By Dr. Sherif Sayed Ahmed, Rohde & Schwarz GmbH & Co. KG, Test and Measurement Division
Fully-automated personnel scanning using a multi-channel millimetre-wave array, with intelligent image-recognition algorithms that perform real-time analysis on raw data, can protect passengers’ safety by identifying threats accurately and clearly, while also preserving travellers’ privacy by eliminating any need to render images on-screen or store data. Dr. Sherif Ahmed of Rohde & Schwarz explains how.
Introduction: Stepping up Airport Security
Airport authorities responsible for preventing attackers boarding planes need an alternative to the basic metal detector gates now widely used at entrances and boarding points. Screening is time consuming and imprecise, as the systems cannot determine the nature or exact location of a perceived threat. Every alarm must be investigated further by a human operative, and many are found to be “false positives”. Moreover, such equipment cannot detect non-metallic weapons like explosives, in particular, which are a major concern for today’s airline operators.
A fast and non-intrusive system is needed, which is able to “see through” clothes in order to detect concealed weapons or other proscribed items such as plastic explosives. Alternatives such as X-rays or ultrasound have a number of disadvantages: since X-rays have an ionising effect on body cells, some passengers – as well as security workers’ representatives – may object to routine or repeated exposure. Ultrasound can only work at extremely close range using a coupling medium, such as a gel, which is obviously impractical in an airline scanning situation.
Scanning with millimetre waves offers an alternative. Some types of millimetre-wave scanners have already entered service in locations such as airports and public buildings. Unlike X-rays, millimetre waves have no ionising effect on organic tissue, and are not harmful to the human body. No physical contact with the body is needed to capture a 3D image, and the scan can be completed quickly to allow high throughput as is required in a busy airport scenario. However, some key challenges must be overcome.
Design to Preserve Privacy
Because millimetre waves do not penetrate the body, the image captured can present a detailed view of the surface of the body beneath clothing. Although this is ideal for detecting almost any concealed object, including non-metallic weapons or explosive materials, there are obvious privacy concerns. Various initiatives have sought to address the issue: some authorities have introduced procedures to restrict the viewing and storage of captured images. However, the potential for breaches of such protocols to occur, or for misinformation about the handling of images to spread, could undermine confidence in the security systems that are intended to protect the travelling public. There is also the possibility for human error, when security staff are responsible for inspecting images individually at checkpoints, which may allow dangerous items to pass through.
Quick Personnel Security Scanner (R&S®QPS) presents a technical solution to these challenges by completely automating the detection of concealed threats. No human-body images are presented on a screen for an operator to view, and no image data is stored. If a concealed object is detected, the system indicates its location on an avatar instead of displaying the passenger’s own body (see Figure 1). Indicating the position of a suspect object in this way can help accelerate subsequent investigation, allowing harmless passengers to proceed quickly and those perceived as a threat to be detained confidently.
To achieve this, Rohde & Schwarz has introduced important new technologies with the R&S®QPS that permit cost-effective full-body scanning and signal processing at near real-time performance.
A variety of approaches are viable for millimetre-wave imaging. Passive systems can be effective for outdoor use, where the background is cool relative to the scanned object. However, in an indoor environment there is less contrast between background and object temperatures. For this reason, a system designed for use inside an airport has to use active illumination whereby a millimetre-wave signal of very low power is emitted towards the person, and receiving units analyse the resulting complex patterns. Known techniques for millimetre-wave imaging with active illumination utilise either mechanical scanning, which is unable to support the fast cycle times needed for high-speed airport-security systems, or dense monostatic antenna arrays that employ large numbers of antennas resulting in prohibitive high system cost.
The R&S®QPS uses a new approach that clusters and positions multiple transmit and receive antennas in a multistatic 2D array. Combined with digital beam-forming, these clusters create an electronically optimised aperture that allows a sparse and therefore low-cost antenna array to provide good image quality at close range. Close range imaging is optimal for human-body scanning due to the illumination limitations caused by the specular reflections out of the human skin at theses frequencies. Thus long –range imaging is becoming impractical to find the small threats of interest.
In addition, the R&S®QPS is designed to operate at higher frequencies than conventional millimetre-wave systems. Operating in the 70-80GHz frequency range allows higher signal bandwidth resulting in superior range resolution. Choosing this frequency range also allowed the development project to take advantage of design knowledge surrounding 77GHz radar systems that is already established in the automotive industry.
Custom Device Development
In the R&S®QPS system, the volume in front of the system is illuminated sequentially by each transmitter, and the complex reflected signals are simultaneously and coherently sampled by all receiver channels. The sampled data is then processed, reflections are calculated, and system error correction is applied. The reconstructed image data is then analysed in near real-time using machine-learning image-recognition algorithms.
To realise the system, Rohde & Schwarz has developed advanced signal sources that generate coherent RF and receiver local-oscillator (LO) signals, which are needed to coherently operate the transmitter and receiver units (Figure 2). The signal source uses Direct Digital Synthesis (DDS) and a highly stable Oven-Controlled Crystal Oscillator (OCXO) to achieve accurate phase stability. After the DDS, the transmit-signal frequency is multiplied to the 20GHz-range and distributed to the clusters. Upon arrival at each chip, the RF and LO signals are quadrupled and distributed to each of the four channels at the operation frequency. The antenna design is optimised to ensure small footprint and high bandwidth, and comprises a differentially-fed dipole, resonant aperture slots and a patch element.
A high level of integration in the RF front-end was critical to achieving a large security scanner comprising multiple imaging clusters that together create a system with over 12,000 channels. Rohde & Schwarz worked with Infineon to produce a custom RF front-end chipset, comprising 4-channel transmitter and receiver chips implemented as MMICs using a cost-effective SiGe:C bipolar process. These chips (see Figure 3) have enabled the performance needed to complete the front and rear scans simultaneously within milliseconds, in order to achieve the throughput rates necessary for a practical air-passenger scanning system.
Each device is able to operate from a single 3.3V supply, with a centre frequency of 75GHz and bandwidth of 10GHz. Power consumption per channel is approximately 150mW for the transmitter, and 180mW for the receiver when fully activated. The devices also integrate thin-film resistors and a metal-insulator-metal-capacitor with Al2O3 dielectric. In the longer term a CMOS-based solution may be considered, subject to improvement of the performance capabilities of CMOS at the required frequency and bandwidth.
A dedicated digital backend unit, including parallel analogue-to-digital conversion and image reconstruction kernels, has been developed (see Figure 4). Four clusters with a signal distribution board, power supply, mechanics, and cooling parts form together one unit. Four of these units are connected to a central board to form a complete array. Two such arrays are connected to an industrial PC via fast PCI-Express connections to create a complete imaging system capable of simultaneous front and rear scanning.
Because threat visibility is crucial for system performance, even the floor of the system, beneath the passenger’s feet, is engineered to help maximise the signal strength at the receiver antenna. The floor is made out of a dielectric layer bounded on metal surface. The design of the structure is made specifically to rotate the signal polarization in order to get co-polarized to the receiver antenna. This solution is based on patented technology and is not trivial to do. The surface is however fully passive and robust for daily operation. Hence the floor is used as a mirror surface in order to extend the illumination coverage of the system and to enhance detection around the ankles (see Figure 5).
Near Real-Time Analysis
Completing the signal chain, a digital backend performs digital down-conversion (DDC) and digital filtering in parallel to minimise measurement time. The data for the digital image is reconstructed on a per-cluster basis, in parallel, to help reduce the extremely high internal data-transfer rates that would otherwise be necessary. Even using this technique, the data-processing load can be as high as 10.6 Tera-Operations Per Second to allow full image reconstruction in under two seconds. The system is able to image features as small as a few millimetres in size, and can show depth variations down to 50 micron.
The image data is analysed automatically using highly optimised and dedicated machine-learning algorithms that are tailored for such security-scanning tasks. Each part of the 3D image is analysed and observed to decide if any location looks anomalous to usual conditions. The algorithms are also trained in a manner that makes them more accurate in finding relevant threats, including but not limited to weapons such as explosives, guns or knives.
Millimetre-wave scanners can detect threats concealed under clothing, but conventional systems revealing body details can potentially violate the privacy and dignity of travellers. Hence high quality imaging in combination with intelligent image processing, can insure the required performance in a convenient operation. By removing any need for security staff to view any of the images captured, and using advanced high-speed electronics capable of near real-time performance to eliminate any need to store such data, the R&S®QPS technology offers an effective solution that can be accepted by the travelling public while enabling faster security checks with fewer errors and greater ability to detect genuine threats.