RADARGRAMMETRIC MEASUREMENTS FROM THE INITIAL MAGELLAN COVERAGE OF PLANET VENUS

In August/September 1990, NASA's Magellan Mission to planet Venus began to image the planet's surface with the help of a Synthetic Aperture Radar system. By May 1991 the initial coverage of the surface of the entire planet was completed in 1789 orbits. Due to gaps, these orbits produced about 1,650 image strips covering 84 percent of the planet's surface. Each orbit resulted in an image strip of a width of 350 pixels and a length of more than 220,000 pixels. Each orbit also resulted in an altimeter trace with a variable instantaneous footprint of about 10 to 30 km in diameter, and radiometry observations with footprints of about 80 km, using the same radar system to obtain these three data sets. The orbit and imaging geometry produce a coverage with single image strips which does limit one's analysis to two dimensions only; the third dimension, terrain elevation, is usually not available from such data. We did attempt, however, to assess a few non-traditional techniques of surface elevation measurements to report on terrain slopes and the elevation of specific terrain features. Single image three-dimensional measurements are based on the assumption that certain features are symmetric, and employ slope information obtainable from shape-from-shading or radar clinometry. We describe the available techniques and assess their accuracy. We illustrate the topographic elevation measurements with a data set of the impact crater Danilova (26.4-degrees-S, 337.2-degrees-E), which we confirm to have a rim height of 1,560 metres, with an uncertainty of +/- 200 metres. We use this data set to also illustrate the shape-reconstruction from shading and apply this technique to an area at 83-degrees-N, 335-degrees-E.