On December 18, 1999, the Clouds and the Earth’s Radiant Energy System (CERES) flight models 1 (FM1) and 2 (FM2) sets of scanning thermistor bolometer sensors were launched into orbit aboard the NASA Terra Spacecraft. The sensors measure earth radiances in the broadband shortwave solar (0.3 µm - 5.0 µm) and total (0.3 µm - >100 µm) spectral bands, as well as in the 8 -12 micrometer water vapor window, narrow-band spectral band. In order to measure sensor response drifts or shifts, inflight blackbody and evacuated tungsten lamp calibration systems were built into the CERES instrumentation. These systems were used to determine the sensor responses during the ground/pre-launch, ground to orbit, and on-orbit phases of the sensor calibrations. Analyses of the pre-launch, vacuum ground calibrations indicated that the CERES sensor responses can change as much as 0.6% between vacuum and ground ambient atmospheric pressure environments. The sensor responses were found to vary directly with the temperature as much as 2% between the 311 K and 270 K thermal environment of the vacuum calibration facility. From the vacuum ground calibration through the on-orbit calibration phases, the Terra Spacecraft CERES broadband total and shortwave sensor responses and in-flight calibration sources maintained their radiance measurement ties to an International Temperature Scale of 1990 (ITS-90) radiometric scale at precision levels approaching ± 0.3% (0.3 Wm-2sr-1). Analyses of the ground and on-orbit calibrations are presented and discussed using built-in, reference blackbody and lamp observations.

The Clouds and the Earth's Radiant Energy System (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 radiances in the broadband shortwave solar (0.3 -5.0 µm) and total (0.3 - >100 pm) spectral bands as well as in the 8 -12 µm water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. In December 1999, the second and third sets of CERES bolometers were launched on the Earth Observing Mission Terra 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 system in order to define in-orbit shifts or drifts in the sensor responses. The shortwave and total sensors are calibrated using the solar radiances reflected from the MAM. Each MAM consists of baffle-solar diffuser plate systems, 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 aluminum mirror segments, which are separated by a Merck Black A absorbing surface, overcoated with silicon dioxide. Thermistors are located in each MAM plate and baffle. The CERES MAM is designed to yield calibration precisions approaching 0.5 percent for the total and shortwave detectors. In this paper, the MAM solar calibration techniques are presented along with on-orbit measurements.

The Clouds and the Earth’s Radiant Energy System (CERES) scanning thermistor bolometers have a response time of approximately 9 ms for 98 to 99% of the signal, after which there is a slow change for the remaining 1 to 2% of the response due to a slow mode. This paper describes the theoretical and experimental procedures used in producing the slow mode coefficients for the CERES Flight Models 1 and 2 instruments aboard the Terra spacecraft, which was launched on December 18,1999. The response behavior for the total thermistor bolometer (0.3 - > 100 µm) and window channel (8-12 µm) were determined by analyzing the internal blackbody calibration ground data while the shortwave thermistor bolometer (0.3 - 5 µm) was determined using shortwave internal calibration source ground data obtained at the TRW calibration facility at Redondo Beach, California. These slow mode coefficients agree with the coefficients obtained by analyzing the in-flight calibration data. A numerical filter removes the effects of the slow mode from the measurements. The method may be applicable to other instruments which have spurious transients.

Spectral Characterizations of the CERES thermistor bolometer is a critical factor in determining precisely the earth's radiant energy budget. CERES bolometers are used in studying the balance between the incoming solar radiation from the sun, the scattered and reflected solar radiation, and the longwave-emitted radiation from the earth. To account for the different frequencies, the CERES instrument contains three thermistor bolometer channels; total channel (0.3 -> 2OO-µm), shortwave channel (0.3 — 5-µm), and window channel (8 — 12-µm). Information on the CERES sensor spectral response is required for their radiometric calibrations. The longwave infrared region (2 - > 2OO-µm) spectral measurements were collected and measured in the TRW Fourier Transform Spectrometer (FTS) vacuum chamber facility. In this paper, in addition to a general review of the concept; the algorithms and procedures are presented which were used to characterize spectrally the sensors' responses. These sensitive modifications are contained both in the shortwave and the longwave regions of the spectral response. Such changes include modifications to the method of analyzing the raw FTS data to produce the longwave region of the spectral response. Other factors which affect the spectral response, such as the Transfer Active Cavity Radiometer's (TACR) calibration data reduction method will also be analyzed.

The Remote Sensing Group at the University of Arizona has used ground-based test sites for the vicarious calibration of airborne and satellite-based sensors. Past work has focused on high-spatial-resolution sensors that are well-suited to the reflectance-, irradiance-, and radiance-based methods. Application of these methods to the recently launched Moderate Resolution Imaging Spectroradiometer (MODIS) with its lower spatial resolution 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 MODIS. The reflectance-based method described here relies on ground-based measurements of the reflectance of both large- and small-scale areas of the test site as well as low-level aircraft data to scale the ground-based measurements to the spatial scale of a MODIS pixel. The radiance-based approach relies on a recently-developed airborne, non-imaging radiometer with an 80-m footprint sampling the test site with a predetermined strategy to account for the differing spatial resolutions. Because this sampling strategy depends upon the test site being use, this work describes the two primary test sites of Railroad Valley Playa in Nevada and White Sands Missile Range in New Mexico and the spectral and spatial effects these sites will have on the calibration of MODIS. Early results from application of the reflectance-based method to MODIS using data from April 2000 indicate that the radiometric response of MODIS has not changed significantly.

We present a spatial analysis for the Visible (VIS), Near Infrared (NIR), Shortwave Midwave Infrared (S/MWIR), and Long Wave Infrared (LWIR) focal planes of Moderate Resolution Imaging SpectroRadiometer (MODIS) PreFlight Model (PFM) on the Terra platform. The analysis includes focal plane detector (channel) alignment, op- tical/electronic cross-talk, and Far Field Response (FFR). The study is performed on pre-launch laboratory Point Spread Response (PSR) data, and three sets of alignment data both taken prior to August 1998. The PSR and alignment data are displayed using the Focal Plane Viewer (FPV), a software utility developed to aid in the study. Channel measurement and band summaries are presented for each focal plane. The findings for optical/electronic cross-talk are based on the PSR and the alignment data. The calculation of Ensquared Energy (EE) from the PSR data characterizes the far-held response for channel 5.

The SpectroRadiometric Calibration Assembly (SRCA) instrument performs spatial, radiometric, and spectral in-flight calibration of the MODIS PFM instrument on the TERRA satellite. In spatial mode, the SRCA is intended to characterize focal plane registration by measuring cross-track spatial shifts of individual detectors (channels), and in-track band centroid shifts. In this paper, we investigate the suitability of the SRCA to evaluate the MODIS MTF on-orbit. Using the SRCA to evaluate the MODIS MTF requires information on the optical quality of the SRCA itself, particularly since it is not designed specifically for MTF measurements. Because the SRCA illumination fills only 1/4 of the optical aperture of the MODIS system, the illumination conditions are significantly different from those of normal MODIS imaging. We characterize the SRCA’s spatial performance by estimating its MTF from the pre-launch SRCA and Integration and Alignment Collimator (IAC) datasets, the IAC being assumed to be of significantly higher quality than the SRCA. This analysis shows that the SRCA, after calibration by comparison to the IAC, may serve as one source for on-orbit MTF measurements of MODIS PFM.

This paper addresses the methodology and algorithm for using the Spectro-Radiometric Calibration Assembly (SRCA) lamp electrical parameters to track its output band radiance change. This allows the use of the SRCA to track MODurate resolution Imaging Spectroradiometer (MODIS) detector gain variation at different orbit positions because the MODIS dn change is attributed to its gain change after subtracting the SRCA output radiance change itself. Pre-launch test data show that using lamp current as a parameter is valid. Orbit data prove that the approach is valid for longer periods of time for all Solar Reflective Bands (SRBs). Data indicate that the MODIS has undetectable gain change at different orbit positions.

In order to increase the repeatability of a radiance calibration sphere source we have developed a technique to digitally control the light source so as to maintain a constant radiance output. In this experiment a data acquisition system was used to read the photocurrents from two detectors that monitor the radiance at different wavelengths, the required change in the lamp current needed to maintain constant radiance was then calculated. Three different radiance control algorithms were tested: single band control, two-band minimum difference control and two-band spectral shape control. An electronic circuit using two moderate precision, digital-to-analog converters and a voltage-controlled power supply was developed to stabilize a small spherical integrating source. The test data show that two (or, by inference, more) spectral bands can be used in the radiance control of a sphere source with different algorithms for calculating the radiance stabilization correction function.

One element of a multi-year calibration program between the National Institute of Standards and Technology (NIST) and the National Aeronautical and Space Administration (NASA) Earth Observing System (EOS) Project Science Office has been the development and deployment of a portable transfer radiometer for verifying the thermal-infrared scales being used for flight-instrument pre-launch calibration. This instrument, the Thermal-infrared Transfer Radiometer (TXR), has been built and the first deployment test was completed successfully, as has been reported previously.1 The 5 µm channel, based on a photovoltaic Indium Antimonide (InSb) detector, so far has demonstrated a pre-deployment to post-deployment uncorrected repeatability of better than 30 mK to 60 mK, which is sufficient to enable intercomparisons at useful uncertainty levels for the EOS program. However, the 10 µm channel, based on a photovoltaic Mercury Cadmium Telluride (MCT) detector, shows uncorrected repeatability levels of about 0.5 K, the response changes being induced by cryocycling. This paper describes the technique that has been developed for correcting these changes. A portable black body check-source travels with the TXR that is used to verify the repeatability during the deployment trip. The check-source, in combination with the stability of the 5 µm channel, is used to restore a higher accuracy scale to the 10 µm channel than would otherwise be possible. This application is analogous to the use of an on-orbit calibration source to check for and correct for launch-induced or degradation-induced flight instrument detector response changes.

Space based remote sensing instruments employing scanning mirrors to acquire data on the earth can experience a radiometric modulation with scan angle (striping) due to polarization effects. Mirrors inherently introduce polarization that depends on the angle of incidence and orientation of the mirror. In the case of the Atmospheric Infrared Sounder (AIRS) the angle of incidence is constant, however the orientation of the mirror changes with scan angle. The polarization of the scan mirror couples with that of the aft optics for spectral separation to produce a radiometric modulation of the signal with scan angle. Data acquired during instrument testing on the polarization of the spectrometer were combined with data obtained for the scan mirror to model the expected radiometric modulation. Results were compared with direct measurements of the modulation obtained during radiometric testing while viewing a large area blackbody. Agreement is very good and shows that the modulation is very small and can be modeled to an accuracy consistent with the radiometric calibration budgets. Both modeled and measured results are presented for representative bands in the instrument as well as a discussion of the modeling techniques and equations used.

Remote sensing of the atmosphere and surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. The current versions of these instruments have two and four detectors per band, respectively, that are scanned across the earth. Large photovoltaic, Hg1-xCdx Te Focal Plane Arrays (FPAs) will permit faster coverage, improved resolution, and improved sensitivity for future designs like the Advanced Baseline Imager (ABI) and the Advanced Baseline Sounder (ABS). However, the transition away from the current small number of detectors requires a technology demonstration of the same or better radiometric precision and uniformity across available FPAs. These measurements, using appropriate flux levels, f-numbers, and readout rates for GOES, are underway at MIT Lincoln Laboratory. Both corrected response from pixel to pixel (residual spatial non-uniformity) and temporal stability of each pixel during the calibration period are required to better than 0. 1 K NEdT. The test set-up and the measurements of dark current and signal performance will be discussed for two arrays.

Remote sensing of the atmosphere and the surface of the earth is performed by the Imager and Sounder instruments onboard the GOES (Geostationary Operational Environmental Satellite) Satellites. By employing large PV Hg1CdTe focal plane array (FPA) detectors, instruments like the Advanced Baseline Imager (ABI) and Advanced Baseline Sounder (ABS) will provide improved update times, resolution, and sensitivity. However, uniformity in the pixel geometry across the array must first be demonstrated in order to maintain the accuracy of weather products at each spot on the ground. This uniformity is particularly important in weather products involving radiance subtractions and ratios from multiple spectral bands employing different detectors. Measurement ofthe spatial response associated with a pixel is important in determining both ground resolution and the effect ofradiance from outside the pixel field-of-view. Therefore, a high precision test set-up has been developed at Lincoln Laboratory to measure both the modulation transfer function (MTF) associated with each pixel in the array and the cross-talk from pixel to pixel. Details of the test set up and initial results of the testing will be discussed.

Global Change Observation Mission (GCOM) is a new generation of earth observation program by NASDA. GCOM aims to derive trends in climate system by long term and systematic measurements of atmosphere, ocean, and land. GCOM-A1 is one of the first generation of GCOM satellites to be launched in 2006, which was formerly called ADEOS-3A. GCOM-A1 will carry atmospheric instruments; two Japanese, Ozone Dynamics Ultraviolet Spectrometer (ODUS), and Solar Occultation Fourier transform spectrometer from Inclined Satellite (SOFIS), and one foreign atmospheric instrument and a GPS occultation instrument.

The Ozone Dynamics Ultraviolet Spectrometer (ODUS) is a satellite-borne, nadir-looking ultraviolet spectrometer for measuring total ozone amount. It will be launched in 2006 onboard Japanese earth observation satellite GCOM-A1 (GCOM : Global Change Observation Mission). The ODUS instrument measures continuous spectrum from 306 to 420nm with 0.5nm spectral step and 20km spatial resolution, using an Ebert-type polychromator and an one-dimensional silicon CMOS array detector, which will improve the accuracy of the retrieved total ozone amount. This paper presents an overview of the ODUS instrument and performance.

Atmospheric composition measurements from satellites are essential for monitoring the earth’s environment. The Ozone Dynamics UV Spectrometer (ODUS) will be launched on the Global Change Observation Mission (GCOM) -A1 satellite in 2006. ODUS covers from 306 to 420 nm back scattered light with 0.5 nm spectral and 20 km spatial resolution using a Fastie-Ebert type polychromator and a one-dimensional UV Si-CMOS array detector. It is a nadir-looking mapping spectrometer with a mechanical scanner, which can acquire global data in one day. It is expected to provide information about total O3, SO2, NO2, BrO, OCIO, HCHO, surface albedo, and aerosol type. Total 03 is inferred from look-up tables calculated with the radiative transfer on multiple solar back scattering. Other constituents are derived in such a way that the deviation of the measured and calculated radiance is minimized. We use the STAR (System for Transfer of Atmospheric Radiation) code for radiative transfer calculation. In recent years, tropospheric O3 measurement has become important for biomass burning and urban air pollution monitoring. The sensitivity of various O3 vertical profiles on the ODUS spectra is studied and tropospheric O3 retrieval algorithm will be presented.

SHOWS is a satellite placed on the circular orbit with the inclination of 18Odeg and the radius of 20,320 km. Its mission is to take infrared imageries of clouds, similar to those taken from the present geo-stationary meteorological satellites. Although its distance to the earth' s surface is 40% of that of the geo-stationary orbit, it is well enough to view the earth as a whole. The radiometer scans the earth along the meridian including the sub-satellite point, and the instantaneous geometric field of view (IGFOV) at the sub-satellite point is a circle with the diameter of 4 km. The concept was derived from our experiences in the present world' s meteorological satellite system and is intended to complement the system by adding 2 such satellites, each placed opposite to the other relative to the earth. It can be launched from Japan by an H-IIA launch vehicle, with the initial injected orbit identical to the geo-stationary transfer orbit (GTO). The radiometer serves not only for the imaging mission but also for the high accuracy attitude determination, by optically sensing the earth's east and west horizons, and stars. The orbit is unexplored yet and poses a radiation concern. As much excess weight as available will be expended to shield semiconductors against the radiation. The sensor data will be sent to the earth with 30kbps. This very low data rate, together with the simple ground antenna mounted only on a single motor will facilitate the direct reception of data by users. A highly integrated data handling system using the Internet has also been conceived.

The NASA Earth Observing Systems’ (EOS) SOlar Radiation and Climate Experiment (SORCE) mission consists of four instruments aboard a small satellite to measure the total solar irradiance (TSI) and solar spectral irradiance from 1 to 2000 nm. Solar irradiance, being the dominant energy source in the Earth's atmosphere, establishes much of the atmosphere's chemistry and dynamics. The SORCE measurements will therefore provide the requisite understanding of one of the primary climate system variables for the NASA EOS program. The SORCE primary science data product will be the TSI and solar spectral irradiance on a 6 hour cadence for a period of 5 years or more. The SORCE science team will study how much the solar irradiance varies, how the solar variability affects the Earth’s atmosphere, and how the magnetic structures on the Sun change its irradiance. The SORCE instruments are the Total Irradiance Monitor (TIM), the Spectral Irradiance Monitor (SIM), the SOLar STellar Irradiance Comparison Experiment (SOLSTICE), and the XUV Photometer System (XPS). The TIM is an active cavity radiometer similar in design to previous cavity radiometers, such as the VIRGO, ACRIM, and ERBS instruments, but with significant improvements in sensor and electrical design. TIM will provide a measurement of TSI directly traceable to SI units with an absolute accuracy of 0.01% and relative accuracy of 0.001% per year. The SIM is a Fery prism spectrometer with an Electrical Substitution Radiometer (ESR) as the reference detector and Si and InGaAs photodiodes as the working detectors. SIM will measure the solar spectral irradiance from 200 nm to 2000 nm with a spectral resolution varying from 0.5 nm to 34 nm, an absolute accuracy of 0.03%, and a relative accuracy of 0.006% per year. The SOLSTICE is an improved version of the UARS SOLSTICE instrument, both being ultraviolet (UV) grating spectrometers with photomultiplier tube detectors. SOLSTICE will measure the solar spectral irradiance from 115 nm to 320 nm with a spectral resolution of 0.1-0.2 nm, an absolute accuracy of 5%, and a relative accuracy of 0.5% per year. The XPS is a set of soft x-ray (XUV) photometers, consisting of Si photodiodes with thin-film filters to select moderate spectral bands. XPS will measure the solar spectral irradiance in the XUV (1-31 nm) and at Lyman-? (121.6 nm) with bandwidths of about 5 nm, an absolute accuracy of 20%, and a relative accuracy of 4% per year. Orbital Sciences Corporation is providing the SORCE satellite, a version of their GALEX spacecraft bus tailored for the SORCE mission. The SORCE satellite is a 3-axis stabilized satellite for pointing the instruments towards the Sun for the primary solar measurements as well as for pointing towards stars for the SOLSTICE in-flight calibrations. The SORCE spacecraft is scheduled for a launch on a Pegasus XL in July 2002 into an orbit with a 645 km altitude and 40° inclination.

The Spectral Irradiance Monitor (SIM) will measure the solar spectral irradiance from 0.2- to 2.0 µm with an accuracy of 300 ppm (1?) and a precision of 100 ppm. The SIM will be launched in 2002 on the EOS SORCE mission (SOlar Radiation and Climate Experiment). This instrument is a Fery prism spectrometer with an Electrical Substitution Radiometer (ESR) as the primary detector, and 4 additional photodiodes detectors. SIM incorporates the following design features. 1) Two independent spectrometer channels coupled with a periscope/calibrator mechanism to monitor changes in prism transmission in-flight. 2) A closed-loop wavelength drive provides precise position knowledge in the spectrometer focal plane. 3) An ESR to maintain the long-term absolute calibration of the instrument. The ESR consists of back-to-back 1x10-mm2 diamond bolometers blackened with nickel phosphorous and mounted at the center of a spherical cavity to increase the effective bolometer blackness. A shutter located in front of the spectrometer entrance slit modulates the solar signal at 0.1 Hz; the ESR synchronously detects light at the shutter fundamental. The bolometer bridge control, electrical power replacement, ESR temperature regulation, prism rotation, and shutter actuation are all under digital signal processor control.

The Total Irradiance Monitor (TIM), to be launched in 2002 on the NASA Earth Observing System (EOS) SOlar Radiation and Climate Experiment (SORCE), will stare at the Sun for five years, and measure the absolute total solar irradiance (TSI). The TIM is an active cavity radiometer with a relative standard uncertainty 100 ppm and a fractional stability of ? 10 ppm/year. The estimated uncertainties are “type B” determined from the parametric uncertainties in a model of the instrument; and the dominant uncertainty will be in the effective aperture area. To obtain such low uncertainty, we: 1. Use metallic NiP as the cavity (diffuse) black. 2. Retrieve the irradiance in the frequency domain. 3. Use phase sensitive detection. 4. Use four separate, duty-cycled cavities. 5. Measure the aperture transmission integral over area. 6. Use diamond thermal/electrical nodes. 7. Use 400 seconds for each completely independent data point for low noise. 8. Use a pulse-width-modulated “standard digital watt” as the onboard standard. 9. Take advantage of the 1 ppm noise level to discover systematic effects. 10. Measure IR shutter radiation from in-flight measurements of dark space. We compare with other TSI measurements on orbit, and as separate shuttle experiments.

The SOLar STellar Irradiance Comparison Experiment II (SOLSTICE II) is a component the NASA Earth Observing System’s Solar Radiation and Climate Experiment mission. SOLSTICE II will use a pair of diffraction grating spectrometers to measure solar irradiance from 115 nm to 320 nm with a spectral resolution of 1 nm and a cadence of 6 hours, with an absolute accuracy of 5%, and with a relative accuracy of 0.5% per year. We will achieve an initial 5% absolute accuracy by calibrating the instrument’s radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Md. Once the instrument is on-orbit, we will track changes in its sensitivity with irradiance measurements of an ensemble of bright, stable, main-sequence B-A stars. SOLSTICE II is an evolution of the SOLSTICE I instrument that is currently operating on the Upper Atmosphere Research Satellite. In this paper we review the basic SOLSTICE concept and describe the characteristics, operating modes, and anticipated performance of the SOLSTICE II instrument.

The forward-looking imaging mode SAR (F-SAR) is much more difficult to achieve high azimuth resolution in comparing with traditional side-looking SAR. It is almost impossible to achieve high azimuth resolution to perform azimuth compression by doing correlation in Doppler-time (f-t) domain like in side-looking SAR. This paper provides a new concept of high azimuth resolution for F-SAR. The Doppler-time histories of F-SAR are expressed as time-Doppler (t-f) format in which the format of the time-Doppler histories is similar with that of side-looking SAR expressed as Doppler-time (f-t) format. There is a very high gradient of time versus Doppler frequency. And the azimuth compression is much more convenient and easy to be performed. The high azimuth resolution can be achieved by doing correlation properly with suitable azimuth references in time-Doppler (t-f) format for F-SAR. The paper will mainly focus on the azimuth resolution in theory.

The Hyperion Imaging Spectrometer is one of three principal instruments aboard the EO-1 spacecraft. Its mission as a technology demonstrator is to evaluate on-orbit issues for imaging spectroscopy and to assess the capabilities of a space- based imaging spectrometer for earth science and earth observation missions. For the latter activity, a science team has been selected, which is complemented by commercial applications teams. This paper will review the design, construction and calibration of the Hyperion instrument. The on-orbit plans and operations will be presented along with updated calibration and characterization measurements.

This paper describes the calibration transfer path from primary standards representing fundamental physical quantities through the calibration radiance source used in Hyperion instrument level absolute calibration. The calibration transfer path and hardware design of the primary and secondary standards and their validation for end-to-end calibration of the sensor are presented. The primary standards reside at the TRW Radiometric Scale Facility and include two high quantum efficiency Silicon photodiode trap detectors; an electrically self-calibrated pyroelectric detector serves as a secondary standard for crosscheck. The end-to-end sensor calibration is accomplished with a Calibration Panel Assembly (CPA) source, which is illuminated by a NIST traceable FEL 1000 transfer standard lamp. An independent crosscheck of the Spectralon reflectance properties is made with a transfer radiometer. An error analysis of the transfer path is presented. The basic strategy of the Hyperion end-to-end calibration is to reduce the size of the sensor responsivity error tree and to provide control of systematic errors as much as possible through cross-calibration.

This paper presents the techniques and results of Hyperion laboratory characterization. Hyperion is a hyperspectral imager scheduled to fly on the Earth-Orbiter 1 (EO-1) spacecraft for the New Millennium project. The other payloads on the spacecraft are ALI (Advanced Land Imager) and AC (atmospheric corrector). The payloads were integrated into the spacecraft at Goddard Space Flight Center (GSFC). An End-to-End imaging test was conducted at GSFC which demonstrated integrity of Hyperion performance after environmental tests. The performance characterization procedures described here include: crosstrack MTF, spectral and spatial co-alignment, spectral wavelength calibration, signal to noise, polarization, spectral response function and scene generation. The characterization was carried out with the TRW Imaging Spectrometer Characterization Facility which is based on a 250 watt QTH lamp, a monochromator, a collimator and a fine pointing mirror. A selection of narrow slits and a knife edge are illuminated at the exit slit of the monochromator for sub-pixel performance characterization parameters such as MTF. Special attention is devoted to the spectral calibration technique using rare earth doped Spectralon panels. This was the technique used at the End-to-End test to verify spectral performance of Hyperion after GSFC environmental tests. It is a particular useful technique when the optical test setup does not allow for the use of a monochromator.

The 10-band scanning radiometer is the main payload of the Chinese polar-orbit FY-1C meteorological satellite. Its instrument configuration, detecting bands, detectors, orbital signal processor and behavior in orbit are described in this paper.

SeaWiFS was launched onboard the OrbView-2 satellite on 1 August 1997. On 4 September 1997, the day of first light for the instrument, SeaWiFS global images were processed automatically using the instrument’s prelaunch calibration and distributed on the World Wide Web. With the first reprocessing of SeaWiFS data in January 1998, the radiometric calibration coefficients for the SeaWiFS visible bands were linked to the water-leaving radiances measured by the Marine Optical Buoy (MOBY). In addition, the calibration coefficient for the 765 nm SeaWiFS infrared band was adjusted to give values consistent with those for an atmosphere with the maritime type of aerosol found in the vicinity of the MOBY buoy. Since the infrared bands were designed to allow the inference of aerosol type for the SeaWiFS atmospheric correction algorithm, this vicarious calibration forces their agreement with the conditions for a known aerosol type. With the second reprocessing in August 1998, temporal changes in the radiometric sensitivities of the SeaWiFS near infrared bands were corrected using lunar and solar measurements. The third SeaWiFS reprocessing in May 2000 introduced small time dependent calibration corrections to some visible bands. Future SeaWiFS reprocessings are scheduled to occur on an annual to biennial basis. With the third reprocessing, the emphasis of the instrument calibration program has shifted to the assessment of the surface truth comparisons used by SeaWiFS, principally those with MOBY.

The Sea-viewing Wide Field of View Sensor (SeaWiFS) was launched during the summer of 1997. While its primary purpose was to provide quantitative data on ocean bio-optical properties at a global scale, its bi-linear gain design allows it to provide data over land as well. Thus, there has been greater interest in understanding the radiometric calibration of the sensor for both gain levels. The Remote Sensing Group of the Optical Sciences Center at the University of Arizona has been using vicarious calibration techniques that rely on ground-based test sites to calibrate a variety of sensors since the mid-1980s. The results of applying these techniques to SeaWiFS are presented here. Three ground-reference data sets are presented, the first from White Sands Missile Range in October 1997, the second from Railroad Valley Playa, Nevada in June 1998, and the third from Railroad Valley Playa in April 2000. The technique used here is a modified version of the reflectance-based method. In this technique, results from ground-based measurements of the surface and atmosphere are used in a radiative transfer code to determine the calibration coefficients for SeaWiFS. The results for all three cases are compared with calibration coefficients derived from the onboard calibration and vicarious calibration approaches used for SeaWiFS as well as to results.

The Rayleigh scattering over a clear ocean is a target which radiance is very well modeled and which enables to calibrate the short wavelengths of remote sensing instruments. But the quality of the calibration strongly depends on the evaluation of the other contributors to the observed Top Of Atmosphere radiance i. e. aerosol scattering and reflection over the sea surface (water color, foam, glint...). However these contributors can be reduced by appropriate viewing conditions. This technique is used to calibrate B1 (051-0.59 µm) and B2 (0.61-0.68µm) channels of HRVIR camera, and B0 (0.4-0.5µm) and B2 channels of VEGETATION camera both of which are aboard SPOT4. This article presents the calibration results obtained during the satellite two years in orbit. The results are compared to: - pre-flight results (integrating sphere) - in-flight results. The in-flight results are provided by: - on board calibration system (lamp and sun sensor) - vicarious calibration over test sites (White Sands, La Crau) - calibration over stable deserts - calibration over the sun glint The analysis of the sensitivity of the calibration to the different parameters used to model the TOA radiance shows the accuracy of such a technique.

The launch of the SPOT 4 satellite in March 1998, has given the opportunity to probe a Modulation Transfer Function (MTF) measurement method relying on bi-resolution images and to perform an in-flight MTF assessment for VEGETATION, a wide field of view sensor. The following paper begins with a presentation of the theoretical basis of the method, putting the stress on the sampling and windowing aspects and on the implicit requirements over the data for this method. The next step gives an overview of the SPOT4 sensors involved, namely HRVIR and VEGETATION and a quick description of the data which have been used. Then, the processing applied to the data as well as its improvement are related. The computations provide 3 curves : the MTF across the track (along the rows of the images), the MTF along the track (along the columns of the images) and the MTF for the 45° direction. The VEGETATION MTF curves obtained are presented and compared to the pre-flight measurements. This enables to conclude to the good stability of the sensor and to suggest further work.

In-orbit calibration is an absolutely necessary and accepted tool to update the pre-flight calibration sets of remote sensing instruments on satellites. Only such a periodical recalibration guarantees the long term quality and accuracy of the data and the reliability of the thematic interpretation. Especially for watching global changes of the ocean coastal zones (phytoplancton, sediments, pollution etc.) using spectroradiometric measurements in the VIS/NIR spectral range we need high radiometric accuracy because of small and often only slightly different signals. The Modular Optoelectronic Scanner (MOS) on the Indian Remote Sensing Satellite IRS-P3 has been showing its capacity in this field for more than 4 years. One reason for this success is the sophisticated in-flight calibration using different methods, first an internal parameter check with lamps and second the absolute recalibration with the sun via spectralon diffusers. These two methods together allow the radiometric recalibration with an uncertainty of ± 0.5% with respect to the initial state and enables us in many cases to recognize which opto-electronical component is responsible for which kind of change in different spectral channels and spatial pixels. Radiation stress, satellite and orbit environment, degradation and surface cleaning effects in vacuum are some items which affect the opto-electronical components in different ways. Comparative investigations of some MOS optical components by experimental simulation of the radiation environment in the 820 km IRS sun synchronous orbit for 1, 2, 3 and 10 years radiation load confirm the in-orbit calibration results. The BRDF of the spectralon sun diffuser, the reflectance of anodized aluminum surfaces and the transmission of the front end quartz window did not change by the radiation stress but the transmittance of the front end optics decreases in the blue spectral region up to 10%. These results will be presented together with the in-orbit calibration results of MOS.

Mirror coatings designed for use on satellite remote sensing instruments were exposed to a simulated geostationary orbit radiation flux of combined ultraviolet, electron, and proton radiation. Reflectance measured in vacuum before, during, and after exposure demonstrated that some mirror coatings are much more resistant to damage than others. The laboratory test results agree with in-orbit mirror temperature increases observed on GOES satellite instruments. The laboratory tests also confirm the necessity of measuring reflectance change in vacuum, due to the rapid recovery of reflectance loss upon exposure to room atmosphere.

This paper discusses the operational in-orbit GOES-8 and GOES-10 imager scan-mirror emissivity trends, as well as their diurnal cycles. The imagers (and sounders) aboard both GOES-8 and GOES-10 experience a variation in scan-mirror emissivity along the east-west scan direction. The most obvious manifestation of this phenomenon is a difference in output between the east and west sides when the insthiments view space, but it is also present in observations of the Earth. The phenomenon is accounted for in the calibration process with an algorithm that makes use ofcoefficients incorporating the variation ofthe scanmirror emissivity with east-west scan angle. The coefficients are derived from measurements of space above the north pole and below the south pole made during GOES station-keeping maneuvers, which are performed a few times a year. Over time, these measurements allow us to compile a trend ofthe east-westemissivityvariation. Operational full-disk images are used to diagnose the diurnal behavior of the residual (after correction) east-west output differences. A comparison between the scan-mirror emissivity of GOES-8 and that of GOES-10 is made to search for patterns related to specific satellites. This paper also reviews how the east-west scan-mirror emissivity coefficients are derived and evaluates the effect ofuncertainties in the band-averaged emissivity measurements on the GOES calibration. An effective scan-mirror temperature is proposed to minimize the residual east-west output differences.

A study of the solar contamination in the radiometric calibration for the NOAA -14, and –15 Advanced Very High Resolution Radiometers (AVHRR) is presented. The solar contamination results in a disagreement between the sensormeasured radiometric output of the onboard blackbody vs. its bulk temperature measured by the platinum resistance thermometers. The sunshield installed on the latest version of the instrument on NOAA-15 may have alleviated but not eliminated the problem. The anomaly can still contribute errors on the order of half a degree in the 3 .7 µm infrared channel. Analysis of the time scale and the spectral characteristics of the radiometric anomaly suggest that the extraneous radiation is a combination of solar radiation and background radiation by an object other than the blackbody. Stray light in earth scenes is also found where the anomaly in the radiometric calibration occurs. The radiometric intensity and spectral characteristics of the stray light are analyzed in order to trace the source of the extraneous radiation. It is found that the effects occur as the spacecraft moves out of the shadow of the earth at a low sun elevation. The intensity of the effect appears to be related to the solar zenith and azimuth angles at the spacecraft.

Attenuated sunlight is a valuable reference for on-board calibration of spacebome instruments that observe reflected sunlight from the earth. Direct viewing of the sun through a perforated plate can provide full aperture, end-to-end calibration. Since the transmissivity of the perforated plate depends only upon its geometry, it is potentially more stable than the diffuse reflectivity of a diffuser plate, particularly when exposed to the space environment. We have observed the sun through a sheet metal plate with a hexagonal array of small holes placed in front of a telescope. A pinhole in the telescope’s focal plane, followed by a spectral filter and a silicon photo-diode, were selected to approximate the IFOV and spectral bands proposed for imagers on future GOES missions. In each observation, the center of the solar image was found to have a smooth, symmetrical maximum, with no significant angular structure due to interference. These observations demonstrated that the perforated plate technique is a promising method for stable, long-term, on-orbit calibration of visible and near IR channels on spacebome optical instruments.

Until now incandescent lamp, sun and moon calibrations have been successfully applied for in-flight calibration of spaceborne Earth observation imaging sensors. The performance development of LEDs in the past decade guided to higher luminous efficiencies, broader spectral coverage, lower degradation of light output over time and lower power consumption. These advantages make LEDs to a candidate for radiometric and spectral calibration of spaceborne spectrometers. For analysing LEDs for space in-flight calibration a set of LEDs has been characterised and a simulation of space radiation quantities (i.e. proton and electron radiation for a polar low-Earth orbit) has been carried out. Additional vacuum tests (outgassing behaviour) demonstrated a possible application of LEDs with epoxy housing for the future space environment. Further on, a concept for long-term temperature stabilisation has been developed for solving the main problem of LED in-flight calibration, i.e. the temperature dependency of the irradiance. Consequently, this study demonstrates that (1) a degradation of LEDs due to space environment is not expected, that (2) long-term temperature stability of LEDs can be ensured, and that (3) the higher blue part of ‘white’ LEDs would best suit ocean-colour scientists needs.

Comparison between actual and potential natural vegetation classifications has been done to identify impact of human activities on vegetation distribution over Asian region. The twelve monthly Normalized Difference Vegetation Index (NDVI) derived from NOAA/AVHRR data (1985-1997), together with climatic data, i.e. air temperature, and precipitation were processed by isoclass unsupervised and maximum likelihood algorithm to get the homogeneous spectral classes for land cover categorizing. Through classification trials, 68 clusters were found as the number of vegetation classes over this region. Moreover, the climatic characteristic value of each class, such as temperature, radiation cloudiness, precipitation, and elevation were extracted from available global dataset to determine the potential natural vegetation. By comparing those classifications, we realized, that India (Southern Asia), and some parts of China (Eastern Asia) were the center of land cover changing. This also appears in croplands of Kazakhstan, Indonesia, Mongolia, Thailand, Russia, Pakistan, Nepal, Myanmar (Burma), and Bangladesh.

Global maps of MODIS pigment products were produced using SeaWiFS and AVHRR data from 2 and 3 July 1998. The global mode value for the Case 2 chlorophyll product is 0.075 mg m-3, while global modes for two empirical chlorophyll algorithms are 0.02 to 0.03 mg m-3 higher. The Case 2 chlorophyll product shows a pronounced bimodal distribution with a secondary mode at around 0.30 mg m-3, which is consistent with high-latitude in situ pigment concentration data from the Southern Ocean. Analysis of spectral ratios of Rrs suggests that the 412 nm channel of SeaWiFS may be 1.5-2.0% too low. Blue-absorbing aerosols are not correctly removed from the imagery of the Mediterranean Sea, causing erroneously high retrievals of gelbstoff (or CDOM) absorption coefficient. A numerical filter to diagnose the presence of Saharan dust is suggested.

The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS data collection takes place via the SIMBIOS Science Team and the NASA Aerosol Robotic Network (AERONET). In addition, SIMBIOS has a calibration and product validation component. The primary purpose of these calibration and product validation activities are to (1) reduce measurement error by identifying and characterizing true error sources such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternative methods when possible, and provides the results to the appropriate project office.

We present the pre-launch infrared calibration of the Geostationary Operational Environmental Satellite (GOES) I-M Imager and Sounder. In addition to contractual performance verification, pre-launch calibration provides necessary information for on-orbit operations. These are system relative spectral response, non-linearity in radiometric response and verification of the accuracy of the on board calibration source. The JR channels are calibrated in a thermal vacuum chamber, under varying instrument operating conditions, with two external, temperature controlled blackbodies. A LN2 controlled target represents cold space, and a variable (200 K to 320 K) temperature target represents the Earth scene. We show methods and results for the following instrument performance parameters: system spectral response, noise, non-linearity and relative accuracy. As there is no absolute radiometric standard, the relative accuracy estimates are between the internal (used for on orbit operational calibration) and external calibration sources. Performance trends versus instrument operating condition and across serial number (SNO3-SNO7) are highlighted. We show residual calibration anomalies and describe probable causes.

The Total Ozone Mapping Spectrometer (TOMS) provides daily global mapping of the total column ozone in the earth’s atmosphere. It does this by measuring the solar irradiance and the backscattered solar radiance in 6 spectral bands falling within the range from 308.6 nm to 360 nm. The accuracy of the ozone retrieval is highly dependent on the knowledge of the transfer characteristics and center wavelength for each spectral band. A 0.1 nm wavelength error translates to a 1.6% error in ozone. Several techniques have historically been used to perform the wavelength calibration of the TOMS instruments. These methods include the use of film and reference spectra from low-pressure spectral line lamps and the use of continuum sources with a narrow-band scanning monochromator. The spectral transfer characteristic of the Flight Model 5 instrument for the QuikTOMS mission was calibrated using a new technique employing a frequency doubled tunable dye laser. The tunable laser has several advantages that include a very narrow spectral bandwidth; accurate wavelength determination using a wavemeter; and the ability to calibrate the instrument system level of assembly (prior methods required that the calibration be performed at the monochromator sub assembly level). The technique uses the output from a diode-pumped solid state Nd:V04 laser that is frequency doubled to provide a continuous wave 532 nm pump laser beam to a Coherent Model 899-01 frequency doubled ring dye laser. The output is directed into the entrance port of a 6-inch diameter Spectralon integrating sphere. A GaP photodiode is used to monitor the sphere wall radiance while a Burleigh Wavemeter (WA-1500) is used to monitor the wavelength of the visible output of the dye laser. The TOMS field of view is oriented to view the exit port of the integrating sphere. During the measurement process the response of the instrument is monitored as the laser source is stepped in 0.02-nm increments over each of the six TOMS spectral bands. Results of the new technique allow establishing the wavelength center to a precision of better than 0.1 nm. In addition to the spectral band measurements, the laser provided a means to calibrate the radiometric linearity of the QuikTOMS instrument and yield new insights into the stray light performance of the complete optical system.

In the characterization of a space-borne wide field-of-view sensor, like Végétation, the multi-angular calibration is strongly complementary to the absolute calibration. It is defined as the process of estimating the sensitivity variations at different points of the Végétation wide field-of-view. This effect has to be integrated in the data processing. Pre-flight measurements were performed before launch, but because of heavy irradiations and aging of the different part of the sensor, it is necessary after launch to check and/or adjust the multi-angular calibration coefficients, gp. For this, the gp coefficients were split into three terms which required different methods: i/ first, the low-frequency term (gpLF) which refer to variation of the optic transmission which slightly decreases when viewing angle increases. The gpLF were verified using acquisitions over 20 desert sites for which TOA reflectances are accurately characterized (from ground measurements and POLDER/ADEOS-1 measurements). No in-flight variation of the gpLF were detected. ii/ second, the high-frequency term (gpHF) which refer to variation of the sensitivity of the elementary detectors. The gpHF were verified statistically using acquisitions over the Antarctica site and were accurately checked for the 4 spectral bands. ii/ third, the medium-frequency term (gpMF) which refer to various kinds of variation (optics, detectors...). The gpMF were verified during the 9pJpWDWLRQ like using the on-board calibration device (lamp profiles) and some small variations were identified (< 0.5% for B0, B2, B3 and ~1% for MIR). This aspect is still under investigation using acquisitions over Antarctica.