Carol J. Bruegge

Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview

The Multi-angle Imaging SpectroRadiometer (MISR) instrument is scheduled for launch aboard the first of the Earth Observing System (EOS) spacecraft, EOS-AM1. MISR will provide global, radiometrically calibrated, georectified, and spatially coregistered imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. Algorithms specifically developed to capitalize on this measurement strategy will be used to retrieve geophysical products for studies of clouds, aerosols, and surface radiation. This paper provides an overview of the as-built instrument characteristics and the application of MISR to remote sensing of the Earth.

A review of reflectance nomenclature used in remote sensing

Field and laboratory radiometers have long been used to characterize the multiangle reflectance properties of homogeneous surface targets. Within the last few years space‐based multiangle spectroradiometers have been deployed, including the Along‐Track Scanning Radiometer‐2 (ATSR), the Polarization and Directionality of the Earths Reflectances (POLDER) instrument, and the Multiangle Imaging SpectroRadiometer (MISR). Other scanning instruments, such as the Moderate Resolution Imaging Spectrometer (MODIS) make use of successive passes to build up multiangle observations. With global monitoring from space, as well as the continuation of traditional observations in the field, advances are being made in research and applications which use these data. There comes a need, with this growth, to standardize reflectance terminology. Success in this endeavor will ensure the integrity of the data products, and facilitate the intercomparison of results. This paper reviews the reflectance terminology that is currently in use by the remote sensing community, including bidirectional reflectance distribution function, bidirectional reflectance factor, hemispherical‐directional reflectance factor, directional‐hemispherical reflectance, hemispherical‐directional reflectance, and bihemispherical reflectance (albedo).

MISR aerosol optical depth retrievals over southern Africa during the SAFARI‐2000 Dry Season Campaign

This paper presents, for the first time, retrievals of aerosol optical depths from Multi-angle Imaging Spectro-Radiometer (MISR) observations over land. Application of the MISR operational algorithm to data taken over southern Africa during the SAFARI-2000 dry season campaign yields results that compare favorably with coincident surface-based measurements taken by the AERONET radiometer network.

MISR: A multiangle imaging spectroradiometer for geophysical and climatological research from Eos

The scientific objectives, instrument concept, and data plan for the multiangle imaging spectroradiometer (MISR), an experiment proposed for the Eos (Earth Observing System) mission, are described. MISR is a pushbroom imaging system designed to obtain continuous imagery of the sunlit Earth at four different view angles (25.8 degrees , 45.6 degrees , 60.0 degrees , and 72.5 degrees relative to the vertical at the Earths surface), in both the forward and aftward directions relative to nadir, using eight separate cameras. Observations will be acquired in four spectral bands, centered at 440, 550, 670, and 860 nm. Data analysis algorithms will be applied to MISR imagery to retrieve the optical, geometric, and radiative properties of complex, three-dimensional scenes, such as aerosol-laden atmospheres above a heterogeneously reflecting surface, nonstratified cloud systems, and vegetation canopies. The MISR investigation will address a number of scientific questions concerning the climatic and ecological consequences of many natural and anthropogenic processes, and will furnish the aerosol information necessary. >

Water vapor column abundance retrievals during FIFE

A variation of the modified Langley plot algorithm is reported here and applied to the retrieval of atmospheric water column abundance from a filtered sunphotometer. In this new methodology an absorption data base (LOWTRAN 7) is used to compute a water abundance versus transmittance curve of growth, rather than the square-root dependence previously assumed. Validation of the technique is provided from an uncertainty analysis, and plans to further validate using Fourier transform spectrometers are detailed. The new sunphotometer technique is used to report total column water vapor during the First ISLSCP Field Experiment (FIFE), and comparisons are made with abundances retrieved via FIFE radiosonde observations. The sunphotometer data can best be utilized, however, to in turn validate data from airborne or in-orbit measurements of water vapor. With these flight sensors, horizontal and topographic variability within the scene can be viewed. An example of the airborne data set is given using an image from the airborne visible infrared imaging spectrometer (AVIRIS), as acquired on August 31, 1990.

Early validation of the Multi-angle Imaging SpectroRadiometer (MISR) radiometric scale

The Multi-angle Imaging SpectroRadiometer (MISR) instrument consists of nine cameras, four spectral bands each, and an on-board calibrator (OBC). Experiments using the latter allow camera radiometric coefficients to be updated bimonthly. Data products are thus calibrated to a stable radiometric scale, even in the presence of instrument response changes. The camera, band, and pixel-relative calibrations are accurately determined using the OBC. Conversely, as the OBC itself is subject to response degradation, MISR also conducts annual field vicarious calibration campaigns. The first of these, conducted in June 2000 at a desert site in Nevada, has been used to establish the present absolute radiometric scale. Validation of this radiometric scale, using AirMISR, shows consistency to within 4%. Following these studies, however, it was determined that MISR radiometry is subject to scene-dependent effects due to ghosting that, for the Nevada test sites, reduces the apparent radiance by 3%. Correction for this effect is required in order to avoid radiometric errors over sites that do not exhibit the same background contrast. Additional studies are in progress, with plans to correct for scene-contrast effects in future Level 1B1 processing.

Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors

For the Multiangle Imaging SpectroRadiometer (MISR), currently under development for the Earth Observing System, we plan to use deployable diffuse reflectance panels to provide a flight calibration of its nine cameras. Near-lambertian reflectance characteristics are desirable to facilitate flat-field camera intercomparisons and to allow each camera to be calibrated under the same illumination levels. Panel spatial and spectral uniformity and stability with time are also required. Spectralon™, a commercially available diffuse reflectance material made from polytetrafluoroethylene (PTFE), has been baselined in the MISR design. To assess the suitability ofthis material, a series of environmental exposure tests were performed. No degradation of the optical properties was apparent following proton bombardment, and stability through UV illumination was satisfactory, provided simple cleaning and handling procedures were implemented. One surprise during the testing, however, was a buildup of several thousand volts of static charge, which developed while simulating a rare pass through an auroral storm. Such a potential for charge buildup is not unique to PTFE, but exists for many thermal control paints used to cover spacecraft exteriors. Further testing of the charged Spectralon failed to produce arcing to the metallic housing frame, and models indicate that charge neutralization will occur after passage through the storm. For these reasons we intend to fly Spectralon as per our original plan.

The Airborne Multi-angle Imaging SpectroRadiometer (AirMISR): instrument description and first results

An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assist in validation of the Earth Observing System (EOS) MISR experiment. Unlike the EOS MISR, which contains nine individual cameras pointed at discrete look angles, AirMISR utilizes a single camera in a pivoting gimbal mount. The AirMISR camera has been fabricated from MISR brassboard and engineering model components and, thus, has similar radiometric and spectral response as the MISR cameras. This paper provides a description of the AirMISR instrument and summarizes the results of engineering flights conducted during 1997.

Improving MODIS surface BRDF/Albedo retrieval with MISR multiangle observations

We explore a synergistic approach to use the complementary angular samplings from the Multi-angle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) to improve MODIS surface bidirectional reflectance distribution function (BRDF) and albedo retrieval. Preliminary case studies show that MODIS and MISR surface bidirectional reflectance factors (BRFs) are generally comparable in the green, red, and near infrared. An information index is introduced to characterize the information content of directional samplings, and it is found that MISR angular observations can bring additional information to the MODIS retrieval, especially when the MISR observations are close to the principal plane. We use the BRDF parameters derived from the MISR surface BRFs as a priori information and derive a posteriori estimates of surface BRDF parameters with the MODIS observations. Results show that adding MISR angular samplings can reduce the relative BRF prediction error by up to 10% in the red and green, compared to the retrievals from MODIS-only observations which are close to the cross-principal plane.

Ultraviolet stability and contamination analysis of Spectralon diffuse reflectance material

A detailed chemical analysis was carried out on Spectralon, a highly Lambertian, diffuse reflectance material. Results of this investigation unambiguously identified the presence of an organic (hydrocarbon) impurity intrinsic to the commercial material. This impurity could be removed by a vacuum bake-out procedure and was identified as the cause of optical changes (degradation) that occur in the material when exposed to UV light. It was found that when this impurity was removed, the Spectralon material was photochemically stable and maintained its reflectance properties even after extensive solar UV exposure.