The SPOT follow-on system whose French acronym is 3S (Suite du Systeme Spot) is a new CNES (French Space Agency) Earth Observation program with reduced development cost as a major objective. In order to reach this goal, the new satellite architecture must lead to an important reduction of the mass, the target being around 500 kg. The reduced mass of the new instrument, that is the essential part with regard to the image quality performances, will have to be taken into account with the aim of using as much as possible the R&D studies that are in progress. The present paper describes a new concept of instrument. The optical system based on a TMA (Three-Mirror Anastigmat) telescope with 3.8 degree Field Of View (FOV) allows the compact architecture necessary to the mass reduction while keeping high performances. The mechanical architecture realizes the deportment of the instrument parts that are submitted to the ground and flight environment while keeping stiffness and stability. The pointing system modifies the direction of the image acquisitions within a plus or minus 30 degree range along track and plus or minus 45 degrees across. The focal plane is composed of two sensors for the acquisition in one panchromatic and four multispectral bands. The processing electronics converts with a 6 Mpixels/s rate the data to 10 bit words in the entire signal dynamic. Finally, the main performances are shown.

SkyMed/Cosmo is a research project aimed to the realization of a space-borne observing system devoted to the monitoring of the Earth surface in the optical and microwave ranges. The project, which is supported by the Italian Space Agency, foresees the utilization of a group of satellites, each of which equipped with a set of instruments allowing different spatial resolutions. The entire system is realized for operational purposes over regional scale (i.e.: disaster monitoring, observation of marine and terrestrial ecosystems), that require a very short re-visitation time. The phenomena to be taken into account for the disaster monitoring are mainly earthquakes, landslides and marine emergencies. The SkyMed/Cosmo system promises to become a useful tool both for the analysis of risks and assessment of damages, and in any case where rapidly changing features (days or weeks) should to be observed. The system will be able to monitor the ground, marine and coastal environments, and to provide useful information about water circulation, water quality, sea bottom vegetation, soil coverage, geology, coastline monitoring, archaeology and cartography. Some of these applications, like those related to marine environment, will require a very high signal to noise ratio as well as accurate radiometric calibration. This work describes the outgoing feasibility study devoted to regional scale environmental applications of remote sensing by the SkyMed/Cosmo optical payload. The analysis of the SkyMed/Cosmo performance and diagnostic capabilities is given and discussed in comparison with those allowed by other satellite instruments.

We present an overview of the Naval EarthMap Observer (NEMO) spacecraft and then focus on the processing of NEMO data both on-board the spacecraft and on the ground. The NEMO spacecraft provides for Joint Naval needs and demonstrates the use of hyperspectral imagery for the characterization of the littoral environment and for littoral ocean model development. NEMO is being funded jointly by the U.S. government and commercial partners. The Coastal Ocean Imaging Spectrometer (COIS) is the primary instrument on the NEMO and covers the spectral range from 400 to 2500 nm at 10-nm resolution with either 30 or 60 m work GSD. The hyperspectral data is processed on-board the NEMO using NRL's Optical Real-time Automated Spectral Identification System (ORASIS) algorithm that provides for real time analysis, feature extraction and greater than 10:1 data compression. The high compression factor allows for ground coverage of greater than 10⁶ km²/day. Calibration of the sensor is done with a combination of moon imaging, using an onboard light source and vicarious calibration using a number of earth sites being monitored for that purpose. The data will be atmospherically corrected using ATREM. Algorithms will also be available to determine water clarity, bathymetry and bottom type.

Current uncertainties in the effects of aerosols and clouds on the Earth radiation budget limit our understanding of the climate system and the potential for global climate change. PICASSO-CENA is a recently approved satellite mission within NASA's Earth System Science Pathfinder (ESSP) program designed to address these uncertainties. The PICASSO-CENA payload includes a lidar and three passive instruments which will provide unique information on the global distribution and properties of aerosols and clouds. PICASSO-CENA will be flown in formation with the EOS PM and CloudSat satellites to provide coincident measurements of atmospheric state, radiative fluxes, and thick clouds. This global suite of measurements will provide a basis for improving the representation of clouds and aerosols in climate models, leading to improved capabilities for predicting climate and climate change. PICASSO-CENA is planned for a three year mission beginning in early 2003 and is being developed within the framework of a collaboration between NASA and CNES.

Japanese Earth observation programs will be described. After the accident of ADOES and stop of JERS-1, the only operating program is TRMM (Topical Rainfall Measuring Mission), which is a joint program with NASA. Future programs can be divided into three categories. One is a long-term global change monitoring programs. ADEOS was the first satellite in this category. Next program is ADEOS2, which will be launched in Nov. 2000. Continuing programs are now called GCOM (Global Change Observation Mission). The first generation of GCOM is divided into two satellites, of which GCOM-A1 will be launched in Feb. 2005 and GCOM-B1 will be launched in Aug. 2005. The second category programs are also dedicated to global change, but more related to process studies. The first mission in this category was TRMM and the second one will be MDS-2 (mission demonstration satellites) which will carry a Mie scattering lidar called ELISE. Other programs include SMILES (Super conducting Millimeter Emission Sounder) and CDL (Coherent Doppler Lidar) on ISS-JEM, Earth radiation mission, etc. The third category programs are composed of high spatial resolution satellites mainly dedicated to land applications. The past MOS-1, MOS-1b and JERS-1 missions were in this category. The next mission is ALOS (Advanced Land Observation Satellite) which will be launched in Aug. 2002. ALOS will carry very high-resolution optical sensors and an L-band synthetic aperture radar.

The Earth Explorer Missions are research/demonstration missions for Earth Observation that are planned for implementation in the frame of the European Space Agency's 'Living Planet' Programme. The program focuses on advancing understanding of different processes that contribute to govern the Earth Systems. One of the four Earth Explorer Missions which was the subject of a Phase A study is the Land-Surface Processes and Interactions Mission (LSPIM). The scientific objectives are the study of land surface processes and their interactions with the atmosphere. It focuses on the measurement of surface characteristics such as albedo, reflectance, bidirectional reflectance distribution function (BRDF) and surface temperature, which are linked to the processes driving bio/geophysical and biochemical variables. To fulfil the mission requirements a hyperspectral imager is proposed as the LSPIM core instrument. The LSPIM imaging spectrometer will provide contiguous spectral coverage in 142 bands within the VIS/NIR/SWIR spectral region with spectral resolution between 10 and 15 nm. Furthermore, TIR observations will be performed by a radiometer in two wavebands. A spatial resolution of 50 m X 50 m with a swath width of 50 km at nadir will be provided. This mission will also have a de- pointing capability for BRDF observations along-track and areal access across-track such that each site of interest can be revisited at 3 days intervals. It is the purpose of this paper to outline the planned spaceborne mission, its scientific objectives and the derived system requirements.

All weather images of sea ice are daily available thanks to passive sensors as SSM/I. They mainly come from the 37 GHz and 19 GHz channel with respectively 35 km and 60 km ground resolution. Mast (standing for millimeter waves aperture synthesis techniques) aims at bringing this ground resolution down to 6 km ((Delta) T equals 1k), by using a larger but fixed 36.5 GHz radiometer with one dimension interferometer techniques. Polarimetry capabilities (measurement of H and V correlation) are also provided at 36.5 GHz with a lower ground resolution, 25 km, and (Delta) T equals 0.1 k. The polarimetric data are combined with the second interferometer channel operating at 18.7 GHz in V polarization in order to retrieve surface wind on ocean.

Pulse limited radar altimeters (Geosat, ERS1/2, Topex/Poseidon) have demonstrated excellent ability in performing measurements of the ocean topography from space with a high degree of accuracy. Data continuity will be ensured through follow on missions like TOPEX-POSEIDON Follow- on and ENVISAT RA2 (developed by ALENIA AEROSPAZIO under ESA contract) in this case providing also the chance for a global Earth topography mapping not more limited to ocean but extended to land and ice regions thanks to innovative design features like resolution adaptivity and robust on board tracking. Earth sciences are now demanding for systems with extensive capability to get topographic measurements over non- ocean surfaces (ice and land regions) but with improved spatial resolution, in the order of 100 - 300 meters respect to the several hundreds of meters provided by nadir looking pulse limited systems. A real step forward in high resolution topography with microwave instrumentation is represented by the application of synthetic aperture and interferometric techniques to the conventional pulse limited altimeter concept, a solution proposed in the literature and extensively exploited by ALENIA AEROSPAZIO in the frame of the ESA studies TOS (Topography Observing Systems) and HSRRA (High Spatial Resolution Radar Altimeter) and proposed in late 1998 for the Earth Explorer Opportunity Mission CRYOSAT. In the high spatial resolution altimeter synthetic aperture processing applied along the direction of motion will allow to improve the resolution in the along track while dual antenna observation geometry will enable reconstruction of surface topography within each synthesized Doppler filter from the phase difference between the radar returns at the two antennas. Thanks to a proper baseline selection, a unique interference fringe can be generated within the observed swath thus avoiding the troubles of phase unwrapping otherwise required in conventional interferometric processing. Aim of this paper is to review the key concepts of the proposed measurement technique and to present the design of the high spatial resolution altimeter operating according to the outlined measurement principle and suitable for a global ocean/ice/land topography mission.

The concept of the small, medium resolution Earth observation camera (14 - 20 m per pixel) using a joint transform optical correlator is proposed. The camera is intended to be used as a secondary payload for low orbit, 3-axis stabilized spacecraft, including ones with moderate attitude stability. The optical correlator is used for a real-time determination of the camera pointing motion for posteriori correction of the image distortions due to attitude unstability. A wide angle lens and long assembly of the line sensors in the focal plane permit to change the view direction without need to move the satellite or camera elements. A possible layout of the optoelectronic unit is described and the results of simulated experiments, expected performances and size estimations are given.

We examine the performance limitations of a four-channel polarimeter in the presence of detection noise for arbitrary light levels. We treat the case where the light exiting the polarimeter is detected by a photon counting sensor at each of the four channels. Specifically, we theoretically describe the propagation of detection noise through the polarimeter calibration matrix. The variances in the four estimated Stokes vector components, both orthogonal intensities and mutual phase delay are theoretically predicted for photon counting noise following Poison statistics, and additive Gaussian detection noise. The variances of these parameters depend on the average number of photons incident on the polarimeter, the root-mean-square read noise, and the polarimeter calibration matrix. This methodology allows for including fixed errors in the polarimeter calibration matrix. Various polarimeter designs, whose calibration matrices are known exactly, are examined for high, low and very low light levels. Theoretical performance curves are shown for various sensor parameters and light levels. The theoretically predicted values are compared to simulated results. Excellent agreement between theory and simulation is shown. The simulation also validates the use of the Gaussian probability density function for the parallel (in-phase) and normal components of the phase fluctuations and results in an accurate theoretical prediction of phase delay fluctuations for arbitrary light levels. The phase delay noise cloud is illustrated for several cases.

In the framework of the scientific co-operation agreement between the Italian Research Council and the Russian Academy of Sciences, a common research program was developed by our two Institutes to analyze the characteristics and the applications of advanced spaceborne optical sensors for the remote sensing of the environment. This program has become a part of the international scientific project PRIRODA. The space module PRIRODA, attached to the inhabited space platform MIR, is the technical base for the project. By means of the optical sensors placed on board of the PRIRODA module, we are studying the geophysical parameters useful for environmental monitoring and resources evaluation, with particular attention paid to: (a) water quality in the coastal zone and particularly near the river estuaries; (b) vegetation stress due to the anthropogenic activities; (c) geophysical studies in areas of geothermal and volcanic activities; (d) estimation and verification of the atmosphere contributions. In order to achieve the aforementioned goals, the Italian side has identified some test sites, located mainly in the Tuscany region (Italy), and the kind of needing data. The following optical sensors, of the PRIRODA module, have being utilized: (1) MOMS-2P. This modular optoelectronic multispectral stereo scanner consists of two sub-units: a threefold stereoscopic imaging system and a four-band multispectral camera with nadir orientation. The instrument parameters were designed in order to fill the gap between existing spaceborne system and airborne photography. (2) MSU-E. This electro-optical scanner is mainly devoted to investigate the reflected solar radiation in the 'atmosphere-Earth surface' system at a spatial resolution of 25 m in three visible and near infrared spectral bands. (3) MSU-SK. This opto-mechanical scanner operates in four adjacent visible and near infrared bands at 120 m of spatial resolution plus one band in the thermal infrared region with 300 m of spatial resolution. An additional feature consists of the possibility to tilt the field of view of the scanner up to 30 degrees in the plane normal to the flight direction. The results obtained from the data acquired by these sensors in March 1997 and February 1998 are discussed and compared with those collected by other aerospace sensors, like the Thematic Mapper, and in field measurements.

The Triana spacecraft will be launched into a halo orbit around L-1, the first Lagrangian point, in late 2000. It will carry an active cavity radiometer for measuring outgoing longwave and reflected solar irradiance from the full disc of the Earth. The reflected solar radiation case is complex and requires computer simulation. The elements of the simulation are described. The purpose of the simulation is to develop a quantitative understanding of the effects of various factors on the measurements to aid in their interpretation. These factors include the variation of albedo over the Earth- atmosphere system, changes of albedo with solar zenith angle and anisotropy of the reflected solar radiation. Finally, there is a diurnal cycle of regional albedo due to clouds varying daily. The active cavity radiometer will also be very useful as a calibration standard for high spatial resolution results from the Earth Polychromatic Imaging Camera (EPIC). The EPIC will provide 10 km resolution imagery every 15 minutes over the Sun-lit face of the Earth, providing information at mesoscale space and time resolution about ozone, aerosols, water vapor and clouds.

With the 3S program (the French acronym for SPOT System Follow-on), CNES intends to continue the SPOT Earth Observation mission with the purpose of achieving severe costs reduction. This increases the need for new, lighter, more compact technologies for the payload. Therefore, CNES has launched preliminary studies in some critical payload domains. The paper deals with the work done in order to demonstrate the feasibility of a small telescope that can be used onboard a mini-satellite. A Three-Mirror Anastigmat (TMA) telescope breadboard has been manufactured using the same technologies that would be required to provide in-orbit stability. The TMA has excellent image quality over its whole 8.4 X 1.4 degree FOV and an intrinsic compactness that makes it much smaller than its focal length: as a result, the whole telescope weighs about 40 kg. The paper particularly focuses on the telescope design, the alignment method and the optical performance under stable laboratory environment. The behavior of the telescope under space environment is described, as well as the tests conducted to validate the computed optical performance under thermal variations and vibrations conditions.

In this presentation we describe flight results for an airborne IR hyperspectral imager used as a test bed for LEISA, a compact spaceborne wedged filter spectrometer. The moderate spectral resolution Linear Etalon Imaging Spectral Array (LEISA) is a low-mass, low-power, low-cost infrared spectral imager for spacecraft applications. LEISA uses a state-of-the- art wedged infrared filter (a linear variable etalon, LVE) in conjunction with a detector array to obtain hyperspectral image cubes. The LEISA concept has been described previously in Reuter et al., 1997, SPIE Vol. 2957, pp 154 - 161, 'EUROPTO Conference on: Advanced and Next-Generation Satellites II.,' 23 - 26 September, 1996, Taormina, Italy. A LEISA type instrument, the Atmospheric Corrector (LAC), will fly on NASA's EO-1 spacecraft to be launched in Dec. 1999. The airborne version of LEISA covers the spectral region from 1.0 to 2.5 microns at a constant resolving power ((lambda) /(Delta) (lambda) ) of approximately 250 (i.e. 4 nm 1.0 microns and 10 nm 2.5 microns). The single pixel spatial resolution is 2 milliradians. This corresponds to 2 meters 1 km altitude and 20 meters 10 km. The instrument has been operated throughout this altitude range. The instrument has a swath width of approximately 29 degrees. A 256 X 256 element NICMOS (Near Infrared Camera Multi-Object Spectrometer) HgCdTe detector array is used as the focal plane. The focal plane is enclosed in a small cryogenic dewar at liquid Nitrogen temperature. Results will be presented for three series of airplane flights: Lubbock Texas (USA) June - September 1997, Lubbock Texas (USA) July - September 1998, Bethlehem Orange Free State (South Africa) March 1999. Issues to be discussed include pre-, and post-flight calibration, image registration and spectral image reconstruction. The relationship of these measurements to future spaceborne hyperspectral imagers will also be discussed.

The use of hyperspectral sensors for geological, agricultural and other remote sensing applications is continually increasing. In addition to airborne sensors, there are now at least four hyperspectral satellite sensors under development. These sensors will be producing a near continual stream of high dimensional data, leading to an obvious analysis bottleneck. Much of the planned analysis of hyperspectral image cubes requires the determination of certain basis spectra called 'end-members.' Once these spectra are found, the image cube can be 'unmixed' into fractional abundances of each material in each pixel. There exist several techniques for accomplishing the determination of these end-members, most of which require the intervention of a trained geologist. This process and the associated computations are often time- consuming. There is a need for automated techniques to allow the quick review of data collected by the sensors. Several different approaches to finding end-members in data will be reviewed, including the Pixel Purity Index, Orasis, and the Iterative Error Estimation methods. A new method, called N- FINDR, which extracts end-members based upon the geometry of convex sets, will be discussed in detail. End-member spectra and abundance maps will be compared to USGS results on AVIRIS data. Data examples from AVIRIS will also be used to compare several of the algorithms.

The IASI instrument (Infrared Atmospheric Sounding Interferometer) is a Fourier-Transform Spectrometer (FTS) providing spectra of the Earth's atmosphere observed from space. The heart of the instrument is a Michelson interferometer (IHOS) equipped with two hollow cube-corners retro-reflectors in place of the classical flat mirrors. The main alignment requirements of the IASI interferometer are the lateral shift, or shear, of the moving cube-corner (seen through the beamsplitter) and the misalignment of its scanning axis: these contributions should not exceed 20 micrometer and 250 (mu) rad respectively during the five years mission in orbit. Thus the most difficult challenge of the IHOS integration on-ground probably is their measurement accuracy, which shall respectively be better than 1 micrometer and 100 (mu) rad. The envisaged characterization method consists in a specific data processing of the fringe patterns created by the interferometer at four different points located in the IHOS Field of View (FoV), corresponding to the IASI instrument pixels. For each acquired interferogram the Optical Path Difference (OPD) created by the interferometer are evaluated using a double Fourier-transform algorithm, and the results are combined together in order to retrieve the apparent trajectory of the mobile cube-corner. This principle was tested on a breadboard interferometer already assembled in the CNES laboratories. The numerical results presented herein tend to demonstrate the efficiency of the method, since the achieved accuracy does not exceed 1.2 micrometer (whatever the cube-corner axial position) and 120 (mu) rad respectively. The main error sources also are discussed.

ATRAS (Atmospheric Radiation Spectrometer) is a nadir-looking Fourier Transform Spectrometer, which is a follow-on instrument of IMG onboard ADEOS satellite. ATRS will have 0.05 (0.1 apodized) spectral resolution over 3 - 16 micron using 4 photo voltaic (InSb and/or PV-MCT) detectors. Objectives of ATRAS are to demonstrate the performance of high spectral resolution IR sounder using FTS technique on (1) monitoring of greenhouse gases, (2) operational temperature and water vapor sounding, and (3) monitoring of earth's radiation budget. ATRAS will have much better radiometric performance compared to the IMG. It was proposed to be launched onboard a Japanese small satellite, MDS-3 (Mission Demonstration Satellite). The mission strategy and results of conceptual design study of ATRAS will be discussed.

Scientists at NASA's Langley Research Center, in collaboration with researchers at Virginia Tech, are developing the next generation of thermal radiation detectors composed of new space-age materials, including carbon-doped Larc-Si and aerogels. In order to accurately model and design these detectors, it is necessary to determine the in situ thermoelectric properties of these new materials, including thin-film effects and contact resistance. The authors present an approach to determine these properties through the use of simultaneous parameter estimation methods in which experimental results obtained from detector prototypes are compared with results predicted from analytical models. Parametric values are varied using an optimization method to minimize the least-squares error between the experimental and model results. A numerical study is presented to validate the use of this approach. Simulated experimental results were produced using a model based on nominal parameter values. These results were then introduced into a parameter estimation algorithm that was able to recover the parameter values without the benefit of a priori knowledge about the material properties. Genetic algorithms, stochastic hill climbers, and a hybrid of the two methods were investigated for use in parameter estimation.

This paper highlights the application of time delay integration charge couple device (TDICCD) and TDICCD focal plane assemblies (FPAs) on real-time earth observation remote sensing satellite. From the angle of the design of space TDICCD cameras, it introduces how to design focal plane assemblies (FPAs) -- including TDICCD device unit (DU) choice, TDICCD field butt, FPAs machine design, electronic focal plane assemblies (EFPAs) video electronic signal processing unit (SPU), TDICCD FPAs temperature control. An example is also given. Two dynamic TDICCD imaging instruments are manufactured using the 'push-broom' imaging principle, and their confident performances are also achieved.

Computer simulation is often adapted to learn the temperature change behavior during the interaction between the pulsed laser and HgCdTe. Although the speed of the computer processor is faster and faster, a proper computation method remains critical for an effective simulation. A main hurdle in the conventional calculation is related to the number of layers, which must be estimated before calculation in this paper, a new method to calculation the temperature of HgCdTe irradiated pulsed laser was proposed. Namely, a few layers were considered firstly until the temperature profile fulfilled a certain condition, then some additional layers were considered. With some minor factors neglected, the calculation is faster than the conventional one. By comparing these two different kinds of calculation, it is suggested the new method can be used for fast simulation of the temperature behavior during the interaction between HgCdTe and pulsed laser.

The wide usage of small satellite imagery, especially its commercialization makes application based on-board compression not only meaningful but also necessary in order to solve the bottleneck between the huge volume of data generated on-board and the very limited downlink bandwidth. In this paper, we propose a method which encodes different regions with different algorithms. We use three shape-adaptive image compression algorithms to be the candidates. The first one is a JPEG-based algorithm; the second one is based on the Object- based Wavelet Transform (OWT) method proposed by Katata; the third adopts Hilbert scanning of the regions of interest followed by one dimensional (1-D) wavelet transform. The three algorithms are also applied to the full image so that we can compare their performance on who rectangular image. We use eight Landsat TM multi-spectral images as our test set. The results show that these compression algorithms have significantly different performance for different regions. For relatively smooth regions, e.g. regions that consist of a single type of vegetation or water areas etc., the 1-D wavelet method is the best; for highly textured regions, e.g. urban areas, mountain areas and so on, the modified OWT method wins over the others; for the whole image, OWT working at whole image mode, which is just an ordinary 2-D wavelet compression, is more suitable. Based on this, we propose a new application based compression architecture which encodes different regions with different algorithms.

In this paper the use of signal representation in signum delta modulation format (SignDM) has been proposed in order to increase the efficiency of the correlation analysis (CA) in real time of short highly noised time series and 2-D data. The SignDM codes are specified by comparing the values of differences between signal samples in PCM formats with a differential zone which is defined a'priori. These codes belong to the set {-1, 0, 1}. In this paper mathematical operations on such codes have been studied. SignDM's sampling rate equals Nyquist's frequency. The following approaches ares suggested to be applied when conducting determination with the help of CA: (1) the frequency of short highly disturbed periodic signals will be specified on the basis of a maximum of the proper power spectrum from their autocorrelation function in the format of SignDM; (2) time shifts between two short and highly noised signals of the same frequency will be defined by the cross- correlation function in SignDM format. Using Wiener & Hinczyn's transform of autocorrelation function increases the effectiveness of the correlation analysis for measuring and processing highly noised periodic signals. In this paper there have been presented the algorithms for the operation of specialized processors used for the correlation analysis of signals in SignDM format, as well as the results of computer simulation for time series N less than or equal to 30, and SNR up to -14dB at the chosen differential zone. These algorithms are regular and economical and especially applicable in on-board data processing for space experiments.

Space-based observations of atmospheric energetics, such as those provided by NASA's Clouds and the Earth's Radiant Energy System (CERES), produce data products intended to be shared with the larger scientific community and merged with other complementary data sets. Meaningful fusion of complementary data requires a well-founded common statistical basis for cited precision and accuracy. A high-level numerical model is available capable of predicting the dynamic opto- electrothermal behavior of CERES-like radiometric channels. The paper reports use of this model to explore the sensitivity of data products to variations in individual optical, thermal and electronic parameters. The optical/thermal radiative part of the model is based on the Monte-Carlo ray-trace (MCRT) method in which millions of rays are traced. Several hours of execution time on a large computer are required to simulate a single scan across the Earth's surface, thus making it impractical to run the simulation for every possible variation of each parameter. A key element of the research involves an effort to determine the minimum number of simulations required to produce statistically meaningful results.

The Eppley pyranometer is widely used to measure shortwave irradiances. This instrument consists of a blackened surface in intimate thermal contact with the hot junction of a thermopile. The cold junction of the thermopile is in thermal contact with a heat sink. Shortwave radiation transmitted through two concentric hemispherical domes is absorbed by the blackened surface. The voltage developed by the thermopile is then interpreted in terms of the shortwave irradiance. Measurements obtained using these instruments are known to be influenced by thermal radiation that produces an offset from the signal that would result solely from the incident shortwave radiation. The thermal radiation emitted and reflected by the filters modifies the net radiation at the detector surface. The ongoing efforts to model these exchanges and to use experimental results to verify the model are described. The parallel experimental effort consists of determining the sensitivity of instrument response to thermal radiation effects. In this effort, thermistors are used to characterize the thermal gradients responsible for the instrument offset. The ultimate goal of the work described is to provide reliable protocols, based on an appropriate instrument model, for correcting measured SW irradiance for variable thermal radiation effects.

ASTER is a remote sensor developed by MITI, Japan, on the platform EOS AM1, recently renamed Terra, fabricated by NASA, USA. The operation of the ASTER sensor will be jointly performed by Japan and USA. Currently, the launch of Terra is scheduled on early October at Vandenberg Launch Site, CA, USA. To keep this target date, ASTER Team is working with NASA. So far, many tests connecting the spacecraft Terra and Ground Segments including ASTER GDS have been conducted and it was shown that the interface between ASTER GDS and NASA is almost ready for Launch. The operation of ASTER will involve both Japan and US sides. Mission operation, which will accept complicated data acquisition schedule will be performed connecting US and Japan, and the test and exercises are being performed. The level 1 data processing will be done at ASTER GDS after having received the Level 0 data from NASA, either by media or by network. After the launch of Terra, ASTER Team expects to have 118 days of the Initial Checkout and the data distribution is planned after this time period.

We investigated the operation temperature dependence of the characteristics of quantum-well infrared photodetector focal plane arrays (QWIP-FPAs) for the 8 - 10 micrometer wavelength region from 65 K to 80 K. We found that a proposed simple circuit model explains the temperature dependence of the DC output and signal intensity of the QWIP-FPA. In this model, we used empirical current-voltage (I-V) characteristics of the QWIP, which was not hybridized with the readout integrated circuit (called 'QWIP itself'), measured at various temperatures and a simplified equivalent circuit model. The signal intensity of the QWIP-FPAs decreases as the temperature increases, while the photo-current of the QWIP itself increases slightly as the temperature increases. The difference between these behaviors is because the bias applied to QWIP in QWIP-FPA varies during the integration cycle and the bias applied to QWIP itself is constant. The noise equivalent temperature difference (NETD) increases from 0.10 K to 0.20 K as the operation temperature increases from 65 K to 80 K, since the signal intensity decreases and the shot noise increases with increasing the dark current.

EOS satellite instruments operating in the visible through the shortwave infrared wavelength regions (from 0.4 micrometer to 2.5 micrometer) are calibrated prior to flight for radiance response using integrating spheres at a number of instrument builder facilities. The traceability of the radiance produced by these spheres with respect to international standards is the responsibility of the instrument builder, and different calibration techniques are employed by those builders. The National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) Project Science Office, realizing the importance of preflight calibration and cross-calibration, has sponsored a number of radiometric measurement comparisons, the main purpose of which is to validate the radiometric scale assigned to the integrating spheres by the instrument builders. This paper describes the radiometric measurement comparisons, the use of stable transfer radiometers to perform the measurements, and the measurement approaches and protocols used to validate integrating sphere radiances. Stable transfer radiometers from the National Institute of Standards and Technology, the University of Arizona Optical Sciences Center Remote Sensing Group, NASA's Goddard Space Flight Center, and the National Research Laboratory of Metrology in Japan, have participated in these comparisons. The approaches used in the comparisons include the measurement of multiple integrating sphere lamp levels, repeat measurements of select lamp levels, the use of the stable radiometers as external sphere monitors, and the rapid reporting of measurement results. Results from several comparisons are presented. The absolute radiometric calibration standard uncertainties required by the EOS satellite instruments are typically in the plus or minus 3% to plus or minus 5% range. Preliminary results reported during eleven radiometric measurement comparisons held between February 1995 and May 1998 have shown the radiance of integrating spheres agreed to within plus or minus 2.5% from the average at blue wavelengths and to within plus or minus 1.7% from the average at red and near infrared wavelengths. This level of agreement lends confidence in the use of the transfer radiometers in validating the radiance scales assigned by EOS instrument calibration facilities to their integrating sphere sources.

The Moon is the only object that is accessible to Earth- orbiting imaging systems, whose brightness is within the dynamic range of most such systems, and whose spectral radiance is potentially knowable to a fraction of a percent. As such, it is a desirable target for radiometric calibration. Several spacecraft teams have begun using or are planning to use lunar observations as part of their calibration process. We examine the data reduction steps that can be used to extract the lunar irradiance from low resolution images of the Moon and qualitatively assess the attendant uncertainties. Images of the Moon provide a precise measure of scattered- light sensitivity. The response integrated over an image is compared to a lunar irradiance model being developed from terrestrial multi-band photometric observations over the 350 - 2500 nm range. For SeaWiFS images, wherein the Moon is about 6 by 20 pixels, the uncertainty in extracting the total lunar signal from the image is about 1% for most bands. A significant source of uncertainty is knowledge of the spacecraft inertial pitch rate, which is currently derived form image analysis. The very low noise in some of the detectors limits knowledge of the zero radiance level to 1/2 of a Data Number. A program is underway to accurately determine at several wavelengths the brightness variations of the Moon associated with Sun-Moon-observer Geometry. Comparisons with Earth-based lunar radiometric observations for relative responsivity (changes of spacecraft instrument gain with time) are consistent to about 1/2 the formal uncertainty. At present, the largest errors in using these data for absolute radiometric calibration are in the lunar radiance model.

The Visible and Infrared Scanner on the Tropical Rainfall Measuring Mission (TRMM/VIRS) is whiskbroom imaging radiometer with two reflected solar bands and three emissive infrared bands. All five detectors are on a single cooled focal plane. This configuration necessitated the use of a paddlewheel scan mirror to avoid the effects of focal plane rotation that arise when using a scan mirror that is inclined to its axis of rotation. System radiometric requirements led to the need for protected silver as the mirror surface. Unfortunately, the SiOx coatings currently used to protect silver from oxidation introduce a change in reflectance with angle of incidence (AOI). This AOI dependence results in a modulation of system level response with scan angle. Measurement of system response vs. scan angle (RVS) was not difficult for the VIRS reflected solar bands, but attaining the required accuracy for the IR bands in the laboratory was not possible without a large vacuum chamber and a considerable amount of custom designed testing apparatus. Therefore, the decision was made to conduct the measurement on-orbit. On three separate occasions, the TRMM spacecraft was rotated about its pitch axis and, after the nadir view passed over the Earth's limb, the VIRS performed several thousand scans while viewing deep space. The resulting data has been analyzed and the RVS curves generated for the three IR bands are being used in the VIRS radiometric calibration algorithm. This, to our knowledge, the first time this measurement has been made on-orbit. Similar measurements are planned for the EOS-AM and EOS-PM MODIS sensors and are being considered for several systems under development. The VIRS on-orbit results will be compared to VIRS and MODIS system level laboratory measurements, MODIS scan mirror witness sample measurements and modeled data.

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) was launched on 1 August 1997, and the first Earth images were taken on 4 September 1997. Regular, daily measurements of the sun, via the onboard diffuser, started on 9 September 1997 and regular, monthly measurements of the moon on November 14, 1997. These lunar measurements, as first reported at EUROPTO'98, provide a highly sensitive method for determining the change in the radiometric sensitivity of SeaWiFS. The prelaunch radiometric calibration used by SeaWiFS was performed in the Spring of 1997 at the spacecraft manufacturer's facility. The calibration measurements were made by a team from the National Institute of Standards and Technology (NIST) and the SeaWiFS Project. The uncertainties in this calibration range from 2% to 3% for the eight SeaWiFS bands. In addition, a set of outdoor measurements of the sun were made at the instrument manufacturer's facility in November 1993, just before the delivery of SeaWiFS to the spacecraft manufacturer. These solar measurements, using the instrument's diffuser, were combined with a separate set of solar radiometer measurements to determine the transmittance of the atmosphere. At the start of on-orbit measurements by SeaWiFS, solar measurements were made again by the instrument. These two sets of measurements make up the transfer-to-orbit experiment. From the ground measurements, the outputs of the SeaWiFS bands on orbit were predicted. For each band, the output from the initial on-orbit measurements agree with the predicted values by 21/2% or less. The uncertainties for the transfer-to-orbit experiment are estimated to be approximately 3% to 4%. From 14 November 1997 to 29 June 1999, SeaWiFS has made 20 measurements of the moon. The analysis of lunar measurements presented here has minor modifications to that presented at EUROPTO'98. The trend lines from the current analysis have been extrapolated back from 14 November 1997 to 4 September 1997 to describe the changes in the radiometric sensitivity over the first 662 days of Earth measurements. The uncertainties in these trend lines are approximately 1%. From these sources, we estimate the overall uncertainty in the SeaWiFS radiances to be about 4%. Based on the lessons learned from the ocean color program that preceded SeaWiFS, the SeaWiFS Program uses a buoy near the Hawaiian Islands to provide 'sea truth' for SeaWiFS. The buoy provides measurements of the spectral radiances leaving the ocean surface. These measurements are compared with those from the 'instrument/atmospheric algorithm system' for SeaWiFS, since an atmospheric model is used to link the spectral radiances at the top of the atmosphere to those at the ocean surface. Using MOBY, the vicarious calibration of SeaWiFS has provided corrections of 3.2% or less to the laboratory calibration coefficients for the instrument. These corrections are applied to the bands in the visible portion of the spectrum. MOBY does not provide a vicarious calibration for the bands in the near infrared.

Optical instruments are normally calibrated with incandescent irradiance or radiance sources. Recently, accurate calibrations using solar radiation have been demonstrated in the visible and near-IR regions (VNIR). The solar-radiation based calibration (SRBC) has major advantage in that the calibration source is the same source used on-orbit by earth- viewing remote sensing sensors such as the ASTER, MODIS, and Landsat 7 ETM+ sensors. In this paper, such a radiometer calibration covering the region between 740 and 2400 nm is presented and compared with lamp-based laboratory calibrations. This work extends the spectral range over which a calibration using solar-radiation has been made.

Ground-reference techniques for the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 are described. The techniques are similar to those used for many years for Landsat-5 Thematic Mapper (TM). Recent results with the Landsat-5 TM are presented, including comparisons with hyperspectral, airborne imaging data. These results show that the Landsat sensor has remained stable within the 5% uncertainty of the ground- reference methods for the last five years. The airborne imagery is also used to show uncertainties due to registration errors, spectral differences, and spatial resolution differences in cross-comparison techniques planned for Earth Observing System sensors. In addition to the use of the traditional methods and test sites, a smaller test site local to the University of Arizona area is being evaluated for use with ETM+. This site, while not as bright, spatially- uniform and large as typical sites, allows more frequent calibrations and hopefully a better understanding of the calibration as a function of time. The selection of the test site, its properties, and example results of calibration of Landsat-5 TM are presented.

The Remote Sensing Group of the Optical Science Center at the University of Arizona has developed a four-band, multi- spectral, wide-angle, imaging radiometer for the retrieval of the bi-directional reflectance distribution function (BRDF) for vicarious calibration applications. The system consists of a fisheye lens with four interference filters centered at 470 nm, 575 nm, 660 nm, and 835 nm for spectral selection and an astronomical grade 1024 X 1024-pixel, silicon CCD array. Data taken by the system fit in the array as a nominally 0.2 degree per pixel image. This imaging radiometer system has been used in support of the calibration of Landsat-5 and SPOT- satellite sensors. This paper presents the results of laboratory characterization of the system to determine linearity of the detector, point spread function (PSF) and polarization effects. The linearity study was done on detector array without the lens, using a spherical-integrating source with a 1.5-mm aperture. This aperture simulates a point source for distances larger than 60 cm. Data were collected as both a function of exposure time and distance from the source. The results of these measurements indicate that each detector of the array is linear to better than 0.5%. Assuming a quadratic response improves this fit to better than 0.1% over 88% of the upper end of the detector's dynamic range. The point spread function (PSF) of the lens system was measured using the sphere source and aperture with the full camera system operated at a distance of 700 mm from the source, thus the aperture subtends less than the field of view of one pixel. The PSF was measured for several field angles and the signal level was found to fall to less than 1% of the peak signal within 1.5-degrees (10 pixels) for the on-axis case. The effect of this PSF on the retrieval of modeled BRDFs is shown to be less than 0.2% out to view angles of 70 degrees. The final test presented is one to assess the polarization effects of the lens system by illuminating the camera system with the same spherical-integrating source with a 50-mm aperture with a linear polarizing filter. The degree of polarization of the system is shown to be negligible for on-axis imaging but to have up to a 20% effect for field angles of 70 degrees. The effect of the system polarization on the retrieval of modeled BRDFs is shown to be up to 3% for field angles of 70 degrees off nadir and solar zenith angle of 70 degrees. Polarization response is therefore found to be the greatest source of error in the system. A method to account for polarization effects in digital camera imagery is proposed.

The land surface for vicarious calibration, e.g. playa, desert sand and snowfield, seemed to be similar to lambertian surface, and the bidirectional reflectance factors were then assumed to be lambertian in may cases of vicarious calibration even it if actually showed the departure from lambertian behavior. Recently, the measurement of bidirectional reflectance distribution function (BRDF) of ground surface becomes important according to the improvement of the vicarious calibration method that leads to more precision coefficients. The scale of in-situ measured BRDF is very different from satellite based BRDF, which generally causes the problem of spatial instability in the in-situ measurement. To estimate the satellite based BRDF, it is better to measure the BRDF at the many points in the ground site. However, most of the previous instruments can measure only a few points at the overpass time of satellite, because the instruments are heavy and need a long time to measure the BRDF. In this study, new BRDF measurement system that has a potential to correct spatial instability using the spectroradiometer and CCD digital camera is proposed, and its capacity and limit are discussed using atmospheric and land surface radiative transfer models.

NASA's first Earth Observing System (EOS) satellite, Terra (formerly known as EOS AM-1), is scheduled for launch in the fall of 1999. This launch will begin a comprehensive monitoring program of solar radiation, the atmosphere, the oceans, and the Earth's continents from a single space-based platform. Specific scientific objectives of Terra include providing the first state distribution of the main Earth- atmosphere coupled parameters; improving our ability to detect human impacts on climate and predicting climate change; providing observations for improving forecasts of the timing and geographical extent of transient climatic anomalies; improving seasonal and interannual predictions; developing technologies for disaster prediction, characterization, and risk reduction from wild-fires, volcanoes, floods, and droughts; and starting long-term monitoring of the change in global climate and environmental change. These objectives are supported by data from five scientific instruments: the Advanced Spaceborne Thermal Emission Radiometer (ASTER), the Clouds and Earth's Radiant Energy System (CERES), the Multi- angle Imaging SpectroRadiometer (MISR), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Measurements of Pollution in the Troposphere (MOPITT) instrument. The raw instrument data will be archived and distributed to the scientific community after capture on the ground and processing to generate scientific data products. The nature of these science data products and their relevance to Earth science will be discussed along with Terra's current status. Terra is managed by Goddard Space Flight Center.

An Advanced Land Imager (ALI) will be flown on the first Earth Observing mission (EO-1) under NASA's New Millennium Program (NMP). The ALI contains a number of key NMP technologies. These include a 15 degree wide field-of-view, push-broom instrument architecture with a 12.5 cm aperture diameter, compact multispectral detector arrays, non-cryogenic HgCdTe for the short wave infrared bands, silicon carbide optics, and a multi-level solar calibration technique. The focal plane contains multispectral and panchromatic (MS/Pan) detector arrays with a total of 10 spectral bands spanning the 0.4 to 2.5 micrometer wavelength region. Seven of these correspond to the heritage Landsat bands. The instantaneous fields of view of the detectors are 14.2 (mu) rad for the Pan band and 42.6 (mu) rad for the MS bands. The partially populated focal plane provides a 3 degree cross-track coverage corresponding to 37 km on the ground. The focal plane temperature is maintained at 220 K by means of a passive radiator. The instrument environmental and performance testing has been completed. Preliminary data analysis indicates excellent performance. This paper presents an overview of the instrument design, the calibration strategy, and results of the pre-flight performance measurements. It also discusses the potential impact of ALI technologies to future Landsat-like instruments.

The Atmospheric Infrared Sounder (AIRS) has been developed for the NASA Earth Observing System (EOS) program for a scheduled launch on the EOS PM-1 spacecraft in December 2000. AIRS, working in concert with complementary microwave instrumentation on EOS PM-1, is designed to provide both new and more accurate data about the atmosphere, land and oceans for application to climate studies and weather prediction. Among the important parameters to be derived from AIRS observations are atmospheric temperature profiles with an average accuracy of 1 K in 1 kilometer (km) layers in the troposphere, humidity profiles to 10% accuracy and surface temperatures with an average accuracy of 0.5 K. The AIRS measurement technique is based on passive IR remote sensing using a precisely calibrated grating spectrometer operating in the 3.7 - 15.4 micrometer region. The instrument concept uses a passively cooled array spectrometer approach in combination with advanced state of the art focal plan and cryogenic refrigerator technology to achieve high performance in a practical long life configuration. The AIRS instrument has successfully completed a comprehensive performance verification program conducted at the Lockheed Martin IR Imaging Systems (LMIRIS) AIRS Test and Calibration Facility (ATCF), which was specially designed for precise spectroradiometric testing of space instrumentation. This paper provides a brief overview of the AIRS mission and instrument design, ATCF test capabilities, along with key results.

AIRS, the Atmospheric Infrared Sounder on the Earth Observing System (EOS) PM spacecraft, is an infrared radiometer which covers the 3.7 - 15.4 micron spectral range with spectral resolving power better than 1000. Performance of the AIRS flight unit will be discussed based on measurements in a thermal vacuum test and calibration facility. Simulated data, based on measured instrument performance and GCM model data, indicate that AIRS, together with the AMSU and HSB microwave radiometers on EOS PM, will achieve retrieval accuracy better than 1K rms in the lower troposphere under clear and partly cloudy conditions. Launch of AIRS on the EOS PM is scheduled for December 2000.

GLI (Global Imager) is a 36 channel visible and infrared radiometer/imager onboard the NASDA/ADEOS-II satellite. The information carried by GLI for the earth-atmosphere system is huge and difficult to be extracted enough efficiently with an operational satellite data analyzing system. We discuss and overview the algorithm development of the GLI Level-2 products at NASDA/EORC. We have several innovations to make the system unique and efficient. GLI simulator and GLI synthetic data sets are among those, which will be useful even for the science and engineering communities of other earth observation satellite systems. We will also overview the current status of the entire GLI project.

Global Change Observation Mission (GCOM) is a follow on mission of ADEOS and ADEOS2. It is under phase A study in NASDA (National Space Development Agency of Japan). GCOM is not satellites but a mission and its concept is to continuously monitor geophysical parameters which are critical to understand global change phenomena, especially phenomena related to climate change and ozone depletion. The first generation of GCOM is now composed of 2 satellites, i.e. GCOM- A1 and GCOM-B1. The target of GCOM-A1 is to monitor greenhouse gases distribution and ozone as well as ozone related constituents from oblique orbit. It is now planned to carry two core instruments, i.e. ILAS2 F/O and ODUS. ILAS2 F/O is a sun occultation sensor using a Fourier transform spectrometer and measures vertical distribution of atmospheric constituents. ODUS is an ultraviolet to visible grating spectrometer and measures total ozone and aerosols. The target of GCOM-B1 is to measure geophysical parameters which are uncertain in today's climate models. Those parameters include, but not limited to, optical thickness of aerosols and clouds, thermal fluxes, carbon fluxes, sink and source of greenhouse gases, etc. GCOM-B1 will carry four core instruments, i.e. SGLI (GLI follow on), AMSR2 (AMSR follow on), alpha-Scat (SeaWinds follow on), and APOLDER (POLDER follow on). Another candidate instrument is ATRAS (IMG follow on). The orbit of GCOM-B1 will be a sun synchronous orbit, which is almost the same as ADEOS2.GCOM-A1 is planned to be launched in Feb. 2005 while GCOM-B1 is planned to be launched in Aug. 2005.

The Ozone Dynamics Ultraviolet Spectrometer (ODUS) is a satellite-borne, nadir-looking ultraviolet spectrometer for measuring total ozone amount. It will be launched in 2005 onboard Japanese earth observation satellite GCOM-A1 (GCOM: Global Change Observation Mission) satellite, which was formerly called Advanced Earth Observation Satellite-3A, ADEOS-3A. The ODUS instrument measures continuous spectrum from 306 to 420 nm with 0.5 nm spectral resolution and 20 km spatial resolution, using an Ebert-type specyrograph and a one-dimensional silicon array detector, which will improve the accuracy of the retrieved total ozone amount. This paper presents an overview of the GCOM-A1 and ODUS instrument, the summary of the evaluation results of the laboratory models.

As part of the ADEOS CAL/VAL program, the authors have studied how to accurately estimate the system spatial resolution characteristics of the Advanced Visible and Near-Infrared Radiometer (AVNIR) in Earth orbit. This report summarizes our study effort for estimating system PSF/MTF characteristics of the AVNIR sensor after launch. Scene structures of a sharp knife-edge with step targets were used with estimation experiments to predict the resolution of the AVNIR sensor. Finding optimal candidate target sites from a display of profiles for the gray level from Earth observation images taken by the AVNIR sensor is very difficult, error-prone and tedious work. Using a newly developed automatic target detection method, we looked for many near-optimal sharp knife edges with step targets in AVNIR operational images. In the data analysis experiments, two estimation methods, the frequency domain method (Fourier transform techniques) and the spatial domain method (spatial convolution techniques), were applied to subscenes of the AVNIR imagery from target sites selected above. The effect of atmospheric degradation was also investigated using atmospheric corrected observation data of the same target sites. The computational results of these methods agree relatively well. AVNIR's estimated spatial resolution for the atmospheric compensation data cases seemed reasonable with respect to design and prelaunch parameters.

TOMS has been the main satellite instrument for measuring the global distribution of the total atmospheric column of ozone since the first one was launched in 1978. The fifth instrument's launch is planned for August 2000. A key scientific objective of the TOMS mission is to monitor the trend of total global ozone, which requires the ability to detect a 1% change in ozone over a decade. This, in turn, requires high calibration accuracy and long-term stability in the TOMS ratio measurements between the solar spectral irradiance and the Earth spectral radiance. The calibration process requires not only knowledge of the radiometric response of the instrument, but also of various instrument characteristics to convert the instrument output to the value of the physical observable being measured. This is due to the fact that the object sources in measurements may have different characteristics from those of the radiometric standards, e.g., intensity, polarization, and spectral distribution; the process of calibration requires a complete set of instrument characteristics, e.g., linearity, spectral bandwidth, and straylight response, to compensate for the difference between the standards and the source being measured. This paper describes methodologies of the TOMS FM-5 prelaunch tests that are relevant to calibration.

Long term orbit missions have the general problem of checking the instrument parameters in order to provide data with a high and equal reliability during the whole mission. Different methods of end-to-end or only partly recalibrations have been used in the past. The behavior of the first pushbroom imaging spectrometer in orbit, the Modular Optoelectronic Scanner MOS, has been observed during 3 years operation in orbit. Two procedures are used to check periodically some parameters: internal calibration with 2 minilamps in each module and external calibration with the sun irradiance onto a white diffuser in front of the entrance optic. The results show different developments of the parameters in the modules: increase of the dark current has not stopped up to now; the sensitivity of MOS-A channels remains constant in the frame of plus or minus 0.5%, while the MOS-B channels reach a constant level after a long period of increasing now. No wavelength shift could be observed. The PRNU of the CCD lines remains inside a plus or minus 1% interval. Spectral variations in the channel sensitivity of MOS-B can be observed and corrected with an uncertainty of plus or minus 0.5%. In general, the in- flight calibration methods allow to characterize the instrument parameters during the mission with an uncertainty of plus or minus 0.5%.

The Spectro-Radiometric Calibration Assembly (SRCA) was calibrated prelaunch in thermal vacuum against a 100-cm spherical integration source for all 20 MODIS solar reflective bands. Two methods of tracking the SRCA radiance change from ground to orbit are addressed. The broadband radiance of the SRCA will be held constant by a temperature-controlled SiPD feedback signal. While in spectral mode a reference SiPD located in the optical path of the SRCA measures the spectral profile of the radiance. The ratio of the reference SiPD signals from two different calibrations indicates a spectral profile change of the SRCA source and, therefore, predicts the SRCA band radiance changes. The second method is based upon monitoring the SRCA lamp resistance, which is non-linearly related to the lamp temperature. This provides a prediction of the SRCA spectral component so that the SRCA radiance variation from the previous calibration will be known. This paper emphasizes prediction of band radiance change. It demonstrates that both approaches track the SIS(100) calibration to within 2% and the inter-comparison of the two methods is within 2%. The success of the tracking will provide a physical transfer from pre-launch to on-orbit.

AATSR is the latest in a series of instruments designed to measure global sea-surface-temperatures to an accuracy of 0.3 K and to monitor global vegetation coverage and cloud properties. It forms part of the payload on ESA's ENVISAT mission due to be launched in 2000. Features new to AATSR include a different cooler system, an improved mechanical structure, and a corrected visible calibration system, VISCAL. The methods and results of the instrument's pre-launch calibration tests are described. These include the field-of- view, visible and infrared radiometric calibrations. The radiometric responses of the visible/near infrared channels were measured, and the in-flight VISCAL unit was calibrated. The calibrations of the thermal infrared channels were verified over a range of target temperatures between 210 K to 315 K and corrections derived for detector non-linearity. Tests were also performed to verify freedom from any significant scan dependent variations or effects due to changes in the thermal environment. Data from the initial calibration run identified a major fault with the instrument's optical alignment, and was vital in establishing the solution for the eventual repair. In addition, the calibration of the visible channels revealed important characteristics affecting the accuracy of the scientific products that would otherwise have been overlooked.

The Clouds and Earth's Radiant Energy System (CERES thermistor bolometers were calibrated using filtered radiances, characterized on an International Temperature Scale of 1990 (ITS-90) derived absolute radiometric scale. Longwave filtered radiances were characterized using the optical and geometric surface properties of the reference Narrow-Field-of-View Blackbody (NFBB), the NFBB temperature measurements from the ITS-90 calibrated platinum resistance thermometers (PRT) embedded in the blackbodies, and the spectral responses of the CERES bolometers. Shortwave filtered radiances were characterized using the cryogenically-cooled Transfer Active Cavity Radiometer (TACR) which was an ITS-90 transfer standard, and using the spectral responses of the bolometers. In ground vacuum facilities, the ITS-90, temperature-based radiometric scale was transferred to the CERES bolometers. As ITS-90 transfer standards, the bolometers were used to characterize the emitted filtered radiances from in-flight systems: (1) the internal calibration module (ICM) which consisted of anodized aluminum blackbodies and tungsten lamp sources; and (2) mirror attenuator mosaic (MAM) which was an aluminum solar diffuser plate, built into the bolometer instrumentation. From the ground [October 1995] through the on-orbit phases [December 1998 - July 1999] of the Tropical Rainfall Measuring Mission (TRMM) Spacecraft CERES instrument mission, the stabilities of the bolometer's responses were assessed from periodical observations of the in-flight calibration systems radiances. Each CERES instrument package consisted of broadband shortwave [0.3 micrometer to 5.0 micrometer], broadband total [0.3 micrometer to greater than 100 micrometer], and narrowband window [8 micrometer and 12 micrometer], scanning thermistor bolometer sensor units; and of in-flight calibration systems. Between the ground and initial on-orbit calibrations, the TRMM CERES bolometers and the built-in, flight calibration system sources maintained their filtered radiance measurement ties to ITS-90 at the plus or minus 0.2 Wm^-2sr^-1 precision level. On-orbit calibration studies indicate that the radiance measurements were stable at the plus or minus 0.2 Wm^-2sr^-1 precision level. The ground and on- orbit calibration results are presented and discussed.

The Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has a worldwide, ongoing ocean color data collection program, plus an operational data processing and analysis capability, SIMBIOS data collection takes place via the SIMBIOS Science Team and the Aerosol Robotic Network (AERONET). In addition, SIMBIOS has a calibration and product validation component. The primary purpose of these calibration and product validation activities are to (1) reduce measurement error by identifying and characterizing true error sources such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternative methods when possible, and provides the results to the appropriate project office, e.g., Centre National D'Etudes Spatialle (CNES) and National Space Development Agency of Japan (NASDA) for Polarization and Directionality of the Earth's Reflectance (POLDER) and Ocean Color and Temperature Sensor (OCTS), respectively. In the near future the Project is looking forward to collaborate with Global Imager (GLI), Ocean Color Imager (OCI) and international entities such as the International Ocean-Colour Coordinating Group (IOCCG) and Space Application Institute (Joint Research Center).

Comparisons of a new class of ultra stable ion-assisted- deposition (IAD) narrow band interference filters fabricated from thin films of refractory metal oxides and SiO₂ have produced 'hard' filters which are radiometrically stable under conditions of extreme environmental stress such as high temperature, humidity, and space. The new IAD interference filter technology can be used to derive detector-based radiometric scales with significantly smaller uncertainties than a source-based radiometric calibration scale. For the ultraviolet, 250 to 400 nm, a calibration transfer spectroradiometer consisting of a small tandem Ebert-Fastie double monochromator and a multifilter spectroradiometer consisting of a series of narrow band IAD filters combined with a NIST quantum efficiency silicon photodiode for the ultraviolet should be capable of defining a detector-based radiometric calibration scale with uncertainties less than 1.0%. The double monochromator and the filter calibration transfer standard spectroradiometer have been used to investigate the radiometric uncertainty and repeatability of three radiance sources; a FEL lamp-diffuser, a xenon arc- diffuser, and the aperture of an internally illuminated sphere.

MHS is an atmospheric humidity sounder providing humidity profile sounding capability in the 89 - 190 GHz range. It is a five channels of self-calibrating microwave radiometer. Together with the complementary instrument AMSU-A, MHS provides the operational microwave sounding capability for the European METOP series and American NOAA-N onwards- meteorological satellites. The MHS instrument is being developed under contract to EUMETSAT. The present paper focuses on the electromagnetic design and development of the 22-cm diameter rotating antenna of the MHS instrument operating at 89, 157, 183 and 190 GHz. The primary feed network so-called Quasi-Optical Network (QON) is an essential part of the overall antenna. It has to provide not only the low-los frequency separation of the four bands but also the proper antenna illumination in amplitude and phase in order to satisfy the required resolution and high beam efficiency. This equipment has been undertaken by the MHS Receiver Team in Toulouse (Matra Marconi Space-France) using state of the art technologies. After a presentation of the antenna concept, the Quasi-Optical Network design is detailed and the electrical measurements results are presented. The antenna radiation pattern measurements at instrument level are used to derive the overall antenna electrical performance.

In the framework of the spaceborne Microwave Humidity Sounder (MHS) project contracted to Matra Marconi Space (MMS) by the Eumetsat organization, very low noise Down Converters were developed in the millimeter wavelengths at 89 GHz, 157 GHz (atmospheric windows) and around 183 GHz (water vapor absorption bands). The achieved noise performance is excellent leading to a significant improvement on the Instrument radiometric sensitivity when compared to the first generation instrument AMSU-B: the noise figures measured over the 0 - 40 degree Celsius temperature range are better than 7 dB for the 157, 183 GHz channels and better than 4.5 dB for the 89 GHz channel. Schottky diode mixers have been retained, as providing nowadays the most proven and efficient solution. At 89 GHz a single balanced configuration with the so-called crossbar solution was selected, whereas in V-band the Sub Harmonically Pumped mixer type was used. In both cases Local Oscillators in W-band were necessary. The sources were constructed around Ka band Dielectric Resonator Oscillators using Whispering Gallery Modes. The four sets of Flight Models have presently been delivered, and represent the state-of-the- art in the field of spaceborne Microwave Remote Sensing built at a Space qualified standard for a continuous meteorological service.

A new era in commercial remote sensing from satellites is beginning, with the emergence of high-resolution cameras that approach the capabilities of aerial photography. The first satellite of the EROS constellation will be launched in a few months and will provide panchromatic images of the Earth at a resolution of 1.8 m. Subsequent units will follow with one meter class panchromatic systems and 3.2 m multi-spectral channels. The constellation will allow high revisit rates and large data collection capacity over most of the Earth. The paper will describe the payloads planned for the series with emphasis on the technological features of the cameras.

For many years, the United States Air Force Research Laboratory (AFRL) has developed algorithms and researched methods for optical tracking and imaging space objects. This effort has been partly limited by the lack of a calibrated on- orbit 'proof' object that can be used to reliably compare predictions to observations. In 1996, AFRL scientists began discussing this problem with the Scientific Research Institute for Precision Device Engineering of the Space Device Engineering Corporation (SDEC), Moscow, Russia. SDEC's own research in this area has been similarly limited. As a result of these discussions, and as a spin-off from related research conducted under AFRL contract, SDEC has constructed a small instrument that can fulfill the role of a non-orbit proof instrument. This free-flying passive satellite, named REFLECTOR, is designed using 32 corner cube retro-reflectors on a simple aluminum frame to ensure reliable return when illuminated from any angle. It is approximately 2 m high and 1 m wide at the base with a mass of only 6 kg. The REFLECTOR satellite has been built and is scheduled for launch as a secondary payload in December 1999. Once deployed, into its near sun-synchronous orbit, it will be observable from any location on Earth. It will be possible to passively acquire and track the satellite (using reflected sunlight) with a telescope as small as 10 cm in diameter. Because the retro- reflectors on the satellite return a large signal, laser tracking and imaging experiments can be done from the ground using small, laboratory-sized lasers. REFLECTOR will provide a 'proof instrument' that will allow the U.S. Air Force and others to test various atmospheric correction techniques.