Showing papers on "Radiometer published in 1995"

An estimate of global primary production in the ocean from satellite radiometer data

Abstract: An estimate of global net primary production in the ocean has been computed from the monthly mean near-surface chlorophyll fields for 1979-1986 obtained by the Nimbus 7 CZCS radiometer. Our model required information about the subsurface distribution of chlorophyll, the parameters of the photosynthesis-light relationship, the sun angle and cloudiness. The computations were partitioned among 57 biogeochemical provinces that were specified from regional oceanography and by examination of the chlorophyll fields. Making different assumptions about the overestimation of chlorophyll by the CZCS in turbid coastal areas, the global net primary production from phytoplankton is given as 45-50 Gt C year"1. This may be compared with current published estimates for land plants of 45-68 Gt C year"' and for coastal vegetation of 1.9 Gt C year"1.

Inter-satellite calibration linkages for the visible and near-infared channels of the Advanced Very High Resolution Radiometer on the NOAA-7, -9, and -11 spacecraft

Abstract: The post-launch degradation of the visible (channel 1: 0.58- 068 microns) and near-infrared (channel 2: approx. 0.72 - l.l microns) channels of the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-7, -9, and -11 Polar-orbiting Operational Environmental Satellites (POES) was estimated using the south-eastern part of the Libyan Desert as a radiometrically stable calibration target. The relative annual degradation rates, in per cent, for the two channels are, respectively: 3.6 and 4.3 (NOAA-7); 5.9 and 3.5 (NOAA-9); and 1.2 and 2.0 (NOAA-11). Using the relative degradation rates thus determined, in conjunction with absolute calibrations based on congruent path aircraft/satellite radiance measurements over White Sands, New Mexico (USA), the variation in time of the absolute gain or slope of the AVHRR on NOAA-9 was evaluated. Inter-satellite calibration linkages were established, using the AVHRR on NOAA-9 as a normalization standard. Formulae for the calculation of calibrated radiances and albedos (AVHRR usage), based on these interlinkages, are given for the three AVHRRs.

Satellite-derived sea surface temperatures: Current status

Abstract: The current status of techniques for deriving sea surface temperatures (SST) from infrared satellite data is reviewed. A short history of the subject is presented, along with a survey of the different techniques employed for obtaining SST from space measurements. The discussion concentrates on the advanced very high resolution radiometer (AVHRR) and similar instruments, as these are found to provide the most useful data for SST estimation. A comparison of different algorithms for deriving SST from the AVHRR instruments shows that there has been little improvement in derivation accuracy over the past decade. The importance of the “first-guess” principle is introduced and shows that some of the more recent “advances” in this field do not address the residual source of error in SST derivation. New algorithms that include coefficients dependent on the variances of the satellite brightness temperature images are found to be difficult to apply to real satellite data. Other new algorithms that include measurements of total atmospheric water vapor column may give a slight improvement in derivation accuracy, but they do not account for anomalous vertical structure in the atmosphere. Directions for future research to improve the accuracy of satellite-derived SST are suggested.

A radiative transfer model for sea surface temperature retrieval for the along‐track scanning radiometer

Abstract: The measurements made by the along-track scanning radiometer are now converted routinely into sea surface temperature (SST). The details of the atmospheric model which had been used for deriving the SST algorithms are given, together with tables of the coefficients in the algorithms for the different SST products. The accuracy of the retrieval under normal conditions and the effect of errors in the model on the retrieved SST are briefly discussed.

Area-averaged vegetative cover fraction estimated from satellite data

Abstract: The relationship was analysed between the vegetation cover factor expressed as a percentage and the area-averaged normalized difference vegetation index (NDVI). On selected days the NDVI was calculated from channel 1 and 2 reflectance data of the National Oceanic and Atmospheric Administration (NOAA—11) satellite's advanced very high-resolution radiometer (AVHRR) for five test areas under agricultural and forestry use. No ground-based reflectance measurements could be made for validation of these data. Therefore the land surface NDVI, which varied with time, and percentage vegetation cover of the test areas were deduced from time-independent but site-specific statistical land use data updated by temporal phenological observations, and from surface-specific reflectance curves published in the literature. The result indicated that the area-averaged NDVI, as obtained from the NOAA—11 radiometer, was less than the value calculated from the land surface NDVI. After correction to reduce the offset of the data, the values would be a suitable indicator of the fraction of vegetation cover.

Polarimetric measurements of sea surface brightness temperatures using an aircraft K-band radiometer

Abstract: Presents the first experimental evidence that the polarimetric brightness temperatures of sea surfaces are sensitive to ocean wind direction in the incidence angle range of 30 to 50/spl deg/. The experimental data were collected by a K-band (19.35 GHz) polarimetric wind radiometer (WINDRAD) mounted on the NASA DC-8 aircraft. A set of aircraft radiometer flights was successfully completed in November 1993. The authors performed circle flights over National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast, which provided ocean wind measurements. The results indicate that passive polarimetric radiometry has a strong potential for global ocean wind speed and direction measurements from space. >

A Validation of a Satellite Cloud Retrieval during ASTEX

Abstract: An algorithm using NOAA-12 AVHRR (Advanced Very High Resolution Radiometer) solar reflectance measurements for retrieving cloud droplet size and optical thickness has been applied to a boundary layer stratocumulus cloud in the vicinity of the Azores on 12 June 1992 during the Atlantic Stratocumulus Transition Experiment (ASTEX). This day was particularly advantageous for validations because of the absence of cirrus or other higher-level clouds during the satellite overpass and the existence of a large relatively uniform stratus cloud dock. Uncertainty estimates for the retrievals are presented along with a discussion of the algorithm. An in-flight absolute calibration of AVHRR channel 1, necessary for accurate optical thickness retrievals, was done concurrently with the retrievals through comparison with a visible radiometer flown on the National Aeronautics and Space Administration's ER-2 and using the stratus cloud as the common reflectance target. Results are compared with in situ microphysica...

Rotational Raman scattering (Ring effect) in satellite backscatter ultraviolet measurements

Abstract: A detailed radiative transfer calculation has been carried out to estimate the effects of rotational Raman scattering (RRS) on satellite measurements of backscattered ultraviolet radiation. Raman-scattered light is shifted in frequency from the incident light, which causes filling in of solar Fraunhofer lines in the observed backscattered spectrum (also known as the Ring effect). The magnitude of the rotational Raman scattering filling in is a function of wavelength, solar zenith angle, surface reflectance, surface pressure, and instrument spectral resolution. The filling in predicted by our model is found to be in agreement with observations from the Shuttle Solar Backscatter Ultraviolet Radiometer and the Nimbus-7 Solar Backscatter Ultraviolet Radiometer.

TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphere range correction algorithm and pre-launch error budget

Abstract: For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted. >

A comparison of Sun photometer derivations of total column water vapor and ozone to standard measures of same at the Southern Great Plains Atmospheric Radiation Measurement site

Abstract: Total column water vapor measurements made over a 1-year period by collocated instruments at the Southern Great Plains Atmospheric Radiation Measurement program site in northern Oklahoma are compared. Microwave radiometer measurements were validated in an earlier program with extensive comparisons to balloon sonde measurements. Sun photometry, using the multifilter rotating shadowband radiometer (MFRSR), was the second technique used to derive water vapor using the 940-nm water band when no clouds were between the radiometer and the sun. Bruegge et al.'s implementation of Reagan et al.'s modified Langley technique was applied in this case. This approach is noteworthy in that it does not rely on a comparison to a standard for calibration. The comparison of 101 one-half-hour to two-hour averages produced a root-mean-square difference of 0.18 cm compared to a mean value of 1.49 cm, or about 12%, with the MFRSR showing an overall bias of +0.03 cm relative to the microwave radiometer data. This is a somewhat higher rms error than other results using the modified Langley technique, but the comparison was made over the entire range of water vapor columns experienced in a year of observations. As a by-product of the water vapor derivation procedure for the MFRSR, ozone column abundance was estimated. Seasonally averaged, these data compare favorably with Dobson measurements from Nashville, Tennessee, which is near the same latitude. A more definitive conclusion regarding the Sun photometer ozone derivation technique requires colocated ozone measurements using an accepted standard approach, however, the technique appears to provide a useful ozone estimate for correcting aerosol optical depth derivations.

TOPEX/Poseidon Microwave Radiometer (TMR). I. Instrument description and antenna temperature calibration

Abstract: The TOPEX/Poseidon microwave radiometer (TMR) is a three-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which is co-aligned with the T/P radar altimeters. The TMR monitors and corrects for the propagation path delay of the altimeter radar signal due to water vapor and nonprecipitating liquid water in the atmosphere. The paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T/sub A/) from the raw digital data. T/sub A/ precision of 0.4 K is predicted on orbit in all expected thermal environments, T/sub A/ accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. These performance figures represent a significant improvement over those of the Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on which TMR is based. The improvements are the result of specific hardware design and calibration changes. Hardware changes include a redesigned feed horn, to reduce impedance mismatches, and the addition of radomes over the feed and sky horns, to reduce thermal variations. Calibration changes involve more extensive temperature cycling and data analysis during thermal/vacuum testing. >

Instrument self-shading in underwater optical measurements: experimental data

Abstract: Self-shading error of in-water optical measurements has been experimentally estimated for upwelling radiance and irradiance measurements taken just below the water surface. Radiance and irradiance data have been collected with fiber optics that terminated with 1°, 18°, and 2π optics housed in the center of a disk that simulated the size of the instrument. Analysis of measurements taken at 500, 600, and 640 nm in lake waters have shown errors ranging from a few percent up to several tens of percent as a function of the size of the radiometer, the absorption coefficient of the medium, the Sun zenith, and the atmospheric turbidity. Comparisons between experimental and theoretical errors, the latter computed according to a scheme suggested by other authors, have shown absolute differences generally lower than 5% for radiances and lower than 3% for irradiances. Analysis of radiance measurements taken with 1° and 18° fields of view have not shown appreciable differences in the self-shading error. This finding suggests that correction schemes for self-shading error developed for narrow-field-of-view radiance measurements could also be applied to measurements taken with relatively larger fields of view.

Surface temperature retrieval in a temperate grassland with multiresolution sensors

Abstract: Radiometric surface temperatures retrieved at various spatial resolutions from aircraft and satellite measurements at the FIFE site in eastern Kansas were compared with near-surface temperature measurements to determine the accuracy of the retrieval techniques and consistency between the various sensors. Atmospheric characterizations based on local radiosonde profiles of temperature, pressure, and water vapor were used with the LOWTRAN-7 and MODTRAN atmospheric radiance models to correct measured thermal radiances of water and grassland targets for atmospheric attenuation. Comparison of retrieved surface temperatures from a helicopter-mounted modular multispectral radiometer (MMR) (∼5-m “pixel”), C-130 mounted thematic mapper simulator (TMS) (NS001, ∼20-m pixel), and the Landsat 5 thematic mapper (TM) (120-m pixel) was done. Differences between atmospherically corrected radiative temperatures and near-surface measurements ranged from less than 1°C to more than 8°C. Corrected temperatures from helicopter-MMR and NS001-TMS were in general agreement with near-surface infrared radiative thermometer (IRT) measurements collected from automated meteorological stations, with mean differences of 3.2°C and 1.7°C for grassland targets. Much better agreement (within 1°C) was found between the retrieved aircraft surface temperatures and near-surface measurements acquired with a hand-held mast equipped with a MMR and IRT. The NS001-TMS was also in good agreement with near-surface temperatures acquired over water targets. In contrast, the Landsat 5 TM systematically overestimated surface temperature in all cases. This result has been noted previously but not consistently. On the basis of the results reported here, surface measurements were used to provide a calibration of the TM thermal channel. Further evaluation of the in-flight radiometric calibration of the TM thermal channel is recommended.

Microwave radiometry and remote sensing of the environment

Abstract: Part 1 Radiometric sensing of atmospheric water vapor and cloud liquid water vapour: clear-air observations of water vapour by ground-based microwave radiometers and Raman lidar, Y. Han et al observed and theoretical atmospheric emission at 20, 30 and 90 GHz - recent results from land- and ocean-based locations, J.B. Snider forward modelling for AMSU, P.J. Rayer profiling of atmospheric water vapour with the millimeter-wave imaging radiometer, J.R. Wang et al assimilation of satellite-derived moisture fields for numerical weather prediction, G. Deblonde et al estimation of errors in the two-beamwidth-antenna method for microwave measurement of atmospheric water vapor, S. Hashimoto improved GPS vertical surveying and GPS sensing of atmospheric water vapour, R. Ware et al using microwave radiometry and space geodetic systems for studies of atmospheric water-vapor variations, G. Elgered et al influence of topospheric electrical path delay on interferometric observations in millimeter radioastronomy, L. Olmi. Part 2 Cloud liquid. Part 3 Radiometric sensing of rain models of precipitation. Part 4 Retrieval of rain. Part 5 Radiometric sensing of the Earth's surface snow and ice. Part 6 Ocean surface. Part 7 Crops and forest. Part 8 New radiometric systems - synthetic aperture radiometers. Part 9 MIMR. Part 10 AMSU. Part 11 Ground-based. (Part contents)

The solar UV radiation environment: measurement techniques and results

Abstract: The Australian Radiation Laboratory (ARL) has been involved for many years in the measurement of solar UV radiation (UVR) using spectroradiometers and a network of broad-band detectors at 16 sites in Australian and Antarctica. Measurement locations range from tropical to polar and as a result, there are many difficulties associated with the maintenance and calibration of the network to ensure an accurate and reliable data collection. Calibration procedures for theious detectors involve comparison with simultaneous spectral measurements obtained using a portable spectroradiometer incorporating a double monochromator, calibrated against traceable standard lamps. Detector-datalogger systems are also intercompared at the Yallambie site for a number of months before installaion at another site. As an additional check on the calibrations, computer models of solar UVR at the earth's surface for days with clear sky and known ozone values are compared with the UV radiometer measurements. The use of chemical, biological and solid state persnal UVR dosimeters is discussed and some measurement results are presented.

Some Aircraft Observations of the Scattering Properties of Ice Crystals

Abstract: Aircraft–borne radiometer measurements made within three cirrus cloud layers are presented together with associated in situ cloud microphysical data. The angular variation of the observed shortwave radiances is simulated using a Monte Carlo multiple-scattering model, making use of a number of scattering phase functions, both theoretical and experimental. It is found that, in general, the laboratory-measured phase functions yield the closest agreement with the aircraft radiances.

Aerosol effects and corrections in the Halogen Occultation Experiment

Abstract: The eruptions of Mt. Pinatubo in June 1991 increased stratospheric aerosol loading by a factor of 30, affecting chemistry, radiative transfer, and remote measurements of the stratosphere. The Halogen Occultation Experiment (HALOE) instrument on board Upper Atmosphere Research Satellite (UARS) makes measurements globally for inferring profiles of NO2, H2O, O3, HF, HCl, CH4, NO, and temperature in addition to aerosol extinction at five wavelengths. Understanding and removing the aerosol extinction is essential for obtaining accurate retrievals from the radiometer channels of NO2, H2O and O3 in the lower stratosphere since these measurements are severely affected by contaminant aerosol absorption. If ignored, aerosol absorption in the radiometer measurements is interpreted as additional absorption by the target gas, resulting in anomalously large mixing ratios. To correct the radiometer measurements for aerosol effects, a retrieved aerosol extinction profile is extrapolated to the radiometer wavelengths and then included as continuum attenuation. The sensitivity of the extrapolation to size distribution and composition is small for certain wavelength combinations, reducing the correction uncertainty. The aerosol corrections extend the usable range of profiles retrieved from the radiometer channels to the tropopause with results that agree well with correlative measurements. In situations of heavy aerosol loading, errors due to aerosol in the retrieved mixing ratios are reduced to values of about 15, 25, and 60% in H2O, O3, and NO2, respectively, levels that are much less than the correction magnitude.

Observation of shoemaker-levy impacts by the galileo photopolarimeter radiometer.

Abstract: The Galileo Photopolarimeter Radiometer experiment made direct photometric observations at 678 and 945 nanometers of several comet Shoemaker-Levy 9 fragments impacting with Jupiter. Initial flashes occurred at (fragment G) 18 July 1994 07:33:32, (H) 18 July 19:31:58, (L) 19 July 22:16:48, and (Q1) 20 July 20:13:52 [equivalent universal time coordinated (UTC) observed at Earth], with relative peak 945-nanometer brightnesses of 0.87, 0.67, 1.00, and 0.42, respectively. The light curves show a 2-second rise to maximum, a 10-second plateau, and an accelerating falloff. The Q1 event, observed at both wavelengths, yielded a color temperature of more than 10,000 kelvin at its peak.

UV-B radiometry and dosimetry for solar measurements.

Abstract: Biologically effective irradiance or dose of solar UV radiation was determined using a spectroradiometer, two broadband radiometers and two types of passive UV-dosimeters. The absolute erythema irradiance and the actinic irradiance were calculated from the solar spectrum measured with the spectroradiometer. It was demonstrated that the erythema irradiance is proportional to the actinic irradiance of solar radiation. The erythema irradiance was also determined using the two broadband radiometers which utilize a filter transmitting erythema spectra. Personal UV-dosimeters such as polysulphone and CR-39 were used to determine the erythema dose for a selected period of time. These results were used to estimate the accuracy of the broadband radiometers and UV-dosimeters. It was found that the results obtained from the broadband radiometers deviate from the absolute erythema irradiance by less than 20% during clear days between the hours of 11:00 and 13:00 Eastern Standard Time (EST) in Australia. The assessment of the erythema dose using passive dosimeters such as polysulphone and CR-39 could introduce an error as high as 40% if the calibration was not performed before undertaking experimental measurements.

Producing Mega-pixel CMB Maps from Differential Radiometer Data

Abstract: A major goal of cosmology is to obtain sensitive, high resolution maps of the Cosmic Microwave Background (CMB) anisotropy. Such maps, as would be produced by the recently proposed Microwave Anisotropy Probe (MAP), will contain a wealth of primary information about conditions in the early universe. To mitigate systematic effects when observing the microwave background, it is desirable for the raw data to be collected in differential form: as a set of temperature differences between points in the sky. However, the production of large (mega-pixel) maps from a set of temperature differences is a potentially severe computational challenge. We present a new technique for producing maps from differential radiometer data that has a computational cost that grows in the slowest possible way with increasing angular resolution and number of map pixels. The required central processor (CPU) time is proportional to the number of differential data points and the required random access memory (RAM) is proportional to the number of map pixels. We test our technique, and demonstrate its feasibility, by simulating one year of a space-borne anisotropy mission.

TOPEX/Poseidon Microwave Radiometer (TMR). II. Antenna pattern correction and brightness temperature algorithm

Abstract: For pt.I see ibid., vol.33, no.1, p.125-37 (1995). The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in the paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, the authors perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 K. the authors also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the /spl sim/40 km TMR footprint to estimate path delay in the /spl sim/3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, the authors discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error. >

Monochromator-based cryogenic radiometry at the NRC

Abstract: A facility is being set up at the National Research Council of Canada for the monochromator-based spectral calibration of transfer standard radiometers using a cryogenic absolute radiometer. General design features and target performance of the new apparatus are discussed. The sources of error inherent to detector calibrations performed using monochromator-based sources are analysed in some detail, with special attention devoted to bandwidth errors, polarization effects and the effect of the angular subtense of the beam. Some detector effects, such as spatial uniformity and temperature dependence, are also discussed. A status report is given on the development of the new cryogenic radiometer facility.

Third comparison of the World Radiometric Reference and the SI radiometric scale

Abstract: A few years ago, we compared for the first time the World Radiometric Reference, which is used as reference for meteorological radiometry, and the Systeme International (SI) radiometric scale realized at the National Physical Laboratory with cryogenic radiometers. We present an improved comparison which used as transfer standard a trap detector calibrated to the NPL primary standard cryogenic radiometer. As a realization of the World Radiometric Reference, we used an electrical substitution radiometer traceable to the WRR and used in solar radiometry. The method of transfer has been improved and the results confirm those of the first comparison but with much lower uncertainty.

Accurate calibration of filter radiometers against a cryogenic radiometer using a trap detector

Abstract: At the Physikalisch-Technische Bundesanstalt filter radiometers are used for the comparison of three primary radiometric standards, an electron storage ring, a black body of known temperature and a cryogenic radiometer. The calibration of the filter radiometers is performed in a two-step process. First, a trap detector is calibrated at the cryogenic radiometer at discrete laser lines. In the second step, this trap detector is used to calibrate the filter radiometers at a spectral comparator. Applying a correction for stray light, the responsivities of the filter radiometers can be measured over a dynamic range of more than eight decades in the wavelength range 420 nm to 1200 nm. To overcome drift effects of the interference filters, the filter radiometers were calibrated every night before and after an application. Following this procedure the responsivity was measured with a relative uncertainty of less than 5 × 10-4 at the 1σ level.

A Comparison of Columnar Water Vapor Retrievals Obtained with Near-IR Solar Radiometer and Microwave Radiometer Measurements

Abstract: A simple two-channel solar radiometer and analysis technique have been developed for setting atmospheric water vapor via differential solar transmission measurements in and adjacent to the 940-nm water vapor absorption band. A prototype solar radiometer developed for the National Oceanic and Atmospheric Administration (NOAA)/Environmental Research Laboratory underwent trial measurements near Boulder, Colorado, and during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment Phase II cirrus intensive field observation program (Coffeyville, Kansas). These measurements provided the convenient opportunity to compare solar radiometer water vapor retrievals with those obtained using NOAA microwave radiometers. The solar radiometer and microwave radiometer retrievals were found to agree to within 0.1 cm most of the time and to within 0.05 cm the majority of the time, yielding a percent difference in the retrievals generally within 10%. Radiosonde soundings, when availab...

Preflight calibration of the POLDER instrument

Abstract: POLDER is a radiometer dedicated to the observation of the polarization and directionality of the Earth's reflectances. It images the Earth with a large field of view (115 degrees) in 8 spectral bands from visible to near infrared. The instrument, developed by CNES will be installed on the ADEOS Japanese satellite to be launched in August 1996. This paper deals with the preflight radiometric and geometrical calibrations of the instrument. It presents the radiometric and geometrical models of the instrument, in relation with its physics. The geometrical calibration consists in determining for each spectral band the mathematical model relating each viewing direction within the instrument field of view to a pixel location in the focal plane. The requirement is to determine the absolute location of the image point with 0.05 pixel accuracy. The radiometric model takes into acount the different characteristics of the instrument, including the polarization capability and the bidimensional detector array. The scientific objectives of POLDER lead to severe radiometric calibration requirements (for instance, +/- 1% on relative low spatial frequency calibration or +/- 0.1% on relative high spatial frequency calibration). The dedicated ground equipments are described (integrating spheres, polarizers, reference radiometer, optical directional reference generator...), as well as the procedures and the first results with their related accuracies.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.