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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
IN-FLIGHT GEOMETRIC CALIBRATION OF THE
LUNAR RECONNAISSANCE ORBITER CAMERA
E. J. Speyerer *, R. V. Wagner *, M. S. Robinson *, D. C. Humm ® K. Becker ©, J. Anderson °, P. Thomas ?
" School of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, Tempe, AZ 85287, USA -
espeyerer@ser.asu.edu
? Space Instrument Calibration Consulting, Annapolis, MD, USA
* Astrogeology Research Program, United States Geologic Survey, Flagstaff, AZ, USA
* Center for Radiophysics and Space Research, Cornell University, Ithaca, NY, USA
Commission IV, WG IV/7
KEY WORDS: Calibration, Registration, Geometric, Sensor, Optical, Distortion, Accuracy, Temperature
ABSTRACT:
The Lunar Reconnaissance Orbiter Camera (LROC) consists of two imaging systems that provide synoptic and high resolution
imaging of the lunar surface. The Wide Angle Camera (WAC) is a seven color push frame imager with a 90° field of view in
monochrome mode and 60° field of view in color mode. From the nominal 50 km polar orbit, the WAC acquires images with a nadir
pixel scale of 75 m for cach visible band and 384 m for the two ultraviolet bands. The Narrow Angle Camera (NAC) consists of two
identical cameras capable of acquiring images with a pixel scale of 0.5 to 1.0 m from a 50 km orbit. Each camera was geometrically
calibrated prior to launch at Malin Space Science Systems in San Diego, California. Using thousands of images acquired since
launch in June of 2009, improvements to the relative and absolute pointing of the twin NACs were made allowing images on the
surface to be projected with an accuracy of 20 meters. Further registration of WAC and NAC images allowed the derivation of a new
distortion model and pointing updates for the WAC, thus enabling sub-pixel accuracy in projected WAC images.
1. INTRODUCTION model for the WAC. This work highlights the LROC team's
efforts and reports the latest results in the calibration process.
1.1 LROC
: 2 ; 1.2 NAC
The Lunar Reconnaissance Orbiter (LRO) is a remote sensing
precursor designed to facilitate scientific and engincering-driven The two NACS, designated as NAC-Left (NAC-L) and NAC-
mapping of the lunar surface for future robotic and human Right (NAC-R), each consist of a 5064 pixel charge coupled
missions [Vondrak et al., 2010]. The spacecraft is equipped with device (CCD) line-array sensors connected to a telescope with a
six science instruments and one technology demonstration 700 mm focal length. Each NAC has a field-of-view (FOV) of
instrument, cach designed to answer key questions about the — 2.85» and is mounted off nadir 1.4? in opposite directions,
Moon's past and present state. The Lunar Reconnaissance providing a 5.7° combined FOV (~10,000 pixels across track)
Orbiter Camera (LROC), was designed to assess meter and with —135 pixels of overlap [Robinson et al, 2010]. The
smaller-scale features to facilitate safety analysis for potential instantaneous FOV for each NAC pixel is 10 pradians,
landing sites and acquire multi-temporal images of the polar providing 50 cm pixels at an altitude of 50 km, which was
regions to enable mapping of the illumination environment common during the first two years of the mission (15
[Robinson et al, 2010; Speyerer and Robinson, 2012]. September 2009 to 11 December 2011).
Additionally, the images from the LROC instrument provide
meter-scale mapping of areas of high science and exploration
interests, high resolution digital elevation models, and global
multispectral views.
The LROC instrument is comprised of three cameras: a pair of
Narrow Angle Cameras (NACs) and a Wide Angle Camera
(WAC) (Figures 1 and 2). Since launch, over 520,000 NAC
images and 235,000 WAC images have been acquired through
15 March 2012. During the early part of the mission, it was
difficult to map project images accurately due to large
unknowns in the absolute position of the spacecraft (sometimes
> 100 m). However, recent precision orbit determination
derived from radiometric data and altimetric crossovers
improved the known position of the spacecraft to within 20
meters [Mazarico et al., 2011]. With this updated ephemeris, the
LROC team is improving the pointing for both the Narrow and
Wide Angle Cameras as well as deriving an improved distortion
Figure 1. One of the two LROC NACs. The gold blanket covers
the telescope and the silver portion is a baffle (27 cm in diam.)
designed to minimize scattered light entering the optics.
1.3 WAC
The WAC is a push frame imager capable of providing images
in seven different color bands (321, 360, 415, 566, 604, 643,
and 689 nm). Light enters the camera through two sets of optics,
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