PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "MGN-V-RSS-5-GRAVITY-L2-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MGN V RSS SPHERICAL HARMONIC MODELS AND DIGITAL GRAVITY MAP DATA V1.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_OBJECT_TYPE = FILE START_TIME = 1992-09-15 STOP_TIME = 1994-10-12 DATA_SET_RELEASE_DATE = 1997-08-01 PRODUCER_FULL_NAME = "WILLIAM L. SJOGREN" DETAILED_CATALOG_FLAG = "N" DATA_SET_DESC = " Data Set Overview ================= The gravitational signature of Venus was determined from velocity perturbations of the Pioneer Venus Orbiter (PVO) and the Magellan spacecraft as measured from the Doppler shift of the S-band and X-band radio tracking signal. The spacecraft were tracked by NASA's Deep Space Network (DSN) at Goldstone, California, Canberra, Australia, and Madrid, Spain. The tracking data were used to determine the spacecraft orbits about Venus, as well as the Venusian gravity field. A detailed description of how the spacecraft orbits were computed and an assessment of their quality can be found in [KONOPLIV&SJOGREN1996]. For information on Magellan gravity investigations see papers in [ICARUSMGN1994]. Data ==== This dataset includes a 120th degree and order spherical harmonic gravitational field model and digital gravity maps of the Bouguer anomalies, free-air gravity anomalies, free-air gravity errors, geoid anomalies, and geoid anomaly errors. There are three data types for the products found on this volume: tabular, array, and image data. The file containing the spherical harmonic coefficients of the Venusian gravity field is in tabular format, with each row in the table containing the degree index m, the order index n, the coefficients Cmn and Smn, and the uncertainties in Cmn and Smn. The digital maps of Bouguer anomalies, free-air gravity anomalies, free-air gravity errors, geoid anomalies, and geoid anomaly errors, are binary images. There are also ASCII 2-D gridded data arrays of all the digital map products, including a byte-scaled image of each of these gridded products. Parameters ========== The gravitational signature of Venus was determined from velocity perturbations on the Pioneer Venus Orbiter and Magellan spacecraft as measured from the Doppler shift of the S-band and X-band radio tracking signal. The Doppler data from the DSN stations were acquired with count intervals of 1 minute and 10 seconds. The free-air gravity anomalies (in milligals, mGals, where 1 mGal = 0.01 mm/s^2) are evaluated at the surface. The free-air gravity errors are also in mGals. Geoid anomalies and errors are in meters. Bouguer anomalies, determined by subtracting the gravitational attraction of the surface topography from the free-air anomaly, are in mGals. Processing ========== The gravity solution consists of over 3,304,000 observations, of which 204,000 were contributed by PVO. The data were divided into independent arcs based on considerations of data coverage and timing of maneuvers. For each arc certain parameters were determined: the spacecraft state (position and velocity), a solar radiation pressure coefficient, Doppler biases for each station over the arc to account for frequency biases, and the mismodeling of the effects of the troposphere and ionosphere on the Doppler signal. The a priori model that was used included the [KONOPLIVETAL1993] gravity model, and included the third-body perturbations due to the Sun, the Earth, and all the planets, the solar radiation pressure perturbation, the solar-induced solid tides, and appropriate relativistic effects. The DE403 planetary ephemeris was used in the analyses. Although each data arc was typically fit to the level of less than 0.1 mm/sec, the data were downweighted in order to attenuate the power of the high degree terms, and account for any systematic mismodeling that might still be present in the data. The solution was also derived using an a priori constraint that was applied where data coverage was poor. This constraint is explained in detail in [KONOPLIV&SJOGREN1996]. Extensive experiments were performed in order to select the a priori weights for the sets of data in the solution -- and care was taken to downweight or delete data that produced spurious signals in the anomaly maps. Ancillary Data ============== 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. 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]. A useful product for understanding the errors in the gravity solution is the gravity field covariance matrix. The gravity field covariance represents the complete error description for the SHG120 gravity field solution, including the coefficient variances (sigmas^2), and the correlations between all the coefficients. It is a product of the least squares solution that derived SHG120. The covariance provides a more complete description of the error associated with the SHG120 gravity field solution, than would be available from the sigmas alone. The covariance matrix is large (~857 MB) since it contains the correlations of each of the solution parameters with all the other parameters. (SHG120 solved for 14756 spherical harmonic coefficients, and the Venusian GM value). The full covariance matrix was used to compute the error maps for the gravity anomalies, and geoid anomalies that are associated with this archive. The covariance matrix is available on request from the PDS Geoscience Node (contact Jim Alexopoulos at 314-935-5365 or jim@wuzzy.wustl.edu) or from JPL (contact Alex Konopliv at 818-354-6105 or ask@krait.jpl.nasa.gov). NOTE: All the ancillary data and information to process and interpret the Doppler data are located at the PDS Geosciences Node, Washington University, St. Louis, MO. Coordinate System ================= The coordinate system for the gravity data reduction is space-fixed earth-mean-equator referenced to J2000. The integration was done in a Venus centered frame. The earth orientation parameters were given by the University of Texas, Center of Space Research, solution EOP (CSR) 95 L 01. Software ========= N/A Media/Format ============ This archive will be available electronically via the World-Wide Web and anonymous FTP transfer: http://pds-geophys.wustl.edu/pds/magellan/harmonics_maps. The archive will also be delivered to the National Space Sciences Data Center (NSSDC) using compact disk write once (CD-WO) media. Formats will be based on standards established by the Planetary Data System (PDS)." CONFIDENCE_LEVEL_NOTE = " Overview ======== The data noise on the PVO Doppler data ranged from 0.3 to 1 mm/s, depending on the arc, and most of the data were at a count interval of 60 seconds. The Magellan Doppler data from the DSN stations had a data noise of <0.1 mm/s with a count interval of 10 seconds. Errors in the free-air gravity anomalies ranged from ~2-12 mGals over the equatorial region (+/- 30 degrees latitude), and 15-21 mGals over the high-latitudes. Errors in the geoid range from ~0.1-2.3 meters. Review ====== The volume containing the Venusian gravity and topography data sets was reviewed by Magellan mission scientists and by PDS. Data Coverage and Quality ========================= The Magellan spacecraft was inserted into orbit about Venus on August 10, 1990. The first three cycles (one cycle is one Venus rotation period of 243 days) were dedicated to SAR imaging and altimetry measurements of the Venus surface. These measurements required the high gain antenna to be pointed to the Venus surface within about thirty minutes of periapsis passage. Thus only high-altitude tracking (>2500 km) was obtained when the high gain antenna was returned to point at Earth. These data still contain some low resolution information on the gravity field and they were used by [MCNAMEEETAL1993]. The altitude of periapsis for the first three cycles is 250 km and is higher than the cycle four, which was dedicated to gravity measurements. Except for the periapsis altitude, the orbit shape for cycle four is identical to the previous cycles and should contain all the gravity information that is in the previous cycles and more. In addition to the first three cycles, the modeling of the solar pressure force for a rotating antenna through periapsis passage is complicated, and solar pressure is a significant force for the higher periapses due to diminished drag. For these reasons, the first three cycles of data are not included in the gravity solution. There was a week of Doppler periapsis data during the period of January 17 to 24, 1992, when the battery was being recharged. The data were at S-Band from the medium gain antenna and periapsis was at 280 km. From April 22 to May 16, 1992 (23 days), the periapsis was at 261 km, and periapsis data at X-Band were acquired for 16 orbits. Other than these two prior blocks of data, all Doppler gravity data from August 1990 to September 1992 were obtained when the spacecraft's altitude was well above 2000 km. From September 15, 1992 to May 24, 1993 (Cycle 4), 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 four or five 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. From May 1993 to August 6, 1993, The Magellan 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 15 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 ODFDR and ATDFDR 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 non-existent for others. 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 provide the very best information on this unique feature. From June 22 to July 14, 1994 (orbits 13298-13635), the Doppler radio tracking on each orbit of Magellan was cut short due to inadvertent spacecraft event sequencing of the thermal control where the spacecraft was turned-off of earth-pointing (loss of radio contact) earlier than it should have been. This was corrected in the next up-load to the spacecraft, making the arcs about another 10 minutes longer. Limitations =========== N/A " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = VENUS END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = MGN INSTRUMENT_ID = RSS END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "ICARUSMGN1994" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MGN-NCFDR1991" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MGN-SFFDR1987" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MGN-ODFDR1988" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MGN-ATDFDR1986" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "KONOPLIVETAL1993" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "KONOPLIV&SJOGREN1996" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MCNAMEEETAL1993" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MOYER1971" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MOYER1987" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "PDSSR1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END