EOS-AM1 is the initial component of NASA's Earth Observing System (EOS). EOS serves as the centerpiece for Mission to Planet Earth (MTPE) and is to provide satellite observations to determine extent, causes, and regional consequences of global climate change. EOS-AM1 is specifically focused on the characterization of terrestrial and oceanic surfaces; clouds, radiation, and aerosols; and the earth's radiative balance. It carries five advanced instruments: advanced spaceborne thermal emission and reflection radiometer (ASTER), clouds and earth's radiant energy system (CERES), multi-angle imaging spectroradiometer (MISR), moderate resolution imaging spectroradiometer (MODIS), and measurements of pollution in the troposphere (MOPITT). They are provided by the Ministry of International Trade and Industry of Japan, NASA's Langley Research Center, Jet Propulsion Laboratory, NASA's Goddard Space Flight Center, and the Canadian Space Agency, respectively. The project is currently in its C/D phase and is maintaining schedule for a June 1998 launch. During the past year all of the instruments and the spacecraft successfully completed their critical design reviews (CDRs) and engineering model fabrication, integration, and testing. Fabrication and integration of flight model hardware is underway. Results of these activities and the current development status are discussed. The EOS-AM1 project is managed by Goddard Space Flight Center.

ASTER is an advanced multispectral imager with high spatial, spectral, and radiometric resolutions for EOS-AM1 spacecraft which will be launched with four other instruments in June 1998. The ASTER instrument covers a wide spectral region from visible to thermal infrared by 14 spectral bands. To meet a wide spectral coverage, optical sensing units of ASTER are separated into three subsystems, that is the visible and near infrared subsystem, the short wave infrared subsystem and the thermal infrared subsystem, depending on spectral region. Moreover, ASTER has an in-track stereoscopic viewing capability by a near infrared band. To acquire the stereo data the VNIR subsystem has two telescopes for the nadir and backward viewings. Several new technologies are adopted as design challenges to realize the high performance. The validity of these new technologies is verified through the evaluation of the engineering model.

ASTER instrument is a high performance spatial imager on board the EOS-AM1 spacecraft, which will be launched in June 1998. The ASTER raw data will be captured by U.S. ground system via TDRSS, processed to level-0 data, and then transferred to the ASTER Ground Data System in Japan for level-1 data products generation. The ASTER Ground Data System consists of the Communication and System Management System (CSMS), ASTER Operation Segment (AOS), Science Data Processing Segment (SDPS), and Direct Receiving Station (DRS). The ASTER Ground Data System adopts a challenging design concept to handle approximately 780 sets of scenes of 14 spectral bands one stereo band (about 80 Gbytes) per day.

The advanced spaceborne thermal emission and reflection radiometer (ASTER) is a multi- spectral imaging radiometer with 14 spectral bands, 60 km imaging swath, and 15 - 90 m spatial resolutions. Since operation of the ASTER instrument will be affected by various constraints such as duty cycle and pointing frequencies, it is necessary to optimize the operation scenario for efficient data acquisition during the 6 year mission period. In addition, many possible combinations of the observation modes of the three ASTER subsystems (VNIR, SWIR, and TIR), which can be operated independently with different gain setting for each spectral band, complicate the data acquisition scenario. There are four data acquisition categories; local observations, regional monitoring, global mapping, and engineering team requests. Local observations will be made in response to data acquisition requests (DARs) from individual investigators. Regional monitoring and global mapping will be scheduled in response to science team acquisition requests (STARs). Prioritization of data acquisition requests will be done using the factors such as user status and observation categories. Three types of schedules; long term schedule (LTS), short term schedule (STS), and one day schedule (ODS) will be generated for ASTER observation activities.

The end-to-end ground data system supporting the advanced spaceborne thermal emission and reflection radiometer (ASTER) consists of elements provided by both Japan ASTER Ground Data System and the highly distributed Earth Observing System Data and Information System (EOSDIS). These two systems must interoperate to provide complex mission operations support and process high-rate (approximately 8 Megabits/sec) data into standard level 1, level 2 and higher data products. The EOS Data and Operations System (EDOS) will provide ground data capture, rate buffering, and level 0 data processing. The EOS Operations Center will provide the operational interface between the Japanese Instrument Control Center and the spacecraft and will monitor the instrument health and safety. The Land Processes Distributed active Archive Center (DAAC) at the EROS Data Center will produce higher-level products based on software provided by the ASTER Science Team and systems provided by the EOSDIS Core System. Higher-level data product quality assurance, as well as U. S. Science Team support for instrument scheduling, will be performed at a science computing facility located at the Jet Propulsion Laboratory. All of these elements are being developed together to assure that this international mission produces data which will serve the needs of the science community.

The clouds and the earth's radiant energy system (CERES) instrument has been developed based on the earth radiation budget experiment (ERBE). The CERES instrument will be flown on the Tropical Rainfall Measurement Mission and on the first two Earth Observation System platforms. The techniques which were used to calibrate the ERBE instruments on the ground and to maintain calibration in orbit to 1% precision are being refined and applied to the design and calibration of the CERES instrument. The same types of on-board calibration devices will be used for CERES as were used on ERBE to determine any changes in the sensors' responses. The TRW Radiometric Calibration Facility which was used for ERBE calibration has been upgraded for CERES by incorporation of a radiometrically characterized black body as a reference and a cryogenic active cavity radiometer as a calibration transfer device for the shortwave calibration system.

Current efforts are directed at creating a high-level end-to-end numerical model of scanning thermistor bolometer radiometers of the type used in the Earth Radiation Budget Experiment (ERBE) and planned for the clouds and the earth's radiative energy system (CERES) platforms. The first-principle model accurately represents the physical processes relating the electrical signal output to the radiative flux incident to the instrument aperture as well as to the instrument thermal environment. Such models are useful for the optimal design of calibration procedures, data reduction strategies, and the instruments themselves. The modeled thermistor bolometer detectors are approximately 40 micrometers thick and consist of an absorber layer, the thermistor layer, and a thermal impedance layer bonded to a thick aluminum substrate which acts as a heat sink. Thermal and electrical diffusion in the thermistor bolometer detectors is represented by a several-hundred-node- finite-difference formulation, and the temperature field within the aluminum substrate is computed using the finite-element method. The detectors are electrically connected in adjacent arms of a two-active-arm bridge circuit so that the effects of common mode thermal noise are minimized. However, because of a combination of thermistor self heating, loading of the bridge by the bridge amplifier, and the nonlinear thermistor resistance-temperature relationship, bridge deflections can still be provoked by substrate temperature changes, even when the change is uniform across the substrate. Of course, transient temperature gradients which may occur in the substrate between the two detectors will be falsely interpreted as a radiation input. The paper represents the results of an investigation to define the degree of vulnerability of thermistor bolometer radiometers to false signals provoked by uncontrolled temperature fluctuations in the substrate.

NASA's clouds and the Earth's radiative energy system (CERES) program is a key component of the Earth Observing System (EOS). Under CERES an array of radiometric instruments will be placed in Earth orbit to monitor the longwave and visible components of the Earth's radiative energy budget. High-level dynamic electrothermal models of these instruments have been formulated under NASA sponsorship. Accurate optical and thermal-radiative characterization of the instruments is assured by a Monte-Carlo-based ray-trace in which tens of millions of rays are traced, and a transient finite-difference formulation involving hundreds of nodes is used to describe thermal and electrical diffusion within the thermistor bolometer sensing elements. The external electronic circuit is also correctly included in the instrument model. The actual CERES instruments will undergo pre-launch calibration in a unique thermal-vacuum radiometric calibration facility equipped with blackbodies, a cryogenically cooled active-cavity radiometer, and shortwave sources. This ground calibration can also be simulated using the high-level, dynamic electrothermal models of the CERES instruments. This offers a quick and inexpensive means of verifying the calibration procedure and anticipating any problems that may arise. The results obtained from these simulations may then be used to predict the regression coefficients in the count-conversion equation used to convert instrument readings into radiance, and to determine which parameters should be included in the count-conversion equation to maximize its sensitivity. The paper presents results of the simulated ground calibrations of the CERES total channel instrument, including predicted values for instrument accuracy during the ground calibrations.

MISR will provide global data sets from Earth orbit using nine discrete cameras, each viewing at unique view directions. The design of this instrument is complete and has been refined following assembly and testing of an engineering model. The engineering model has been invaluable in identifying correctable design flaws, in resolving subsystem interface issues early in the program, and in providing the science team with as-built performance data to be used in the algorithm development. MISR will fly with an on-board calibrator consisting of Spectralon diffuse panels and photodiode detector standards. Both the use of Spectralon and flight detector standards have been developed by the MISR team. Currently the engineering team is assembling and testing the flight cameras, and the data teams are preparing for the post-launch geometric and radiometric calibration of the instrument, as well as developing algorithms to provide the science products. With a 3.3 Mb orbital average data rate, and global coverage each nine days, processing will be automated and standardized. Deliverables include calibrated, registered data sets, as well as aerosol/land surface, and cloud parameters.

The EOS/MISR instrument contains nine cameras; each camera focal plane consists of four closely spaced, linear CCD arrays. Each of the four arrays is preceded by a narrow bandpass filter covering a distinct spectral region. Testing of two engineering model cameras revealed spatial and spectral optical crosstalk levels not predicted by stray light analysis. A detailed investigation of filter coating anomalies and the filter assembly/CCD geometry elucidated several mechanisms responsible for optical crosstalk. These mechanisms, as well as recommendations concerning the design of future focal planes, are presented in this paper.

The MODIS instrument is currently under development at Hughes Aircraft Company, Santa Barbara Research Center. The MODIS is scheduled to fly on the Earth Observing System suite of spacecraft with the first launch in mid 1998. The MODIS engineering model (EM) was completed in May of 1995 and has undergone extensive performance characterization testing. This paper covers the highlights of that testing. Results from engineering model (EM) tests demonstrate that the MODIS instrument will meet its performance objectives, offering excellent radiometric, spatial, and spectral performance while maintaining high precision and accuracy over the mission life.

The need is identified for a unified approach to the preflight and in-flight absolute radiometric calibration of satellite sensors, which does not depend on the accurate transfer of lamp and detector calibrations from the laboratory to orbit. Such an approach is described that uses the sun to provide the link between preflight solar-radiation-based calibration, in-flight solar- diffuser-based calibration, and vicarious calibration. An example is given of each of these methods and uncertainty budgets are provided. It is shown that an uncertainty, with respect to solar exo-atmospheric irradiance, of less than 3%, one sigma, can be attained for each method and that each can, if needed, be related to national laboratory standards. In addition to providing the link between preflight and in-flight on-board calibrations, this unified approach also fills the critical need to relate these calibrations to those of the radiometers used for vicarious calibration and data-product, field-validation measurements, by also referencing them to the same solar-irradiance scale.

In the frame of the JERS1 System Verification Program, conducted by MITI-NASDA, CNES and CERT proposed to use the knowledge of the SPOT high resolution sensors (HRV) calibration to deduce the in-flight calibration of the visible and near infrared bands of the JERS1 optical sensor (OPS). The idea is to perform an intercalibration of the cameras using close in time acquisitions over stable desert areas, regularly acquired for SPOT/HRV multitemporal calibration studies. In spite of operational difficulties to achieve such a 'rendezvous', it has been possible to obtain a cloud free OPS acquisition over one of our SPOT sites with a couple of SPOT3 acquisitions, two days before and three days after the JERS1 overpass. This paper describes in detail the method used to perform this intercalibration and its estimated accuracy. Such a method, already experimented with other satellites, provides a good intercalibration useful for a combined use of SPOT and JERS1 data. As far as OPS absolute calibration is concerned, discrepancies of 14 to 17%, according to the spectral band, can be noticed in comparison with the absolute calibration coefficients provided with the image data which could indicate a sensitivity decrease of the sensor from the ground measurements.

In-flight absolute radiometric calibration is critical for multi-temporal and multi-sensor data comparisons. In the case of vicarious calibration of optical sensors based on ground-level measurements, the test site must be well characterized in spatial, radiometric, spectral, and temporal domains. Remotely sensed data acquired at other wavelengths can contribute to a baseline understanding of ground targets and provide insight into the usefulness of such targets for in-flight calibration of optical sensors. With these considerations in mind, multi-temporal ERS-1 SAR data have been obtained for White Sands, New Mexico, and Lunar Lake and Railroad Valley playas in Nevada. This paper reports on an initial examination of these SAR image data sets and the significant pattern changes observed in the scenes. It is concluded that surface roughness, soil moisture and run-off are major factors giving rise to the observed scene characteristics.

For earth observation instruments accurate on ground calibration is necessary to characterize sensor parameters that are used for the in orbit calibration. TNO Institute of Applied Physics has developed an absolute radiometric calibration facility for the calibration of both complete instruments and diffusers. The facility operates in the wavelength range of 240 - 2400 nm. It incorporates a specially designed spectral irradiance source and a so called Brewster polarization. For accurate diffuser calibration a detector assembly is developed. In this paper the calibration facility with its features is described in some detail and a few calibration results are given.

Global change has now become one of the most important problems for human kind. The major problems are global warming, stratospheric ozone depletion, tropical forest decrease, desertification, acid rain and decrease of bio-diversity. Among them, global warming and ozone depletion are the most urgent and critical problems for human beings. In order to solve these problems, accurate and comprehensive knowledge of the state of the art should be obtained. Polar orbiting earth observation satellite programs now being conducted under international coordination are programs aimed to solve these problems. They are composed of three kinds of series: i.e., ADEOS series by NASDA, Japan; EOS series by NASA, USA; and ENVISAT series by ESA, Europe. ADEOS is the first satellite of this series and will be launched by NASDA on August 1996. There are 8 sensors on-board ADEOS. They are provided by 6 agencies from 3 countries. The scientific objective of ADEOS is to contribute to the understanding of global environment, especially global warming and stratospheric ozone depletion.

AVNIR is a high spatial resolution imager on ADEOS with four multispectral bands and one panchromatic band covering the visible and the near-infrared range. AVNIR also has multi- view angle observation (pointing) function with the range of plus or minus 40 degrees from nadir. These two characteristics of AVNIR, high spatial resolution observation and stereoscopic observation, enable it to produce high quality land cover/use maps and digital elevation models (DEM). Besides these two practical products AVNIR is expected to observe scientific parameters including space reflectance and surface reflectance which would play an important role in assessing the radiation budget at the earth surface. Also AVNIR is expected to produce several science data sets for monitoring coastal environment, volcanic activities as well as terrestrial ecosystems.

Ozone layer observation will be conducted with the solar occultation sensor ILAS (improved limb atmospheric spectrometer) on board the ADEOS (Advanced Earth Observing Satellite; to be launched in August 1996) to provide vertical profiles of ozone, methane, water vapor, nitrogen dioxide, nitric acid, and nitrous oxide from absorption measurements in the infrared region, and temperature and pressure profiles from measurements of absorption due to oxygen molecules in the visible region. Optical properties of stratospheric aerosol and polar stratospheric clouds (PSCs) are also derived from visible and infrared extinction measurements. Using the ILAS flight model, optical performance data was obtained from the experiments with a gas cell and a black body light source. Field experiments have been planned for the post-launch validation, which includes field campaigns using large balloons at Kiruna (Sweden) and ground-based remote sensors at Kiruna, Alaska, Syowa Station and other locations. This paper briefly describes the ILAS instrument, its performance evaluation, laboratory experiments to determine the instrument function, data processing algorithms and validation plans.

The present status and scientific expectation of the Ocean Color and Temperature Scanner (OCTS) is outlined. The OCTS is an optical radiometer devoted to the frequent global measurement of ocean color and sea surface temperature with IFOV of approximately 700 m and swath of 1400 km. The main scientific objectives of OCTS are briefly presented, describing the model atmosphere-ocean system that is used for deriving the level 2 standard products.

Experiments on the earth-satellite-earth laser long-path absorption measurements of atmospheric trace species will be carried out with the retroreflector in space (RIS) for the Advanced Earth Observing Satellite (ADEOS). The RIS is a single-element hollow retroreflector with an effective diameter of 0.5 m, which was designed for spectroscopic measurement in the infrared region. The ground system for the experiment employs two single-longitudinal-mode pulsed CO₂ lasers. High-resolution atmospheric absorption spectra are measured by using the Doppler shift of the return beam caused by the satellite movement. Vertical profiles of O₃ and CH₄, and column contents of CFC12, HNO₃, CO, N₂O, etc. will be obtained from the measured spectra.

The high spatial resolution and global coverage of a spaceborne microwave scatterometer make it a powerful instrument to study phenomena ranging from typhoons to El Nino Southern Oscillations which have regional and short term economic and ecological impacts as well as effects on long term and global climate changes. In this report, the application of scatterometer data, by itself, to study the intensity and the evolution of a typhoon is demonstrated. The potential of combining wind vector and precipitable water derived from two spaceborne sensors to study the hydrologic balance in the tropics is discussed. The role of westerly wind bursts as a precursor of anomalous warming in the equatorial Pacific is investigated with coincident data from microwave scatterometer, altimeter, and radiometer.

A new TOMS requirement is to measure total ozone trends of 1% per decade. The ozone calibration depends on knowledge of diffuser plate reflectance and spectrometer wavelength changes. Absolute diffuser reflectance changes are now measured with a reflectance calibration assembly, containing a UV Hg-phosphor lamp. Three diffusers are flown to permit frequent solar calibrations and infer relative diffuser changes when exposed at different rates. Finally, an ambiguity in the wavelength monitor was corrected. These three changes to the original Nimbus-7 TOMS design are expected to produce 0.5% ozone trend data, based on pre launch test results.

The European Space Agency is preparing the next generation of earth observation satellites to be launched during the next decade. Two classes of missions have been defined: the 'Earth Watch' missions of pre-operational nature and the research-oriented 'Earth Explorer' missions featuring satellites dedicated to particular scientific research fields. Nine potential missions have been identified that are the subject of technical and scientific studies in view of the selection of a reduced set of missions to be studied at phase A level: atmospheric chemistry, atmospheric dynamics, atmospheric profiling, earth radiation, gravity, magnetometry, precipitation, surface processes and topography. Technical activities are currently devoted to the definition of the satellite systems and their payloads. In particular, the main instruments are studied at pre-phase A level to assess their feasibility, produce first conceptual designs, and identify areas requiring further study. In parallel, new technologies needed for the realization of the instruments are being identified and are the subject of a technical pre- development effort.

The space module PRIRODA is the technical base for the international scientific project PRIRODA. It will be launched and attached at the end of 1995 to the inhabited space platform MIR. By means of the optical sensors placed on board the PRIRODA module, we are going to study the following topics: (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 perform the goals mentioned above, the Italian side has identified some test sites mainly in the Tuscany region and the data from the following optical sensors of the PRIRODA module will be utilized: (1) ISTOK-1 -- the instrument is a 64 channel infrared spectro-radiometer in the band of 4 to 16 micrometers. (2) MOS-OBZOR -- this imaging spectrometer is dedicated to the investigation of the reflected solar radiation in the visible and near infrared. (3) OZON-M -- the instrument is suitable to the investigation of the spatial structure of the infrared radiation of the ocean surface and the atmosphere. (4) MSU-E -- this electro-optical scanner operates at a spatial resolution of 25 m in three visible and near infrared spectral bands. (5) 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. Taking into account the characteristics of these sensors and utilizing both experimental data available now and coming from similar sensors placed on aircraft or spacecraft and suitable models, we are estimating the possible applications of such new sensors.

The paper presents the infrared atmospheric sounding interferometer (IASI) being developed at CNES TOULOUSE. IASI is a part of the operational meteorological payload managed by EUMETSAT. The baseline platform for the first flight model of the instrument is the METOP-1 polar platform to be launched during 2001. IASI consists of a Fourier transform spectrometer (FTS) based on a Michelson interferometer coupled to an integrated imaging system which allows characterization of cloudiness inside the FTS field of view. This instrument will provide spectra of high radiometric quality at 0.5 cm-1 resolution from 15.5 micrometer to 3.62 micrometer, with global coverage twice per day at 25 km horizontal resolution.

A new generation of submillimeter wave limb sounders require multiband real time spectrometer backends with high constraints on mass, size and power consumption. A new concept, the multiband chirp transform spectrometer (MBCTS) was developed for this purpose. The MBCTS was chosen to be suitable for the Advanced Microwave Atmospheric Sounder (AMAS), a European instrument, which is currently in the breadboarding phase and is planned to be flown in 1999. The basic concept and the development status of the multiband CTS are described.

A polarimetric C-band airborne SAR has been developed in the Netherlands. The system makes use of a phased array antenna with solid state amplifiers. The project consists of two phases, a definition phase and a realization phase. The definition phase consisted of the actual realization of a SAR research system, which made its first successful test flight in November 1990. The research system is based on the concept of a wide beamwidth antenna, rigidly fixed to the aircraft. Pulse compression and a high PRF ensure sufficient sensitivity in this system, which is equipped with a 160 Watt peak pulse power solid state transmitter. The processing is done off-line. In the current realization phase the polarimetric system is under its first airborne tests. Its design is based on the experience gained with the research system. The system makes use of a phased array with dual polarized patch radiators and will be equipped with solid state amplifiers. The technology used and the expected system performance are representative for future spaceborne systems. This paper gives an overview of the SAR research system and the results obtained with this system. The polarimetric system design and use are discussed. Apart from the use as an advanced polarimetric airborne SAR, their is the perspective of using this system as a demonstrator for ESA's future ASAR system.

The paper describes the environmental tests to be carried out on the scientific instrumentation to be flown on the M-55 Geophysika in the frame of the APE Program. The instruments, developed by different European research institutes, are for remote sensing and in situ measurements of the major components of the Earth's stratosphere. The paper presents the technological activities that ENEA (Ente Nazionale per le Nuove Tecnologie l'Energia e l'Ambiente) is carrying out in its laboratories to verify the correspondence of the various instruments to meet the requirements for airborne application. The reference documents used have been the RTCA/DO-160C and the MDB (Myasishchev Design Bureau) specifications.

The spacecraft of Cassini Mission will be launched towards Saturn in 1997 in order to study the physical structure and chemical composition of Saturn as well as all its moons. To this end many instruments will be mounted on the spacecraft; one of these is the Cassini Radar. Cassini Radar is a cooperative project between National Aeronautics and Space Administration (NASA/Jet Propulsion Laboratory (JPL) and Italian Space Agency (ASI). ASI has committed to Alenia Spazio the design, integration and test of the radio frequency subsystem, while the digital subsystem is the responsibility of JPL. Cassini Radar is a multimode instrument able to operate in an imaging mode (0.85 and 0.425 MHz bandwidth), a scatterometer mode (0.106 MHz bandwidth), and a radiometer mode (100 MHz bandwidth). These modes will be used to acquire images, topographic profile, backscatter reflection coefficient, and sense brightness temperatures of the surface of Titan. Main test results are reported and discussed to demonstrate that the instrument satisfies the mission requirements.

The goal of the OMEGA instrument (planned to fly the Mars'96 Orbiter) is to monitor the past and present evolution of Mars through visible and infrared spectral mapping of its surface. The spectral range (0.5 to 5.1 micrometers) includes signatures of major and minor components of both the surface and the atmosphere of Mars. The spectral and spatial resolutions required and the high signal to noise ratio lead to a three channel instrument: (1) A visible spectrometer (whiskbroom type) with a bidimensional silicon array (288 by 384 elements 23 by 23 micrometers) and a refractive telescope illuminating a holographic grating. (2) A two-channel infrared spectrometer (pushbroom type) based on linear InSb array (128 elements). A reflective telescope and a scanning device give the imaging capability. The IR detectors, cooled at 77 K were developed in France by the Societe Anonyme de Telecommunication (SAT) for this instrument with adapted geometry and specific two band filters. A specific electronic was developed for this instrument, especially one digital electronics based on a transputer associated to a digital signal processor in order to obtain a high efficiency, error free, data compression. After its space qualification, the instrument was fully calibrated at the Institut d'Astrophysique Spatiale (IAS) Orsay.

The atmospheric infrared sounder (AIRS) is a high spectral resolution IR spectrometer. AIRS, together with the advanced microwave sounding unit (AMSU) and the microwave humidity sounder (MHS), is designed to meet the operational weather prediction requirements of the National Oceanic and Atmospheric Administration (NOAA) and the global change research objectives of the National Aeronautics and Space Administration (NASA). The three instruments will be launched in the year 2000 on the EOS-PM spacecraft. Testing of the AIRS engineering model with start in 1996. An extensive effort of data simulation and retrieval algorithm development has been used to define the AIRS instrument functional requirements and to demonstrate that the combined AIRS/AMSU/MHS data meet the temperature retrieval accuracy of 1 K rms in 1 km thick layers and water vapor profiles with 20% rms accuracy in 2 km layers in the troposphere under all-weather day, night and cloudy conditions. Assimilation of data with this accuracy into global circulation models is required to achieve a positive forecast impact. The AIRS instrument represents a major step forward in satellite based remote sensing technology. In particular, improvements in second generation PV:HgCdTe detector array/readout technology coupled with a rapid advance in long life, low vibration, Stirling/pulse tube cryocooler design have been instrumental. This paper describes the overall hardware design and performance and provides a brief status of the development effort.

The microwave limb sounder (MLS) for NASA's Earth Observing System Chemistry Mission series of satellites is currently under development, with launch planned for 2002 on EOS- CHEM. This sensor builds on heritage from the successful MLS on NASA's Upper Atmosphere Research Satellite (UARS), which was first to demonstrate the microwave limb sounding technique for measuring the abundance of trace species in the Earth's atmosphere from an orbiting satellite. Its relevance to the atmospheric chemistry and global change objectives of Mission to Planet Earth is discussed along with details of the instrumentation being developed.

The Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is a small spectrometer obtaining solar irradiance measurements in the ultraviolet, 120 to 420 nm. This radiation, especially at wavelengths below 300 nm, is absorbed by the earth's middle and upper atmosphere providing the dominant energy input. Even small changes in this radiation field will have an impact on the atmosphere's composition, temperature, and dynamics. The challenge is to measure solar irradiance with better than 1% relative accuracy over arbitrarily long time periods. The SOLSTICE has the unique capability of observing both the Sun and early type stars using the same optics and detectors. Only apertures and integration times are altered to cover a dynamic range of nine orders of magnitude. Individually these 'standard' stars should vary by only small fractions of a percent over time periods of thousands of years, and the ensemble average flux from twenty or thirty of these stars is an even more stable reference. We describe the SOLSTICE technique and results from the Upper Atmosphere Research Satellite (UARS) SOLSTICE that has made daily observations since October 1991. A second generation SOLSTICE is now being designed for the Earth Observing System (EOS), to be flown within the next seven years, hopefully overlapping the UARS mission.

Given the emergence of numerous Earth orbiting probes, regional data centers that introduce advanced technologies in an evolutionary fashion are quickly becoming necessary to control burgeoning development costs, to minimize operations costs, and to test riskier technologies. In the past and present, the Joint Typhoon Warning Center (JTWC) has provided invaluable feedback to NASA about the standard operations for covering four of the six major tropical cyclone basins, (North and South Indian Oceans, and the western North Pacific and Southwest Pacific Oceans). Since the termination of U.S. Air Force (USAF) weather reconnaissance flights in 1987, position fixes, intensity determination and intensity forecasts of tropical cyclones and other severe storms are based on heavy utilization of satellite derived image products. During 1988 a total of 2,044 satellite fixes were made on western North Pacific tropical cyclones, 117 fixes on tropical cyclones in the North Indian Ocean and 1,144 for those in the southern hemisphere (Plante and McMorrow 1988). NASA and the U.S. Air Force have developed a multi-satellite receiving and analysis system to support the JTWC. The system is capable of tracking and analyzing multiple storms simultaneously, requesting and scheduling of resources from five other remote facilities and providing information on storm positions and intensities. This interactive meteorological data display and image analysis system (MIDDAS) has been developed by operational forecasters; thus, the system incorporates elements designed to reduce manpower and provide analysis tools which integrate satellite observations from five satellites (GMS-4, GMS-5, NOAA-10, NOAA-12, NOAA-14, DMSP-44, DMSP-46) together with in situ measurements from the Global Telecommunications System (GTS). The system description, data analysis capabilities, and the required technologies for the next development stage are presented.

EOSDIS, the data and information system being developed by NASA to support interdisciplinary earth science research into the 21st century, will do more than manage and distribute data from EOS-era satellites. It will also promote the exchange of data, tools, and research results across disciplinary, agency, and national boundaries. This paper describes the options that data providers will have for interacting with the EOSDIS Core System (ECS), the infrastructure of EOSDIS. The options include: using the ECS advertising service to announce the availability of data at the provider's site; submitting a candidate data set to one of the Distributed Active Archive Centers (DAACs); establishing a data server that will make the data accessible via ECS and establishing Local Information Manager (LIM) which would make the data available for multi-site searches. One additional option is through custom gateway interfaces which would provide access to existing data archives. The gateway, data server, and LIM options require the implementation of ECS code at the provider site to insure proper protocols. The advertisement and ingest options require no part of ECS design to reside at the provider site.

In 1998 the National Aeronautics and Space Administration (NASA) will launch the first of a series of Earth Observation System (EOS) spacecraft designed to study the environment. The moderate resolution imaging spectroradiometer (MODIS) is a key EOS instrument. Current plans are to fly a series of six MODIS instruments on the EOS-AM and -PM satellite series. The operational life span for the EOS effort is fifteen years (1998 - 2012). Processing the data from the EOS platform and the MODIS instrument will require state of the art jumps in computing, storage and local and wide area networks. The continuous raw data rate from MODIS will average 10 megabits per second or approximately 110 gigabytes per day. The total product storage capacity for the MODIS data and products is estimated to be 650 gigabytes (GB) per day and 230 terrabytes (TB) per year. This paper focuses on the Team Leader Computing Facility (TLCF) which will be used to develop, integrate, optimize, test and validate the operational versions of the MODIS software. Approaches that achieve the high network bandwidth and high performance computing are needed to support MODIS software development and testing on global MODIS data sets. Candidate technologies are evaluated in light of the above requirements on the TLCF.

A simulation system allows analyses of specific sensor configurations. Application areas for such an approach are: (1) design and optimization of optical sensors for specific (well known) applications, (2) sensitivity of different data products to position and calibration errors, (3) test of retrieval algorithms, and (4) mission support. The simulation system consists of mathematical and physical models to simulate the passage of electromagnetic radiation from the source of the emission to the sensor. The key part of this simulation system is a ray tracer interacting with a digital terrain model around an ellipsoid, it also includes sensor opto- electronics and spacecraft orbit calculation. Other parts are the simulation of the planetary atmosphere and the hardware of the sensor with additional retrieving algorithms. The usefulness of the simulation system for the optimization has been shown with a three-line CD sensor for the determination of cloud position and speed. For the simulation system clouds are assumed as a partly transparent, uniformly moving body, modeled by a digital terrain model over the surface. Furthermore a simple radiometric model was implemented: A cloud has a constant transmission, the ground surface and the clouds have constant albedos. The simulation shows that some conditions are mandatory for the determination of cloud speed and height, especially the movement of the sensor has to be an accelerated one. This will be reached though a highly elliptical orbit of the satellite. However, the determination will not be possible near pericentre.

This new satellite radar altimeter concept uses on-board real-time partially coherent processing to realize an along-track impulse response shape and position which are not degraded by terrain slope or elevation. The key innovation is delay compensation, analogous to range curvature correction in a burst mode synthetic aperture radar. The detected outputs of many bursts are incoherently integrated to accumulate more than one hundred equivalent looks. The along-track footprint size is on the order of 200 - 300 meters. The radar equation for the delay/Doppler radar altimeter has an h^(-5/2) dependence on height, which is more efficient than the corresponding h^(-3) factor for a pulse-limited altimeter. The radiometric response obtained by the new approach would be 10 dB stronger than that of the TOPEX/POSEIDON altimeter, for example, if the same hardware were used in the delay/Doppler mode. The concept is a candidate small satellite instrument for earth observation, with particular suitability for precision altimetry of coastal and polar ice sheets.

High quality electronic imaging systems are highly demanding in more and more resolutions. For Earth observation applications through space-borne imaging pushbroom systems, high pixel count focal planes are required. They are commonly obtained thanks to butting several CCD linear image sensors, mechanically in one focal plane, or through optical linear beam splitters. These solutions involve high precision positioning and thermal stability to insure perfect imaging line reconstruction. Optical beam splitting avoids blind space between elementary devices, and thus is preferred. However it leads to a heavy and large camera. The new linear image sensor developed by Thomson-CSF is made of 12,000 pixels. This monolithic CCD allows us to cover the whole field of view of a camera in the SPOT 5 mission, and drastically simplifies the whole space-borne instrument. It features on-chip anti- blooming function on each 6.5 micrometer by 6.5 micrometer square pixel, and is mounted in a dual in line ceramic package, specially designed for high flatness and mechanical stability. Main characteristics of this new high resolution CCD linear image sensor are high output data rate up to 40 MHz, 80 dB large dynamic range, very low lag, as well as high electro-optical performances in photo-response uniformity, low dark current and noise. The paper details the CCD architecture, and reports on electro-optic, geometrical and mechanical performances.

In this paper we present preliminary results from our research on CMOS active pixel image sensors (APS) suitable for space applications where radiation hardness, random pixel readout, low power consumption and minimum volume is of particular importance. The APS detector structure offers important advantages compared to CCD sensors that are used in most of today's scientific imagers. The design, layout and some preliminary test results from photogate and photodiode pixel structures, including in-pixel buffer amplifiers and correlated double sampling circuitry, is presented. Both single detector elements and three imaging arrays of 32 by 32 pixels have been developed and tested. The development has been carried out using standard CMOS DLP/DLM technology with 1.2 micrometer design rules.

An evolutionary technique using genetic algorithm is employed to find the position of a data collecting platform (DCP) on the surface of the Earth, according to the Doppler shift of the DCP's transmission frequency, as measured by a satellite. The method uses the orbital attributes of the satellite (i.e., its position in regard to the center of the Earth, and its speed); the Doppler deviation measured by the satellite; and the expected Doppler deviation that should have been measured by the satellite for the candidate DCP under ideal conditions (perfect oscillators and no ionospheric interference). The thrust of this method is that the only model that has to be embedded into it is the calculation of the expected Doppler deviation measured by the satellite for the candidate positions of a DCP. While this calculation can be easily performed, traditional methods have relied on the inverse approach, which is computationally harder to work out. This research is work-in-progress. The results presented here are derived from noiseless synthetic data. Our final target is to apply the model on data obtained from an array of DCPs deployed over the Brazilian territory, and relayed to the ground by the first Brazilian data collecting satellite.

Present large space-based remote sensing systems, and those planned for the next two decades, remain dichotomous and custom-built. An integrated architecture might reduce total cost without limiting system performance. An example of such an architecture, developed at The Aerospace Corporation, explores the feasibility of reducing overall space systems costs by forming a 'super-system' which will provide environmental, earth resources and theater surveillance information to a variety of users. The concept involves integration of programs, sharing of common spacecraft bus designs and launch vehicles, use of modular components and subsystems, integration of command and control and data capture functions, and establishment of an integrated program office. Smart functional modules that are easily tested and replaced are used wherever possible in the space segment. Data is disseminated to systems such as NASA's EOSDIS, and data processing is performed at established centers of expertise. This concept is advanced for potential application as a follow-on to currently budgeted and planned space-based remote sensing systems. We hope that this work will serve to engender discussion that may be of assistance in leading to multinational remote sensing systems with greater cost effectiveness at no loss of utility to the end user.

Satlantic and Orbital Sciences Corporation have developed a concept for a novel satellite scatterometer mission designed to retrieve the global marine wind fields on a daily basis. Using a constellation of 5 - 8 small satellites in low earth orbit, each equipped with a Ku-band scatterometer, ocean basin wide winds could be observed synoptically on time scales consistent with 12 hour weather forecasts. Traditional approaches to design of marine wind scatterometers have been based on the concept of providing a swath, e.g., 500 km wide for ERS-1, which produces bands of dense (25 km spacing, 50 km resolution) observations over the oceans. The observation frequency varies according to repeat cycles, orbit phasing and latitude. On the other hand, global forecast models generally have grids at the same or lower resolution, but evenly spaced. The mismatch in the measured fields with those of the models has reduced the utility of using satellite derived winds for operational data assimilation. The present concept is based on the premise of sampling the winds on a scale more consistent with the current generation of medium term forecast models. The observations would be assimilated into these models, and would provide more stable solutions or model states on which to provide forecasts. The scatterometers are designed to be scanning, pencil beam systems, configured with three beams looking to each side. The scanning is carried out in a fashion that allows a patch on the ocean to be observed at 4 different azimuthal angles, thus providing good resolution of directional ambiguity. Data processing is very simple, avoiding range gating or Doppler processing. The resulting downlink data rates are also low enough to allow the design of a simple, low-cost ground segment that would be designed for ease of assimilation of the global wind fields into operational models in near to real-time.

Current architectural and design trade techniques often carry unaffordable alternatives late into the decision process. Early decisions made during the concept exploration and development (CE&D) phase will drive the cost of a program more than any other phase of development; thus, designers must be able to assess both the performance and cost impacts of their early choices. The Space Based Infrared System (SBIRS) cost engineering model (CEM) described in this paper is an end-to-end process integrating engineering and cost expertise through commonly available spreadsheet software, allowing for concurrent design engineering and cost estimation to identify and balance system drives to reduce acquisition costs. The automated interconnectivity between subsystem models using spreadsheet software allows for the quick and consistent assessment of the system design impacts and relative cost impacts due to requirement changes. It is different from most CEM efforts attempted in the past as it incorporates more detailed spacecraft and sensor payload models, and has been applied to determine the cost drivers for an advanced infrared satellite system acquisition. The CEM is comprised of integrated detailed engineering and cost estimating relationships describing performance, design, and cost parameters. Detailed models have been developed to evaluate design parameters for the spacecraft bus and sensor; both step-starer and scanner sensor types incorporate models of focal plane array, optics, processing, thermal, communications, and mission performance. The current CEM effort has provided visibility to requirements, design, and cost drivers for system architects and decision makers to determine the configuration of an infrared satellite architecture that meets essential requirements cost effectively. In general, the methodology described in this paper consists of process building blocks that can be tailored to the needs of many applications. Descriptions of the spacecraft and payload subsystem models provide insight into The Aerospace Corporation expertise and scope of the SBIRS concept development effort.

The first Brazilian satellite, one out of the family of three data collection satellites -- SCD, has been successfully operating as an automatic collector of environmental data acquired by a set of data collection platforms (PCD) distributed in the Brazilian territory. The data have been used by the scientific community for several applications and studies such as tropical forest regeneration, ozone layer, greenhouse effect, drifting buoys and so on. This paper addresses the software system developed for the Data Collection Mission Center, on tasks of storing, processing and distributing environmental data transmitted by SCD1. The analysis and development of this software were oriented to fulfill the needs of both the Mission Center operators and the users of the environmental data. Techniques for designing user interface centered on user needs were applied.

Acousto optical spectrometers (AOS) have become an attractive alternative to filterbanks or autocorrelators for applications in radioastronomy and in heterodyne as well as in laboratory spectroscopy. Due to continuous improvements, AOSs have now achieved a performance and reliability level that makes this technology applicable for airborne or spaceborne missions. A first fully space qualified AOS was built at the University of Cologne for the Submillimeter Wave Astronomy Satellite (SWAS) to be launched in fall 1995. The SWAS-AOS has a large bandwidth of 1.4 GHz covered by 1365 channels, with a center frequency of 2.1 GHz. Only 11 mW rf white noise input power is required for simultaneous saturation of all channels. The design is optimized for very high stability and allows operation within a temperature range from minus 5 to plus 30 degrees Celsius at temperature variations of up to 2 degrees Celsius/hour. The total weight is 7.2 kg including electronics, the power consumption is 5.4 watts including data pre-averaging electronics and dc-dc converter losses. The performance was verified also after complete vibrational and thermal vacuum environmental testing. For future projects with large bandwidth requirements or with multichannel systems the AOS technology can also be used to fabricate array spectrometers. Such array AOS offers the unique option to multiply the available bandwidth without multiplying the hardware accordingly. Especially for spaceborne applications this is an extremely useful development because weight, power consumption as well as costs increase only very moderately. At present the first prototype with four independent 1 GHz channels is in development. This array AOS will have a total bandwidth of 4 GHz covered by 4000 channels, and will be available in 1996.

The paper describes the development of a remote sensing instrument manufactured utilizing CCD sensors. The main peculiarity of the DARWIN (digital aircraft resources and wildlife imaging) is that it is assembled with commercial electronic components. Consequently it is low cost to produce and easy to maintain and to find the spare parts. The first prototype is presented, named QSM (quick sensing machine), it was developed in the period 1987/90, then a later prototype named DARWIN is presented and the foreseen up grading of the instrument consequent to the managing of the first two prototypes and to the availability on the market of new and more up-dated electronic components. The instrument has four channels and the spectral band of each channel can be selected changing the optical filter mounted in front of each sensor. For its lightweight and its low power consumption it can also be installed on an ultralight platform (ULM). Also a simple, economic and flexible system is described composed by the DARWIN, an ULM with GPS and a software able to manage the images, based on a PC. Finally, a market survey and the possible users are indicated.

The paper introduces an innovative architecture for the on-board units that are responsible to provide the data interface, control and processing capability normally allocated in separated electronics boxes in the data handling subsystem of the space system. A new solution for the attitude control of the space vehicle has been studied and developed and the utilization of this technological growth, in particular that concerns the GPS receiver, is matter for novel architecture. This new approach also involves in general the small satellite ground segment product as matter of a dedicated development approach. Small and medium satellites are considered an attractive solution for the low cost scientific experimentation, communication or remote sensing satellites. The functional and performance capability of the studied on-board units and ground segment are assessed in tight conjunction with the evolution of the European and the USA market. The design of these units has to be based on few and simple driving requirements, directly derived from the new modified scenario: (1) The limited budgets available for space system. (2) The quick mission data return, i.e., low development time by specific and tailored system development tools. The quick availability of data to scientists/user is requested without jeopardizing the maximum and guaranteed scientific or commercial return. The proposed system is then given thinking to an architecture based on a high degree of modularity (and reuse of existing library of modules) thus allowing to keep down costs and to speed up the time to market. The design ground rules are so established in order to cope with the following performance: (1) capability to adapt with few impacts the system interfaces, in particular for attitude sensors and actuators that are tightly mission dependent; (2) easy adaptation of on board computational performances and memory capacity (including mass memory storage capability); (3) definition of a hierarchical and modular software design for the same rationale explained for the hardware.

The interest for an integrated autonomous guidance and navigation control package, satisfying to different mission requirements with a common architecture, is becoming very attractive in the perspective to reduce cost mission and to provide significant benefits when measurements noise conditions may change during the mission and safety critical spacecraft operations are involved. In this paper Laben and Honeywell present an interesting approach to integrate an attitude GPS space receiver and a miniaturized inertial measurement unit (MIMU), to enhance the performances of both sensor systems. In traditional G&NC systems, based on Inertial Navigation Sensor (INS) Measurements, long term drift, affecting zero point stability of gyroscope and accelerometer, are integrated over time during the measurement process, resulting in an increasing attitude and navigation error. These errors can be reduced by periodic reset, shifting the problem to the need of on board accurate and precise absolute position and attitude references. A convenient way to overcome such limitation is here discussed making profit of Laben experience, matured as a company leader in on board data handling and space qualified GPS receiver systems, and by Honeywell as a world leader manufacturer of guidance and navigation packages. The approach would be a guideline for a novel scheme of G&NC architecture where a GPS receiver, performing both attitude and orbit determination, and a MIMU that includes three ring laser gyro and three accelerometers, are integrated in a common unit. In such a system, the measurements performed by the sensors are numerically filtered, removing high side frequency bandwidth noise components, to provide accurate and reliable input data for the attitude and navigation algorithms that will be executed by the embedded guidance computer. The results of such elaboration will be directly the actuation values to drive the space vehicle under both operative and non operative conditions, according to control laws established for the specific mission. With respect to other papers on the subject, the present introduces the characterization of the noise performances of the GPS tensor receiver, the first space qualified GPS receiver performing attitude and orbit determination -- designed and manufactured by Laben, and the analysis of the error model of the MIMU designed and manufactured by Honeywell. From the discussion of such error models a scheme for the sensors data fusion based on an extended Kalman filter is then proposed.

Increasing interest in small satellites has generated a need for detailed information regarding the capabilities and costs of such systems. Of specific interest are comparisons of the cost- effectiveness of small satellites for remote sensing applications with the more traditional large satellites. An additional issue is whether small satellite acquisition philosophies actually enable cost reductions at acceptable levels of risk. To address these issues a series of studies has been conducted at The Aerospace Corporation over the past five years. A large database of cost, technical, and performance characteristics has been assembled for small satellites actually flown or in late stages of development. A small satellite cost model (SSCM) has been derived from this database with cost-estimating relationships (CERs) based largely on small satellite performance characteristics. Associated risk analysis techniques and cost-engineering models (CEMs) have also been formulated, with the SSCM functioning as the central engine for cost analysis of small satellite approaches. These and other tools are applicable to a wide range of small remote sensing satellite analyses.

Using a new microsat called MightySat II as a platform, Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager (FTHSI) to be operated from a spacecraft. This payload will also be the first to fly on the Phillips Laboratory MightySat II spacecraft series, a new, innovative approach, to affordable space testing of high risk, high payoff technologies. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have shown that the instrument is capable of separating a wide range of subtle spectral differences. Variations in the return from the Georges Bank and shoals are discernible and various types of coastal grasses (sea oats and spartina) can be isolated against a sand background.

The Ozone Layer Monitoring Experiment (OLME) is one of the primary payloads for the Chilean FASat-Alfa microsatellite. The objective of this experiment is to measure solar backscattered ultra violet (SBUV) radiation at several wavelengths, with the purpose to retrieve total ozone content, specially over Chile and the Antarctica. The OLME instrument must be light, small and relatively cheap, to meet the mission constraints, so a simple unorthodox design is pursued, using CCDs and interference filters, together with non-imaging detectors. The design concepts behind this new approach to ozone remote sensing are presented, together with the processing procedures developed to retrieve ozone content from radiance measurements.

This paper concerns the development of a compact low-mass wide angle CCD camera for Earth observation. The project will take advantage of the opportunity of a low-cost, short time scale launch to produce early data demonstrating the technology available for 'small cloud' detection, and is of particular interest to future Earth Observing satellite instruments involved in the measurement of sea surface temperature. The 2.5 kg camera will operate at visible wavelengths, and use a large format CCD to achieve 250 m by 250 m resolution on the ground, and with a field of view of plus or minus 8.5 degrees (equivalent to 194 by 144 km on the ground). The challenge in the designing and building of this instrument has been the need to maintain performance with restrictions on the available space, mass, power, and schedule. This paper describes the scientific objectives, system, and detailed design of the camera.

SCIAMACHY (scanning imaging absorption spectrometer for atmospheric cartography) is a spectrometer with a wavelength range stretching from the UV (240 nm) to the NIR (2380 nm), addressing the trace gases important in the ozone cycle and the gases involved in global climate change. It is scheduled to fly on ESA's ENVISAT, launch 1998/99. In concept the instrument is similar to the recently launched instrument GOME on ERS-2, which is a scaled- down version of SCIAMACHY. For the UV and visible wavelengths the RL 1024 SR detector arrays of EG&G Reticon are employed, similar to those of GOME. For the detection of wavelengths beyond 1000 nm (not present in GOME) InGaAs is selected as detector material. Up to 1600 nm the lattice-matched In_.53Ga_.47As is used. For the longer wavelengths, strained-layer In_xGa_1-xAs, with x between 0.60 and 0.83, has been developed by Epitaxx Inc. (New Jersey). Emphasis during the development was on extending the sensitivity to longer wavelength, whilst keeping the dark current (noise) within acceptable levels. In this paper we present the design and the first performance results of the flight models for all InGaAs focal plane arrays (FPAs). Measurements of the spectral response, dark current and noise are presented, in combination with characteristics of the application-specific capacitive trans-impedance amplifier multiplexer. The performance data of the NIR FPAs have been incorporated in the SCIAMACHY-instrument simulator. As an example of its use, the predicted sensitivity to retrieve CO, N₂O and CH₄ abundances is given.

The POLDER (polarization and directionality of earth reflectances) sensor has been developed by the French Centre National d'Etudes Spatiales (CNES) to be flown on board NASDA's ADEOS polar orbiting satellite. The POLDER sensor is a wide-field-of-view (114 degrees, 2400 km swath), moderate resolution (6 multiplied by 7 km² at nadir) and multi-spectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the Earth-atmosphere-ocean system. In addition to the classical measurement and mapping capabilities of a narrow-band multispectral imaging radiometer, POLDER will provide new opportunities by measuring polarized reflectances and observing target reflectances from up to 14 directions along a single satellite pass and from more directions by multi-pass combination. Both the bidirectional reflectance distribution function (BRDF) and the bidirectional polarization distribution function (BPDF) are sampled and determined from the POLDER data. The bulk of the observations performed with the POLDER sensor are expected to contribute to advances in climate-related research on aerosol cycling, aerosol-cloud-radiation interactions, the Earth radiation budget, ocean primary production and continental biosphere dynamics as well as to other environmental studies.

The Mission to Planet Earth will provide long term measurements of the Earth as a global system; the first series of missions is already under development. The second series of missions will continue the measurement baseline while evolving beyond today's technology. The EOS-AM2 mission is the second in the EOS-AM series of remote sensing spacecraft and is scheduled to launch in 2004. The measurements being considered for the EOS-AM2 mission include the Earth's radiation budget and atmospheric radiation, global land use, and land cover change, local-scale ecological and biogeochemical processes, global aerosol distribution and cloud properties, top-of-atmosphere, cloud, and surface angular reflectance functions, surface albedo, aerosol, and vegetation properties, as well as biological and physical processes on land and the ocean. Several options are being explored to deploy the set of instruments to carry out these measurements, including single spacecraft as well as multiple spacecraft configurations. The driving requirements contributing to the choice of a spacecraft configuration include measurement continuity, coverage, resolution, repeat cycle, and calibration.