Data Set Information
DATA_SET_NAME MGN V RDRS DERIVED GLOBAL VECTOR DATA RECORD V1.0
DATA_SET_ID MGN-V-RDRS-5-GVDR-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : The Global Vector Data Record (GVDR) is a sorted collection of scattering and emission measurements from the Magellan Mission. The sorting is into a grid of equal area 'pixels' distributed regularly about the planet. For data acquired from the same pixel but in different observing geometries, there is a second level of sorting to accommodate the different geometrical conditions. The 'pixel' dimension is 18.225 km. The GVDR is presented in Sinusoidal Equal Area (equatorial), Mercator (equatorial), and Polar Stereographic (polar) projections. The GVDR is intended to be the most systematic and comprehensive representation of the electromagnetic properties of the Venus surface that can be derived from Magellan data at this resolution. It should be useful in characterizing and comparing distinguishable surface units. Parameters : The Magellan data set comprises three basic data types: echoes from the nadir-viewing altimeter (ALT), echoes from the oblique backscatter synthetic aperture radar (SAR) imaging system, and passive radiothermal emission measurements made using the SAR equipment. The objective in compiling the GVDR is to obtain an accurate estimate of the surface backscattering function (sometimes called the specific backscatter function or 'sigma- zero') for Venus from these three data types and to show its variation with incidence (polar) angle, azimuthal angle, and surface location. The ALT data set has been analyzed to yield profiles of surface elevation [FORD&PETTENGILL1992] and estimates of surface Fresnel reflectivity and estimates of meter-scale rms surface tilts by at least two independent methods [FORD&PETTENGILL1992; TYLER1992]. The 'inversion' approach of [TYLER1992] provides, in addition, an empirical estimate of the surface backscatter function at incidence angles from nadir to as much as 10 degrees from nadir in steps of 0.5 degrees. Statistical analysis of SAR image pixels for surface regions about 20 km (across track) by 2 km (along track) provided estimates of the surface backscatter function over narrow angular ranges (1-4 degrees) between 15 and 50 degrees from normal incidence [TYLER1992]. By combining results from several orbital passes over the same region in different observing geometries, the backscatter response over the full oblique angular range (15-50) could be compiled. In fact, the number of independent observing geometries attempted with Magellan was limited, and some of these represented changes in azimuth rather than changes in incidence (or polar) angle. Nevertheless, data from many regions were collected in more than one SAR observing geometry. Histograms of pixel values and quadratic fits to the surface backscattering function over narrow ranges of incidence angle were computed by [TYLER1992]. Passive microwave emission by the surface of Venus was measured by the Magellan radar receiver between ALT and SAR bursts. These measurements have been converted to estimates of surface emissivity [PETTENGILLETAL1992]. With certain assumptions the emissivity derived from these data should be the complement of the Fresnel reflectivity derived from the ALT echo strengths. In cases where the two quantities do not add to unity, the assumptions about a simple dielectric (Fresnel) interface at the surface of Venus must be adjusted. Processing : The processing carried out at the Massachusetts Institute of Technology (MIT) to obtain altimetry profiles and estimates of Fresnel reflectivity and rms surface tilts has been described elsewhere [FORD&PETTENGILL1992]. In brief it involves fitting pre-computed templates to measured echo profiles; the topographic profiles, Fresnel reflectivities, and rms surface tilts are chosen to minimize differences between the data and templates in a least-squares sense. The estimates of emissivity require calibration of the raw data values and correction for attenuation and emission by the Venus atmosphere [PETTENGILLETAL1992]. These data have been collected by orbit number on a set of compact discs [FORD1992] and into a set of global maps, also distributed on compact disc [FORD1993]. ALT and SAR data have been processed at Stanford University using 'inversion' methods, whereby the radar equation is converted to a matrix-vector relationship and that is solved (using least-squares techniques) to obtain an 'empirical' scattering function for each radar echo [TYLER1992]. In the case of ALT data, stability of the solution requires considerable attention; in the case of SAR data, it is the variability of the surface scattering itself that leads to the most uncertainty. Stanford has also independently confirmed the results of the MIT processing of emissivity data but without developing separate algorithms. At MIT the primary input data were ALT-EDR tapes from the Magellan Project. The echoes themselves were used to obtain the altimetry and fitting results. SAR block quantization data were used to obtain a coarse estimate of high-angle scattering efficiency and that was applied to adjust the near-nadir Fresnel reflectivity. Raw radiometry measurements from the SAR burst headers were the input to the emissivity calculations. At Stanford ALT-EDR tapes were the input for calculation of near-nadir empirical backscattering functions. For oblique backscatter, C-BIDR tapes from the Magellan Project and F-BIDR files obtained via Internet from Washington University were the input products. Output was collected on an orbit-by-orbit basis into a product known as the Surface Characteristics Vector Data Record (SCVDR). The SCVDR has been delivered to the Magellan Project for orbits through 2599; processing of data beginning with orbit 2600 and continuing through the end-of-Mission is pending completion of the first version of the GVDR. Tabulated Data : The GVDR data set comprises several 'tables' of results based on analysis of each of the data types described above. These include: (1) Image Data Table (2) Radiometry Data Table (3) MIT ALT Data Table (4) Stanford ALT Data Table (1) Image Data Table -------------------- This table contains results from analysis of SAR image strips. The results are parameterized by the azimuth angle, the incidence (polar) angle, and the polarization angle. Quantities include the number of image framelets used to compute the scattering parameters; the median, the mode, and the one-standard-deviation limits of the pixel histogram; and the three coefficients and the reference angle of the quadratic approximation to sigma-zero as a function of incidence angle. (2) Radiometry Data Table ------------------------- This table contains results from MIT analysis of the radiometry data. The results are parameterized by the azimuth angle, the incidence angle, and the polarization angle. The results include the number of radiometry footprints used to compute the estimate of thermal emissivity, the emissivity, and its variance. (3) MIT ALT Data Table ---------------------- This table contains results derived from the MIT altimetry data analysis. The results include the number of ARCDR ADF footprints used in computing the estimates of scattering properties for the pixel and estimates (and variances) of radius, rms surface tilt, and Fresnel reflectivity from the ARCDR. (4) Stanford ALT Data Table --------------------------- This table contains results from the Stanford analysis of altimetry data. Results include the number of SCVDR footprints used in computing the estimates of surface properties for this pixel, the centroid of the Doppler spectrum, the derived scattering function and the angles over which it is valid, variance of the individual points in the derived scattering function, and results of fitting analytic functions to the derived scattering function. Ancillary Data : Ancillary data for most processing at both MIT and Stanford was obtained from the data tapes and files received from the Magellan Project. These included trajectory and pointing information for the spacecraft, clock conversion tables, spacecraft engineering data, and SAR processing parameters. For calibration of the radar instrument itself, Magellan Project reports (including some received from Hughes Aircraft Co. [BARRY1987; CUEVAS1989; SE011]) were used. Documentation on handling of data at the Jet Propulsion Laboratory was also used [BRILL&MEISL1990; SCIEDR; SDPS101]. Coordinate System : The data are presented in gridded formats, tiled to ensure that closely spaced points on the surface occupy nearby storage locations on the data storage medium. Four separate projections are used: sinusoidal equal area and Mercator for points within 89 degrees of the equator, and polar stereographic for points near the north and south poles. The projections are described by [SNYDER1987]; IAU conventions described by [DAVIESETAL1989] and Magellan Project assumptions [LYONS1988] have been adopted. Software : A special library and several example programs are provided in source code form for reading the GVDR data files. The general- purpose example program will serve the needs of the casual user by accessing a given GVDR quantity over a specified region of GVDR pixels. More advanced users may want to write their own programs that use the GVDR library as a toolkit. The library, written in ANSI C, provides concise access methods for reading every quantity stored in the GVDR. It conveniently handles all geometric and tiling transformations and converts any compressed quantities to a standard native format. The general purpose program mentioned above provides an example of how to use this library. Media/Format : The GVDR will be delivered to the Magellan Project (or its successor) using compact disc write once (CD-WO) media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992].
DATA_SET_RELEASE_DATE 1994-05-13T00:00:00.000Z
START_TIME 1990-08-01T12:00:00.000Z
STOP_TIME 1993-12-31T11:59:59.000Z
MISSION_NAME MAGELLAN
MISSION_START_DATE 1989-05-04T12:00:00.000Z
MISSION_STOP_DATE 1994-10-12T12:00:00.000Z
TARGET_NAME VENUS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGN
INSTRUMENT_NAME RADAR SYSTEM
INSTRUMENT_ID RDRS
INSTRUMENT_TYPE RADAR
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Overview : The GVDR is intended to be the most systematic and comprehensive representation of the electromagnetic properties of the Venus surface that can be derived from Magellan data at this resolution. Nevertheless, there are limitations to what can be done with the data. Review : The GVDR will be reviewed internally by the Magellan Project prior to release to the planetary community. The GVDR will also be reviewed by PDS. Data Coverage and Quality : Because the orbit of Magellan was elliptical during most of its mapping operations, parts of the orbital coverage have higher resolution and higher signal-to-noise than others. Cycle 1 Mapping --------------- During Mapping Cycle 1, periapsis was near 10 degrees N latitude at altitudes of approximately 300 km over the surface. The altitude near the poles, on the other hand, was on the order of 3000 km. For all data types this means lower confidence in the results obtained at the poles than near the equator. Further, the spacecraft attitude was adjusted so that the SAR antenna was pointed at about 45 degrees from nadir near periapsis; this was reduced to near 15 degrees at the poles. The objective was to compensate somewhat for the changing elevation and to provide scattering at higher incidence angles when the echo signal was expected to be strongest. The ALT antenna, at a constant 25 degree offset from the SAR antenna, followed in tandem but at angles which were not optimized for obtaining the best altimetry echo. During Mapping Cycle 1 almost half the orbits provided SAR images of the north pole; because of the orbit inclination, ALT data never extended beyond about 85N latitude in the north and 85S in the south. No SAR images of the south pole were acquired during Mapping Cycle 1 because the SAR antenna was always pointed to the left of the ground track; the Cycle 1 SAR image strip near the south pole was at a latitude equatorward of 85S. Cycle 2 Mapping --------------- During much of Mapping Cycle 2, the spacecraft was flown 'backwards' so as to provide SAR images of the same terrain but with 'opposite side' illumination. This adjustment also meant that the SAR could image near the Venus south pole (but not near the north pole). The ALT data continued to be limited to latitudes equatorward of 85N and 85S. Cycle 3 Mapping --------------- During Mapping Cycle 3 the emphasis was on obtaining SAR data from the same side as in Cycle 1 but at different incidence angles (for radar stereo). In fact, most data were acquired at an incidence angle of about 25 degrees, which meant that the ALT antenna was usually aimed directly at nadir instead of drifting from side to side, as had been the case in Cycle 1. These Cycle 3 data, therefore, may be among the best from the altimeter. Dynamic range in SAR data was larger than in Cycle 1 because the incidence angle was fixed rather than varying to compensate for the changing spacecraft height. All Cycles ---------- It is important to remember that, since the SAR and ALT antennas were aimed at different parts of the planet during each orbit, building up a collection of composite scattering data for any single surface region requires that results from several orbits be integrated. In the case of data from polar regions, where only the SAR was able to probe, there will be no ALT data. When scheduling or other factors interrupted the systematic collection of data, there may be ALT data for some regions but no comparable SAR or radiometry data (or vice versa). Note that for all Cycles outages played an important role in determining coverage. For example, although a goal of Cycle 3 radar mapping was radar stereo, early orbits were used to collect data at nominal incidence angles that had been missed during Cycle 1 because of thermal problems with the spacecraft. A transmitter failure during Cycle 3 caused a loss of further data. It is not within the scope of this description to provide detailed information on data coverage. Limitations : Both the template fitting approach and the inversion approach will have their limitations in estimating overall surface properties for a region on Venus. The template calculation assumes that scattering is well-behaved at all incidence angles from 0 to 90 degrees and that a template representing that behavior can be constructed. The Hagfors function [HAGFORS1964] used by MIT, however, fails to give a finite rms surface tilt if used over this range of angles, so approximations based on a change in the scattering mechanism must be applied [HAGFORS&EVANS1968]. The inversion method [TYLER1992] is susceptible to noise at the higher incidence angles and this will corrupt solutions if not handled properly. Users of this data set should be aware that radar echoes are statistically variable and that each result has an uncertainty. A nominal nadir footprint can be assigned to altimetry results, but this footprint is biased near periapsis because the ALT antenna is rotated about 20 degrees from nadir (during Cycle 1). Over polar regions in Cycle 1, the ALT antenna is rotated about 10 degrees to the opposite side of nadir. A more important consideration in polar regions is that the area illuminated by the ALT antenna is approximately 100 times as large as near periapsis because of the higher spacecraft altitude. The region contributing to echoes in polar regions -- and therefore the region over which estimates of Fresnel reflectivity and rms surface tilts apply -- is much larger than at periapsis.
CITATION_DESCRIPTION Maurer, M. J., MGN V RDRS DERIVED GLOBAL VECTOR DATA RECORD V1.0, MGN-V-RDRS-5-GVDR-V1.0, NASA Planetary Data System, 1994
ABSTRACT_TEXT This data set contains the Magellan Global Vector Data Record (GVDR), a sorted collection of scattering and emission measurements from the Magellan Mission. The sorting is into a grid of equal area 'pixels' distributed regularly about the planet. For data acquired from the same pixel but in different observing geometries, there is a second level of sorting to accommodate the different geometrical conditions. The 'pixel' dimension is 18.225 km. The GVDR is presented in Sinusoidal Equal Area (equatorial), Mercator (equatorial), and Polar Stereographic (polar) projections.
PRODUCER_FULL_NAME MICHAEL J. MAURER
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