Paul R. Spyak

Vicarious Radiometric Calibrations of EOS Sensors

Abstract Four methods for the in-flight radiometric calibration and cross calibration of multispectral imaging sensors are described. Three make use of ground-based reflectance, irradiance, and radiance measurements in conjunction with atmospheric measurements and one compares calibrations between sensors. Error budgets for these methods are presented and their validation is discussed by reference to SPOT and TM results and shown to meet the EOS requirements in the solar-reflective range.

Evaluation of the Applicability of Solar and Lamp Radiometric Calibrations of a Precision Sun Photometer Operating Between 300 and 1025 nm

Over a period of 3 years a precision Sun photometer (SPM) operating between 300 and 1025 nm was calibrated four times at three different high-mountain sites in Switzerland, Germany, and the United States by means of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2varsigma-statistical plus systematic errors) of the calibration constants V(0) (lambda), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the UV-A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared spectral region. For SPM channels within strong water-vapor or ozone absorption bands a modified Langley-plot technique was used to determine V(0) (lambda) with a lower accuracy. Within the same period of time, we calibrated the SPM five times using irradiance standard lamps in the optical labs of the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Switzerland, and of the Remote Sensing Group of the Optical Sciences Center, University of Arizona, Tucson, Arizona. The lab calibration method requires knowledge of the extraterrestrial spectral irradiance. When we refer the standard lamp results to the World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengths. The largest disagreement (4.4%) is found for the channel centered at 610 nm. The results of these intercomparisons change significantly when the lamp results are referred to two different extraterrestrial solar irradiance spectra that have become recently available.

Thermal-infrared field radiometer for vicarious cross-calibration: characterization and comparisons with other field instruments

A four-band (8.2 to 9.2, 10.5 to 11.5, 11.5 to 12.5, and 8 to 14 mm), prototype, thermal-IR radiometer, model CE 312 (CE 312 is the company model number. In previous papers, the CE 312 was called the CLIMAT (conveyable low-noise IR radiometer for measurements of at- mosphere and ground-surface targets)), with a built-in radiance refer- ence is been fabricated by CIMEL Electronique (Paris, France) for use as a field instrument. The instrument is briefly described, laboratory char- acterization is detailed, and its field measurements are compared with those from three other radiometers. The CE 312s main characteristics are linearity of better than 0.8%, field of view of 9.5 deg; noise-equivalent temperature difference of 0.06 to 0.2 K (depending on the band) for brightness temperatures of 0 to 75°C; SNR greater than 1100 for the broadband and greater than 400 for the other bands for brightness tem- peratures between 10 and 80°C; and repeatability of the measured radi- ance smaller than 0.35% after four field campaigns, corresponding to 0.2 K in terms of brightness temperature. Field measurements were con- ducted over different periods during 1996 at Jornada Experimental Range, New Mexico, Lunar Lake and Railroad Valley, Nevada, and Lake Tahoe, California. The CE 312 compares quite favorably with the other instruments: the brightness temperature at two different sites compared to within 0.3 K with two instruments. These measurements show that the CE 312 thermal-IR radiometer is very stable for ambient temperatures varying between 15 and 60°C and that the availability of several filters in the thermal-IR region can help tremendously to improve the accuracy of the radiance determination.

Scatter from particulate-contaminated mirrors. part 1: theory and experiment for polystyrene spheres and λ=0.6328 μm

The scattered light resulting from polystyrene spheres residing on mirrors was measured at λ = 0.6328 μm, and results are compared to that predicted by a modified Mie theory. The method for cleaning the samples, counting and measuring particles, the measurement procedure, and the theoretical model employed to predict the scatter from contaminants on mirrors are discussed. The comparisons between theory and experiment indicate that the theory predicts the forward scatter, but the backscatter predictions are not as successful. The indication is that the developed model can accurately predict the scatter from dust on mirrors.

Radiometric measurement comparisons using transfer radiometers in support of the calibration of NASA's Earth Observing System (EOS) sensors

EOS satellite instruments operating in the visible through the shortwave infrared wavelength regions (from 0.4 micrometer to 2.5 micrometer) are calibrated prior to flight for radiance response using integrating spheres at a number of instrument builder facilities. The traceability of the radiance produced by these spheres with respect to international standards is the responsibility of the instrument builder, and different calibration techniques are employed by those builders. The National Aeronautics and Space Administrations (NASAs) Earth Observing System (EOS) Project Science Office, realizing the importance of preflight calibration and cross-calibration, has sponsored a number of radiometric measurement comparisons, the main purpose of which is to validate the radiometric scale assigned to the integrating spheres by the instrument builders. This paper describes the radiometric measurement comparisons, the use of stable transfer radiometers to perform the measurements, and the measurement approaches and protocols used to validate integrating sphere radiances. Stable transfer radiometers from the National Institute of Standards and Technology, the University of Arizona Optical Sciences Center Remote Sensing Group, NASAs Goddard Space Flight Center, and the National Research Laboratory of Metrology in Japan, have participated in these comparisons. The approaches used in the comparisons include the measurement of multiple integrating sphere lamp levels, repeat measurements of select lamp levels, the use of the stable radiometers as external sphere monitors, and the rapid reporting of measurement results. Results from several comparisons are presented. The absolute radiometric calibration standard uncertainties required by the EOS satellite instruments are typically in the plus or minus 3% to plus or minus 5% range. Preliminary results reported during eleven radiometric measurement comparisons held between February 1995 and May 1998 have shown the radiance of integrating spheres agreed to within plus or minus 2.5% from the average at blue wavelengths and to within plus or minus 1.7% from the average at red and near infrared wavelengths. This level of agreement lends confidence in the use of the transfer radiometers in validating the radiance scales assigned by EOS instrument calibration facilities to their integrating sphere sources.

Scatter from particulate-contaminated mirrors. part 4: properties of scatter from dust for visible to far-infrared wavelengths

It is shown that the far-infrared scatter from smooth mirrors can be dominated by the scatter from just a few very small particles or defects. This emphasizes the necessity for good cleaning techniques and good clean-room procedures. Several effects of this finding are discussed, as well as several other related topics: the cleanliness required for the scatter to be dominated by a mirrors surface microroughness; a proposed specification for low-scatter infrared mirrors; incident angle invariance of clean and contaminated mirrors; the shape ofthe bidirectional reflectance distribution function (BRDF) curves; and the relation between surface cleanliness level, clean-room cleanliness class, and BRDF.

Initial Results of the Bidirectional Reflectance Characterization Round-Robin in Support of EOS

Laboratory measurements of the bidirectional reflectance distribution function (BRDF) of diffusely reflecting samples are required to support calibration in the Earth Observing System (EOS) programme of the National Aeronautics and Space Administration (NASA). To assess the ability of the instrument calibration laboratories to perform accurate BRDF measurements, a round-robin comparison with the National Institute of Standards and Technology (NIST) as the central laboratory was initiated by the EOS Project Science Office. The comparison parameters, which include measurement wavelength, spectral bandwidth, illumination and viewing geometry, sample type and alignment, and data format, were selected in consultation with the participants. The participants were selected based on their roles as metrology laboratories with direct connections to EOS or other international Earth remote-sensing satellite programmes. This paper briefly describes the format of the comparison, the status of the first round, and some preliminary results.

In-flight radiometric calibration of ASTER by reference to well-characterized scenes

ASTER will be calibrated in the laboratory by reference to sources traceable to NRLM and NIST standards and through the use of transfer radiometers. Partial aperture on-board calibration systems will be used in the solar-reflective range and an on-board blackbody source will be used in the infrared. An important independent source of calibration data will be provided through the in-flight radiometric calibration of ASTER by reference to well- characterized scenes. The latter is the subject of this paper. Methods that make use of ground reflectance and radiance measurements made simultaneously with atmospheric measurements at selected sites and used as input to radiative transfer codes are described. The results of error analyses are presented indicating that, depending on the method used, the predicted uncertainties fall between +/- 2.8% and +/- 4.9%, for the solar-reflective range. In the thermal infrared, the goal is an uncertainty of less than 1 K. A method that provides in-flight cross calibrations with other sensors also is described.

Short-wave infrared transfer radiometer for the calibration of the Moderate-Resolution Imaging Spectrometer and the Advanced Spaceborne Thermal Emission and Reflection Radiometer

A short-wave infrared (700-2500-nm) radiometer has been designed and built to calibrate and cross calibrate spherical-integrating sources used in the calibration of satellite sensors residing on NASAs Earth Observing System platforms. We describe the design, predicted and measured performance, and calibration of the transfer radiometer.

A 400 nm to 2500 nm absolute spectral radiance comparison using filter radiometers

An integrating-sphere source developed by MTL Systems, Inc. was calibrated for them by a commercial standards laboratory traceable to the National Institute of Standards and Technology (NIST), resulting in values of spectral radiance in the 400 nm to 2500 nm spectral region. The spectral radiance of the MTL sphere source was then measured using three filter radiometers, also NIST-traceable, that were developed for the Earth Observing System (EOS). In the visible and near-infrared, the values from two Si-photodiode filter radiometers and the supplied calibration values agree within their mutual uncertainties, but in the short-wave infrared, the agreement between the filter-radiometer results and the calibration values is less satisfactory. The reason for this discrepancy is not understood, and additional measurements should be performed.