Polarization refers to the direction of travel of an electromagnetic wave vector’s tip: vertical (up and down), horizontal (left to right), or circular (rotating in a constant plane left or right). The direction of polarization is defined by the orientation of the wave’s electric field, which is always 90°, or perpendicular, to its magnetic field.
A radar antenna can be designed to send and receive electromagnetic waves with a well-defined polarization. By varying the polarization of the transmitted signal and receiving several different polarized images from the same series of pulses, SAR systems can gather detailed information on the polarimetric properties of the observed surface, which can reveal the structure, orientation and environmental conditions of the surface elements. For example, linearly oriented structures such as buildings or ripples in the sand tend to reflect and preserve the coherence (same linear direction) of the polarimetric signal. Randomly oriented structures such as tree leaves scatter and depolarize the signal as it bounces multiple times. Multiple polarizations and wavelength combinations provide different and complementary surface information.
Imaging radars can have different polarization configurations. A single-polarization system, or “single-pol,” transmits and receives a single polarization, typically the same direction, resulting in a horizontal-horizontal (HH) or vertical-vertical (VV) imager. (The first designation is the transmit direction and the second is receive.) A dual-polarization system, or “dual-pol,” might transmit in one polarization but receive in two, resulting in either HH and HV or VH and VV imagery. Dual polarization provides additional detail about surface features through the different and complementary echoes.
A quad-pol system would alternate between transmitting H and V waves and would receive both H and V, resulting in HH, HV, VH and VV imagery. To operate in quad-pol mode, however, the radar must pulse at twice the rate of a single- or dual-pol system since the transmit polarization has to alternate between H and V pulse by pulse. As this type of operation can cause interference between the received echoes, a variant of quad-pol known as quasi-quad-pol can be used. Quasi-quad-pol mode operates two dual-pol modes simultaneously: an HH/HV mode in the lower bounds of the transmit frequency band, and a VH/VV mode in the upper portion. Because the frequencies are different, the two modes don’t interfere with each other, but for this same reason, the observed data are mutually incoherent, or have no phase relationship with each other.
While most spaceborne radar systems are linearly polarized, it is also possible to create a signal that’s circularly polarized on transmission. This is typically done by simultaneously transmitting H and V signals that are phase shifted by 90°. The resulting wave’s electric field vector tip draws a circular path as it rotates between the offset amplitudes. Various combinations of right-circular and left-circular polarization transmit and receive configurations allow synthesizing single-, dual-, and quad-pol mode data from circular-polarized observations. A hybrid version that transmits a circularly polarized wave (R or L) and receives H and V is known as compact-pol. Compact-pol combines the desirable properties of dual-pol, e.g., discrimination between oriented and random surfaces, while better balancing the power between the receive channels.
The complex backscatter data received in the various polarimetric combinations are related to the electrical and geometric properties of the observed surface. For example, the surface roughness and moisture content of soils both contribute to the returned signal, and the ratio of HH to VV is an indicator of moisture content. Bare surfaces have a weak depolarizing effect, while vegetation canopies are highly depolarizing. Applications of polarimetry are widespread and include agriculture, forestry, geology, hydrology, oceanography, coastal zones and disaster response. For NISAR, the quantification of biomass is an important mission objective.
NISAR will provide multiple polarization modes across its two radar bands, a 24 cm wavelength L-SAR and a 10 cm wavelength S-SAR. Dual-pol (HH/HV or VV/VH) is expected to be available for global observations every cycle, with the potential for quad-pol in India and the United States. Over land surfaces, the transmit polarization will principally be horizontal, and receive will be over both vertical and horizontal polarizations. For a limited set of targets, the NISAR mission will make fully polarimetric measurements (quad-pol) by alternating between transmitting H and V and receiving both H and V (HH, HV, VH, VV). In general, NISAR has available single-pol (HH or VV), dual-pol (HH/HV or VV/VH), and compact-pol (RH/RV) on both bands. S-band may provide quasi-quad-pol (HH/HV and VH/VV) and L-band may provide quad-pol (HH/HV/VH/VV).