Polarimetry
The study and measurement of the polarization state of radar signals. SAR polarimetry (PolSAR) uses different combinations of transmitted and received polarizations (HH, HV, VH, VV) to extract information about surface structure, vegetation type, and soil moisture.
Overview
Polarimetry in radar remote sensing measures how the polarization state of transmitted microwave radiation changes upon interaction with the Earth's surface. By transmitting and receiving radar signals in different polarization orientations — horizontal (H) and vertical (V) — SAR systems can characterize the physical structure and scattering properties of surface features in ways that single-polarization imagery cannot.
How It Works
A polarimetric SAR transmits pulses in horizontal (H) and/or vertical (V) polarization and records the reflected signal in both polarizations, producing up to four channels: HH (transmit H, receive H), VV, HV, and VH. Dual-pol systems (e.g., Sentinel-1: VV+VH) provide two channels, while quad-pol (full polarimetric) systems record all four, enabling complete characterization of scattering behavior.
Decomposition techniques extract physical meaning from polarimetric data. Freeman-Durden decomposes the signal into surface scattering (single bounce from flat surfaces), double-bounce scattering (from vertical structures), and volume scattering (from randomly oriented scatterers like forest canopies). The Cloude-Pottier decomposition uses eigenvalue analysis of the coherency matrix to classify scattering mechanisms without physical assumptions.
Key Facts
- HH, VV, HV, and VH are the four polarization channels — HV means transmit horizontal, receive vertical.
- Cross-polarization channels (HV, VH) are generated primarily by volume scattering in vegetation and are sensitive to biomass.
- Sentinel-1 operates in dual-pol mode (VV+VH), while missions like ALOS-2 PALSAR-2 and RADARSAT-2 offer full quad-pol capability.
- Freeman-Durden decomposition separates surface, double-bounce, and volume scattering contributions.
- The ESA BIOMASS mission (P-band, launched 2024) uses full polarimetry specifically for global forest biomass mapping.
Applications
Crop Classification
Different crop types produce distinct polarimetric signatures based on plant structure, enabling more accurate crop mapping than single-polarization data.
Forest Biomass Estimation
Volume scattering from canopy correlates with biomass, while the HV cross-polarization channel is particularly sensitive to vegetation structure.
Soil Moisture Mapping
Decomposing surface scattering from vegetation scattering enables more accurate soil moisture retrieval under agricultural canopies.
Sea Ice Classification
Different ice types (first-year, multi-year, ridged) produce distinct polarimetric signatures, enabling automated classification for navigation support.
Limitations & Considerations
Quad-pol acquisition halves the swath width compared to single-pol because the system must alternate between transmit polarizations, reducing coverage. Cross-pol channels (HV, VH) have lower signal-to-noise ratio than co-pol channels. Interpretation of polarimetric parameters requires specialized expertise. Decomposition techniques make assumptions about scattering mechanisms that may not hold in all environments. Processing full polarimetric data is computationally intensive and requires careful calibration.
History & Background
Polarimetric SAR research began in the 1980s with airborne systems like NASA/JPL's AIRSAR. The theoretical framework was established by Cloude and Pottier (entropy-alpha decomposition, 1997) and Freeman and Durden (three-component decomposition, 1998). Spaceborne polarimetric data became available with ALOS PALSAR (2006), RADARSAT-2 (2007), and TerraSAR-X. The ESA BIOMASS mission (2024) represents the first dedicated spaceborne polarimetric P-band SAR for global forest biomass estimation.
Related Terms
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