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].

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