Improvised explosive devices made from common, inexpensive components are a serious threat to the lives of service personel and civilians in many parts of the world. Critical to reducing this threat is the development of tools and methods to effectively detect, locate, and prevent the initiation of IEDs at standoff distances. The fact that most IEDs employ radio receivers and/or low-cost electronic circuits suggests a relatively straight forward way of identifying and disabling these devices.

All active electronic devices radiate some electromagnetic energy, either intentionally or unintentionally. These electromagnetic emissions can be used to detect and locate electronic devices. Because wireless receivers must always be active and include at least some high frequency circuitry, they are usually good unintentional transmitters. Using a variety of inexpensive wireless receivers typical of the type that is often used to detonate command-initiated improvised explosive devices (IEDs), Missouri S&T has demonstrated the ability to detect, identify and locate these devices based on their unintentional radiated emissions.

Active electronic circuits generate electromagnetic emissions. Eliminating or disguising radiated emissions is very difficult for a realistic circuit design, especially low-cost designs. Over the past 15 years, researchers in the Missouri S&T EMC Laboratory have developed a number of tools and techniques to identify, locate and characterize sources of electromagnetic emissions in electronic products, even when those sources are in the vicinity of many other intentional and unintentional RF transmitters.

To demonstrate how electromagnetic emissions might be captured, processed and used to identify a particular electronic device, RF emissions were measured from a variety of devices typically associated with command-initiated IEDs. Devices included the receivers for remote-controlled toys, wireless doorbells, cordless phones, car alarms, and cell phones. A typical captured signal from a receiver is shown below.

The short-term FFT in this figure shows the magnitude of the measured electromagnetic radiation as a function of frequency (0-500 MHz, y-axis) and time (0-40 mS, x-axis). The unintentional radiation from this receiver is clearly visible in the short-term FFT. This radiation can be used to identify and locate the device. Similar, unique signals were measured on all of the devices we studied, allowing easy differentiation between the devices. Once identified, these devices are relatively easy to jam, locate and defeat.

To illustrate the unique nature of the RF emissions from various devices, we sampled these emissions and slowed them down by a factor of 100,000. This process converts radio frequencies to audio frequencies so that they can be "heard". These audio signals demonstrate very convincingly that these radio receivers can be uniquely identified from their unintentional radiated emissions. These files can be found at http://emclab.mst.edu/electronic_device_identification.html.

Reception of the relatively weak electromagnetic signals from the radio receivers at great distances, while challenging, can be accomplished through specialized antenna designs, measurement equipment, and signal processing techniques. The ability to receive extremely weak signals with some a priori knowledge of signal characteristics has been clearly demonstrated in other projects at the Missouri S&T Electromagnetic Compatibility Laboratory.

Detection and location of the transmitter, as well as the receiver, is also possible, but our research shows that most transmitters only radiate significantly when they are actively communicating with the receiver, presumably to initiate the explosive device. Also, the signal from the transmitter may be weaker than the unintentional emissions from the receiver in many practical situations.

While detection and location of the transmitter may not be useful for locating the IED before initiation, it may be useful for locating the position of the individual responsible for placing and activating the device. Another possibility for locating the individual responsible for placing the IED is by detecting the unintentional electromagnetic emissions from a video or digital camera. Many terrorists record their work. Preliminary measurements show that emissions from a video or digital camera can be readily detected and differentiated from other devices.