Improvised explosive devices (IEDs) represent an increasingly deadly threat to both ground forces and civilians. As opposed to traditional warfare activities where the enemy is readily identifiable, insurgents have concentrated their efforts on building and supplying the explosives and trigger mechanisms needed to wage IED-based attacks--typically along roadsides targeting troop transport and supply line vehicles.

While early IEDs were primitive, using such things as wires, magnets, or pressure sensors to trigger an explosion, they have grown more sophisticated in their deployment. Contemporary versions often utilize wireless technology, allowing them to be triggered remotely by means of such devices as cellular telephones, radio-controlled (RC) toy remote controls, or even garage-door openers. To counter this dangerous use of wireless technology, the United States military has relied on high-power RF jammers to prevent wireless receivers onboard an IED from effectively detecting the trigger signal. To ascertain the effectiveness of these jammers, however, requires proper test equipment and measurement techniques.

This use of low-cost consumer wireless technologies in battlefield environments (or even in Homeland Security applications) requires detection and differentiation of RF signals to understand which are legitimate and which are threats that must be mitigated. Countermeasure responses for threat signals are usually accomplished by means of high-power RF jammers. Unfortunately, RF jammers are not an ideal solution, since they can affect a wide range of frequencies, thereby jamming intended as well as unintended signals from electronic devices and creating “signal pollution.” This can lead to problems with remotely operated aerial drones, Global Positioning System (GPS) receivers, communications and sensing systems, and other radio-based functions needed by military personnel in the field. One promising solution is the use of spectrum analyzers and long-duration storage systems to record, analyze, and create new waveforms and complex RF environments in support of more effective IED countermeasure technologies.

The Joint Improvised Explosive Device Defeat Organization (JIEDDO) was established in 2006 to address the IED threat using a combination of intelligence, training, and technology. Originally formed as the Army’s IED Task Force, the group has transformed into a combined joint service, interagency, multi-national program with a goal of leveraging all available resources and technologies in a coordinated campaign to defeat the IED threat.

Major investments have been made to develop and improve jammers for a variety of platforms, including ground vehicles, portable systems, unmanned aerial vehicles (UAVs), and unmanned ground vehicles (UGVs). In fact, market analyst Forecast International predicts a $28.4 billion market for RF jammers and other electronic warfare (EW) equipment over the next decade, representing more than 45,000 system sales. Initial efforts to evaluate the performance of IED jamming solutions were relatively simple (Fig. 1).

Jammer performance was determined by the system’s ability to prevent wireless activation from different distances and geometries between the jammer, IED, and detonating device. This basic pass/fail test approach, while effective at establishing a benchmark for a jamming system’s capability, did not provide the information needed to understand why a system would not work under certain conditions or what improvements would be most effective in enhancing performance (other than by adding more power).

To understand jammer effectiveness and reduce system development time, a number of key factors must be considered, such as the jamming-to-noise ratio, jamming thresholds, and jamming latency. These factors are needed for different signals or determining which jamming waveforms are most effective. It is also useful to consider how a device is being jammed. And it is critical to figure ways to maintain jammer effectiveness while maximizing interoperability with other mission critical assets.

An integrated RF capture and playback system offers strong capabilities for documenting, analyzing, and recreating complex RF environments. The latest spectrum analyzers offer wide calibrated acquisition bandwidth and wide dynamic range along with a digital output that can provide a full 16-b in-phase/quadrature (I/Q) digitized RF signal data stream. Real-time frequency triggering functionality allows the use of frequency or statistical event markers to identify spectrum of interest while it is being recorded.

A long-duration RF signal storage system can continuously record a spectrum analyzer’s signal stream for hours or days, without gaps or interruptions. The ability to time-stamp data and place event markers in the data stream works together with the spectrum analyzer to provide deep insight and rapid signal of interest collection. Data analysis tools can then be used to analyze information using event marker functionality to locate intermittent and hard-to-find signals, extract and export signals for detailed analysis, create signal libraries, and create repeatable custom dynamic spectrum loops for testing and evaluation. This combination helps to reduce the time needed to develop effective countermeasure solutions.

Analyzing potential threat environments often requires capturing RF signals in a complex, cluttered RF environment. This typically requires a spectrum analyzer with at least 100-MHz acquisition bandwidth and about 75 dB or better spurious-free dynamic range (SFDR). Another key requirement is a real-time processing engine that allows users to trigger on multiple time-varying signals and small signals in the presence of large signals, or trigger on signals within signals for the purpose of marking events in recorded data.

Scanning-type signal analyzers with narrow instantaneous bandwidths are not well suited for this type of signal capture since it requires prior knowledge of the signal type. Instruments with real-time processing capability have a higher probability of signal intercept and are able to capture very brief threat signals. An instrument with a digital phosphor display can also be useful for this type of signal capture. This type of display can show complex time varying signals, even in the presence of higher power signals (Fig. 2). This makes it possible to discern “suspect” signals even if they are short duration and are at frequencies close to powerful emitters. To classify signals during the development of countermeasure devices, pulse measurement analysis is a useful capability--as are frequency, amplitude, and phase versus time measurements.

A high-capacity storage system can accurately store uninterrupted signals captured by a real-time spectrum analyzer for hours or days depending on acquisition bandwidth. Ideally, it should be capable of recording full 16-b I and Q data streams to the full real-time spectrum analyzer bandwidth, typically more than 100 MHz. Collecting long-duration recordings of IED jammer scenarios makes it possible to conduct detailed analysis of events leading to and following a specific IED action. The capability of flagging data with event triggers or provide accurate time-stamps enhances the speed of analysis using such functions as rapid search on events.

A continuous playback generator (CPG) makes it possible to replay all or any part of the recorded spectrum with full 16-b I and Q precision using a vector signal generator (VSG). This capability allows segments of spectrum containing signals of interest to be recreated at will. The recreated spectrum can be replayed into the spectrum analyzer for additional analysis, into another signal analysis system, or used to test system performance in a repeatable environment.

By capturing and recording uninterrupted data streams and event markers, detailed analysis can be rapidly performed on a variety of criteria during the IED remote activation. Collected data can be time stamped with an external IRIG B receiver to high precision and event markers can be recorded during the data collection process. Tools such as these facilitate the process of finding signals of interest in the captured data files.

Hours or days of signal capture can lead to large amounts of data. For analysis, it is important to have tools capable of handling large volumes of data that can complement existing signal-analysis tools, such as MatLab and XMidas. As shown in Figs. 3 and 4, a general-purpose signal-analysis package can be used to locate signals, correlate signals with captured or created library signals, and evaluate signals using a variable persistence using a temperature gradient color density. By replaying the capture in “slow motion” or frozen in time, designers can perform detailed analysis of transient events and other signal details.

Also useful is the ability to create, modify, and export waveforms. Collected signals can be extracted and exported to other analysis tools, manipulated to simulate impairments and other effects, or created from scratch. These complex signals can then be created and transmitted at any desired frequency using a CPG and VSG. For further system testing and evaluation, an RF editor can be used to create complex time variable spectrum. The first step is to create a library of captured or created signal segments. Next, complex sequences containing multiple individually time varying signals can be created. Individual signals can be modified to simulate impairments such as multipath effects and noise. Using this approach, IED jammer scenarios can be created with a wide variety of wireless signals and test conditions.

Signal capture and playback tools can provide a better understanding of jammer technologies. Designers can use these tools to evaluate different jamming scenarios and the effects of different waveforms on mitigating various RF threats. Ideally, this information can lead to the development of sophisticated waveforms that can be used at lower power levels, thus achieving effective jamming without disrupting the performance of non-threat systems.

In the field, these tools can be used to record an entire stimulus-response sequence for evaluation of different IED jammer systems, although field testing can be expensive and time consuming. The cost for outdoor test facilities, government vehicles, and multiple personnel can quickly add up. Through use of a signal capture system, test scenarios can be more easily created using captured and/or created signal information. Bench top or anechoic chamber test and evaluation systems can be implemented with the capability to simulate most real-world situations.

A more practical approach to in-field testing is to perform analysis in a laboratory environment (Fig. 5). In a laboratory setting, a signal capture and playback system can be used to capture, analyze, and playback signals from jammers, IED transmitters, and receivers, either individually or at the same time. Other unrelated radio communication, RF sensor technologies, and/or GPS signals can be injected or used to evaluate collateral effects or improve the receiver sensitivity or selectivity in a given environment. Although field testing remains a critical step for the final verification of technologies and support certification, moving technology development to a more controlled laboratory bench offers a lower cost of test for research of new methods and optimizing jammer performance.

For engineers tasked with mitigating the modern warfare threats of IEDs, emerging tools make it possible to understand complex RF environments in detail. New waveforms can be easily and quickly created, evaluated, and their effectiveness tested. Existing and new jammer systems can be evaluated using repeatable scenarios. Improving jammer performance will reduce causalities and help to minimize adverse effects on other mission support elements.