NSSDCA ID: PSPG-00877
Availability: Archived at NSSDC, accessible from elsewhere
Time span: 1997-07-31 to 2001-06-30
This description was generated automatically using input from the Planetary Data System.
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].
This dataset includes spherical harmonic gravitational field models 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.
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. The free-air gravity values are given by the radial (or vertical) acceleration at the reference surface. This is the radial gradient of the gravity potential. The reference surface is a sphere with radius 6051.0 km. The GM for the potential expansion is 324858.5897 km^3/s^2 (from the 120th degree and order gravity field solution). The bouguer map is the actual free-air gravity minus the theoretical free-air gravity from the topography with no compensation. The acceleration difference is at the reference surface of 6051.0 km. The reference radius for the topography field is 6051.848044 km.
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.
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].
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
The topography model of Venus was produced by a spherical harmonic analysis of the most complete set of Magellan altimetry data, augmented by Pioneer Venus and Venera data. A detailed report on this harmonic analysis and its scientific implications is in [RAPPAPORT&PLAUT1994].
This dataset includes a 360th degree and order spherical harmonic topography model, and a digital map of the topography. 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 topography field is in tabular format, with each row in the table containing the degree index m, the order index n, and the coefficients Cmn and Smn. The digital map of topography is a binary image. There is also an ASCII 2-D gridded data array of the digital image, including a byte-scaled image of the gridded product.
The topography of Venus was determined from altimetry data obtained by the Magellan spacecraft and augmented by Pioneer Venus and Venera data. The mean radius of Venus was found to be 6051.848 km.
The starting point for building the set of data that was analyzed to produce the topography spherical harmonic model was the GTDR (Global Topography Data Record) files which contain maps of Magellan altimetry data [FORD&PETTENGILL1992]. Gaps in the data were filled using a) Pioneer Venus Orbiter altimetry merged with Venera 15/16 altimetry data; and b) the 120 X 120 model of [KONOPLIVETAL1993], providing a complete set of data referenced to Venus body-fixed reference frame. These data were projected on a rectangular grid of 0.25 X 0.25 degrees and the harmonic coefficients were computed using quadrature procedure [BALMINO1993].
Magellan altimetric data and documentation can be found in numerous published datasets: Global Topography Data Record (GTDR); Altimetry and Radiometry Composite Data Record (ARCDR); and Altimeter Experiment Data Record (ALT-EDR).
A Venus body-fixed reference frame, as defined by [DAVIESETAL1992B], was used. Software ======== N/A
These data are available on-line from the Planetary Data System (PDS) at:
http://pds-geosciences.wustl.edu/mgn/mgn-v-rss-5-gravity-l2-v1/mg_5201/
Questions and comments about this data collection can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Gordon H. Pettengill | Data Provider | Massachusetts Institute of Technology | ghp@space.mit.edu |
Mr. William L. Sjogren | General Contact | NASA Jet Propulsion Laboratory | wls@nomad.jpl.nasa.gov |