Shore-based polarized infrared images of ship target and background were obtained in the MAPTIP measurement series, using vertical and horizontal polarization filters. These images have been analyzed to yield degree of polarization and polarization contrast enhancement for targets with sea and sky background Strong vertical polarization is observed in the sea surface emission near the Brewster Angle in the 8 - 12 micrometer (LWIR) band, whereas horizontal polarization due to reflection is usually dominant in the 3 - 5 micrometer (MWIR) band. Ship targets at broadside aspect show a degree of polarization less than 5%. An enhancement of contrast by up to 30% in the LWIR (less in the MWIR) is obtained by polarization suppression of sea background. The contrast enhancement in the MWIR is counteracted by the horizontal polarization of reflected sunlight.
Sea radiance in the mid and far infrared shows a considerable degree of polarization which affects observed target-to-background contrast. An improvement in contrast is achieved with horizontal (s-plane) polarization filtering to suppress sea surface emission. Visibility and range affect the contrast in both polarizations. Scenes recorded during the MAPTIP measurement series off the coast of the Netherlands with the oceanographic ship HrMs Tydeman show decrease in contrast with range and better contrast for horizontal polarization against sea background. A simple mathematical model is presented relating contrast to extinction and path radiance which increase with increasing path length or worsening visibility.
Realistic inversions of lidar signals for extinction using the Klett technique must take into account both the theoretical limitations of the inversion and the experimental constraints of the hardware. A simple test of the Klett inversion algorithm s19 is performed with and without experimental limitations and uncertainties. The effect of limited accuracy in the digitization of the lidar return and limited dynamic range is presented. A simplified technique for detecting clouds in the presence of low visiblity is developed, and some limitations are presented.
In previous papers we have presented measurements of degree of polarization in the mid and far infrared both in near-horizon sun-glint and in the adjacent sea background radiance. The polarization has been related to the Fresnel reflection and emission coefficients and compared with predictions. These measurements have been extended to include target-to-background polarization contrast measurements. Target polarization was small, but sea surface emission showed moderate degree of polarization in the p-plane (vertical). Considerable ship- background contrast improvement was achieved by polarization filtering in the horizontal plane. This effect was greater in the far infrared. Values are given for the degree of polarization of target and background and the contrast improvement factor due to polarization filtering.
In support of a continuing program of evaluation and experimental validation of FLIR Tactical Decision Aid performance codes, a series of measurements has been made of ship radiance temperature distributions together with sea and sky backgrounds. The measurements have been made at ranges from one quarter to one mile off the coastline in Monterey Bay, using a land- mounted Agema 780 dual band Thermovision radiometric sensor, with computer data acquisition and storage. The target ship was the research vessel Point Sur carrying a full suite of meteorological instruments and an array of thermal sensors for ship surface temperature distribution. Rawinsonde balloons were released to obtain vertical temperature and humidity profiles for path correction using LOWTRAN. The normal skin emissivity was measured in a separate experiment. The current data band consists of 898 stored radiometric frames containing ship images including starboard, port, bow and stern aspects, together with sea and sky background frames with varied zenith angle. These files are available for false color display and analysis in a variety of formats.
Commercial frame grabber technology in IBM PC compatible computers has been adapted to allow direct digital input of infrared search and track data at rates of up to 10 mbytes per second, permitting real-time processing and display of false color thermal images. Examples of single frame displays and background suppression by frame subtraction are shown. On- board processed Fourier spectra and Fourier power spectra of selected frame lines are shown. Curve-fitted representations are compared for clear air, cloud and land clutter, and a commercial aircraft at close range.
The performance of the Tactical Environmental Support System FUR tactical decision aid program UFLR was reported upon previously. In using this code it was assumed that the temperature difference between ship and sea is fixed. In actuality the temperature difference changes with sea state, aspect, cloud cover, and visibility conditions. In this article a simple predictive temperature contrast taking these effects into account is described and validated against experimental data. A modified ThA code UFLRB including this input is compared with observed ranges. The modified procedure shows up to 36% improvement in detection range prediction at night.
The NPS-IRST,a modification of the Advanced Demonstration Model of the Navy AN/SAR-8 InfraRed Search and Target Designation system, mounted in a rooftop location at the Naval Postgraduate School, Monterey, has been used to record background scene information. High data-rate tape-recording with reduced-rate playback is used for data processing and a 'framegrabber' board for image display and processing. Irradiance probability density function, pulse width probability density function, and autocorrelation function plots are displayed for five background types selected from false-color image displays.