The Atmospheric Lidar (ATLID) is the backscatter lidar instrument developed for ESA, under the prime contractorship of MATRA MARCONI SPACE France. This kind of lidar has been selected for flight on an ESA Earth Explorer satellite, and will be based on ATLID concept and technologies. It is part of a multi-payload mission, named Earth Radiation, dedicated to the Earth radiative transfer study for climatology. The lidar will provide information on the atmosphere, such as cloud cover, top height of all cloud types and planetary boundary layer, thin cloud extent, optical depth and polarization. The instrument features a pulsed diode-pumped Nd-YAG laser (1.06 micrometers wavelength) together with a one-axis scanning 60 cm lightweight telescope. A technology pre-development program has been performed in order to raise the maturity of the instrument design. Elegant breadboard models have been realised and submitted to environmental tests. The laser transmitter, the laser thermal control subsystem (capillary-pumped two-phase loop), the diode laser power supply, the avalanche photodiode detection chain, the narrow-band filter, the scan mechanism, and the telescope lightweight primary mirror (C-SiC) have been breadboarded in the frame of the programme. The instrument design and performance have also been consolidated with regards to the successful hardware results.

SAGE III is part of the Mission to Planet Earth's Earth Observatory System with a first launch in the summer of 1998. SAGE III will provide long term monitoring of atmospheric species such as ozone and aerosols which play an important role in global environmental and climatic changes. This paper will briefly describe the goal of the SAGE III experiment, the instrument design, and the development of the processing algorithm for routine data processing to produce scientifically important data products for the science community.

SFINX (SRON's Fabry-perot INterferometer eXperiment) consists of a 65 - 90 micrometer wavelength spectrometer based on a Fabry-Perot interferometer, and is equipped with both a conventional Ge:Ga photon detector operating at 4 K, and a novel high-temperature superconductor (HTS) bolometer detector operating at 87 K. The spectral resolution is about 8000, or 0.015 cm^-1, comparable with the width of the thermal emission lines of the stratospheric species under study. Target molecules are OH, HCl, HO₂, and possibly more. The SFINX instrument now under development will fly as a piggy-back instrument on a stratospheric balloon together with the MIPAS-B2 instrument of IMK/FZK (Karlsruhe, Germany), and can be regarded as a proof of concept for a satellite application. Because of its low satellite resource demands, for a satellite application a SFINX like instrument has great advantages with respect to Fourier transform spectrometers or heterodyne receivers. In particular, the HTS bolometer detector can be cooled by mechanical coolers which are presently available in space- qualified versions, thereby avoiding the use of liquid cryogen.

A 120 X 5 degree instantaneous field-of-view Wide Angle Earth Sensor has been developed for use on missions that are prohibited from viewing the Earth with the payload sensor. Developed especially for NASA's WIRE (Wide-Field Infrared Explorer) mission, the WAES consists of two Sensor Heads and an Electronics Control Unit. On the WIRE mission the sensor heads are arranged back to back 180 degrees apart such that they can monitor one axis of rotation, while a sun sensor is used for the other axis. This is of extreme importance to these missions, as the telescope features a detector that is cooled by a solid hydrogen cryogen. Accidental viewing of the Earth or sun would cause the cryogen to boil off and end the mission prematurely. The predicted accuracy of 1.5 degrees exclusive of radiance errors and 3.5 degrees inclusive of radiance errors.

We report on continuing work at ITT to develop imaging spectrometers for future remote sensing systems operating in geosynchronous earth orbit (GEO). Preliminary notional requirements for hyperspectral imaging from GEO were established and compared with available technology. A high level trade study defined by evaluation of a signal-to-noise based figure of merit, cost and technical risk factors selected tunable filter technology for development into a breadboard visible near-infrared imaging spectrometer. We built and tested a breadboard system using an Acousto- optical Tunable Filter that is currently being field tested. Our analysis shows an imaging Fourier Transform Spectrometer offers best overall value for thermal infrared imaging spectrometers in GEO.

The uncooled Thermal Imaging Radiometer (TIR) utilizes a single microbolometer detector array to obtain calibrated infrared images in four spectral bands. The spectral bands and bandwidths are user selectable from 8 to 12 microns. The TIR is a portable system designed for both indoor and outdoor use. It is controlled by a laptop computer which also displays the fully calibrated spectral images in real time with both color and gray scale options. The TIR scans 360 degrees and provides continuous imaging of the surrounding scene on the laptop display. Selected images can also be saved for radiometric analysis. Early test results and thermal images from the TIR will be presented.

The opto-electronic system for Earth observing from the space based on the tunable acousto-optic filter and the time-position counter of separate photons is considered. The possibility to reach the high spectral and spatial resolutions without wide-aperture optics and complex systems of control is shown.

The NASA Scatterometer (NSCAT) was launched in August 1996 aboard the Japanese Advanced Earth Observation System-I (ADEOS-I) as part of the NASA Earth Probes program. NSCAT is an active microwave remote sensor designed to measure winds over the ocean from space. NSCAT can measure vector (speed and direction) winds over 90% of the Earth's ice-free oceans every two days. Such data is important in weather prediction and air-sea interaction studies since winds modulate all air-sea fluxes. The Ku-band NSCAT is a follow-on to the Seasat scatterometer but has improved resolution and coverage. NSCAT is a Doppler scatterometer with 600 km wide swaths on either side of the nadir track. In comparison the C-band ERS-1/2 Active Microwave Instrument scatterometer system has only a single swath. A follow-on to NSCAT, known as SeaWinds, is in development and will be launched in 1999. The on-orbit performance of NSCAT is superb with initial data products delivered to the science team within two months of the start of data collection. This paper provides a brief overview of the NSCAT mission and its status and briefly discusses the results of the calibration and validation study and some new applications of NSCAT data.

Polar sea ice plays an important role in the global climate and other geophysical processes. Although spaceborne scatterometers such as NSCAT have low inherent spatial resolution, resolution enhancement techniques can be utilized to make NSCAT data useful for monitoring sea ice extent in the Antarctic. Dual polarization radar measurement parameters, A and B, are used to identify sea ice and ocean pixels in composite images where A is (sigma) ^o normalized to 40 degrees and B is the incidence angle dependence of (sigma) ^o. In particular, the copol ratio and the vertical polarization B values contain useful information about the presence of sea ice. A first estimate of the sea ice extent is obtained through an automated linear discrimination that assigns the decision boundary based upon the properties of the bivariate distribution. This is used to obtain estimates of the statistics needed to perform a more accurate Mahalanobis distance discrimination. Ice edge detection noise reduction is performed through region growing and erosion/dilation techniques. The algorithm is applied to NSCAT data. The resulting edge closely matches the NSIDC SSM/I derived 30 percent ice concentration edge.

A detailed evaluation of NASA scatterometer (NSCAT) data has recently been performed to determine the error characteristics of this data type and its applicability to ocean surface analysis and numerical weather prediction. The first component of this evaluation consisted of both subjective and objective comparisons of NSCAT winds to ship and buoy observations, Goddard EOS (GEOS) and National Centers for Environmental Prediction (NCEP) wind analyses, European Space Agency European Remote Sensing Satellite wind vectors, and Special Sensor Microwave Imager wind speeds. This was then followed by a series of data assimilation and forecast experiments using the GEOS-1 data assimilation system (DAS), the prototype for the GEOS-2 DAS, and an earlier version of the NCEP DAS, that was operational in 1995. This paper will provide a brief summary of these experiments and a few illustrations of the results obtained.

The NASA Scatterometer (NSCAT) is designed to make measurements of the normalized radar backscatter coefficient ((sigma) ^o) of the ocean's surface. The measured (sigma) ^o is a function of the viewing geometry and the surface roughness due to wind-generated waves. By making multiple measurements of the same location from different azimuth angles it is possible to retrieve the near-surface wind speed and direction with the aid of a Geophysical Model Function (GMF) which relates wind and (sigma) ^o. The wind is estimated from the noisy (sigma) ^o measurements using maximum likelihood techniques. The probability density of the measured (sigma) ^o is assumed to be Gaussian with a variance that depends on the true (sigma) ^o and therefore the wind through the GMF and the measurements from different azimuth angles are assumed independent in estimating the wind. In order to estimate the accuracy of the retrieved wind, we derive the Cramer-Reo (CR) bound for wind estimation from scatterometer measurements. We show that the CR bound can be used as an error bar on the estimated wind. The role of geophysical modeling error in the GMF is considered and shown to play a significant role in the wind accuracy. Estimates of the accuracy of NSCAT measurements are given along with other scatterometer geometries and types.

The recently launched NASA Scatterometer (NSCAT) estimates the wind speed and direction of near-surface ocean wind. Several possible wind vectors are identified for that location of the earth known as a cell. Typically, the speeds of the possible wind vectors are the same, but the directions are very different. The correct wind must be distinguished from these in a step called ambiguity removal. Unfortunately, ambiguity removal algorithms are subject to error. Because the true wind is not known, where these errors occur is difficult to determine, and there is little information about how to detect the errors in this removal step. One method we have developed to assess the accuracy of the ambiguity removal algorithm is to compare the point-wise retrieved wind to wind retrieved using a model. The model is fit to the point-wise retrieved wind to determine if the observed wind is realistic or containing possible ambiguity removal errors The algorithms' performance achieves its goal to identify at least 95 percent of the regions that contain errors. The algorithm provides a very simple tool to indicate regions of possible ambiguity removal errors in the point-wise retrieved winds for NSCAT data. This paper describes this algorithm and its performance for real NSCAT data. It also outlines an algorithm to correct some of the errors. This is done by either choosing the alias closest to the model-fit or by simply replacing the erroneous wind vectors with those produced by the model-fit.

Traditional scatterometer wind estimation inverts the model function relationship between the wind and backscatter at each resolution element, yielding a set of ambiguities due to the many-to-one mapping of the model function. Field-wise wind estimation dramatically reduces the number of ambiguities by estimating the wind for many resolution elements, simultaneously, using a wind field model that constrains the spatial variability of the wind. In this paper four wind field models are presented or use in field- wise wind estimation. The models considered include tow standard expansions, a data-driven model, and a model based on geophysical constraints on the pressure field. Model accuracy, as a function of the number of model parameters, is reported for each model. This accuracy is evaluated using NSCAT JPL nudged L2.0 data. Because of the inherent compromise between computational complexity of high-order models and the imprecise fit of low-order models, automated classification schemes are developed to identify a priori whether a region will be well modeled by a simple wind model. Classification is performed through hypothesis testing on raw NSCAT data and point-wise estimates.

Field-wise scatterometer wind estimation determines the vector wind at many resolution elements simultaneously by estimating the parameters of a wind field model from the scatterometer measurements. According to simulations, it results in more accurate estimates than traditional point- wise estimation, which estimates the vector wind one resolution element at a time. Further, field-wise estimation produces fewer ambiguities than point-wise estimation. Field-wise estimation necessitates locating the local minima of a high-dimensional objective function. Conventional optimization techniques can be employed if initial search points within the capture regions of the local minima can be found. We develop and evaluate two novel approaches that determine initial search points and locally optimize them to produce field-wise estimates. The utility of each algorithm is demonstrated using simulated NASA Scatterometer data.

The distributed batch controller (DBC) supports scientific batch data processing. Batch jobs are distributed by the DBC over a collection of computing resources. Since these resources may be widely scattered the DBC is well suited for collaborative research efforts whose resources may not be centrally located. The DBC provides its users with centralized monitoring and control of distributed batch jobs. Version 1 of the DBC is currently being used by the TOVS Polar Pathfinder project to generate Arctic atmospheric temperature and humidity profiles. Profile generating jobs are distributed and executed by the DBC on workstation clusters located at several sites across the US. This paper describes the data processing requirements of the TOVS Polar Pathfinder project, and how the DBC is being used to meet them. It also describes Version 2 of the DBC. DBC V2 is implemented in Java, and utilizes a number of advanced Java features such as threads and remote method invocation. It incorporates a number of functional enhancements. These include a flexible mechanism supporting interoperation of the DBC with a wider variety of execution resources and an improved user interface.

The National Snow and Ice Data CEnter (NSIDC) Distributed Active Archive Center (DAAC) provides data and information on snow and ice processes, especially pertaining to interactions among snow, ice, atmosphere and ocean, in support of research on global change detection and model validation, and provides general data and information services to the cryospheric and polar processes research community. THe NSIDC DAAC is an integral part of the multi- agency-funded support for snow and ice data management service at NSIDC. The Moderate Resolution Imaging Spectroradiometer (MODIS) will be flown on the first Earth Observation System (EOS) platform in 1998. The MODIS Instrument Science Team is developing geophysical products form data collected by the MODIS instrument, including snow cover and sea ice products which will be archived and distributed by the NSIDC DAAC. The MODIS snow and ice mapping algorithms will generate global snow, lake ice and sea ice cover products on a daily basis. These products will augment the existing record of satellite-derived snow cover and sea ice products that began about 30 years ago. The characteristics of these products, their utility, and comparisons to other data sets are discussed. Current developments and issues are summarized.

Calibration of optical sensors onboard earth observation satellites has been a major issue for several years. A collection of techniques has been developed involving modelization, surface or airplane based measurements of surface and atmosphere characteristics. The present study has the following objectives: conceive and develop a repository for all available data sets related to optical sensor calibration, including selection of commonly used sites, characteristics of the surface, climatology of the atmosphere, ground- to air-based measurements as well as results of calibration of various sensors over these sites. The main goal is to unify the use of these sites to make inter comparison of results easier and the calibration of forthcoming sensors more accurate at little extra expenses. Conception of the database and its environment will identify the type of data to gather, the format to use, the updating frequency, as well as the theoretical background necessary to the use of these data. For instance, information will be given about spectral behavior of the surface or about how to make measurements at various resolution easy to compare.

The in-flight calibration program for the EOS-AM1 Multi- angle Imaging SpectroRadiometer (MISR) includes on-orbit calibration, characterization of instrument properties and a calibration integrity process. One of the primary activities of the in-flight radiometric calibration and characterization group responsible for the MISR calibration program at the Science Computing Facility is to produce a data file called the Ancillary Radiometric Product (ARP). Radiance scaling and conditioning processing at the distributed active archive center, as well as other science data product generation, proceed using ARP parameters. The parameters that make up the ARP include pre-flight data which give an account of the instrument radiometric response, along with other instrument descriptors. Radiometric response will be maintained and updated throughout the mission.

The multi-angle imaging spectroradiometer (MISR) cameras completed detailed calibration and characterization testing a year ago, as reported in earlier literature. Since that time the cameras have been assembled onto a common flight optical bench, along with photodiode detector standards and diffuse calibration targets. The orderly multiplexing of high-rate data streams from nine camera, twenty-four calibration photodiode channels, and engineering measurements of temperatures and voltages into packets has been verified. Camera fields-of-view clearances have been established, and a verification of camera and photodiode relative response established. These verification tests of the instrument have been followed by shipment of the instrument to the spacecraft integrator facility, where testing continues. Even for the simplest of experiments, the insight learned into the functionality of the instrument has been invaluable. This paper reviews a sampling of these test and lessons learned, from the simplest verification experiments, to the complex camera boresight determinations.

The scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY), to be launched on the European polar platform ENVISAT at the end of 1999, will measure sun- and moonlight which is either transmitted, reflected or scattered by the Earth atmosphere. The double spectrometer is designed for the ultraviolet, visible and near IR wavelength region, covering that range with a resolution of 0.24 nm to 1.5 nm. It was conceived to improve our knowledge and understanding of a variety of issues of importance to chemistry and physics of the Earth atmosphere. Scientific objectives are to study ozone hole chemistry, troposphere- stratosphere exchange and tropospheric pollution. This will be achieved by a combined limb, nadir and occultation observation strategy. The SCIAMACHY instrument and operational concept is finalized approaching now the on- ground calibration phase. The planned wavelength and radiometric calibration measurements, including a detailed characterization of the instrument polarization behavior, will be fundamental for the required high data accuracy. Additionally the in-flight calibration and monitoring concept will allow a proper correction of instrument ageing effects. This study describes pre-flight and in-flight calibration the approaches to be employed to maintain a high radiometric and spectral accuracy of the SCIAMACHY measurements throughout its life are reported.

According to the results of SCARAB Flight Model 1 on orbit, some modifications will be introduced in the on board calibration procedure of Flight Model 2. After having described briefly the instrument and the mission, this paper will detail: the on board procedure for the first flight model (FM1); the different problems encountered during the FM1 mission; the proposed new on board calibration procedure; the global error budget.

The purpose of this study is to propose and study algorithms for off-line trend monitoring and change-point detection for calibration coefficient data streams. This type of algorithm is suitable for monitoring different sensors e.g. AVHRR, ETM+ and MODIS. Based on these algorithms we can produce flags for he instruments indicating normal and abnormal behavior. These algorithms can also help to discover trends and features in the calibration data. Some of these algorithms will be used by the Landsat-7 ETM+ Image Assessment System and the EOS AM1 MODIS calibration system for modeling the gain behavior of the instruments over time. Mathematically we used quadratic splines and statistical concepts for building incremental models.

Errors can occur in laboratory measurements when the response of a bandpass-filtered radiometer extends into an atmospheric absorption region. Atmospheric models, such as MODTRAN3, can be valuable tools that allow optical measurement in these regions to be accurately analyzed. Comparisons of MODTRAN3-predicted and laboratory-measured atmospheric transmittance have been made to help establish the validity of MODTRAN3 for use in modeling short-path length, low resolution, optical effects over the absorption band near 1380 nm. MODTRAN3-predicted transmittance is shown to be within 4 percent of the measured data and well within 2 percent foremost of the water band. The spectroradiometric measurement of the water-vapor absorption band, its description, and its comparison to the MODTRAN3 prediction are presented. Also presented are examples of errors that can occur when an instrument response extends into this region.

Pre-launch instrument calibration testing of the current series of GOES Imagers and Sounders has shown a systematic non-linearity in the radiometric response of the SWIR channels of approximately 0.2 percent. Possible mechanisms for the observed non-linearity include; inherent InSb detector non-linearity, non-constant temperature error on the blackbody calibration target, relative spectral response error, and electronics non-linearity. A calibration model common to the GOES and POES radiometers is presented. The sensitivity of the SWIR channels' non-linear response to errors in spectral response is derived. Systematic shift of spectral response center wavenumber of approximately 10 cm^-1 are found to induce a 0.1 percent non-linearity in response. Pre-launch calibration coefficients from recent NOAA radiometers are analyzed. Operational calibration errors caused by incorrect quadratic coefficients are found to be as large as 0.15K for warm scenes.

Simulations of a technique to measure the spectral response functions (SRFs) of the Atmospheric IR Sounder (AIRS) detectors is presented. The AIRS detectors will be illuminated by the output of a blackbody modulated by a Fourier transform spectrometer. The resulting interferograms measured by the AIRS detectors are then Fourier transformed to produce SRFs. Separate simulations were performed for the central and wing SRF regions. The impact of resolution, finite field of view apodisation, and nonlinear detector response on the SRF full-width, central frequency, and wing response were investigated.

Several MODIS cloud product algorithms are being developed at the University of Wisconsin for the generation of day-1 products after the launch of MODIS. MODIS Airborne Simulator (MAS) radiometric data collected form NASA's ER-2 platform is being used to simulate MODIS spectral bands for testing and refinement of the cloud product algorithms. Spectral characterization is an important component of the MAS calibration. MAS LWIR bands are spectrally characterized in ambient conditions using a monochromator and are corrected for source spectral shape and atmospheric attenuation. An atmospheric correction based on LBLRTM forward model transmittances demonstrates that strong spectral absorption features, such as Q-branch CO₂ absorption near 13.9 micrometers , are effectively removed from the spectral measurements with the aid of a small spectral position correction. Comparisons of MAS in-flight data to well- calibrated HIS instrument data indicate that MAS LWIR spectral calibration drift over time is less than 5 percent of FWHM. The MODIS CO₂ cloud top height retrieval shows small dependence on the spectral characterization, with retrieved cloud top height changing by less than 0.5 km in response to a 5 percent spectral position change. This is within the tolerance of other error sources in the cloud top properties algorithm.

Cosine collector designs based on internally baffled integrating spheres have been difficult to evaluate in the past due to the expense and difficulty of building the spheres and measuring their angular response. A Monte Carlo model has been developed that enables integrating sphere designs to be evaluated relatively quickly and efficiently. The model was applied to an integrating sphere employing an internal conical baffle. The angular response and overall throughput of the sphere as predicted by the model are presented and discussed. Construction of the sphere was recently completed and the relative angular response was measured in the field. A solar radiometer was used to determine the solar irradiance at the entrance aperture of the sphere, enabling the response of the sphere to be found as a function of the solar zenith angle. Results of the measurement are presented and compared with the model results.

Airborne radiometric instruments are often used to collect calibrated radiance data, whether for producing remotely- sensed imagery, for use in vicarious calibration, or for atmospheric correction. Typically, these radiometers are calibrated in a laboratory environment using source whose spectral outputs are traceable to some established, man-made standard. In the field, these devices are used with a different source: solar radiation. The use of solar radiation as a calibration source should therefore be considered when calibration radiometers used to collect energy in the solar reflective region. This paper presents a novel method of calibration which makes use of scattered solar radiation as the source. This technique is particularly applicable for airborne radiometers intended to view low-reflectance surfaces, since the magnitude and spectral distribution of the collected energy is very similar to that of skylight, especially at shorter visible wavelengths. The method is applied to visible and near-IR bands of a Barnes Modular Multispectral 8-channel Radiometer. A sensitivity study was performed for the method and an associated uncertainty analysis is presented. The calibration results are compared to a second, more established solar-based method whose source is directly- transmitted solar irradiance.

A spectral polarimeter with an autotracking mount to obtain atmospheric parameters required for the vicarious calibration of satellite sensors has been modified to work with anew computer and electronic components. The instrument has 12 bands covering the visible through the short-wave IR. There are 9 bands from 400 nm to 1100 nm which use a silicon detector, and 3 bands from 1100 nm to 2500 nm which use a temperature-stabilized, lead-sulfide detector. The instrument's operation was verified by using it as a solar radiometer and collecting Langley plot data. These were compared to data taken concurrently by a well-characterized, manually-pointed radiometer with 10 visible and near-IR channels. In addition, the effect of the gaseous transmittance on the retrieved optical depths of the short- wave IR bands are presented. The data are obtained by finding the band-averaged transmittance for each filter under several atmospheric and view conditions using the output from MODTRAN3.

A four-band, prototype, thermal-IR radiometer with a built- in radiance reference has been fabricated by CIMEL Electronique, Paris, France, for use as a field instrument. This paper briefly describes the instrument and discusses laboratory characterization measurements and results, including spectral response, linearity of better than 0.8 percent, field of view of 9.5 degrees, noise-equivalent temperature difference of 0.06-0.2 degrees C for temperatures of 0 to 75 degrees C, signal-to-noise ratio greater than 1100 for the broad band and greater than 400 for the other bands for temperatures between 10 and 80 degrees C, nonrepeatability of less than 0.35 percent after four field campaigns, and absolute calibration.

The clouds and the Earth's Radiant Energy System (CERES) spacecraft scanning thermistor bolometers will measure earth-reflected solar and earth-emitted longwave radiances, at the top-of-the-atmosphere. The measurements are performed in the broadband shortwave and longwave spectral regions as well as in the 8-12 micrometers water vapor window over geographical footprints as small as 10 kilometers at the nadir. The CERES measurements are designed to improve our knowledge of the earth's natural climate processes, in particular those related to clouds, and man's impact upon climate as indicated by atmospheric temperature. November 1997, the first set of CERES bolometers is scheduled for launch on the Tropical Rainfall Measuring Missions (TRMM) Spacecraft. The CERES bolometers were calibrated radiometrically in a vacuum ground facility using absolute reference sources, tied to the International Temperature Scale of 1990. Accurate bolometer calibrations are dependent upon the derivations of the radiances from the spectral properties of both the sources and bolometers. In this paper, the overall calibration approaches are discussed for the longwave and shortwave calibrations. The spectral response for the TRMM bolometer units are presented and applied to the bolometer ground calibrations in order to determine pre-launch calibration gains.

Scarab Flight Model is a four channels scanning radiometer, launched in February 1994, with the same ERBE scientific mission. It worked perfectly during one year, aboard a Russian satellite, METEOR 3 N degrees 7. Data wee very consistent with ERBE results. Calibration of FM2 and Spare Model is described. The calibration comprises three phases: solar/diffuser short wave source; blackbodies long wave sources under vacuum; integrating sphere short wave source. Results are described and a comparison between the tow last methods is established.

The basic physical principles underlying three common techniques for the vicarious calibration of the post-launch performance of meteorological satellite sensor are briefly reviewed. The techniques considered are: (a) using 'radiometrically stable' desert calibration targets which yield relative degradation rates; (b) congruent path aircraft/satellite radiance measurements which yield absolute calibrations; and radiative transfer model simulation methods which yield absolute calibrations. The applications of the three techniques will be illustrated, using the visible and near-IR channels of the Advanced Very High Resolution Radiometer flown on the NOAA polar-orbiting operational environmental satellites as an example. The establishment of inter-satellite calibration linkages, and cross-satellite sensor calibration will be briefly mentioned.

SCIAMACHY is one of the payloads for the ENVISAT satellite of the European Space Agency ESA. The instrument is a German-Dutch co-production. The launch of ENVISAT is foreseen in 1999. SCIAMACHY is an atmospheric chemistry instrument with wavelength coverage from 240 nm up to 2.4 micrometer and allows nadir viewing, limb viewing, solar occultation and observation of solar reference spectra over an onboard diffuser. The instrument uses eight linear array detectors with 1024 pixels, each, yielding a spectral simultaneity factor of more than 8000, spanning a large wavelength range from UV to NIR. The instrument has a two- mirror scan system, allowing nadir and limb scans. The instrument has different optical efficiencies for different polarization orientations of incoming radiation. An onboard Polarization Monitoring Device is used to monitor the polarization state of the incoming radiation so that the observed instrument output can be corrected for polarization effects. Accurate pre-flight and in-flight calibration of SCIAMACHY is essential for the observational objectives of the instrument, and a vacuum facility with external optical stimulators has been developed in order to allow the instrument to be calibrated under representative environmental conditions.