We outfitted a polarimetric microgrid sensor with standard c-mounted lenses and observed inconsistencies in the linear polarimetric response across the focal plane, particularly in the corners. We provide details of our polarimetric image calibration process to mitigate these inconsistencies and display processed results. Standard dark correction and gain calibration improved intensity uniformity, but polarized Mueller calibration was needed to correct variability in polarimetric response at the pixel level. We improved our Mueller calibration process through the addition of an Arduino-controlled rotating analyzer to capture uniform polarimetric scenes. This made the Mueller data collection process significantly easier, more repeatable, and resulted in improved polarimetric Stokes products. Details of the device and examples of how it improved results are shared.
Oil thickness is a crucial parameter in the characterization of oil spills for environmental impact. The feasibility of using active microwave sensors to measure thickness was addressed in a series of microwave scatterometer experiments performed by Simrad Marine A/S in a wave tank at the Nansen Environmental Remote Sensing Center. The thickness of the oil layer was maintained at levels similar to the thick part of an oil spill (0.1 - 1 mm). The measurements showed the capability of active microwave sensors to measure oil spill thickness when the oil type is known. In addition to thickness characterization, the experiment studied the effects of oil viscosity, incidence angle, wind speed, wind angle, microwave frequency, and polarization. The backscatter contrast was observed to be greater for lower incidence angles which indicates that the ERS-1 viewing geometry is optimum for the detection and measurement of thick oil slicks. A thickness-dependent backscatter model was developed which included the effects of oil viscosity, composite surface effects, and oil-water reflectivities. The model viscous effects saturated when the oil thickness was greater than the viscous boundary layer thickness. This explained the observed C-VV backscatter contrast saturation for low viscosity diesel oil at thicknesses greater than 0.15 mm. The model predicted contrast saturation at greater thicknesses for the higher viscosity oils. The data showed this trend but the measurements did not extend to thicknesses which tested the model completely.
The Environmental Research Institute of Michigan (ERIM) has developed a unique ground- based, portable, synthetic aperture radar (SAR). This SAR images targets in their natural backgrounds without the expense of an airborne sensor and with higher performance (bandwidth, resolution) than existing airborne systems. A horizontal 36-foot long aluminum truss supports a rail and an antenna cartridge, which is moved along the rail to allow synthetic aperture focusing. The system is fully-polarimetric and has collected data over the frequency band of 400 - 1300 MHz resulting in a nominal resolution of 0.17 m in range and 0.5 m in cross-range. The low frequency range of the system allows for penetration of soil (to shallow depths) as well as foliage and the system has been used to collect images of buried and foliage- obscured targets. The ground imagery collected to date includes steel oil drums buried at depths of up to one-meter. Both the drums as well as the disturbances due to digging the holes are visible in the imagery. Foliage imagery includes portions of a Lear jet under a mature hardwood forest. Due to the low frequency and wide bandwidth of the sensor (400 - 1300 MHz), obscured objects are clearly visible in the SAR imagery. Other responses in the foliage imagery are due to the dihedral-like ground-trunk reflections.
The present bistatic radar system is polarimetric and operates coherently over the 30-1300 MHz band; it is able to scan foliage in angle to ascertain spatial variations in the transmission properties of the foliage that can degrade the ability of the radar to focus a foliage-obscured object. The system is ground-based, with one antenna attached to a carriage that can move 10 m horizontally along a rigid track and a second, fixed-tripod antenna. Angular and frequency-correlation measurements are presented and discussed.
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