Showing papers on "Spectroradiometer published in 2010"

An Evaluation of the Use of Atmospheric and BRDF Correction to Standardize Landsat Data

Abstract: Normalizing for atmospheric and land surface bidirectional reflectance distribution function (BRDF) effects is essential in satellite data processing. It is important both for a single scene when the combination of land covers, sun, and view angles create anisotropy and for multiple scenes in which the sun angle changes. As a consequence, it is important for inter-sensor calibration and comparison. Procedures based on physics-based models have been applied successfully with the Moderate Resolution Imaging Spectroradiometer (MODIS) data. For Landsat and other higher resolution data, similar options exist. However, the estimation of BRDF models using internal fitting is not available due to the smaller variation of view and solar angles and infrequent revisits. In this paper, we explore the potential for developing operational procedures to correct Landsat data using coupled physics-based atmospheric and BRDF models. The process was realized using BRDF shape functions derived from MODIS with the MODTRAN 4 radiative transfer model. The atmospheric and BRDF correction algorithm was tested for reflectance factor estimation using Landsat data for two sites with different land covers in Australia. The Landsat reflectance values had a good agreement with ground based spectroradiometer measurements. In addition, overlapping images from adjacent paths in Queensland, Australia, were also used to validate the BRDF correction. The results clearly show that the algorithm can remove most of the BRDF effect without empirical adjustment. The comparison between normalized Landsat and MODIS reflectance factor also shows a good relationship, indicating that cross calibration between the two sensors is achievable.

Field-based spectral reflectance measurements of seasonal snow cover in the Indian Himalaya

Abstract: In the present study, spectroradiometer (350-2500 nm) experiments are carried out in the field to understand the influence of snow grain size, contamination, moisture, ageing, snow depth, slope / aspect on spectral reflectance and to determine the sensitive wavelengths for mapping of snow and estimation of snow characteristics using satellite data. The observations suggest that, due to ageing and grain-size variation, the maximum variations in reflectance are observed in the near-infrared region, i.e. around 1040-1050 nm. For varying contamination and snow depth, the maximum variations are observed in the visible region, i.e. around 470 and 590 nm, respectively. For the moisture changes, the maximum variations are observed around 980 and 1160 nm. Based on the spectral signatures of seasonal snow, the normalized difference snow index (NDSI) is studied, and snow indexes, such as grain and contamination indexes, are proposed. The study also suggests that the NDSI increases with ageing, grain size and moisture content. The NDSI values remain constant with variations in slope and aspect. Attempts are made to estimate seasonal snow characteristics using multispectral Advanced Wide Field Sensor (AWiFS) Indian Remote Sensing (IRS-P6) and Moderate Resolution Imaging Spectroradiometer (MODIS) Terra satellite data and validated with snow-meteorological observatory data of the study area.

Mapping Forest Background Reflectance in a Boreal Region Using Multiangle Compact Airborne Spectrographic Imager Data

Abstract: Forest background, consisting of understory, moss, litter, and soil, contributes significantly to optical remote sensing signals from forests in the boreal region. In this paper, we present results of background reflectance retrieval from multiangle high-resolution Compact Airborne Spectrographic Imager sensor data over a boreal forest area near Sudbury, ON, Canada. Modifications of the background by white and black plastic sheets at two sites provide two extreme limits for the development and testing of an algorithm for retrieving the background information from multiangle data. Measured background reflectances in red and near-infrared bands at six sites in the vicinity of these modified sites are used to validate the algorithm. We also explore the effect of uncertainties in the input forest structural parameters on this retrieval. The results document: 1) capability of the algorithm to retrieve meaningful background reflectance values for various forest stand conditions, particularly in the low to intermediate canopy density range; 2) the effect of background bidirectional reflectance distribution function on retrieved values; 3) performance of the algorithm using data with different cross angle values; and 4) verification of the internal consistency of the geometric-optical 4-Scale model used. The results provide an important platform for the operational estimation of the vegetation background reflectance from the bidirectional reflections observed by the Multiangle Imaging Spectroradiometer instrument.

An improved method for estimating surface fine particle concentrations using seasonally adjusted satellite aerosol optical depth.

Abstract: Using satellite observations of aerosol optical depth (AOD) to estimate surface concentrations of fine particulate matter (PM2.5) is a well-established technique in the air quality community. In this study, the relationships between PM2.5 concentrations measured at five monitor locations in the Baltimore, MD/Washington, DC region and AOD from Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-Angle Imaging Spectroradiometer (MISR), and Geostationary Operational Environmental Satellite (GOES) were calculated for the summer of 2004 and all of 2005. Linear regression methods were used to determine the direct quantitative relationships between the satellite AOD values and PM2.5 concentration measurements. Results show that correlations between AOD and surface PM2.5 concentrations range from 0.46 to 0.84 for the analyzed time period. Correlations with AOD from MODIS and MISR were higher than those from GOES, likely because of variations in the algorithms used by the different instruments. To...

Remote sensing of radiative and microphysical properties of clouds during TC4: Results from MAS, MASTER, MODIS, and MISR

Abstract: The Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) and MODIS/Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Airborne Simulator (MASTER) were used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds at 50 discrete wavelengths between 0.47 and 14.2 microns (12.9 microns for MASTER). These observations were obtained from the NASA ER-2 aircraft as part of the Tropical Composition, Cloud and Climate Coupling (TC4) experiment conducted over Central America and surrounding Pacific and Atlantic Oceans between 17 July and 8 August 2007. Multispectral images in eleven distinct bands were used to derive a confidence in clear sky (or alternatively the probability Of cloud) over land and ocean ecosystems. Based on the results of individual tests run as part of the cloud mask, an algorithm was developed to estimate the phase of the clouds (liquid water, ice, or undetermined phase). The cloud optical thickness and effective radius were derived for both liquid water and ice clouds that were detected during each flight, using a nearly identical algorithm to that implemented operationally to process MODIS Cloud data from the Aqua and Terra satellites (Collection 5). This analysis shows that the cloud mask developed for operational use on MODIS, and tested using MAS and MASTER data in TC(sup 4), is quite capable of distinguishing both liquid water and ice clouds during daytime conditions over both land and ocean. The cloud optical thickness and effective radius retrievals use five distinct bands of the MAS (or MASTER), and these results were compared with nearly simultaneous retrievals of marine liquid water clouds from MODIS on the Terra spacecraft. Finally, this MODIS-based algorithm was adapted to Multiangle Imaging SpectroRadiometer (MISR) data to infer the cloud optical thickness Of liquid water clouds from MISR. Results of this analysis are compared and contrasted.

Validation of JAXA/MODIS sea surface temperature in water around Taiwan using the Terra and Aqua satellites

Abstract: The research vessel-based Conductivity Temperature Depth profiler (CTD) provides underwater measurements of the bulk sea surface temperature (SST) at the depths of shallower than 5 m. The CTD observations of the seas around Taiwan provide useful data for comparison with SST of MODIS (Moderate Resolution Imaging Spectroradiometers) aboard Aqua and Terra satellites archived by JAXA (Japan Aerospace Exploration Agency). We produce a high-resolution (1 km) MODIS SST by using Multi-Channel SST (MCSST) algorithm. There were 1516 cloud-free match-up data pairs of MODIS SST and in situ measurements during the period from 2003 - 2005. The difference of the root mean square error (RMSE) of satellite observations from each platform during the day and at night was: 0.88°C in Aqua daytime, 0.71°C in Aqua nighttime, 0.71°C in Terra daytime, and 0.60°C in Terra nighttime. The total analysis of MODIS-derived SST shows good agreement with a bias of 0.03°C and RMSE of 0.75°C. The analyses indicate that the bias of Aqua daytime was always positive throughout the year and the large RMSE should be attributed to the large positive bias (0.45°C) under diurnal warming. It was also found that the bias of Terra daytime was usually negative with a mean bias of -0.41°C; its large RMSE should be treated with care because of low solar radiation in the morning. Key word: SST, Taiwan, MODIS, Satellite observation

Comparison of MISR and MODIS land surface albedos: Methodology

Abstract: [1] The broadband white sky surface albedo (bihemispherical reflectance) products available from the Moderate Resolution Imaging Spectroradiometer (MODIS) are compared at regional and continental scales with similar products generated from the Multiangle Imaging Spectroradiometer (MISR) land surface bidirectional reflectance factor (BRF) parameters. This paper describes the methodology applied to derive MISR white sky albedos over four spectral broadbands of interest, namely, 0.3–0.7 μm, 0.4–1.1 μm, 0.7–3.0 μm, and 0.3–3.0 μm, as well as an evaluation of the strategy adopted to compare the MODIS and MISR products. The results are very encouraging since the two data sets show very good statistical agreement over large areas and over a full year of measurements, despite the many differences that exist in the suite of algorithms applied to retrieve these surface quantities from each of these instruments separately.

Quality assurance of solar UV irradiance in the Arctic.

Abstract: The first Arctic intercomparison of three solar ultraviolet (UV) spectroradiometers and two multifilter radiometers was held in May and June 2009 at Ny-Alesund, Svalbard, Norway. The transportable reference spectroradiometer QASUME acted as reference instrument for this intercomparison. The measurement period extended over eleven days, comprising clear sky and overcast weather conditions. Due to the high latitude, measurements could be performed throughout the day during this period. The intercomparison demonstrated that the solar UV measurements from all instruments agreed to within ±15% during the whole measurement period, while the spectroradiometer from the Alfred-Wegener Institute agreed to better than ±5%. This intercomparison has demonstrated that solar UV measurements can be performed reliably in the high-latitude Arctic environment with uncertainties comparable to mid-latitude sites.

The effect of the global UV irradiance measurement accuracy on the single scattering albedo retrieval

Abstract: . The possibility of measuring aerosol optical absorption properties in the UV spectral range such as single scattering albedo (SSA), using remote sensing techniques, is currently an open scientific issue. We investigate the limitations on calculating column average SSA using a combination of global UV spectral measurements (that are comon in various UV monitoring stations worldwide) with radiative transfer modeling. To point out the difficulties in such a retrieval we have used the travelling reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) results from 27 visits to UV monitoring stations around Europe. We have used the QASUME instrument as relative reference, analyzing absolute differences and also temporal and spectral deviations of UV irraidances, that are used as basic input for the SSA retrieval. The results comparing the mean SSA derived by all instruments, measuring synchronous UV spectra, showed that 5 were within ± 0.02 difference from the SSA calculated from the QASUME instrument, while 17 were within ± 0.04, for the Solar zenith angle of 60 degrees. As for the uncertainty that has been calculated using the 2σ standard deviation of the spectral measurements, a mean 0.072 and 0.10 (2σ) uncertainties have been calculated for 60° and 30°, respectively. Based on the fact that additional uncertainties would be introduced in the SSA retrieval from AOD model input accuracy, assymetry parameter assumptions, we show that only very few instrumnents could be able to detect long term SSA changes. However, such measurements/results ar useful in order to retrieve SSA at UV wavelengths, a product needed for various applications such as, inputs for modeling radiative forcing studies and satellite retrieval algorithms.

Spectral Solar Radiation Measurements and Models for CPV Module Production Estimation

Abstract: The photocurrent produced by a multijunction III–V cell is limited by the component cell of the union which yields the lowest photocurrent. The induced energy losses are quantified by the Spectral Radiation Matching Factor (SRMF), defined as the ratio between the energy produced by a III–V multijunction cell and the energy that could be produced if the subcells were current matched at any moment. Accurate SRMF estimations generally require accurate measurement of solar spectrum. Another method is to measure the spectrum using three “isotype” cells, or single‐junction cells that have the same spectral response as each of the three component cells in the multi‐junction stack. That method is validated against photocurrents calculated from spectral measurements (by spectroradiometer) and broadband direct beam measurements (by pyrheliometer). Those comparisons show agreement in the results above 98% under clear‐sky conditions. The difference between the two independent SRMF calculations is within 1% margin. The application of this method to the spectral measurements realized in Madrid in 2009 leads to estimate an annual current spectral mismatching of around 7%.

3-mm wave spectroradiometer for studies of atmospheric trace gases

Abstract: We developed a high-sensitivity receiving and measuring facility designed for observations of the ozone O3 and carbon monoxide CO spectral lines by the method of ground-based remote sensing of the atmosphere in the 90–116 GHz frequency range. The measurement and calibration procedure is described in detail. The results of observations of the O3 and CO emission lines at frequencies 110.836 amd 115.271 GHz, respectively, are presented. The sensitivity level of the receiving and measuring facility, which was implemented in the experiment using a filter-bank analyzer and a Fourier spectrum analyzer is determined.

Measurement of the middle-atmosphere temperature profile using a ground-based spectroradiometer facility

Abstract: The spectroradiometer facility for ground-based thermal sensing of the middle atmosphere is developed and manufactured. Observation of the atmospheric self-radiation in the range including two lines (27_ and 29_) of spin-rotational transitions of molecular oxygen is performed. The atmospheric-temperature profiles in the altitude interval 10–55 km are recovered from the observation data.

Lookup table based pixel-by-pixel atmospheric correction method of remote sensing images

Abstract: The invention discloses a lookup table based pixel-by-pixel atmospheric correction method of remote sensing images The method establishes a lookup table of atmospheric correction factors under the conditions of different optical thicknesses of aerosols, solar zenith angles, sensor zenith angles and ground elevations by off-line calculation of a 6S atmospheric radiative transfer model and performs pixel-by-pixel atmospheric correction on MODIS (Moderate-Resolution Imaging Spectroradiometer) remote sensing images on the basis of the lookup table The invention can effectively improve the effect on the atmospheric correction of the MODIS images because of spatial distribution differences of atmospheric conditions, sensor positions and the like

Overview of the iCATSI multi-pixels standoff chemical detection sensor and the MR-i imaging spectroradiometer

Abstract: ABB Bomem is expanding its line of infrared remote sensing products with the addition of a new imaging spectroradiometer. The instrument is modular and support several configurations. One of its configurations is a multipixels sensor optimised for differential acquisition in the VLWIR to support research related to chemical detection. In that configuration, the instrument is equipped with a dual-input telescope to perform optical background subtraction. The resulting signal is the differential between the spectral radiance entering each input port. The other configuration is a general purpose imaging spectroradiometer designed to acquire the spectral signature of rapid events and fast targets in infrared. Overview of the design and results from tests and first field trials will be presented.

Determination of tropospheric aerosol characteristics by spectral measurements of solar radiation using a compact, stand-alone spectroradiometer

Abstract: We developed a method for characterizing atmospheric properties from ground-based, spectral measurements of direct and scattered solar radiation under clear sky conditions. A compact spectroradiometer is employed for radiation measurement in the wavelength range between 350 and 1050 nm with a resolution of 10 nm. Spectral matching of measured and simulated spectra yields a set of optical parameters that describe optical characteristics of tropospheric aerosols. We utilize the radiative transfer code MODTRAN4 for constructing realistic atmospheric models. Details of the system calibration, analysis procedure, and the results of its performance test are described.

Ten years of MISR observations from Terra: Looking back, ahead, and in between

Abstract: The Multi-angle Imaging SpectroRadiometer (MISR) instrument has been collecting global Earth data from NASA's Terra satellite since February 2000. With its nine along-track view angles, four visible/near-infrared spectral bands, intrinsic spatial resolution of 275 m, and stable radiometric and geometric calibration, no instrument that combines MISR's attributes has previously flown in space. The more than 10-year (and counting) MISR data record provides unprecedented opportunities for characterizing long-term trends in aerosol, cloud, and surface properties, and includes 3-D textural information conventionally thought to be accessible only to active sensors.

Spatial Statistical Data Fusion for Remote Sensing Applications

Abstract: Data fusion is the process of combining information from heterogeneous sources into a single composite picture of the relevant process, such that the composite picture is generally more accurate and complete than that derived from any single source alone. Data collection is often incomplete, sparse, and yields incompatible information. Fusion techniques can make optimal use of such data. When investment in data collection is high, fusion gives the best return. Our study uses data from two satellites: (1) Multiangle Imaging SpectroRadiometer (MISR), (2) Moderate Resolution Imaging Spectroradiometer (MODIS).

Results and lessons from a decade of Terra MODIS on-orbit spectral characterization

Abstract: Since its launch in December 1999, the NASA EOS Terra MODIS has successfully operated for more than a decade. MODIS makes observations in 36 spectral bands from visible (VIS) to longwave infrared (LWIR) and at three nadir spatial resolutions: 250m (2 bands), 500m (5 bands), and 1km (29 bands). In addition to its on-board calibrators designed for the radiometric calibration, MODIS was built with a unique device, called the spectro-radiometric calibration assembly (SRCA). It can be configured in three different modes: radiometric, spatial, and spectral. When it is operated in the spectral modes, the SRCA can monitor changes in Sensor spectral performance for the VIS and near-infrared (NIR) spectral bands. For more than 10 years, the SRCA operation has continued to provide valuable information for MODIS on-orbit spectral performance. This paper briefly describes SRCA on-orbit operation and calibration activities; it presents decade-long spectral characterization results for Terra MODIS VIS and NIR spectral bands in terms of chances in their center wavelengths (CW) and bandwidths (BW). It is shown that the SRCA on-orbit wavelength calibration capability remains satisfactory. For most spectral bands, the changes in CW and BW are less than 0.5 and 1 nm, respectively. Results and lessons from Terra MODIS on-orbit spectral characterization have and will continue to benefit its successor, Aqua MODIS, and other future missions.

From Primary Standard to Mobile Measurements - Overview of the Spectral Irradiance Calibration Equipment at PTB

Abstract: The Spectral Irradiance Calibration Equipment (SPICE) at PTB incorporates several radiometric installations. They can be flexibly combined for a large variety of measurement requirements. The instrumentation consists of high-temperature blackbodies as primary radiometric standards, high-quality spectroradiometer systems and a variety of auxiliary measurement equipment. Besides the realization and dissemination of spectral irradiance to various international customers, SPICE enables PTB to investigate new developments in the field of source-based spectroradiometry. Several successfully finished international intercomparisons and cooperations validate the quality and flexibility of SPICE.

Light source evaluation apparatus, light source evaluation system, light source adjustment system, and method for evaluating a light source

Abstract: Provided are a light source evaluation device, a light source adjustment system, a light source evaluation system, and a light source evaluation method whereby it is possible to evaluate the characteristics of a solar simulator, which is a light source for measuring the characteristics of a solar cell, without creating a reference cell or pseudo cell tailored to the spectral sensitivity of a solar cell to be measured. Said evaluation is performed by calculating an evaluation value of the characteristics of the light emitted by a solar simulator in comparison to natural sunlight on the basis of the spectral irradiance of the light emitted by a solar simulator as measured by a spectroradiometer, the spectral irradiance of natural sunlight, and the pre-measured spectral sensitivity of the solar cell to be measured.

Light source evaluation device, light source adjustment system, light source evaluation system, and light source evaluation method

Abstract: Provided are a light source evaluation device, a light source adjustment system, a light source evaluation system, and a light source evaluation method whereby it is possible to evaluate the characteristics of a solar simulator, which is a light source for measuring the characteristics of a solar cell, without creating a reference cell or pseudo cell tailored to the spectral sensitivity of a solar cell to be measured. Said evaluation is performed by calculating an evaluation value of the characteristics of the light emitted by a solar simulator in comparison to natural sunlight on the basis of the spectral irradiance of the light emitted by a solar simulator as measured by a spectroradiometer, the spectral irradiance of natural sunlight, and the pre-measured spectral sensitivity of the solar cell to be measured.

Future VIIRS enhancements for the integrated polar-orbiting environmental satellite system

Abstract: The Visible/Infrared Imager Radiometer Suite (VIIRS) is the next-generation imaging spectroradiometer for the future operational polar-orbiting environmental satellite system. A successful Flight Unit 1 has been delivered and integrated onto the NPP spacecraft. The flexible VIIRS architecture can be adapted and enhanced to respond to a wide range of requirements and to incorporate new technology as it becomes available. This paper reports on recent design studies to evaluate building a MW-VLWIR dispersive hyperspectral module with active cooling into the existing VIIRS architecture. Performance of a two-grating VIIRS hyperspectral module was studied across a broad trade space defined primarily by spatial sampling, spectral range, spectral sampling interval, along-track field of view and integration time. The hyperspectral module studied here provides contiguous coverage across 3.9 - 15.5 μm with a spectral sampling interval of 10 nm or better, thereby extending VIIRS spectral range to the shortwave side of the 15.5 μm CO 2 band and encompassing the 6.7 μm H 2 O band. Spatial sampling occurs at VIIRS I-band (~0.4 km at nadir) spatial resolution with aggregation to M-band (~0.8 km) and larger pixel sizes to improve sensitivity. Radiometric sensitivity (NEdT) at a spatial resolution of ~4 km is ~0.1 K or better for a 250 K scene across a wavelength range of 4.5 μm to 15.5 μm. The large number of high spectral and spatial resolution FOVs in this instrument improves chances for retrievals of information on the physical state and composition of the atmosphere all the way to the surface in cloudy regions relative to current systems. Spectral aggregation of spatial resolution measurements to MODIS and VIIRS multispectral bands would continue legacy measurements with better sensitivity in nearly all bands. Additional work is needed to optimize spatial sampling, spectral range and spectral sampling approaches for the hyperspectral module and to further refine this powerful imager concept.

Characterization of material reflectance variation through measurement and simulation

Abstract: The characterization of material reflectance properties is important in the analysis of hyperspectral and polarization imagery as well as accurate simulation of such images. This paper merges the results of empirical reflectance property (spectral pBRDF) measurements with detailed model based simulations. The empirical data are collected with a laboratory spectroradiometer as well as an RIT-developed spectro-polarimetric imaging goniometer. The modeling uses an adaptation of RIT's Digital Imaging and Remote Sensing Image Generation (DIRSIG) model to capture the radiative transfer in rough surfaces with micron-scale features. Measurements and model results for several man-made materials under various conditions are presented.

A new imaging FTIR spectroradiometer

Abstract: According to the characteristic of single-element detector and non-imaging spectroradiometer, a new imaging FTIR spectroradiometer system was developed for spectral data acquisition This system is composed of a spectroradiometer, a synchronous controller and a scanning device. Using the data interface of spectroradiometer, spectral radiometric calibration can be achieved for the system. The image resolution is 500 x 500 pixels, spectral range is 667-5000 cm(-1), spectral resolution is 1 cm(-1), and space Field of view is 150 degrees, Instant Field of View is 0.3 degrees. Experiments were held for actual data acquisition and data analysis was made. The analysis result indicates that the proposed system is adequate for non-realtime imaging spectral data acquisition

Solar cell evaluation device and light source evaluation device used therefor

Abstract: PROBLEM TO BE SOLVED: To compensate nonlinearity of spectral sensitivity of a solar cell to an irradiation light quantity.SOLUTION: A light source evaluation device 10 performing a light quantity calibration of a solar simulator (illumination light source) 3 in a solar cell evaluation device 1 captures irradiation light of the solar simulator 3 by a spectroradiometer 13 and gains spectral irradiance L(λ) thereof. A spectral sensitivity measurement device 5 gains spectral sensitivity Pi(λ)=P(λ,L(λ)) of a solar cell 2 by changing intensity of white bias light in a plurality of i stages and performing emission line irradiation from a spectral light source each time. A calculation unit 14 calculates an effective value of an illuminance level of illumination light from the solar simulator 3 actually acting on photoelectric conversion, from the spectral irradiance L (λ) and spectral sensitivity Ps(λ) selected for each wavelength λ from the spectral sensitivity Pi(λ) according to the difference between spectral emission illuminance of a reference solar radiation S(λ) and the spectral irradiance L(λ); and performs feedback control of the light quantity of the solar simulator 3.

Complete affordable system for simultaneous VIS/NIR and MWIR/LWIR spectral atmospheric transmittance measurements (ATMS)

Abstract: During the second half of the '70's Ben-Shalom et al.1 developed a system for research of the spectral transmittance of the atmosphere in the infrared range on nearly horizontal atmospheric paths between 2.5 and 14 microns. The system configuration is bi-static: a source (of known emission) and a measuring sensor station linked by line of sight. Comparison of the measured radiation with the known emission gives the fraction of transmitted radiation. The longest path published there was a respectable 44 Km. and provided significant data. However, the system required a very high temperature (2400K) short lived (~3 hours) emitter element, home-built projection optics, and a cumbersome closed cycle water cooling system for the 6 KWatt source. The sensor end of the system was a Circular Variable Filter (CVF) based spectroradiometer using liquid nitrogen (LN2) cooled detectors. The signal processing electronics was based on a synchronous detection method using a chopper at the source and a radio-transmitted reference signal in phase with the chopper. In this paper we describe the Atmospheric Transmittance Measurement System (ATMS) recently built by CI and presently undergoing reliability and accuracy tests. Its main advantages over the old system are: it is built of only commercial off-the-shelf items (COTS), it can measure in both the IR and visible ranges simultaneously, it is cost effective and easy to use and maintain. The calibration method and transmittance measurement algorithm are also described. A transmittance measurement of a nearly horizontal sea level path of 6.5 Km. is shown here as an example for the whole spectral range of 0.4 to 14 microns in less than very clear and dry weather conditions. The result shows agreement with the MODTRAN model on the spectral behavior but at the time of publication we are still investigating the accuracy of the actual transmittance values. We believe that the ATMS can be used for longer paths, based on the signal to noise ratio encountered at 6.5 Km. In addition and in contrast to the old '70's system, which could be used only for measurements above 1 Km. paths, the ATMS is built and calibrated so that it can measure transmittance through short paths of few tens of meters.

Characterization of a double monochromator

Abstract: Optical spectroradiometers used to measure and monitor the radiance output of uniform sources must be thoroughly characterized. The viability of the use of an instrument for such purposes is based upon the establishment of knowledge of its radiometric responsivity characteristics. The NASA Goddard Space Flight Center Radiometric Calibration Laboratory (RCL) has commissioned a new spectroradiometer for use in measurements of irradiance and radiance sources. The spectroradiometer is comprised of a commercial scanning grating, Czerny-Turner double monochromator. This spectroradiometer has been used to make measurements on a number of irradiance and radiance sources over the wavelength range of 300 to 2400 nm. Instrument characterization included determination of stability, functional wavelength calibration and scattered light performance. Comparison measurements were also made with other radiometers. The data gathered from these measurements is presented, analyzed, and discussed.