An imaging system with three-dimensional (3-D) capability can be implemented by using a stepped frequency radar which synthesizes a two-dimensional (2-D) planar aperture. A 3-D image can be formed by coherently integrating the backscatter data over the measured frequency band and the two spatial coordinates of the 2-D synthetic aperture. This paper presents a near-field 3-D synthetic aperture radar (SAR) imaging algorithm. This algorithm is an extension of the 2-D range migration algorithm (RMA). The presented formulation is justified by using the method of the stationary phase (MSP). Implementation aspects including the sampling criteria, resolutions, and computational complexity are assessed. The high computational efficiency and accurate image reconstruction of the algorithm are demonstrated both with numerical simulations and measurements using an outdoor linear SAR system.

3-D radar imaging using range migration techniques

A system of two aspheric lenses is described, which efficiently converts a collimated Gaussian beam to a flattop beam. Departing from earlier designs, both aspheric surfaces were convex, simplifying their fabrication; the output beam was designed with a continuous roll-off, allowing control of the far-field diffraction pattern; and diffraction from the entrance and exit apertures was held to a negligible level. The design principles are discussed in detail, and the performance of the as-built optics is compared quantitatively with the theoretical design. Approximately 78% of the incident power is enclosed in a region with 5% rms power variation. The 8-mm-diameter beam propagates approximately 0.5 m without significant change in the intensity profile; when the beam is expanded to 32 mm in diameter, this range increases to several meters.

Design and performance of a refractive optical system that converts a Gaussian to a flattop beam.

This paper presents a 3-D near-field imaging algorithm that is formulated for 2-D wideband multiple-input-multiple-output (MIMO) imaging array topology. The proposed MIMO range migration technique performs the image reconstruction procedure in the frequency-wavenumber domain. The algorithm is able to completely compensate the curvature of the wavefront in the near-field through a specifically defined interpolation process and provides extremely high computational efficiency by the application of the fast Fourier transform. The implementation aspects of the algorithm and the sampling criteria of a MIMO aperture are discussed. The image reconstruction performance and computational efficiency of the algorithm are demonstrated both with numerical simulations and measurements using 2-D MIMO arrays. Real-time 3-D near-field imaging can be achieved with a real-aperture array by applying the proposed MIMO range migration techniques.

Three-Dimensional Near-Field MIMO Array Imaging Using Range Migration Techniques

We describe the design and performance of an infrared imaging spectrograph that was first used as an airborne sensor in October, 1995. This instrument, called the spatially-enhanced broadband array spectrograph system (SEBASS), is intended to explore the utility of hyperspectral infrared sensors for remotely identifying solids, liquids, gases, and chemical vapors in the 2 to 14 micrometers 'chemical fingerprint' spectral region. The instrument, which is an extension of an existing non-imaging spectrograph uses two spherical-faced prisms to operate simultaneously in the atmospheric transmission windows found between 2.0 and 5.2 micrometers and between 7.8 and 13.4 micrometers (LWIR). ALthough the SEBASS instrument is designed primarily for use from an aircraft platform, it was used in March 1996 for a tower-based collection.

LWIR/MWIR imaging hyperspectral sensor for airborne and ground-based remote sensing

A diode-pumped, solid-state laser (52) of a laser system (50) provides ultraviolet Gaussian output (54) that is converted by a diffractive optical element (90) into shaped output (94) having a uniform irradiance profile. A high percentage of the shaped output (94) is focused through an aperture of a mask (98) to provide imaged shaped output (118). The laser system (50) facilitates a method for increasing the throughput of a via drilling process over that available with an analogous clipped Gaussian laser system. This method is particularly advantageous for drilling blind vias (20b) that have better edge, bottom, and taper qualities than those produced by a clipped Gaussian laser system. An alternative laser system (150) employs a pair of beam diverting galvanometer mirrors (152, 154) that directs the Gaussian output around a shaped imaging system (70) that includes a diffractive optical element (90) and a mask (98). Laser system (150) provides a user with the option of using either a Gaussian output or an imaged shaped output (118).

Beam shaping and projection imaging with solid state uv gaussian beam to form vias

A two-element holographic optical system is described for converting an elliptically shaped Gaussian-profile laser beam into a rectangularly shaped beam that is uniform in amplitude and phase in the nearfield. Theoretical analysis, design considerations, and experimental results are presented for a compact converter system of less than 6 cm in length.

Holographic conversion of a Gaussian beam to a near-field uniform beam

An analytic expression is derived for the angle-dependent polarimetric bidirectional reflectance distribution function (BRDF) of a glossy coating consisting of a Lambertian reflector covered by a specular dielectric layer. This model conforms to all the requirements of a real BRDF and explains the angular dependence of the polarized BRDF measurements of a glossy paint. Two common assumptions about the properties of paint coatings are shown to be incorrect: Multiple scattering by paint pigments will seldom completely depolarize the reflected energy, and, even when it does, the first surface and the volume scattering cannot be completely separated based on polarization alone. Guidelines for using the model to separate first-surface and volume scattering components are proposed.

Polarimetric bidirectional reflectance distribution function of glossy coatings

Infrared multispectral sensors are being investigated as a means for day and night target detection. Infrared multispectral sensors would exploit high spectral band-to-band correlation to reject high background clutter. An infrared Fourier transform spectrometer-based field measurement system was used to collect spectral signature data of targets and desert scrub and sand backgrounds from a 100 foot tower at White Sands Missile Range. The measurements include target-to-background spectral contrast, subpixel targets, background spectral correlation, and background spatial power spectra. The measurements have been analyzed to determine multispectral signal-to-clutter ratios versus target, background, diurnal, and weather variations, background correlation versus temperature clutter variations, and spectral correlation versus spatial scale. These measurements contribute to the expanding target and background infrared hyperspectral signature database. The results of the analysis demonstrate the utility and robustness of infrared multispectral techniques for target detection.

Target detection in desert backgrounds: infrared hyperspectral measurements and analysis

Migration processing exactly accounts for the wavefront curvature over the imaged scene. Migration processing is therefore capable of forming high resolution SAR images when the data is acquired over a large synthetic aperture collection angle. Because migration processing requires phase compensation to a line corresponding to the nominal SAR flight path, the phase history is chirped over a very large bandwidth, requiring a very high sample rate to prevent aliasing in the frequency spectrum. The sample rate is determined by the size of the synthetic aperture collection angle. When migration processing is applied to a spotlight-mode SAR, this sampling rate can be much higher than that required for normal spotlight-mode processing. In this latter case, the phase history is motion compensated to scene center and the sample rate is determined by the spot size. Higher sampling rates result in large FFTs and may cause range ambiguity problems. ERIM has pursued the development of a variation on migration processing, which we call the Frequency domain Replication and Downsampling (FReD) algorithm, which enables the acquisition of data at normal spotlight-mode rates and which does not require the computation of FFTs any larger than those required for normal spotlight-mode processing. The FReD algorithm is based on the fact that when a discrete, aliased spectrum is replicated a sufficient number of times, the resultant spectrum will contain the desired signal spectrum. What we call the FReD algorithm is discussed in two articles by Prati, et al. Subsequent processing steps extract the desired portion of the spectrum to form an image. This paper will review migration processing, discuss the FReD algorithm and present expressions for the number of operations required for its implementation. Migration-processed, spotlight-mode SAR imagery derived from airborne collected data and demonstrating the utility of FReD are presented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Migration processing of spotlight SAR data

A series of infrared hyperspectral field measurements was made at Wright Patterson Air Force Base and the US Army White Sands Missile Range by the Joint Multispectral Program (JMSP) between November 1993 and June 1994 In these experiments, a highly sensitive Fourier transform spectrometer (FTS) was used to collect data from test panels, military and civilian vehicles, and various types of natural backgrounds Post-collection data analyses are being conducted by the JMSP to assess the potential of thermal multispectral processing for detecting and classifying low-contrast ground targets in natural clutter environments One target material of special interest is CARC paint, which is currently applied to US Army vehicles in various colors to create woodland and desert camouflage patterns CARC-painted test panels were observed in a wide variety of backgrounds and weather conditions during all of the JMSP experiments It is shown here that certain fine-scale spectral features of this paint can support reliable two-color discrimination of CARC-coated test panels in different natural backgrounds, even under low contrast and high clutter conditions The paper also examines environmental variations in two key parameters that determine spectral detectability; specifically, the observed target-background spectral contrast signature (which provides the required coloring), and the background spectral correlation (which provides for multiband clutter suppression)

Kenneth K. Ellis

Papers

Holographic conversion of a Gaussian beam to a near-field uniform beam

Polarimetric bidirectional reflectance distribution function of glossy coatings

Target detection in desert backgrounds: infrared hyperspectral measurements and analysis

Migration processing of spotlight SAR data

Analysis of infrared hyperspectral measurements by the joint multispectral program