Polarimetric Synthetic Aperture Radar
Measurement Types
Related resources
Polarimetric Synthetic Aperture Radar
Synthetic Aperture Radar (SAR) is a remote sensing method that produces radar imagery by emitting successive radio frequency (RF) waves and receiving their reflected echoes. SAR polarimetry, known as PolSAR, refers to the measurement and analysis of the polarisation state of these radio waves, which provides additional information on the surface structure, geometry and material properties of the Earth’s surface. PolSAR enables the enhanced classification of terrain types, vegetation, and urban features through the analysis of different polarisation responses. 1) 3) 4) 5) 6)
Polarisation is a property of all electromagnetic plane waves, referring to the alignment and regularity of their electric and magnetic components. Radar emission can be polarised in different directions, for example, single direction, horizontal-horizontal (HH) or vertical-vertical (VV), where the first letter designates the transmit direction and the second the receiving. Dual polarisation systems transmit in one direction, while receiving in both, producing either HH/HV or VV/VH imagery. Quad-pol systems alternate between transmitting H and V waves, and receive both, producing HH, HV, VV, and VH imagery.
The enhanced information provided by PolSAR is possible due to the scattering of radio waves with different surfaces. The four primary scattering effects in PolSAR are:
- Surface scattering occurs on flat surfaces, such as roads, dry soil or calm water, where the radar signal reflects away from the sensor, resulting in strong VV polarisation and low backscatter.
- Volume scattering occurs in vegetation, ice and snow, where the radar signal scatters multiple times inside the medium. This results in high backscatter in cross-polarisation and allows polarimetric SARs to retrieve biomass and canopy structure information.
- Double bounce scattering occurs when a radar signal reflects off of perpendicular surfaces, producing strong backscatter, especially in HH/VV polarisation.
- Helix scattering occurs in complex metallic structures, detectable only by quad-pol systems. 1) 4) 5) 6)
Example Products
Polarimetric Decomposition Products
Polarimetric decomposition products break down SAR backscatter into different scattering mechanisms, with surface, volume and double-bounce scattering being the most common. This data can be visualised as RGB imagery by assigning each scattering component a colour. 2) 3) 4) 7)
Polarimetric decomposition is most effective for urban monitoring through its identification of double-bounce scattering, forestry studies through volume scattering, and water body reflections through surface scattering. The use of these three scattering mechanisms is known as Freeman-Durden Decomposition. 2) 4) 5) 6) 7)
Extending upon the three component model, Yamaguchi decomposition, known as the four component model, introduces helix scattering to the other scattering components. This allows better discrimination of human-made metallic objects such as bridges or power lines, although it requires quad-pol SAR data which is not provided by all polarimetric SAR satellites. 6) 7)
Alternatively, Cloude-Pottier decomposition, also known as the entropy-alpha anisotropy method, does not assume specific scattering models, but instead classifies scattering randomness using anisotropy to determine the dominant scattering type for each area of the target. Values near zero entropy indicate simple scattering (e.g. surface scattering), whereas values approaching one signify complex scattering, such as double-bounce and volume scattering. Cloude-Pottier decomposition is highly applicable in soil moisture and land cover classification, although it does not separate backscatter into physical components. 6) 7)
Finally, Pauli decomposition visualises polarimetric differences in RGB format by separating SAR data into HH-VV, HV/VH and HH+VV components. This enables quick and intuitive land cover interpretation but does not physically separate scattering mechanisms, instead highlighting polarimetric differences. 6) 7)
Land Cover Classification Maps
A key Level 3 data product in SAR polarimetry is the creation of land classification maps, where variables are mapped to a uniform space-time grid. These maps utilise decomposition methods to identify scattering patterns to classify land cover types in the target area, supporting applications in land use monitoring, forestry and agriculture. 1) 6) 7)
Forest Biomass and Vegetation Height Maps
SAR Polarimetry can also be used to map vegetation height and density, and is achieved by classifying the observed scattering mechanisms received from known forested areas. Each scattering pattern provides information on different components of the forest area. For example, surface scattering indicates bare soil exposure and ground layer characteristics, volume scattering indicates canopy density and foliage structure, and double bounce scattering indicates the presence of denser biomass, such as tree trunks. 1) 6) 7)
Related Missions
Sentinel-1
Sentinel-1 is a dual-satellite constellation consisting of Sentinel-1A (launched April 2014) and Sentinel-1C (launched December 2024). The Sentinel-1 satellites are identical, each carrying a C-Band Synthetic Aperture Radar (C-SAR) instrument with dual-polarisation capability. The mission aims to provide sea and ice monitoring, oil spill and ship surveillance, land surface monitoring, and mapping of forests, waters and soil.
NISAR
The NASA-ISRO Synthetic Aperture Radar (NISAR) mission is a planned cooperative effort between NASA and ISRO. It is a radar imaging satellite carrying both L-band and S-band SARs, each possessing quad-pol capability. The L-band SAR will monitor landscape topography and heavily forested areas, while the S-band SAR will monitor soil moisture and provide basic vegetation mapping.
ROSE-L
The Radar Observing System for Europe in L-band (ROSE-L) is a planned ESA SAR mission. It will feature 12 dual-polarised antenna subarrays, and offer quad-pol SAR imaging capabilities. It aims to collect ground motion and high-resolution soil moisture information on vegetated areas, provide vegetation biomass data, and map land cover.
ALOS, ALOS-2 & ALOS-4
The Advanced Land Observing Satellite series is the Japanese Aerospace Exploration Agency (JAXA) aimed at observing and monitoring disaster-hit areas, forests, sea-ice, and monitoring infrastructure displacement. ALOS, ALOS-2 and ALOS-4 all carry versions of the PALSAR (Phased Array type L-band Synthetic Aperture Radar) instrument. The sensors can operate in single, dual and full (quad) polarisation modes, as well as an experimental compact pol mode included on ALOS-2.
RADARSAT
RADARSAT is the Canadian C-band SAR series, with RADARSAT-1 launching in 1995, followed by RADARSAT-2 in 2007. The RADARSAT Constellation Mission (RCM) is a three-satellite constellation launched in 2019. RADARSAT-1 could operate with single or dual polarisation, while RADARSAT-2 and RCM can operate in single, dual or quad polarisation modes.
RADARSAT Constellation Mission (RCM)
COSMO SkyMed
COSMO SkyMed (Constellation of Small Satellites for Mediterranean basin Observation) is the Italian X-band SAR series launched in 2007, and followed by COSMO SkyMed Second Generation (CSG) in 2019. COSMOS-SkyMed operates in single or dual polarisation modes, while CSG can also operate in quad-pol mode.
RISAT-1
Radar Imaging Satellite-1 (RISAT-1) was the first imaging satellite independently developed by ISRO. Aiming to deploy all-weather, day and night SAR imagery, it carried a single SAR instrument, known as RISAT-SAR. This instrument featured both co- and cross-polarisation, as well as a quad-pol stripmap imaging mode. These features enabled the monitoring of a variety of parameters in fields such as vegetation monitoring, agriculture, forestry, soil moisture mapping and sea ice tracking.
SAOCOM
The Argentine MicroWave Observation Satellite (SAOCOM) is a dual-satellite constellation owned and operated by the Agrentine Space Agency (CONAE). Both satellites carry an identical L-band polarimetric SAR, which aims to provide all-weather SAR imagery, aid studies in agriculture, forestry, fishery, weather and land use, and to develop soil moisture maps. SAOCOM-SAR provides quad-pol imagery across all modes.
References
1) Basics of SAR Polarimetry, 10 June 2024, URL: https://natural-resources.canada.ca/maps-tools-and-publications/satellite-imagery-elevation-data-and-air-photos/educational-resources/tutorial-radar-polarimetry/basics-sar-polarimetry/9583
2) ESA, and Skywatch. “Sentinel-1 Toolbox.” March 2015, URL: https://step.esa.int/docs/tutorials/S1TBX%20Polarimetry%20Tutorial.pdf
3) Ghosh, Sujata, and Nidhi Chaubey. Analysis of Different SAR Decomposition Methods using EOS-4 Polarimetric Data for Urban and Natural Features. Advanced Data Processing Research Institute, 6 November 2023, URL: https://incaindia.org/images/uploads/Paper%20-%2042.pdf
4) Natural Resources Canada, and Carleton University. “SAR Polarimetry.” URL: https://dges.carleton.ca/courses/IntroSAR/Winter2019/SECTION%203%20-%20Carleton%20SAR%20Training%20-%20SAR%20Polarimetry%20%20-%20Final.pdf
5) “Polarimetry | Get to Know SAR – NASA-ISRO SAR Mission (NISAR).” Nisar, URL: https://nisar.jpl.nasa.gov/mission/get-to-know-sar/polarimetry/
6) Pottier, Eric. “Introduction to SAR Polarimetry.” Hungarian Space Office, 4 September 2017, URL: https://eo4society.esa.int/wp-content/uploads/2021/04/2017Land_D2T2-P_Pottier_Polarimetry.pdf
7) Pottier, Eric. “SAR POLARIMETRY Basics Concepts, Advanced Concepts and Applications.” European Space Agency, 21 January 2013, URL: https://earth.esa.int/eogateway/documents/20142/37627/PolSAR-2013-Basic-Concepts.pdf