Improvised explosive devices (IEDs) are rudimentary bombs that generally consist of commonly found non-military materials. Although common IED threats include roadside bombs, suicide bombers are another emerging problem in the IED arena. Suicide bombers carrying personal-borne IEDs (PBIEDs) are extremely hard to detect or stop. Because no IED emplacement is necessary, a suicide bomber can quickly strap on an IEDladen vest and move to the kill zone. Placed in the proper urban environment, this weapon is capable of inflicting serious structural damage and killing hundreds of people in mere seconds.

Research has been done to detect magnetic materials used in IEDs in urban environments using a wireless sensor network. Using the magnetic detectors in the wireless sensor nodes, magnetic behaviors and patterns are analyzed to differentiate a person carrying an IED and a person possessing magnetic material like jewelry or keychains. Using wireless sensor nodes instead of standard metal detectors enables the detectors to remain hidden to outside observers. The small nodes easily blend into the indigenous environmental settings to provide stealth.

All IEDs require a power source to initiate the weapon. Most initiators are battery- operated electrical devices, but there are other means of initiation. Spring-loaded initiators require no electrical power to function. The IED initiator detonates the weapon and begins the bombing sequence. Common initiators are blasting caps and fuse igniters. Electrical initiators can be triggered in various ways, including a button, radio frequency, and optical. The IED switch arms the weapon after the initiator sequence begins. The switch could be an arming switch, fuse, or both for redundancy. Once the IED is armed, the internal circuit is complete and detonation occurs shortly thereafter.

One of the first solutions to detect IEDs was to detect the frequency spectrum used by the IED initiator devices. By correctly analyzing the frequency spectrum used by the remote triggers, troops successfully jammed the frequencies and prevented IEDs from being triggered. Electromagnetic pulse jamming also destroyed IED circuitry.

Unmanned Aerial Vehicles (UAVs) use mounted cameras to take pictures of probable IED areas and then come back for more images. The Buckeye camera mounted on a UAV uses an electro-optical sensor capable of producing threedimensional images. Using imagery software or the human eye, the pictures are analyzed against pictures from the same area, but taken at a different time. Scrutinizing the images to the nearest pixel, experts can determine if suspicious IED activity has occurred in a region.

Magnetic Wireless Sensor Networks

A wireless sensor network (WSN) is formed from a series of wireless network nodes or motes, generally in an ad-hoc configuration. Each node contains a small processor to handle sensing duties. Nodes are able to relay information using a predetermined routing protocol such as ZigBee. Due to the wireless constraints, each WSN node needs a self-contained power source such as batteries.

A standard WSN uses the nodes for their physical sensing capabilities in conjunction with a base station, which receives information from the nodes and passes it to another source to process the data. Since the base station receives input from the WSN nodes, it has higher power requirements and must always coordinate data delivery out of the network. Each node contains sensing capabilities appropriate for the network application and needs. Nodes cannot process or analyze the information, but can forward information to either another node or the base station.

The mesh-networking feature of the motes allows them to communicate with each mote in the network. Additional motes can be added to the network or motes can be removed from the network seamlessly. The magnetic detection capability within the motes uses a twoaxis magnetic field sensor to detect electronic voltage perturbations around the sensor. The passive infrared sensors detect dynamic changes in the thermal radiation environment within immediate vicinity of the sensor. The mote also contains a dormant microphone to detect acoustic changes within its environment. Each mote contains four magnetic and passive infrared sensors placed within a cubicle housing to provide nearly 360-degree coverage.

Placing a WSN by entry and exit points of urban buildings provides a stealthy means of detecting IED materials. The accuracy of the mote detectors allows observers to distinguish normal routines from suspicious IED activities. The WSN can be set up to alert security officials of possible IED activity, and used in conjunction with standard surveillance methods to provide a more complete and accurate depiction of actual activities taking place in real time.

WSN Deployment

In IED detection, the network looks for patterns of activity that appear suspicious and raises alerts when a certain level of confidence has been achieved in the prediction. Ferrous materials compose a large number of IEDs, making magnetic sensors a logical choice for detecting IEDs. However, magnetic sensors alone may not be sufficient in confirming IED presence because the network may be susceptible to false positives (the network falsely detecting IEDs) or false negatives (failure of the network to detect IED). Using a combination of different sensor modalities could mitigate both possibilities.

The urban environment presents many challenges. Large crowds provide many variables unbeknownst to the planning process. For example, the presence of a metal shopping cart in a grocery store is a common occurrence; thus, another reason to use metallic sensors in conjunction with other detection characteristics. Another issue is the emplacement of the wireless sensor nodes. Although relatively small, they must be carefully placed to avoid accidental detection.

To find the optimum deployment scenario, various tests were conducted using the motes, 18" orange safety cones to elevate the motes from the ground, and steel buckets and staples to simulate metallic IED material. The initial setup kept the metal bucket in a fixed position and the mote was walked along a straight-line path over the bucket. The spacing was too great and the motes had trouble detecting magnetic material unless extremely close to the mote.

To test if large amounts of metal at a specified distance would give the same magnetic reading as smaller amounts of metal at a closer distance, a keychain was placed 6" from the mote. The keychain gave readings just as strong as a bucket placed 3' away from the mote.

Another test used a basic rectangular configuration of motes placed at fivemeter intervals. A steel bucket was traversed through various paths around the motes. These various paths often produced dead spots. The mote was then placed 1.5' away from a wall. Metal was placed at varying heights of the wall to determine how high the metal could be detected by the sensor mote. Results showed that a 2.5' height was the maximum distance that still provided consistent results.

In another test, two motes represented an entrance or doorway to an urban building. The motes were placed at 2' intervals. This interval was later increased to 4, 8, and 12'. A subject carrying a metal bucket traversed the network. Final tests were conducted with two and three subjects traversing the network with differing amounts of metal. The 2 and 4' configurations were tested as a means of providing network redundancy and avoiding blind spots inside the mote area. The motes were able to detect strong magnetic signals from the bucket and provided many data points for detection by the mote software.

The mote intervals were then expanded to 8 and 12'. The 8' configuration still provided reliable and consistent results. The 12' configuration showed some readings, but was not consistently able to detect metal from 6' away. So, an 8' interval between motes was optimal for the six-mote network, providing redundancy of motes while avoiding blind spots within the network (see figure).

Overall, a wireless sensor network using only magnetic detection is not a complete solution for the IED problem. The strengths of a WSN include low power requirements, adaptability, and relative ease of use. For controlling entrances to buildings such as shopping malls, places of worship, or office buildings, a cost-effective WSN implementation would be possible. It would require a configuration that leaves no holes in detection and provides redundancy to prevent network failure.

This article was written by Lieutenant Matthew P. H. O’Hara of the United States Navy. For more information on the Navy’s IED detection technologies, click here.

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