An Airborne imaging Multispectral Polarimeter (AROSS-MSP)

Transport of sediment and organisms in rivers, estuaries and the near-shore ocean is dependent on the dynamics of waves, tides, turbulence, and the currents associated with these interacting bodies of water. We present measurements of waves, currents and turbulence from color and polarization remote sensing in these regions using our Airborne Remote Optical Spotlight System-Multispectral Polarimeter (AROSS-MSP). AROSS-MSP is a 12-channel sensor system that measures 4 color bands (RGB-NTR) and 3 polarization states for the full linear polarization response of the imaged scene. Color and polarimetry, from airborne remotely-sensed time-series imagery, provide unique information for retrieving dynamic environmental parameters relating to sediment transport processes over a larger area than is possible with typical in situ measurements. Typical image footprints provide area coverage on the water surface on the order of 2 square kilometers with 2 m ground sample distance. A significant first step, in advanced sensing systems supporting a wide range of missions for organic UAVs, has been made by the successful development of the Airborne Remote Optical Spotlight System (AROSS) family of sensors. These sensors, in combination with advanced algorithms developed in the Littoral Remote Sensing (LRS) and Tactical Littoral Sensing (TLS) Programs, have exhibited a wide range of important environmental assessment products. An important and unique aspect of this combination of hardware and software has been the collection and processing of time-series imaging data from militarily-relevant standoff ranges that enable characterization of riverine, estuarine and nearshore ocean areas. However, an optimal EO sensor would further split the visible and near-infrared light into its polarimetric components, while simultaneously retaining the spectral components. AROSS-MSP represents the third generation of sophistication in the AROSS series, after AROSS-MultiChannel (AROSS-MC) which was developed to collect and combine time-series image data from a 4-camera sensor package. AROSS-MSP extends the use of color or polarization filters on four panchromatic cameras that was provided by AROSS-MC to 12 simultaneous color and polarization data channels. This particular field of optical remote sensing is developing rapidly, and data of this much more general form is expected to enable the development of a number of additional important environmental data products. Important examples that are presently being researched are: minimizing surface reflections to image the sub-surface water column at greater depth, detecting objects in higher environmental clutter, improving ability to image through marine haze and maximizing wave contrast to improve océanographie parameter retrievals such as wave spectra and water depth and currents. These important capabilities can be supported using AROSS-MSP. The AROSS-MSP design approach utilizes a yoke-style positioner, digital framing cameras, and integrated Global Positioning System/Inertial Measurement Unit (GPS/IMU), with a computer-based data acquisition and control system. Attitude and position information are provided by the GPS/IMU, which is mounted on the sensor payload rather than on the airframe. The control system uses this information to calculate the camera pointing direction and maintain the intended geodetic location of the aim point in close proximity to the center of the image while maintaining a standoff range suitable for military applications. To produce high quality images for use in quantitative analysis, robust individual camera and inter-camera calibrations are necessary. AROSS-MSP is optimally focused and imagery is corrected for lens vignetting, non-uniform pixel response, relative radiometry and geometric distortion. The cameras are aligned with each other to sub-pixel accuracy for production of multichannel imagery products and with the IMU for mapping to a geodetic surface. The mapped, corrected image data are analyzed to produce data products, such as, color and polarization imagery, degree and angle of linear polarization imagery and time-series products, such as directional wave spectra, currents and bathymetry.