The following report was released in April 3, 2014 by the National Institute for Justice and was produced under contract by ManTech Advanced Systems International.

Through-the-Wall Sensors for Law Enforcement: Best Practices

• 19 pages
• March 2014

The National Institute of Justice (NIJ) Sensor, Surveillance, and Biometric Technologies (SSBT) Center of Excellence (CoE) has undertaken a best practices report of through-the-wall sensor (TTWS) devices for operation by law enforcement and first responder agencies in the United States. These devices use a form of radar to detect movement behind barriers. The ability to sense the presence of individuals through common building materials can be useful during rescue operations, law enforcement operations and other tactical scenarios.

This report provides advice, tactics and information related to the use of TTWS in operational settings. The information provides law enforcement individuals and organizations with a better understanding of the capabilities and limitations of available TTWS equipment. When put into practice, an agency can make the most of the technology and improve the outcome and safety of operational scenarios in which it is deployed. The best practices report focuses on the use of commercially available TTWS devices suitable for law enforcement or emergency response applications.

A Frequently Asked Questions section is included that provides answers to common questions or concerns that criminal justice practitioners may have before purchasing, during training or while using TTWS devices. The report provides key advice that should be considered each time a TTWS is deployed, including advice and information on training, basic measurement practices and known limitations.

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What is the purpose of Through-the-Wall Sensors (TTWS)?

TTWS devices are designed to increase situation awareness of law enforcement and first responders. TTWS devices should not be used as the only contributing factor for entry or justification for a cause of action.

Who can use TTWS?

TTWS operation is limited to law enforcement, emergency rescue or firefighting organizations. This equipment may be operated only for law enforcement applications, the providing of emergency services and necessary training operations.

What does the Federal Communications Commission (FCC) have to do with TTWS?

The FCC governs all devices that emit radio waves, and regulates which frequencies and power levels the devices are allowed to operate. Only TTWS devices that have undergone testing and received FCC certification are permitted to be used in the U.S. Note that federal agencies do not fall under FCC jurisdiction and are instead governed by the National Telecommunications and Information Administration (NTIA).

Does the FCC approve the use of TTWS?

Yes. TTWS operators or their agencies must have a license in the Public Safety Radio Pool under Title 47 Code of Federal Regulations, Part 90. TTWS operation can also be granted under an Experimental License to organizations conducting research, development or testing.

Are there any special steps necessary before a criminal justice agency can purchase a TTWS device?

No. The FCC does not regulate or control the sale of TTWS devices, but sets forth the rules under which they can be used. Only prior possession of the above-mentioned public safety radio license is required.

There is a vendor/website selling a TTWS device not mentioned in this report. Can my criminal justice agency purchase and use it?

Yes and no. Yes you can purchase it; however, only TTWS devices that have received FCC certification are permitted to be used in the U.S. The FCC does not regulate or control the sale of TTWS devices, but sets forth the rules under which they can be used. As a result, some vendors are selling TTWS devices that have not undergone proper certification or are imported from international manufacturers. FCC certification can be verified by requesting the FCCID from the vendor and using the FCCID to search the FCC database for authorized equipment (https://apps.fcc.gov/oetcf/eas/reports/GenericSearch.cfm).

Are there other lawful considerations prior to, or during use?

Most states require a warrant before using a TTWS device on a building, unless there is an active situation where there is no expectation of privacy, such as a known hostage or standoff situation.

Are there any health risks associated with TTWS device operation?

No. TTWS devices can be considered low power radios for health and safety purposes.

Will TTWS devices interfere with cellphones or radios?

No. The frequencies utilized are not within connected or neighboring bands with other communication devices. In addition, TTWS devices transmit at low power outputs.

How do TTWS devices work?

TTWS devices operate via electromagnetic signal (i.e., radio waves). TTWS devices transmit RF energy and monitor the reflected energy to detect motion. This concept is similar to the manner in which police use a traffic speed gun to monitor the speed of a moving vehicle.

Are there different kinds of TTWS devices? What is the difference between 1D vs. 2D TTWS devices?

Some devices provide one-dimensional (1D) information, which includes range and status (moving/breathing) of a target, while others provide a two-dimensional (2D) graphical display of multiple targets. This display can include the general shape and layout of a room, other solid structures within a room, and can detect the number and placement of items moving within a structure. Most 2D devices will only detect up to three persons in motion at one time.

What will TTWS devices detect? Animals? Objects such as fans?

TTWS devices will detect all motion, including trees and animals such as household pets. Smaller animals have a weaker radar cross-section and are not always detected. Even motion from inanimate sources, like fans and moving curtains, can trigger TTWS device detection.

Be aware of the environment, including your own team’s position. Even when positioned behind the TTWS, movement from team members can sometimes result in false signals.

Will TTWS devices detect an injured or sleeping person who is not moving?

TTWS devices report the ability to detect breathing. This is accomplished through detecting the motion of the chest upon inhale/exhale. However, successful detection is dependent on the amount of motion and its relative orientation to the device. As a result, a person who is a shallow breather, resting or in a prone position (e.g., sleeping) might not be detected.

Will TTWS devices detect more than one person?

Yes and no. All TTWS devices detect the signals from multiple targets. However, 1D devices only display the range to the target with the strongest RF return signal. Occasionally, an operator may see the display flash back and forth between two ranges, which can be an indication of more than one target in a room. This is not a reliable indicator, however, and should only be considered in an advisory capacity. 2D TTWS devices that present a map will display multiple targets up to the device limit (typically three limited to different simultaneous targets).

Do TTWS devices show an image of the person or object within a structure?

No. TTWS technology uses radar signals, not visible light or infrared optical methods; therefore, the devices are extremely limited in spatial details.

Where and what types of structures and places are TTWS devices less useful?

Detecting persons in houses is easier than apartments. Apartments, such as public housing with thick walls, often use rebar and metal doors. Newer office buildings and office construction use metal wall studs, which might interfere with measurements. Mobile homes are often constructed with solid aluminum exterior walls and therefore will block radar signals; they are the least suitable for TTWS.

Do TTWS devices operate through metal surfaces?

No. TTWS devices will not penetrate solid metal surfaces including walls, doors and home siding. This limitation even includes thin metal surfaces, like metal-backed insulation or some mobile home walls. Structures with internal support, such as rebar or chicken wire, may or may not allow operation, depending on the spacing of the metal as well as placement of the TTWS with respect to the metal.

How thick of a wall will TTWS devices penetrate?

TTWS devices easily penetrate most common building materials, including concrete, wood, brick, glass, adobe, etc., with thickness of up to 12 inches. The thicker the wall, the more signal loss (attenuation); this will result in longer measurement time (i.e., lag in the display of relevant results). The thickness and type of materials can affect the accuracy of the measurement as well.

Can TTWS devices detect through multiple walls?

TTWS devices can detect through multiple walls, floors and ceilings up to the depth noted within operational parameters. If the building structure is known, a trained operator can often determine whether a person is in a back room or a front room.

What can be done to better detect through home or office walls?

Some TTWS devices are designed to be held in a horizontal mode during operation; however, rotating the device 90 degrees (vertical) will often help an operator miss wall studs or other blocking objects.

How long should a scan take?

Although the depth of the scan affects the time of detection, it should take less than a minute for results from current commercially available systems.

What types of items affect TTWS device operation?

TTWS have limited field of views and can be blocked by certain items (e.g., metal); moving to various points along a wall can only help to develop a more complete understanding of the interior.

TTWS detects all motion, including trees and animals; therefore, always be aware of the whole environment.

Porous concrete blocks can absorb water from humid environments, which can sometimes affect the operation of a TTWS.

Do not move the device while scanning. The TTWS radar works by relying on the reflected signal returning to its exact point of origin to produce accurate measurements.

If one receives a jumping measurement (e.g., Moving and then Breathing; 7 feet distance and then 20 feet distance), then it is likely caused by metal interference (rebar, insulation backing).

Are there times when a TTWS device would not detect a person?

TTWS devices may miss a person standing immediately against the other side of the barrier wall, interpreting them as part of the wall by mistake.

TTWS devices cannot detect people hiding behind metal objects such as refrigerators, washers, dryers, etc.

Poured concrete walls thicker than 12 inches, such as those in parking garages, are too thick for TTWS to detect motion.

How large an area do TTWS devices cover? How far away does a TTWS device detect people?

Each type of TTWS device has different operational parameters, including field of view and distance. Detection range is dependent on exterior (and interior) wall material type and thickness, as well as the nominal range of the specific device. Currently, commercially available systems have Fields of View of 120 to 160 degrees and Maximum Ranges of 15 to 20 meters (50 to 65 feet).

Can a TTWS device scan a whole house or room?

TTWS devices have a conical field of view that will incorporate motion from both azimuth and elevation. TTWS devices detect moving targets within a specified angle and out to a certain distance. This enables an entire room to be covered in a single scan. A cone will extend +/- degrees in both the horizontal and vertical axis from the scanned point. This means that both walls and the ceiling of multiple story structures will be scanned. Scanning a two-story house will detect motion on the second floor, as well as on ground level. Depending on the size of the room being scanned, TTWS devices will likely detect motion beyond the room to potentially include the far exterior of the structure.

Taking multiple measurements at different places will establish a more reliable understanding of the interior structure.

Can TTWS devices operate in a stand-off mode, without holding them against a wall?

Some TTWS devices have been designed to easily operate in a standoff mode, while other TTWS devices will often work successfully in a standoff mode if the device can be kept motionless by the operator.

Do I need training to use a TTWS?

Yes. All operators should receive instructional training from the vendor or from an experienced user (i.e., train-the-trainer approach). In addition, although some interfaces are simple and appear to require no advanced training, practicing and learning how to use a TTWS device is always recommended so that you can interpret the signal and use the device effectively in the field. As with all tools and devices, practicing in the same manner and environment as the device would be used operationally is recommended.

How often should the batteries in a TTWS device be maintained?

Although some TTWS devices have a low-battery indictor, there is often a short time period between low battery being displayed and the device running out of power. Replacing batteries often and having replacement batteries handy is recommended. Many TTWS devices recommend a specific type of battery for optimal performance (e.g., Energizer lithium L91 batteries).

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