Primary radiometric standards of incoherent radiationin the 0.04-25 micron range, developed and maintained in the USSR, are described. The standards are based on a synchrotron radiation source, high- and low-temperature blackbodies and absolute radiometer. The basic characteristics of the standards are given and the prospects for the construction of the cryogenic blackbody and absolute radiometer to be used for low-intensity calibrations are discussed. The methods and calibration systems for aerospace instrument calibrations against the radiometric standards are mentioned.

The imaging instrument and main subassemblies, and the calibration device on board the Vegetation remote sensing mission are described. The possible error sources are examined and the final performances are compared with specifications. A brief description of operational use is given.

The Medium Resolution Imaging Spectrometer (MERIS) is intended to be flown on the First European Polar Platform (PPF) scheduled for launch in 1997. The instrument designed for ocean monitoring, is capable of transmitting 15 spectral bands programmable in width and position across the extended visible domain 400nm - 1050nm. A spatial resolution of 250m is reached on-ground, with a wide field-of-view (FOV) of 82 degrees. To achieve high radiometric performances for the images, an in-flight calibration system is required with data correction on-board in real time. Calibration coefficients will be applied on raw images to reach both a spatial and spectral uniformity of 0.05 percent, and an absolute accuracy of 2 percent.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) will limb-sound the atmosphere by measuring its midinfrared emission with an apodized resolution of 0.05/cm. Given the current expected temperature variations of the instrument over the orbit, MIPAS will require a deep space reference measurements after every altitude scan, and a combined deep space and blackbody measurement at a lower frequency. For an adequate acquisition efficiency, low resolution calibration measurements will be performed, at 0.2/cm or 1/cm.

A new radiometic calibration system was developed for the prelaunch calibration of Optical Sensors (OPS) of Japanese Earth Resources Satellite (JERS) to be launched in 1992. OPS consists of eight bands from 520 nm to 2400 nm. The primary standard of the calibration system was fixed-point blackbodies of copper, silver, zinc, lead, and tin. The transfer standard was two variable temperature blackbody furnaces used at temperatures from 232 C to 1362 C. The spectral radiances were transferred to a large internally illuminated integrating sphere with an inner diameter of 1 m and an aperture of 280 mm in diameter. The OPS was calibrated against the large integrating sphere. Uncertainty of the spectral radiances of the variable temperature blackbody furnaces was estimated to be +/- 1.3 percent and that of the large integrating sphere to be better than +/- 3 percent.

A detailed description is given of an internally irradiated integrating sphere which supplies the radiance necessary for prelaunch calibration of the optical sensors mounted on the Japanese Earth Resources Satellite. The sphere is 1 m in diameter and has an aperture of 280 mm over which uniform radiance is expected. The sphere has 12 halogen lamps of 500 W operated at 100 V dc, giving the optical sensors nearly maximum spectral radiance for each spectral band from the earth's surface. Nonuniformity of the spectral radiance over the aperture is very small, enabling all the elements of each band to be simultaneously calibrated. A double-grating spectrometer is used to calibrate the integrating sphere against transfer standards.

Liquid helium-cooled electrical substitution radiometers (ESRs) provide irradiance standards with demonstrated absolute accuracy at the 0.01 percent level, spectrally flat response between the UV and IR, and sensitivity down to 0.1 nW/sq cm. We describe an automated system developed for NASA - Goddard Space Flight Center, consisting of a cryogenic ESR illuminated by servocontrolled laser beams. This system is designed to provide calibration of single-element and array detectors over the spectral range between 257nm in the UV to 10.6 microns in the IR. We also describe a cryogenic ESR optimized for black body calibrations that has been installed at NIST, and another that is under construction for calibrations of the CERES scanners planned for Eos.

This paper presents an analysis both of the NOAA-11 calibration system performance and of the changes of the solar diffuser reflectivity as measured by this system. In particular, after two years of inflight operation, the NOAA-11 system's performance is either on par or exceeding preflight expectations with none of the NOAA-9 problems evident. The solar diffuser displays a wavelength dependent degradation of about 0.5 percent per year at 404 nm to about 2 percent at 185 nm which is consistent with previous diffuser reflectivity changes observed on NIMBUS-7 and NOAA-9.

The Shuttle Solar Backscatter Ultraviolet (SSBUV) spectrometer instrument provides regular in-orbit calibration checks on the SBUV/2 ozone/solar irradiance monitoring instruments which are being flown routinely on NOAA operational satellites. The goal of the long-term ozone monitoring program is to detect possible changes in stratospheic ozone with a two sigma accuracy of approximately 1 percent over the course of a decade. This translates into a requirement that the SSBUV instrument be calibrated to a one sigma precision of 1 percent at the wavelengths used for ozone monitoring. We have previously shown that the precision of the SSBUV calibrations is such that we can meet this requirement. Here we discuss SSBUV radiometric sensitivity changes occurring as a result of the first two Space Shuttle flights. Finally, we present and compare SSBUV solar irradiance measurements taken during these flights.

The use of a solar-diffuser panel is a desirable approach to the on-board absolute radiometric calibration of satellite multispectral sensors used for earth observation in the solar reflective spectral range. It provides a full aperture, full field, end-to-end calibration near the top of the sensor's dynamic range and across its entire spectral response range. A serious drawback is that the panel's reflectance, and the response of any simple detector used to monitor its reflectance may change with time. This paper briefly reviews some preflight and on-board methods for absolute calibration and introduces the ratioing-radiometer concept in which the radiance of the panel is ratioed with respect to the solar irradiance at the time the multispectral sensor is viewing the panel in its calibration mode.

The use of an on-board solar diffuser has been proposed to monitor the in-flight calibration of satellite sensors. This paper presents the preliminary specifications and design for a ratioing radiometer, to be used to determine the change in radiance of the solar diffuser. The issues involved in spectral channel selection are discussed and the effects of stray light are presented. An error analysis showing the benefit of the ratioing radiometer is included.

If the solar spectral irradiance and the orientation and directional reflectance of a solar diffuser are known, then the spectral radiance of the diffuser is readily calculated and it can be used for the accurate absolute calibration of a satellite sensor. However, the solar diffuser is exposed during in-flight satellite sensor calibration to high-energy ultraviolet irradiance, particle impacts and atomic oxygen effects. This paper describes desirable solar diffuser characteristics and the results of proton and UV irradiation on the directional-hemispheric spectral reflectance, the bidirectional spectral reflectance factor and the polarization properties of candidate diffuser materials.

The Multi-angle Imaging SpectroRadiometer (MISR) plans to use deployable diffuse reflectance panels to provide periodic radiometric calibrations of its nine cameras while in-flight. Near-Lambertian reflectance characteristics are desirable to facilitate flat-field camera intercomparisons. Also required is panel spatial and spectral uniformity, and stability with time. Spectralon, a commercially available polytetrafluoroethylene (PTFE) compound, has been baselined in the MISR design. To assess the suitability of this material, a series of degradation tests were planned and implemented. These included UV vacuum exposure and proton bombardment tests which simulated the exposure levels to be encountered during the mission life. Proton levels are now considered too low to be of concern, but UV vacuum tests demonstrate sensitivity to material contamination. Material investigations have concluded that hydrocarbons are present in the bulk of the material, and that plastic packaging materials can introduce additional surface-layer contamination. It is found however, that these unwanted elements can be eliminated through vacuum pumping at elevated temperatures. Exposure to a UV source, while in vacuum, is again planned for a set of targets which have been vacuum baked. This will assess the stability of the pure PTFE form.

The sun is the intended source of radiation for a solar diffuser for the calibration of spacecraft radiometric sensors. Two unwanted sources have also been identified: (1) solar radiation reflected from the earth and scattered by the atmosphere, and (2) solar radiation reflected from surrounding spacecraft structures. This paper describes the determination of the stray radiation incident on the diffuser for the High Resolution Imaging Spectrometer (HIRIS). Finally, the optimization of the calibration geometry is considered.

Three methods for determining an absolute radiometric calibration of a spacecraft optical sensor are compared. They are the well-known reflectance-based and radiance-based methods and a new method based on measurements of the ratio of diffuse-to-global irradiance at the ground. The latter will be described in detail and the comparison of the three approaches will be made with reference to the SPOT-2 HRV cameras for a field campaign 1990-06-19 through 1990-06-24 at the White Sands Missile Range in New Mexico.

Absolute reflectance-based radiometric calibrations of Landsat-5 Thematic Mapper (TM) are improved with the inclusion of a method to invert optical-depth measurements to obtain aerosol-particle size distributions, and a non-Lambertian surface reflectance model. The inverted size distributions can predict radiances varying from the previously assumed jungian distributions by as much as 5 percent, though the reduction in the estimated error is less than one percent. Comparison with measured diffuse-to-global ratios show that neither distribution consistently predicts the ratio accurately, and this is shown to be a large contributor to calibration uncertainties. An empirical model for the surface reflectance of White Sands, using a two-degree polynomial fit as a function of scattering angle, was employed. The model reduced estimated errors in radiance predictions by up to one percent. Satellite calibrations dating from October, 1984 were reprocessed using the improved methods and linear estimations of satellite counts per unit radiance versus time since launch were determined which showed a decrease over time for the first four bands.

Calibrated solar radiometer intercepts allow spectral optical depths to be determined for days with intermittently clear skies. This is of particular importance on satellite sensor calibration days that are cloudy except at the time of image acquisition. This paper describes the calibration of four solar radiometers using the Langley-Bouguer technique for data collected on days with a clear, stable atmosphere. Intercepts are determined with an uncertainty of less than six percent, corresponding to a maximum uncertainty of 0.06 in optical depth. The spread of voltage intercepts calculated in this process is carried through three methods of radiometric calibration of satellite sensors to yield an uncertainty in radiance at the top of the atmosphere of less than one percent associated with the uncertainty in solar radiometer intercepts for a range of ground reflectances.

The calibration method reported here makes use of the reflectances of several large, uniform areas determined from calibrated and atmospherically corrected SPOT Haute Resolution Visible (HRV) scenes of White Sands, New Mexico. These reflectances were used to predict the radiances in the first two channels of the NOAA-11 Advanced Very High Resolution Radiometer (AVHRR). The digital counts in the AVHRR image corresponding to these known reflectance areas were determined by the use of two image registration techniques. The plots of digital counts versus pixel radiance provided the calibration gains and offsets for the AVHRR. A reduction in the gains of 4 and 13 percent in channels 1 and 2 respectively was found during the period 1988-11-19 to 1990-6-21. An error budget is presented for the method and is extended to the case of cross-calibrating sensors on the same orbital platform in the Earth Observing System (EOS) era.

NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) and associated ground-based data have been collected at NOAA/NESDIS, on a daily basis and for 600 days, using five stations within the continental United States in the NOAA solar radiation (SOLRAD) monitoring network. The data have been filtered to include only uniformly overcast conditions and analyzed along the lines described by Paris and Justus (1988). Results from this first long-term pilot operational application of the method are presented. The method is potentially useful for establishing yearly-averaged trends in the radiometric gain of AVHRR Channels. The relatively small data base examined here suggests a precision in the 600 day mean gain of 5 percent or worse, with a significant part of this uncertainty being driven by poor knowlege of the bidirectional reflectance properties of clouds. The results suggest that the method in its present formulation has insufficient precision to be used as a primary method for the measurement of in-orbit gains of reflected-solar radiometers aboard polar orbiting satellites. Intrinsic limitations to the precision and time resolution of the method are discussed, and suggestions are offered for improving the precision of future results.

The absolute radiometric calibration of the NS001 Thematic Mapper Simulator reflective channels was examined based on laboratory tests and in-flight comparisons to ground measurements. The NS001 data are calibrated in-flight by reference to the NS001 internal integrating sphere source. This source's power supply or monitoring circuitry exhibited greater instability in-flight during 1988-1989 than in the laboratory. Extrapolating laboratory behavior to in-flight data resulted in 7-20 percent radiance errors relative to ground measurements and atmospheric modeling. Assuming constancy in the source's output between laboraotry and in-flight resulted in generally smaller errors. Upgrades to the source's power supply and monitoring circuitry in 1990 improved its in-flight stability, though in-flight ground reflectance based calibration tests have not yet been performed.

A convenient technique that has been used to calibrate, in-flight, a helicopter-mounted Daedalus multispectral scanner is described. It used four large canvas panels laid out in a square with a Spectralon panel as a reference. A calibrated Barnes modular multispectral radiometer, carried on a 2.2-m boom, was rotated around a 2.5-m high tripod at the center of the square. The radiometer sampled the four large panels and the Spectralon panel once every two minutes. Atmospheric spectral transmittance measurements were made using a filter radiometer on an autotracking mount during the morning of the flight. The reflectance and optical depth data were used in an atmospheric radiative transfer code to predict the spectral radiances at the scanner. The calibration was completed by comparing the image digital counts to the predicted spectral radiances.

The factors affecting the spectral composition of radiation reaching a distant observer from a natural object, and thus determining its apparent color, are experimentally studied. A method to calculate the apparent color is examined in which the spectral radiance of a distant object is first measured at zero distance and variations in the apparent radiance are then studied as a function of the distance. Sample results are given.

The Cloud and the Earth's Radiant Energy System (CERES) program continues the long term monitoring of the Earth's energy budget begun by the Earth Radiation Budget Experiment (ERBE) scanning radiometer instruments. The radiometic ground calibration sources employed for ERBE were designed to cover the very large (all Earth) field of view of the non-scanning radiometers. The ERBE radiometer ground and flight calibration proved to be more accurate than the requirement. The ground calibration sources to be used for CERES will be optimally designed to calibrate the much more narrow field of view of the scanning radiometer to improve on the absolute calibration performance. In addition, the shortwave calibration will be made in narrow bands to eliminate uncertainty in the spectral shape of the shortwave calibration source. Each shortwave band will be absolutely calibrated by transfer to a blackbody using a cryogenic active cavity radiometer fitted with the same telescope optics as the CERES radiometers.

Enhancements which have been made in the Radiometric Calibration Facility (RCF) of the CERES payload are described. These include narrow field blackbody and wide field of view blackbody sources, an active cavity radiometer, and a shortwave reference source. They permit the RCF to calibrate the CERES instruments to better than +/- 0.5 percent absolute radiometric accuracy in the 3.5 to above 50 micron wavelength region and to +/- 1.0 percent in the 0.3-3.5 micron region.

The solar calibration instrumentation and approaches to the scanning radiometers in the ERBE experiment are described in detail. Emphasis is given to evaluating the stability of the mirror attenuator mosaic (MAM) solar diffusing plate. Flight and ground MAM calibration measurements are presented and compared.

The Clouds and Earth's Radiant Energy System (CERES) scanning radiometer is planned as the Earth radiation budget instrument for the Earth Observation System, to be flown in the late 1990's. In order to minimize the spatial sampling errors of the measurements, it is necessary to select design parameters for the instrument such that the resulting point spread function will minimize spatial sampling errors. These errors are described as aliasing and blurring errors. Aliasing errors are due to presence in the measurements of spatial frequencies beyond the Nyquist frequency, and blurring errors are due to attenuation of frequencies below the Nyquist frequency. The design parameters include pixel shape and dimensions, sampling rate, scan period, and time constants of the measurements. For a satellite-borne scanning radiometer, the pixel footprint grows quickly at large nadir angles. The aliasing errors thus decrease with increasing scan angle, but the blurring errors grow quickly. The best design minimizes the sum of these two errors over a range of scan angles. Results of a parameter study are presented, showing effects of data rates, pixel dimensions, spacecraft altitude, and distance from the spacecraft track.

Earth Observation System (EOS) optical multispectral imaging sensors provide images of the earth at various spectral and spatial resolutions, in the visible (VIS) and infrared (IR) regions of the solar spectrum. Accurate knowledge of extraterrestrial solar spectral irradiance and its variations with time, are needed to trace sensor calibration in space, and for the development of terrestrial atmospheric models needed in data validation. A brief review of the extraterrestrial solar VIS/IR spectral irradiance available in the literature will be reviewed, and the need to develop an extraterrestrial solar spectral irradiance for the EOS studies will be pointed out. The solar calibration of the Earth Radiation Budget Experiment (ERBE), earth-viewing sensors will be discussed. Observed variations in the solar constant (solar irradiance, at the mean sun-earth distance of one astronomical unit, integrated over all wavelengths), and solar spectral irradiance with solar activity and its implications for EOS studies also will be discussed.

A new technique for improving the accuracy of radiance calibrations for large-area integrating-sphere sources has been investigated. Such sources are used to calibrate numerous aircraft and spacecraft remote sensing instruments. Recent measurements performed at NIST and NASA Goddard Space Flight Center have demonstrated that the uncertainty of sphere-source radiance measurements can be improved from the present 5 to 10 percent level to a 1 to 2 percent level. Silicon detectors with bandpass filters mounted in front of them and calibrated for absolute spectral responsivity can be used to confirm and to monitor the absolute radiance of a sphere source.