Radiometric Calibration with a Field Spectroradiometer

The NIST (National Institute of Standards and Technology) states in the Guidelines for Radiometric Calibration that state-of-the-art electro-optical sensors designed for today’s space-based applications require thorough, system-level radiometric calibrations to characterize the instrument and to ensure that all mission objectives are met.

Calibration is the means of describing and quantifying the performance of a sensor. Radiometric calibration refers to the calibration of a sensor measuring electromagnetic radiation and atomic particle radiation. To conduct this calibration an observed measurement is compared to a known measurement.

Vicarious radiometric calibration is the process of comparing synchronous in-situ measurements to satellite data to correct sensor drift. This type of calibration is required due to the inaccessibility of the sensor on a satellite. These calibrations require highly sensitive UV-Vis-NIR spectroradiometers for in-situ measurements like those developed and manufactured at Spectral Evolution.

Calibration is important because it allows for correction of data, and for timely and accurate data interpretations. It also allows for the identification of performance limitations and ensures success. Many applications, such as the effects of human activities on surface ecology, vegetation monitoring, climate studies, and ocean research depend on the long-term stability of satellite radiometric calibrations to produce useful estimates of variability. Satellite sensors are calibrated pre-launch in the laboratory using integrating spheres. The radiometric and spectral performance of sensors degrades over time post-launch due to the change in environment, sensor use, and aging.

↑ Graphic representation of one of NASA’s Landsat remote sensing satellites.

Vicarious radiometric calibration (VRC) uses in-situ measurements over pseudo-invariant calibration sites at the time of satellite or airborne overpass. Calibration sites can be natural (desert, ocean) or man-made (target tarps of different reflectance values). The ideal site for VRC is a location where surface reflectance properties are constant and there is minimal atmospheric condition variation. These measurements capture the “true” radiance at the ground that corresponds to a pixel in the image captured by the sensor. This measurement allows you to calculate a correction coefficient for the entire image.

NIST-traceable calibrated field spectroradiometers such as the RS-3500, PSR+, NaturaSpec™, and RS-8800 are excellent for radiometric calibration. Instruments like the Naturaspec provide the high spectral resolution and sensitivity required to calibrate the most advanced satellites. The sensors fast and easy operation allows you to take higher volume of scans during the limited satellite overpass time adding to their utility and the integrity of your calibration.

Sample spectra collected using the Naturaspec with an 8-degree optic lens on black top pavement. The red line represents the solar irradiance measurement. The blue line represents the surface reflectance ratio taken. Similar uniform surfaces are often used for satellite radiometric calibration.

Spectral Evolution’s UV-Vis-NIR field-portable spectroradiometers are rugged and offer the best combination of high resolution and high sensitivity resulting in the most precise spectral data from any field instrument available on the market. The wide variety of accessories & fore optics makes our instruments the most versatile spectroradiometers for all types of measurements, including reflectance, radiance, irradiance, & transmittance.