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Polarimetry

A radar antenna can be designed to transmit and receive electromagnetic waves with a well-defined polarization, which is defined as the orientation of the electric field vector in the plane orthogonal to the wave propagation direction. By varying the polarization of the transmitted signal, SAR systems can provide information on the polarimetric properties of the observed surface. These polarimetric properties are indicative of the structure of the surface elements within a resolution element. Oriented structures such as buildings or naturally aligned features (e.g. sand ripples) respond preferentially to oriented polarizations and tend to preserve polarimetric coherence, whereas randomly oriented structures lead to depolarization of the scattered signals.
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A polarimetric radar can be designed to operate as a single-pol system, where the there is a single polarization transmitted and a single polarization received. A typical single-pol system would transmit horizontally or vertically polarized waveforms and receive the same (giving HH or VV imagery). A dual-pol system might transmit a horizontally or vertically polarized waveform and measure signals in both polarizations in receive (resulting in HH and HV imagery). A quad-pol or full-pol system would alternate between transmitting H-and V-polarized waveforms and receive both H and V (giving HH, HV, VH, VV imagery). To operate in quad-pol mode requires a pulsing of the radar at twice the rate of a single- or dual-pol system since the transmit polarization has to be alternated between H and V in a pulse-by-pulse manner to enable coherent full-polarized data acquisitions. Since this type of operation can cause interference between the received echoes, a variant of quad-pol known as quasi-quad-pol can be used, whereby two dual-pol modes are operated simultaneously: an HH/HV mode is placed in the lower portion of the allowable transmit frequency band and a VH/VV mode is operated in the upper portion. Being disjoint in frequency, the modes do not interfere with each other. However, the observed HH/HV and VH/VV data are mutually incoherent.

While most spaceborne systems are linearly polarized, it is also possible to create a circularly polarized signal on transmit, whereby the tip of the electric field vector is rotating in a circle as it propagates. This is typically implemented by simultaneously transmitting equal amplitude H and V signals that are phase shifted by 90 degrees. Various combinations of right-circular and left-circular polarization configurations on transmit and receive allow synthesizing single-, dual-, and quad-pol mode data from circular-polarized observations.
This is relevant as recent work has emphasized the benefits of hybrid polarization, where a circularly polarized wave is transmitted and H and V signals are received. The dual-pol instance of this mode is known as compact-pol. Compact-pol captures many of the desirable scattering properties of a dual-pol system, e.g. discriminating between oriented and random surfaces, while better balancing the power between the receive channels.

Classical radar polarimetry is trying to relate the complex backscatter observed in various polarimetric combinations to the electrical and geometric properties of the observed surfaces in order to extract meaningful information. Observation-based empirical work, as well as theoretical modeling, helps establish these relationships. For example, over soils, surface roughness and moisture both contribute to the backscattered amplitude, but it can be shown that HH and VV images have similar responses to roughness, such that the ratio HH/VV is primarily an indicator of moisture content. As another example, bare surfaces have a weak depolarizing effect, while vegetation canopies generally are highly depolarizing. So, a joint examination of the dual-pol channels HH and HV can distinguish these surface types.
For this mission, quantifying biomass is an important measurement objective. Empirical relationships have been developed that allow mapping of radar backscatter amplitude to the amount of biomass present in an image resolution cell. The relationship varies with vegetation type and environmental conditions (e.g. soil moisture and roughness), but with multiple polarizations and repeated measurements, the biomass can be determined with sufficient accuracy.