Millimeter-wave imaging techniques and systems have been developed at the Pacific Northwest National Laboratory (PNNL), Richland, WA, for the detection of concealed weapons and contraband at airports and other secure locations. These techniques were derived from microwave holography techniques that utilize phase and amplitude information recorded over a two-dimensional aperture to reconstruct a focused image of the target. Millimeter-wave imaging is well suited for the detection of concealed weapons or other contraband carried on personnel since millimeter-waves are nonionizing, readily penetrate common clothing material, and are reflected from the human body and any concealed items. In this paper, a wide-bandwidth three-dimensional holographic microwave imaging technique is described. Practical weapon detection systems for airport or other high-throughput applications require high-speed scanning on the order of 3 to 10 s. To achieve this goal, a prototype imaging system utilizing a 27-33 GHz linear sequentially switched array and a high-speed linear scanner has been developed and tested. This system is described in detail along with numerous imaging results.

Three-dimensional millimeter-wave imaging for concealed weapon detection

In synthetic aperture radar (SAR) interferometry, the phase differences between two different sensor positions are used to estimate the terrain topography. Although it is possible in this way to find a three-dimensional (3D) surface representation, the distribution of the different scatterers in the height direction at a fixed range and azimuth position remains unknown. Contrary to this, tomographic techniques enable a real geometric resolution capability in the height direction and introduce new possibilities for many applications and inversion problems. Even misinterpretations in SAR images caused by layover and foreshortening effects can be solved by the tomographic processing. In this paper, the successful experimental realization of polarimetric airborne SAR tomography is demonstrated for the first time. The authors present the concept of aperture synthesis for tomographic imaging for the case of a multibaseline imaging geometry and discuss the constraints arising from the limited number of flight tracks. They propose a method for reduction of the height ambiguities associated to the irregular and undersampled spatial distribution of the imaging positions. Finally, they address the experimental requirements for polarimetric airborne SAR tomography and show experimental results using a multibaseline data set acquired in L-band by DLR's experimental SAR (E-SAR) of a test-site near Oberpfaffenhofen, Germany.

First demonstration of airborne SAR tomography using multibaseline L-band data

In this paper for the first time the successful experimental realisation of airborne SAR tomography is demonstrated. The authors present the concept of aperture synthesis for 3D-imaging for the case of the multibaseline imaging geometry and discuss the limits arising from the limited number of flight tracks and their irregular spatial distribution. Finally they address the experimental requirements for SAR tomography and show some experimental results using a multibaseline data set acquired in L-band by DLR's experimental SAR (E-SAR) on a testsite near Oberpfaffenhofen/Germany.

The authors use range profiles as the feature vectors for data representation, and they establish a decision rule based on the matching scores to identify aerospace objects. Reasons for choosing range profiles as the feature vectors are explained, and criteria for determining aspect increments for building the database are proposed. Typical experimental examples of the matching scores and recognition rates are provided and discussed. The results demonstrated can be used for comparison with other identification methods. >

Using range profiles as feature vectors to identify aerospace objects

This paper provides principles and applications of the sparse microwave imaging theory and technology. Synthetic aperture radar (SAR) is an important method of modern remote sensing. During decades microwave imaging technology has achieved remarkable progress in the system performance of microwave imaging technology, and at the same time encountered increasing complexity in system implementation. The sparse microwave imaging introduces the sparse signal processing theory to radar imaging to obtain new theory, new system and new methodology of microwave imaging. Based on classical SAR imaging model and fundamental theories of sparse signal processing, we can derive the model of sparse microwave imaging, which is a sparse measurement and recovery problem and can be solved with various algorithms. There exist several fundamental points that must be considered in the efforts of applying sparse signal processing to radar imaging, including sparse representation, measurement matrix construction, unambiguity reconstruction and performance evaluation. Based on these considerations, the sparse signal processing could be successfully applied to radar imaging, and achieve benefits in several aspects, including improvement of image quality, reduction of data amount for sparse scene and enhancement of system performance. The sparse signal processing has also been applied in several specific radar imaging applications.

Sparse microwave imaging: Principles and applications

Results of a study demonstrating the feasibility of broadband and speckle-free projective imaging of a complex shaped scattering object in the 6-17 GHz range with centimeter resolution are presented. It is shown how angular, spectral, and polarization diversities can be combined in the data acquisition process with a unique target-derived reference technique to access the three-dimensional Fourier space of the scatterer cost-effectively. Imagea retrieval algorithms, based on the projection slice theorem and knowledge of object symmetry, are utilized in obtaining images of a model aircraft with near optical resolution. The implications for high-resolution three-dimensional tomographic imaging radar networks are discussed.

Prospects for three‐dimensional projective and tomographic imaging radar networks

A procedure for accessing the 3-D Fourier space of a conducting or nondispersively scattering object by angular (aspect) and spectral diversity is described. Cost-effectiiie data acquisition in the microwave regime is achieved by substituting spectral degrees of freedom for the more costly angular degrees of freedom set by the number of observation points defining the imaging aperture. A novel target de/Lived Aeietence (TDR) technique is utilized to generate a synthetic phase reference signal emanating from a point on the target achiev-ing thereby a 3-D teri4te44 Fourciek hotognam re-cording arrangement that has many practical advantages. Application of the ptojection-slice theorem, derivable from the multidimensional Fourier transform, to the accessed Fourier space volume is shown to allow the retrieval of 3-D image detail. This can be achieved either tomogra-phically in slices _(or cross-sectional outlines) or as shown here in the form of a projection image of the object scattering function. Examples of micro-wave data acquisition, data normalization for an undesirable range dependent phase term via the TDR method, and image reconstruction are presented for a complex conducting test object. The results shown herald a new generation of high resolution 3-D imaging radars and can be applied in NDE (non-destructive evaluation), biomedical imaging, and remote sensing applications.

T. H. Chu

Papers

Prospects for three‐dimensional projective and tomographic imaging radar networks

Tomographic And Projective Reconstruction Of 3-D Image Detail In Inverse Scattering