Seasonal changes in relative C-band backscatter of northern forest cover types

Reports the results of an experiment performed to investigate the changes of C-band microwave backscatter as a function of season in northern forests. The purpose was to determine whether seasonal changes can be used to increase the information content of single-polarization C-band SAR data. Data were acquired in four consecutive seasons along the same east-west line with a pixel spacing of 3.9 m (azimuth) by 4.7 m (range) with incidence angles ranging from 45° to 75°. Calibration was carried out within each scene, allowing seasonal changes in relative backscatter and absolute dynamic range to be studied. The investigation demonstrated that the entire dynamic range of mean C-HH backscatter values of forest stands was never more than about 6 dB. The range exhibited seasonal variations, from only 3.5 dB in February, to 6.0 dB in May. The seasonal changes in dynamic range of the nondeciduous softwoods are hypothesized to be dominated by changes in the dielectric constant of the woody and foliar parts of the trees. Seasonal changes of deciduous backscatter relative to the softwoods allows multitemporal SAR data to be used to distinguish between hardwood and softwood species