The security checks passengers undergo before boarding flights are changing, the aim being to scan more passengers while reducing waiting times - and, of course, improving threat detection to ensure safety and security. Walk-through metal detectors (WTMD) are a familiar pre-flight security check. Although familiar, they are far from ideal: frequent “false positives” oblige security staff to perform many manual checks that are labor-intensive and detract from the traveler’s experience. Of more concern, the equipment cannot detect plastic or liquid explosives or ceramic blades.
As threats are evolving, so must the security techniques, while achieving important practical and privacy goals:
- Privacy is a key concern. New equipment and procedures must respect the privacy and dignity of travelers and not place security staff in awkward or stressful situations.
- Safety is critical. Scanners cannot use radiation harmful to passengers or security staff.
- Checks need to be completed quickly, as the average time from front door to departure lounge is an important metric for airports.
- Cost is always an issue. The equipment must be affordable and not require additional staff.
- A less obvious concern is size; space in busy airports is scarce and a premium. A security system involving large or power-hungry equipment, extra rooms or dedicated security areas is impractical.
mmWave scanners meeting these requirements offer a better alternative to the traditional WTMD and are already being introduced at leading airports around the world. From the passenger’s perspective, the difference between the new scanner and the familiar metal-detector gate is the requirement to stand still for a few seconds inside the machine, facing the scanner with the arms away from the body. In addition to detecting a wider range of threats involving non-metallic objects or substances, the new scanners reduce the rate of “false positives,” which shortens the average time to screen each passenger.
To protect the privacy and comfort of airline customers and ensure a stress-free working environment for security staff, the new scanners take advantage of advanced machine-learning techniques to avoid inspecting actual body images. They use several technical innovations to meet the goals for scan time, equipment size and cost.
The Optimum mmWave Band
Scanning with mmWave is intrinsically well-suited to airport security. They have no ionizing effect on the body’s cells and are considered harmless to staff and passengers. In comparison, ultrasonic scanning, although harmless, has a very short range; a coupling medium - usually a gel - is required to ensure image quality. This is obviously impractical in an airport.
Wavelengths in the 1 to 10 mm range are suitable for non-contactless scanning and allow a suitable combination of penetration depth and spatial resolution to detect objects airline passengers may seek to conceal beneath clothing. In choosing the best wavelength, there is a trade-off between penetration and spatial resolution. A spatial resolution of about 2 mm is considered adequate for security applications, so E-Band (60 to 90 GHz with wavelengths of 5 to 3.3 mm) provide reliable object recognition and adequate penetration to reach the surface of the skin. Within this band, working in the 70 to 80 GHz range allows equipment designers to use existing components and knowledge from automotive radar applications, which shortens the development time.
Passive vs. Active Scanning
mmWave images can be captured passively by detecting the characteristic radiation of an object and the natural background radiation. This is suitable for equipment used outdoors, where the background radiation temperature is typically below 100oK. Indoors, however, the radiometric contrast between the object and background is much lower. Although this can be addressed using cooled detectors, passively detected radiation can be confused with thermal noise, resulting in a lack of depth information about the object. Obviously, this is not ideal when the goal is to classify objects quickly and accurately and identify concealed weapons, while reliably avoiding false positives.
For this reason, active scanning is preferred for security systems. This involves illuminating the subject by transmitting low-power mmWave radiation. With the high water content of human tissue, the body acts as a strong reflector. Accurately characterizing the reflections enables the system to identify various objects concealed against the skin. With active scanning, antenna positioning is critical to minimize the effects of unwanted reflections. Where conventional industrial applications using active mmWave imaging operate at far-field distances, long-range imaging is impractical to find small threats in security applications. Hence, airport security scanners operate at close range.
3D from 2D
To reconstruct an accurate 3D image of a scanned object requires sampling a 2D aperture with a broadband measurement signal at each selected transmitter-receiver combination. The transmitter/receiver design must be optimized both for depth and spatial resolution. For example, the high range resolution needed to identify thin objects, such as plastic explosives formed in sheets, requires a large signal bandwidth and corresponding short pulse duration.
There are several ways to achieve the required spatial resolution. Conventional mechanical scanning is not well-suited to the fast cycle times needed for the mass screening of airline passengers. The typical alternative involves dense monostatic antenna arrays that require large numbers of transmitter/receiver units, resulting in equipment that is extremely expensive. Rohde & Schwarz overcame this challenge by combining synthetic aperture algorithms from radio astronomy with virtual aperture techniques to create a new form of multistatic 2D array. Such arrays comprise multiple clusters of transmit and receive antennas in a novel array architecture. Digital beamforming algorithms are applied to weight each antenna with suitable phase and amplitude factors to create an electronically optimized aperture. Using a cost-effective sparse antenna array, the resulting system can achieve good image quality at close range, with minimal ambiguities.
In the multistatic array, each transmitter sequentially illuminates the volume in front of the system, with all receive antennas activated simultaneously to ensure coherent sampling of the reflected field. Subsequent processing calculates the reflections and applies the necessary error correction. Compared to a monostatic array, multistatic imaging requires significantly fewer channels, while parallelizing the data acquisition to allow near real-time performance. The resolution of the Rohde & Schwarz scanning system is approximately a half-wavelength, namely 2 mm. For a given image resolution, the speed of a digital beamforming system depends mainly on the number of measurements and the complexity of the image formation algorithms. This enables system performance to be improved by leveraging successive generations of DSP ICs with higher clock speeds and greater computational parallelism.
To realize the system, Rohde & Schwarz developed signal sources that generate coherent RF and receiver local oscillator (LO) signals, which are needed to coherently operate the transmitters and receivers. 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 20 GHz range and distributed to the clusters. At each chip, the RF and LO signals are quadrupled to the operating frequency and distributed to each of the four channels. The antenna design is optimized to ensure a small footprint and high bandwidth. Figure 1 shows a transmit/receive panel which integrates approximately 100 transmit and 100 receive antennas.
Based on the innovative sparse-array design, a full-size body scanner can be realized with about 12,000 channels. Although this is far less than would be required to achieve a similar system using conventional antenna array and imaging knowhow, many discrete front-ends would be needed using current commercial RFICs, which have been developed for systems with few channels. Practical space constraints demand higher integration, and the RF front-end must be closely integrated with the RF signal source and antenna to minimize interface losses at mmWave frequencies. As suitable modules were not available commercially, Rohde & Schwarz worked with Infineon to produce a custom RF front-end chipset comprising a four-channel transmitter/receiver MMIC fabricated with Infineon’s SiGe:C bipolar process. The MMICs are carefully packaged to maintain the bandwidth and reduce production cost (see Figure 2).
Protecting Privacy with AI
The security system can image features as small as a few millimeters and can show depth variations down to 50 microns. The reconstruction block automatically analyzes the image data using dedicated and optimized machine-learning algorithms, tailored for security scanning. Each part of the 3D image is analyzed and observed to decide if any location looks anomalous to usual conditions. The algorithms are also trained to be more accurate finding relevant threats, including but not limited to explosives, guns and knives.
The accuracy of these machine-learning algorithms enables the system to reliably identify prohibited items based solely on the data analysis, and no visible body image is created at any point in the system. Any detected threat or unusual object is highlighted on-screen to security staff by indicating the location on an avatar (see Figure 3). While protecting privacy, this also provides a reliable guide for security staff to quickly deal with a situation appropriately. The captured mmWave data is discarded as soon as the analysis is complete.
mmWave scanning is an effective threat detection technology. To realize a practical and cost-effective solution for airport security, Rohde & Schwarz has addressed the technical and privacy challenges. The resulting scanner uses innovations such as multistatic imaging with sparse antenna arrays, advanced multi-channel MMICs and RF modules, high performance parallel processing and machine-learning techniques.
The Rohde & Schwarz QPS family are the first commercial security scanners to achieve these exceptional technical advances, quickly receiving praise from airport security agencies worldwide (see Figure 4). Advanced mmWave technology will provide passengers with shorter security queues, less intervention by security staff and safe flights.