project. The exciter generated a sky-level signal which was provided to the Transmitter Subsystem for the spacecraft uplink signal. Based on predicts from the CMC, the BVE provided a sky-level uplink signal to either the low-power or the high-power transmitter. It was tunable under command of the DCO (Digitally Controlled Oscillator). The diplexer in the signal path between the transmitter and the feed horn for all antenna types (used for simultaneous transmission and reception) could be configured such that it was 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. DSCC Downlink Subsystem ----------------------- The Downlink Subsystem consisted of three groups of equipment: the closed-loop receiver group, the open-loop receiver group, and the RF monitor group. Closed-Loop Receivers: The current closed-loop group, called the Downlink Tracking and Telemetry Subsystem (DTK), consisted of the Downlink Controller, the Receiver and Ranging Processor (RRP), and the Telemetry Processor (TLP) assemblies. The DTT was currently based around the Block V Receiver (BVR) equipment. The BVRs generated downlink carrier and downlink range phase data (not Doppler counts and ranging units, as had been the case before early 2003), and delivered these (phase) data directly to the Project. The DTT could simultaneously support as many downlink channels as could be assigned by the NMC, up to the total number of RRPs available at a given complex (allowing for the reception of several different frequencies/wavelengths/bands, or different polarizations of the same downlink band). The only other constraint was that any selected downlink band/bands had to be supported by that antenna. The closed-loop receivers provided the capability for rapid acquisition of a spacecraft signal, and telemetry lock-up. In order to accomplish signal acquisition within a short time, the receivers were predict driven to search for, acquire, and track the downlink automatically. Rapid acquisition precluded manual tuning, though that remained as a backup capability. The BVRs utilized FFT analyzers for rapid lock-up. The downlink predicts were generated by the NSS and then transmitted to the CMC, which sent them to the Receiver Subsystem where two sets could be stored. The receiver started acquisition at the beginning of a track (pass), or at an operator-specified time. The BVRs could also be operated from the NMC without local operators attending them. The receivers also sent performance and status data, displays, and event messages to the NMC. With the BVRs, the simulation (SIM) synthesizer signal was used as the reference for the Doppler extractor. The synthesizer was adjusted before the beginning of the pass to a frequency that was appropriate for the channel (i.e., within the band) of the incoming signal; and would generally remain constant during the pass. The closed-loop receiver AGC loop could be configured to one of three settings: narrow, medium, or wide. It was configured such that the expected amplitude changes were accommodated with minimum distortion. The loop bandwidth (2BLo) was configured such that the expected phase changes could be accommodated while maintaining the best possible loop SNR. Open-Loop Receivers: The open-loop Radio Science Receiver (RSR) was a dedicated receiver that got a downconverted signal (about 300 MHz), filtered the signal to limit its bandwidth (to 265-375 MHz, centered at 320 MHz), and then further downconverted (to a center frequency of 64 MHz) and digitized the signal. The RSR filters were specified by their bandwidths, desired resolution, and offset from the predicted sky frequency. The open-loop receivers operated in both a link-assigned and a stand-alone mode. In the link-assigned mode, the NMC received monitor data from the RSR for incorporation into the data set for tracking support, and provided a workstation from which the RSR could be operated. RSRs that were not assigned to a link could be operated in a stand-alone mode without interference to any activities in progress at the complex. Monitor data were not sent to the NMC by RSRs operating in the stand-alone mode. DSCC Transmitter Subsystem -------------------------- The Transmitter (TXR) Subsystem accepted a sky-level frequency exciter signal from the Uplink (Exciter) Subsystem exciter. This signal was routed via the diplexer through the feed horn to the antenna, where it was then focused and beamed to the spacecraft. The Transmitter Subsystem power capabilities ranged from 18 kW to 400 kW, for S- and X-band uplink. Power levels above 20 kW were available only at 70-m stations. DSCC Tracking Subsystem ----------------------- Beginning in early 2003, all the Tracking Subsystem functions were incorporated within the Uplink Subsystem (UPL) and the Downlink Tracking and Telemetry Subsystem (DTT) -- the DTK was eliminated. DSCC Frequency and Timing Subsystem ----------------------------------- The Frequency and Timing Subsystem (FTS) provided all of the frequency and timing references required by the other DSCC subsystems. It contained four frequency standards, of which one was prime and the other three were backups. Selection of the prime standard was done via the CMC. Of these four standards, two were hydrogen masers followed by clean-up loops (CUL) and two were cesium standards. These four standards all fed the Coherent Reference Generator (CRG), which provided the frequency references used by the rest of the complex. FTS also provided the frequency reference to the Master Clock Assembly (MCA), which in turn provided time to the Time Insertion and Distribution Assembly (TID), which provided UTC and SIM-time to the complex. JPL's ability to monitor the FTS at each DSCC was limited to the station-calculated Doppler pseudo-residuals, the Doppler noise, the RSR, the SSI, and to a system that used the Global Positioning System (GPS). GPS receivers at each DSCC received a one-pulse-per-second signal 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 was reported to JPL, where a database was kept. The clock offsets stored in the JPL database were given in microseconds; each entry was a mean reading of the measurements from several GPS satellites, and a time tag associated with the mean reading. The clock offsets that were provided included those of SPC 10 relative to UTC (NIST), SPC 40 relative to SPC 10, etc. Optics - DSN ============ Performance of the DSN ground stations depended primarily on size of the antenna and capabilities of the electronics. These are summarized in the following set of tables. 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 34-m Transmit BWG HEF -------- ----- ----- ----- Frequency (MHz) 2110- 2025- N/A 2120 2120 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) 20-100 20 N/A Receive ------- Frequency (MHz) 2270- 2270- 2200- 2300 2300 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 & R L or R L or R System Temp (K) 20 31 38 DSS X-Band Characteristics 70-m 34-m 34-m Transmit BWG HEF -------- ----- ----- ----- Frequency (MHz) 8495 7145- 7145- 7190 7190 Wavelength (m) 0.035 0.042 0.042 Ant Gain (dBi) 74.2 66.9 67 Beamwidth (deg) N/A 0.074 Polarization L or R L or R L or R Tx Power (kW) 20 20 20 Receive ------- Frequency (MHz) 8400- 8400- 8400- 8500 8500 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 & R L & R L & R System Temp (K) 20 30 20 NB: The 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 was 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 were used in spacecraft radio science experiments, the details of transmitter and antenna performance rarely impacted the results. Calibration - DSN ================= Calibrations of hardware systems were carried out periodically by DSN personnel; these ensured that systems operated at required performance levels -- for example, that antenna patterns, receiver gain, propagation delays, and Doppler uncertainties met specifications. No information on specific calibration activities was 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 performed a series of calibrations to ensure that systems met specifications for that operational period. Included in these calibrations was measurement of receiver system temperature in the configuration to be employed during the pass. Results of these calibrations were recorded in (hard copy) Controller's Logs for each pass. Operational Considerations - DSN ================================ The DSN was a complex and dynamic 'instrument.' Its performance for Radio Science depended 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 could be carried out in several ways. For details see the subsection 'DSCC Antenna Mechanical Subsystem' in the 'Subsystem' section. Binary pointing was the preferred mode for tracking spacecraft; pointing predicts were provided, and the antenna simply followed those. With CONSCAN, the antenna scanned conically about the optimum pointing direction, using closed-loop receiver signal strength estimates as feedback. In planetary mode, the system interpolated from three (slowly changing) RA-DEC target coordinates; this was 'blind' pointing since there was no feedback from a detected signal. In sidereal mode, the antenna tracked a fixed point on the celestial sphere. In 'precision' mode, the antenna pointing was adjusted using an optical feedback system. In addition, it was possible on most antennas to freeze the z-axis motion of the subreflector to minimize phase changes in the received signal. DSCC Downlink Tracking and Telemetry Subsystem ---------------------------------------------- The diplexer in the signal path between the transmitter and the feed horns on all antennas could be configured so that it was out of the received signal path in order to improve the signal-to-noise ratio in the receiver system. This was known as the 'listen-only' or 'bypass' mode. Closed-Loop vs. Open-Loop Reception ----------------------------------- Radio Science data could be collected in two modes: closed- loop, in which a phase-locked loop receiver tracked the spacecraft signal, or open-loop, in which a receiver sampled and recorded a band within which the desired signal presumably resided. Closed-loop data were collected using Closed-Loop Receivers, and open-loop data were collected using Open-Loop Receivers in conjunction with the Full Spectrum Processing Subsystem (FSP). See the Subsystems section for further information. Closed-Loop Receiver AGC Loop ----------------------------- The closed-loop receiver AGC loop could be configured to one of three settings: narrow, medium, or wide. In general, it was configured so that expected signal amplitude changes were accommodated with minimum distortion. The loop bandwidth was typically configured so that expected phase changes could be accommodated while maintaining the best possible loop SNR. Coherent vs. Non-Coherent Operation ----------------------------------- The frequency of the signal transmitted from the spacecraft could 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 were known as the coherent (or 'two-way') and non-coherent ('one-way') modes, respectively. Mode selection was made at the spacecraft, based on commands received from the ground. When operating in the coherent mode, the transponder carrier frequency was derived from the received uplink carrier frequency with a 'turn-around ratio' as expressed in the table below: Uplink Downlink Turn-Around Band Band Ratio --------------------------------- X X 880/749 X S 240/749 In the non-coherent mode, the downlink carrier frequency was derived from the spacecraft's on-board, crystal-controlled oscillator. Either closed-loop or open-loop receivers (or both) could be used with either spacecraft frequency reference mode. Closed-loop reception in the two-way mode was usually preferred for routine tracking/navigation. Occasionally the spacecraft operated coherently such that one ground station did the transmitting, and a second/different ground station received the 'downlink' signal -- this was referred to as the 'three-way' mode. Media Calibration System ------------------------ The Earth's atmosphere contributes phase and amplitude noise to the spacecraft radio signal received at a ground station. Each DSCC had a GPS receiver subsystem to calibrate for both the ionosphere and troposphere (both wet and dry components), along the zenith direction. This subsystem also measured the temperature, pressure, humidity, wind speed and direction, and Faraday rotation. Location - DSN ============== Accurate spacecraft navigation using radiometric data required knowledge of the locations of the DSN tracking stations. The coordinate system in which the locations of the tracking stations were expressed needed to be consistent with the reference frame definitions used to provide Earth orientation calibrations. The International Earth Rotation Service (IERS) had established a terrestrial reference frame for use with Earth orientation measurements. The IERS issued a new realization of the terrestrial reference frame each year. The definition of the coordinate system changed slowly as the data improved, and as ideas about how best to define the coordinate system developed. The overall changes from year to year were at the level of as few centimeters. The 1993 version of the IERS Terrestrial Reference Frame (IRTF1993) was most used for DSN station locations. The DSN station locations were determined by use of VLBI measurements, and by conventional and GPS surveying. Tables of station locations were available in either Cartesian or geodetic coordinates. The geodetic coordinates were referred to a geoid with an equatorial radius of 6378136.3 m, and a flattening factor f=298.257, as described in IERS Technical Note 13. The DSN Station Locations in ITRF1993 Cartesian reference frame at epoch 1993.0 (assuming subreflector-fixed configuration) were as follows: Antenna x(m) y(m) z(m) ------------------------------------------------ DSS 12 -2350443.812 -4651980.837 +3665630.988 DSS 13 -2351112.491 -4655530.714 +3660912.787 DSS 14 -2353621.251 -4641341.542 +3677052.370 DSS 15 -2353538.790 -4641649.507 +3676670.043 DSS 16 -2354763.158 -4646787.462 +3669387.069 DSS 17 -2354730.357 -4646751.776 +3669440.659 DSS 23 -2354757.567 -4646934.675 +3669207.824 DSS 24 -2354906.528 -4646840.114 +3669242.295 DSS 25 -2355021.795 -4646953.325 +3669040.628 DSS 26 -2354890.967 -4647166.925 +3668872.212 DSS 27 -2349915.260 -4656756.484 +3660096.529 DSS 28 -2350101.849 -4656673.447 +3660103.577 DSS 33 -4461083.514 +2682281.745 -3674570.392 DSS 34 -4461146.720 +2682439.296 -3674393.517 DSS 42 -4460981.016 +2682413.525 -3674582.072 DSS 43 -4460894.585 +2682361.554 -3674748.580 DSS 45 -4460935.250 +2682765.710 -3674381.402 DSS 46 -4460828.619 +2682129.556 -3674975.508 DSS 49 -4554231.843 +2816758.983 -3454036.065 (Parkes) DSS 53 +4849330.129 -0360338.092 +4114758.766 DSS 54 +4849434.496 -0360724.062 +4114618.570 DSS 55 +4849525.318 -0360606.299 +4114494.905 DSS 61 +4849245.211 -0360278.166 +4114884.445 DSS 63 +4849092.647 -0360180.569 +4115109.113 DSS 65 +4849336.730 -0360488.859 +4114748.775 DSS 66 +4849148.543 -0360474.842 +4114995.021 The DSN Station Locations in ITRF1993 Geodetic reference frame at epoch 1993.0 (assuming subreflector-fixed configuration) were as follows: latitude longitude height Antenna deg min sec deg min sec (m) ---------------------------------------------------------- DSS 12 35 17 59.77577 243 11 40.24697 962.87517 DSS 13 35 14 49.79342 243 12 19.95493 1071.17855 DSS 14 35 25 33.24518 243 6 37.66967 1002.11430 DSS 15 35 25 18.67390 243 6 46.10495 973.94523 DSS 16 35 20 29.54391 243 7 34.86823 944.71108 DSS 17 35 20 31.83778 243 7 35.38803 937.65000 DSS 23 35 20 22.38335 243 7 37.70043 946.08556 DSS 24 35 20 23.61492 243 7 30.74701 952.14515 DSS 25 35 20 15.40494 243 7 28.70236 960.38138 DSS 26 35 20 8.48213 243 7 37.14557 970.15911 DSS 27 35 14 17.78052 243 13 24.06569 1053.20312 DSS 28 35 14 17.78136 243 13 15.99911 1065.38171 DSS 33 -35 24 1.76138 148 58 59.12204 684.83864 DSS 34 -35 23 54.53984 148 58 55.06236 692.71119 DSS 42 -35 24 2.44494 148 58 52.55396 675.35557 DSS 43 -35 24 8.74388 148 58 52.55394 689.60780 DSS 45 -35 23 54.46400 148 58 39.65992 675.08630 DSS 46 -35 24 18.05462 148 58 59.08571 677.55141 DSS 49 -32 59 54.25297 148 15 48.64683 415.52885 DSS 53 40 25 38.48036 355 45 1.24307 827.50081 DSS 54 40 25 32.23152 355 44 45.24459 837.60097 DSS 55 40 25 27.45965 355 44 50.51161 819.70966 DSS 61 40 25 43.45508 355 45 3.51113 841.15897 DSS 63 40 25 52.34908 355 45 7.16030 865.54412 DSS 65 40 25 37.86055 355 44 54.88622 834.53926 DSS 66 40 25 47.90367 355 44 54.88739 850.58213 Measurement Parameters - DSN ============================ Open-Loop System ---------------- Output from the Open-Loop Receivers (OLRs), as sampled and recorded by the Radio Science Receiver (RSR), was a stream of 1-, 2-, 4-, 8-, or 16-bit I (In-Phase) and Q (Quadrature-Phase) samples. The spacecraft transmitted an RF signal to an antenna, where the signal was downconverted to IF. The RSR selected an IF signal for a particular frequency band and passed it through a digitizer (where it was attenuated and then mixed with timing information). The signal was then decimated, filtered (to I&Q samples), and then multiplied by the signal from a numerically controlled oscillator. Finally, the RSR reduced the bandwidth and sample rate of the samples, and truncated the results (thus creating an offset of -0.5 in the output data). The samples of data were packed into SFDU blocks (nominally containing a single second's worth of data), and a header was attached to provide the following associated data for the record: - time tag for the first sample in the data block - data source identification (DSS, RSR, and sub-channel), and frequency band - data sample resolution (bits per sample) and rate (samples per second) - filter gain, ADC RMS amplitude, and attenuation - frequency and phase polynomial coefficients Closed-Loop System ------------------ Since mid 2003, closed-loop data were recorded and provided in Tracking and Navigation Files (TNFs). The TNFs were comprised of SFDUs that had variable-length, variable-format records with mixed typing (i.e., can contain ASCII, integer, and floating-point items in a single record). These files all contained entries that included measurements of Doppler, range, and signal strength, along with status and uplink frequency information. Acronyms and Abbreviations ========================== 1PPS One Pulse per Second ACS Antenna Control System ADEV Allan Deviation AGC Automatic Gain Control APA Antenna Pointing Assembly BPF Band Pass Filter BWG Beam Wave Guide CFE Common Front End CONSCAN Connical Scanning D/L downlink dBi decibel relative to isotropic DDC Digital Down Converter DSCC Deep Space Communications Complex DSN Deep Space Network DSS Deep Space Network Station ESA European Space Agency ESOC European Space Operations Centre FTS Frequency and Timing subsystem HEF High Efficiency HGA High Gain Antenna HSB High-Speed BWG IFMS Intermediate Frequency Modulation System IVC Intermediate Frequency Selection Switch JPL Jet Propulsion Laboratory LCP Left Circular Polarization LGA Low Gain Antenna LNA Low Noise Amplifier LPF Low Pass filter MB Medium band Mbit Mega bit MOLA Mars Orbiting Laser Altimeter MRS Mars Express Radio Science N/A not applicable NASA National Aeronautics and Space Administration NMC Network Monitor and Control NNO New Norcia ODF Orbit Data File OLR Open Loop Receiver PDS Planetary Data System PI Principal Investigator pwr power rcvrs receivers RCP Right Circular Polarization RF Radio Frequency RFDU Radio Frequency Distribution Unit RIV Radio Science IF-VF Downconverter rms root mean square RSR Radio- Science Receiver RSS Radio Science Subsystem SIM SNR Signal-Noise-Ratio SNT System Nolise Temperature SPC Signal Processing Center sps samples per second STAT Science Time Analysis Tool TCXO Temperature Controlled Crystal Oscillator TID Time Insertion and Distribution Assembly TNF Tracking and Navigation File TWOD Two-way dual-frequency mode TWOS Two-way single-frequency mode TWTA Traveling wave tube amplifier Tx Transmitter U/L uplink UNK unknown UTC Coordinated Universal Time VDP VME Data Processor VeRa Vensu Express Radio Science VEX Venus Express VF Video Frequency VME Versa Module Eurocard (standard bus) w watt