Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical free-space laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radio-frequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the mars laser communications demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.

The Earth Observing One (EO-1) satellite, a part of National Aeronautics and Space Administration's New Millennium Program, was developed to demonstrate new technologies and strategies for improved Earth observations. It was launched from Vandenburg Air Force Base on November 21, 2000. The EO-1 satellite contains three observing instruments supported by a variety of newly developed space technologies. The Advanced Land Imager (ALI) is a prototype for a new generation of Landsat-7 Thematic Mapper. The Hyperion Imaging Spectrometer is the first high spatial resolution imaging spectrometer to orbit the Earth. The Linear Etalon Imaging Spectral Array (LEISA) Atmospheric Corrector (LAC) is a high spectral resolution wedge imaging spectrometer designed to measure atmospheric water vapor content. Instrument performances are validated and carefully monitored through a combination of radiometric calibration approaches: solar, lunar, stellar, Earth (vicarious), and atmospheric observations complemented by onboard calibration lamps and extensive prelaunch calibration. Techniques for spectral calibration of space-based sensors have been tested and validated with Hyperion. ALI and Hyperion instrument performance continue to meet or exceed predictions well beyond the planned one-year program. This paper reviews the EO-1 satellite system and provides details of the instruments and their performance as measured during the first year of operation. Calibration techniques and tradeoffs between alternative approaches are discussed. An overview of the science applications for instrument performance assessment is presented.

The EO-1 spacecraft, launched November 21, 2000 into a Sun-synchronous orbit behind Landsat 7, hosts advanced technology demonstration instruments, whose capabilities are currently being assessed by the user community for future missions. A significant part of the program is to perform data comparisons between the Hyperion Imaging Spectrometer and Advanced Land Imager (ALI) payloads on the EO-1 spacecraft with the Landsat 7 ETM+ sensor. To perform the absolute radiometric comparison, an aggregation method was developed and used to combine the proper portions of the Hyperion 10 nm bands to emulate the broader multispectral response of the ALI and ETM+. The aggregation capability allows radiometric comparisons between different instruments with different spectral response functions. This paper reviews the aggregation methodology, and presents absolute comparison results. A demonstration of the replication of portions of ALI and ETM+ scenes through a synthesis of Hyperion hyperspectral data is also provided. The capability of synthesizing broad band data from a hyperspectral sensor will enable users to test the selection of broad bands for future missions by using various combinations of Hyperion bands.

The Advanced Land Imager (ALI) is the primary instrument on the first Earth Observing Mission (EO-1), under NASA's New Millennium Program. On-orbit absolute radiometric calibration of this instrument relies primarily on solar calibration and involves pointing the ALI at the Sun with the aperture cover closed; deploying a Spectralon diffuser over the secondary mirror; and exercising a variable area subaperture located in the cover. The bimonthly solar calibration data is supplemented by daily internal reference lamp observations, monthly lunar observations, and periodic observations of ground truth targets. Each of the above techniques will be reviewed and the results compared to prelaunch calibration parameters. Additionally, the stability of the ALI during the course of its first year on orbit will be presented.

The EO-1 Advanced Land Imager (ALI) was developed under NASA's New Millennium Program. The ALI is validating new technologies and measurement techniques for application to future wide field-of-view, non-cryogenically cooled, pushbroom Earth observing systems. Extensive pre flight measurements of the BRDF for the optical elements were made. These data were used in a system level analysis to assess the impact of stray light on flight observations. A model was developed to estimate the error in measured scene radiance as a function of the ratio of scene radiance to background radiance. This effect can be significant for scenes with high contrast. A quantification of the ALI stray light characteristics based on preflight measurements and flight observations will be presented. The impact of stray light on the radiometric accuracy of in-flight data will be compared to the model.