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. For information on Magellan gravity investigations see papers in [ICARUSMGN1994].  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. [KONOPLIV&SJOGREN1996] provides the most detailed description of the Magellan gravity experiment.    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.  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].  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.   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.  All the Doppler residuals are two-way (round trip), where the Doppler shift is the shift between the outgoing and returned frequency. The Doppler residuals can be converted into velocity residuals of the spacecraft relative to Earth using:  (Doppler residual * c) / (2 * frequency),  where c is the velocity of light, 2.997925E11 mm/sec, and the frequency is 8.43E09 Hz, which reduces to 1 Hz Doppler being equivalent to 17.78 mm/sec (range-rate).  The LOS data during cycle 4 (Sept. 92 - May 93), cycle 5 (Aug. 93 - April 94) and cycle 6 (April 94 - Oct. 94) have been produced using a 120th degree and order spherical harmonic model (MGN120P) of the Venusian gravity field, 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, which has only a central term and without any spherical harmonic coefficients).    Ancillary Data : N/A    Coordinate System : The coordinates extracted from the JPL ODP (Orbit Determination Program) and given in the data files for the spacecraft position and velocity are: space-fixed cartesian, Venus-centered, referenced to the Venus true equator and prime meridian at the epoch time given.    Software : N/A    Media/Format : The Cycle 4, 5 and 6 LOSAPDR dataset will be available electronically via the World-Wide Web and anonymous FTP transfer. 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).

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 archive will be reviewed by PDS and planetary scientists familiar with the dataset, prior to release to the planetary community.    Data Coverage and Quality : There were 990 LOSAPDRs for cycle 4 (orbits 5827-7615) 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. There were 1679 LOSAPDRs produced at JPL and then validated at the Geophysics Subnode for cycles 5 and 6 combined. There were 736 LOSAPDRs for cycle 5 (orbits 8606-12248) and 943 for cycle 6 (orbits 12250-14992).    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.    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.    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.    June 22 to July 14, 1994 ------------------------ During this period (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