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