Could Radar Be a More Cost-Effective Security Screening Alternative to X-Rays?

By: Damien Clarke
Lead Consultant

10th October 2019

5 minute read

Home » antenna

A key task in the security market is the detection of concealed threats, such as guns, knives and explosives. While explosives can be detected by their chemical constituents the other threats are defined by their shape. A threat detection system must, therefore, be able to produce an image of an object behind an opaque barrier.

X-rays are probably the most commonly known technology for achieving this and they are widely used for both security and medical applications. However, while they produce high-quality images, x-ray machines are not cheap and there are health concerns with their frequent use on or in the vicinity of people.

An alternative to x-rays often used at airports for full-body screening are microwave imaging systems. These allow the detection of concealed objects through clothes though the spatial resolution is relatively low and objects are often indistinguishable (hence the requirement for a manual search). The ability to detect and identify concealed items can, therefore, be improved by using a high-frequency mm-wave (60 GHz) system.

Plextek has investigated this approach through the use of a Texas Instruments IWR6843 60 – 64 GHz mm-wave radar which is a relatively inexpensive consumer component that could be customised to suit many applications. However, a single radar measurement only contains range information and not angle information. It is, therefore, necessary to collect multiple measurements of an object from different viewpoints to form an image. This is achieved through the use of a custom 2D translation stage that enables the radar to be automatically moved to any point in space relative to the target object. In this example, radar data was collected across a regular grid of 2D locations with millimetre spacing between measurements.

This large set of radar measurements can then be processed to form an image. This is achieved by analysing the small variations in the signal caused by the change in viewpoint when the object is measured from different positions. The set of range only measurements is then extended to include azimuth and elevation as well. In effect, this process produces a 3D cube of intensity values defining the radar reflectivity at each point in space. A slice through this cube at a range corresponding to the position of the box allows an image to be formed of an object that is behind an (optically) opaque surface.

In this case, a cardboard box containing a fake gun was used as the target object. Clearly, a visual inspection of this box would not reveal the contents, however, 60 GHz mm-waves can penetrate cardboard and therefore an image of the concealed object can be produced. In this case, the resulting image of the contents of the box clearly shows the shape of the concealed gun.

This example simulates the detection of a gun being sent through the post and automatic image analysis algorithms would presumably be capable of flagging this box for further inspection. This would remove the need for human involvement in the screening process for each parcel.

A more mature sensor system using this approach could be produced that did not require the manual scanning process but used an array of antenna instead. It would also be possible to produce similar custom systems that were optimised for different target sets and applications.

 

Acknowledgement

This work was performed by Ivan Saunders during his time as a Summer student at Plextek before completing his MPhys at the University of Exeter.

A key task in the security market is the detection of concealed threats, such as guns, knives and explosives. While explosives can be detected by their chemical constituents the other threats are defined by their shape. A threat detection system must, therefore, be able to produce an image of an object behind an opaque barrier.

X-rays are probably the most commonly known technology for achieving this and they are widely used for both security and medical applications. However, while they produce high-quality images, x-ray machines are not cheap and there are health concerns with their frequent use on or in the vicinity of people.

An alternative to x-rays often used at airports for full-body screening are microwave imaging systems. These allow the detection of concealed objects through clothes though the spatial resolution is relatively low and objects are often indistinguishable (hence the requirement for a manual search). The ability to detect and identify concealed items can, therefore, be improved by using a high-frequency mm-wave (60 GHz) system.

Plextek has investigated this approach through the use of a Texas Instruments IWR6843 60 – 64 GHz mm-wave radar which is a relatively inexpensive consumer component that could be customised to suit many applications. However, a single radar measurement only contains range information and not angle information. It is, therefore, necessary to collect multiple measurements of an object from different viewpoints to form an image. This is achieved through the use of a custom 2D translation stage that enables the radar to be automatically moved to any point in space relative to the target object. In this example, radar data was collected across a regular grid of 2D locations with millimetre spacing between measurements.

This large set of radar measurements can then be processed to form an image. This is achieved by analysing the small variations in the signal caused by the change in viewpoint when the object is measured from different positions. The set of range only measurements is then extended to include azimuth and elevation as well. In effect, this process produces a 3D cube of intensity values defining the radar reflectivity at each point in space. A slice through this cube at a range corresponding to the position of the box allows an image to be formed of an object that is behind an (optically) opaque surface.

In this case, a cardboard box containing a fake gun was used as the target object. Clearly, a visual inspection of this box would not reveal the contents, however, 60 GHz mm-waves can penetrate cardboard and therefore an image of the concealed object can be produced. In this case, the resulting image of the contents of the box clearly shows the shape of the concealed gun.

This example simulates the detection of a gun being sent through the post and automatic image analysis algorithms would presumably be capable of flagging this box for further inspection. This would remove the need for human involvement in the screening process for each parcel.

A more mature sensor system using this approach could be produced that did not require the manual scanning process but used an array of antenna instead. It would also be possible to produce similar custom systems that were optimised for different target sets and applications.

Acknowledgement

This work was performed by Ivan Saunders during his time as a Summer student at Plextek before completing his MPhys at the University of Exeter.

Further Reading

By Marcus C. Walden

Abstract: This paper presents the antenna G/T degradation incurred when communications systems use very inefficient receive antennas. This work is relevant when considering propagation predictions at HF (2-30 MHz), where it is commonly assumed that antennas are efficient/lossless and external noise dominates over internally generated noise at the receiver. Knowledge of the antenna G/T degradation enables correction of potentially optimistic HF predictions. Simple rules-of-thumb are provided to identify scenarios when receive signal-to-noise ratios might be degraded.

Read more…

By Marcus C. Walden

Abstract: This paper describes the design and characterization of a frequency-scanning meanderline antenna for operation at 60 GHz. The design incorporates SIW techniques and slot radiating elements. The amplitude profile across the antenna aperture has been weighted to reduce sidelobe levels, which makes the design attractive for radar applications. Measured performance agrees with simulations, and the achieved beam profile and sidelobe levels are better than previously documented frequency-scanning designs at V and W bands.

Read more…

All 'Things' to be considered in IoT

All ‘Things’ to be Considered in IoT

Richard Emmerson - Senior Consultant, Communications Systems

By: Richard Emmerson
Senior Consultant, Communications Systems

3rd May 2017

Home » antenna

So you have a great idea for an internet connected ‘Thing’. You’ve done the business plan, you’ve raised some investment, or maybe you’re staking your own money. All you have to do now is connect your ‘Thing’ to ‘The Internet of Things (IoT)’ and get the product into the market.

Well, there are a few things you should consider before you jump in.

How will the ‘Thing’ connect?

Surely that’s simple, everyone’s using LoRa (long-range, low-power radio), so I can buy some LoRa modules, connect them to my ‘Thing’ and I’m done.

Well, yes and no.

Range, Data Rate & Power Consumption

With any communications system, there is a direct trade-off between range, data rate and power consumption. LoRa is potentially a great system for IoT. When properly designed, it can achieve long range (typically 2 km in urban areas and line of sight in rural areas) and have a battery life that can last for years. However, data rates are limited to between 0.3 kbps to 50 kbps; with the longest range achieved at the lowest data rate. In the EU 868 MHz band, the duty cycle is also limited to 1%, meaning, at the lowest data rate, only 51 user bytes can be sent every 245 seconds. This is fine for a smoke alarm but unsuitable for a security camera.

For higher data rate applications, a 3G or 4G modem module may be a better choice, provided the power is available. For power limited systems, there is also the Narrowband IoT system, which uses the 4G mobile network with data rates between 20-250 kbps and offers impressive battery life.

What about base stations?

BaseStationLoRa can be used in a peer-to-peer mode (communication between nodes). To connect to the internet though requires some kind of base station. This could be a local base station installed in the home or office, or a wide area base station (LoRa-WAN). You might choose to supply customers with their own low-cost base stations, or take advantage of public networks such as ‘The Things Network’.

An alternative may be to use the ’Sigfox’ system. This has similar performance to LoRa but for a small subscription fee accesses an international network of base stations owned and managed by Sigfox. Unlike Sigfox and cellular systems, LoRa has the advantage that if there is no coverage then you can simply add your own base station.

What about the Antenna?

AntennaBoardThe antenna is a key part of any wireless system and is an area where many developers face problems. In order to work efficiently, antennas need an effective area which is made up of the antenna itself and the circuit board it is connected to. Look carefully at the datasheet for that tiny 868 MHz ‘chip’ antenna and you are likely to see that it requires a PCB of approximately 90 mm length.

However, this poses a problem for small devices operating at 868 MHz, as the antenna is unlikely to be efficient, and that 2km range you expected just reduced to 500 m or less. The antenna may also become de-tuned by the presence of breaks in the PCB ground plane, nearby components and caseworks require a matching network to compensate for these effects. For really small devices, it may be worth considering Bluetooth, which with its higher operating frequency of 2.4 GHz requires a smaller PCB, and, with the release of Bluetooth 5, can be used for local area networks.

So that’s it?

Well, not quite. LoRa and Sigfox use the licence free 868 MHz ISM band in Europe and 915 MHz band in the US. Both of which are prone to interference from other users. There is also the platform, encryption, data ownership, and regulatory approvals to consider.

So you have a great idea for an internet connected ‘Thing’. You’ve done the business plan, you’ve raised some investment, or maybe you’re staking your own money. All you have to do now is connect your ‘Thing’ to ‘The Internet of Things (IoT)’ and get the product into the market.

Well, there are a few things you should consider before you jump in.

How will the ‘Thing’ connect?

Surely that’s simple, everyone’s using LoRa (long-range, low-power radio), so I can buy some LoRa modules, connect them to my ‘Thing’ and I’m done.

Well, yes and no.

Range, Data Rate & Power Consumption

With any communications system, there is a direct trade-off between range, data rate and power consumption. LoRa is potentially a great system for IoT. When properly designed, it can achieve long range (typically 2 km in urban areas and line of sight in rural areas) and have a battery life that can last for years. However, data rates are limited to between 0.3 kbps to 50 kbps; with the longest range achieved at the lowest data rate. In the EU 868 MHz band, the duty cycle is also limited to 1%, meaning, at the lowest data rate, only 51 user bytes can be sent every 245 seconds. This is fine for a smoke alarm but unsuitable for a security camera.

For higher data rate applications, a 3G or 4G modem module may be a better choice, provided the power is available. For power limited systems, there is also the Narrowband IoT system, which uses the 4G mobile network with data rates between 20-250 kbps and offers impressive battery life.

What about base stations?

BaseStationLoRa can be used in a peer-to-peer mode (communication between nodes). To connect to the internet though requires some kind of base station. This could be a local base station installed in the home or office, or a wide area base station (LoRa-WAN). You might choose to supply customers with their own low-cost base stations, or take advantage of public networks such as ‘The Things Network’.

An alternative may be to use the ’Sigfox’ system. This has similar performance to LoRa but for a small subscription fee accesses an international network of base stations owned and managed by Sigfox. Unlike Sigfox and cellular systems, LoRa has the advantage that if there is no coverage then you can simply add your own base station.

What about the Antenna?

AntennaBoardThe antenna is a key part of any wireless system and is an area where many developers face problems. In order to work efficiently, antennas need an effective area which is made up of the antenna itself and the circuit board it is connected to. Look carefully at the datasheet for that tiny 868 MHz ‘chip’ antenna and you are likely to see that it requires a PCB of approximately 90 mm length.

However, this poses a problem for small devices operating at 868 MHz, as the antenna is unlikely to be efficient, and that 2km range you expected just reduced to 500 m or less. The antenna may also become de-tuned by the presence of breaks in the PCB ground plane, nearby components and caseworks require a matching network to compensate for these effects. For really small devices, it may be worth considering Bluetooth, which with its higher operating frequency of 2.4 GHz requires a smaller PCB, and, with the release of Bluetooth 5, can be used for local area networks.

So that’s it?

Well, not quite. LoRa and Sigfox use the licence free 868 MHz ISM band in Europe and 915 MHz band in the US. Both of which are prone to interference from other users. There is also the platform, encryption, data ownership, and regulatory approvals to consider.

Save

Further Reading

Save

By Marcus C. Walden

Abstract: The design of a 16-element waveguide array employing radiating T-junctions that operates in the Ku band is described.

Amplitude weighting results in low elevation sidelobe levels, while impedance matching provides a satisfactory VSWR, that are both achieved over a wide bandwidth (15.7-17.2 GHz). Simulation and measurement results, that agree very well, are presented. The design forms part of a 16 x 40 element waveguide array that achieves high gain and narrow beamwidths for use in an electronic-scanning radar system.

I. INTRODUCTION: Design equations for optimum horn antennas have long been established [1]. This concept has been employed in an electronic-scanning ground surveillance radar system for nominal elevation beamwidths of 10° and 20°. Unfortunately, for narrower beamwidths, the optimum horn becomes impractically long for a man-portable system (e.g. for a desired 5° beamwidth, the length is ~1.5 m).

Because the antenna forms part of a 40-element array that defines the azimuth beamwidth, a 16-element waveguide array with a corporatefeed structure was selected to achieve the desired 5° elevation beamwidth with a short physical length.

Read more…