This study investigates the use of thematic class correspondence in the fusion of hyperspectral data with higher spatial resolution synthetic aperture radar (SAR) data. A thematic map derived from the SAR imagery is used to introduce spatial information into the hyperspectral imagery, a spatial-spectral fusion. Because the underlying physical processes measured by the imaging systems substantially differ, classes derived from one may have partial or no relationship to classes from the other. In our approach, SAR-derived class contributions to a mixed hyperspectral pixel are weighted in the fusion process based on their correspondence with spectral classes. Unconstrained and weighted least squares solutions for the resulting linear system are described. A comparison of fusion results is presented with and without use of thematic content.
Polarimetric synthetic aperture radar (POLSAR) provides additional information about the scatterers and clutter in a scene over that of single-band SAR. A fully polarimetric sensor contains four imaging channels that, when properly calibrated, can indicate the type of scatterers present. For example, it is possible to discriminate between trihedral-, dihehdral-, and dipole-like scatterers. The orientation of the scatterers can also be extracted. Based upon this additional information, hypotheses can be generated about the objects in the scene that are richer than those generated from single-band data. Combinations of transmission and reception with antennae that ideally represent orthogonal, balanced polarimetric states generate the four channels of the POLSAR system. In practice, the antenna elements are not perfect; crosstalk and imbalances exist between them, so that calibration is necessary. This paper addresses the calibration of POLSAR data, and introduces some new approaches to this problem. These include a novel gradient descent algorithm for crosstalk removal and the application of a rotating dihedral to the calibration of a sensor with receiver characteristics that are transmit-state dependent. The sensitivity of the Cloude polarimetric decomposition to varying amounts of crosstalk and imbalance in an imperfectly calibrated data set is also discussed.
The complex phase history of the synthetic aperture radar (SAR) image can be broken up into a series of overlapping crossrange sub-apertures from which time-dependent information can be extracted from the scatterers that make up the image. The change in the image characteristics from one sub-aperture window to the next can be correlated to generate a stability map of the scene; i.e., areas that change rapidly as the sensor viewpoint changes can be identified. These changes in pixel characteristics over the aperture may be due to many sources: scatterers interfering with each other differently as acquisition geometry is swept out, scatterer sources being created and destroyed as the geometry changes, and aspect- dependent scatterers (e.g. dihedrals) being interrogated differently over the width of the aperture. The stability analysis reported earlier for the peak features has been extended to arbitrary pixels in the SAR image, and the application of this analysis to other features of interest can be made. An example using measured MSTAR data of the stability of a characterized scatterer from the Slicey phantom, imaged in a self-obscuring state, is presented in the context of the stability analysis. Further generalizations of the approach to polarimetric SAR are also presented.
We demonstrate how a realistic scene with high spatial and spectral resolution can be synthesized from color aerial photography and AVIRIS hyperspectral imagery. A review of techniques for image fusion is first presented. Image processing techniques were developed to extract a digital elevation model from stereo aerial photography in order to correct temporal shading differences between two image sets and to fuse aerial photography with AVIRIS imagery. A unique contribution is the explicit inclusion of corrections for topography and differing solar angles in the fusion process. Color infrared photography and an AVIRIS image from a site near Cuprite, Nevada, were used as a test of an improved high-frequency modulation (HFM) technique for the creation of hybrid images. Comparison of spectra extracted from the synthesized image with library spectra demonstrated that our methodology successfully preserved the spectral signatures of the ground surface.
Many techniques have been developed and demonstrated to combine low spatial resolution multispectral imagery with relatively high resolution panchromatic imagery to accomplish multispectral 'sharpening'. We investigate here a particular approach, high frequency modulation, and show that care must be exercised in the design of spatial weighting functions in order to optimize the resulting fused image. Mathematical relationships are derived showing that, under reasonable assumptions, the modulation equation leads to a specific and intuitively satisfying form of the weighting function. Examples of sharpened multispectral images are presented demonstrating the predicted improvement when attention is given to the spatial weighting function.
The simulation of imagery that will be produced by a new sensor is an important element in the design process. To properly account for a new sensor's spatial and spectral degradation, it is necessary to supply as input to the simulation a relatively high spatial and spectral representation of the type of scene to be imaged. The performance of the sensor under design is then evaluated by comparison of the simulated degraded image with the input scene and/or by comparison with simulated images from existing sensors. In this paper we demonstrate how realistic scene with high spatial and spectral resolution can be synthesized from color aerial photography and AVIRIS hyperspectral imagery. Image processing techniques for fusion of aerial photography with AVIRIS imagery that take into account the topography of the scene are described. Sample composite images for a site near Cuprite, Nevada, are presented along with evidence that spectral signatures are preserved after applying the synthesis procedure.