Radargrammetry on three planets: Mapping the Solar System's hidden corners

Synthetic Aperture Radar (SAR) can provide useful images where passive optical imaging cannot, either because the microwaves used can penetrate atmospheric clouds, because active imaging can "see in the dark," or both. These abilities have led to the use of SAR on several planetary missions, including the Magellan at Venus (Radar Investigation), the Cassini at Titan (RADAR), and the Chandrayaan-1 and LRO lunar orbiters (Mini-RF). As participants in these missions, we have developed radargrammetry software and techniques and used them to map the Solar System’s darkest and cloudiest places. Radargrammetry—the science of making geometric measurements from SAR images, analogous to photogrammetry—yields digital topographic models (DTMs) from stereopairs, and can improve the positional accuracy of map products by bundle adjustment to ground control (usually derived from a global altimetry data set). To achieve these ends, we prepare the images and supporting data by using the USGS cartographic software ISIS, then use commercial stereo software (SOCET SET®) to control the images, create DTMs by automatic image matching, and interactively edit the results to remove matching errors. To use SOCET SET, it was necessary to write “sensor model” software describing the transformation between pixels and ground coordinates for each instrument. Magellan and Cassini images are provided in a map-projected and often mosaicked format, so the sensor model must first identify when a given pixel was observed and undo the map projection to recover the range and Doppler shift that were originally observed. These observables are then related to world coordinates by the principles of SAR image formation, taking into account elevation and adjustments to the spacecraft trajectory. Unprojected Mini-RF images are available so only the final step is needed in the Mini-RF sensor model. We have also written a Mini-RF sensor model for ISIS, making it possible to produce controlled mosaics in this free software system. SOCET SET and the stereo display hardware it uses remain essential for DTM production, but are available through the Planetary Photogrammetry Guest Facility that the USGS operates on behalf of NASA. In our presentation we will show examples of Venus, Titan, and lunar DTMs as well as controlled mosaics of Mini-RF images, and will discuss our efforts to quantify the precision, accuracy, and resolution of our products. These quality factors depend on the image resolution and also on the viewing geometry. In general images can be matched to a fraction of their resolution, leading to DTM vertical precisions of a few tens (Magellan, Mini-RF zoom mode) to a few hundreds of meters (Cassini, Mini-RF baseline mode). Horizontal resolutions of 5-10 pixels can generally be achieved, yielding a 1.5 order of magnitude resolution improvement over Magellan altimetry data, and offering the potential to fill the multi-kilometer gaps between LRO altimetry profiles near the lunar equator with elevation posts every 50-100 m. Cassini RADAR stereopairs sample only a few percent of Titan but are of inestimable value because they provide a full map view of geologic features whereas other topographic data sources such as altimetry provide only isolated elevation profiles.