The payload of the EOS AM-1 spacecraft consists of five scientific instruments: the Advanced Spaceborne Thermal Emission Radiometer (ASTER), the Clouds and the Earth 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 EOS AM-1 instruments will collect data on the physical and radiative properties of clouds (ASTER, CERES, MISR, MODIS); on air-land and air-sea exchanges of energy, carbon, and water (ASTER, MISR, MODIS); and on vertical profiles of greenhouse gases (MOPITT). The EOS AM-1 spacecraft and its instruments are fully integrated and launch is expected in mid-1999.

Fourier Transform Spectrometer (FTS) techniques are being used to characterize the spectral performance of the Clouds and the Earth's Radiant Energy System (CERES) Flight Models. Each Flight Model contains three scanning thermistor bolometer radiometric channels. These channels measure the broadband filtered radiances in the shortwave (0.3 - 5.0 micron), total (0.3 - greater than 100 micron) and 8 - 12 micron water vapor window regions. Accurate characterization of the end-to-end spectral performance is necessary to both determine the filtered radiances from standard sources during radiometric calibration, and to unfilter on-orbit radiance measurements. In order to meet the high calibration requirements deemed necessary by the CERES Science Team, a new technique which measures the broad-band end-to-end spectral response of the CERES sensors with high relative accuracy was necessary.

The ASTER system is a multispectral imager which covers a spectral range from visible to thermal infrared light by combining three subsystems composed of four telescopes. To ensure the high-quality data products concerning to the geolocation and band-to-band matching performance, the geometric registration is needed. This paper describes the geometric validation procedure for a multi-telescope imager with a cross-track pointing function. The strategy for the maintenance of database files and the preparation a GCP library is also shown.

ASTER is a multispectral imager which covers wide spectral region from visible to thermal infrared with 14 spectral bands, and will fly on EOS-AM1 in 1999. To meet this wide spectral coverage, ASTER has three optical sensing subsystems (multi-telescope system), VNIR, SWIR and TIR. This multi- telescope configuration requires highly refined ground processing for the generation of Level-1 data products that are radiometrically calibrated and geometrically corrected. A prototype Level-1 processing software system is developed to satisfy these requirements. System design concept adopted includes; (1) 'Automatic Processing,' (2)'ALL-IN-ONE-CONCEPT' in which the processing is carried out using information included in Level-0 data product only, (3) 'MODULE INDEPENDENCE' in which only process control module independently control other modules to change any operational conditions. (4) 'FLEXIBILITY' in which important operation parameters are set from an external component to make the processing condition change easier. The adaptability and the performance of the developed software system are evaluated using simulation data.

ASTER is an imaging remote sensor developed by MITI, Japan, on the platform EOS-AM1 fabricated by NASA, USA. The operation of the ASTER sensor will be jointly performed by Japan and USA. The ASTER Ground Data System (ASTER GDS) will be responsible for keeping track of the sensor status, sending data acquisition schedule and for processing ASTER Level 1 and higher level data. The preliminary design concept was discussed in the literature. The ASTER GDS seems to have several challenging aspects such as: (1) The ASTER GDS handles huge data volume. (2) The ASTER GDS accepts flexible mission operation, including cloud prediction. (3) The ASTER GDS makes a correction for the Band-to-Band misregistration, including inter-telescope registration. (4) User Interface including data search by browse data which will be shown to user electronically, and data acquisition request and data processing request. According to the Data Exchange Principle, the ASTER Data will be delivered at the lowest possible price. Therefore, user community of remote sensing is expected to widely spread and to have a chance to familiarize the remote sensing data. (5) ASTER GDS has a Direct Receiving station which is capable to receive and process high rate data downlinked by X-band, in real time. In this paper, challenging aspects (1) and (2) of the ASTER GDS will be discussed in detail. The issue relating to (3) was discussed in the reference.

The EOS Chemistry mission is the third of the 'flagship' missions of NASA's Earth Sciences Enterprise, following EOS AM and EOS PM. Design work on the Chemistry spacecraft has started under NASA contract NAS5-32954. This spacecraft is the second of the EOS Common Spacecraft, EOS PM being the first one. The design has recently been successfully completed with spacecraft CDR this June, and it is now in the fabrication phase for the PM mission. This design of the spacecraft will be summarized. The Chemistry mission will carry four new scientific instruments. These instruments place some new requirements on the spacecraft to accommodate them, but these are satisfied with relatively small 'kits' that are added to the Common Spacecraft design. This design of the Chemistry configuration of the Common Spacecraft will be presented. We have explored the cost effectiveness of small and medium satellite architectures for hypothetical future Earth Science missions. These cost comparisons include the life cycle cost of space, launch and ground segments of each complete mission and are based on actual costs of existing missions. We find that the lowest cost approach under a wide variety of assumptions is to use a medium size spacecraft carrying a suite of instruments.

The science need for remotely sensed soil moisture has been well established in the hydrologic, climate change and weather forecasting communities. There also have been a number of programs that have demonstrated the feasibility of using long wave microwave sensors for estimating soil moisture. These have ranged from truck mounted sensors, to intensive airborne campaigns with science objectives. Based on this history of truck and aircraft experiments, the science community has settled on a soil moisture product that meets the following criteria: a two day global repeat, a measured layer of 5 cm of soil, a footprint of 20 to 30 km, and an absolute accuracy of plus or minus 4% volumetric soil moisture. The principal sensor to accomplish this is an L-band passive microwave radiometer. A soil moisture mission is being proposed for the NASA Earth Systems Science Pathfinder (ESSP) mission which has very real constraints of a limited budget which includes the launch vehicle, and a three year award to launch time schedule. Within the past few years there have been a number of mission concepts proposed that meet the challenge of getting a very large antenna in space in order to realize a spatial resolution on the ground that meets the science and applications needs. This paper describes some of the alternative concepts considered to meet these unusual requirements and the ways to solve the very large antenna challenge, and the criteria used to choose the final design for an ESSP proposal. The paper also discusses the alternatives considered to obtain the necessary ancillary data for characterizing the surface roughness, the surface temperature and the attenuation affects of vegetation.

The European Space Agency's mandate include to undertake research, development and demonstration of Earth Observation related space technologies. The MetOp satellites, which prototype is planned for 2003, will be the European contribution to the world meteorological polar satellite system, replacing the 'morning' satellites, provided by the U.S. until then. MetOp will embark a NOAA payload, plus European advanced instruments. The satellites are being developed by ESA in cooperation with EUMETSAT, which has the overall responsibility of the European Polar System EPS, which main objectives are to furnish data for operational meteorology and climate monitoring. Some instruments on MetOP will provide detailed information on the atmospheric temperature/humidity profiles, essential for weather forecasting. Other parameters will be available from the scatterometer and the global ozone instrument, continuation of the ERS series. The use of MetOp data is expected to contribute to the improvement of meteorological and other applications, plus to Earth Sciences research. Industrial activities for the manufacturing of the satellites started in February 1998, under the leadership of MMS (F).

Millimeter-wave radiometry of the earth's surface from Low Earth Orbit (LEO) with a resolution of a few km requires antenna apertures several meters across and sub-second scanning times. Fulfilling these requirements with a mechanically scanned real-aperture antenna presents formidable mechanical challenges. An attractive alternative described here is to use synthetic aperture techniques employing a sparse-array of antennas that trade the mechanical complexity of real-aperture imaging for the electrical complexity of synthetic aperture imaging. We present results of an ESA- sponsored study aimed at seeking the optimum technique for high performance synthetic aperture mm-wave radiometry from LEO.

SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) is a contribution to the ENVISAT-1 satellite, which is to be launched in spring 2000. The SCIAMACHY instrument is designed to measure sunlight transmitted, reflected and scattered by the Earth's atmosphere or surface. The instrument measures simultaneously from the UV to the NIB spectral spectral region (240 - 2380 nm). Observations are made in alternate nadir and limb viewing geometries and also for solar sunrise and lunar moonrise occultation. Inversion of the SCIAMACHY measurements will provide the following: the amount and distributions of some important trace gases O₃, BrO, OClO, ClO, SO₂, H₂CO, NO₂, CO, CO₂, CH₄, H₂O, N₂O, p, T, aerosol, and radiation flux profiles, cloud cover and cloud top height. Combination of the near simultaneous limb and nadir observations enables the tropospheric column amounts of O₃, NO₂, CO, CH₄, H₂O, N₂O, SO₂, and H₂CO to be detected. SCIAMACHY will provide new insight into the global behavior of the troposphere and the stratosphere.

Radiometric Calibration is an essential activity, both on- ground and in-flight, for the correct operations of any Radiometer System. In the frame of M.I.M.R. (Multi-Frequency Imaging Microwave Radiometer) project, Officine Galileo participated from the early phases with system activities relevant to calibration and with development of Calibration devices. In particular the breadboard of the in-flight calibrator during phase B, and the Targets for on-ground radiometric characterization of M.I.M.R. during present Demonstrator phase, were designed, manufactured and tested. This paper describes the two Targets, working at fixed cryogenic temperature (Fixed temperature Target, FT) and at temperature settable from cryogenic to ambient (Variable temperature Target, VT) that have been used for the M.I.M.R. Demonstrator test campaign. Moreover the in-flight calibrator, that was also used in this campaign, is described.

Since 1994 Alenia Aerospazio has been involved as Prime Contractor, under European Space Agency contract, in the development and testing of Multifrequency Imaging Microwave Radiometer (MIMR) Demonstrator Model. This activity is to be considered as part of a wider design and development phase, started in 1991, aimed to define MIMR instrument that was and it is still required to fulfill in a more efficient and accurate way the scientific objectives in the various radiometric application fields. Besides parallel activities aimed at a deeper 'technological' investigation on those areas considered critical for the flight instrument, the MIMR Demonstrator program has been successfully completed through the functional validation and radiometric calibration campaign performed on an integrated system sub-assembly and held in Farnborough (UK) at RSI Meteorological Office Thermal Vacuum Facility in February 1998. This paper provides a description of the MIMR Demonstrator system and the relevant test set-up, the testing approach and the processing logic are also presented. The most relevant analyses and results complete the content of this paper.

Alenia Aerospazio Remote Sensing Division started in 1986 the study of microwave radiometers under Italian Space Agency fundings, and since 1989 the definition and development of radiometric systems under European Space Agency (ESA) contracts. In particular the Multifrequency Imaging Microwave Radiometer (MIMR) and the ENVISAT Microwave Radiometer (MWR) were both developed by the European Industry, with Alenia Aerospazio as Prime Contractor. MWR is an instrument designed and developed as part of the Envisat-1 satellite scientific payload, with Alenia Spazio engaged in the phase C-D as instrument Prime Contractor, leading an industrial consortium of European and American companies. The Flight Model of the Instrument has been delivered to ESA at the end of July 1997, after completion of test and calibration activities. Given the MWR in-flight calibration concept, a specific pre-flight calibration and characterization activity was performed to define a radiometer mathematical model and a relevant ground characterization database including all model coefficients. The model and its database will be used by on-ground processing during instrument in-flight operation to retrieve the antenna-measured temperature. Standing its complexity and iterative measurement concept, the pre-flight characterization and calibration of the instrument is the key aspect of its development phase. Within this paper the key instrument design topics are summarized, and after a summary overview of the overall flight model qualification campaign, emphasis will be on the pre-flight calibration and characterization activities and radiometric performance achievements among several test phases.

ENVISAT Microwave Radiometer (MWR) is an instrument designed and developed as part of the Envisat-1 satellite scientific payload, with Alenia Aerospazio engaged in the phase C-D as instrument Prime Contractor, leading an industrial consortium of European and American companies. The Flight Model of the Instrument was delivered to ESA at the end of July 1997, after successful completion of design, test and calibration activities. An Engineering Model of the instrument was also developed and completed in March 1997. The MWR output products are of prime importance for wind/wave products of the Radar Altimeter (RA-2) Instrument, part of the Envisat-1 payload, providing correction of atmospheric propagation data. The products are also useful for direct evaluation of brightness temperature in order to characterize polar ice, land surface properties and sea surface temperature. In order to achieve the required accuracy and sensitivity performance, an in- flight two-point calibration concept is adopted, with hot and cold calibration reference points for each frequency channel. Periodically the measurements of earth scene radiation are interrupted to allow the measurement of an on-board calibration load and of the deep cold space. The overall ground calibration tasks were performed through an iterative sequence of measurement and relevant model corrections, with an extensive instrument calibration in a thermal-vacuum environment, to derive the final radiometer model coefficients and to verify its performance in the expected in-flight environment. To achieve the required instrument calibration accuracy, extremely accurate blackbody target sources were required, in order to simulate the Earth scene and the deep space (for cold calibration), as seen by the radiometer during its in-flight mission. The definition, development and characterization of such blackbody targets were key aspects to achieving the required stimulus accuracy for proper calibration of the instrument. These tasks were jointly performed by the UK Meteorological Office (UKMO) and Alenia Aerospazio, and lead to the final calibration of the instrument with successful results. Within this paper an overview of the instrument calibration will be given; emphasis will be on the trade-offs and design requirements derived for the Calibration Targets, on their design and calibration, and finally on the achieved results and instrument performance.

This paper introduces the Hypersat and Mission Demonstration Satellite programs, which are small satellite programs in the National Space Development Agency of Japan (NASDA). The goals of the programs are to establish sophisticated microelectronics and mechanics technology; to achieve low cost, high performance satellite; and to quickly respond to the variety of requests from end users. One author has an idea for a high-resolution optical imager which is small and light enough to be launched on this small satellite. This idea is presented in detail. A wide swath strip mode operation SAR design is also introduced.

This paper reports the results of the preliminary design and testing of the Panchromatic Remote Sensing Instrument for Stereo Mapping (PRISM) Bread-Board Model (BBM). PRISM is one of the major mission instruments to be installed on the Advanced Land Observing satellite (ALOS) which is expected to launch in early 2003 on a Japanese H-IIA vehicle. The PRISM system is designed to provide accurate data for making and updating maps on the 1/25,000 scale. That requires the PRISM system to be a high performance sensor with spatial resolution of 2.5 m and a wide swath of 70 km. To realize both capabilities, the optical system utilizes three-mirror, off axis anastigmatic designs with more than 28,000 detector elements which is realized by using multiple CCD senors. PRISM has three independent telescopes for stereoscopic imagery. These telescopes are arranged in order to obtain forward, nadir, and backward looking data. A set of three images is used for Digital Elevation Model (DEM) extraction. The current BBM phase of the project is underway to confirm the feasibility of the design. At the same time, the preliminary design is being used to understand the functions and characteristics of entire PRISM system.

The Phased Array type L-band Synthetic Aperture Radar (PALSAR) on the Advanced Land Observing Satellite (ALOS) is considered to be one of the follow-on sensors of JERS-1 SAR. PALSAR is designed to achieve high radiometric performance as well as observation flexibility, in addition to the data continuity of JERS-1. It has a beam steering capability using an active phased array antenna, and a multi-polarization capability. In order to achieve high radiometric performance, the system parameters have been carefully designed, and some internal calibration procedures have been investigated. Based on the current design, PALSAR can acquire the data from 8 to 60 degrees of incidence angle. A noise equivalent backscattering coefficient is from -30 to -25 dB depending on the incidence angle. The required radiometric stability is within 1 dB over one scene. The status of development is currently the Bread Board Model (BBM) phase, and NASDA has manufactured the antenna system and tested it both electronically and mechanically. This paper describes the PALSAR system design as well as some results from BBM development.

Infrared (IR) remote sensing imaging applications require high-performance Focal Plane Arrays (FPAs) operating in all ranges of the IR spectrum. Short wavelength (SWIR; 1 to 3 micrometer), middle wavelength (MWIR; 3 to 5 micrometer), mid- long wavelength (MLWIR; 6 to 8 micrometer), long wavelength (LWIR; 8 to 14 micrometer), and very long wavelength (VLWIR; greater than 14 micrometer). These diverse spectral bands require high performance detectors and Read Out Integrated Circuits (ROICs) to perform the multi-spectral mission requirements. Significant progress in the design and fabrication of HgCdTe detector arrays and Read Out Integrated Circuits (ROICs) over the past few years has led to the demonstration of high resolution, low noise and large format reliable FPAs. Hybrid FPAs have been measured and their performance parameters are presented. Focal Plane Array D* performance values have been obtained for a multitude of spectral ranges and configurations that include; (1) (lambda) _c equals 1.8 micrometer, 12 X 256 arrays operating at 295 K with median D* approximately 1.4 X 10¹² cm Hz^1/2/W, (2) (lambda) _c equals 2.4 micrometer, 12 X 256 arrays operating at 250 K with median D* equals 1.6 X 10¹² cm Hz^1/2/W, detectors used are grown by MBE on lattice matched CdZnTe, (3) PACE-1 detectors with (lambda) _c equals 2.5 micrometer, 1024 X 1024 arrays operating at 115 K with peak D* of 2.3 X 10¹³ cm Hz^1/2/W at a background flux (phi) _b equals 1.2 X 10¹¹ ph/cm²- s, (4) MBE HgCdTe on Silicon MWIR detectors have been fabricated and the detector R_oA performance for (lambda) _co approximately 5.0 micrometer is in the 10⁶ to 10⁷ ohm-cm² range at 78 K. (5) MBE HgCdTe on CdZnTe detectors, ((lambda) _c equals 15.8 micrometer at 65 K), 128 X 128 array operating at 40 K with peak D* of 2.76 X 10¹¹ cm Hz^1/2/W at a background flux (phi) _b equals 8.0 X 10¹⁵ ph/cm²-s. High performance 640 X 480 arrays imaging in the MWIR band have been fabricated on CdZnTe and PACE-1 substrates. The performance of these and additional FPAs will be presented.

Described is thermal radiation detector conceived for possible deployment on GERB (Geostationary Earth Radiation Budget). It consists of a linear array of 256 elements, each 60 micrometer square and separated by a 3-micrometer gap. Each element is the active junction of a single-junction-pair zinc- antimonide/platinum thermopile. The reference junction is mounted on an isothermal substrate, and the active junction is thermally isolated from the substrate by a thin layer of parylene. The detector is mounted on one wall of a wedge- shaped, mirrored cavity intended to increase the effective absorptivity and improve the spectral flatness of the detector through multiple reflections. A dynamic opto-electrothermal model of the detector/cavity combination has been formulated in order to facilitate its optimal design. The optical part of the model is based on a Monte-Carlo ray trace that takes into account diffraction at the entrance slit as well as the diffuse and specular components of reflectivity of the cavity surfaces. Heat absorption and diffusion through the thermopile structure has been modeled using the finite element method. The model has been used to validate a method for eliminating optical cross-talk among elements of the array through post- processing of data.

In October 1997, the TRWIS III sensor was mounted into a small airplane for the purpose of collecting hyperspectral data over a variety of scenes in Ventura County, California, a largely agricultural area about 100 km from Los Angeles. The resulting hyperspectral 384 band data was analyzed using two different approaches. The first was a physically based procedure using the ratios of spectra selected based upon ground truth. Ratios between images in different bands is a way to emphasize the spectral difference and minimize the effect of illumination. The spectral bands selected were in the vicinity of the near infrared 'red edge' chlorophyll feature. The second procedure is an image processing procedure to transpose the image cube using an orthogonal subspace projection of the hyperspectral data cube. In general, a transformation over the full spectral region of the data (from approximately 400 nanometers to 2.45 micrometers) did not give results separating the tree types as well as the physically based ratio method. However, if the spectral region was restricted to 20 to 30 bands in vicinity of the red edge feature, then similar vegetation separation was achieved. In this paper, the analysis using both procedures will be discussed and compared.

We have developed a laboratory breadboard spectropolarimetric hyperspectral imaging sensor for operation in the visible-near IR waveband to demonstrate its potential for future airborne and spaceborne systems. An existing spatially modulated imaging Fourier transform spectrometer was modified by the addition of polarization analyzer components. Images collected by the focal plane array vary spatially in one dimension and spectrally in the other. A scanning slit simulates a pushbroom scanning mode for the second spatial dimension. Polarimetric sensing is accomplished using two liquid crystal variable retarders in tandem followed by a linear polarizer. To recover the complete Stokes vector four images, each with distinct combinations of retardances, are collected for each slit position. Sample images are presented.

Previous papers have described the concept behind the MightySat II.1 program, the satellite' Fourier Transform imaging spectrometer's optical design, and the design for the hyperspectral imaging payload. Initial qualification testing of the payload has been completed. All component level qualification tests have been finished. The solid block optics, interferometer, camera and telescope where all successfully tested and a payload Critical Deign Review was passed. Early optical testing of the monolithic interferometer has shown that it has the designed spectral resolution of less than 100 cm^-1. Bench testing of a custom VME data interface board that operates the sensor in a variety of spatial and spectral resolution modes can transfer data satisfactorily at data rates up to 24.3 Mbytes/sec over a VSB bus to spacecraft solid state memory. Problems in manufacturing the hardened C-40 processors has caused a change to an unhardened version of the C-40 using tantalum foil for protection. This still allows all hyperspectral 'smart' imaging spectrometer demonstrations including a 10:1 data compression technique. The payload is scheduled to be delivered in April 1999 for integration on to the spacecraft bus.

On May 5, 1994, President Clinton made the landmark decision to merge the Nation's military and civil operational meteorological satellite systems into a single, national system capable of satisfying both civil and national security requirements for space based remotely sensed environmental data. Convergence of these programs is the most significant change in U.S. operational remote sensing since the launching of the first weather satellite in April 1960, and marks a significant departure from eight earlier attempts over the last twenty years to combine these previously separate programs. For the first time, the U.S. government is taking an integrated approach to identify and meeting the operational satellite needs of both the civil and national security communities. The joint program formed as a result of President Clinton's direction is known as the National Polar-orbiting Operational Environmental Satellite System (NPOESS), and it is expected to provide up to $650 million in government cost savings through the year 1999 and up to $1.8 billion over the life of the program.

A new, photon-sensitive, imaging array, the active pixel sensor (APS) has emerged as a competitor to the CCD imager for use in star and target trackers. The Jet Propulsion Laboratory (JPL) has undertaken a program to develop a new generation, highly integrated, APS-based, multipurpose tracker: the Programmable Intelligent Microtracker (PIM). The supporting hardware used in the PIM has been carefully selected to enhance the inherent advantages of the APS. Adequate computation power is included to perform star identification, star tracking, attitude determination, space docking, feature tracking, descent imaging for landing control, and target tracking capabilities. Its first version uses a JPL developed 256 X 256-pixel APS and an advanced 32-bit RISC microcontroller. By taking advantage of the unique features of the APS/microcontroller combination, the microtracker will achieve about an order-of-magnitude reduction in mass and power consumption compared to present state-of-the-art star trackers. It will also add the advantage of programmability to enable it to perform a variety of star, other celestial body, and target tracking tasks. The PIM is already proving the usefulness of its design concept for space applications. It is demonstrating the effectiveness of taking such an integrated approach in building a new generation of high performance, general purpose, tracking instruments to be applied to a large variety of future space missions.

In the last years two new kinds of microwave radiometers are being studied for Earth observation: aperture synthesis interferometric radiometers and polarimetric radiometers. The first ones are formed by an array of small antennas whose outputs are cross-correlated and then, properly processed to obtain a map of the apparent brightness temperature of the whole scene being imaged. One- and two-dimensional systems have been studied by some space agencies, e.g. ESTAR by NASA, and MIRAS by ESA, as a solution that avoids the implementation of large steerable antennas at low frequencies (L-band), while reaching a relatively high spatial resolution: about 20 - 30 Km. More recently preliminary studies of mm-wave systems have also been studied to improve the spatial resolution achieved by today's radiometers. On the other hand, polarimetric radiometers are formed by a dual-polarization antenna. The real and the imaginary parts of the complex cross-correlation computed from the H/V outputs leads to the third and fourth Stokes parameters of the incoming thermal radiation, which are basically related to roughness state of the surface being imaged. At present, a number of studies are being conducted to establish the relationship with the wind direction over the sea surface. The performance analysis of those systems requires the modeling of the apparent brightness temperature map of the Earth and/or sea surface that would be imaged at the microwave and the mm-wave frequencies, which is the object of this paper.

This paper presents results from the ATSR-2 atmospheric Correction EXperiment (ACEX), a collaborative campaign between CSIRO and the University of Nottingham held at two uniform land sites in Australia in April and May 1997 in which the surface-leaving radiance was measured in the three VIS/NIR bands of the ATSR-2 sensor as it passed over aboard the ERS-2 satellite. The sites, which are permanently instrumented to monitor the surface radiation budget components, and the campaign instruments, measurement procedure and analysis are described. The results agree with other calibrations of the ATSR-2 shortwave channels to within a few percent.

Solar diffuser based monitors are the preferred method for on- board calibration for short wavelength regions of Radiometric Earth Remote Sensing instruments where spectral matching and long term stability are paramount. This paper describes an aluminum integrating sphere, with internal photo-diode monitoring, being developed for the on-board short wavelength, (0.32 - 4 micrometer) calibration monitor of the GERB instrument. GERB will image the earth surface from geostationary orbit over a bandwidth of 0.32 - 30 micrometer and is mounted on the Meteosat Second Generation (MSG) spin stabilized satellite resulting in a very rapidly rotating field of view of GERB (100 RPM). The adopted arrangement for the integrating sphere is described and its performance illustrated with supporting test data and optical modeling. Comparisons with the ATSR-2, MS20 flat tile system are made and recommendations for future calibration systems, drawn.

The calibration of space borne observations is required for converting the engineering unit (digital count) into the relevant physical unit (radiance). The Meteosat spacecraft series has an on-board black body calibration mechanism, in which black bodies with known temperatures can be viewed. However, an absolute on-board calibration of the optical system is not possible, as part of the system consisting of the front optics is excluded from the calibration path. Nevertheless, on-board black body calibration will be used operationally on Meteosat-7 and may prove useful for detecting trends in the sensor sensitivity. For absolute calibration of the infrared channels, vicarious techniques are presently used, comprising the use of external reference data like sea surface temperatures for the IR channel or radiosonde observations for the WV channel. In such vicarious calibration technique the external reference data are converted into expected radiances at the top of the atmosphere using a radiative transfer model. The expected radiances are then related to the observed counts, the relation being the vicarious calibration coefficient. Black body calibrations have been performed for a short period on Meteosat-4, until a failure of the mechanism forced the black body calibrations to be suspended. During commissioning of the Meteosat-7 spacecraft black body calibrations have been resumed, and the results show a relatively good relation between the black body and the vicarious calibration. During Meteosat's eclipse seasons in autumn the temperatures of the black bodies respond directly to the diurnal variation of the ambient spacecraft temperature. Black body calibrations may be used to crosscheck or estimate the vicarious calibration. The calibration of the IR channel may be validated using satellites with an on-board calibration, like polar orbiting satellites as the NOAA series. In a preliminary study NOAA-14 is used for a cross- satellite comparison with a few orbits in 1996 and 1997. The NOAA-14 channel 4 and 5 observations were used for collocation with the Meteosat-6 IR channel. First results indicate a good agreement of the Meteosat calibration with satellite cross calibrations for both channels. The next series of Meteosat spacecraft (Meteosat Second Generation) will also have a black body calibration mechanism on board, which has an improved functionality with respect to the present system. As with the operational Meteosat spacecraft the black body mechanism excludes the front optics in its optical path, yet it is envisaged that the black body calibration can be used for absolute calibration if a correction model is applied. The present vicarious calibration techniques will be available for monitoring the operational calibration, or for assisting it.

The Modular Optoelectronic Scanner MOS was developed at the Institute of Space Sensor Technology/Berlin of the German Aerospace Center (DLR) and specially designed for observations of medium scale effects of the system surface-atmosphere. MOS consists of the two VIS/NIR imaging spectrometers MOS-A and MOS-B and the SWIR camera MOS-C. It was launched on March 21, 1996 on board the Indian Remote Sensing Satellite IRS-P3 together with the Indian Wide Field Scanner WIFS and an X-ray instrument. Two different in-orbit calibration devices are integrated into the MOS equipment: (1) the internal calibration system based on two minilamps and (2) the sun calibration based on spectralon diffusers for absolute radiometric recalibration and long-term stability check of the sensitivity. Thus it is possible to determine the actual relative calibration data with an accuracy of about 0.5%. The interpretation of the calibration data of the MOS-IRS mission in orbit for two years shows that all detector elements really are working normally. The behavior of the sensitivity of all elements of a CCD-line is nearly identical. Altogether, the sensitivity of the MOS-A channels remains constant in an interval of plus or minus 0.7%, increases by different amounts for the MOS-B channels up to 6% and decreases for MOS-C about 1%. The results of the in-orbit calibrations are the basis for a consistent interpretation of the remote sensing measurements of the environment.

Recently a new class of instruments, that uses a detector array to measure spectra for multiple points on the ground, has become available. These instruments build up an image by a pushbroom technique. The realization of the maximum potential of these Array Imaging Spectrometers is dependent on the ability to correct pixel to pixel variations in gain, bias, dark current and linearity of the detector array. Residual calibration striping in each band along the temporal axis, is usually the limiting noise source in these sensors, not photon or system electronic noise. Indeed, performance calculations or measurements which do not take this important noise source into account seriously overestimate the performance of Array Imaging Spectrometers. In this paper, the calibration requirements for Array Imaging Spectrometers, in general, and the procedure used to calibrate the Airborne Hyperspectral Imager (AHI) will be discussed. Examples from the Airborne-Hyperspectral Imager (AHI) will be used to illustrate the residual error and characterize its effect.

Measurements of the lunar surface in the visible and near infrared wavelength regions provide a new and intriguing method of determining changes in the sensitivities of Earth observing radiometers. Lunar measurements are part of the calibration strategy for the instruments in the Earth Observing System (EOS) and part of the calibration strategy for the Sea Viewing Wide Field of View Sensor (SeaWiFS). SeaWiFS was launched on August 1, 1997. The first SeaWiFS images of the Earth were taken on September 4, 1997, and the first lunar measurements were made on November 14, 1997. We describe the results from the initial nine lunar measurements by SeaWiFS, extending to July 10, 1998. The time series for the lunar images show changes in the sensitivities of SeaWiFS bands one through five (412 to 555 nm) to be very small over the eight month interval. For band 6 (670 nm), the decrease in sensitivity over seven months is 1/2%. For bands 7 and 8 (765 and 865 nm), the decreases are 11/2% and 5% respectively. These changes, with reduced scatter in the results, are also found in the band ratios. The instrument changes can be seen in the SeaWiFS data products. Using the lunar time series, plus data from diffuser and ocean surface measurements, a time-dependent correction for the changes in the sensitivities of bands 6, 7, and 8 has been applied in the SeaWiFS data processing algorithm.

The Moon is the only natural object outside the Earth's atmosphere that is within the dynamic range of most imaging instruments on Earth-orbiting spacecraft. The excellent photometric stability of the Lunar surface will allow its use as a long-term instrument calibration source once the dependence of Lunar spectral radiance on phase and libration angles are well characterized. A program to provide this characterization is underway. Observations are being made in 23 bands within 350 - 950 nm, 7 of which correspond closely with spacecraft instrument bands. Observations in nine bands within 950 - 2500 nm began recently. Although at this time the absolute Lunar radiance model is preliminary and uncertainties are larger than most instrument calibration goals, changes in spacecraft instrument sensitivity can be precisely monitored and absolute calibration can be applied retroactively as the accuracy of the Lunar spectral radiance model improves. Several space-based imaging systems have already begun using the Moon for calibration and the EOS AM-1 platform will make periodic attitude maneuvers for Lunar and space calibration.

The U.S. Geological Survey and Northern Arizona University have established a program of lunar photometry with the dedicated Robotic Lunar Observatory (ROLO). This program has the potential to calibrate any past or future visible-band image from a geostationary satellite that contains the moon. As an early application of this technique, a visible-band image of the moon has been obtained from Japan's Geostationary Meteorological Satellite (GMS-5) under viewing and illumination conditions that allow it to be directly compared with a calibrated multi-band image of the moon taken nearly simultaneously from the ground by ROLO. The results demonstrate that the lunar method can potentially calibrate the sensor to an accuracy of a few percent over a wide range of the radiances typical of cloud-free land scenes. However, the inadequate calibration and probable cloud contamination of the particular ground image used preclude an accurate absolute calibration in this case. The merits of this new technique relative to other techniques of calibrating current operational visible-band sensors are discussed and the spectral considerations peculiar to the calibration of broadband sensors are pointed out.

SPOT4, the fourth satellite of the SPOT family remote sensing satellites, was launched on the 20th of March 1998. During the first months, we calibrate the two identical on-board cameras named HRVIR (because of the added Mid Infra-Red channel) and VEGETATION, a wide field of view radiometer providing 1.15 kilometers resolution measurements in the same designed channels as HRVIR (B2, B3 and MIR), and we evaluate the quality of the images. Radiometric calibration results are presented in this paper. Different methods are applied based on the experience gained with SPOT1, 2, 3 and POLDER: (1) pre- launch measurements, (2) on-board calibration system, (3) vicarious calibration over test sites, (4) inter-SPOT calibration over desert areas, (5) calibration over the molecular scattering, (6) inter-cameras calibration between HRVIR1 and HRVIR2, (7) inter-cameras calibration between HRVIR and VEGETATION. The accuracy of each calibration procedure is estimated. The measurements are combined in a model that minimizes errors and provides the camera sensitivity as a function of time.

This paper presents a new method for receiver calibration of interferometric radiometers. It is based on the distributed noise injection mechanism and it consists of fully calibrating the baseline error terms of the central antennas (which share the same noise source) while keeping only separable error calibration for the distant ones. This improves the accuracy of the shortest baselines, which are the most significant ones due to the smoothness of the brightness temperature to measure. Simulations show that, compared to previously reported methods, the improvement on the radiometric resolution can be as high as 3.9. The robustness against frequency response mismatch between receivers is improved by a factor 2.9.

A snowfield is a candidate as a target for vicarious and cross calibrations of Advanced Spaceborne Thermal Emission and Reflection (ASTER) radiometer. In previous our studies, the uncertainties of snowfields for these calibrations were estimated, and the field experiments for a vicarious calibration over the snowfield have been carried out using Landsat TM data. These studies show the enough potential of a snowfield as a target for in-flight radiometric calibrations but also the problem for snow bidirectional reflectance distribution (BRDF) effects. The snow surface was assumed to be Lambertian in previous studies. However, the snow surface shows departure from Lambertian behavior and high reflectance, therefore the multiple scattering between atmosphere and non- Lambertian snow surface can not be ignored. In this study, snow BRDF and atmospheric radiative transfer are simulated using doubling-adding method that has a potential to calculate multiple scattering between non-Lambertian surface and atmosphere, and then the snow BRDF effects on these calibrations are estimated. The result shows that the radiative transfer code that account the multiple scattering between surface and atmosphere is necessary for the ASTER in- flight radiometric calibrations over the snowfield in the cases of the large angle solar zenith angle, large snow particle size and etc.

The SPOT4 remote sensing satellite was successfully launched at the end of March 1998. It was designed first of all to guarantee continuity of SPOT services beyond the year 2000 but also to improve the mission. Its two cameras are now called HRVIR since a short-wave infrared (SWIR) spectral band has been added. Like their predecessor HRV cameras, they provide 20-meter multispectral and 10-meter monospectral images with a 60 km swath for nadir viewing. SPOT4's first two months of life in orbit were dedicated to the evaluation of its image quality performances. During this period of time, the CNES team used specific target programming in order to compute image correction parameters and estimate the performance, at system level, of the image processing chain. After a description of SPOT4 system requirements and new features of the HRVIR cameras, this paper focuses on the performance deduced from in-flight measurements, methods used and their accuracy: MTF measurements, refocusing, absolute calibration, signal-to-noise Ratio, location, focal plane cartography, dynamic disturbances.

This paper describes designing and testing the ground-base multi-spectral sensor using off-axis three-mirror reflective optics for imaging optics, and dichroic mirrors for separation of spectral bands. The off-axis three-mirror optics provides an obstruction free FOV (Field Of View) with a wide spectral range and high spatial resolution over a wide FOV. The three- mirror anastigmat consists of two aspherical mirrors and a spherical mirror. The design is configured for a telecentric flat focal plane. The dichroic mirrors separate spectral range into three visible bands, a near infrared band, an MWIR (Middle Wavelength InfraRed) band and an LWIR (Long Wavelength InfraRed) band. Relay optics is used to adjust multiplication factors for infrareds. The 10,000 element CCD sensors are used for three visible bands and the near infrared band. The 960 element HgCdTe linear arrays are used for the MWIR and the LWIR bands. The results of experiments show that the design goals are successfully achieved. Among them are a wide spectral band from 0.4 micrometer to 10 micrometer, with spatial resolution of 10 (mu) rad over 5.7 degrees FOV for visible and near infrared bands and 100 (mu) rad over 5.7 degrees for the MWIR and the LWIR bands. The overall MTF at the center of the FOV are more than 0.3 at Nyquist frequency for all bands.

A conceptual design is given for the space borne sensor to measure the three dimensional distribution of trace gases in the troposphere. The sensor makes spectral measurements of trace gas absorption lines in the surface reflected solar radiation. Only a few absorption lines are selected by a narrow band pass filter and scanned by a tunable etalon with the resolution better than 0.1 cm^-1. The sensitivity and vertical resolution of this sensor system to the variation the trace gas amount are examined with using radiative transfer model that includes the single scattering of solar radiation by air molecules and aerosols. Some examples of measurements are shown with a ground-based sensor for water vapor vertical profile.

A great effort is actually devoted to dispose of sensors with increased spectral, spatial, and radiometric resolution, improvements that are ended to obtain quantitative and accurate information of the observed scenes. Topics dealing with the different aspects of sensor calibration are, in this framework, increasingly important. We investigated some relevant problems connected with sensor calibration: the flat- field correction, and the SNR estimate. In a past work we have shown a new algorithm devoted to off-line flat-field correction, that has been shown to correctly work on images gathered by matrix-detectors. In the same work we had also shown a novel approach to SNR evaluation. In this paper we discuss how our model for flat-field correction behaves when applied to data acquired by scanning devices. For images gathered by these sensors we developed a model which correctly predicts the appearance of a spatially-coherent pattern of disturbances, with a characteristic cross-track shape. We also show that our flat-field procedure is able to properly correct the images observed by scanning detectors. Theory and data- reduction algorithms were tested with images acquired by multispectral and hyperspectral imaging systems.

It is envisioned that large, lightweight, optical mirrors for use in space will require onboard instrumentation for measuring and controlling the gross aspects of the mirror surface figure. An optical heterodyne array is well-suited for the figure sensing portion of this operation. The basic principles of heterodyne wave front sensing are reviewed and the advantages of this approach are highlighted. We describe the development of a dedicated circuitry module for integration with a sensor array for performing heterodyne array phase measurements.

Large, lightweight optical mirrors for space applications will require systems for controlling the gross aspects of the mirror surface figure. We have developed an autonomously operating wavefront measurement system based on dual- wavelength heterodyne distance measurement techniques combined with heterodyne array imaging. A key feature of the technique is variable optical path length measurement sensitivity that allows the system to measure surface figure errors ranging from thousands of wavelengths down to fractions of a wavelength. Automatic sensitivity tuning combined with completely digital phase measurement over the entire surface allows the integration of this device in automated sensing and optical correction schemes. It is particularly suited for the measurement and control of spatially and temporally variable optical surfaces of the kind that would be found in a space- based adaptive optics system. In this paper we demonstrate aspects of the automated super-heterodyne system on an optical surface with surface profile structure height that varies from micron to mm sized features. We describe the hardware and software design of the heterodyne based measurement system that enable it to choose the appropriate synthetic wavelength to measure different areas of the surface profile. Spatial processing methods are discussed that allow the system to merge segments of large scale images that have been measured at different synthetic wavelengths.

We examine the low light level performance of a four-channel polarimeter with a single photon counting sensor associated with each channel. Specifically, we investigate how the detection noise propagates through the polarimeter calibration matrix and influences the estimation of I₁ and I₂, the intensities in two orthogonal polarization states and, (delta) , the phase delay between the two orthogonal states. The error in estimating the phase delay is first derived in the small angle approximation for low light levels. The approximation is removed by deriving a general formalism valid for arbitrary light levels including very low light levels. A specific polarimeter design is examined and quantitative examples are shown and discussed for that polarimeter.

Meteor-3M(2)/TOMS-5 is a cooperative joint mission between the Russian Space Agency (RSA) and the United States (US) National Aeronautics and Space Administration (NASA). A US Total Ozone Mapping Spectrometer (TOMS) instrument is scheduled to be flown aboard a Russian Meteor-3M satellite in the year 2000. The main science objectives of the mission are to continue global total-ozone measurements to monitor long-term change in global total ozone, to understand processes related to the Antarctic ozone hole, and to improve the understanding of the processes that govern global total ozone. Secondary objectives are to measure aerosol amounts (dust, smoke, volcanic ash, and sulfates) and SO₂. This paper describes the Meteor-3M(2) spacecraft, the TOMS-5 experiment, operations of Meteor- 3M(2)/TOMS-5, and plans for data processing, data archiving and distribution.

The conception and results of modernization of the EUV radiation spectrometer for use in the Solar Patrol Mission in the framework of Russian project. 'The creation of permanent space patrol of soft X-ray and hard UV solar radiation' are presented. The EUV spectrometer is one of three devices developed for the above patrol together with soft X-ray and EUV radiometer and spectrometer. The main purpose of the Solar Patrol Mission is the permanent monitoring of the solar activity including the periods of solar flares which will allow the variation of absolute fluxes of ionizing radiation and its spectral composition to be determined. These data are of great importance for solving the problems for solar- terrestrial relations.

In the report we consider a method of dynamic aberration measurement in the frameworks of the problem of minimization of difference between potential and operational resolution of the earth observation satellite systems. The method is based on statistical analysis of the current image at the output of the image receiver. The results of the measurement are recalculated into control signals of the actuators in the adaptive feedback loop.

The Moderate Resolution Imaging Spectroradiometer (MODIS) will employ a solar illuminated diffuser as an on-board calibration device. The solar diffuser is Labsphere Spectralon^TM. The bidirectional reflectance factor (BRF) of the solar diffuser panel is needed for the on-orbit reflectance calibrations. The BRF of the solar diffuser panel was measured using the Santa Barbara Remote Sensing scattering goniopolarimeter, which is a relative device, so a known BRF standard was required. The standard was a Spectralon sample that was calibrated by the National Institute of Standards and Technology. Its BRF was calibrated in the 400 to 1800 nm region. Total uncertainty estimates for the MODIS solar diffuser BRF were less than 1.0% for all wavebands other than the longest one, which was centered at 2100 nm. The estimated uncertainty for the 2100-nm waveband was 1.5%. Hapke scattering theory was used to generate models for the BRF. For the MODIS solar diffuser models, the root-mean-square of the fit values were less than 0.3%, for wavebands other than 2100 nm. They were less than 1.1% for the 2100-nm waveband.

The National Aeronautics and Space Administration (NASA) is studying options for future space-based missions, building upon the measurements to be made by the first series of Earth Observing System (EOS) missions. One mission under consideration is the NPOESS Preparatory Project (NPP), a cooperative mission of NASA and the National Polar-orbiting Operational Environmental Satellite System (NPOESS). This mission would utilize new instrument technologies being developed by the NPOESS, with additional NASA requirements, to continue certain measurements from the first series of EOS missions. By flying in the 2005 time period, NPP would provide an early demonstration and validation of new instrument technologies and algorithms in support of future NPOESS missions and extend the critical time series measurements of EOS.

Given the current choices of (1) an ever increasing population of large numbers of satellites in low-earth orbit (LEO) constellations for commercial and military global coverage systems, or (2) the alternative of smaller count geosynchronous satellite system constellations in high-earth (HEO), of higher cost and complexity, a number of commercial communications and military operations satellite systems designers are investigating the potential advantages and issues of operating in the mid-earth orbit altitudes (MEO) (between LEO and HEO). At these MEO altitudes both total dose and displacement damage can be traded against the system advantages of fewer satellites required. With growing demand for higher bandwidth communication for real-time earth observing satellite sensor systems, and NASA's interplanetary and deep space virtual unmanned exploration missions in stressing radiation environments, JPL is developing the next generation of smart sensors to address these new requirements of: low-cost, high bandwidth, miniaturization, ultra low-power and mission environment ruggedness. Radiation hardened/tolerant Active Pixel Sensor CMOS imagers that can be adaptively windowed with low power, on-chip control, timing, digital output and provide data-channel efficient on-chip compression, high bandwidth optical communications links are being designed and investigated to reduce size, weight and cost for common optics/hybrid architectures.

This paper details the Chinese Academy of Space Technology (CAST) position and recent development in space remote sensing activities nd gives the author's view of the accomplishment of European Space Agency (ESA) in this field. Space remote sensing technology is a useful scientific and technical means to realize sustainable development of the society of mankind now and future. Peace and development are the common desires of the people across the world and become the current of the present era. Nowadays, cooperation in scientific research and technical works among different countries has been the trend. 'Open the door to the outside world' policy adopted by the Chinese Government gives great warranty and strong motivation for cooperation and communication between China and the world. CAST as a leading space group of China has a strong willing to cooperate with ESA, NASA and other nation's space bodies. At the last part of this paper, author's view about the opportunity of cooperation in space remote sensing technology between ESA and CAST is put forth.

The EnviSat-1 satellite will embark an innovative radar altimeter. The calibration of the measurements of range from this instrument will be performed using novel techniques. The range measurement will be calibrated absolutely by establishing the actual geocentric sea-level along the sub- satellite tracks. These tracks are located in a limited and well-controlled region in the western Mediterranean and will include a number of fully-equipped individual sites which will provide higher confidence in the overall analysis, combined with data from the whole area at lower weight. The determination of the geocentric sea-level is performed using tide gauges and geodetic means such as leveling and floating GPS receivers. The altimeter sea-level is derived from the altimeter range corrected for propagation effects and sea- state bias, and a precise restitution of the trajectory of the satellite. These measurements comprise three vectors: range, orbital height and sea-surface height. The difference between orbital-height minus range, and sea-surface height provides the bias. The backscatter coefficient measured by previous altimeters has not been absolutely calibrated. An emerging application of the RA-2 in investigation of surface properties has identified the need to perform this calibration. A number of techniques are under study to determine the feasibility of meeting this need, including the use of well-controlled natural targets, the use of the altimeter receiver as a passive radiometer in order to determine its gain and the use of a transponder to return a precisely known return echo power to the radar.