Several advances in the spacecraft industry have resulted in a new class of smaller spacecraft. The relative affordability of these spacecraft makes them attractive for consideration for several types of satellite missions. Studies have been performed in recent years examining the cost effectiveness of small satellite architectures utilized specifically for low Earth orbit (LEO) scientific missions. The results of these studies vary depending upon the assumptions used and the nature of the m missions addressed. This paper explores the cost effectiveness of small and large satellite architectures for post-2002 Mission to Planet Earth missions. The cost comparison examines the life cycle cost of the space, launch and ground segments of each complete mission. The analysis is based on spacecraft bus costs and historical spacecraft and launch vehicle reliabilities as well as estimations of the costs of oversight, integration and testing and operations for each of the missions. The analysis examines the architecture, single or multiple sensor platforms, best suited to orbit several sensors for LEO scientific missions.

Following the launch of the SPOT4 satellite, and preliminary in-flight tests, the operational phase of the VEGETATION system is under way. The mission, system and orbital payload are described. Emphasis is put on the excellent image quality, which fulfills all users' requirements and design goals: in-flight measured image performances are presented and compared to on-ground predictions. VGT-P, VGT-S1 and VGT-S10 image products delivery is presently starting and full operation of the system is scheduled by the end of 1998.

NASA's New Millennium Program selected a Doppler Wind Lidar (DWL) using heterodyne detection for the EO-2 mission scheduled for launch in 2001. The Space Readiness of Coherent Lidar Experiment (SPARCLE) will involve a 100mj, pulsed 2 micron laser and a 25 cm wedge scanner. This mission represents the first time a DWL will have flown in space to make tropospheric wind measurements. In addition to demonstrating that the technology can operate in a space environment, the SPARCLE will also provide data sets that are critical to the validation of system performance models and the selection of optimal scanning patterns required for future data products. Extensive computer simulations have been conducted for SPARCLE and some of the follow-on mission concepts that would lead to a fully operational instrument. Global weather forecasting models have been used to provide realistic representations of cloud, water vapor, winds and their effects on DWL data product accuracy and coverage. Results from these simulation efforts indicate that a Coherent DWL can be built and operated to provide virtually full tropospheric coverage, clouds permitting.

Landsat-7 will carry the enhanced thematic mapper plus (ETM+) as its payload. This instrument is a derivative of the Thematic Mapper (TM) instruments flown on the Landsat 4 and 5 spacecraft. Key changes to the instrument include a new 15 meter panchromatic band, a higher spatial resolution thermal band and two new solar calibrators to improve the radiometric calibration of the reflective bands.

The primary instrument of the first Earth Orbiter satellite (EO-1) under NASA's New Millennium Program will be an Advanced Land Imager (ALI), with multispectral and imaging spectrometer capabilities. The principal motivation for this mission is to flight-validate advanced technologies which are relevant to the next-generation of Earth Science Systems Program Office science needs. The ALI telescope is a reflective triplet design employing silicon carbide mirrors with a 15 degree cross-track field of view. There are three imaging technologies in the ALI. The first is a multispectral panchromatic array with 10 spectral bands in the visible and near IR and short wave IR. The two additional imaging technologies are the Wedge Imaging Spectrometer (WIS) and the Grating Imaging Spectrometer (GIS) that each provides a continuous range of wavelength selections from 0.4 to 2.5 micrometers . Elements of the WIS and GIS were developed but due to budgetary and schedule constraints, and some performance issues, were not included in the flight assembly. The paper will present details of the ALI design and status.

The Medium Resolution Imaging Spectrometer (MERIS), developed under European Space Agency (ESA) contract, for the ENVISAT 1 Polar Orbit Earth Mission belongs to a new generation of Ocean Color sensors which aim is to improve the knowledge of some crucial processes of our planet. The instrument development is currently carried out by an international team led by AEROSPATIALE under the ENVISAT prime contractor ship of DORNIER. MERIS is a pushbroom instrument measuring the radiance of the Earth in 15 programmable spectral bands between 390 nm and 1040 nm over a 1150 km swath width. The wide spatial extent is obtained by 5 cameras with a field of view of 14 degrees each located in a fan shape configuration. This paper presents MERIS FM instrument performance verification plan, some of the involved test set ups an the measured performances.

Board information system 'STEP-E' intended for identification, energy measurements and determination of space-time response of the magnetosphere and space eager particles is presented. It operates together with telescopic detectors, which are installed from outside satellite. As a result of processing of detector signals, non-stationary streams in the manner of sharp change of the stream intensity in comparison with the average intensity of the stream reveal. These non-stationary streams carry information about process under investigation. The methods of corresponding pulse impositions, defining intensity of non-stationary pulse streams of eager particles and finding correlation relationship between intensities of casual particles streams and parameters of electrical and magnetic fields were used under the developed system 'STEP-E'.

The Optical Assembly (OA) for the Multispectral Thermal Imger (MTI) program has been fabricated, assembled, and successfully tested for its performance. It represents a major milestone achieved towards completion of this earth observing EO imaging sensor that is to be operated in low earth orbit. Along with its wide field of view, 1.82 degrees along-track and 1.38 degrees cross-track, and comprehensive on-board calibration system, the pushbroom imaging sensor employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 micrometers . The OA has an off-axis three-mirror anastigmatic telescope with a 36-cm unobscured clear aperture. The two key performance criteria, 80 percent enpixeled energy in the visible and radiometric stability of 1 percent 1 (sigma) in the visible/near-IR and short wavelength IR, of 1.45 percent 1 (sigma) in the medium wavelength IR, and of 0.53 percent 1 (sigma) long wavelength IR, as well as its low weight and volume constraint drive the overall design configuration of the OA and fabrication requirements.

Global wind profiles are needed for a wide range of meteorological applications. Since the 1980's, observing system simulation experiments have been conducted in order to evaluate the potential impact of space-based wind profiler data on numerical weather prediction, and to evaluate trade-offs in lidar design. These experiments indicated tremendous potential for satellite lidar observations to improve atmospheric analyses and forecast. More recent experiments are aimed at assessing the precise requirements for space-based lidar wind profile data and to evaluate the potential for alternative technologies.

A Refocusing Mechanism (REM) has been designed and built for the SEVIRI instrument, the imager in the Meteosat Second Generation (MSG) satellite. The purpose of the mechanisms is in-orbit focal adjustment from its geostationary assembly which forms part of SEVIRI. As the MSG satellite observes the earth and its atmosphere from its geostationary orbit, the REM will be used to compensate for satellite structural distortions due to gravitational-, thermal- and aging effects. It is designed for frequent use. It enables in- orbit refocusing in 1.4 micrometers steps over a range of 2 mm with virtually no spurious rotations and translations. The REM was designed to support a large 3.9 kg mirror module in an ARIANE 5 rocket launch environment without the use of clamping mechanisms. Further major design drivers were the limited volume and the strict mass requirement. The REM function is realized in two modules. A guide module, based on elastic elements provides the required stroke within the strict requirements on lateral deviation and tilt. An actuator module with a motor, a planetary gearbox and a recirculating rollerscrew provides the force for translation of the mirror assembly. Coupling of the two modules has been realized by a dedicated leverarm structure.

In 1991 Carl Zeiss started a program to develop powerful telescopes for airborne and spaceborne earth-observation telescopes. This article summarizes some of the main result of this program. To emphasize the importance of these activities a short historical review was added.

This paper is devoted to the presentation of an Integrator Dewar Cooling Assembly dedicated to high frequency space applications. SAGEM SA has been a manufacturer of IR InSb and HgCdTe detectors for a long time. These detectors cover a large spectral range. The capability to use HgCdTe photovoltaic detectors for heterodyne applications at 10.6 micrometers has been demonstrated in the frame of ESA and CNES contracts. SAGEM SA has recently developed a new concept of heterodyne receiver, totally integrated, operating at variable temperatures down to 77K, using HgCdTe or InSb photovoltaic detectors. This receiver is an innovative product due to its small volume, its low weight and its low electrical consumption. The miniaturization of this product the latter to be used in space applications, specially for the earth observation missions. The performance of such a receiver with respect of the electrical bandwidth is presented in order to compare it with a receiver for terrestrial or airborne applications based on the use of a laboratory HF dewar.

The Satellite Attitude Sensor (SAS) utilizes IR bolometer arrays to perform the role of a staring horizon sensor with good accuracy and a wide field of view that avoids mechanical scanning. An innovative concept has resulted in simple and inexpensive design suitable for the requirements of both low and high altitude orbits. Although SAS was designed especially for Earth-observing micro-satellites, it is applicable to satellites with geosynchronous or highly elliptical orbits and the update rate is sufficient for use with a spinning satellite or a rocket. The ability of the sensor to operate in both spinning and 3-axis stabilized modes will allow it to fulfill the needs of large variety of scientific and remote sensing applications. Laboratory test are in process to verify the design.

An activity is underway to establish the feasibility of using the Moon as an irradiance standard to calibrate the visible channel of the GOES Imagers. This channel is calibrated prior to launch but there are no facilities in the instrument for in-flight calibration of the visible band. The Moon is with in the Imager field of view for a few days disturbance of the operational observations. This makes it relatively easy to compare instruments on different GOES separate NASA-sponsored program, ground-based telescopic measurements of the Moon are being made in several spectral bands between 0.34 and 0.95 micrometers to establish the Moon as a spectral radiance standard over a wide range of illumination and liberation angles. Histograms of images of the Moon can be converted to total lunar irradiance using prelaunch calibration. This will eventually be compared with the predicted total radiance for the Moon from ground measurement to update the calibration. There are over 100,000 pixels in a GOES image of a full Moon, which enables a precise measurement. Imager test data from GOES 10 was analyzed and preliminary estimates of the performance of the two different analysis approaches are presented. This paper includes a comparisons of the relative calibration of the Moon at two times, a few days apart, which have significant changes in the Moon's illumination. This data is used to assess the ability of the model to track these changes assuming the instrument was stable over these few days. Further work will be required to improve performance of the analysis of these space data, in the ground model and in the use of this type of data to make an absolute calibration of the Imager.

Each Imager and Sounder on the present generation of GOES satellites has an eight-channel visible detector array. These visible arrays perform star-sensing measurements to establish inertial attitude references so that earth observations can be registered to the earth's latitude and longitude coordinates. The Imager's visible array also performs earth observations. We have used the archived signal levels of star measurements to estimate the long-term throughput loss in these channels of the Imager and Sounder on GOES 8 and GOES 9. An exponential decay rate was determined for each sensor by averaging the values derived from each of approximately 30 stars over a time interval of at least 500 days. Large degradations in image quality occur during local night, when direct sunlight enters the optical ports of the two sensor. Therefore, we have deleted observations made during the 10 hour interval around midnight, satellite time, from our analysis. Variable stars and stars with low signal-to-noise ratios were also excluded. The annual throughput losses for the four sensors, derived from measured star signal levels, range from 3.8 percent to 9.6 percent.

Understanding the sources of uncertainty in GOES Imager IR data is important to meteorologists and scientists who develop meteorological products. One component of radiometric uncertainty that is not well characterized, unlike noise and calibration errors, arises from the sensors's MTF. To understand this effect it is necessary to know the amount of power at high spatial frequencies in a typical scene. The sensor MTF, however, acts as a lower pass filter on the scene spatial frequency content, passing low frequencies and attenuating higher frequencies. To study the effect of the higher spatial frequencies in a scene, a model of both sensor MTF and scene spatial frequency content has been developed. The scene model is based on data from the Modis Airborne Simulator (MAS), a 50 channel radiometer- imager flown aboard a NASA ER-2. The MAS sensor has a 50 m IFOV at nadir, compared to the GOES channel radiometer- imaging flown aboard a NASA ER-2. The MAS sensor has a 50 m IFOV at nadir, compared to the GOES 4 km IFOV. The data sets from which the scene model was developed contain various combinations of land and clouds from several flights. Overlapping power spectral densities from the two sensors validate the use of MAS data for a GOES scene model at high spatial frequencies. The sensor MTF model is based both on measurements made during pre-launch testing and on theoretical calculations from sensor f-number, detector size and electronics filtering. The MTFs of Imager channels 2 and 4 are compared. Their difference is applied to the scene power spectra to evaluate the average radiometric error due to MTF differences.

The edge of the mono is used as a high contrast target to perform a visible 'knife-edge' MTF test on a digital imaging system in geostationary orbit. An image of the moon is taken in the camera's normal scanning mode, and traces across the sharpest edge are used to form an edge spread function (ESF). The ESF is then sued to produce an MTF estimate. In a second trial, the imaging system stares as the lunar edge drifts by, creating an edge spread function with a much higher effective spatial sampling rate. In each case, a technique of combining and resampling traces is employed to adapt the knife-edge MTF technique for use with sampled data. The resulting MTF curves track ground test frequencies to within five percent. The phase transfer function is also extracted, and the process is repeated in the north/south direction. The functions are combined to produce a 2D optical transfer function (OTF) which is used as an inverse filter to restore raw images via deconvolution. The approach thus offers a means of testing the MTF and OTF of orbiting image acquisition devices as well as enhancing satellite imagery.

Examination of future NOAA Geostationary Operational Environment Satellite (GOES) IR sounder requirement suggested replacing the present GOES I-M series filter wheel instrument with a high-resolution FTIR interferometer. NOAA and MIT Lincoln Laboratory initiated a design and test program replacement feasibility. In collaboration with NASA and ITT Aerospace, a brassboard-version GOES High-Resolution Interferometer Sounder (GHIS) developed under this pathfinder program was installed inside an earlier generation sounder at ITT Aerospace. This paper describes the suite of tests performed while operating the GHIS brassboard at room temperature inside the sounder. Results from the tests effort highlight key issues involved in characterizing FTIR interferometer performance for GOES sounder applications.

The Emergency GOES Imager study responds to the potential need for a small, back-up imager for weather observations in the event of failure of one or more of the current GOES satellites. The Emergency GOES Imager (EGI) is designed to be compact and lightweight. Minimal spatial resolution is required in the visible and IR band for the purpose of synoptic forecasts. The ground resolution requirement is 16 km for the 10.2 to 11.2 micrometers IR band and 4 km for the 0.5 to 0.7 micrometers visible band. Due to the small size of the instrument, the EGI has the potential to be deployed either alone on a small launcher or as an auxiliary payload on a larger satellite. The overall size of the EGI is dependent on the orientation of the satellite because of the dependence on amount of solar shielding required for the cooler, and the choice of coolers for specific satellite orientations. Although the EGI design is for an emergency system, the design utilizes recent technology in the form of both a linear IR focal plane array, in front of its constant-motion mirror, and a visible CCD array with a staring-format. The IR array has the potential to present a technical challenge to array manufacturers in the area of calibration, assuming a 0.1 K NEDT. We discuss the means by which the emergency requirements are met with this small and simple system, define the limiting technologies in the design, and explore modifications necessary to expand these requirements.

The Moderate Resolution Imaging Spectrometer (MODIS) will be one of the primary instruments observing the earth on the Earth Observing System (EOS) scheduled for launch in 1999. The relative spectral response of MODIS is an important part of the radiometric calibration train. A double filter grating monochromator anomalous spectral response was observed in some MODIS bands but not in others. Special tests were made which illustrated a spectral cyclic structure which should not have been present. A computer model was built of the double monochromator that enabled small perturbations to be introduced for the first and second monochromator sections. A parametric study illustrated that the anomalous spectral response behavior could be duplicated by assuming a constant and a variable offset between the first and second monochromator sections. The cause of this was found and a simple solution was implemented. The solution was to have the first monochromator section have a larger spectral slit function than the second section. Selected data will be shown to illustrate the phenomena.

The Spectro-Radiometric Calibration Assembly (SRCA) is briefly described. This paper discusses one of the SRCA functions -- spatial calibration to determine the channel position shift along-scan. The band centroid shift along- track will be discussed in Part II. Discussion in this part mainly focuses on the algorithm, the methodology, and the stability of the calibration signals. The algorithm and the methodology includes: a model of the image motion as a function of data sample and the centroid calculation to determine the detector/band positions. The SRCA stability is examined by checking the data inconsistencies between redundant samples, sub-samples, and the two mirror sides. Also described is the handling of the dark-reading for the thermal bands, which is affected by background energy for the opaque part of the reticle. A series of results are presented, which compare the relative position shift and co- registration between channels, bands, and Focal Plane Assemblies. Test results are illustrated for calibration under different environments and by different instruments. These results indicate that the SRCA is a stable an sensitive calibrator which provides consistent results. The overall uncertainty analysis will be presented in Part II.

This paper discusses the SRCA spatial calibration for the band centroid shift along-track. It is the second part of spatial calibration, which is for along-track. Next is a discussion of the linear range of weighted Y, which is the moment arm in the centroid computation, and the effect of MODIS/SRCA magnification change on the Y value. Also described is the relative calibration between channels under different environments and by different instruments. An overall summary of uncertainty sources and spatial calibration accuracy are presented.

The Moderate Resolution Imaging Spectroradiometer (MODIS) will be one of the primary instruments observing the earth on the Earth Observing System (EOS) scheduled for launch in 1999. MODIS polarization performance characterization was required for the 0.4 to 0.6 micrometers (VIS), 0.6 micrometers to 1.0 micrometers (NIR), and 1.0 micrometers to 2.3 micrometers (SWIR) regions. A polarized source assembly (PSA) consisting of a collimator with a rotatable Ahrens polarizer was used to illuminate MODIS with a linearly polarized beam. MODIS signal function having two-cycles per 360 degrees prism rotation signal function was expected. However, some spectral bands had a distinct four-cycle anomalous signal. The expected two-cycle function was present in all regions with the four-cycle anomaly being limited to the NIR region. Fourier analysis was very useful tooling determining the cause of the anomaly. A simplified polarization model of the PSA and MODIS was generated using Mueller matrices-Stokes vector formalism. Parametric modeling illustrated that this anomaly could be produced by energy having multiple passes between PSA Ahrens prism and the MODIS focal plane filters. Furthermore, the model gave NIR four-cycle magnitudes that were consistent with observations. The IVS and SWIR optical trans had birefringent elements that served to scramble the multiple pass anomaly. The model validity was demonstrated with an experimental setup that had partial aperture illumination which eliminated the possibility of multiple passes. The four-cycle response was eliminated while producing the same two-cycle polarization response. Data will be shown to illustrate the four-cycle phenomenon.

The Moderate Resolution Imaging Spectroradiometer (MODIS) will be one of the primary instruments observing the earth on the Earth Observing System (EOS) scheduled for launch in late 1999. The solar diffuser BRF characterization is required for MODIS on-orbit reflectance calibration. The system reflectance calibration accuracy requirement is 2 percent covering a spectral region of 400 to 2300 nm. An internal flow down specification of 1 percent was allotted to characterization of the BRF. The SBRS scattering goniometer will be described. The source is a quartz-halogen lamp. Multiple field stops, aperture stops, and baffle masks were experimentally optimized to reduce scattered light to acceptable levels. In addition the room was made 'light tight'. Glan Thompson and Wollaston polarizers were used in the illuminating and viewing arms, respectively. Three sets of detectors were used to cover the 400 to 2300 nm range: PMT, Ge, and PbS. The rotary and translation stages used to move the solar diffuser, polarizers, optical filters, and detectors being computer controlled, which permitted measurements to be made remotely. This scattering goniometer is a relative device, so the flight solar diffuser is characterized by measuring it relative to a diffuser which was characterized by NIST. The transfer to the NIST standard was done before and after solar diffuser characterization. BRF measurements were made at five wavelengths and nine illumination angles with one out-of-plane observation angle. Multiple BRF measurements were made to determine repeatability and spatial uniformity. Measured BRF data will be presented.

The MODIS will be one of the primary instruments observing the earth on the EOS scheduled for launch in 1999. The on orbit radiometric calibration requirement for 3.5 micrometers to 14.3 micrometers spectral region is 1 percent with the sea surface bands at 0.5 percent. The on board calibrator blackbody (OBC BB) has a V groove cavity construction with polished low reflectance surfaces which provides an effective cavity emittance of 0.997 which has been characterized with a ground calibration blackbody system having an emittance > 0.999. The OBC BB provides an on orbit end to end system level radiometric calibration. Normally it operates at MODIS ambient temperature with a secondary mode of being heated to 315 K and then transitioning to ambient. Its temperature is monitored with 12 glass encapsulated thermistors whose initial temperature calibration is within 0.013 K traceable to NIST standards. The standard deviation of the 12 sensors during the cool down transition varies between 0.08 to 0.03 K with a weighted effective standard deviation of 0.042 K. The temperature profile will be graphically illustrated temporally and spatially. The relationship of MODIS viewing geometry related to BB is illustrated.

A key milestone in NASA's Mission to Planet Earth project was achieved with the completion of Environmental Qualification testing of the Protoflight Model (PFM) Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Completing this task paved the way for MODIS to be integrated onto the EOS AM spacecraft which is to be launched as the keystone of the EOS system. Qualification of the PFM MODIS instrument required conducting an extensive test program in four different test facilities. Accomplishing environmental qualification testing, while meeting the stringent contamination and operational requirements for the MODIS instrument, required us to address a variety of issues and tasks. The main tasks included: developing special ground support test equipment, developing special tenting and handling equipment to protect the instrument from being contaminated during off-site environmental vibration an electromagnetic compatibility testing, designing and developing a state-of-the-art thermal-vacuum test chamber, and defining detailed test operations to fully characterize the instrument's electrical, optical and mechanical performance before, during and after each environmental test sequence. Selected penalty test were streamlined for characterizing the instrument whenever design changes or improved test techniques were incorporated to ensure all requirements had been met while maintaining a fully qualified instrument.

The Clouds and Earth Radiant Energy System (CERES) instrument is designed to measure the Earth's radiation budget and also to make measurements from which the anisotropy of reflected solar radiation can be computed. The instrument design, which is based on the Earth Radiation Budget Experiment (ERBE), and its operations are described. The instrument can scan in elevation and azimuth simultaneously. The azimuthal rotation is important for gathering data to describe the anisotropy of the reflected solar radiance field. The ground vacuum calibration facility ties the calibration of the instrument to the International Temperature Scale of 1990. In-flight calibration sources are included to maintain and demonstrate the required 1 percent accuracy of each mission. Flight operations to achieve the accuracy are also discussed. The CERES Proto-Flight Model is flying on the Tropical Rainfall Measurement Mission spacecraft and successive models are scheduled to fly aboard the EOS/AM-1 and EOS/PM-1 platforms. The objectives of each flight of the instrument are discussed.

Each Clouds and the Earth's Radiant Energy System (CERES) instrument contains three scanning thermistor bolometer radiometric channels. These channels measure broadband radiances in the shortwave (0.3 —5.0 rim), total (0.3 - <100 j.tm) and 8 — 12 im water vapor window regions. Ground-based radiometric calibration of CERES instruments is conducted by TRW, Redondo Beach, CA using their state of the art Radiometric Calibration Facility (RCF). Among the many features of the RCF are longwave and shortwave calibration systems. RCF blackbodies tied to the International Temperature Scale of 1990 ( ITS'90) are used for longwave calibrations. A unique Shortwave Reference Source (SWRS) allows calibration in the 0.4 to 2.0 m region. Additionally, a cryogenically cooled Transfer Active Cavity Radiometer (TACR) allows the transfer of the RCF longwave blackbody source calibration onto the shortwave RCF source. The CERES Internal Calibration Module (1CM) contains blackbody sources for the Total and Window radiometric channels and an evaporated quartz-tungsten lamp for the Shortwave channel. The 1CM sources are used to monitor the radiometric stability of the CERES instruments over their lifetime. Currently three CERES flight instruments (Proto-Flight Model, Flight Model 1, and Flight Model 2) have undergone radiometric calibration in TRW's RCF. This paper presents the timeline of radiometric stability for the PFM instrument since its original calibration in the RCF. This timeline includes spacecraft level thermal-vacuum testing and initial on-orbit data. Keywords: CERES, TRMM, EOS, Radiometry, Calibration

The Clouds and the Earth's Radiant Energy System (CERES) instrument has scanning thermistor bolometers that measure broadband radiances in the shortwave, total and 8-12 micron water vapor window regions. On November 27, 1997, the CERES Protoflight model (PFM) instrument was launched aboard the Tropical Rainfall measuring Mission spacecraft. In December 1998, the CERES FLight models I and II instruments are scheduled for launch on the Earth Observing System-AM1 platform. The instrument generally operates in three scan modes; crosstrack normal, rotating azimuth normal and rotating azimuth short modes, while measuring the earth reflected and emitted radiances. The sensor measurements have shown a dependency on observation geometry during each of these scan modes of operation. At each elevation observation angle, the zero radiance offsets of the sensors were measured on the ground using end caps and a constant radiance reference source, consisting of a curved strip blackbody. On-orbit, offsets were determined from observations of cold space. This paper describes the procedures and facilities used to determine the zero radiance offsets. The offset values calculated from ground and in-flight data for TRMM sensors, as well as the ground measurements for the FM1 and FM2 sensors are presented.

The CERES spacecraft scanning thermistor bolometers measure earth-reflected solar and earth-emitted longwave radiances, at the top-of-the-atmosphere. The bolometers measure the earth radiance in the broadband shortwave solar and total spectral bands as well as in the 8-12 micrometers water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. November 1997, the first set of CERES bolometers was launched on the Tropical Rainfall Measuring Mission spacecraft. Ground vacuum calibrations define the initial count conversion coefficients that are used to convert the bolometer output voltages into filtered earth radiances. The mirror attenuator mosaic (MAM), a solar diffuser plate, was built into the CERES instrument package calibration using the solar radiances reflected from the MAM's. Each MAM consists of baffle-solar diffuser plate system, which guide incoming solar radiances into the instrument fields of view of the shortwave and total wave sensor units. The MAM diffuser reflecting type surface consists of an array of spherical, overcoated with silicon dioxide. Thermistors are located in each MAM plate and baffle. The CERES MAM is designed to yield calibration precisions approaching 2 percent for the total and shortwave detectors. In this paper, the MAM solar calibration data reduction techniques are presented along with on-orbit measurements.

A new shortwave calibration standard known as transfer active cavity radiometer (TACR) was developed by TRW to overcome the problem of nonuniform wall temperatures of the integrating sphere in ground calibration of the Earth Radiation Budget Experiment (ERBE) scanning sensors. The TACR is cryogenically cooled near 5 K, and is calibrated using the Narrow Field-of-view blackbody source at different temperatures. The TACR is used to transfer the blackbody radiometric scale of International Temperature Scale of 1990 to the shortwave calibration standard called the shortwave reference source (SWRS). The SWRS is used as the primary source for calibrating spacecraft shortwave thermistor bolometers and the shortwave portion of the total thermistor bolometers. This paper describes the TACR's design and its uses in calibrating the CERES shortwave and total bolometers in the shortwave spectral region between 0.4 to 2.0 micrometers . On November 27, 1997, the CERES Proto-flight model instrument was launched aboard the Tropical Rainfall Measuring Mission spacecraft. During December 1998, the CERES flight models 1 and 2 will be launched aboard the EOS AM spacecraft. The resulting gain values for shortwave and total scanning sensors obtained by using the SWRS ground calibration data are detailed and discussed.

All physical measuring devices have finite response times determined by the characteristics of their optical, electrical, and thermal components. In a system which is not time-critical, one merely waits for the system to stabilize, and then reads the measurement. In a system which is in motion, such as the CERES scanners, the spatial location of any measurement is dependent upon the response time of the detector. In order for the remotely sensed data to be accurately geolocated, the systematic delays between the time-of-observation of the scene and the time-of-measurement must be determined. The first CERES instrument was launched aboard the Tropical Rainfall Measuring Mission (TRMM) platform from Japans Tanegashima Island Space Center on 28 November 1997. The next two CERES instruments are scheduled for dual launch on the EOS/AM-1 platform in late 1998. In this paper we specifically address the delay between the time of observation of a scene, and its actual measurement as determined from Point Response Function Source data taken in the CERES calibration chamber at TRW in Redondo Beach; and we compare the theoretically generated Point Spread Functions to these measurements. The agreement between the measured and theoretical contours in excellent, in the total and shortwave channels. For all 3 instruments, the calculated time-delay of 0.023ms to the centroid,and the measured peak delay of 0.022ms are very consistent. The result of on-orbit coastline detection algorithms are currently being analyzed in comparison with the PSF delays used in the geolocation algorithms in order to further validate the proper geolocation of the measured data.

The first CERES instrument was launched on NASA/NASDA's Tropical Rainfall Measurement Mission (TRMM) platform Nov. 28, 1997. More CERES instruments will soon be launched on NASA's Earth Observing System (EOS) satellites. CERES measures top of the atmospheric radiative fluxes using microprocessor controlled scanning radiometers. The CERES EOS-AM1 Instrument Simulator will serve as a testbed for the testing of custom instrument commands intended to solve in- flight anomalies of the instruments which could arise during the CERES EOS-AM1 mission. The concept of using an identical but non-flight qualified microprocessor and electronics ensemble linked to a virtual instrument PC running modeling software results in a relatively inexpensive simulation system capable of high fidelity. The first CERES Instrument Simulator resembles the TRMM CERES instrument sufficiently well to allow benchtop testing and functional verification of microprocessor loads for TRMM instrument unloads, circuitry identical to the flight unit's twin microprocessors and telemetry interface to the supporting spacecraft electronics and two personal computers (PC) connected to the I/O ports that control azimuth and elevation gimbals. Flight simulation software consists of the unmodified TRW developed Flight Code and Ground Support Software specific to the platform under study which serves as the instrument monitor and also links NASA/TRW developed engineering models of the gimballed instrument. The CERES engineering development software models were modified to provide a virtual instrument running in real-time on a second PC linked to the flight microprocessors instrument control ports. The cost of the electronics and development of such a simulation system is dwarfed by the cost to develop the actual flight software, which has already been invested in building the actual instrument. Thus for the simulator, flight software re-use is very cost effective. The concept of using dedicated electronics and specific flight software matched to the instrument being studied allows for high fidelity simulations of each instrument system. The differences in flight code that represent the final flight configurations from platform to platform are accurately modeled. The overall task of tailoring the simulation system hardware to a given instrument platform reduces to the art of interfacing and programming a pair of embedded microprocessors linked to a pari of commercially available PCs.

Clouds and the Earth's Radiant Energy System (CERES) is a NASA investigation to examine the role of clouds in the radiative energy flow through the Earth-atmosphere system. The first CERES scanning radiometer was launched on November 27, 1997 into a 35 degree inclination, 350 km altitude orbit, on the Tropical Rainfall Measuring Mission (TRMM) spacecraft. The CERES instrument consists of a three channel scanning broadband radiometer. The spectral bands measure shortwave, window and total radiation reflected or emitted from the Earth-atmosphere system. Each Earth viewing measurement is geolocated to the Earth fixed coordinate system using satellite ephemeris, Earth rotation and geoid, and instrument pointing data. The interactive CERES coastline detection system is used to assess the accuracy of the CERES geolocation process. By analyzing radiative flux gradients at the boundaries of ocean and land masses, the accuracy of the scanner measurement locations may be derived for the CERES/TRMM instrument/satellite system. The resulting CERES measurement location errors are within 10 percent of the nadir footprint size. Precise pointing knowledge of the Visible and IR scanner is required for convolution of cloud properties onto the CERES footprint; initial VIRS coastline results are included.

Since 1978, long-term variations in the total solar irradiance have been monitored using spacecraft radiometers, at the 0.01 percent precision level. The irradiance measurements were performed from the Earth Radiation Budget Satellite (ERBS), Nimbus-7, Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS), European Retrievable Carrier, Solar and Heliospheric Observatory (SOHO), and the Space Shuttle Atmospheric Laboratory for Applications and Science (ATLAS) spacecraft platforms. Radiometer response can drift or shift at precision levels of a few hundreds of a percent. In-flight calibration sources are not available to detect radiometer response changes at radiometric accuracy or precision instrumental drifts or shifts which may be incorrectly interpreted as solar irradiance changes while consistent trends among the different measurements sets wee used to detect long-term irradiance variability components. In this paper, 1991-1998 corresponding ERBS, UARS, SOHO, and ATLAS irradiance measurements are inter-compared with each other as well as with the ERBS empirical irradiance fit. The empirical irradiance fit is based upon 10.7-cm solar radio flux and photometric sunspot index, indices of solar magnetic activity. Analyses of recent data sets identified no long- term shifts and drifts in the ERBS, SOHO, or UARS data sets. The typical value of the total solar irradiance is approximately 1365 Watts per meter squared.

The EO-1 Advanced Land Imager (ALI) is the first earth- orbiting instrument to be flown under NASA's New Millennium program. The ALI employs novel wide-angle optics and a multispectral and panchromatic spectrometer. EO-1 is a technology verification project designed to demonstrate comparable or improved Landsat spatial and spectral resolution at a substantial cost saving. This paper provides an overview of ground calibration and performance assessment of the ALI. Included are techniques for calibrating and assessing spatial, spectral, and radiometric parameters for this instrument. Additionally, the pre-flight technique used to verify in-flight solar calibration is discussed.

Comparison of the spectral resolving power of different types of spectrometer can be facilitated by the use of the spectral modulation transfer function (SMTF). It is the Fourier transform of the instrument spectral response function. The SMTF's of the two imaging spectrometers included in the Advanced Land Imager of the EO-1 satellite are shown as examples. The SMTF offers a way to specify the spectral resolution required for the detection of known spectral features. It can also be used to determine the ultimate resolution that can be achieved through processing signals form any given instrument and observation.

Imaging spectrometers for the earth observation on unmanned satellites usually work for some years without a chance of direct inspection of their performance data other than by ground-based data evaluation, given and determined during the integration and calibration procedure on ground. Radiance sources, integrated in the instrument assembly itself, can be used for periodic check of the stability or the changes in some instrument parameters, which influence the reliability of the remotely sounded data. Beside absolute recalibrations with the extraterrestrial sun irradiance via a diffuser, such relative calibrations with internal methods and means provide actual data of the instrument state every time. On the basis of the experience of in-flight calibrations with miniaturized lamps in the multichannel spectrometers MKS and the imaging spectrometer MOS a lot of test procedures were made to verify these lamps for purposes of the in-flight calibration of the High Resolution Imaging Spectrometer (HRIS) with an extended spectral range up to 2.35 micrometers . This paper describes the laboratory test during the HRIS phase-A study and some examples of in-flight calibrations during the orbit missions are discussed.

THe general configuration of the Enhanced Thematic Mapper Plus that will fly on Landsat 7 closely follows that of the whiskbroom Thematic Mapper scanners on Landsats 5 and 7. Design modification include changes to the on-board calibration system. The addition of solar calibrators reduces the need to rely on the internal calibrator for long-term calibration of the reflective bands, but miniature incandescent lamps are still used to monitor short-term variations in system response. Internal calibration is accomplished by means of an oscillating shutter than passe immediately ahead of the prime focal plane each time the scan mirror reverse direction. Mounted on the shutter is a miniature projection system fed by a system of sapphire light rods that transit light from stationary, redundant tungsten lamps mounted at the center of oscillation of the moving assembly. The area of the shutter adjacent to the optics is painted flat black. Its temperature is continuously monitored, and it provides the reference radiance level for DC-restoration of all bands. A mirror on the shutter reflects light from a blackbody to provide an additional reference level for the thermal band.

An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assist in validation of the Earth Observing System (EOS) MISR experiment. The airborne instrument is built to the same performance specifications as MIST. Both instruments view the earth at nine discrete view angles, provide data products which are radiance scaled to Systeme International units, registered among the view angles, and geolocated. Whereas on-orbit MIST will acquire a global data set every nine days, The aircraft version is restricted to a target size of 9 by 11 km per aircraft run. AirMISR does, however, have the advantages of offering a means to testbed new mission procedures or camera designs, provide data sets which predate the MISR launch, and a means of returning to the laboratory to update the camera calibrations. This paper provides the AirMISR laboratory calibration results, as well as an example of a vicarious calibration exercise.

The Remote Sensing Group at the University of Arizona has provided vicarious calibration results for all three SPOT- series satellites carrying the HRV cameras. The HRVs are high spatial-resolution sensors that are well-suited to the reflectance-, irradiance-, and radiance-based methods. The SPOT-4 platform has a similar sensor, the HRVIR camera, that now includes a band at approximately 1630 nm. The SPOT-4 platform also has a new sensor, Vegetation, that has much lower spatial resolution than the HRVIR sensor, and thus poses a challenge for vicarious calibration. This work presents the modifications that must be made to reflectance- , irradiance-, and radiance-based approaches in order to use them for the Vegetation. This paper also describes a proposed procedure for such a calibration along with an uncertainty analysis due to spectral mismatch and spatial misregistration. The work also shows how such cross- calibration can be done using a cross-calibration between Landsat-5 TM and SPOT-3 HRV using White Sands Missile Range. The results show TM can be cross-calibrated using HRV to provide calibration coefficients which are within 1 percent of those obtained from a reflectance-based approach.

To aid in calibrating and monitoring the performance of the NASA scatterometer (NSCAT), a calibration ground station (CGS) was operated in White Sands, New Mexico from mid November 1996 through February 1997. The CGS was used to verify the proper operation of the NSCAT system including transmitted power, frequency, pulse width, and receiver gain. It was also used to track spacecraft attitude variation, and to measure the antenna gain balance between different beams. This paper will describe the basic operation of the CGS, and the principle results obtained during the calibration period. The CGS is a transmit/receive system which was used to record pulses from NSCAT, and to transmit pluses back to NSCAT. The CGS data was synchronized with NSCAT telemetry, and processed for timing, frequency, and gain information. Timing discrepancies indicated significant spacecraft attitude variations beyond the values reported in telemetry. Gain discrepancies showed a small ascending/descending difference. The cause of this difference is not clear at this time.

This work shows how to estimate the characteristics of atmospheric aerosols using space-based observations of polarization and radiance taken by POLDER equipped on the satellite ADEOS. The retrieval algorithms for aerosols are based on light scattering simulations of polarization field. The model aerosol has taken into account the heterogeneous grain model with Maxwell-Garnett mixing rule for typical oceanic and water-soluble aerosol models. It is found that polarization information is useful to retrieve aerosol properties. As a result, aerosol distribution over the Pacific ocean near Japan is obtained, and we found that aerosol characteristics change in temporal and regional scales.

European Space Agency's GOMOS instrument is a pr of the ENVISAT-1 satellite, which will be launched in 1999. The GOMOS instrument will measure ozone and other trace gas densities in the stratosphere with stellar occultation technique. The data inversion of the GOMOS instrument is a non-linear problem, which can be solved with iterative least squares routines. In the statistical inversion theory the Bayesian approach to the problem involves the computation of the whole posteriori distribution instead of iteratively locating the maximum of it. We have studied different MCMC methods for this purpose. The choice of a suitable MCMC method is crucial for the convergence of the Markov chain. We have found the basic Metropolis algorithm with an adaptive proposal distribution most promising for the GOMOS data processing. This MCMC method allows us easily to study the sensitivity of the solution. Moreover, our examples show that sometimes a better estimate for the interesting quantity is achieved by computing the expectation value instead of the maximum likelihood solution.

SPOT Earth observation satellites are based upon the PUSH- BROOM acquisition principle whereby a CCD linear array acquires images row by row, perpendicularly to the satellite track, with columnwise displacement being generated by the satellite's motion on its orbit.

In the last decade imaging spectrometers for Earth observation were developed by different government agencies because of the ability to use the full information of a spectrum as a major tool in the study of physical and biological processes of the Earth. Instead of a few relatively broad spectral bands, the new imager concepts provide the detection of many contiguous narrow spectral bands by applying the technology of 2D detectors. The step from 1D to 2D solid-state imagers makes it necessary to include the typical behavior of these advanced imagers on the signal-to-noise (SN) ratio analysis. A simulation of maximum and minimum radiances for typical spectral signatures of the Earth and the verification of these radiances with Modular Optoelectronic Scanner data give the source for the calculation of electronics generated at the imager's detector. Different sensors will be compared concerning water-minimum and vegetation-maximum signals, and the degradation of the SN ratio due to image-smear of 2D solid-state imagers is demonstrated. A developed smear parameter gives a tool for analyzing the spectrometer performance during the conceptual phase.

As a testbed to evaluate instrument specifications and data reduction techniques for future satellite atmospheric sounders, an airborne Fourier transform interferometer sounder has been developed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The NPOESS Airborne Sounder Testbed Interferometer (NAST-I) collects atmospheric sounding from NASA's high-altitude ER-2 aircraft. Flown at a nominal 20km altitude, collected data is meant to simulate spaceborne atmospheric sounding. The NAST interferometer generates high quality interferograms over the 3.7-16.1 micrometers IR range with a resolution of 0.25cm^-1. Interferograms are collected in real-time by an embedded TI TMS320C32 digital signal processing chip housed in an industrial grade 133MHz Pentium PC. Three channels of analog interferogram data and one channel of telemetry data are acquired by four, 16-bit analog to digital converters. Oversampling and decimation are then used to produce high-fidelity samples at a reduced rate and to relax analog filter design requirements. This paper focuses on the signal processing aspects of the instrument and discuss test flight data collected during the instrument's initial flight test.

Conventionally, science data centers are supply driven - what they do and how they do it are determined entirely by what data sets they hold. The DAAC was no different. Between 1994 and 1997, the DAAC reinvented itself by re-examining its customers, changing the focus from supply driven to demand driven and achieved breakthrough results. This is the 'business model' for science data centers. This article describes the DAAC's business model and the supply-driven data management practices.

In this paper, a general philosophy about feature windows based applications and a windows-based application are presented for analysis, algorithm development, testing and validation studies for signal, image and data processing (SIDP) for Space-based Surveillance applications. This dedicated facility is called AUG-SIDP. It performs several specialized tasks such as blur estimation, restoration, CFAR detection, clutter modeling, registration, pixel and data level fusion, target tracking and classification. It is still in the development and testing phase.

A desirable feature of scientific data systems is that suspect and bad data be identified before release to the user community. This is a challenging task within the NASA Earth Observing System (EOS), due to the large volume and the complexity of data produced and the near real time mode between data production and distribution. The EOS data and information system (EOSDIS) provides the computing and network facilities, to support the generation and archival of data products from EOS satellites. EOS Quality Assessment (QA) methodology integrates a) the automated detection of certain types of suspect data, b) the capability of EOSDIS to alert the instrument team scientists and processing facility personnel to suspect data, c) the extraction of this data from the archives for QA purposes and the subsequent storage of QA results within EOSDIS, and d) the organization, archival and display of all of these QA result in a user-friendly format for the user community. Development of EOS QA methodology is described through discussion of the organization of QA information within the EOSDIS database, the components of operational QA, suggestions of how users may exploit the provided QA parameters and the support EOSDIS will provide users in utilizing QA information.

Improving the ground resolution of SPOT 5 compared with SPOT 4 involves multiplying the data rate by 4. This made it necessary to seek a new image compression algorithm able to significantly increase the compression ratio while complying with the image quality requirements of SPOT users. We finally selected a DCT based algorithm embedded in a regulation loop in order to obtain a constant rate at the compressor output. This algorithm was first tested using simulated images. Quantitative and qualitative analyses were carried out at different development stages. Finally, a wide range of images chosen from the SPOT image database was used for validation purpose. This led us to propose several optimizations which have now been thoroughly tested. The result is an almost lossless compression algorithm, which will be used on both SPOT5 and HELIOS II, the French space agency's forthcoming Earth observation missions.

THe ASTER Emergency Backup System (AEBS) provides limited post AM-I platform launch capability for data ingest, processing and distribution of ASTER level 1 and level 2 products to the ASTER Science Team. This system is designed based on a request for the Goddard Space Flight Center's Earth Science Data and Information System, Project Manager when the Earth Observing System Data and Information Systems Core System contractor slipped delivery schedules.

The complex nature of satellite communication and remote sensing computer system requires the NASA to predict the cost and performance of its computer systems during all system life cycle phases. The EOS data and operations systems (EDOS) simultaneously captures multiple science and telemetry data stream downlinked from EOS satellites in CCSDS format, and demultiplexes the raw data streams into time ordered, annotated CCSDS packet files which are distributed to the EOS data active archive centers, and to other agencies for further processing. The EDOS also provides real-time forward link services for command and telemetry data. This paper reports summary level results of EDOS modeling analyses and discusses the assumptions, algorithms, data sources and applications of the models from both Government and contractor perspectives. For example, the EDOS Acceptance Tests for Reliability, Maintainability, and Availability (RMS) requirements rely on the detailed results of the RMS models.

The Ocean Color CCD Imager, as a remote sensor of the first ocean satellite of CHina, will be launched. At present, the development of its prototype has been completed. This paper describes the laboratory absolute calibration for the prototype of Ocean Color CCD Imager. The calibration system was established. It is made up the spectral radiometer, spectral irradiance standard lamp, standard, the White Diffuser Plate and a 1.5 meter diameter integrating sphere source. The major calibrated contents include spectral responsibility, saturation spectral radiance, noise equivalent spectral radiance and dynamic range. The results of the experiment are presented and discussed in this paper. The estimated uncertainty of the absolute calibration is about 6 percent.

A simple and robust model is presented for describing the extinction coefficient of stratospheric aerosols in the UV- visible range. In a first version, four basic volcanic modes are distinguished. The parameters of the vertical profiles take the latitudinal dependence into account. Future improvements are presented.

The EO-1 Advanced Land Imager (ALI) is the first earth- orbiting instrument to be flown under NASA's New Millennium program. The ALI employs novel wide-angle optics and a multispectral and panchromatic spectrometer. EO-1 is a technology verification project designed to demonstrate comparable or improved Landsat spatial and spectral resolution with substantial mass, volume, and cost savings. This paper provides as overview of in-flight calibration and performance assessment of the Advanced Land Imager. Include dare techniques for calibrating and assessing focus and MTF using long, straight man-made objects and monitoring of radiometric linearity and offsets using an internal calibration source, standard Earth references scenes, and solar and lunar observations.

The Medium Resolution Imaging Spectrometer (MERIS), developed by the European Space Agency (ESA) for the Envisat polar orbit Earth mission, belongs to a new generation of ocean colour sensors which will yield a major improvement in the knowledge of such a crucial processes as the ocean contribution to the carbon cycle. The global mission of MERIS will have a major contribution to scientific projects which seek to understand the role of oceans and ocean productivity in the climate system and our ability to forecast change through models. Secondary objectives of the MERIS mission will be directed to the measurement of atmospheric parameters associated with clouds, water vapour and aerosols in addition to land surface parameters, important in particular for the understanding of vegetation processes. MERIS measures the radiance reflected from the Earth's surface in the visible and near infra-red part of the spectrum. Data are transmitted in fifteen spectral bands of programmable width and location. The instrument features two spatial resolution and several observation and calibration modes selectable by ground command. This paper gives an overview ofthe instrument mission, its concept and data products.

Keywords: MERIS, ENVISAT, Imaging Spectrometer, Ocean colour