Standards institute improves remote-sensing instrument calibration

Accurate ground and space-based measurements of IR and optical radiation originating from the Earth and stars are critical for improving numerical weather prediction, monitoring the rate and magnitude of climate change, and solving fundamental problems in cosmology. Meeting the stringent measurement requirements for these applications requires that IR and optical sensors are accurately calibrated relative to the International System of Units (SI) as maintained and disseminated by national metrology institutes, such as the National Institute of Standards and Technology (NIST).

NIST has developed specialized capabilities for calibrating remote-sensing instruments that measure IR and optical radiation, which are provided by the NIST Facility for Spectral Irradiance and Radiance Calibrations using Uniform Sources (SIRCUS).^1,2 They consist of a suite of frequency-tunable lasers, integrating spheres, and absolute trap detectors (see Figure 1). Laser light is coupled into an integrating sphere through an optical fiber to produce uniform, quasi-Lambertian, and highly radiant monochromatic sources. A trap detector that has been calibrated for absolute irradiance responsivity measures the irradiance (Wm⁻²) of these sources. The radiance (Wm⁻²sr⁻¹) is determined from the aperture diameters of the detector and integrating sphere and the distance between the two apertures.

Figure 1.Schematic of the National Institute of Standards and Technology Facility for Spectral Irradiance and Radiance Calibrations using Uniform Sources consisting of a suite of frequency-tunable lasers, integrating spheres, and absolute trap detectors.

Using these tunable sources of known irradiance and radiance, SIRCUS can accurately calibrate sensors for irradiance or radiance responsivity as a function of wavelength. It can also characterize out-of-band and stray-light contamination for filter radiometers, spectrometers, and spectrographs.³ And, for sensors not easily shipped to NIST, such as satellite sensors and astronomical telescopes, NIST has developed a portable version of SIRCUS called Traveling SIRCUS that can be deployed to a customer's facility for calibrations. SIRCUS calibrations are traceable to NIST primary standards for the watt and the meter with relative expanded uncertainties as low as 0.1% (k = 2).

Two recent collaborations involving NIST scientists demonstrated the utility of SIRCUS for the radiometric calibration of remote-sensing instruments. The first involved a team of scientists from NIST, Ball Aerospace, the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), Raytheon, and Northrop Grumman, who performed the first system-level calibration of an operational satellite sensor. The sensor, known as the Visible Infrared Imager Radiometer Suite (VIIRS),⁴ operates from the IR to the visible and will produce many key environmental data products when it is flown as part of the Joint Polar Satellite System (JPSS), formerly known as the National Polar-orbiting Operational Environmental Satellite System (NPOESS). Previous testing of VIIRS at the component level in 2008 raised concerns about the sensor's ability to meet the stringent accuracy requirements for the 20-year climate data record for ocean color, which is used to assess ocean health and carbon storage. The system-level calibration performed with Traveling SIRCUS confirmed that the radiometer's filters cause small leaks of radiometric flux between sensor bands, a phenomenon known as crosstalk. However, the amount of crosstalk is small enough that VIIRS scientists should be able to use the calibration results to make corrections in measurements made by VIIRS on orbit. Traveling SIRCUS was also employed to study the internal on-orbit calibration system used by VIIRS for the visible through shortwave IR bands. While the data analysis is still in progress, the comparison between measured and predicted signals will provide critical information about the performance of the on-orbit calibration system.

In another recent deployment of Traveling SIRCUS, scientists from NIST, Jet Propulsion Laboratory, and the California Institute of Technology collaborated on the prelaunch calibration of the Cosmic Infrared Background Experiment (CIBER)⁵ sensor that flew in June 2009 and June 2010. CIBER was designed to search for anisotropies in the cosmic IR background by measuring fluctuations at wavelengths and spatial scales where first-light galaxy signals are expected to be detectable and discriminated from foregrounds. SIRCUS calibration provided the most accurate available measurements of the CIBER sensor's absolute spectral response. This is critical because the CIBER experiment's success is strongly dependent on the instruments' quantitative calibration to enable subtraction of unwanted stellar (zodiacal) and solar background radiation.

Both of these successful calibrations illustrate the potential for tunable laser systems such as SIRCUS to improve the calibration and characterization of satellite sensors while reducing testing time and cost. They also offer a paradigm for future laser-based testing of satellite sensors. Additionally, they represent a fruitful collaboration between different government agencies, satellite sensor companies, universities, and data product analysts.