Sentinel-1 can collect several different images from the same series of pulses by using its antenna to receive specific polarisations simultaneously. Sentinel-1 is a phase-preserving dual polarisation SAR system. It can transmit a signal in either horizontal (H) or vertical (V) polarisation, and then receive in both H and V polarisations. Dual polarisation Level-1 Single Look Complex (SLC) products contain complex values. In addition to the backscatter intensity that can be measured from each single polarisation, the inter-channel phase information allows to perform enhanced analysis of backscattering properties.
Targets on the ground have distinctive polarisation signatures reflecting different polarisations with different intensities and converting one polarisation into another. For example, volume scatterers (e.g. forest canopy) have different polarisation properties than surface scatterers (e.g. sea surface). Polarimetric techniques such as polarimetric target decomposition (eigenvector-based, model-based, ...) allow the separation of different scattering contributions and can be used to extract information about the scattering process. Dedicated dual-Pol SAR classification techniques allow improved classification of point targets and distributed target areas.
The following RADARSAT-2 image of Flevoland in the Netherlands shows the difference in intensities from VH (left) and VV (centre). The composite RGB (colour) image on the right was created using the VV channel for red, VH channel for green and the ratio |VV| / |VH| for blue.
Figure 1: Intensities from different polarimetric channels (image courtesy of MDA and CSA)
Figure 2: Image acquired by Sentinel-1A on 2017/11/01 over Brittany region, France, with VV intensity image, VH intensity image, and RGB color composite. Copernicus Sentinel data (2017) processed by CLS.
Applications of polarimetry include:
- agriculture: crop type identification, crop condition monitoring, soil moisture measurement
- forestry: biomass estimation, species identification and fire scar mapping
- hydrology: monitoring wetlands and snow cover
- oceanography: sea ice identification, coastal wind field measurement, oil spill detection
- security: ship detection and classification
With Sentinel-1, dual polarisation acquisitions are potentially collected over land worldwide as well as over priority coastal zones. The Sentinel-1 Observation Scenario is made available by ESA Here. This scenario provides maps of revisit and coverage on each acquisition mode, including the dual polarisation ones. The availability of dual polarisation Sentinel-1 data makes it possible to setup global and routine applications using polarimetric analysis.
For an introduction to polarimetric concepts, please see the Radar Polarimetry chapter of the Fundamentals of Remote Sensing tutorial from the Canadian Centre for Remote Sensing (CCRS). For an in-depth theory course on polarimetry, please see the ESA POLSARPRO Polarimetry Tutorial.