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NASA NTRS1992

SAR data processing for the Magellan prime mission

The Magellan prime mission involves mapping the planet of Venus once around its rotational axis. The Magellan synthetic aperture radar (SAR) data processing facility processes all SAR data collected by the Magellan spacecraft into image data on an orbit-by-orbit basis. About 1700 million bits of radar data were collected. A complete description of the Magellan SAR Data Processing Facility is provided with emphasis on key design features of the data processors that satisfied the project data processing requirements. A summary of the attained data processing performance is included, as well as a brief discussion of some of the constraints and considerations regarding the applicability of the processors to meeting the data processing goals anticipated for the follow-on mission phases (i.e., cycles II, III, and beyond).

Leung, K.↗
NASA NTRS1991

Tectonic evolution of Lavinia Planitia, Venus

High resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. Lavinia is a broad lowland over 2000 km across, centered at about 45 deg S latitude, 345 deg E longitude. Herein, the tectonic evolution of Lavinia is discussed, and its possible relationship to processes operating in the planet's interior. The discussion is restricted to the region from 37.3 to 52.6 deg S latitude and from about 340 to 0 deg E longitude. One of the most interesting characteristics of Lavinia is that the entire region possesses a regional tectonic framework of striking regularity. Lavinia is also transected by a complex pattern of belts of intense tectonic deformation known as ridge belts. Despite the gross topographic similarity of all of the ridge belts in Lavinia, they exhibit two rather distinct styles of near surface deformation. One is composed of sets of broad, arch-like ridges rising above the surrounding plains. In the other type, obvious fold-like ridges are rare to absent in the radar images. Both type show evidence for small amounts of shear distributed across the belts.

Squyres, Steven W.↗
NASA NTRS1993

Magellan Mission

The Magellan spacecraft was launched from Cape Kennedy on 4 May 1989 and was inserted into orbit around Venus on 10 Aug. 1990. The Magellan spacecraft carries a radar instrument that makes synthetic aperture radar (SAR) images of the surface, measures the altitude of the Venusian surface directly below the spacecraft, and obtains radiometric observations of the surface. Radar and radiometric observations of the Venusian surface commenced on 15 Sep. 1990 and continued until 15 Sep. 1992. Gravity observations began on 24 Sep. 1992 and will continue until late May 1993. The radar observations produced SAR images and surface topography for 99 percent of the surface. These radar observations support the objective of improving the knowledge of the geological history of Venus by analysis of surface morphology and the processes that control them. The gravity observations that are being conducted now support the Magellan objective of improving the knowledge of the geophysics of Venus, principally its density distribution and dynamics. Also, Magellan generated more digital planetary image data than all previous planetary missions.

Thompson, Thomas W.↗
NASA NTRS1993

(abstract) Radiophysical Properties of Venusian Impact Craters

An analysis of 222 large (greater than 20-km-diameter) impact craters on Venus using both cycle 1 and cycle 2 Magellan data is being conducted to determine the radiophysical properties (i.e., backscatter cross section, emissivity, reflectivity, rms slope) of the craters and to search for correlations with target region properties and subsequent geological history.

morphology↗
NASA NTRS1992

Mapping Magellan's prospects

An overview is presented of the mapping cycles of Venus performed to date by NASA's Magellan spacecraft. Attention is given to the operating aspects of controlling the different orbits about the planet to obtain the desired mapping data.

Doody, Dave↗
NASA NTRS1993

Density of impact craters on tessera, Venus

After the plains, tessera is the most abundant terrain on Venus. It occupies about 10 percent of the Venusian surface, forming the continent-like blocks and small islands above the adjacent plains. Tessera is a result of tectonic deformations of some precursor terrain. However, the nature of that precursor, as well as the causes and mechanisms of its formations, are under debate. Any models considering tessera terrain involve estimation of tessera age, either relative or absolute. It is well known that the important information on the age of a planetary surface comes from impact crater statistics. The Magellan global overview of Venus with improved resolution provides an opportunity to gather data on impact craters in amounts large enough for statistically reliable estimations of crater density for different terrains. Our study of impact crater density on tesserae compared to the surrounding terrains has a goal to determine whether it is higher, lower, or the same and to interpret it in terms of the tessera age and processes involved.

Ivanov, M. A.↗
NASA NTRS2003

A Statistical Analysis of Corona Topography: New Insights into Corona Formation and Evolution

Extensive mapping of the surface of Venus and continued analysis of Magellan data have allowed a more comprehensive survey of coronae to be conducted. Our updated corona database contains 514 features, an increase from the 326 coronae of the previous survey. We include a new set of 106 Type 2 or stealth coronae, which have a topographic rather than a fracture annulus. The large increase in the number of coronae over the 1992 survey results from several factors, including the use of the full Magellan data set and the addition of features identified as part of the systematic geologic mapping of Venus. Parameters of the population that we have analyzed to date include size and topography.

Stofan, E. R.↗
NASA NTRS1993

Comparative hypsometric analysis of both Earth and Venus topographic distributions

Previous studies have compared the global topographic distribution of both planets by means of differential hypsometric curves. For the purpose of comparison, the terrestrial oceanic load was removed, and a reference base level was acquired. It was chosen on the basis of geometric considerations and reflected the geometric shape of the mean dynamical equilibrium figure of the planetary surface in both cases. This reference level corresponds to the well-known sea level for the Earth; for Venus, given its slow rate of rotation, a sphere of radius close to the mean, median and modal values of the planetary radii distribution were considered and the radius value of 6051 km arbitrarily taken. These studies were based on the low resolution (100 x 100 sq km) coverage of Venus obtained by the Pioneer Venus altimeter and on the 1 deg x 1 deg terrestrial topography. But, apart from revealing the distinct contrast existing between the Earth's bimodal and Venus' strong unimodal topographic distribution, the choice of such a reference level is inadequate and even misleading for the comparative geophysical understanding of the planetary relief distribution. The present work reinvestigates the comparison between Earth and Venus hypsometric distribution on the basis of the high-resolution data provided, on one hand, by the recent Magellan global topographic coverage of Venus' surface, and on the other hand, by the detailed NCAR 5 x 5 ft. grid topographic database currently available for the Earth's surface.

Rosenblatt, P.↗
NASA NTRS1995

(abstract) Venus Gravity Field

A global gravity field model of Venus to degree and order 75 (5772 spherical harmonic coefficients) has been estimated from Doppler radio tracking of the orbiting spacecraft Pioneer Venus Orbiter (1979-1992) and Magellan (1990-1994). After the successful aerobraking of Magellan, a near circular polar orbit was attained and relatively uniform gravity field resolution (approximately 200 km) was obtained with formal uncertainties of a few milligals. Detailed gravity for several highland features are displayed as gravity contours overlaying colored topography. The positive correlation of typography with gravity is very high being unlike that of the Earth, Moon, and Mars. The amplitudes are Earth-like, but have significantly different gravity-topography ratios for different features. Global gravity, geoid, and isostatic anomaly maps as well as the admittance function are displayed.

Venus gravity field model gravity-topography ratio↗
NASA NTRS1989

Planetary exploration in the 1990's

Planned and proposed near-term missions for planetary exploration are outlined. The history of planetary missions is reviewed and the characteristics of proposed future missions are discussed. The missions include the Magellan mission to map Venus, the Mars Observer mission, the Lunar Geochemical Orbiter, the Lunar Observer mission, the second and third phases of the Venus Orbiter, the future of the Voyager mission, and the Galileo mission to study the rings and satellites of Jupiter. Other missions include the Ulysses solar polar mission, the Cassini Saturn and Titan probes, the Comet Rendezvous Asteroid Flyby mission, the Mars Sample Return Mission, and the Comet Nucleus Sample Return.

Blanchard, Douglas↗
NASA NTRS1991

Multiple image SAR shape-from-shading

A technique for combining radar image shape-from-shading with stereo radargrammetry to produce terrain surface models using multiple SAR images is described. This technique is expected to be of use to reconstruct surface shape from Magellan images of planet Venus, and to refine the results of terrestrial radar measurements. Local variation in pixel shading is an indicator of terrain slope changes. This variation in pixel shading offers an opportunity for increasing the detail of terrain mapping over that which is available from stereo radargrammetry alone. Shape-from-shading can potentially provide a relative change in height at each pixel. This leads to a dense set of height measurements and a more faithful rendition of the local terrain shapes. However, shape-from-shading needs some type of boundary values or external terrain low-frequency information to succeed. These can be obtained from stereo or from altimeter measurements.

Thomas, J.↗
NASA NTRS1990

A new treatment of the albedo radiation pressure in the case of a uniform albedo and of a spherical satellite

The purpose of this paper is to present a model for the radiation pressure acceleration of a spherical satellite, due to the radiation reflected by a planet with a uniform albedo. A particular choice of variables allows one to reduce the surface integrals over the lit portion of the planet visible to the satellite to one-dimensional integrals. Exact analytical expressions are found for the integrals corresponding to the case where the spacecraft does not 'see' the terminator. The other integrals can be computer either numerically, or analytical in an approximate form. The results are compared with those of Lochry (1966). The model is applied to Magellan, a spacecraft orbiting Venus.

Borderies, Nicole↗
NASA NTRS1992

Electrostatic effects on dust particles in space

The star scanner of the Magellan spacecraft experienced operational anomalies continuously during Magellan's journey to Venus. These anomalies were attributed to the presence of dust particles in the vicinity of the spacecraft. The dust particles, which were originated from the surface of thermal blankets, were liberated when the electrostatic force acting on them was of sufficient magnitude. In order to verify this hypothesis, an experimental program was initiated to study the mechanisms responsible for the release of dust particles from a spacecraft surface. In the experiments, dust particles were immersed in a plasma and/or subjected to ultra-violet irradiation. Results showed that the charging state of a dust particle was strongly dependent on the environment, and the charge on a dust particle was approximately 10(exp 3) elementary charges. Consequently, in the space environment, electrostatic force could be the most dominant force acting on a dust particle.

Leung, Philip↗
NASA NTRS1992

Orbit determination for Magellan and Pioneer 12 using same-beam interferometry

Simultaneous tracking of two spacecraft in orbit about a distant planet, by two widely-separated earth-based radio antennas, provides more accurate positioning information than can be obtained by tracking each spacecraft separately. A demonstration of this tracking technique, referred to as Same-Beam Interferometry (SBI), is in progress using the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the earth-based tracking antennas. The plane-of-sky position difference between spacecraft is precisely determined by double-differenced phase measurements. This data type complements line-of-sight Doppler. Data were collected from Magellan and Pioneer 12 on Aug. 11-12, 1990, shortly after Magellan was inserted into Venus orbit. Orbits for both spacecraft were fit with one day data arc using Doppler and SBI data and compared to orbits fit to only Doppler data. The Doppler plus SBI orbits show improved orbit-to-orbit consistency over the Doppler-only orbits.

Folkner, W. M.↗
NASA NTRS1995

New observations of Bolivian wind streaks by JPL Airborne SAR: Preliminary results

In 1993 NASA's Jet Propulsion Laboratory Airborne Synthetic Aperture Radar system (AIRSAR) was deployed to South America to collect multi-parameter radar data over pre-selected targets. Among the sites targeted was a series of wind streaks located in the Altiplano of Bolivia. The objective of this investigation is to study the effect of wavelength, polarization, and incidence angle on the visibility of wind streaks in radar data. Because this is a preliminary evaluation of the recently acquired data we will focus on one scene and, thus, only on the effects of wavelength and polarization. Wind streaks provide information on the near-surface prevailing winds and on the abundance of winderodible material, such as sand. The potential for a free-flyer radar system that could provide global radar images in multiple wavelengths, polarizations, and incidence angles requires definition of system parameters for mission planning. Furthermore, thousands of wind streaks were mapped from Magellan radar images of Venus; their interpretation requires an understanding of the interaction of radar with wind streaks and the surrounding terrain. Our experiment was conducted on wind streaks in the Altiplano of Bolivia to address these issues.

Blumberg, Dan G.↗
NASA NTRS1993

Venus gravity and topography: 60th degree and order model

We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

Konopliv, A. S.↗