Data Set Information
DATA_SET_NAME MGN V RSS LINE OF SIGHT ACCELERATION PROFILES V1.0
DATA_SET_ID MGN-V-RSS-5-LOSAPDR-L2-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : Line of Sight Acceleration Profile Data Records (LOSAPDR) consist of data from Doppler tracking of the orbiting spacecraft. The relative motion of the spacecraft and the earth-based radio receiver is measured very precisely, and known motions are removed a priori (i.e. earth rotation, planetary motions, spacecraft orbital motion, solar pressure, drag), leaving small velocity changes caused by variations in the mass distribution of the planet. The residual Doppler frequency shifts are linearly proportional to the component of velocity in the Earth direction. Numerical differentiation of these velocity residuals with respect to time produces line-of-sight (LOS) gravity. These measures are accelerations at spacecraft altitude which can be modeled for geophysical interpretation. Note: JPL documents describing the Magellan gravity experiment, including the acquisition, processing, and quality of the LOSAPDR data, are available from the PDS Geosciences Node, Washington University, St. Louis, MO. Parameters : Users of these data should be aware of particular events and tracking station operations to properly reduce the complex motions inherent in the Doppler signals. Many of the spacecraft parameters can be found in the Navigation Constants Document, [MGN-NCFDR1991]. There is a small force record, [MGN-SFFDR1987], which lists the times and duration of the momentum dumps. There were several maneuvers and their times and durations are given in the Maneuver Profile Listing (MPL) data product. The spacecraft orientations are all listed in the Spacecraft Attitude During Hide Maneuver Listing (SADHML) data product. Note: The MPL and SADHML hardcopy listings are located at the Washington University, PDS Geosciences Node, St. Louis, MO. On every orbit the spacecraft orientation changed with the high gain antenna pointing at Venus or at the Earth or some other direction to keep the spacecraft temperature within safe bounds. The transmitter ramp rates and initiation times are listed in the ODFDR (Orbit Data File Data Record) and ATDFDR (Archival Tracking Data File Data Record) [MGN-ODFDR1988; MGN-ATDFDR1986]. The equations to incorporate the ramp data as well as the complete theoretical Doppler observable are given in [MOYER1971; MOYER1987]. Processing : The raw X-Band uplink/downlink Doppler observations have been fit with a model which accounts for theoretical motions and other signal effects. The difference between the observed or measured Doppler and the theoretical model are Doppler residuals. These are small systematic variations and are attributed to unmodeled gravity field perturbations. The Doppler residuals are then spline fitted and analytically differentiated to produce line-of-sight (LOS) accelerations. The spacecraft position in orbit at each acceleration point is noted at a specific altitude, latitude and longitude. The LOS accelerations during cycle 4 (Sept. 92 - May 93) were produced with a Venusian gravity model having only a central term (i.e. GM only, without any spherical harmonic coefficients). The LOS data during cycle 5 (Aug. 93 - April 94), and beyond, have been produced using a 40th degree and order spherical harmonic model (MGN40E), which causes the Doppler residuals to be much smaller and thus the LOS accelerations to be also small (i.e. on the order of a few milligals as compared to 50-80 milligals with the GM only model). None of the cycle 5+ data are included here. Ancillary Data : N/A Coordinate System : N/A Software : N/A Media/Format : The Cycle 4 LOSAPDR dataset will be delivered to the Magellan Project (or its successor) using compact disk write once (CD-WO) media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992].
DATA_SET_RELEASE_DATE 1994-06-01T00:00:00.000Z
START_TIME 1992-09-15T12:00:00.000Z
STOP_TIME 1993-05-28T12:00:00.000Z
MISSION_NAME MAGELLAN
MISSION_START_DATE 1989-05-04T12:00:00.000Z
MISSION_STOP_DATE 1994-10-12T12:00:00.000Z
TARGET_NAME VENUS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGN
INSTRUMENT_NAME RADIO SCIENCE SUBSYSTEM
INSTRUMENT_ID RSS
INSTRUMENT_TYPE RADIO SCIENCE
NODE_NAME Geosciences
ARCHIVE_STATUS SUPERSEDED
CONFIDENCE_LEVEL_NOTE Overview : The Doppler residuals were differentiated to produce LOS acceleration profiles. These LOS profiles however have distortions in them due to viewing geometry, variations in spacecraft altitude and the least squares reduction process. With the high degree and order spherical harmonics, similar profiles can be generated that are free from most of the above effects, but may suffer from loss of high frequency components that remain in the raw data. The LOS data should be used for refinements when small features are important to the analyst, but otherwise the gravity field as defined by the harmonics is the most valid. Review : The LOSAPDR will be reviewed internally by the Magellan Project prior to release to the planetary community. The LOSAPDR will also be reviewed by PDS. Data Coverage and Quality : During cycle 4 there were 866 LOS acceleration profiles (LOSAPDRs) produced at JPL and then validated and archived at the Planetary Data System's Geophysics Subnode at Washington University, St. Louis, MO. There was 360 degrees of longitude coverage with periapsis altitude near 180 km. During cycle 5, the coverage was redundant and there was a gap in the longitude coverage. Some profiles were acquired across apoapsis (550 km altitude) while others were across periapsis. January 17 to January 24, 1992 ------------------------------ There was a week of Doppler periapsis data during January (17-24) 1992 when there was battery recharging. The data were at S-Band from the medium gain antenna and periapsis was at 280 km. April 22 to May 16, 1992 ------------------------ From April 22 to May 16, 1992 (23 days) with periapsis at 261 km there were sixteen orbits of X-Band periapsis data acquired. August 1990 to September 1992 ----------------------------- Other than these two prior blocks of data, all Doppler gravity data from August 1990 to September 1992 were taken when spacecraft altitudes were well above 2000 km. September 15, 1992 to May 24, 1993 ---------------------------------- The first block of high quality and high resolution gravity data were acquired. Periapsis was lowered to 180 km and coverage across periapsis was obtained continuously for 360 degrees of longitude. Periapsis latitude remained essentially constant at 10 degrees N latitude and good resolution was maintained within +/- 30 degrees (i.e. 40 degrees N to 20 degrees S where spacecraft altitudes reached ~450 km). There were only a few days where no data were obtained, but in general excellent data coverage was maintained providing at least two orbits per day and much of the time 4 to 5 orbits per day. All of these were X-Band uplink and downlink having a noise level of 0.1 mm/sec. In addition there were also two or more orbits with S-Band coverage each day. The amount of Doppler observations for each orbit has been tabulated for this entire period and is located at the National Space Science Data Center (NSSDC) and PDS Geosciences Node, Washington University, St. Louis, MO. All the ancillary information to process these data is also available at the above locations. May 1993 to August 6, 1993 -------------------------- The MGN spacecraft successfully completed an aerobraking sequence that nearly circularized its orbit. This new orbit provided high resolution data in the high latitude regions which previously had poor gravity model determination. The orbit had a periapsis altitude of 180 km and an apoapsis of 550 km and provided ~300 - 400 km altitudes over the high latitude regions. The orbital period changed from 195 minutes to 94 minutes, so there were approximately fifteen orbits per day. Because of operational sequencing of the onboard computers and the necessity to orient the spacecraft for thermal conditions, the first good coherent X-Band data were acquired on Aug 19, 1993. There were some short S-Band data blocks starting on August 6, 1993. The data during aerobraking are on ODF and ATDF files, but were not used for any of the gravity experiment reductions. The Doppler data acquired during aerobraking were all S-Band at relatively high altitudes. There were no data near periapsis since the spacecraft was oriented with its high gain antenna pointed along the spacecraft velocity vector and not toward the earth. The daily coverage in this orbit was excellent with data for several orbits every day. However orbital geometry of the Earth and Venus have made the coverage redundant for some longitudes and none existent for others. Note that radio tracking should continue until March 1995 to have complete 360 degrees of longitude coverage. January 6 to January 26, 1994 ---------------------------------- There was a solar conjunction on January 16, 1994 and as a result data from January 6, 1994 to January 26, 1994 are very noisy and essentially non-usable for gravity extraction. The data immediately after solar conjunction have low altitude passes over Maxwell Montes and provides the very best information for this unique feature. Limitations : N/A
CITATION_DESCRIPTION Sjogren, W. L., MGN V RSS LINE OF SIGHT ACCELERATION PROFILES V1.0, MGN-V-RSS-5-LOSAPDR-L2-V1.0, NASA Planetary Data System, 1994
ABSTRACT_TEXT Line of Sight Acceleration Profile Data Records (LOSAPDR) consist of data from Doppler tracking of the orbiting spacecraft. The relative motion of the spacecraft and the earth-based radio receiver is measured very precisely, and known motions are removed a priori (i.e. earth rotation, planetary motions, spacecraft orbital motion, solar pressure, drag), leaving small velocity changes caused by variations in the mass distribution of the planet. The residual Doppler frequency shifts are linearly proportional to the component of velocity in the Earth direction. Numerical differentiation of these velocity residuals with respect to time produces line-of-sight (LOS) gravity. These measures are accelerations at spacecraft altitude which can be modeled for geophysical interpretation.
PRODUCER_FULL_NAME WILLIAM L. SJOGREN
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