This PDF file contains the front matter associated with SPIE Proceedings Volume 7081, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

The NIST role in supporting our Nation's climate research is described. The assembly of climate data records over decadal time scales requires assimilating readings from a large number of optical sensors deployed in space and on the Earth by various nations. NIST, in partnership with NASA and NOAA, develops and disseminates the calibration tools and standards to ensure that the measurements from these sensors are accurate, comparable, and tied to international standards based on the SI system of units. This effort helps to provide confidence that the small decadal changes in environmental variables attributed to climate change are not an artifact of the measurement system. Additionally, it ensures that the measurements are physics based and thus comparable to climate models.

A vacuum compatible integrating sphere was built to operate inside a thermal vacuum chamber. This paper presents the design and test results for a 1.65 meter diameter vacuum compatible integrating sphere with a 1.0 meter diameter exit port and approximately 10kW of internal tungsten lamps. Liquid nitrogen is used as cooling medium to remove the heat generated by these lamps. There are no moving parts inside the vacuum chamber. The radiance is monitored with two filter-wheel detectors, one TE-cooled silicon and one TE-cooled germanium, as well as a TE-cooled silicon array spectrometer. All three detectors are located outside the thermal vacuum chamber and view the sphere radiance through fiber optic cables. The system was tested inside a thermal vacuum chamber at NASA Goddard Space Flight Center before commissioning in the 5.5 meter thermal vacuum chamber at Space Applications Centre in Ahmedabad, India. Results of tests of radiance uniformity, radiance levels, and radiance stability are presented. Comparisons of the filter radiometers with the array spectrometer are also presented.

The Remote Sensing Group (RSG) at the University of Arizona Optical Sciences Center has been performing high accuracy laboratory calibration for over 20 years. An integral part of this laboratory calibration is the implementation of very accurate and repeatable transfer radiometers. This work highlights the design and characterization of one such radiometer. This particular radiometer is essentially an updated version of a previous radiometer designed and characterized by the RSG (Spyak 2000) with particular attention being paid to increased portability, ease of use and autonomous operation. This work also covers the characterization of this radiometer, including radiometric calibration, field of view, and general performance.

Pre-launch performance has been characterized on the EOS-C camera: capable of Earth observation at 2.5 m resolution and 20 km swath width. Topics discussed in this paper include measurements of system modulation transfer function (MTF) and pixel lines-of-sight (LOS); radiometric and spectral calibration; end-to-end imaging.

Satellite instruments operating in the reflective solar wavelength region often require accurate and precise determination of the Bidirectional Reflectance Distribution Function (BRDF) of laboratory based diffusers used in their pre-flight calibrations. In this paper we present gray Spectralon BRDF measured using a monochromatic broadband source at ultraviolet, visible and near-infrared wavelengths. By comparing these results, we quantitatively examine the wavelength and geometrical scatter properties of gray-scale Spectralon. The Spectralon diffusers with specified hemispherical reflectances of 2%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 99% were measured using P and S incident polarized light over a range of incident and scatter angles. The measurements are compared, and the influence of material composition on the BRDF is described. The future application of gray-scale Spectralon in the calibration of spaceborne sensors is described. All data were obtained using the out-of-plane scatterometer located in NASA's Goddard Space Flight Center's Diffuser Calibration Facility. The results are NIST traceable.

The Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging SpectroRadiometer (MISR) instruments, currently in orbit, take radiometrically accurate pictures of the earth. These two instruments represent two types of earth observing cameras. MODIS is a cross track scanner while MISR is a pushbroom imager. An overview of the pre-launch ground optical testing equipment of MODIS and MISR is presented.

The Modulation Transfer Function (MTF) is a standard measure of the spatial quality of an imaging sensor. The MTF is calculated by a normalized Fourier Transform of a Point Spread Function (PSF), which is a two-dimensional function. For simplicity of calculation, a one-dimensional PSF, or Line Spread Function (LSF) is utilized in the along-scan direction. The along-scan direction LSF model is sub-divided into the component level LSFs and the proper LSF is constructed by the design specification information. To construct the modeled LSF, a spatial convolution is performed for the three major components: optical, detector, and integration time LSFs. The optical LSF is calculated using Zemax. The detector LSF is modeled as a rectangular function of the nominal detector size. Similar to the detector LSF, the integration time LSF is modeled as a rectangular function by using the sampling frequency and the integration pulse duration time. The modeling LSF is compared with the measured LSF from a prelaunch ground calibration device, the Integrated Alignment Collimator (IAC). Because the IAC test slit has a width of 0.1 MODIS IFOV, an extra step pulse convolution is added to the final LSF model. The comparison results show an excellent agreement between the modeled and measured LSFs in the spatial domain and MTFs in the frequency domain for selected reflective solar bands (RSB).

The Spectro-Radiometric Calibration Assembly (SRCA) is one of the on-board calibrators for the MODIS instrument. The SRCA is operated in three modes: spectral, spatial, and radiometric. The spatial mode is used to track the changes in band-to-band registration both along-scan (band and detector) and along-track (band) and the MTF in the along-scan direction for all 36 MODIS bands over the MODIS lifetime. In the SRCA spatial mode, a rectangular knife-edge reticle, located at the focus of the SRCA collimator, is imaged onto four MODIS Focal Plane Assemblies (FPA). The reticle is illuminated by a spherical integration sphere and a glow-bar so that all bands can have an appropriate signal level. When the MODIS scan mirror rotates, the illuminated knife-edge scans across the bands/detectors. In addition, there are five electronic phase-delays so that the sampling spacing is reduced to 1/5 of the detector size, which results in dense data points. After combining detector responses from all phase-delays, a combined bell-shaped response profile is formed. The derivative of the detector response to the knife-edge is the Line Spread Function (LSF). In the frequency domain, the Modulation Transfer Functions (MTF) are calculated from the normalized Fourier transform of the LSF. The MTF results from the SRCA are validated by the pre-launch results from the Integrated Alignment Collimator (IAC) and a SRCA collection performed in the Thermal Vacuum (TV). The six-year plus on-orbit MTF trending results show very stable responses in the VIS and NIR FPAs, and meet the design specifications. Although there are noticeable MTF degradations over the instrument lifetime in bands 1 and 2, they are negligible with the large specification margins. In addition, a similar relationship is found between the band locations in the VIS and NIR FPAs versus MTF values.

Since their respective launches in December 1999 and May 2002, NASA's EOS Terra and Aqua MODIS have successfully operated on-orbit for more than 8.5 and 6 years. MODIS collects data using a two-sided scan mirror over a large scan angle range. It has 36 spectral bands with wavelengths ranging from visible (VIS) to long-wave infrared (LWIR). The scan mirror is made of a polished, nickel-plated beryllium base coated with high purity silver, which is then over-coated with the Denton proprietary silicon monoxide and silicon dioxide mixture. The scan mirror's reflectance was characterized pre-launch using its witness samples, and the response versus scan angle (RVS) was measured at the sensor system level. In this study, we present an assessment of the MODIS scan mirror's on-orbit degradation by examining changes in spectral band responses over each sensor's mission lifetime. On-orbit results show that the scan mirror's optical properties for both Terra and Aqua MODIS have experienced significant degradation in the VIS spectral region. The degradation is mirror side dependent as well as scan angle dependent. For MODIS spectral bands at longer wavelengths, the degradation is relatively small. For Terra MODIS, the VIS spectral band degradation is noticeably different between the mirror sides, with bands 8 and 9 showing the largest differences, up to 10%. In contrast, the mirror side differences in Aqua MODIS are much smaller for all spectral bands. Also illustrated in this paper are examples of the mirror side differences, before and after on-orbit correction.

MODIS has 36 spectral bands with wavelengths in the visible (VIS), near-infrared (NIR), short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR). It makes observations at three nadir spatial resolutions: 0.25km for bands 1-2 (40 detectors per band), 0.5km for bands 3-7 (20 detectors per band), and 1km for bands 8-36 (10 detectors per band). The VIS, NIR, and SWIR are the reflective solar bands (RSB), which are calibrated on-orbit by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM). The bi-directional reflectance factor (BRF) of the SD provides a RSB calibration reference and its on-orbit changes are tracked by the SDSM. In addition, MODIS lunar observations are regularly scheduled and used to track the RSB calibration stability. On-orbit observations show that the changes in detector response are wavelength and scan angle dependent. In this study, we focus on detector-to-detector calibration differences in the MODIS VIS/NIR spectral bands, which are determined using SD and lunar observations, while the calibration performance is evaluated using the Earth view (EV) level 1B (L1B) data products. For Aqua MODIS, the detector calibration differences and their impact are also characterized using standard ocean color data products. The current calibration approach for MODIS RSB carries a band-averaged response versus scan angle (RVS) correction. The results from this study suggest that a detector-based RVS correction should, due to changes in the scan mirror's optical properties, be implemented in order to maintain and improve the current RSB L1B data product quality, particularly, for several VIS bands in Terra MODIS.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Earth Observing System (EOS) Aqua platform has 9 spectral bands with center wavelengths from 412nm to 870nm that are used to produce the standard ocean color data products. Ocean color products require a stability of the radiometric calibration on the order of 0.2%, which surpasses the official requirements for the MODIS reflective solar bands. The primary calibration source for the MODIS reflective solar bands is the on-board solar diffuser. For the ocean color bands, the SD calibration is performed with an attenuation screen to prevent saturation. The ocean color products are calculated using supplemental sun beta angle corrections (with a magnitude of about 0.5%) for the MODIS Aqua solar diffuser measurements in the ocean color bands. The initial corrections were derived using a three-year time series of solar diffuser measurements. This paper presents an update to these corrections for Aqua using a six-year time series, and describes the effect of these new corrections on the resulting calibration coefficients. The corrections are also described for the MODIS on Terra. The magnitude of the corrections for Terra is significantly less than for Aqua, and the sign of the response to the beta angle in Terra is opposite to that of Aqua.

MODerate resolution Imaging Spectroradiometer (MODIS), operated on both Terra and Aqua spacecrafts, measures the Earth scenes with 36 spectral bands allocated into four Focal Plane Assemblies (FPAs). Mis-registration between the spectral bands and FPAs was observed, which will lessen the data quality and reduce the accuracy of science products generated with multiple spectral bands located on different FPAs. An approach using ground targets, developed and validated in our previous work, is an alternative way for characterizing the MODIS Band-to-Band Registration (BBR), by calculating the centroid location difference of same dark targets observed by each band in its own field of view. The long term time series of spatial shift, not only for the band but also for the detector, are presented over sensor's operation years (year 2000-2007 for Terra MODIS and year 2002-2007 for Aqua MODIS). With this ground target approach, the spatial performances of both Terra and Aqua MODIS are evaluated. The results show that spatial shifts are small except they are relatively large between bands on the warm FPA and cold FPA of Aqua MODIS. The discrepancy between detectors is quite small and mainly attribute to the systematic error of the approach. Moreover, the long term results reveal an annual variation for some high resolution bands.

MODIS is a key instrument on-board both EOS Terra and Aqua satellites launched in December 1999 and May 2002, respectively. It has 16 thermal emissive bands (TEB) with a wavelength range from 3.5 to 14.4μm and 10 detectors in each band. The TEB detectors are located on cold focal plane assembles nominally controlled at 83K. TEB on-orbit calibration is performed for each band, detector and mirror side on a scan-by-scan basis with the dominant gain response determined through observations of a temperature controlled on-board blackbody (BB). The noise equivalent temperature difference (NEdT) is a key parameter to characterize the calibration uncertainty. Values of the NEdT are normally monitored at a fixed BB temperature of 285K for Aqua and 290K for Terra. Variations of NEdT with BB temperature are determined using the BB warmup/cooldown cycle scheduled every three months for a BB temperature range between 270 and 315K. In this study, we extend TEB detector NEdT characterization to temperatures below the BB temperature range. This approach uses detector responses to the interior of the instrument cavity, which varies in temperature between 210 and 270K depending on the viewing portion of the cavity inner surface. This study examines the stability of the cavity inner surface responses. Comparison of the NEdT obtained from cavity and BB measurements shows good agreement at the same temperature of 270K. Finally, the results of both Terra and Aqua NEdT determined from cavity measurements are presented.

Stars are regularly observed in the visible channels of the GOES Imagers for real-time navigation operations. However, we have been also using star observations off-line to deduce the rate of degradation of the responsivity of the visible channels. We estimate degradation rates from the time series of the intensities of the Imagers' output signals when viewing stars, available in the GOES Orbit and Attitude Tracking System (OATS). We begin by showing our latest results in monitoring the responsivities of the visible channels of the Imagers on GOES-8, -9, -10, -11 and -12. Unfortunately, the OATS computes the intensities of the star signals with approximations suitable for navigation, not for estimating accurate signal strengths, and thus we had to develop objective criteria for screening out unsuitable data. With several layers of screening, our most recent trending method yields smoother time series of star signals, but the time series are populated by a smaller pool of stars. With the goal of simplifying the task of data selection and to retrieve stars that have been rejected in the screening, we tested a technique that accessed the raw star measurements before they were processed by the OATS. We developed formulations that not only produced star signals more suitable for monitoring the changes in the Imager's outputs from views of constant-irradiance stellar sources, but also gave more information on the radiometric characteristics of the visible channels. We present specifics of this technique together with sample results. We discuss improvements in the quality of the time series that allow for more reliable inferences on the gradually changing responsivities of the visible channels. We describe further contributions of this method to monitoring of other performance characteristics of the visible channel of an Imager.

Desert-based vicarious calibration plays an important role in generating long-term reliable satellite radiances for the visible and near-infrared channels of Advanced Very High Resolution Radiometer (AVHRR). Lacking an onboard calibration device, the AVHRR relies on reflected radiances from a target site, e.g., a large desert, to calibrate its solar reflective channels. While the radiometric characteristics of the desert may be assumed stable, the reflected radiances from the target can occasionally be affected by the presence of clouds, sand-storms, vegetation, and wet surface. These contaminated pixels must be properly identified and removed to ensure calibration performance. This paper describes an algorithm to remove the contaminated pixels from AVHRR measurements taken over the Libyan Desert based on the characteristics of consistent Normalized Difference Vegetation Index (NDVI) land-cover stratification. We first apply a NDVI histogram-determined threshold to screen pixels contaminated with vegetation in each individual AVHRR observation. Our analyses show that the vegetation growth inside the desert target has negligibly small impact on the AVHRR operational calibration results. Two criteria based on the maximum NDVI compositing technique are then employed to remove pixels contaminated with clouds, severe sand-storms or wet sand surface. Compared to other cloud screening methods, this algorithm is capable of not only identifying high reflectance clouds, but also removing low reflectance of wet surface and nearly indifferent reflectance of severe dust storms. The use of clear pixels appears to improve AVHRR calibration accuracy in the first three-four years after satellite launch.

The Remote Sensing Group at the University of Arizona has developed an automated methodology and instrument suite to measure the surface reflectance of the vicarious calibration test site at Railroad Valley, Nevada. Surface reflectance is a critical variable used as one of the inputs into a radiative transfer code to predict the top-of-atmosphere radiance, and inexpensive and robust ground-viewing radiometers have been present at the site since 2004. The goal of the automated approach is to retain RSG's current 2-3% level of uncertainty while increasing the number of data sets collected throughout the year without the need for on-site personnel. A previous study was completed to determine if the number and positions of the four radiometers were adequate to spatially sample the 1-km² large-footprint site at Railroad Valley. The preliminary study utilized one set of panchromatic data from Digital Globe's QuickBird satellite. Results from this one day showed that the positions of the four ground-viewing radiometers adequately sample the site. The work presented here expands in a spectral and temporal sense by using high-spatial-resolution data from Ikonos, QuickBird, and Landsat-7 ETM+ to determine if the locations of the ground-viewing radiometers correctly sample the site. The multispectral capability of these sensors is used to establish if there are any spectral effects, which will also help RSG to determine what spectral bands should be chosen for the new ground-viewing radiometers that are currently in development for the automated test site at Railroad Valley.

The METOP-A satellite Infrared Atmospheric Sounding Interferometer (IASI) Level 2 products comprise retrievals of vertical profiles of temperature and water vapor. The L2 data were validated through assessment of their error covariances and biases using radiosonde data for the reference. The radiosonde data set includes dedicated launches as well as the ones performed at regular synoptic times at Lindenberg station (Germany). For optimal error estimate the linear statistical Validation Assessment Model (VAM) was used. The model establishes relation between the compared satellite and reference measurements based on their relations to the true atmospheric state. The VAM utilizes IASI averaging kernels and statistical characteristics of the ensembles of the reference data to allow for finite vertical resolution of the retrievals and spatial and temporal non-coincidence. For temperature retrievals expected and assessed errors are in good agreement; error variances/rms of a single FOV retrieval are 1K between 800 - 300 mb with an increase to ~1K in tropopause and ~2K at the surface, possibly due to wrong surface parameters and undetected clouds/haze. Bias against radiosondes oscillates within ±0 5K . between 950 - 100 mb. As for water vapor, its highly variable complex spatial structure does not allow assessment of retrieval errors with the same degree of accuracy as for temperature. Error variances/rms of a single FOV relative humidity retrieval are between 10 - 13% RH in the 800 - 300 mb range.

A new type of remote sensing instrument based upon the Fabry-Perot interferometric technique has been developed at NASA's Goddard Space Flight Center. Fabry-Perot interferometry (FPI) is a well known, powerful spectroscopic technique and one of its many applications is to be used to measure greenhouse gases and also some harmful species in the atmosphere. With this technique, absorption of particular species is measured and related to its concentration. A solid Fabry-Perot etalon is used as a frequency filter to restrict the measurement to particular absorption bands of the gas of interest. With adjusting the thickness of the etalon that separation (in frequency) of the transmitted fringes can be made equal to the almost constant separation of the gas absorption lines. By adjusting the temperature of the etalon, which changes the index of refraction of its material, the transmission fringes can be brought into nearly exact correspondence with absorption lines of the particular species. With this alignment between absorption lines and fringes, changes in the amount of a species in the atmosphere strongly affect the amount of light transmitted by the etalon and can be related to gas concentration. The instrument that we have developed detects the absorption of various atmospheric trace gases in direct or reflected sunlight. It can be used as ground based, airborne and satellite sensor for gases such as carbon dioxide (1570 nm), oxygen (762 nm and 768 nm lines sensitive to changes in oxygen pressure and oxygen temperature) and water vapor (940 nm). Our current goal is to develop an ultra precise, inexpensive, ground based device suitable for wide deployment as a validation instrument for the Orbiting Carbon Observatory (OCO) satellite. We show measurements for CO₂ and, O₂, , compare our measurements to those obtained using other types of sensors and discuss some of the peculiarities that must be addressed in order to provide the very high quality column detection required for solving problems about global distribution of greenhouse gases and climatological models. The recent long term experimental data on CO₂ and O₂ detection in atmosphere using Fabry-Perot technique are presented and discussed.

The Atmospheric Infrared Sounder (AIRS), launched on the EOS Aqua spacecraft on May 4, 2002, is a grating spectrometer with 2378 channels in the range 3.7 to 15.4 microns. Spectra from grating spectrometers are capable of unsurpassed absolute radiometric accuracy, which makes them excellent potential sources for climate data records used in climate trending analyses. However, because each channel has its own detector and (partially) its own electronics, some individual channels can suffer from higher noise than other channels and, in extreme cases, can fail completely. Radiometric quality non-uniformity can complicate error and noise estimation for some products. In particular, crosscalibration with other instruments, frequency interpolation, and frequency shifting are made more difficult. AIRS level 1B data are calibrated spectral radiances from each channel. This paper describes results of creating level 1C spectra from level 1B data, where radiances from channels determined to be very noisy are replaced with values determined by taking advantage of channel-to-channel correlations. Several methods are evaluated and validated.

The scientific objectives of the CLimate Absolute Radiance and REfractivity Observatory (CLARREO) new start recommended by the National Research Council Decadal Survey prioritize high accuracy measurements of infrared spectra, tested for systematic error, tied to international measurements standards, and suitable for testing long-term climate forecasts (of 10 years or more). We present the results from a realistic proof-of-concept study for this mission concept and examine the prospects of testing and improving long-term climate forecasts from ensembles of coupled General Circulation Models (GCMs) such as those participating in the Intergovernmental Panel on Climate Change 4th Assessment Report (IPCC-4AR).

NASA's anticipated plan for a mission dedicated to climate (CLARREO) will hinge upon the ability to fly absolute standards that can provide the basis to meet stringent requirements on measurement accuracy For example, instrumentation designed to measure spectrally resolved infrared radiances will require high-emissivity calibration blackbodies having absolute temperature uncertainties of better than 0.045 K (3 sigma). A novel scheme to provide absolute calibration of temperature sensors, suitable for CLARREO on-orbit operation, has been demonstrated in the laboratory at the University of Wisconsin. The scheme uses the transient temperature signature obtained during the phase change of different reference materials, imbedded in the same thermally conductive medium as the temperature sensors - in this case the aluminum blackbody cavity. Three or more reference materials can be used to assign an absolute scale to the thermistor sensors over a large temperature range. Using very small quantities of phase change material (<1/250^th the mass of the cavity), melt temperature accuracies of better than 10 mK have been demonstrated for Hg, H₂O, and Ga, providing calibration from 233K to 303K. The flight implementation of this new scheme will involve special considerations for packaging the phase change materials to ensure long-term compatibility with the containment system, and design features that help ensure that the on-orbit melt behavior in a microgravity environment is unchanged from pre-flight full gravitational conditions under which the system is characterized.

We present a method to characterize the emissivity of a spaceborne blackbody and the instrument line-shape (ILS) of a spectrometer using a quantum cascade laser (QCL) based reflectometer. QCLs allow the realization of on-orbit reflectometry that directly observes blackbody surface properties. We present experimental data verifying that the QCL reflected radiance signal can be measured by an Earth-observing spectrometer. The QCL can also be used to realize a monochromatic, spatially uniform source of infrared radiation to measure the spectrometer's ILS, which can be inverted to obtain diagnostic information about the integrity of the detector and nonlinearities in the detector signal-chain.

A new call for Core Earth Explorer Ideas was released by the European Space Agency in March 2005. The Call focused on the global carbon and water cycles, atmospheric chemistry and climate, as well as the human element as a cross cutting issue. The proposals were peer reviewed by scientific panels, and also appraised technically and programmatically by ESA. This paper describes the Earth Explorer cycle and gives an overview of the six candidate missions selected for assessment studies.

The Multi-angle Imaging SpectroRadiometer (MISR) has been acquiring global cloud and aerosol data from polar orbit since February 2000. MISR acquires moderately high-resolution imagery at nine view angles from nadir to 70.5°, in four visible/near-infrared spectral bands. Stereoscopic parallax, time lapse among the nine views, and the variation of radiance with angle and wavelength enable retrieval of geometric cloud and aerosol plume heights, height-resolved cloud-tracked winds, and aerosol optical depth and particle property information. Two instrument concepts based upon MISR heritage are in development. The Cloud Motion Vector Camera, or WindCam, is a simplified version comprised of a lightweight, compact, wide-angle camera to acquire multiangle stereo imagery at a single visible wavelength. A constellation of three WindCam instruments in polar Earth orbit would obtain height-resolved cloud-motion winds with daily global coverage, making it a low-cost complement to a spaceborne lidar wind measurement system. The Multiangle SpectroPolarimetric Imager (MSPI) is aimed at aerosol and cloud microphysical properties, and is a candidate for the National Research Council Decadal Survey's Aerosol-Cloud-Ecosystem (ACE) mission. MSPI combines the capabilities of MISR with those of other aerosol sensors, extending the spectral coverage to the ultraviolet and shortwave infrared and incorporating high-accuracy polarimetric imaging. Based on requirements for the nonimaging Aerosol Polarimeter Sensor on NASA's Glory mission, a degree of linear polarization uncertainty of 0.5% is specified within a subset of the MSPI bands. We are developing a polarization imaging approach using photoelastic modulators (PEMs) to accomplish this objective.

Technology changes in detector development and the significant improvement of manufacturing accuracy in combination with the permanent engineering research influences the spaceborne sensor systems, which are focused on Earth observation and remote sensing. Developments in focal plane technology, e.g. the combination of large TDI lines, intelligent synchronisation control, fast readable sensors and new focal plane and telescope concepts are the key developments for new remote sensing instruments. This class of instruments disposes of high spatial and radiometric resolution for the generation of data products for mapping and 3D GIS VR applications. Systemic approaches are essential for the design of complex sensor systems based on dedicated tasks. The system-theoretical description of the instrument inside and a simulated environment is the basic approach for the optimisation process of the optical, mechanical and electrical designs and assembly. Single modules and the entire system have to be calibrated and verified. The traceability of the performance-related parameters from the single sensor up to the flight readiness of the instrument forms the main focus inside such complex systems. In the future it will also be possible to prove the sensor performance on wafer level before assembly. This paper gives an overview about current technologies for performance measurements on sensor, focal plane assembly (FPA) and instrument level without the optical performance of the telescope. The paper proposes also a technology, which can be used for sensor performance measurements on wafer level.

A concept has been developed for a satellite sensor system capable of global observation of the location, extent and brightness of night-time lights at a spatial resolution suitable for the delineation of primary features within human settlements. Nightsat should be capable of producing a complete cloud-free global map of lights on an annual basis. We have used a combination field spectra of outdoor lighting, moderate resolution color photography of cities at night from the International Space Station, and high-resolution airborne camera imagery acquired at night to define a range of spatial, spectral, and detection limit options for a future Nightsat mission. Primary findings of our study are that Nightsat should collect data from a near-synchronous orbit in the mid-evening with 50 to 100 m spatial resolution, detection limits in the range of 10^-8 watts/cm²/sr/um, and a capacity for in-flight radiometric calibration. Although panchromatic low-light imaging data would be useful, multispectral low-light imaging data would provide valuable information on the type or character of lighting; potentially stronger predictors of variables, such as ambient population density and economic activity. The Nightsat mission concept is unique in its focus on observing a human activity, in contrast to traditional Earth observing systems that focus on natural systems.

With the increased emphasis on monitoring the Earth's climate from space, more stringent calibration requirements are being placed on the data products from remote sensing satellite instruments. Among these are stability over decade-length time scales and consistency across sensors and platforms. For radiometer instruments in the solar reflectance wavelength range (visible to shortwave infrared), maintaining calibration on orbit is difficult due to the lack of absolute radiometric standards suitable for fight use. The Moon presents a luminous source that can be viewed by all instruments in Earth orbit. Considered as a solar diffuser, the lunar surface is exceedingly stable. The chief diffculty with using the Moon is the strong variations in the Moon's brightness with illumination and viewing geometry. This mandates the use of a photometric model to compare lunar observations, either over time by the same instrument or between instruments. The U.S. Geological Survey in Flagstaff, Arizona, under NASA sponsorship, has developed a model for the lunar spectral irradiance that explicitly accounts for the effects of phase, the lunar librations, and the lunar surface reflectance properties. The model predicts variations in the Moon's brightness with precision ~1% over a continuous phase range from eclipse to the quarter lunar phases. Given a time series of Moon observations taken by an instrument, the geometric prediction capability of the lunar irradiance model enables sensor calibration stability with sub-percent per year precision. Cross-calibration of instruments with similar passbands can be achieved with precision comparable to the model precision. Although the Moon observations used for intercomparison can be widely separated in phase angle and/or time, SeaWiFS and MODIS have acquired lunar views closely spaced in time. These data provide an example to assess inter-calibration biases between these two instruments.

The Moon plays an important role in the radiometric stability monitoring of the NASA Earth Observing System's (EOS) remote sensors. The MODIS and SeaWIFS are two of the key instruments for NASA's EOS missions. The MODIS Protoflight Model (PFM) on-board the Terra spacecraft and the MODIS Flight Model 1 (FM1) on-board the Aqua spacecraft were launched on December 18, 1999 and May 4, 2002, respectively. They view the Moon through the Space View (SV) port approximately once a month to monitor the long-term radiometric stability of their Reflective Solar Bands (RSB). SeaWIFS was launched on-board the OrbView-2 spacecraft on August 1, 1997. The SeaWiFS lunar calibrations are obtained once a month at a nominal phase angle of 7°. The lunar irradiance observed by these instruments depends on the viewing geometry. The USGS photometric model of the Moon (the ROLO model) has been developed to provide the geometric corrections for the lunar observations. For MODIS, the lunar view responses with corrections for the viewing geometry are used to track the gain change for its reflective solar bands (RSB). They trend the system response degradation at the Angle Of Incidence (AOI) of sensor's SV port. With both the lunar observation and the on-board Solar Diffuser (SD) calibration, it is shown that the MODIS system response degradation is wavelength, mirror side, and AOI dependent. Time-dependent Response Versus Scan angle (RVS) Look-Up Tables (LUT) are applied in MODIS RSB calibration and lunar observations play a key role in RVS derivation. The corrections provided by the RVS in the Terra and Aqua MODIS data from the 412 nm band are as large as 16% and 13%, respectively. For SeaWIFS lunar calibrations, the spacecraft is pitched across the Moon so that the instrument views the Moon near nadir through the same optical path as it views the Earth. The SeaWiFS system gain changes for its eight bands are calibrated using the geometrically-corrected lunar observations. The radiometric corrections to the SeaWiFS data, after more than ten years on orbit, are 19% at 865 nm, 8% at 765 nm, and 1-3% in the other bands. In this report, the lunar calibration algorithms are reviewed and the RSB gain changes observed by the lunar observations are shown for all three sensors. The lunar observations for the three instruments are compared using the USGS photometric model. The USGS lunar model facilitates the cross calibration of instruments with different spectra bandpasses whose measurements of the Moon differ in time and observing geometry.

A methodology for long-term radiometric cross-calibration between the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) sensors was developed. The approach involves calibration of near-simultaneous surface observations between 2000 and 2007. Fifty-seven cloudfree image pairs were carefully selected over the Libyan desert for this study. The Libyan desert site (+28.55°, +23.39°), located in northern Africa, is a high reflectance site with high spatial, spectral, and temporal uniformity. Because the test site covers about 12 kmx13 km, accurate geometric preprocessing is required to match the footprint size between the two sensors to avoid uncertainties due to residual image misregistration. MODIS Level 1B radiometrically corrected products were reprojected to the corresponding ETM+ image's Universal Transverse Mercator (UTM) grid projection. The 30 m pixels from the ETM+ images were aggregated to match the MODIS spatial resolution (250 m in Bands 1 and 2, or 500 m in Bands 3 to 7). The image data from both sensors were converted to absolute units of at-sensor radiance and top-of atmosphere (TOA) reflectance for the spectrally matching band pairs. For each band pair, a set of fitted coefficients (slope and offset) is provided to quantify the relationships between the testing sensors. This work focuses on long-term stability and correlation of the Terra MODIS and L7 ETM+ sensors using absolute calibration results over the entire mission of the two sensors. Possible uncertainties are also discussed such as spectral differences in matching band pairs, solar zenith angle change during a collection, and differences in solar irradiance models.

The NASA Ocean Biology Processing Group's Calibration and Validation Team uses SeaWiFS on-orbit lunar calibrations to monitor the radiometric response of the instrument over time. With almost eleven years of lunar measurements (more than 124 monthly observations) available for this analysis, the Cal/Val Team has undertaken an investigation of the optimum function to use in fitting the time series and the fidelity of resulting radiometric corrections that are applied to the ocean data. Two aspects of the on-orbit behavior of SeaWiFS show changes over time: the long-term radiometric response for each band and the dependence of the individual detector response in each band on the varying focal plane temperatures. Since band 8 (865 nm) shows the greatest changes in response over time, the analysis has concentrated on that band. The initial goal of the SeaWiFS on-orbit calibration effort has been to use a single function to fit the mission-long lunar time series. To date, that goal has been met by using a pair of simultaneous decaying exponential functions with short-period and long-period time constants. As late mission observations were added to the time series (beyond seven years into the mission), the long-term radiometric trend has been approaching a linear function of time. Consequently, the long-term trend is starting to bias the fit for the first three years of the mission. The Cal/Val team has addressed this issue by introducing a radiometric epoch into the time series fitting functions, where the best fit for the early mission is provided by exponential functions with periods of 200 and 2500 day and the best fit for the late mission is provided by an exponential with a 400-day time constant and a linear function (or an exponential with a 40,000-day time constant). A complication in optimizing these fits is that the dependence of the detector response on varying focal plane temperatures began changing approximately seven years into the mission. Analyses of periodic residuals in the lunar calibration time series in the latter part of the mission show that either the temperature-dependence of the detector response or the overall thermal environment of the instrument is changing over time. The Cal/Val Team has used correlations between these residuals and the focal plane temperatures to evaluate revisions to the temperature corrections for the detector response. Complications in computing these revised temperature corrections are that the behavior of the temperature corrections is not readily described by an analytical function and that the long-term radiometric fits compensate, to an extent, for changes in the temperature corrections. In order to develop an improved calibration model for SeaWiFS, the Cal/Val Team has developed a methodology for simultaneously fitting the long-term radiometric trend of each band and the change in the temperature-dependence of the individual detector responses. This work shows the increased fidelity of the calibration derived simultaneously for the long-term radiometric trend and the focal plane temperature response compared to the sequential derivations of these corrections.

MODIS is one of the major instruments for the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) missions. It is on-board both the EOS Terra and Aqua spacecrafts, launched in December 1999 and May 2002, respectively. Each MODIS provides spectral observations in multiple angular views of reflectance at the top of atmosphere (TOA) around the globe. This study focuses on one visible (0.65μm) band and one near-IR (0.84μm) band to examine the variations of observed TOA reflectances due to the impact of the bi-directional reflectance distribution function (BRDF). Two highly uniform ground sites in the Libyan Desert and Antarctica are selected. The variation of reflectance as a function of view zenith angle at a fixed solar zenith angle is studied based on reflectances obtained from multiple granules over passing each site. The variation of reflectance as a function of solar zenith angle at a fixed view zenith angle is studied based on reflectances collected from 16-day repeatable orbits, which have the same view geometry relative to each site. Results show that variations of the near nadir reflectances at the desert site are close to the Lambertian pattern while those at the Dome C. site are strongly anisotropic. Comparison of a simple Lambertian and a BRDF correction is made in terms of their effectiveness in removing reflectance trending variations. The BRDF correction is based on a semi-empirical model consisting of two kernel-driven components. Results show that both corrections are able to significantly remove trending variations at the desert site, which produce a remarkably low variability of less than 2% relative to the linear fit. The normalized reflectance trends show that from year 2000 to 2007, the visible and near-IR bands dropped only 1.7 and 1.2% in total, respectively.

From the Landsat program's inception in 1972 to the present, the earth science user community has benefited from a historical record of remotely sensed data. The multispectral data from the Landsat 5 (L5) Thematic Mapper (TM) sensor provide the backbone for this extensive archive. Historically, the radiometric calibration procedure for this imagery used the instrument's response to the Internal Calibrator (IC) on a scene-by-scene basis to determine the gain and offset for each detector. The IC system degraded with time causing radiometric calibration errors up to 20 percent. In May 2003 the National Landsat Archive Production System (NLAPS) was updated to use a gain model rather than the scene acquisition specific IC gains to calibrate TM data processed in the United States. Further modification of the gain model was performed in 2007. L5 TM data that were processed using IC prior to the calibration update do not benefit from the recent calibration revisions. A procedure has been developed to give users the ability to recalibrate their existing Level-1 products. The best recalibration results are obtained if the work order report that was originally included in the standard data product delivery is available. However, many users may not have the original work order report. In such cases, the IC gain look-up table that was generated using the radiometric gain trends recorded in the NLAPS database can be used for recalibration. This paper discusses the procedure to recalibrate L5 TM data when the work order report originally used in processing is not available. A companion paper discusses the generation of the NLAPS IC gain and bias look-up tables required to perform the recalibration.

The National Landsat Archive Production System (NLAPS) has been the primary processing system for Landsat data since U.S. Geological Survey (USGS) Earth Resources Observation and Science Center (EROS) started archiving Landsat data. NLAPS converts raw satellite data into radiometrically and geometrically calibrated products. NLAPS has historically used the Internal Calibrator (IC) to calibrate the reflective bands of the Landsat-5 Thematic Mapper (TM), even though the lamps in the IC were less stable than the TM detectors, as evidenced by vicarious calibration results. In 2003, a major effort was made to model the actual TM gain change and to update NLAPS to use this model rather than the unstable IC data for radiometric calibration. The model coefficients were revised in 2007 to reflect greater understanding of the changes in the TM responsivity. While the calibration updates are important to users with recently processed data, the processing system no longer calculates the original IC gain or offset. For specific applications, it is useful to have a record of the gain and offset actually applied to the older data. Thus, the NLAPS calibration database was used to generate estimated daily values for the radiometric gain and offset that might have been applied to TM data. This paper discusses the need for and generation of the NLAPS IC gain and offset tables. A companion paper covers the application of and errors associated with using these tables.

The WorldView-1 high spatial resolution commercial imaging satellite was launched on September 18, 2007. WorldVew-1 contains a single panchromatic band with a spectral range of 400-900 nm capable of collecting half-meter pixels on the ground with a swath width of 17.6 km. The instrument is a pushbroom scanner with a focal plane of 36400 detectors, six time-delayed integration (TDI) exposure rates, and bi-directional scanning. All of the detectors must be calibrated for each of the twelve TDI/scan direction combinations. A pre-launch radiometric calibration was performed on the instrument using a full-aperture integrating sphere and NIST-traceable transfer radiometers. The test data are used to determine the absolute radiometric calibration of the instrument in units of radiance per count and also generate the relative pixel-to-pixel gain and offset values for the non-uniformity correction. The relative gain and offset values are updated on orbit using uniform desert targets. Image quality metrics for banding and streaking are applied to the uniform scenes comparing calibration factors derived pre- and post-launch showing reduced values when using the updated calibration factors. Relative radiometric performance assessment of imagery collected in the first nine months on orbit shows that WorldView-1 has excellent radiometric image quality with banding better than 0.55% and streaking better than 0.23%. Long term trends show that banding and streaking are not changing significantly with time.

From October 1984 through August 2005, the NASA Earth Radiation Budget Satellite (ERBS)/Earth Radiation Budget Experiment (ERBE) nonscanning active cavity radiometers (ACR) were used to monitor long-term changes in the earth radiation budget components of the incoming total solar irradiance (TSI), earth-reflected TSI, and earth-emitted outgoing longwave radiation (OLR). From October 1984 through September 1999, using on-board calibration systems, the ERBS/ERBE ACR sensor response changes, in gains and offsets, were determined from on-orbit calibration sources and from direct observations of the incoming TSI through calibration solar ports at measurement precision levels approaching 0.5 Watts-per-squared-meter, at satellite altitudes. On October 6, 1999, the on-board radiometer calibration system elevation drive failed. Thereafter, special spacecraft maneuvers were performed to observe cold space and the sun in order to define the post-September 1999 geometry of the radiometer measurements, and to determine the October 1999-September 2003 ERBS sensor response changes. Analyses of these special solar and cold space observations indicate that the radiometers were pointing approximately 16 degrees away from the spacecraft nadir and on the antisolar side of the spacecraft after the elevation drive failure. The special observations indicated that the radiometers' responses were stable at precision levels approaching 0.5 Watts-per-squared-meter. In this paper, determinations of the measurement geometry [sensor pointing direction] and of the radiometers' gain and offset are presented. These determinations will permit the accurate processing of the October 1999 through August 2005 ERBE data products at satellite and top-of-the-atmosphere altitudes.

The Remote Sensing Group (RSG) at the University of Arizona has performed high-accuracy radiometric calibration in the laboratory for more than 20 years in support of vicarious calibration of space-borne and airborne imaging sensors. Typical laboratory calibration relies on lamp-based sources which, while convenient to operate and control, do not simulate the solar spectrum that is the basic energy source for many of the imaging systems. Using the sun as a source for preflight radiometric calibration reduces uncertainties caused by the spectral mismatch between the preflight and inflight calibration, especially in the case in which a solar diffuser is the inflight calibration method. Difficulties in using the sun include varying atmospheric conditions, changing solar angle during the day and with season, and ensuring traceability to national standards. This paper presents several approaches using the sun as a radiometric calibration source coupled with the expected traceable accuracies for each method. The methods include direct viewing of the solar disk with the sensor of interest, illumination of the sensor's inflight solar diffuser by the sun, and illumination of an external diffuser that is imaged by the sensor. The results of the error analysis show that it is feasible to achieve preflight calibration using the sun as a source at the same level of uncertainty as those of lamp-based approaches. The error analysis is evaluated and compared to solar-radiation-based calibrations of one of RSG's laboratory-grade radiometers.

A new heliostat facility at Ball Aerospace and Technologies Corporation (BATC) in Boulder, CO will allow the use of the sun as the source in the calibration of earth observing sensors. The solar spectrum is the basic energy source for such instruments; therefore it is advantageous to perform initial ground radiometric calibrations using the sun. Using this method for preflight radiometric calibration reduces uncertainties caused by the spectral mismatch between the preflight and in-flight calibration, especially in the case in which a solar diffuser is the in-flight calibration method. This method also reduces stray light concerns as the instrument diffuser is measured in situ with the same radiance level it sees on orbit. This paper presents the design of a heliostat test facility which tracks the sun and directs the solar beam into a thermal vacuum chamber, allowing the instrument under test to be kept in a safe, clean and controllable environment. Design considerations that affect the uniformity and transmission of the system are discussed. The opto-mechanical logistics of creating a heliostat that will deliver a 13-inch solar beam into a thermal vacuum chamber are also presented. This facility is currently under construction at BATC and is expected to be operational by the end of 2008.

The Clouds and the Earth's Radiant Energy System (CERES) is the only program currently measuring the global Earth Radiation Budget (ERB) from space. Two CERES units are located on the EOS Terra platform and two more are placed on the EOS Aqua satellite. Each of the four operational CERES instruments uses three broadband radiometric scanning telescopes: the shortwave (SW 0.3 - 5μ), total (0.3 - >100μ), and window (8 - 12μ) channels. Rigorous pre-launch ground calibration and in-flight calibration is performed on each CERES unit to achieve an accuracy goal of 1% for SW flux and 0.5% for outgoing LW flux.

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002. AIRS has demonstrated in-flight NIST traceability and high radiometric accuracy. This accuracy is achieved in orbit by transferring the calibration from a Large Area Blackbody (LABB) to the On-Board Calibrator (OBC) blackbody during preflight testing. The LABB theoretical emissivity is in excess of 0.9999 and temperature uncertainty is less than 30 mK. The LABB emitted radiance is NIST traceable through precision Platinum Resistance Thermometers (PRTs) located on the internal surfaces. The radiometric accuracy predictions for AIRS based on the OBC, LABB, and pre-flight measurements give an accuracy of 0.2K, 3 sigma. AIRS pre-flight calibration coefficients have not changed in flight, preserving the link between observations and pre-flight calibration and characterization. An update is being considered that will improve accuracy and preserve traceability.

In this paper a new technique of objects measurement based on multi-wavelength lidar system has been proposed and developed to make horizontal-path laser measurements of objects. The two or more wavelengths laser transmitter operates within and adjacent to the sensitive bands exhibited by the characteristics of each object, the result could be used to establish inversion models of the laser transmitting backscatter signals. The application value and the key techniques of the spectral lidar are analyzed. The lidar wavelength selection method is studied and a hyperspectral experiment had been down to testify the feasibility of the theory and the lidar detection is simulated. Also issues to approach the final goal of this new technique are discussed.

We are presenting new achievements in single photon counting altimeter simulator. The existing planetary altimeter simulator proposed for operational range 400 to 1400 km with one meter range has been extended by multiple target situation - simple clouds model and by some basic interaction with terrain profile map. The new design cooperating with map system and results of the photon counting laser altimeter simulator are presented. The simulator is designed to be a theoretical and numerical complement for a Laser Altimeter Technology Demonstrator of the space borne laser altimeter for planetary studies built on our university. The simulator is useful complement for any photon counting altimeter both for altimeter design and for measured data analysis.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Earth Observing System (EOS) Aqua platform has 9 spectral bands with center wavelengths from 412nm to 870nm that are used to produce the standard ocean color data products. Ocean scenes usually contain high contrast due to the presence of bright clouds over dark water. The MODIS has been characterized for straylight effects prelaunch. In this paper, we derive Point-Spread Functions for the MODIS Aqua ocean bands based on the prelaunch Near-Field Response measurements. We use Harvey-Shack coefficients derived by the system vendor Santa Barbara Remote Sensing. The crucial step in the derivation of the Point-Spread Function is the normalization of the Harvey-Shack coefficients relative to the center pixel. The straylight contamination of ocean scenes is evaluated based on artificial test scenes. Furthermore, the dependence of top-of-atmosphere radiances and ocean color products on proximity to a cloud is analyzed, and a straylight correction algorithm is proposed.

We present preliminary experimental results, sensitivity measurements and discuss our new CO₂ lidar system under development. The system is employing an erbium-doped fiber amplifier (EDFA), superluminescent light emitting diode (SLED) as a source and our previously developed Fabry-Perot interferometer subsystem as a detector part. Global measurement of carbon dioxide column with the aim of discovering and quantifying unknown sources and sinks has been a high priority for the last decade. The goal of Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) mission is to significantly enhance the understanding of the role of CO₂ in the global carbon cycle. The National Academy of Sciences recommended in its decadal survey that NASA put in orbit a CO₂ lidar to satisfy this long standing need. Existing passive sensors suffer from two shortcomings. Their measurement precision can be compromised by the path length uncertainties arising from scattering within the atmosphere. Also passive sensors using sunlight cannot observe the column at night. Both of these difficulties can be ameliorated by lidar techniques. Lidar systems present their own set of problems however. Temperature changes in the atmosphere alter the cross section for individual CO₂ absorption features while the different atmospheric pressures encountered passing through the atmosphere broaden the absorption lines. Currently proposed lidars require multiple lasers operating at multiple wavelengths simultaneously in order to untangle these effects. Our current goal is to develop an ultra precise, inexpensive new lidar system for precise column measurements of CO₂ changes in the lower atmosphere that uses a Fabry-Perot interferometer based system as the detector portion of the instrument and replaces the narrow band laser commonly used in lidars with the newly available high power SLED as the source. This approach reduces the number of individual lasers used in the system from three or more to one-considerably reducing the risk of failure. It also tremendously reduces the requirement for wavelength stability in the source putting this responsibility instead on the Fabry-Perot subsystem.