Millimetre-wave scanners protect our privacy in airports

New airport security screening technology using millimetre waves have to potential to improve threat detection without using body images which invade personal privacy, writes Dr. Sherif Sayed Ahmed

Scanner 1Long airport security queues are not fun for anyone involved. Waiting times can be stressful for travellers, who would rather be relaxing airside. The security staff themselves can feel pressure to work more quickly, which may lead to errors. And airport operators understand the need to maintain both security and the customer experience at a consistently high level.

On the other hand, airports must satisfy the general expectation that security should be faultless in preventing weapons or other dangerous or illicit objects being carried onto aircraft.

Security scanners now entering service in airports such as London City Airport make use of millimetre-wave technology to accelerate throughput and enhance threat detection while safeguarding the privacy of travellers.

Active scanning with millimetre waves is not harmful to the body, and can determine the presence and location of a threat accurately thereby reducing “false positives” that oblige security staff to carry out additional time-consuming checks.

Some millimetre-wave scanners have been trialled, but cycle times are slow and more importantly the systems can capture detailed images of the surface of the body underneath clothes.

Clearly, this is unacceptable to airport customers, and may also place unwanted data-protection obligations on authorities.

An ideal millimetre-wave scanner should be able to analyse the results collected in real-time, and raise an alarm if there is cause for concern, or discard the collected data immediately if the result is a “pass”.

The time taken to scan the entire body should be short, and RF transmitted power should be very low to provide the best possible assurance of safety for operators and customers. Moreover, an acceptable means of reporting suspected threats is needed, which does not invade the traveller’s privacy.

For example, a millimetre-wave scanner developed by Rohde & Schwarz uses active scanning, and by operating at frequencies in the 70-80GHz range, which is higher than conventional millimetre-wave systems, it can achieve the required detection resolution.

The system comprises two scanning panels, each containing eight groups of four transceiver clusters. The two panels are positioned to scan the front and rear of the body simultaneously, as shown in figure 1.

The scanner has a multi-static two-dimensional transceiver array that is designed to give a faster cycle time.

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. This processed data is then analysed in near real-time using machine-learning image-recognition algorithms.

In total, the system has over 12,000 RF channels. A high level of integration was needed to achieve the high throughput needed to complete front and rear scans within a few milliseconds, and to realise a reliable system within acceptable size and cost constraints.

Rohde & Schwarz worked with Infineon Technologies to produce a custom RF front-end chipset comprising 4-channel transmitter and receiver chips, illustrated in figure 2.

Scanner 2These chips are implemented as monolithic microwave ICs (MMICs) using a SiGe:C bipolar process, and integrate thin-film resistors and a metal-insulator-metal-capacitor with Al2O3 dielectric. Power consumption is approximately 150mW per channel for the transmitter and 180mW for the receiver, when fully activated. From a safety perspective, the RF transmitted power is hundreds or even thousands of times lower than that of a mobile phone, and so poses no risk to travellers or security staff.

Transceivers comprising the RF front-end chipset and digital back-end device are assembled as units that comprise four clusters with a signal distribution board, power supply, mechanics, and cooling system.

Four of these units are then connected to a central board to form a complete array. The complete imaging system comprises two of these arrays for simultaneous front and rear scanning. The two arrays are connected to an industrial PC via fast PCI-Express connections.

In addition, the floor of the system, beneath the passenger’s feet, is engineered using patented technology to help maximise the signal strength at the receiver antenna. It is designed as a passive system, featuring dielectric and conductive layers, to co-polarise the signal to the receiver antenna. This effectively creates a mirror surface that extends the illumination coverage of the system and enhances detection around the ankles.

The systems digital processing capability includes parallel analogue-to-digital conversion and image reconstruction kernels. When a scan is performed, the collected data is reconstructed on a per-cluster basis to minimise internal data-transfer rates.

Operating at up to 10.6 Tera operations/s, the full image is reconstructed in under two seconds. The system can image features as small as a few millimetres in size, and can show depth variations down to 50 micron.

The 3D-image data is analysed automatically using highly optimised and dedicated machine-learning algorithms. These algorithms are tailored for security-scanning tasks, and trained to ensure extremely accurate detection of relevant threats, including but not limited to weapons such as explosives, guns or knives.

If a suspected threat is detected, its location is indicated on a symbolic graphic of the human body, or avatar thereby eliminating any need to present actual body images.


Dr. Sherif Sayed Ahmed works for Rohde & Schwarz 


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