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.

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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.

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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.

Spolarisation

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Animation showing four different polarization states: horizontal, left-circular, vertical, right-circular. Credit: Davidjessop CC BY-SA 4.0

Polarisationmeaning in Physics Electrostatics

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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.

Whether called a forest fire, bush fire, or wildfire, uncontrolled fires have a devastating impact on communities and natural resources. Fast and effective detection is a key to protecting infrastructure and ensuring communities’ safety. While it seems that fires should be easy to see, there are often obstacles that get in the way. First, smoke obscures what the fire-fighters can see – both on the ground and from the air. Also, aerial imagery can see only the tops of trees and can give a false impression of how large a fire is on the ground.

Polarisationmeaning

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SWIR (Shortwave Infrared) refers to a non-visible range of light falling between 1400-3000 nm in wavelength. SWIR light is same as visible light which reflects from minerals, man-made and natural structures due to the unique nature of the object, the resultant SWIR light shows shadow and contrast in its imagery (contrast depends upon the radiometric resolution of a sensor). Unlike visible light imagery, SWIR light image is not in colour, makes objects easily recognizable and yields one of the tactical advantages of the SWIR, namely, object or individual identification.

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WorldView-3’s spectral bands allow for precise mineral identification and chemical measurements. Energy from light is either transferred to molecules of matter or reflected away from them. Based on the mineral content, different materials absorb specific wavelengths of light and reflect others.

SWIR can be used for heat detection which may results from wildfire, volcanic eruptions, heat produced by factory machinery or anthropogenic activity.

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The SWIR imagery available from WorldView-3 was designed with geological applications in mind. Electronic absorptions in the SWIR wavelengths can be used for detecting materials containing anion groups such as Al-OH, Mg-OH, Fe-OH, Si-OH, carbonates, ammonium, and sulphates – many of which are indicator minerals in the mining industry. Exposed outcrops are manifestations of potential mineral ores or sub-surface deposits. While the geological and mining industries spend millions of dollars to identify potential mining sites during their exploration phase, WorldView-3 SWIR data can cut costs and increase efficiency by narrowing the potential area before field verification is planned.

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.

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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).

Polarisationin physics

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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.

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Identifying man-made materials In SWIR wavelength, man-made materials reflect differently and show unique nature which is not possible in visible light spectrum, helps to distinguish materials that look similar to the naked eye. There are many reasons to understand the different types of man-made materials that are present in a particular area. Other examples of using SWIR to accurately classify man-made materials include an urban planner who needs to know building roof types to plan for fire mitigation, a transportation/logistics manager who needs to know road types to plan for material deliveries to job sites, or an analyst who needs to understand materials being used in new construction to determine the purpose of the construction. SWIR can answer these questions with satellite data that has not been available until now.