The moderate resolution imaging spectroradiometer (MODIS) is a scanning radiometer that will fly as a facility instrument on the NASA polar-orbiting earth observing system (EOS) spacecraft. The first MODIS instrument is scheduled for launch in 1998 on the first EOS-AM spacecraft. MODIS is designed to provide critical data necessary to monitor global change and provide information vital to understanding the Earth as a system. This paper provides an overview of the MODIS requirements and system design. The operation of the instrument is described from photons in to formatted data out. Brief descriptions of the key functional subsystems of the instrument are provided. Predicted performance is summarized for critical areas including radiometric sensitivity and calibration accuracy, modulation transfer function pointing accuracy, and spectral band registration.

A radiometric math model (RMM) has been developed for use as an analytical tool to establish design requirements and to model performance of the moderate resolution imaging spectroradiometer (MODIS) instrument design. The MODIS instrument is scheduled for flight on the first earth observing system (EOS) AM spacecraft in mid-1998. The model represents a significant advancement in the state of the art in radiometric simulation techniques. Characteristics of the MODIS optical design, focal plane assembly (FPA), scanning and electrical parameters, and the on-board calibrators are input to the model as predicted, estimated, or measured. Primary outputs of the model are radiometric sensitivity [signal- to-noise ratio (SNR) and noise equivalent temperature (NE(Delta) T)] and radiometric accuracy. This paper describes the theory of the model, the input and output parameters, and the major program modules, and presents predictions for the current MODIS design.

There are always a wide variety of obstacles to writing and maintaining engineering documentation. To combat these problems, documentation generation can be linked to the process of engineering development. The same graphics and communication tools used for structured system analysis and design (SSA/SSD) also form the basis for the documentation. The goal is to build a living document, such that as an engineering design changes, the documentation will `automatically' revise. `Automatic' is qualified by the need to maintain textual descriptions associated with the SSA/SSD graphics, and the need to generate new documents. This paper describes a methodology and a computer aided system engineering toolset that enables a relatively seamless transition into document generation for the development engineering team.

The moderate-resolution imaging spectroradiometer (MODIS) instrument is currently being designed at Santa Barbara Research Center to be flown on board the earth observation system (EOS) polar orbiting spacecraft. The MODIS measures the scene radiance in 36 spectral bands from the visible to the long wave infrared regions with high radiometric accuracy and an instantaneous field of view (FOV) of 250, 500, and 1000 meters at nadir. Two on board microprocessors are used to control the instrument subsystems, and supervise the data acquisition and packet formatting. This paper describes the MODIS multiprocessor architecture, the on board data processing, the command and control subsystem, and the flight software necessary to run the instrument with a high degree of reliability.

A combination MODIS optics characteristics, short back focal length, and relatively distorting optics, has required major revisions in techniques used earlier to characterize effective focal length (EFL) and modulation transfer function (MTF) in the thematic mapper (TM) project. This paper compares measurement approaches used to characterize TM optics and revised methodology intended to characterize MODIS optics at an integration and assembly level.

The ground support equipment (GSE) used for test and evaluation of MODIS is currently in the detailed design phase. The GSE consists of optical stimulus sources and calibrator, electronic interfaces to simulate the spacecraft, and the hardware and software infrastructure for control, analysis, and archive functions. This paper focuses on the system test equipment (STE) that encompasses the spacecraft simulation and the infrastructure support. The STE is unique in that it has been designed to support both engineering and acceptance testing of a complex system. Common test set features have been incorporated into modules that are designed for cohesiveness.

The clouds and the earth's radiant energy system (CERES) experiment will provide consistent data bases of radiation and cloud fields. The CERES instrument consists of a scanning thermistor bolometer package with built-in flight calibration systems. Two bolometer packages will be launched on the earth observing system (EOS) platforms to measure the Earth/atmosphere-reflected solar shortwave and Earth/atmosphere-emitted long wave radiances with measurement accuracy goals approaching 1.0% and 0.5%, respectively. In each package, there are three different bolometers. All bolometers will be calibrated in a unique TRW vacuum facility equipped with blackbodies, a cryogenically cooled active-cavity radiometer, shortwave sources, and other specialized calibration devices. The blackbodies are tied to the International Temperature Scale of 1990 (ITS'90). Using math models, the calibration measurements will define the instrument filtered gains and offsets. This paper outlines the CERES instrument design and radiometric calibrations.

Stability of the solar constant makes the Sun an attractive on-orbit calibration source for radiometers operating at visible and near IR wavelengths. Direct viewing of the Sun provides a radiance or irradiance that is significantly above the dynamic range of most earth observing system (EOS) radiometers, thereby requiring attenuated viewing of the Sun. To provide radiometric repeatability, the attenuator used must be stable over time at all in-band wavelengths, uniformly flood the radiometer aperture and field of view, and be invariant over the range of solar viewing angles. The Earth Radiation Budget Experiment (ERBE) radiometers flown in the mid-1980s carried a mirror attenuator mosaic (MAM) to attenuate the solar energy. This device, consisting of specularly reflective, closely packed concave hemispheres with a black mask covering the area between the spheres, was successfully used to calibrate the ERBE shortwave (0.3 to 3.5 micrometers ) and total (0.3 to > 50 micrometers ) radiometer channels. For CERES, the calibration accuracy requirements have been tightened (+/- 1% shortwave, +/- 0.5% total channel, end-of-life, 1 (sigma) ). While the stability and uniformity demonstrated by the ERBE MAM are sufficient for CERES, the variation with solar incidence angle is not. Improvements to the ERBE design have been made for CERES and sample MAMs have been fabricated and tested. The results of this study as well as the features and details of the MAM design are addressed.

The clouds and the Earth's radiant energy system (CERES) program continues the long term monitoring of the Earth's radiant energy budget begun by the Earth Radiation Budget Experiment (ERBE) scanning radiometer instruments. The CERES instrument contains three thermal detector based radiometers with broadband spectral responses. The relative spectral responses must be characterized at far infrared wavelengths out to 200 micrometers in support of absolute radiometric calibration. This will be accomplished with a Fourier transform spectrometer as a spectral source, relay optics and a vacuum chamber containing the sensors. This facility currently under development for the CERES program will measure end-to-end sensor spectral response relative to a spectrally flat well characterized reference detector also located in the vacuum chamber. Facility design and controls on the measurement process to assure spectral accuracy are discussed.

The multi-angle imaging spectroradiometer (MISR) instrument is currently under development for flight on the first earth observing system platform, EOS-AM1, to be launched in 1998. The instrument will obtain global multi-angle imagery at nine separate view angles, using a separate charge-coupled-device pushbroom camera at each angle. Images will be obtained at 443, 555, 670, and 865 nm with spatial sampling, selectable in-flight, ranging from 275 m to 2.2 km. Data from the instrument will be used to retrieve the optical properties of tropospheric aerosols over land and ocean, to study the bidirectional reflectance properties of the Earth's surface and clouds, and to measure terrain topography and cloud heights. This paper reviews the MISR science objectives, presents an update to some of the instrument design parameters, and discusses the status of the instrument design and development. Test results from a recently built `brassboard' prototype camera are discussed.

The design of an Earth remote sensing sensor, such as the multi-angle imaging spectroradiometer (MISR), begins with a set of science requirements that determine a set of instrument specifications. It is required that the sensor meet these specifications across the image field, over a range of sensor operating temperatures, and throughout mission life. In addition, data quality must be maintained irrespective of bright objects, such as clouds, within the scene, or out-of-field glint sources. During the design phase of MISR, many refinements to the conceptual design have been made to insure that these performance criteria are met. These design considerations are the focus of this paper. Spectral stability with field angle, scene polarization insensitivity, and UV exposure hardness have, for example, been enabled through a telecentric optical design, a Gaussian shaped filter spectral profile used in conjunction with a Lyot depolarizer, and contamination prevention through consideration of material choices and handling procedures. Spectral, radiometric, and MTF stability of the instrument assures the scientific community that MISR imagery can be used for highly accurate aerosol, bi-directional reflectance distribution function (BRDF), and cloud studies.

Many techniques exist for statistically estimating measurement uncertainties in radiometric instruments. One method, the fidelity interval, is addressed here and applied to simulated calibration data for the multi-angle imaging spectroradiometer (MISR). The fidelity interval is the region within which the true radiance is expected to lie with a given probability and thus provides a measure of the calibration uncertainty. Calibration data are reduced using a least- squares approach to provide the gain and offset coefficients used in data reduction. It is from these calibration measurements that the fidelity interval is estimated. The technique allows us to optimize our calibration test plan through simulations which vary the number of radiometric levels, the radiometric range available for testing, and the number of data repetitions.

With increasing awareness of the potential for changes in the earth's environment through natural and artificial mechanisms comes an enhanced desire to globally monitor more regions of the atmosphere. The chemical state of the troposphere is recognized as a significant area, although one where it is extremely difficult to make measurements due to the interfering effects of clouds and the nearby surface. The measurements of pollution in the troposphere (MOPITT) instrument uses the principle of correlation spectroscopy to measure carbon monoxide (CO) amounts at three levels in the troposphere utilizing thermal radiation at 4.7 micrometers and the total column amount of CO and methane (CH₄) using reflected sunlight around 2.4 micrometers . The MOPITT instrument will fly on the AM-1 platform of NASA's Earth observing system (EOS) program in mid-1998. The instrument is being funded by the Canadian Space Agency (CSA) and the data processing by NASA. MOPITT has an international science team with members from Canada, the USA, and the UK.

SAGE III has been selected as part of the earth observing system for flight on the aerosol and chemistry satellites missions beginning in the year 2000. During lunar and solar occultation, SAGE III will measure vertical profiles of O₃, NO₂, H₂O, NO₃, OClO, temperature, and aerosols from cloud tops through the stratosphere, and of O₃ through the mesosphere. This paper describes the lineage of SAGE III, its science objectives, current instrument design, details of phase B testing and analysis, expected performance, and its contributions to monitoring global change and to meeting other EOS objectives.

The advanced spaceborne thermal emission and reflection radiometer (ASTER), a multi- spectral imaging radiometer with 14 spectral bands, is a research facility instrument that will be launched in 1998 on NASA's EOS-AM1 platform. Characteristics of the ASTER data can be summarized as (1) wide spectral coverage from the visible to thermal infrared regions, (2) multispectral thermal infrared data with high spectral and spatial resolution and (3) stereoscopic capability in the along track direction. ASTER is currently being designed to meet the requirements given by the ASTER science team.

This paper presents the preliminary design results of the visible and near-infrared radiometer (VNIR) of the ASTER instrument for the EOS-AM1 program. The VNIR is a spaceborne radiometer employing push broom scanning with 5,000 element CCD sensors. The VNIR provides image data with high spatial resolution of 15 X 15 m, and has the pointing capability of +/- 24 deg in the cross-track direction to obtain a wide swath. The VNIR also provides stereoscopic image data in band 3 (0.76 - 0.86 micrometers ) with nadir- and backward- looking telescopes for topographical studies and mapping of the earth.

ASTER in the shortwave infrared spectral region is an imager to cover the 1.6 to 2.5 micrometer wavelength region. The spatial resolution is 30 m and the spectral region is divided into 6 bands. To realize the imager, some technology breakthrough was required. This paper deals with the challenge in the development of ASTER in the shortwave infrared spectral region.

The advanced spaceborne thermal emission and reflection radiometer (ASTER) to be mounted on the U.S.'s EOS-AM1 polar orbiting platform, scheduled for launch in 1998 by NASA, will be part of a remote sensing equipment complex whose purpose is to locate mineral resources and monitor the earth's environment. The ASTER, a Japanese mission, consists of three optical sensors -- the visible and near-infrared radiometer (VNIR), the short wavelength infrared radiometer (SWIR), and the thermal infrared radiometer (TIR), which has 5 spectral bands in the thermal infrared region (8 - 12 micrometers ). The TIR, which is the focus of this paper, is expected to provide high temperature and ground resolution in its acquisition of surface temperature information from the ground, oceans, and clouds. Such information will be useful in monitoring volcano activity, desertification, forestation and flora distribution, and the global climate as a whole. The TIR will also acquire information on the thermal radiation spectrum that will be useful in classifying rock formations and composition.

ASTER is an advanced multispectral optical imager with a high spatial resolution, and covers a wide spectral region from visible to thermal infrared. In addition, ASTER has a stereoscopic viewing capability in the along-track direction. Excellent observational performances are expected by trying several technical challenges. High radiometric resolutions will be achieved by employing pushbroom scanning in visible, near infrared, and short wave infrared bands without sacrificing spatial resolutions. An adoption of active coolers will enhance the performance of short wave and thermal infrared bands. Major predicted observational performances of ASTER in the EM design phase are described.

ASTER is composed of three radiometers for separate wavelength regions: visible and near infrared radiometer (VNIR) and short wavelength infrared radiometer (SWIR) both in the solar reflection region, and thermal infrared radiometer (TIR) in the thermal emission region. Each radiometer will be calibrated before launch on the ground, and after launch in orbit. This paper describes the calibration plan of ASTER radiometer.

An airborne ASTER simulator (AAS) is being developed by the Geophysical Environmental Research Corporation (GER) to study land surface temperature and emittance in the thermal infrared. Laboratory tests in October 1992 at NASA's Stennis Space Center (SSC) measured the AAS's spectral, approximate NEdT, and approximate spatial response characteristics. The spectral FWHM for most channels is smaller than 0.3 micrometers ; the NEdT for most TIR channels is better than 0.4 K; and the nominal IFOV is 5 mrad. Flight data was collected over Cuprite and Goldfield, Nevada and near Valencia, California in November 1992. The silicified and opalized zones at Cuprite could be discriminated using decorrelation-stretch images. AAS decorrelation-stretch images agree, qualitatively, with data from NASA's thermal infrared mapping spectrometer (TIMS). These results indicate the AAS may be a good tool for remote sensing studies of geological materials. Lower noise detector arrays and linear variable (optical) filters for the TIR channels will be tested in flights over Cuprite, Nevada later this year. These and other improvements may reduce the NEdT and improve the signal-to-noise ratio.

SeaWiFS, the sea-viewing wide field-of-view sensor, will bring to the ocean community a welcomed and improved renewal of the ocean color remote sensing capability that was lost when the Nimbus-7 coastal zone color scanner (CZCS) ceased operating in 1986. Because of the role of phytoplankton in the global carbon cycle, data obtained from SeaWiFS will be used to assess the ocean's role in the global carbon cycle, as well as in other biogeochemical cycles. SeaWiFS data will be used to help determine the magnitude and variability of the annual cycle of primary production by marine phytoplankton and to determine the distribution and timing of spring blooms. The observations will help to visualize the dynamics of ocean and coastal currents, the physics of mixing, and the relationship between ocean physics and large-scale patterns of productivity. The data from SeaWiFS will help fill the gap in ocean biological observations between those of CZCS and those of the moderate resolution imaging spectrometer (MODIS) on the Earth Observing Satellite-A (EOS-A).

A new method for performing a preflight calibration of an optical remote sensing instrument with an on-board solar diffuser calibration system is presented. The rationale, method, advantages, disadvantages, error sources, and expected accuracies are discussed. The method was applied to the SeaWiFS sensor to be flown on the SeaStar Satellite.

A cross-calibration radiometer covering the 400 to 900 nm range with a silicon `trap' detector has been designed and built. The radiometer was designed for comparing the output of preflight radiometric calibration sources for the MODIS and ASTER EOS sensors. These sources will be large aperture spherical-integrating sources with tungsten-halogen-bulb internal illumination. The two spheres will be calibrated using standards from NIST and NRLM. The radiometer is intended to compare the output of the spheres at seven wavelengths corresponding to MODIS bands, some of which overlap ASTER bands. The wavelength selection is by highly blocked nominal 10 - 15 nm wide interference filters. The radiometer detectors, apertures, and filters are held at a controlled temperature slightly above ambient. The design and calibration of the radiometer are described.

The Land Processes Distributed Active Archive Center (DAAC) is archiving and processing precursor data from airborne and spaceborne instruments such as the thermal infrared multispectral scanner (TIMS), the NS-001 and thematic mapper simulators (TMS), and the advanced very high resolution radiometer (AVHRR). The instrument data are being used to construct data sets that simulate the spectral and spatial characteristics of the advanced spaceborne thermal emission and reflection radiometer (ASTER) and the moderate resolution imaging spectrometer (MODIS) flight instruments scheduled to be flown on the EOS-AM spacecraft. Ames Research Center has developed and is flying a MODIS airborne simulator (MAS), which provides coverage in both MODIS and ASTER bands. A simulation of an ASTER data set over Death Valley, California has been constructed using a combination of TMS and TIMS data, along with existing digital elevation models that were used to develop the topographic information. MODIS data sets are being simulated by using MAS for full-band site coverage at high resolution and AVHRR for global coverage at 1 km resolution.