4.1.5 C-band and X-band SAR
SAR systems operating at shorter wavelengths (C-band: 5.6 cm; X-band: 3.1 cm) typically reflect from the surface and top layer of the forest (leaves and twigs) and thus provide information about canopy structure. While the contrast between forest and low vegetation generally is less distinct compared with longer wavelength SAR, the use of two polarisations improves discrimination. X-band SAR data can be acquired at a spatial resolution better than 5 metres, which allows more detailed characterisation of forest canopy structure and although still regarded as research, has potential to provide information about forest degradation (e.g., selective logging (Baldauf, 2013)).
Frequent time series of C-band SAR data has demonstrated capacity for detection of changes in forest cover, and has potential for use for early warning of forest clearing. For forest-related C-band applications, data collected at dual-polarisation (including one cross-polarisation channel) is a critical requirement. To avoid confusion with changes occurring in other land cover classes, change detection can be applied relative to a pre-determined forest area derived e.g. from optical or L-band SAR data.
Sentinel-1A and -1B (successfully launched in 2014 and 2016) are C-band core missions. They will provide intra-annual observations of all global land areas, with potential higher frequency observations over selected countries or regions. Data from the Sentinel 1 missions are being distributed with a free and open data policy, making it an attractive option for monitoring forest activities in the tropics. Data can be accessed through the Copernicus data hub as well as the Alaska SAR Facility .
Amongst non-core missions, a full global coverage of X-band SAR data have been collected by the TanDEM-X satellite constellation.