Field observation of surface conditions using LCTF spectropolarimeter

Abstract

Polarimetric analysis of solar rays reflected from the Earth's surface is expected to play an important role in future Earth environment observation. Research on an imaging spectropolarimeter using a liquid crystal tunable filter (LCTF), which is able to measure the polarimetric properties at selected wavelengths of solar rays reflected from land or water surfaces, has been conducted over the past five years at NAL for such analysis. Efforts are now under way to put this sensor to practical use, for airborne and ultimately space-based Earth environment remote sensing. This paper first presents the principle and construction of an LCTF spectropolarimeter which senses radiation in the 400-720 nm wavelength band. Next, an outline of an onboard observation system that incorporates an LCTF spectropolarimeter and its performance characteristics obtained by laboratory tests are presented. Third, the apparatus and procedures for the field experiment using such observation system are described, and the area for the field experiments is shown. Spectral characteristics of solar rays reflected from the observed spots are then shown by relative radiance as the analyzed results of experimental data and spectral images at various wavelengths and polarization angles are also shown as further analyzed results. It is made clear from the experimental results that solar rays reflected from targets with differing characteristics have different spectropolarimetric properties. Moreover, the result of the flight experiment conducted preliminarily to confirm the operational functions of the observation system in a flight environment is shown. Finally it is concluded that the way has been paved for determining the surface conditions from the properties of the images acquired by the LCTF spectropolarimeter.