PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2001-12-07, STEFANIE LAWSON;" RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "CLEM1-L-LWIR-3-RDR-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = " CLEMENTINE LWIR BRIGHTNESS TEMPERATURE V1.0" START_TIME = 1994-02-27T07:04:02.174Z STOP_TIME = NULL DATA_OBJECT_TYPE = IMAGE DATA_SET_RELEASE_DATE = 2002-07-01 PRODUCER_FULL_NAME = "STEFANIE L. LAWSON" CITATION_DESC = "Lawson, S. L., and B. M. Jakosky, Clementine LWIR Brightness Temperature Archive, CLEM1-L-LWIR-3-RDR-V1.0, NASA Planetary Data System, 2002." DETAILED_CATALOG_FLAG = "N" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_SET_TERSE_DESC = "The Clementine Long-Wave Infrared (LWIR) Brightness Temperature data set contains calibrated brightness temperature images that provide information on physical properties of the lunar surface." DATA_SET_DESC = " Data Set Overview ================= The scientific payload on the Clementine spacecraft included a Long-Wave Infrared (LWIR) camera with a single passband of width 1.5 microns centered at a wavelength of 8.75 microns. The LWIR spatial resolution ranged from 200 m per pixel near the poles to 55 m per pixel at the equator. Contiguous pole to pole imaging strips were obtained with approximately 10% overlap between adjacent frames. However, significant longitude gaps exist between successive orbital passes. During the systematic mapping phase of the Clementine mission, approximately 220,000 thermal- infrared images of the lunar surface were obtained. This data set contains LWIR images where the observed radiance has been converted to brightness temperature, which provides information on various physical properties of the lunar surface. Processing ========== Preflight calibration for the LWIR instrument was performed at Lawrence Livermore National Laboratory in an effort to measure camera characteristics such as radiometric sensitivity, gain and offset scale factors, temporal/spatial noise, and the dependence of dark-noise on focal plane array (FPA) temperatures. Several steps were involved in the generation of brightness temperature values which included converting measured data number (DN) values to radiance values; identifying and eliminating bad pixels; correcting for pixel response variation across the detector array; determining the zero-flux background of the instrument; comparing LWIR measured radiance of the Apollo 17 landing site to in situ temperature measurements in order to derive absolute calibration adjustments; and finally, converting measured radiance values to brightness temperatures via the Planck function [LAWSONETAL2000]. The first step in the routine to determine brightness temperature from LWIR images involved converting measured DN values (ranging from 0 to 255) to equivalent radiance values through a preflight calibration equation. This equation corrected for the changing gain and offset states used throughout an imaging orbit in order to account for the increase in surface thermal emission near the equator. The primary uncertainty in the calibration was the subtraction of a zero-flux radiance. The dark-frame signal was extremely large and varied with time in orbit and probably with lens and FPA temperatures. Through the 1.5 hours of a lunar imaging orbit, the heat input to the spacecraft by the cryocooler increased the surrounding temperature and decreased the cooling effectiveness, resulting in rising FPA temperatures with time. This zero-level background increase was much less pronounced in the first month of systematic mapping than in the second. At the mid-lunar- mapping-mission orbital-correction burn, fuel was expended and the overall thermal mass of the spacecraft was reduced. This resulted in an increased operational temperature and caused the LWIR background counts generated from the pixel dark current and the thermal emission from within the optical path to increase. Thus, after the mid-lunar-mapping orbital burn, the number of LWIR images taken during an orbital pass was reduced to avoid saturation of the detector. In order to subtract a zero-flux radiance from the LWIR images, a series of space-looking frames (away from the Sun, Moon, and Earth) was used that were acquired at the beginning and end of many lunar mapping orbits. The increasing background level through an orbit was accounted for by fitting a line, on a pixel-by-pixel basis, to the pre-mapping space and post-mapping space radiance values as a function of time. The next step in the process was to create bad-pixel maps and flat fields. After subtracting a zero-level image from each lunar image, hundreds of lunar images from a single orbit were averaged together on a pixel-by-pixel basis. Thus a lunar mean image and an associated lunar standard deviation image were generated. Lunar images at latitudes higher than 70 deg were neglected in the calculation of the mean image because they were often either saturated (due to excessively high gain states) or they had values less than zero after the zero-level image subtraction. Bad pixels were identified on the mean and standard deviation images as pixels that did not vary (low or zero standard deviation), varied randomly (high standard deviation), or were pegged at high values such that their dynamic range was limited (high mean). Pixels with low means also appeared bad on lunar images. An average of approximately 11% of the 16,384 detector array pixels was characterized as consistently bad. There were approximately 10 orbits where at least one map had more than 20% bad pixels. In order to correct for pixel response variation across the detector array, a flat field frame was created by multiplying the lunar mean image and the bad-pixel map, smoothing over the bad pixels, and normalizing the resultant image to unity. For an IR camera, the FPA signal comes not only from the signal generated from the lunar surface, but also from dark current generated internally within each pixel detector and from thermal emission generated from the camera optical path. For the LWIR the magnitude of these contributions was very large and varied both through an orbit and through the mission. Thus three separate bad-pixel maps and flat fields were required for each orbit. The number created was constrained by the number of lunar images over which to average. The minimum number of images to average was controlled by the need to eliminate structure due to scene variations that result from surface topography. The maximum number of images to average was limited by the varying thermal emission of the lunar surface through an orbit; the measured radiance values were much higher near the equator than near the poles. To create the mean and standard deviation images, latitude bins were averaged together in the following way: 70S to 20S, 20S to 20N, and 20N to 70N. Lunar images from latitudes higher than 70 degrees used flat fields and bad-pixel maps from adjacent latitude bins. Inflight absolute calibration was accomplished by comparing the LWIR-derived temperatures at the Apollo 17 landing site to temperatures determined in situ from the heat-flow experiment. Although in situ temperatures were also measured at the Apollo 15 site, LWIR frames are available only for the Apollo 17 site. The difference between Apollo temperatures and LWIR temperatures was approximately 17 K, corresponding to a 22% difference in radiance. This 22% change in radiance reflects both the limitations of the preflight calibration and the differences between prelaunch and inflight calibrations. Therefore, all of the LWIR-derived radiance values were multiplied by 0.82. Brightness temperatures were then calculated using the Planck function assuming unit emissivity. The bad pixel routine did not identify all bad pixels due to their shifting locations from image to image. The few remaining bad pixels were eliminated via a sigma-filter routine that replaced pixels that were greater than 3-sigma away from the mean of a 3 x 3 pixel region around each pixel. Finally, the LWIR temperatures were corrected for the varying Sun-Moon distance throughout the mission. " CONFIDENCE_LEVEL_NOTE = " Preflight calibration data were acquired with an automated calibration facility at Lawrence Livermore National Laboratory. The preflight calibration measurements included radiometric sensitivity; FPA uniformity; gain and offset scale factors; temporal/spatial noise; dark noise dependence on FPA temperatures, integration times, input voltage levels, and spectral response of FPA; optical distortion map; point spread function; electronic warm-up time; and cryocooler cool-down time. Absolute uncertainties were found from the zero-level radiance determination and are constant in radiance but vary in equivalent temperature throughout an orbit, being lower near the equator where the surface temperatures are higher. For the entire mission, a typical 2-sigma uncertainty is 7 K or greater. Absolute uncertainties were propagated on a pixel-by-pixel basis throughout the calibration routine, and then image-averaged. Adjacent LWIR frames overlap and there is consistently less than a 1% difference in brightness temperature between overlapping regions. Thus, the statistical uncertainty of a single measurement is much less than the absolute uncertainties through an orbit. The LWIR camera cannot measure temperatures below approximately 150 K. In the calibration routine, a zero-level image is subtracted from each lunar radiance image. Sometimes this results in pixel values less than zero, particularly near the poles where the temperatures are low. All pixel values that are less than zero in radiance are set to 0.0001 and then converted to temperature. This results in temperatures around 45 K that are not real and have large associated uncertainties. Temperatures below 150 K are not to be trusted. Several orbits worth of data were not reduced, either because there were no space images taken for that orbit or because the detector saturated. LWIR images from 95% of the orbits from the first month of lunar mapping and from 77% of the orbits from the second month of lunar mapping were reduced. Thus data from a total of 86% of the 266 Clementine systematic mapping orbits was reduced. For more information on the LWIR calibration routine, see [LAWSONETAL2000]. " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = MOON END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = SKY END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = CLEM1 INSTRUMENT_ID = LWIR END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "LAWSONETAL2000" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END