The Astrogeology Program of the U.S. Geological Survey is pleased to announce the availability of a reduced-resolution version of the Clementine near-infrared (NIR) mosaic. Processed to 500 m/pixel spatial resolution, an ISIS cube version of this multispectral mosaic can be downloaded in east and west hemispheres (70° N to 70° S) here on the USGS Astrogeology Research Program web site. This First Release marks the beginning of systematic processing and impending release of a series of full-resolution, multispectral tiles or quads on archive media and, in the future, through the PDS Map-a-Planet web site.
In May and June of 1994, the NASA/DoD Clementine Mission acquired global, 11-band, multispectral data of the lunar surface using the ultraviolet-visible (UVVIS) and near-infrared (NIR) camera systems (e.g., Nozette et al, 1994; McEwen and Robinson, 1996). The global 5-band UVVIS Digital Image Model (DIM) of the Moon at 100 m/pixel was released to the Planetary Data System (PDS) in 2000 (Eliason et al., 1999). The NIR DIM has six spectral bands (1100, 1250, 1500, 2000, 2600, and 2780 nm) and will be delivered in 996 quads at 100 m/pixel (303 pixels/degree) in early 2004. This First Release NIR mosaic is processed to 500 m/pixel, has 6 spectral bands, and is presented in Simple Cylindrical map projection for the east (0° to 180° E) and west (180° to 360° E) lunar hemispheres. The NIR data have been radiometrically corrected, geometrically controlled to the Clementine 750-nm mosaic, and photometrically normalized to form seamless, uniformly illuminated mosaics of the lunar surface. The first four NIR bands (1100 to 2000 nm) have also been normalized to reflectance based on the approach previously applied to the calibrated UVVIS global mosaics (Pieters et al., 1999). The 2600 nm and 2780 nm NIR buands are provided as calibrated Clementine digital numbers (in counts/ms).
Details of the processing of the NIR mosaics have been described in several publications: Lucey et al., 1998, LPS XXIX, #1576 (25 KB, PDF); Lucey et al., 2000, LPS XXXI, #1273 (55 KB, PDF); and Eliason et al., 2003, LPS XXXIV, #2093 (210 KB, PDF). Processing of the global mosaics based on the radiometric and photometric procedures described in these references resulted in the "standard processing" global mosaics presented above.
For these products, we used the radiometric camera model described in Lucey et al. (2000) along with a thermal background correction described in Eliason et al. (2003). Residual radiometric artifacts remain in these data and are clearly visible when a ratio of two NIR bands is generated. The artifacts are attributed to 1) the radiometric camera model having residual errors in the values associated with the instrument operating modes (gain, offset, and exposure values); 2) residual errors existing in the thermal background correction; and 3) the shading properties of the sensor array changing with the thermal conditions of the camera that have not been properly characterized.
In addition to the major radiometric and photometric processing, empirically derived frame offset corrections were applied to produce a second version of the global mosaics with reduced variability across camera modes and adjacent orbits. This additional processing resulted in the "empirically corrected" mosaics and is described in more detail below. Similar before-and-after comparisons are shown for the Aristarchus Plateau region and the eastern portion of the South Pole-Aitken basin region. Finally, 1500/2000-nm ratios of the western hemisphere and the Aristarchus Plateau are shown to illustrate the magnitude of the changes between the calibrated and empirically corrected versions of the NIR mosaics.
Frame Offset Corrections
After radiometric and photometric calibrations were applied to the NIR data (e.g., Eliason et al., 2003), additional corrections were derived to adjust for remaining problems in the characterization of specific camera modes and the drift of radiometric properties over the two month Clementine observation period. Problems in the existing NIR calibrations (standard processing version) were apparent in both NIR albedo images and ratio images of NIR and NIR to UVVIS bands. Though such problem areas were typically associated with 10° latitude strips where specific camera settings were poorly characterized, no systematic correction for these residual errors was apparent based on comparisons of image means with gain and offset camera mode values. As a result, a set of empirical corrections was developed to reduce within- and between-orbit variations for each NIR band as described below.
To identify Clementine NIR frames with residual calibration errors, ratio images were created by dividing each radiometrically and photometrically calibrated NIR band to the USGS calibrated 750 nm UVVIS data. To make computational processing more efficient, the NIR data were divided by longitude into twelve 30° wide strips at a resolution of 1 km/pixel. Several thousand "truth" image frames were then selected for each NIR band of each strip based on orbits and camera mode states that demonstrated the least amount of radiometric variability across NIR/UVVIS ratio images. Image frames that were not included in the selected truth sets were then adjusted by computing an offset for each frame based on color ratio differences in overlapping regions of the frame being corrected and the surrounding truth-set frames (frame offset = derived ratio offset * 750 nm albedo). This procedure was applied to each longitude strip to derive a global set of offset frame corrections for each NIR band.
Derived frame offset corrections for each NIR band were then applied to a global mosaic of Clementine NIR data at 2 km/pixel resolution. New ratio images of the NIR channels over the UVVIS 750 nm data were created from the frame offset-corrected NIR data to evaluate results. Uniformity across camera mode settings and orbits was improved in the resulting ratio images for most regions. Offset correction values for NIR frames that were identified as unimproved by the automated offset correction algorithm due to within-image gradients or lack of adequate surrounding truth images were either eliminated from the offset corrections or adjusted manually based on visually matching values with surrounding frames in the ratio images.
After applying the best set of frame offset corrections derived through the above steps, a final small offset correction was then computed for every image frame in the Clementine global mosaic to correct for residual between-orbit variations. This was carried out by differencing the global ratio of each NIR band over the 750 nm band by the same data after applying a longitudinal median filter. The width of the longitudinal median filter was varied until a filter was identified that captured color offsets across orbits while minimizing the influence of geologic albedo and color boundaries on the subsequently derived offset corrections. A frame specific offset for all six NIR bands was derived from the mean difference of the pre- and post-filter ratio images over each frame's coverage (frame offset = filter specific constant * ratio offset * 750 nm albedo). Derived frame offsets were then smoothed as a function of latitude within each orbit and added to the previous offset correction values.
The final result of the offset deviation processing described above was a single text file containing offset corrections by NIR band for every image frame in the Clementine 2 km mosaic. A backplane containing the area of each Clementine frame is included in the ISIS cube files of each 0.5 km/pixel NIR mosaic. These offsets values were then subtracted from the radiometrically corrected NIR data to create the offset-corrected 0.5 km/pixel mosaics.
A final multiplicative correction equivalent to the month1-to-month2 correction applied to the calibrated global UVVIS data was also applied to the offset corrected NIR mosaic (frame based NIR calibrations were made relative to the 750 nm UVVIS data before this final multiplicative correction had been applied).
Apollo 16 Normalization
Apollo 16 soil measurements previously used to calibrate the UVVIS data (Pieters et al., 1999 [1.5 MB, PDF ];) were convolved through the first four NIR filter transmission curves. The longest NIR wavelengths (2600 and 2780 nm bands) were not normalized to these soil measurements because reflectance information was not available and may be complicated by the presence of thermal emission signatures. The values obtained from the soil spectra were then compared with the same Apollo 16 landing site location used to calibrate the UVVIS data to obtain a reflectance normalization coefficient for the first four bands of the NIR data (1100 - 2000 nm). Multiplicative correction factors derived from the standard processing data and the offset corrected NIR data were consistent to ~ 2% or better for all four bands. Due to a decrease in orbit to orbit variability around the Apollo 16 landing site, values derived from the final offset corrected mosaics were used for the normalizations of both datasets and are presented in Table 1 below.
Apollo 16 Calibration Coefficients
Notes on Usage
This First Release of the Clementine NIR mosaic represents a significant improvement in radiometric calibrations over the original mission calibrations generated by Hye Sook Park (Lawerence Livermore Laboratory) from preflight calibration of the instrument. Assessement of the inflight image showed calibrations gain and offset states with up to 10% residual errors in addition to the camera suffering from a drifting additive offset which differs throughout an orbit and from orbit to orbit (Lucey et al., 1997). In spite of the improvements over the original calibration, residual errors remain and these influence the usage of the data. The presence and approximate magnitudes of such errors can be identified in the ratio images of various bands of the global NIR data where color variations change across orbits and camera modes that cannot be attributed to surface materials. Though such color variations are less apparent in the frame corrected version of the NIR data, some residual patterns due to imperfect calibrations are known to exist in this version of the data as well. As a result, it may be useful to compare both versions of the NIR mosaics when conducting regional-scale analyses to identify local radiometric variations introduced by imperfect data calibrations. Due to the potential for such spatial variation in the radiometric calibrations, it is advantageous to conduct spectral analyses of the data over relatively large regions that include several different orbits and camera modes.
- Weller et al., 2007 USGS Lunar Orbiter Digitization Project: Updates and Status, LPS #2092 abstract for Lunar and Planetary Science XXXVIII (2007) [30 KB, opens in a new window]
- Rosiek et al., 2006, The Unified Lunar Control Network 2005 USGS Open-File Report 2006-1367 [opens in a new window]
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- Gaddis et al., 2003, http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1459.pdf">Reviving Lunar Orbiter: Scanning, Archiving, and Cartographic Processing at USGS, LPS #1459 abstract for Lunar and Planetary Science XXXIV (2003) [260 KB, opens in a new window]
- Gaddis et al., 2001, Cartographic Processing of Digital Lunar Orbiter Data, LPS #1892, abstract for Lunar and Planetary Science XXXII (2001) [1.6 MB, opens in a new window]
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- Hansen, Thomas P., 1970, Guide to Lunar Orbiter Photographs, NASA SP-242. See Astrogeology's NASA SP-242 - Guide to Lunar Orbiter Photographs for an online version of this publication.