WO2008109946A1

WO2008109946A1 - Three-dimensional millimeter-wave imaging system

Info

Publication number: WO2008109946A1
Application number: PCT/AU2008/000342
Authority: WO
Inventors: John David Bunton
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2007-03-12
Filing date: 2008-03-12
Publication date: 2008-09-18

Abstract

Methods (400), apparatuses, systems (100) and computer program products for 3D imaging using millimeter-wave radiation are described. Millimeter-wave radiation is transmitted continuously (412) from a fan-beam antenna (122) to actively illuminate a field of view (110). Millimeter-wave radiation is received (414) using at least one other fan-beam antenna (120) to scan the field of view (110) passively. The fan beams of the antennas (120, 122) are geometrically orthogonal, intersecting fan beams. Components of the transmitted millimitre-wave radiation and the received millimeter- wave radiation are cross correlated (170, 416) to generate cross-correlated output at each fan beam intersection point in the field of view (110). A 3D image (132) is generated (130, 418) from the cross-correlated outputs. At least one (200) of the fan-beam antennas is adapted to mechanically scan a field of view (110) using a fan beam.

Description

THREE-DIMENSIONAL MILLIMETER-WAVE IMAGING SYSTEM

RELATED APPLICATION

The present application claims the benefit of the earlier filing date of Australian Provisional Patent Application No. 2007901276 filed on 12 March 2007 in the name of Commonwealth Scientific and Industrial Research Organisation and entitled "Three- Dimensional Millimeter- Wave Imaging System", the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to millimeter- wave imaging systems and, more particularly, to millimeter-wave imaging systems utilizing active scanning.

BACKGROUND Interest in millimeter- wave imaging systems generally operating around 35 or 94GHz is increasing. Such imaging systems have less spatial resolution than optical or infrared systems due to the much longer wavelength. However, such millimeter- wave systems have a significant advantage over optical and infrared sensor systems in their ability to see through clouds, fog, smoke and clothing materials.

Millimeter-wave imaging systems are either too slow in operation or expensive. Some systems have attempted to use passive imaging. Such passive systems detect the power level received at a pixel via an autocorrelation or total power measurement. Such systems involve large moving components, multiple receivers, multiple receivers followed by complex processing, or full focal plane arrays. In satellite systems for earth imaging, a linear phased array places a row of pencil beams on the ground from above, and movement of the satellite provides scanning in the orthogonal direction. Another passive approach involves cross-correlating between two orthogonal fan beams.

Thus, a need clearly exists for an improved millimeter- wave imaging system. SUMMARY

In accordance with an aspect of the invention, there is provided a method of three- dimensional (3D) imaging using millimeter-wave radiation. The method includes the steps of: transmitting millimeter-wave radiation continuously from a fan-beam antenna to actively illuminate a field of view; receiving millimeter- wave radiation using at least one other fan-beam antenna to scan the field of view passively, the fan beams of the transmitting and receiving antennas being geometrically orthogonal, intersecting fan beams; cross-correlating at least components of the transmitted millimeter- wave radiation and the received millimeter-wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view; and generating a 3D image from the cross-correlated outputs. At least one of the fan-beam antennas is adapted to mechanically scan a field of view using a fan beam. The transmitting and receiving steps are performed in orthogonal directions defining a scan range, and an intersection region of the orthogonal fan beams is able to cover any point in the scan range.

The components may include a downconverted version of the received radiation and a downconverted version of a transmit signal prior to transmitting the millimeter wave radiation.

The scan range may determine the field of view, and a beam width of each fan-beam in a narrow direction may determine an angular resolution of the image.

The receiving step to scan the field of view passively may be implemented using a plurality of fan beams.

The method may further include the step of controlling the geometrically orthogonal, intersecting fan beams.

The at least one fan-beam antenna adapted to mechanically scan a field of view comprises a pill-box antenna that may include: a metal housing with an elongated aperture in at least one side of the housing; a curved primary reflector surface located 8 000342

-3-

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 being designed to rotate and providing one- dimensional beam scanning in a narrow direction of the other fan beam. The polarization rotating transreflector may include a planar metallic reflector, and a grid of closely spaced wires, the grid spaced n><λ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimeter wave radiation.

The method may further include the steps of: measuring lag between the transmitted millimeter- wave radiation and the received millimeter- wave radiation; and eliminating leakage of the transmitted millimeter- wave radiation from the received millimeter- wave radiation. The leakage may be eliminated dependent upon the measured lag being at or near zero lag.

The transmitting antenna is preferably a fan-beam antenna adapted to mechanically scan a field of view using a fan beam.

In accordance with another aspect of the invention, there is provided an apparatus for three-dimensional (3D) imaging using millimeter- wave radiation. The apparatus includes: a fan-beam antenna for transmitting millimeter- wave radiation continuously to actively illuminate a field of view; at least one other fan-beam antenna for receiving millimeter- wave radiation to scan the field of view passively, the fan beams of the transmitting and receiving antennas being geometrically orthogonal, intersecting fan beams, at least one of the fan-beam antennas being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; a correlator for cross-correlating at least components of the transmitted millimitre-wave radiation and the received millimeter- wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view; and an output system for generating a 3D image from the cross-correlated outputs. -A-

In accordance with yet another aspect of the invention, there is provided a system for three-dimensional (3D) imaging using millimeter-wave radiation. The system includes: a module for transmitting millimeter- wave radiation in a fan beam continuously to actively illuminate a field of view; a module for receiving millimeter- wave radiation in at least one fan beam to scan the field of view passively, the fan beams of the transmitting and receiving means being geometrically orthogonal, intersecting fan beams, at least one of the transmitting and receiving means being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; a module for cross-correlating at least components of the transmitted millimirre-wave radiation and the received millimeter- wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view; and a module for generating a 3D image from the cross-correlated outputs.

In accordance with a further aspect of the invention, there is provided a computer program product having a computer readable medium storing a computer program for three-dimensional (3D) imaging using millimeter- wave radiation. The computer program product includes: a computer program code module for controlling transmission of millimeter- wave radiation in a fan beam continuously to actively illuminate a field of view; a computer program code module for controlling reception of millimeter- wave radiation in at least one fan beam to scan the field of view passively, the scanning operations of the transmission and the reception being controlled, the fan beams of the transmitting and receiving means being geometrically orthogonal, intersecting fan beams, at least one of the transmitting and receiving means being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; a computer program code module for receiving and processing cross-correlated output at each fan beam intersection point in the field of view from a correlator that cross-correlates at least components of the transmitted millimitre-wave radiation and the received millimeter- wave radiation; and a computer program code module for generating a 3D image from the cross-correlated outputs.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are described hereinafter with reference to drawings, in which:

Fig. 1 is a schematic diagram showing the arrangement and operation of a system for three-dimensional (3D) imaging using millimeter-wave radiation in accordance with an embodiment of the invention;

Fig. 2 is a perspective view of a pill-box antenna of the type disclosed in United States Patent Application Publication No. 20060049980;

Fig. 3 is a schematic diagram showing an XF correlator that may be practiced in the system of Fig. 1;

Fig. 4 is a flow diagram showing the method of 3D imaging using millimeter- wave radiation;

Figs. 5 A and 5B are an active 187-GHz digital image of a subject with no concealed objects and a digital photograph of a person with the 187 GHz image of Fig. 5 A superimposed on the person showing the subject has no concealed objects in the field of view, respectively; and

Figs. 6A and 6B are an active 187-GHz digital image of a subject with a concealed metallic knife and a digital photograph of a person with the 187 GHz image of Fig. 6 A superimposed on the person showing that the subject has a concealed metallic knife, respectively.

DETAILED DESCRIPTION Methods, an apparatuses, systems and computer program products for three-dimensional (3D) imaging using millimeter-wave radiation are disclosed. In the following description, numerous specific details are set forth, including particular bandwidths, antenna configurations, operating frequencies, pillbox antenna dimensions, and the like. However, in the light of this disclosure, it will be apparent to a person skilled in the art that changes may be made to the embodiments without departing from the scope and spirit of the invention.

1. Overview The embodiments of the invention provide an active millimeter- wave imaging system that has increased sensitivity and can achieve acceptable imaging speeds. Further, the embodiments of the invention have fewer radiofrequency (RP) components, mainly an active transmitter and a passive receiver, and hence costs are reduced. The embodiments of the invention employ active system "radar" at short range, and correlation techniques are used to generate depth information, thereby enabling the elimination of foreground and background objects, for example. In the embodiments, continuous radiation millimeter (mm) wave radiation is used in transmission rather than pulsed radiation. The continuous transmit signal maximizes sensitivity. Two antennas are used to generate scanning orthogonal fan beams that intersect within a region of interest. One antenna transmits a broadband signal and the other antenna receives. The transmit signal is correlated with the receive signal, generating a number of lags (cross correlations with different delays between the two).

Fig. 4 broadly outlines a method 400 of three-dimensional (3D) imaging using millimeter- wave radiation. Processing commences in step 410. In step 412, millimeter- wave radiation is transmitted continuously from a fan-beam antenna to actively illuminate a field of view. In step 414, millimeter- wave radiation is received using at least one other fan-beam antenna to scan the field of view passively. The fan beams of the transmitting and receiving antennas are geometrically orthogonal, intersecting fan beams. The transmitting and receiving steps are performed in orthogonal directions defining a scan range, and an intersection region of the orthogonal fan beams is able to cover any point in the scan range. At least one of the fan-beam antennas is adapted to mechanically scan a field of view using a fan beam. In step 416, at least components of the transmitted millimitre-wave radiation and the received millimeter- wave radiation are cross correlated to generate cross-correlated output at each fan beam intersection point in the field of view. In step 418, a 3D image is generated from the cross-correlated outputs. Processing terminates in step 420. While only a single pass is shown in Fig. 4, it will be apparent to one skilled in the art in view of Fig. 4 that steps 412 to 418 may be repeatedly applied. The method is described in greater detail hereinafter with reference to Figs. 1 to 3, 5 and 6.

The embodiments of the invention use at least two scanning fan-beam antennas and can scan the field of view or scene in azimuth and elevation using one or more mechanically scanning antennas. One implementation of a fan-beam antenna adapted to mechanically scan a field of view involves a pill-box antenna having a rotatable sub-reflector. Such a fan-beam antenna is disclosed in United States Patent Application Publication No. 20060049980 (USSN 10/ 518758) published in the name of Archer et al on 09 March 2006 and entitled "Real-time, cross-correlating millimeter-wave imaging system", which is incorporated herein by reference. Such antennas generate orthogonal fan beams, one for transmitting millimeter- wave radiation and another for receiving such radiation. The scan direction is at the intersection of the fan beams. Scanning with a subreflector is advantageous since doing so significantly reduces the moving mass, and hence safety is increased and cost can be reduced. The cross correlation with orthogonal fan beams that are scanned by subreflector movement, together with the use of active imaging, forms a bistatic radar. This greatly increases the sensitivity of the system. With an active system, high sensitivity can be achieved with a single transmitter and a single receiver. A correlator and matched filter measure multiple lags giving range discrimination, which allows a three-dimensional image to be generated. This permits fixed background and foreground sources to be distinguished from wanted targets.

More preferably two scanning pill-box antennas can be used. The antennas transmit noise or chirp signals on one antenna and receive on the other. The received signal is correlated with the transmitted signal, which generates multiple lag signals. Each lag corresponds to a different range bin. Scanning implemented by rotation of a subreflector greatly reduces the moving mass allowing safe operation at high scanning rates. Further, this produces a cost advantage compared to systems that use flapping or rotating mirrors, which have a size comparable to the full aperture of an antenna.

In an alternative embodiment of the invention, instead of a single receiver, extra receivers can be added to the receiving antenna, thereby increasing speed and sensitivity, or reducing the mechanical scanning speed. A phased-array receiving system may be used as the receiving antenna in place of the mechanical scanning antenna.

Because separate transmit and receive antennas are employed, the system uses continuous rather than pulsed illuminations. Continuous illumination increases sensitivity. Further any antenna coupling is distinguishable for distant target returns by range discrimination. Antenna coupling generates a response at small or zero lag values, while targets generate a response at large lag values. In the embodiments of the invention, millimeter radiation is normally transmitted and received on the same polarisation. However, cross polarisation operation is possible. Using the same polarisation provides greater sensitivity.

2. Active System

Fig. 1 illustrates an active millimeter- wave imaging system 100 in accordance with an embodiment of the invention. The system 100 includes a transmitting fan-beam antenna 122, a receiving fan-beam antenna 120 (at least one of which is adapted to mechanically scan the field of view using a fan beam), a digital correlator 170, and a computer or other computing and control device 130, 132. The transmitting fan-beam antenna 122 transmits millimeter- wave radiation continuously to actively illuminate the field of view 110. The other fan-beam antenna 120 receives millimeter-wave radiation to scan the field of view 110 passively. The fan beams of the transmitting and receiving antennas 122, 120 are geometrically orthogonal, intersecting fan beams. The digital correlator 170 cross correlates at least components of the transmitted millimeter- wave radiation and the received millimeter-wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view. The cross-correlated outputs from the correlator 170 are provided to an output system 130, 132 for generating one or more3D images from the cross-correlated outputs. The output system is preferably a computer 130 with a suitable display 132 or other device for producing the 3D millimetre-wave image. In the embodiment of Fig. 1, the digital correlator 170 is implemented using an FPGA, but other implementations are possible without departing from the scope and spirit of the invention.

In Fig. 1, a Gunn oscillator 180 is coupled to a frequency doubler (2X) to produce a

186.85 GHz transmitting signal. While a particular operating frequency is specified, this is for purposes of description only and other frequencies can be practiced. The Gunn oscillator 180 is optionally controlled by the computer 130 via a control signal 182 (indicated by a dashed arrow between the oscillator 180 and the computer 130). A 1OdB coupler 190 splits the transmitting signal, which is coupled to the transmitting fan beam antenna 122 and a low noise amplifier (LNA). Likewise, received millimeter radiation is provided to a corresponding LNA. As noted hereinbefore, millimeter radiation is normally transmitted and received on the same polarisation, but cross polarisation operation is possible. Using the same polarisation provides greater sensitivity. The outputs of these two LNAs are provided to respective mixers 142 and 144, which are also coupled to a local oscillator (LO), which in the example shown in Fig. 1 has a frequency of 47 GHz). The output of mixers 142 and 144 are provided respectively to LNAs 152 and 154. The output of the LNAs 152 and 154 are provided to respective filters 162 and 164, which provide their filtered outputs as inputs to the digital correlator 170.

Millimeter- wave radiation is generated by the frequency-doubled Gunn oscillator 180, which may be chirped from 186.85 to 187.10 GHz and may produce approximately 2 mW of CW power, for example. The 250 MHz of bandwidth (BW) allows the time-of- flight to be estimated to within 4 ns, and hence distance to be estimated to within 0.6 meters. A fraction of the 187 GHz signal is passed to the receiver (LNA, 142, 152, 180) with the remainder fed to the horizontal pillbox antenna 122. This antenna 122 produces a vertical fan beam focused at a given distance (e.g., 5 meters) from the system 100. The second (vertical) antenna 120 receives radiation from a horizontal fanbeam and passes the radiation to the second receiver (LNA, 144, 154, 164). The IF signals from the two receivers are cross-correlated by the correlator 170, and a correlated output signal (e.g. voltage) is produced that is proportional to the magnitude of the reflected radiation at the intersection of the two fan beams. Using this approach, a 2D-plane is covered by scanning the beam from each antenna in one direction.

For example, the antennas can be constructed from dimensionally stable aluminium

(MIC-6) to minimize warping of the parallel plates. The subreflector thickness of 4.7mm allows only 150μm clearance on either side of the subreflector (not shown in Fig. 1; see Fig. 2). The dimensions of the pillbox antennas in this embodiment are approximately 700x600x50 mm with an aperture of 490x5 mm. Again, these dimensions are not intended to be restrictive, but are given by way of example.

The system may be practiced using two InP low-noise amplifiers and an InP sub- harmonically pumped mixer, which make use of the 4th harmonic of the local oscillator (LO) 140. The LO 140 can be an 11.75GHz dielectric resonator oscillator multiplied four times to a frequency of 47GHz (corresponding to a 4th harmonic frequency of 188 GHz at the mixer). The LO power can be boosted through a GaAs amplifier (not shown) before being passed to the mixers 142, 144.

The I and Q channels of the received signal may undergo 6OdB of amplification and be filtered by 0.5GHz high pass and 8GHz low pass filters 162, 164, before being passed through 250 MSample/s Analogue-to-Digital Converters ADCs (not shown in Fig. 1 between the filters 162, 164 and the correlator 170) and then fed into the FPGA-based multi-lag correlator 170. The correlator 170 features 16 lags with 4 ns resolution, which corresponds to 0.6 m resolution in target distance. Fig. 3 illustrates an XF correlator of the type that can be used to implement the correlator 170. In Fig. 3, the input from the receiver for received millimeter- wave radiation following ADC is labelled Antenna 1. The delay nD may be set to set to zero, and this signal is coupled to each of the mixers X 320. Only six delay elements D 310 are shown in Fig. 3 for ease of illustration. The input from the receiver for transmitted millimeter- wave radiation following ADC is labelled Antenna 2, which is coupled to a sequence of delays 310. Seven mixers X 320 are shown for ease of illustration. The digitized input from the receiver for transmitted millimeter- wave radiation is coupled to one of the mixers and the sequence of delays D 310. The delays D310 are coupled to a respective one of the mixers 320. The outputs of the seven mixers 320 are coupled to respective accumulators A 330.

In the system 100 of Fig. 1, the source produces much higher powers than those emitted from blackbodies within the scene and can be considered as an object at very high temperature (typically thousands of Kelvin). If the source is incoherent and physically large, active millimeter-wave imaging is essentially equivalent to passive imaging with the surroundings at very high temperatures, and hence results in much greater contrast within the image. Because of the much higher effective temperature of the surroundings, millimeter waves generated by objects within the scene are of less significance in active imaging. The overwhelming contribution to the received signal from a pixel is the millimeter- wave reflectivity of the object in that pixel.

If the source is a point source or physically small, active imaging becomes more complicated because the reflected signal is significantly dependent on the orientation of the object being imaged. An object oriented so that the incoming radiation reflects directly towards the receiver produces large reflected signals compared to objects with other orientations. Flat surfaces at non-normal incidence may simply reflect millimeter- waves away from the imager and may not be visible in the image. Possible solutions are to:

(i) use a physically large source, (ii) use multiple sources, (iii) rotate the object being imaged and/or (iv) compress the dynamic range of the image so that the specular reflections do not dominate the image. One advantage of active imaging is the potential to obtain range information in the scene. By transmitting noise or a chirped signal and utilizing a multi-lag correlator, the time-of- flight of the transmitted signal can be estimated, and this allows the distance to each object to be calculated.

3. Example of Mechanically Scanning Fan Beam Antenna

In this embodiment of the invention, two pillbox antennas are placed at 90 degrees relative to each other in the system for three-dimensional (3D) imaging using millimeter- wave radiation. Each antenna generates a fan beam that can be scanned by rotating the subreflector of the antenna. The fan beams are orthogonal, and high sensitivity is achieved where the beams overlay. Millimeter-wave radiation is transmitted continuously from one of the fan-beam antennas to actively illuminate a field of view. For example, the transmission antenna transmits a wideband signal at mm-wave frequencies, e.g. 0.1 GHz to 1.0 GHz. The transmit signal is split and part of the signal goes to the transmit antenna and the other part goes to one of a matched pair of receiving chains. LNAs normally have different gains and noise figures.

Fig. 2 illustrates a pillbox antenna 200 for producing a millimeter-wave fan-beam has a shaped primary reflector 234 coupled to a single feed-horn 230, 232 via a rotating sub- reflector 220, which provides beam scanning as the sub-reflector 220 spins. More than one sub-reflector may be practiced, with at least one sub-reflector rotating to provide beam scanning. The sub-reflector 220 is disc-like in form. The antenna 200 includes a metal housing 210 with a radiating aperture 212 formed in one side of the metal housing. The length of the radiating aperture 212 is approximately 200 wavelengths and the width of the aperture 212 is approximately one wave length. Other dimensions may be practiced without departing from the scope and spirit of the invention. The direction of the electric field at the aperture is indicated by an arrow 214. Located within the metal housing 210 is the primary reflector surface 234 coupled to the tapered wave guide feed- horn 230 with a wave guide input/output 232 oppositely positioned relative to the radiating aperture 212 within the housing 210. At the bottom of the tapered wave guide feed-horn 330 within the metal housing 210 is the rotating sub-reflector 220 for one dimensional beam scanning. Further details of this antenna's configuration are described in United States Patent Application Publication No. 20060049980.

Millimeter- wave radiation is received using at least one other fan-beam antenna to scan the field of view passively. As noted above, the fan beams of the transmitting and receiving antennas are geometrically orthogonal, intersecting fan beams. An intersection region of the orthogonal 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 a narrow direction determines an angular resolution of the image. As noted above, the transmitting antenna is a fan-beam antenna adapted to mechanically scan a field of view using a fan beam.

The transmit and receive chains use the common oscillator so that the transmit and receive signals are phase coherent. The baseband outputs are filtered 162, 164 and digitized (not shown). The digital signals are provided to the correlator 170, which may be of the XF or FX type. For a single receiver system, the XF system is preferred. The baseband signal may be real or complex, but here an IQ downconverter gives a complex baseband signal. Both components of the baseband signal are digitized and passed to the correlator 170). That is, the transmitted millimitre-wave radiation and the received millimeter-wave radiation are cross correlated to generate cross-correlated output at each fan beam intersection point in the field of view. The transmit and receive signals are multiplied together and averaged to give the cross correlation for zero lag. Delays are introduced into the transmit signal, and cross correlations for multiple lags are generated. For a target at distance x, the cross correlation is maximum for a lag of 2x/c nanoseconds, where c is the velocity of light in meters per nanosecond.

The lag between the transmitted millimeter- wave radiation and the received millimeter- wave radiation is measure, and leakage of the transmitted millimeter- wave radiation is eliminated from the received millimeter- wave radiation. The leakage is eliminated dependent upon the measured lag being at or near zero lag. The distance resolution of the system depends on the bandwidth at the input to the correlator. For a bandwidth BW, the time resolution is 1/BW, and the distance is 0.15 BW meters with BW in gigahertz (GHz). For example, using a bandwidth BW of 250 MHz, the resolution is 0.6 meters.

A 3D image can be generated from the cross-correlated outputs. This may be done by passing the correlations to a computer for display. In the computer, algorithms can be used to distinguish information contained in the correlation lags to distinguish targets from background, or to form a 3D image or any simpler representation. The computer can also be programmed to control the positioning of the fan beams via control signals.

For low power applications, the system can used for close-range surveillance. For example, when scanning a crowd, the range discrimination allows individual people to be distinguished. Thus, each person can be processed separately to determine whether a person has anything concealed under the person's clothing. For portal entry applications, the range discrimination allows the foreground and background targets to be eliminated.

High-power systems have a much greater range than the 10 meters to 100 meters of low power systems. However, this introduces a near-far problem. For use in applications such as aircraft landing radars, the transmit antenna is preferably place vertically and generates a horizontal fan beam; preferably, this is the slow scan direction. In horizontal flight, the fan beam intersects the ground at approximately constant range. Thus, the return signal strength varies slowly, allowing the gain variation to be catered for by slowing changing the receive gain or transmit power. The antenna coupling causes greater interference for distant targets as their returns are weakest, but this is compensated for by the fact that discrimination provided by the correlator is greater in large ranges.

The embodiments of the invention have a number of applications. One application is for surveillance to detect concealed items. This includes portal-entry or area surveillance to detect explosives and/or weapons concealed under clothing on a person. Other applications include, at higher transmit power levels the millimeter-wave 3D imaging system can be used for wide-area intrusion detection and as an aid for aircraft landing. In such applications, the ability of mm- wave radiation to penetrate fog and clothing is important in allowing the detection of objects that are visually or optically obscured.

Fig. 5 A illustrates a digital image of a subject with no concealed objects produced by the system 100 of Fig. 1. Fig. 5B illustrates a digital photograph of the subject/person with the 187 GHz image of Fig. 5 A superimposed on the person, showing the subject has no concealed objects. In contrast, Fig. 6A illustrates are a digital image of the same subject/person with a concealed metallic knife under the person's clothing. Fig. 6B shows the person with the 187 GHz image superimposed on the person, showing that the subject has a concealed metallic knife. Thus, the active system 100 of Fig. 1 can be advantageously applied in surveillance systems.

4. Alternative Embodiments

Other modifications of the system can be implemented. One alternative embodiment involves using additional receivers coupled to the receive antenna, so that the receive antenna scans multiple lines of the image at once. This can be used to increase imaging rate and/or sensitivity. In one form of this embodiment, a full phased array can be used as the receiver, and the imaging rate is equal to the scan rate of the transmit antenna.

Still further, in additional embodiments of the invention, mechanical scanning can be implemented in a number of ways, including movement of a feed on a cylindrical Luneburg Lens, or in a focal plane of a cylindrical reflector (either prime or secondary focus).

5. Computer Implementation

The embodiments of the invention are preferably at least partly computer implemented. In particular, the processing or functionality of Fig. 1 to 6 can be implemented as software, or a computer program, executing a computer. The method or process steps for 3D imaging using millimeter-wave radiation may be effected at least partly by instructions in software including relevant data that are carried out by the computer. The software may be implemented as one or more modules for implementing the process steps. A module is a part of a computer program that usually performs a particular function or related functions. Also, as described hereinbefore, a module can also be a packaged functional hardware unit for use with other components or modules.

In particular, the software may be stored in a computer readable medium. Relevant storage device(s) include: a floppy disc, a hard disc drive, a magneto-optical disc drive, CD-ROM, magnetic tape or any other of a number of non- volatile storage devices well known to those skilled in the art. The software is preferably loaded into the computer from the computer readable medium and then carried out by the computer. A computer program product includes a computer readable medium having such software or a computer program recorded on it that can be carried out by a computer. The use of the computer program product in the computer preferably effects advantageous apparatuses in accordance with the embodiments of the invention.

The software may be encoded on a CD-ROM or a floppy disk, or alternatively could be read from an electronic network via a modem device connected to the computer, for example. Still further, the software may be loaded into the computer system from other computer readable medium including DVD, magnetic tape, a ROM or integrated circuit, a magneto-optical disk, a radio or infra-red transmission channel between the computer and another device, a computer readable card such as a PCMCIA card, and the Internet and Intranets including email transmissions and information recorded on websites and the like. The foregoing is merely exemplary of relevant computer readable mediums. Other computer readable mediums may be practised without departing from the scope and spirit of the invention.

In the foregoing manner, methods, apparatuses, and computer program products for three-dimensional (3D) imaging using millimeter- wave radiation are disclosed. While only a small number of embodiments are described, it will be apparent to those skilled in the art in view of this disclosure that numerous changes and/or modifications can be made without departing from the scope and spirit of the invention.

Claims

The claims defining the invention are as follows:

1. A method of three-dimensional (3D) imaging using millimeter- wave radiation, the method including the steps of: transmitting millimeter-wave radiation continuously from a fan-beam antenna to actively illuminate a field of view; receiving millimeter- wave radiation using at least one other fan-beam antenna to scan the field of view passively, the fan beams of the transmitting and receiving antennas being geometrically orthogonal, intersecting fan beams, at least one of the fan-beam antennas being adapted to mechanically scan a field of view using a fan beam, the transmitting and receiving steps being performed in orthogonal directions defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; cross-correlating at least components of the transmitted millimeter- wave radiation and the received millimeter- wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view; and generating a 3D image from the cross-correlated outputs.

2. The method according to claim 1, wherein said components include a downconverted version of said receive radiation and a downconverted version of a transmit signal prior to transmitting said millimeter wave radiation.

3. The method according to claim 1, wherein the scan range determines the field of view, and a beam width of each fan-beam in a narrow direction determines an angular resolution of the image.

4. The method according to claim 1, wherein the receiving step to scan the field of view passively is implemented using a plurality of fan beams.

5. The method according to claim 1, further including the step of controlling the geometrically orthogonal, intersecting fan beams.

6. The method according to claim 1, wherein the at least one fan-beam antenna adapted to mechanically scan a field of view comprises a modified pill-box antenna including: 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 being designed to rotate and providing one- dimensional beam scanning in a narrow direction of the other fan beam.

7. The method according to claim 6, wherein the polarization rotating transreflector includes: a planar metallic reflector; and a grid of closely spaced wires, the grid spaced n*λ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimeter wave radiation.

8. The method according to claim 1, further including the steps of: measuring lag between the transmitted millimeter-wave radiation and the received millimeter- wave radiation; and eliminating leakage of the transmitted millimeter- wave radiation from the received millimeter-wave radiation.

9. The method according to claim 8, wherein the leakage is eliminated dependent upon the measured lag being at or near zero lag.

10. The method according to claim 1 or 6, wherein the transmitting antenna is the fan-beam antenna adapted to mechanically scan a field of view using a fan beam.

11. An apparatus for three-dimensional (3D) imaging using millimeter- wave radiation, the apparatus including: a fan-beam antenna for transmitting millimeter- wave radiation continuously to actively illuminate a field of view; at least one other fan-beam antenna for receiving millimeter- wave radiation to scan the field of view passively, the fan beams of the transmitting and receiving antennas being geometrically orthogonal, intersecting fan beams, at least one of the fan-beam antennas being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; a correlator for cross-correlating at least components of the transmitted millimitre-wave radiation and the received millimeter-wave radiation to generate cross- correlated output at each fan beam intersection point in the field of view; and an output system for generating a 3D image from the cross-correlated outputs.

12. The apparatus according to claim 11, wherein said components include a downconverted version of said receive radiation and a downconverted version of a transmit signal prior to transmitting said millimeter wave radiation.

13. The apparatus according to claim 11, wherein the scan range determines the field of view, and a beam width of each fan-beam in a narrow direction determines an angular resolution of the image.

14. The apparatus according to claim 11, wherein the at least one other fan-beam antenna uses a plurality of fan beams .

15. The apparatus according to claim 11, further including a controller for controlling the geometrically orthogonal, intersecting fan beams.

16. The apparatus according to claim 11, wherein the at least one fan-beam antenna adapted to mechanically scan a field of view comprises a pill-box antenna including: 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 being designed to rotate and providing one- dimensional beam scanning in a narrow direction of the other fan beam.

17. The apparatus according to claim 16, wherein the polarization rotating transreflector includes : a planar metallic reflector; and a grid of closely spaced wires, the grid spaced nxλ/4 from the planar metallic reflector, where n is an odd integer and λ is a wavelength of the millimeter wave radiation.

18. The apparatus according to claim 11, further including: a module that measures lag between the transmitted millimeter-wave radiation and the received millimeter- wave radiation; and a module that eliminates leakage of the transmitted millimeter- wave radiation from the received millimeter-wave radiation.

19. The apparatus according to claim 18, wherein the leakage is eliminated dependent upon the measured lag being at or near zero lag.

20. The apparatus according to claim 11 or 16, wherein the transmitting antenna is the fan-beam antenna adapted to mechanically scan a field of view using a fan beam.

21. A system for three-dimensional (3D) imaging using millimeter- wave radiation, the system including: means for transmitting millimeter-wave radiation in a fan beam continuously to actively illuminate a field of view; means for receiving millimeter-wave radiation in at least one fan beam to scan the field of view passively, the fan beams of the transmitting and receiving means being geometrically orthogonal, intersecting fan beams, at least one of the transmitting and receiving means being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; means for cross-correlating at least components of the transmitted millimitre- wave radiation and the received millimeter-wave radiation to generate cross-correlated output at each fan beam intersection point in the field of view; and means for generating a 3D image from the cross-correlated outputs.

22. The system according to claim 21, wherein said components include a downconverted version of said receive radiation and a downconverted version of a transmit signal prior to transmitting said millimeter wave radiation.

23. The system according to claim 21 , wherein the receiving means uses a plurality of fan beams.

24. The system according to claim 21, further including means for controlling the geometrically orthogonal, intersecting fan beams.

25. The system according to claim 21, further including: means for measuring a lag between the transmitted millimeter-wave radiation and the received millimeter- wave radiation; and means for eliminating leakage of the transmitted millimeter- wave radiation from the received millimeter-wave radiation.

26. The system according to claim 25, wherein the leakage is eliminated dependent upon the measured lag being at or near zero lag.

27. The system according to claim 21 , wherein the transmitting means includes the fan-beam antenna adapted to mechanically scan a field of view using a fan beam.

28. A computer program product having a computer readable medium storing a computer program for three-dimensional (3D) imaging using millimeter-wave radiation, the computer program product including: computer program code means for controlling transmission of millimeter- wave radiation in a fan beam continuously to actively illuminate a field of view; computer program code means for controlling reception of millimeter- wave radiation in at least one fan beam to scan the field of view passively, said scanning operations of said transmission and said reception being controlled, the fan beams of the transmitting and receiving means being geometrically orthogonal, intersecting fan beams, at least one of the transmitting and receiving means being adapted to mechanically scan a field of view using a fan beam, the orthogonal transmitting and receiving fan beams defining a scan range, and an intersection region of the orthogonal fan beams being able to cover any point in the scan range; computer program code means for receiving and processing cross-correlated output at each fan beam intersection point in the field of view from a correlator that cross-correlates at least components of the transmitted millimitre-wave radiation and the received millimeter- wave radiation; and computer program code means for generating a 3D image from the cross- correlated outputs.