Seasonal comparison of HUTSCAT ranging scatterometer and ERS-1 SAR microwave signatures of boreal forest zone

A set of ERS-1 SAR images along with airborne nonimaging ranging scatterometer (HUTSCAT) measurements and in situ surveys has been obtained from the Sodankylä test site (center latitude = 67.41 N, center longitude = 26.58 E) in Northern Finland. A total of five measurement campaigns were organized during 1991–1993. Nineteen test lines have been selected from the test site to represent different land-use catogories. The landuse in the test area consists of open areas (agricultural fields, bogs, and clear-cut areas) and sparsely forested areas (mires, pine, and mixed forests). Microwave signatures representing the test lines have been extracted from ERS-1 SAR images and HUTSCAT measurements. The behavior of these signatures has been compared with each other and with our boreal forest semiempirical backscattering model. A set of extensive field measurements (snow depth, density, wetness, coverage, and snow water equivalent) made on the test lines are used in the various analyzes. The results indicate that the behavior of ERS-1 SAR microwave signatures is similar to that of HUTSCAT even in the presence of forest canopies. Also, the deviations of microwave signatures for various land-use classes behave similarly. This allows us to use the boreal forest semiempirical backscattering model based on HUTSCAT data to divide the ERS-1 backscattering signal into two contributions: 1) backscattering contribution from the top layer of vegetation canopy and 2) backscattering contribution from the canopy, ground, and ground–canopy reflections. By using the boreal forest semiempirical model, the behavior of these contributions is also observed in various soil conditions (wet ground, frozen ground, dry snow, and wet snow). These results explain some aspects of the boreal forest backscattering mechanism in the presence of snow cover and wet soil, which have not been experimentally investigated before.