RADARGRAMMETRY ON THREE PLANETS 
R.L. Kirk and E. Howington-Kraus 
Astrogeology Program, U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, Arizona 86001 USA - rkirk@usgs.gov 
Commission IV, WG IV/7 
KEY WORDS: Spacebome Remote Sensing, Synthetic Aperture Radar (SAR), Radargrammetry, Visualization, Geology, 
Topographic Maps, Adjustment, Sensor Models 
ABSTRACT: 
Synthetic Aperture Radar (SAR) can provide useful images in situations where passive optical imaging cannot, either because the 
microwaves used can penetrate atmospheric clouds, because active imaging can "see in the dark," or both. We have participated in 
the NASA Magellan mission to Venus in the 1990s and the current NASA-ESA Cassini-Huygens mission to Saturn and Titan, which 
have used SAR to see through the clouds of Venus and Titan, respectively, and have developed software and techniques for the 
production of digital topographic models (DTMs) from radar stereopairs. We are currently preparing for similar radargrammetric 
analysis of data from the Mini-RF instrument to be carried to the Moon on both the ISRO Chandrayaan-1 and NASA Lunar 
Reconnaissance Orbiter (LRO) missions later in 2008. These instruments are intended to image the permanently shadowed areas at 
the lunar poles and even see below the surface to detect possible water ice deposits. In this paper, we describe our approach to 
radargrammetric topographic mapping, based on the use of the USGS ISIS software system to ingest and prepare data, and the 
commercial stereoanalysis software SOCET SET (® BAE Systems), augmented with custom sensor models we have implemented, 
for DTM production and editing. We describe the commonalities and differences between the various data sets, and some of the 
lessons learned, both radargrammetric and geoscientific. 
1. INTRODUCTION 
Synthetic Aperture Radar (SAR) can provide useful images in 
situations where passive optical imaging cannot, either because 
the microwaves used can penetrate atmospheric clouds and 
even peek beneath the solid surface, because active imaging can 
"see in the dark," or both. Past planetary applications of SAR 
have exploited its cloud-penetrating abilities, as part of the 
Magellan mission to Venus and the Cassini/Huygens 
investigation of Saturn’s giant satellite Titan. Beginning in 
2008, closely related instruments on two lunar orbiters will use 
polarimetric radar to image the permanently shadowed areas of 
the lunar poles and search for subsurface ice deposits. We have 
participated in data analysis for all four of these planetary 
imaging radars, focusing on the generation of map products. 
We describe the techniques for radargrammetry (precisely 
analogous to photogrammetric analysis of passive optical 
images, but based on the different geometric principles by 
which radar images are formed) that we have developed and 
applied to these data sets. This work encompasses both the 
production of controlled image products by bundle adjustment 
(solution for improved image orientation parameters and ground 
coordinates of features, based on measurements of 
corresponding features in the images) and the production of 
digital topographic models (DTMs) by automated and/or 
manual identification of dense sets of image correspondences. 
Our approach to radargrammetry is to make synergistic use of 
digital cartography software written in-house at the USGS with 
a commercial softcopy stereo system. 
2. VENUS 
2.1 Magellan Mission and Data 
Venus is Earth’s sister planet in terms of size and position in the 
solar system, but hardly an identical twin. The surface is 
hidden by a dense carbon dioxide atmosphere with cloud decks 
of sulfuric acid, under which the pressure at the surface is more 
than 90 atmospheres and the temperature is 735 K. The 
Magellan spacecraft entered Venus orbit in 1990 with a radar 
imager/altimeter as its only instrument and high resolution 
global mapping of the surface as its primary goal. By 1992, 
Magellan had made three complete cycles of polar orbits, each 
cycle covering the full range of longitudes. During this time the 
spacecraft obtained SAR images in S band (12.6 cm >.) covering 
>96% of the planet at a ground sample distance of 75 m/pixel 
(Saunders et al., 1992). Images taken with a decreased look 
angle from vertical, primarily during Cycle 3, provide stereo 
coverage of 17% of the planet when combined with images 
with same-side illumination from earlier in the mission 
(primarily Cycle 1). The stereo geometry of these images is 
extremely favorable, allowing elevation measurements with an 
estimated vertical precision (EP) of ~10 m (Leberl et al., 1992). 
Opposite-side coverage was obtained over a greater area with 
even stronger stereo geometry, and can be useful in mapping 
areas of low relief such as the lowland plains. Magellan also 
obtained radar altimetry data at a horizontal resolution of 10x25 
km, but photogrammetric analysis of the stereoimagery can 
yield topographic maps with a horizontal resolution more than 
an order of magnitude superior to that of the altimeter. 
The SAR data from each Magellan orbit were recorded as an F- 
BIDR (Full-resolution Basic Image Data Record), a ~20x 17,000 
km strip with 75 m pixel spacing. The BIDR data set was not 
distributed widely, but was archived on magnetic tape and later 
on CD-ROMs, with copies at the NASA Planetary Data System