The accuracy of spaceborne sensors measuring reflected solar radiance can be affected by multiple factors. First, instruments with complex optics are sensitive to polarization. The response of such instruments is characterized before launch; however, sensitivity to polarization can change on orbit significantly. None of the existing on-orbit sensors has the ability to monitor its sensitivity to polarization on orbit. Another factor is the degradation of optics, particularly in blue wavelength range below 500 nm. Currently, there is no reliable method to access spectral changes in the optics of instruments on orbit. The third factor contributing to changes in on-orbit calibration is the instrument response to stray light. The prior method of correcting radiometric measurement for polarization effects was based on vicarious calibration to the SeaWIFS instrument, which was designed not to be sensitive to polarization.

The Ground-to-Space Laser Calibration (GSLC) System will be capable of calibrating the following: sensitivity to polarization, degradation of optics, and response to stray light of spaceborne reflected solar sensors. The concept is based on using an accurate ground-based laser system pointing at and tracking the instrument on orbit during nighttime and clear atmosphere conditions. The GSLC system will be applicable to instrument calibration in both low-Earth and geostationary Earth orbits (LEO and GEO, respectively).

The calibration is to be performed by the accurate ground-based laser system, which will transmit laser light to the sensor on orbit. The laser signal will be within instrument dynamic range, with well-known wavelength and polarization state (near 100% linear polarization, known polarization direction). The instrument response to the same light intensity with varying polarization constitutes its sensitivity to polarization.

The unique feature of the GSLC system is that it is a space-focused application located on the ground. This makes it easily accessible for maintenance and development. The advantage and uniqueness of the GSFC concept is that all calibration and verification measurements are relative, but they address the most important instrument characteristics on orbit.

This work was done by Constantine Lukashin and Bruce Wielicki of Langley Research Center. LAR-18202-1

NASA Tech Briefs Magazine

This article first appeared in the January, 2015 issue of NASA Tech Briefs Magazine.

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