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Real-time, cross-correlating millimetre-wave imaging system

Real-time, cross-correlating millimetre-wave imaging system description/claims

The present invention relates generally to millimetre imaging systems and in particular to a realtime millimetre imaging system for detecting millimetre wave radiation and generating a corresponding image.

Millimetre-wave imaging systems produce a picture of a scene by detecting thermally generated radiation in the 30-300 GHz range, which is emitted or reflected by objects in the field of view of the instrument. Such systems offer advantages over equivalent instruments detecting infrared and visible light, because the millimetre-wave radiation can penetrate low visibility and obscuring conditions (e.g., caused by clothing, walls, clouds, fog, haze, rain, dust, smoke, sandstorms) without the high level of attenuation that occurs at the other noted wavelengths. This is particularly the case in specific “windows” for atmospheric transmission of radio waves that occur between 90 and 110 GHz and between 210 and 250 GHz.

Millimetre-wave imaging systems may be used in a range of important applications such as: aids to aircraft landing; collision warning in air, land and sea transport; detection and tracking of ground based vehicular traffic; covert surveillance for intruders, contraband and weapons. In such applications, the availability of real-time, “movie-camera” like imaging is highly desirable. However, for such systems to find wide acceptance in the commercial market-place, the sensing instrumentation must be light in weight, small in size, and affordable in cost.

A range of millimetre-wave imaging systems have been reported, but fail to meet the size, weight, and cost requirements for wide commercial acceptance of the technology, while at the same time offering real-time moving images. Such systems use two distinct technologies: mechanical scanning of the beam of a single antenna, and two-dimensional arrays.

Mechanical scanning of the beam of a single antenna connected to a single receiving system is performed in a raster pattern over a scene to detect the emitted radiation and produce a map or image of the brightness. The angular resolution of the resultant image is determined by the width of the antenna beam, whereas the scan angle determines the field of view. Rapid real-time imaging is difficult or inadequate, because physically large and cumbersome antenna elements (required to achieve high angular resolution) must be moved quickly at high rates.

Two-dimensional arrays of electrically-small antennas and integrated receivers sample the magnitude of the received millimetre-wave signal at the focal plane of an antenna system. This information is then used to produce a snap-shot of the brightness in the field of view of the instrument. In any given plane, the angular resolution of the resultant image is determined by the number of elements across the array and the outer dimensions of the array. In contrast, the field of view is determined by the beam-width of the individual antenna-array elements. Rapid real-time imaging can be achieved with these systems. However, this occurs at the expense of large numbers (1000's) of millimetre-wave receiving sub-systems and complex electronic phase shifting and amplitude weighting networks. Because of the large number of receivers required, heterodyne systems are avoided (in view of the local oscillator distribution problems) in favour of direct detection systems, with the attendant problems of gain stability and poorer sensitivity. Coherent local oscillator distribution to such a large number of millimetre-wave heterodyne receivers presents significant difficulties.

Thus, a need clearly exists for an improved real-time millimetre-wave imaging system capable of producing real-time, movie-like imaging, in which the system is more compact, less complex, and less expensive to produce.

In accordance with a first aspect of the invention, an image is formed from millimetre waves. To do so, a field of view is scanned using two geometrically orthogonal, intersecting co-polarized fan beams to receive millimetre-wave radiation. The components of received millimetre-wave radiation from the two fan beams are cross-correlated. The polarizations of the electric fields of the two fan beams are arranged to be substantially parallel in alignment. This may be achieved by polarization rotation filtering of the millimetre-wave radiation received in one of the fan beams. The two fan beams may be scanned in azimuth and elevation defining a scan range. The intersection region of the two fan beams is able to cover any point in the scan range. The scan range determines the field of view and a beam width of each fan beam in the narrow direction determines an angular resolution of the image. The cross-correlated output is measured at each point in the field of view to produce a map of the brightness. The position of the two geometrically orthogonal, intersecting fan beams may be controlled to generate the cross-correlated output at each fan beam intersection point in the field of view. Preferably, the scanning is implemented using a dual fan-beam antenna. The dual fan-beam antenna may have two modified pill-box antennas and a polarization rotator to change the direction of the incident polarization for one of the modified pill-box antennas. An image may be formed from millimetre waves of a different polarization by having a polarization rotator to change the direction of the incident polarization for a different modified pill-box antenna, only one polarization rotator being used at any time.

In accordance with a second aspect of the invention, millimetre-wave radiation is received. A field of view is scanned using a fan beam to receive millimetre-wave radiation. Polarization of incident millimetre-wave radiation is rotated through 90 degrees, and the field of view is scanned using another fan beam to receive the polarization-rotated millimetre-wave radiation. The fan beams intersect and are geometrically orthogonal to each other, yet the radiation is co-polarized. The fan beams are provided by respective fan-beam antennas. Each such antenna may include a modified pill-box antenna. Preferably, the modified pill-box antenna includes: a metal housing with an elongated aperture in at least one side of the housing, a curved primary reflector surface located within the housing and opposite the aperture, a feed horn within the housing, and one or more sub-reflectors for coupling the feed horn to the primary reflector surface. At least one of the sub-reflectors is designed to rotate, providing one-dimensional beam scanning in the narrow direction of the fan beam. The polarization rotation for a fan beam may be implemented using a polarization rotating transreflector. Preferably, the transreflector includes: a planar metallic reflector, and a grid of closely spaced wires. The wires are preferably spaced n×λ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimetre-wave radiation. The polarization rotating transreflector may be positioned at a 45 degree angle relative to the aperture of the second fan-beam antenna and at a substantially 45 degree angle relative to the direction of incident millimetre-wave radiation. The polarization rotation for a fan beam may be switched by exchanging a polarization rotating transreflector and a planar metallic reflector, both aligned in the same way. An exchange may be effected by turning a polarization rotating transreflector by 180 degrees to use its back surface as a planar metallic reflector. An exchange may be effected by making the wires of a polarization rotating transreflector out of a material that has a switchable conductivity.

In accordance with a third aspect of the invention, millimetre wave radiation is received for generating an image. To do so, millimetre wave radiation is received in accordance with first and second fan beams. The first and second fan beams are geometrically orthogonal to each other and intersecting. The millimetre wave radiation received in accordance with the second fan beam is co-polarized with the millimetre wave radiation received in accordance with the first fan beam. Components of the millimetre wave radiation received in accordance with the first and second beams are downconverted to generate respective intermediate frequency (IF) signals. The IF signals are cross-correlated. The resulting cross-correlated signal is filtered to provide a value proportional to brightness at each point in the scene. The received millimetre wave radiation may be amplified in accordance with the first and second beams prior to the step of downconverting.

In accordance with a fourth aspect of the invention, millimetre-wave imaging is disclosed. To do so, millimetre-wave radiation is received. The receiving includes: receiving millimetre-wave radiation by scanning a field of view using a fan beam, rotating the polarization of incident millimetre-wave radiation through 90 degrees, and receiving the polarization-rotated millimetre-wave radiation by scanning a field of view using another fan beam. The fan beams intersect and are geometrically orthogonal to each other. The received millimetre-wave radiation is processed. The processing step includes: receiving components of millimetre-wave radiation from the antenna received in accordance with the fan beams, downconverting respective components of the received millimetre wave radiation received to generate respective intermediate frequency (IF) signals, cross-correlating the IF signals; and filtering the resulting cross-correlated signal. The filtered, cross-correlated signal is proportional to the brightness at each point in the field of view as the antenna beams are scanned. In this way, an image of the scene may be built up. The scanning of each fan beam may be independently controlled as required so that the image can be generated from the filtered, cross-correlated output signal which provides a value proportional to the brightness of the scene at each point in said field of view.

A small number of embodiments are described hereinafter with reference to the drawings, in which:

FIG. 1 is a radiation pattern of two crossed fan beam antennas in accordance with the embodiments of the invention;

FIG. 2 is a simplified block diagram of a real-time millimetre-wave imaging system in accordance with an embodiment of the invention;

FIG. 3 is a perspective view of an example of a pill-box antenna for implementing a scanned-beam imaging system in accordance with another embodiment of the invention;

FIG. 4 is a perspective view of a combination of two pill-box antennas and a metallic reflector for producing a dual-scanning beam antenna with co-polarized far-field response in accordance with a further embodiment of the invention;

• Provisional Patent
• Utility Patent

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