WO2009090406A1

WO2009090406A1 - Microwave imaging system

Info

Publication number: WO2009090406A1
Application number: PCT/GB2009/000132
Authority: WO
Inventors: Amit Mehta
Original assignee: Uws Ventures Ltd
Priority date: 2008-01-18
Filing date: 2009-01-19
Publication date: 2009-07-23
Also published as: GB0800882D0; GB201012635D0; GB2469002A

Abstract

A microwave imaging system for imaging a defined region (12), the system comprising a plurality of portable active RFID tags (10) distributed around said region (12) for generating a plurality of RF signals (20) and directing said signals (20) to said defined region (12) and for receiving RF signals from said defined region (12), and means for transmitting the characteristics of said received signals (20) to a remote processing station (14) through a wireless communication channel, extracting image data from said received signals and constructing a corresponding image.

Description

MICROWAVE IMAGING SYSTEM

This invention relates generally to a microwave imaging system and, more particularly, to a microwave imaging system employing RFID technology.

In response to the continued threat of terrorism, inspection of persons and other items for the presence of weapons and other threatening items has become essential at security checkpoints, such as those found at airports, concerts, sporting events and other venues of private and public facilities potentially at risk from terrorist attacks. Conventional security inspection systems tend to include one or more of physical inspection by security personnel, metal detectors and X-ray imaging systems. However, physical inspection by security personnel is tedious and invasive, and metal detectors are prone to false alarms, and in any event, are not capable of discriminating between different metallic objects. Furthermore, X-ray systems pose a potential health risk and are relatively expensive.

In response to the need for improved security inspections systems, various microwave imaging systems have been proposed as an alterative to the conventional systems described above. Microwave radiation is generally defined as electromagnetic radiation having wavelengths between radio waves and infrared waves. An advantage of microwave radiation over X-ray radiation is that microwave radiation is non-ionizing, and therefore poses no known health risks to people for short-term exposure at moderate power levels. In addition, over the spectral band of microwave radiation, most dielectric materials, such as clothing, paper, plastic and leather are nearly transparent. Therefore, microwave imaging systems have the ability to penetrate clothing to image items concealed by clothing.

US patent application publication no. 2006109160 describes a microwave imaging system having one or more fixed scanning panels in relation to a portal through which a human subject is able to walk. Each scanning panel is provided with an array of microwave antenna elements that are capable of being programmed with a respective direction coefficient to direct microwave radiation from a microwave source toward a position on the target, and the antenna elements are further capable of being programmed to receive reflected microwave illumination reflected from that position on the target. A processor measures the intensity of reflected radiation to determine the value of a pixel within an image of the target and multiple heams can be directed towards the target to obtain corresponding pixel values for use by the processor in constructing the image.

Whilst such a system is effective in alleviating some of the problems associated with conventional security imaging systems, it can be cumbersome to install and extremely inconvenient to move once installed. Thus, there is no possibility of reconfiguring such a system dynamically in order to image another region. Furthermore, it would be desirable, at least under some circumstances, for the imaging system to be invisible, so as to avoid putting a potential suspect on notice that they are being security checked.

It is therefore an object of the present invention to provide an improved microwave imaging system that addresses the above problems and aims.

In accordance with the present invention, there is provided an imaging system for imaging a defined region, the system comprising: a plurality of portable active RFID tags distributed around said region for generating a plurality of RF signals and directing said signals into said defined region and for receiving RF signals reflected from said defined region; and

- means for extracting image data from said received signals and constructing a corresponding image.

In a preferred embodiment, each of said RF signals comprises one or more RF pulses, and more preferably each of said RF signals comprises a plurality of linear frequency modulated pulses, preferably transmitted by an antenna array, which in combination form a directional beam of a predetermined beamwidth as well as bandwidth.

In a preferred embodiment, the system comprises a (preferably remotely located) controller for selectively controlling one or more respective parameters of said RF signals, which parameters may include pulse width and/or the delay in transmission to any of said antennas. The controller is preferably connected to the RFID tags through a separate wireless link. Preferably said controller is equipped with a graphical user interface, and is configured to be operated by a skilled operator.

Each said portable RFID tags may comprise a remotely programmable memory and said controller is preferably configured to selectively reprogram said memory so as to control one or more respective parameters of said RF signals.

Preferably, said image data extraction means comprises implementations of Synthetic Aperture Radar imaging algorithms running on the remote controller and optimized dynamically by the operator, with the help of the graphical user interface mentioned.

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating the principal components of a system according to an exemplary embodiment of the invention; and

Figure 2 is a schematic diagram illustrating an RFED tag for use in the system of Figure 1.

RFID (Radio-frequency identification) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. Most commonly used passive RFID tags contain at least two parts. One is an integrated circuit for storing information. The second is an antenna for receiving and re-transmitting the signal, said signal being modified during the interval between reception and retransmission in accordance with the stored information. The proposed extended RFID tags, being active, will additionally have the capability to transmit microwave pulses, and store the characteristics of the echo of these pulses from a target, thus acting as the front- end of a rudimentary low-power short-range Radar.

Referring to Figure 1 of the drawings, an imaging system according to an exemplary embodiment of the present invention comprises a plurality of portable RFID tags 10, each of which is carried by a security agent or similar. The tags 10 are not conspicuous and therefore, can be carried without the knowledge of a target to be scanned. Such security agents would typically be present at airports, railway stations, football games and other large gatherings of people, where they can position themselves around an area 12 to be scanned.

Thus, the area to be scanned can be relatively easily changed simply by moving the respective security personnel carrying the tags 10. One or more stationary RFID tags 11 may also be provided at the venue, as required. A remote central controller 14 controls the overall operation of the system thus configured.

Referring in addition to Figure 2 of the drawings, each RFID tag 10 comprises an RF source 16 for generating RF pulses. The RF pulses thus generated are preferably linear FM pulses, wherein each pulse is a modulated pulse with the carrier frequency rising linearly from the beginning to the end of the pulse. Typically, for a 0.1 microsecond pulse, the carrier frequency might rise linearly from 24 GHz at the start of the pulse to 25 GHz at the end of the pulse.

The RF source thus generates an RF signal and feeds it to an array of antenna elements 18 (say 3 x 3). Thus, a linear FM pulse is fed to each antenna element 18 of the array (with suitable respective delays), and the signals subsequently transmitted by the antenna elements 18 into the region 12 to be imaged, in combination, form a directional beam 20 of a predetermined beamwidth and bandwidth.

The beams 20 thus transmitted into the region 12 to be scanned are reflected back to the antenna elements 18 of each array and the return signals captured at the receiving antenna 19 (which are relatively very weak) need to be downconverted and processed before information about the reflector (e.g. a concealed metallic weapon) can be extracted. For this purpose, each tag 10 includes a LNA (low noise amplifier) 22, LO (local oscillator) 24 and downconverter 26 for capturing the return signal, and an analogue-to-digital converter 28 and digital signal processor 30 for extracting the above-mentioned information. This information may be relayed to a remote reader where images can be built up and analysed. The data from a single tag will contain too little information to build up an image, since the aperture size of the antenna array is small, and the power level is also low. However, working in unison, the tags provide a synthetic aperture which is large enough to obtain a useful image. The construction of the image from the obtained data can be realized by the well-known synthetic aperture radar image processing techniques. It will be appreciated by a person skilled in the art that it is relatively easy for the tags to identify their relative positions to the controller, by means of, for example, phase interferometry for direction and round-trip delay measurement for range. These will be constantly monitored by one of the stationary tags (called 'ranging tag'), and the data will be relayed to the controller. For this scheme to work, the tags will carry unique identifying codes to identify themselves to the ranging tag.

In principle the minimum range will be defined by pulse width (for simplicity, reception should begin after transmission ends), while the maximum range will be defined by available power, however it is expected that for the application of interest the system will be used at a range of 5 - 30 metres. The role of the 'reader' of a conventional RFID system is here played by the remote computing and visualization station (which includes the central controller 14) that may be in the form of, for example, a suitable pc or laptop computer.

Finally, each tag 10 includes a simple digital circuit 32 and remotely programmable memory 34 to control the operation of the tag 10. The remote central controller is preferably configured to reprogram the memory 34 of each tag so as to change the sequences of signal transmission/reception, as well as modulation parameters (e.g. beam width, delays to the various antenna elements, etc). This enables a user to concentrate on a particular region, or have a broad view, or rapidly switch beam directions etc. The communication between the controller and the tags is achieved through a separate RF link 36 with its own antenna 38, operating preferably at the ISM band around 2.45 GHz. It may be noted that with rapid advances in CMOS-RF technology taking place, it is expected that the entire circuitry on the tag other than the antennas may be implemented as a single CMOS chip, bringing the cost of such an active tag close to the cost of a typical currently used passive RFID tag. Even using off-the-shelf MMIC and digital components, the cost of a tag is expected to be small enough to bring the complete system cost far below the price of any presently offered microwave imaging system.

In a typical environment an intended recipient tag may not be in the line of sight of the controller. In this case data may easily be relayed from one tag to another to the correct recipient, using the unique identifying codes of the tags.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A microwave imaging system for imaging a defined region, the system comprising: a plurality of portable active RFID tags distributed around said region for generating a plurality of KF signals and directing said signals to said defined region and for receiving RF signals reflected from said defined region; and means for transmitting the characteristics of said received signals to a remote processing station through a wireless communication channel, extracting image data from said received signals and constructing a corresponding image.

2. A system according to claim 1, wherein each of said RF signals comprises one or more RF pulses.

3. A system according to claim 2, wherein each of said RF signals comprises a plurality of linear frequency modulated pulses which, in combination, form a directional beam of a predetermined beamwidth and bandwidth.

4. A system according to claim 3, wherein said pulses are transmitted from an antenna array.

5. A system according to any one of claims 1 to 4, comprising a controller for selectively controlling one or more respective parameters of said signals.

6. A system according to claim 5, wherein said parameters include pulse width and/or the delay in transmission to any of the elements of said antenna array.

7. A system according to claim 5 or claim 6, wherein said controller is equipped with a graphical user interface for aiding an operator in setting said parameters.

8. A system according to claim 5 or claim 6, wherein said portable RFID tags comprise a remotely programmable memory and said controller is configured to selectively reprogram said memoiy so as to control one or more respective parameters of said RF signals.