The European Space Agency has launched with greta success a series of geostationary meteorological satellites and of remote sensing polar orbiters. The plans include the continuation of those missions by flying or preparing enhanced and innovative instruments on ENVISAT, MSG and METOP platforms, which will be followed by the Earth Watch and Earth Explorers missions as from the next decade.

The Ozone Monitoring Instrument-Imaging Spectrometer (OMI and ImS) is a trace gas monitoring instrument in the line of GOME. OMI provides nadir viewing UV-visible spectroscopy with a resolution sufficient to separate out the various absorbing trace gases present in a wavelength range of 270 to 780 nm. This includes O₃, SO₂, BrO, OCIO, HCHO, NO₂, NO₃ and water vapor. O₂ is also measured and is used to derive cloud top heights and cloud cover. The ImS is different as compared to the GOME in the sense that a 2D CCD is used as detector instead of a 1D diode array. The first dimension of the CCD is used to detect spectral information and the second dimension is used to image the swath perpendicularly to the track of the satellite. The concept allows to have a wide swath together with good horizontal resolution, has a polarization scrambler and room temperature detectors. A special add-on channel measures, at reduced resolution, polarization in three directions for the full spectral range. This channel is meant to enhance aerosol and PSC data products. The ImS is intended for flight on the ESA/Eumetsat METOP satellites.

One of the instruments to be launched on ENVISAT around the turn of the century is the Scanning Imaging Absorption spectroMeter for Atmospheric Cartography (SCIAMACHY). This eight-channel spectrometer covers the wavelength range of 240 nm to 2385 nm and will allow the first space-borne simultaneous atmospheric measurements in the ultraviolet, visible and near-infrared. Furthermore, since the instrument is capable of measuring spectra of the sun light scattered/transmitted by the Earth and its atmosphere is nadir, limb and sun-occulation mode, it will allow obtaining 3D images of atmospheric trace gases and aerosol, with global coverage in three days. These data will greatly enhance insight in the dynamics and the long-term behavior of the constituents of the Earth atmosphere. The instrument is designed and built as a joint Dutch/German project, funded and supervised by the respective National Space Agencies. The Dutch partners, TNO-TPD, Fokker Space and SRON, are responsible for the Optical and Radiant Cooler Assemblies, the German main contractor Dornier for the Electronic Assembly. At present SCIAMACHY is well in its C/D phase: the structural models have already demonstrated their capability to withstand the launch vibrations and loads, the electronic, the optical and the radiant cooler assembly are all nearing their completion, calibration facilities have been built or updated, and the verification of their requirements has been planned in detail. This contribution will give an overview of the current status of the instrument and point out some of the challenges one had to face. In particular we will focus on the optical assembly, the heart of SCIAMACHY.

The medium imaging spectrometer (MERIS), developed by the European Space Agency (ESA) for the ENVISAT-1 polar orbit Earth mission, belongs to a new generation of ocean color sensors which will yield a major improvement in the knowledge of such a crucial processes as the ocean contribution to the carbon cycle. MERIS measures the radiance reflected from the Earth's surface in the visible and near infrared part of the spectrum. Data are transmitted in fifteen spectral bands of programmable width and location. The instrument features tow spatial resolution and several observation and calibration modes selectable by ground command. The instrument development is currently carried out by an international team led by AEROSPATIALE under ENVISAT prime contractor ship of DORNIER. The development of the instrument has now reached a status where the instrument has been proven to be compliant with the scientific requirements. This paper gives an overview of the instrument, its design with emphasis given to the acquisition and on-board processing chains. A summary of the major performance sand interface budgets is also provided.

GOMOS is a medium resolution spectrometer designed to measure the concentrations of, and monitor the trends in, stratospheric ozone with very high accuracy and to observe other atmospheric trace gases. Using the star occultation technique, GOMOS combines the features of self-calibration, high vertical resolution and good global coverage. Due to its high sensitivity down to 250 nm, which is one of its main design drivers, GOMOS can measure ozone profiles from 15 km to 90 km. Accessible altitude ranges, accuracies and global coverage are optimum on the night side. In addition, it can measure atmospheric turbulence, which is of interest for understanding the vertical exchange of energy in the Earth's atmosphere. The main mission, instrument and the equipment requirements and their implications on the design and technology choices are presented. The GOMOS design has been completed and validated, and the hardware and software development is currently in its final stage with the flight model equipment in manufacturing. Equipment and subsystems level tests performed have all confirmed so far that the design meets and even exceeds the requirements with respect to several parameters.

SCIAMACHY has been selected for the ESA environmental satellite ENVISAT with the objective to carry out atmospheric research in the UV, VIS, and IR spectral range. The most innovative parts of the instrument are the low- noise InGaAs semiconductor focal plane arrays covering the 1.0-2.4 micrometers wavelength range. For the first time InGaAs focal plane arrays with an extended wavelength range have become space qualified. In this paper theory and measurement of the dark current and noise behavior of these detectors is presented. Each InGaAs focal plane array consists of a 1024 pixel linear photo-detecting sliver and two 512 pixel multiplexing read-out chips. Each multiplexer contains 512 individual charge transimpedance amplifier and correlated double sampling circuits. A cylindrical lens, integrated in the detector housing, focuses the light on detector in the cross-dispersion direction. The InGaAs composition of the detectors is tuned to match the required wavelength range. Measurements have been performed of the dark current and noise as function of temperature and bias voltage in order to relate their performance to theory presented in this paper. InGaAs detectors sensitive to 2400 nm wavelength achieve dark current levels as low as 20-100 fA per detector pixel area of 1.25 (DOT) 10^-4 cm² at an operating temperature of 150 K and a bias voltage of 2 mV. Lower temperatures further reduce the dark current but also decrease the quantum efficiency at long wavelengths, yielding no net gain in performance. The development programme of these SCIAMACHY detectors has been carried out by Epitaxx Inc., for and in cooperation with the Space Research Organization Netherlands.

EOS-AM1 is the first component of NASA's Earth Observing System (EOS). As centerpiece to Mission to Planet Earth, EOS will provide satellite observations to determine the extent, causes, and regional consequences of global climate change. EOS-AM1 is intended to obtain information about the physical and radiative properties of clouds; air-land and air-sea exchanges of energy, carbon, and water; measurements of important trace gases in the atmosphere; and volcanology. It carries five advanced instruments: advanced spaceborne thermal emission and reflection radiometer (ASTER) provided by the Ministry of International Trade and Industry of Japan, Clouds and Earth's Radiant Energy System provided by NASA's Langley Research Center, Multi-angle Imaging SpectroRadiometer provided by the Jet Propulsion Laboratory, Moderate Resolution Imaging SpectroRadiometer provided by NASA's Goddard Space Flight Center, and Measurements of Pollution in the Troposphere provided by the Canadian Space Agency. The project is currently in its D Phase and is maintaining schedule for a June 1998 launch. Fabrication of Flight Model hardware is being completed and integration and subsystems testing is underway. During the next six months, all instruments will be delivered to the spacecraft contractor for integration with the spacecraft bus. System- level compatibility, performance, and environmental testing will follow. The ambitious science objectives, associated data quality and instrument/spacecraft technology considerations, and the current development status will be discussed. The EOS-AM1 project is managed by Goddard Space Flight Center.

EOS AM-2, scheduled to launch in 2004, is the second mission in the EOS AM series within NASA's Mission to Planet Earth program. The EOS AM-2 mission will measure the Earth's radiation budget and atmospheric radiation, global land use, land cover change, local-scale ecological and biogeochemical processes, global aerosol distribution and cloud properties, top-of-atmosphere, cloud, and surface angular reflectance functions, surface albedo, aerosol, and vegetation properties, as well as biological and physical processes on land and the ocean. The baseline instrument complement for the EOS AM-2 mission includes five instruments: the Landsat Advanced Technology Instrument, which will continue the Landsat series of measurements; advanced versions of three instrument that will have flown on the EOS AM-1 mission: the Advanced Moderate Resolution Imaging SpectroRadiometer, the Advanced Multi-angle Imaging SpectroRadiometer, and the clouds and the earth radiant energy system; and a new instrument, the Earth Observing Scanning Polarimeter. Several options are being explored to deploy this instrument complement, including several single spacecraft configurations as well as multiple spacecraft configurations. The driving requirements contributing to the choice of a spacecraft configuration include measurement continuity, coverage, resolution, geolocation, co- registration, repeat cycle, and calibration; important constraints are cost and launch vehicle availability.

Processing system designers must make substantial changes to accommodate current and anticipated improvements in remote sensing instruments.Increases in the spectral, radiometric and geometric resolution lead to data rates, processing loads and storage volumes which far exceed the ability of most current computer systems. To accommodate user expectations, the data must be processed and made available quickly in a convenient and easy to use form. This paper describes design trade-offs made in developing the processing system for the moderate resolution imaging spectroradiometer, MODIS, which will fly on the Earth Observing System's, AM-1 spacecraft to be launched in 1998. MODIS will have an average continuous date rate of 6.2 Mbps and require processing at 6.5 GFLOPS to produce 600GB of output products per day. Specific trade-offs occur in the areas of science software portability and usability of science products versus overall system performance and throughput.

At the same time that we develop new sensors we also need to produce algorithms and processing systems to analyze the data in an operational mode shortly after launch. To develop and test the algorithms and processing systems we need test data. Yet new sensors are often designed to produce combinations of measurements that have never been made before. To resolve this dilemma for the moderate resolution imaging spectroradiometer (MODIS) to be launched in 1998 on the Earth Observing System (EOS) AM platform we are producing synthetic data to test the programing and operational aspects of the algorithms and processing systems. Both MODIS and the resulting synthetic data provide measurements at 36 wavelengths ranging from the visible well into the infrared day and night over the entire globe. The test data covers many of the physical conditions MODIS will observe with a full range of surface and atmospheric characteristics over land and sea with correct instrument and orbital characteristics. The data is sufficiently representational of the radiances MODIS would observe that the processing algorithms run to completion in a reasonable manner and use computing resources similar to those expected with real flight data. Although the simulation is not detailed enough to support theoretical investigations it has proven invaluable in implementing new concepts into operational code. So far we have provided hundreds of gigabytes of data covering many test cases. We describe requirements for synthetic data, tell how the data is produced, what characteristics it models, what limitations it has and what sorts of tests it supports. We show examples of the resulting data sets and describe our plans for future improvements. Sample synthetic MODIS data sets are available and we tell where and how to obtain them.

Planned for launch in June 1998, the Earth Observing System (EOS) AM1-spacecraft will carry five instruments which will be placed into a polar, sun-synchronous, 705 km orbit. EOS AM-1 will cross the equator at 10:30 am local time when daily cloud cover is typically at a minimum over land, such that surface features can be more easily observed. The Ministry of International Trade and Industry of Japan is providing the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument. The ASTER instrument is a high performance spatial imager which has three sensors. The interval between the nearest two orbits is 172 km at the equator. The swath of sensors is 60 km. Therefore ASTER must have pointing capability in order to cover the whole surface of the earth. ASTER accept xARs form many users. Because of constrained spacecraft data capacity, ASTER's data acquisition scheduling algorithm which will perform the most effective scheduling from the inputs which consist of many xARs and which is derived from prioritization information.

The Land Surface Processes and Interaction Mission is one of the candidates for the future Earth Explorer Missions, which are planned to be launched beyond the year 2003 after the ENVISAT and METOP programs. The scientific requirements for such a mission can be met by PRISM, a hyperspectral imager studied by ESA, which makes simultaneous measurements of reflectance and temperature of selected sites of the Earth. PRISM is designed to be flown in a polar sun-synchronous orbit, providing coregistered images in the visible and short-wave infrared, with a spectral resolution of about 10 nm, and the thermal IR range from 3.5 to 12.3 micrometers , divided into 4 spectral bands with a typical width of 1 micrometers . PRISM will make simultaneous observation of the same scene in all spectral bands with tight coregistration. The spatial sampling on ground is about 50 m over a swath of 50 km. Across-track pointing capability is available for convenient accessibility of the observation sites. The paper describes the main mission and instrument requirements with emphasis on the radiometric requirements. Results of the feasibility studies will be given. The most important results of the related technological developments will also be discussed.

PRISM is a spaceborne hyperspectral imager for a future land surface research mission, whose prime objective is the observation of biophysical processes at a local to regional scale. PRISM is designed for a dedicated medium-size satellite in a polar sun-synchronous 11:00 h orbit, and will provide coregistered spectral images in tow spectral regions: from the visible to short-wave IR range with a spectral resolution of about 10 nm and two bands in the thermal IR from 10.3 micrometers to 12.3 micrometers . The presented instrument concept comprises four modules with separate interfaces to the platform: the optical, calibration, cooler and electronics modules. The optics module design is based on a pushbroom type of imaging spectrometer in which the entire field of view is imaged on four detector arrays. The long-wavelength arrays are cooled by tow pairs of Stirling cycle coolers. The instrument layout and platform accommodation are optimized to meet the high radiometric accuracy requirement. The key element of the instrument is the pointing unit, whose mirror is protruding over the platform edge for a wide across track coverage and or access to the three on-board characterization units and to cold space. The pointing unit will provide global accessibility in 3 days. A platform rotation in pitch will enable BRDF measurements of ground test sites by varying along track pointing angles.

For the post 2000 time frame, the ESA has defined candidate missions for Earth Observation. In the class of the Earth Explorer missions, dedicated to research and demonstration missions, the Land-Surface Processes and Interactions Missions involves a dedicated satellite carrying a single optical payload named PRISM. PRISM is a push broom multispectral imager providing high spatial resolution images in the whole optical spectral domain. It provides an access on any site on Earth within at maximum 3 days. In addition, the mission will be able to provide multi- directional observations by combining instrument depointing capabilities and satellite maneuvering. The instrument radiometric performance reach a high level of accuracy by involving on-board calibration capabilities. This paper presents the results of one of the two pre-feasibility studies awarded by ESA, led by AEROSPATIALE and concerning the PRISM payload.

The Linear Etalon Imaging Spectral Array (LEISA) represents a new class of hyperspectral cameras which use non- dispersive thin film filters as wavelength selective elements. The simplicity and versatility of these instruments make them attractive for spaceflight use. LEISA currently operates in the shortwave IR spectral region, but the design is adaptable to operation at wavelengths from visible to longwave IR.

Satellite long term observation scenario has been studied. Earth observation strategy in NASDA consists of following three objectives. Those are (1) monitoring, modeling and prediction of future earth environment by data assimilation, (2) promote satellite data utilization for earth resource management and sustainable development, (3) natural disaster monitoring, modeling and prediction. Satellite measurement form polar, sun-synchronous orbit should be the first priority because of the whole globe coverage. The ADEOS series planned as the major one starts from ADEOS in 1996, ADEOS-II in 1999, ADEOS III in 2003 and follow on. Diurnal variation measurement is also essential for climate change studies especially precipitation and cloud/radiation forcing. ATMOS series has been studied for these mission requirements. ATMOS series has been studied for these mission requirements. ATMOS will be take an inclined orbit to separate diurnal cycle. Newly developed sensors will use experimental platforms such as SPace Stations, small test satellite and airplanes.

The NASDA successfully launched the ADEOS at 10:53 a.m./01:53 a.m. on August 17, 1996 from Tanegashima Space Center. The main objective of ADEOS is to contribute to elucidation of phenomena of the earth system through integrated observation of geophysical parameters using a number of sensors. ADEOS was placed into the final proper orbit on September 8 and the function of the bus system and the mission instruments are now being checked out. The initial mission checkout of ADEOS will continue for 90 days until the middle of November. ADEOS is functioning normally as of September 19, 1996.

ADEOS-2 is a successor of the ADEOS, which was successfully launched at August 17, 1996 from Japan. Its main purpose is to contribute to the global change issues, such as global warming, ozone depletion and carbon cycle.It has 5 main sensors; global imager, advanced microwave scanning radiometer, sea winds, polarization and directionality of the Earth's reflectances, and improved Lim atmospheric spectrometer-II. In this mission, collaboration with exiting international research projects is heavily emphasized such as global energy and water cycle experiment, and climate variability and predictability of the world climate research program, International Geosphere-Biosphere Program, global ocean observing systems and global climate observing system. Its present status will be concisely summarized.

The design of the global imager (GLI) on board the ADEOS-II satellite is discussed to realize an efficient radiometer for monitoring the Earth's surface and lower atmosphere. The GLI has 36 channels allocated over a wide spectral range between 0.38 micrometers and 12 micrometers for retrieving various geophysical parameters important for climate systems studies, such as cloud and aerosol microphysical parameters, ocean color pigments, vegetation indices, snow/ice microphysical parameters, and so on. Land surface and cloud detections are further enhanced by its six 250 m channels spectrally similar to LANDSAT/TM channels. The science issues relevant for the GLI mission are also briefly discussed.

Precipitation Radar (PR) is a key instrument on the Tropical Rainfall Measuring MIssion (TRMM), which is a joint US/Japan space program. The PR is the first rain radar in space. The characteristics of the PR are high sensitivity, low side lobe level an the high speed electrical beam scanning. On- orbit calibration of the PR instrument will be performed by temperature monitoring of the components, internal loop calibration, and the overall system gain calibration using Active Radar Calibrator placed on the ground. Development and protoflight testing of PR has been completed. PR is now integrated to the TRMM spacecraft and TRMM observatory testing is now under way at Goddard Space Flight Center of NASA. In this paper, systems design, system parameters, on- orbit operation, calibration and the performance test results of PR are reported.

This paper introduces outline of Japanese high-resolution earth observation satellite called ALOS, which NASDA plan to launch in 2002. Main mission objectives comprise DEM generation for GIS, environmental and hazard monitoring. NASDA have completed the investigation of users' requirements and preliminary design of hardware for the ALOS. As a result, the ALOS will equip both optical and microwave sensors to fulfill the requirements. DEM will be generated by the optical sensor with stereoscopic observation capability by 'three-line-sensor' with 2.5 m resolution. Multi-spectral information will be acquired by AVNIR-II, a multi-spectral optical sensors with 5m resolution and 4 bands. The microwave sensor, a L-band synthetic aperture radar (SAR), which is a follow-on of the JERS-1/SAR, has capabilities of look-angle change and the ScanSAR mode.

Advanced visible and near infrared radiometer type 2(AVNIR- 2) is a high resolution land observation sensor, which will be loaded on ALOS. AVNIR-2 is composed of a multispectral and panchromatic subsystem. THese former subsystem is recently named as AVNIR-2; same with the old instrument name. The latter subsystem is named PRISM. The multispectral subsystem has ground resolution of 10m and four spectral bands which have same spectral range with that of AVNIR loaded on ADEOS. PRISM has three line detectors and characteristics of B/H equals 1.0 and ground resolution of 2.5m. By means of these characteristics, topographic maps with contour of 5m elevation interval are planned to be produced from those panchromatic data. Because observing data rate of PRISM is very high, on-board lossy data compression will be applied in order to reduce down link data rate. In this report, influences of the on-board lossy compression for terrain elevation measurements are evaluated. As the results, it is clear that: (1) measurement error of terrain elevation does not increase by on-board lossy data compression, (2) measurement error of terrain elevation decreases by on-board lossy data compression under the condition of turbid atmosphere and (3) measurement error of terrain elevation decreases in spite of atmospheric conditions when block coding is used as a lossy compression method.

The second generation instrument of the German Modular Optoelectronic Multispectral/Stereo Scanner (MOMS-02) was first flown aboard space shuttle flight STS-55 in the frame of the German D2 mission in 1993. During the mission approximately 8 mio. km² of data have been recorded from a mean orbit altitude of 296 km resulting in a GIFOV of 4.5 X 4.5 m² in the nadir panchromatic and 13.5 X 13.5 m² in the panchromatic for/aft and multispectral bands. The spatial and spectral information content, estimated by validation methods, confirms the newly designed band-design. Residual image distortions occurred due to malfunctioning electronic modules and due to a missing stable thermal environment during the D2 mission. MOMS-02 was refurbished and will acquire data in a preoperational manner aboard the Russian PRIRODA module mounted on the MIR space station for a duration of 18 months. An orbit altitude of about 380-405 km will result in a spatial resolution of 5.5 X 5.5 m² in the nadir looking panchromatic band and 16.5 X 16.5 m² in for/aft stereo and multispectral modules. First data are expected by the end of Sept. '96. An accompanying intensive calibration and validation program with partners around the world will be conducted to verify and guarantee high data quality as well as to provide up-to- date calibration coefficients for the users.

The solar extreme ultraviolet EUV and x-ray radiation is the main source of energy in the upper atmosphere and ionosphere of Earth. However the permanent satellite monitoring of the EUV and soft x-ray radiation still does not exist. This fact exclusively connected with technical difficulties of space measurements and calibration in this spectral range. The project is based on the experience in the development, creating and exploration of radiometers and grating spectrometers for measurements of absolute solar ionizing fluxes on the Soviet satellites. The main methodological characteristics of the measurements and information processing are given.

The multi-angle imaging spectroradiometer (MISR) will provide global data sets from Earth orbit using nine pushbroom cameras, each viewing in a fixed, unique direction. Data will be acquired for day-lit portions of the orbit at an average rate of 3.3 Mbits s^-1 for the entire six year mission. Automated ground processing will make use of the instrument radiometric, spectral, and geometric calibrations, to produce registered images at the nine view angles. This, the Level 1 product, provides top- of-atmosphere scene radiances, weighted by the spectral band profile for the instrument. Initially, processing will proceed with pre-flight determined radiometric response coefficients. In-flight radiometric calibration of the sensor will then provide monthly updates to these coefficients, to account for degradation which may occur during the mission. THe spectral response profiles are invariant in time, and are provided only by the pre-flight measurements. These include an out-of-band spectral calibration of each channel. These spectral data are used as input to the radiometric calibration of the instrument, and also to produce certain Level 2 products for which an out- of-band correction is made. This paper describes the calibration program, with emphasis on results from the recently completed pre-flight calibration.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) consisting of a visible to near infrared radiometer, a shortwave infrared (SWIR) radiometer and thermal infrared radiometer will be onboard the Earth Observing System's (EOS) AM-1 platform. The characteristics of ASTER have been published in several papers. In particular, the calibration plan for ASTER has been described in detail. One of the important issues for the calibration plan of ASTER is the determination of a set of calibration coefficients using preflight calibration, onboard calibration, cross-calibration and vicarious calibration data. In order to establish a method for determination of a set of calibration coefficients, a preliminary field campaign was conducted at Lunar Lake and Railroad Valley Playas in Central Nevada in the USA in June 1996. The procedures and methods used and the data collected during the field campaign are briefly described here together with the current plans for ASTER calibration activities and a method for determining a set of calibration coefficients.

The visible and infrared scanner (VIRS), one of three primary sensors on the Tropical Rainfall Measuring Mission (TRMM), has completed its development and test phase at Santa Barbara Remote Sensing and has ben delivered to the Goddard Space Flight Center where it has been integrated on the TRMM spacecraft. VIRS is a five band imaging radiometer with bandpasses similar to those of the Advanced Very High Resolution Radiometers that have flown on the NOAA series of satellites for the last 18 years. VIRS will can a +/- 45 degree swath with a 2.11 kilometer IFOV at nadir from the non-sun-synchronous 350 kilometer TRMM orbit. All five bands will be cooled to 107K at mission start using a passive radiative cooler. The two reflected solar bands will be calibrated on orbit using a solar diffuser. This paper discusses ground calibration and characterization results and proposed post-launch radiometric calibration procedures for the VIRS data.

PRISM is a future spaceborne hyperspectral imager, to operate in the spectral range from 450 nm to 12.3 micrometers . The PRISM instrument designer faces a challenging requirement - the absolute radiometric accuracy of the instrument in the range from 450 nm to 2350 nm shall be better than 2 percent of the measured radiance. This requirement can only be met using highly accurate characterization sources and a thermally stable instrument. In the DSS PRISM concept a calibration module is located separate from the optical module on an adjacent platform panel. The characterization sources are accessed via a pointing mirror, which is protruding over the platform edge. THis configuration allows for a wide across track coverage and access to the three on- board characterization units and to cold space. The three on-board characterization units are an aperture plate with small hole apertures for direct sun viewing, a reflective diffuser and a blackbody with selectable heater levels. Laser sources illuminating the diffuser serve as spectral references. The high accuracy in the VNIR/SWIR range is achieved by subsequent characterization measurements using the aperture plate and the diffuser: the first serves as absolute radiation reference for a limited amount of pixels, whereas the later will provide uniform illumination of all pixels and thus allows to correlate the sensitivities of the absolutely measured pixels to the others.

A study has been carried out for the European Space Agency (ESA) on strategies for radiometric and spectral calibration of data produced by optical sensors observing Earth surface from Earth orbit. The study relates specifically to data produced by high-resolution imaging spectrometers and imaging radiometers, such as the PRISM instrument which is currently under development by ESA. The typical instrument specification includes 50m spatial resolution on ground, with 10nm spectral resolution through the visible/near-IR and short-wave IR spectral wavebands, plus imaging in a few selected wavebands in the thermal IR. This paper is limited to discussion of the first step in the radiometric calibration process, in which raw digitized data from the space instrument are converted to accurate values for at- sensor radiances. Relative spectral response information, for each resolved spectral band, must accompany the calibrated radiance data. The calibration process, for at- sensor radiances, requires accurate characterization of the space-instrument response characteristics. Part of the specification for the PRISM instrument is given as an example of performance requirements, and a possible design form for PRISM is briefly described as an introduction to discussion of the relevant response characteristics. Critical pre-flight and in-flight characterization requirements are noted, and methods for in-flight characterization are discussed briefly, including use of on- board hardware and space-views, and vicarious calibration using ground target areas.

Two complementary methods for in-flight polarization calibration of the POLDER instrument are described. One makes use of unpolarized targets such as clouds, and the other one of very strongly polarized targets such as sunglint over the ocean. Errors budget and the airborne validation are presented.

The Global Ozone Monitoring Experiment (GOME) was launched onboard the ERS2 satellite in April 1995. In order to study atmospheric trends over a very long time, the instrument was calibrated on ground very accurately. Obviously the accuracy has to be guaranteed during the lifetime of the instrument. Therefore, since the instrument in space is susceptible to changes and degradation, changes in the instrument performance have to be monitored, as was done during the GOME in-orbit validation phase.

Since mid-87, the histograms of all the SPOT cloud-free images processed in Toulouse (France), Kiruna (Sweden) and by SICORP (USA) have been stored in a data base. Thanks to the continuous effort made to calibrate the SPOT HRVs, the observed reflectances for the four SPOT spectral bands can be estimated. Reflectances can be visualized on world maps for a given period and are mainly used to tune the viewing gains. This calibration procedure is now operational for SPOT2 and SPOT3: the new programming center automatically adjusts the on-board electronic gains according to the landscape to optimize the image dynamics while avoiding saturation. This paper presents the basic design of the base, from the reception of histograms to the creation of gain files. Reflectance maps show the data base contents in terms of world coverage. The accuracy of the computed reflectances is discussed and the problem of geographic areas with no data in the base in considered. We also briefly describe other SPOT Histogram Data Base applications such as sizing an optical sensor, and searching for uniform areas for relative calibration or stable areas for absolute temporal calibration. Finally we discuss future enhancements such as using data from other sensors like VEGETATION, which will fly on SPOT4 and would provide information for the mid- infrared band to optimize SPOT4 HRVIR image acquisition.

Typically, the surfaces used in radiative transfer codes to predict the radiance at a satellite sensor are assumed to be lambertian and spatially-homogeneous. Of course, surface bidirectionality and surface-surround effects are second order terms, but improvements in vicarious calibration procedures, as well as more challenging accuracy requirements, require that these effects be included. This paper examines the effect of surface BRDF and spatial inhomogeneity on retrieved calibration coefficients for the SPOT HRV cameras from the well-known reflectance-based calibration approach. This calibration method has primarily relied on test sites at White Sands Missile Range in New Mexico, USA and La Crau, France. BRDF effects are studied using multispectral measurements of the bi-directional reflectances of both sites used in a Fourier series expansion in azimuth to model the surface BRDF. This Fourier series expansion follows the architecture of the successive- orders-of-scattering radiative transfer code and is easily introduced as a new boundary condition. These BRDF models are used to reprocess past calibration data and the results are compared to those obtained by assuming the surface to be lambertian. In addition to surface BRDF, spatial- inhomogeneity of the surface reflectance is examined. The surrounding area's surface reflectance is derived from the SPOT imagery and included in the radiative transfer computations suing in house modifications to the 6S code. These results are compared to those which exclude adjacency effects.

The in-flight radiometric calibration of satellite multispectral sensor for earth and atmospheric observations can be conveniently based on solar diffusers. Theoretically, a knowledge of the spectral bi-directional scatter distribution function (BSDF) of the diffuser panel, and the solar incidence angle is all that is needed to allow the retrieval of the earth albedo in the observed direction. At the request of the ESA, the Centre Spatial de Liege, with the support of Officine Galileo as subcontractor, is currently designing a high-versatility high-accuracy BSDF measurement set-up with application to the calibration of space solar diffusers. This instrument will allow a BSDF measurements uncertainty within 1 percent for any angle in the wavelength range from 200 nm to 2400 nm. Vacuum measurements, polarization analysis capabilities and thermalization of the test sample between 200K and 300K are other unique features of this set-up.

Rayleigh scattering targets over clear oceans under large solar and viewing angles have proved their efficiency to calibrate remote sensing instruments in the blue channels. To obtain a good accuracy, this method needs an evaluation of the different contributors participant in the total TOA signal: aerosol, atmospheric conditions, foam, water reflectance. The near infrared is used to estimate the aerosol content which is after transfered in the band to be calibrated depending on the considered aerosol type. Atmospheric conditions and foam are evaluated with ECMWF data: water vapor content, surface pressure, wind speed. Two methods are described for the spectral transfer of the aerosol optical thickness and compared using different SPOT acquisitions: one using the absolute calibration coefficient and the other interband calibration coefficient. The absolute calibration coefficients are computed in the blue and green channels of SPOT3.

Thermal infrared and microwave satellite radiometers are typically calibrate in-flight using a space view as a radiometric zero, and a view to a blackbody as a full-scale reference view. This study refers specifically to the high- resolution dynamics limb sounder, an infrared filter radiometer under development for the NASA EOS-Chem payload, but some aspects of it are of general interest for radiometric calibration. Instruments of this type typically have scan patterns generated by a plane scan mirror near the front of the optical train. The calibration views are thus acquired with a slightly different optical configuration from each other, and from the required scene views, raising questions about scan-dependent gain and polarization effects. The present paper establishes a formalism for handling polarization issues in radiometers, adapted to a geometrical ray-tracing modeling approach. Using this formalism, it is shown that the gain errors are substantially cancelled provided the scanning geometry permits a space view with a very similar optical arrangement to the required scene views, and the blackbody, intermediate mirror and scan mirror can be maintained at the same temperature. This cancellation follows from the application of Kirchoff's law, and is thus a robust conclusion. Explicit general expressions are given for the residual errors arising with non-zero temperature differences.

The ability to conduct in-flight absolute radiometric calibrations of ocean color sensors will determine their usefulness in the decade to come. On-board calibration systems are often integrated into the overall system design of such sensors and have claimed uncertainly levels from 2-3 percent, but independent means of system calibration are desirable to confirm that such systems are operating properly. Vicarious methods are an attractive means of this verification. Due to the high sensitivity of ocean color sensors, the use for bright reflectance surfaces often results in sensor saturation. Low reflectance targets, such as water bodies, should therefore be used. This paper presents the results of sensitivity studies of the reflectance- and radiance-based approaches when applied to a water target and method uncertainties for calibrations of the Sea-Viewing Wide Field-of-view Sensor (SeaWiFS). The paper also present the results of a field campaign which took place at Lake Tahoe in June 1995. This lake represents a typical oligotrophic water body and has the advantage of being located at a high elevation where tropospheric aerosol loading is low. Aircraft-based radiance data and surface measurements of reflectance are sued to calibrate SeaWiFS- simulated bands from Advanced VIsible and Infrared Imaging Spectrometer (AVIRIS) data. Atmospheric characterization is obtained using solar extinction measurements, surface-level atmospheric pressure readings, and columnar gaseous absorber amounts at sensor overpass. The measured radiances are transferred to the top of the atmosphere using a radiative transfer code which fully computes the contributions of multiple scattering by the atmosphere. The results are compared to those obtained form a laboratory-based calibration of AVIRIS.

The ALOS is a Japanese sun-synchronous earth observing satellite scheduled to be launched in 2002. ALOS carries both optical and microwave high resolution imaging sensors, i.e. the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM), the AVNIR-2, and the PALSAR, mainly for cartographic use, environmental and hazard monitoring, the earth resources investigations. PALSAR is an advanced-type follow-on SAR of the JERS-1/SAR and developed jointly by NASDA and MITI/JAROS. PALSAR is operated at an L-band frequency with parallel and cross polarization in a variety of beam selection modes providing different spatial resolutions, i.e. 8m/10m and 20m, incidence angles, and swath widths. In addition, the ScanSAR mode enables to serve a wide region up to 350 km with a low resolution ecology, hydrology, glaciology, oceanography, etc., together with the use of radar interferometry techniques for precise measurements of surface topography and its changes caused by earthquakes, volcanic activities, etc.

Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager to be operated from a spacecraft. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have indicate that the instrument is capable of separating a wide range of subtle spectral differences. The concept design for the payload has been completed and hardware is in fabrication for an engineering model.