Search

In June 2007, a spherical integrating source was calibrated in the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center's (GSFC) Calibration Facility as part of the prelaunch characterization program for the NPOESS Preparatory Program (NPP) Ozone Mapping and Profiler Suite (OMPS) instrument. Before shipment to the instrument vendor, the sphere radiance was measured at the Remote Sensing Laboratory at the National Institute of Standards (NIST) and then returned to the NASA Goddard facility for a second calibration. For the NASA GSFC calibration, the reference was a set of quartz halogen lamps procured from NIST. For the measurement in the Remote Sensing Laboratory, the reference was an integrating sphere that was directly calibrated at NIST's Facility for Spectroradiometric Calibrations (FASCAL). For radiances in the visible and near-infrared (400 nm to 1000 nm), the agreement between the NASA GSFC calibration and the validation measurements at the Remote Sensing Laboratory was at the 1 % level. For radiances in the near ultraviolet (250 nm to 400 nm), the agreement was at the 3 % level.

For the past decade, the Marine Optical Buoy (MOBY), a radiometric buoy stationed in the waters off Lanai, Hawaii, has been the primary in-water oceanic observatory for the vicarious calibration of U. S. satellite ocean color sensors, including the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometers (MODIS) instruments on the National Aeronautics and Space Administration's (NASA's) Terra and Aqua satellites. The MOBY vicarious calibration of these sensors supports international effort to develop a global, multi-year time series of consistently calibrated ocean color data products. A critical component of the MOBY program is establishing radiometric traceability to the International System of Units (SI) through standards provided by the U. S. National Institute of Standards and Technology (NIST). A detailed uncertainty budget is a core component of traceable metrology. We present the MOBY uncertainty budget for up-welling radiance and discuss additional considerations related to the water-leaving radiance uncertainty budget. Finally, we discuss approaches in new instrumentation to reduce the uncertainties in in situ water-leaving radiance measurements.

Absolute stellar photometry is based on 1970s terrestrial measurements of the star Vega calibrated by using the Planck radiance from a Cu fixed-point blackbody. Significant advances in absolute radiometry have been made in the last 30 years that offer the potential to improve both terrestrial and space-based absolute stellar photometry. These advances include new high-temperature blackbody standards, absolute cryogenic radiometry, solid-state optical radiation sources, improved atmospheric transmittance modeling, and laser-based radiometric calibration. We describe the possible use of these new technologies for ground-based calibration of standard stars and their impact on stellar photometry, including present efforts to achieve highly accurate measurements from the ultraviolet to the near infrared for cosmological applications.

Determination of the water-leaving spectral radiance using in-water instrumentation requires measurements of the upwelling spectral radiance (Lu) at several depths. If these measurements are separated in time, changes in the measurement conditions result in increased variance in the results. A prototype simultaneous multi-track system was developed to assess the potential reduction in the Type A uncertainty in single set, normalized water-leaving radiance achievable if the data were acquired simultaneously. The prototype system employed a spectrograph and multi-track fiber-coupled CCD-detector; in situ in-water tests were performed with the prototype system fiber-coupled to a small buoy. The experiments demonstrate the utility of multi-channel simultaneous data acquisition for in-water measurement applications. An example of the potential impact for tracking abrupt responsivity changes in satellite ocean color sensors using these types of instruments as well as for the satellite vicarious calibration is given.

The water-leaving spectral radiance is a basic ocean color remote sensing parameters required for the vicarious calibration. Determination of water-leaving spectral radiance using in-water radiometry requires measurements of the upwelling spectral radiance at several depths. The Marine Optical System (MOS) Remotely Operated Vehicle (ROV) is a portable, fiber-coupled, high-resolution spectroradiometer system with spectral coverage from 340 nm to 960 nm. MOS was developed at the same time as the Marine Optical Buoy (MOBY) spectrometer system and is optically identical except that it is configured as a profiling instrument. Concerns with instrument self-shadowing because of the large exterior dimensions of the MOS underwater housing led to adapting MOS and ROV technology. This system provides for measurement of the near-surface upwelled spectral radiance while minimizing the effects of shadowing. A major advantage of this configuration is that the ROV provides the capability to acquire measurements 5 cm to 10 cm below the water surface and is capable of very accurate depth control (1 cm) allowing for high vertical resolution observations within the very near-surface. We describe the integrated system and its characterization and calibration. Initial measurements and results from observations of coral reefs in Kaneohe Bay, Oahu, extremely turbid waters in the Chesapeake Bay, Maryland, and in Case 1 waters off Southern Oahu, Hawaii are presented.

As part of an effort to reduce uncertainties in the radiometric calibrations of integrating sphere sources and standard lamp irradiance sources, the Goddard Space Flight Center (GSFC) Radiometric Calibration Facility's (RCF) primary radiometer was characterized at the NIST facility for Spectral Irradiance and Radiance Calibrations with Uniform Sources (SIRCUS). Based on those measurements, a nominal slit scattering function was developed for the radiometer. This allowed calculations of band averaged spectral radiances and irradiances for the radiometer's measurements of sphere and standard lamp sources, respectively. From these calculations the effects of bandwidth and spectral stray light were isolated for measurements in the blue spectral region. These effects, which depend on the spectral distribution of the source being measured, can be as large as 8% for measurements at 400 nm. The characterization results and a correction algorithm for these effects are presented here.

Organic materials in the oceans have spectral signatures based on their light-scattering properties. These optical properties are related to bio-physical and bio-chemical data products such as the concentration of phytoplankton chlorophyll-α through bio-optical algorithms. A primary quantity of interest in ocean color research is the water-leaving spectral radiance L w (λ), often normalized by the incident solar flux. For quantitative studies of the ocean, derivation of the relationship between the optical properties and physically meaningful data products is critical. There have been a number of recent advances in radiometry at the National Institute of Standards and Technology that directly impact the uncertainties achievable in ocean-color research. These advances include a new U.S. national irradiance scale; a new laser-based facility for irradiance and radiance responsivity calibrations; and a novel tunable, solid-state source for calibration and bio-optical algorithm validation. These advances, their relevance to measurements of ocean color, and their effects on radiometrically derived ocean-color data products such as chlorophyll-α are discussed.

Sun photometers are used to characterize the radiative properties of the atmosphere. They measure both the incident solar irradiance as well as the sky radiance (from scattered incident flux). Global networks of sun photometers provide data products such as aerosol optical thickness derived from these measurements. Instruments are typically calibrated for irradiance responsivity by cross-calibration against a primary reference sun photometer and for radiance responsivity using a lamp-illuminated integrating sphere source. A laser-based facility for Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) has been developed at the National Institute of Standards and Technology. Sensors can be calibrated in this facility for absolute spectral irradiance and radiance responsivity with combined expanded ( k = 2) uncertainties ranging from 0.15% to 0.25%. Two multi-channel filter radiometers used in the Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) program of the National Aeronautics and Space Administration (NASA) at the Goddard Space Flight Center (GSFC) were calibrated for radiance and irradiance responsivity using conventional approaches and using laser-illuminated integrating spheres on SIRCUS. The different calibration methods are compared, the uncertainties are evaluated, and the impact on remote sensing applications is discussed.

We provide an historical overview of NIST research and development in radiometry for space-based remote sensing. The applications in this field can be generally divided into two areas: environmental and defense. In the environmental remote sensing area, NIST has had programs with agencies such as the National Aeronautical and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) to verify and improve traceability of the radiometric calibration of sensors that fly on board Earth-observing satellites. These produce data used in climate models and weather prediction. Over the years, the scope of activities has expanded from existing routine calibration services for artifacts such as lamps, diffusers, and filters, to development and off-site deployment of portable radiometers for radiance- and irradiance-scale intercomparisons. In the defense remote sensing area, NIST has had programs with agencies such as the Department of Defense (DOD) for support of calibration of small, low-level infrared sources in a low infrared background. These are used by the aerospace industry to simulate ballistic missiles in a cold space background. Activities have evolved from calibration of point-source cryogenic blackbodies at NIST to measurement of irradiance in off-site calibration chambers by a portable vacuum/cryogenic radiometer. Both areas of application required measurements on the cutting edge of what was technically feasible, thus compelling NIST to develop a state-of-the-art radiometric measurement infrastructure to meet the needs. This infrastructure has led to improved dissemination of the NIST spectroradiometric quantities.

Development of a new spectral irradiance scale realization at the National Institute of Standards and Technology (NIST) requires a careful and complete calibration of a high- temperature blackbody between 1200 K and 2800 K. Filter radiometers have been calibrated to measure the spectral radiance (or radiance temperature) of the blackbody. Using a monochromator, the blackbody spectral radiance will then be used to determine the spectral irradiance distributions of primary and secondary spectral irradiance lamp standards at NIST. In this paper, a calibrated pyrometer and V((lambda) ) filter radiometer will be used to evaluate the high-temperature blackbody for determination of the optimum calibration method for the blackbody. The blackbody apparatus will be described in detail. When the power supply is the sole source of current control, the blackbody current is stable to within 0.056%, resulting in 0.72% for the uncertainty in the blackbody spectral radiance. To achieve our goal of 0.1% in the final blackbody radiance, a blackbody current stability of 0.007% is required. Due to the day-to-day variations in the current, calibrations of the blackbody msut be made frequently. Several feedback control options are recommended as possible solutions for improving both the short term and long term current stability.

This paper reviews the research and design of high temperature blackbody sources for the temperature interval from 2000 K to 3000 K. Sources with large apertures are addressed specifically, as these are well suited to the important problem of spectral irradiance scale realizations.