Ask any of our veterans returning from active duty in Iraq or Afghanistan, and especially those who had earlier seen action in other combat theatres. They will tell you that warfare has changed radically in recent decades.
For one thing, the enemy doesn’t necessarily wear an easily identifiable uniform, hence the prevalence of improvised explosive devices (IEDs) that are set off using RF from readily available transmitters. IEDs have dictated greatly increased vigilance in terms of electronic spectrum monitoring.
The growing use of IEDs has led to the concept of forced dependence between ground and air forces. For example, consider an Army convoy traveling in a hostile area. In the past, such convoys were by and large able to defend themselves. But now you might need Navy aircraft flying along with the convoy to create a shroud of electronic interference to protect against cell-phone use in any area where the convoy travels, augmenting the jamming that the convoy itself is already deploying.
Threats from IEDs have caused the armed forces to invest heavily in electronic warfare capabilities, and this in turn has created new test requirements, both in the field and for developers of military systems. In this article, we’ll look at the current landscape in test for military applications. We will also look at how test has changed for the military itself.
The first major theme of military test in such an environment is that of interoperability between the various communications and jamming systems. A military vehicle driving in a hostile area will use side-scan radar looking for weapons caches of surface-to-air missile (SAM) launchers, grenade launchers, and other potential threats. At the same time, the vehicle has to communicate back to its home base, receive GPS signals, and also have countermeasures in place to detect IEDs. All of these systems have to interoperate smoothly without causing interference with each other.
An important technology that has come to the fore in interoperability testing for military applications is RF capture and playback. This technology is useful not only for scanning for threats, but also for testing the interoperability of field systems. All too often, the systems that implement side-scan radar, GPS, and radio communications are developed independently of each other. They must be integrated at military proving grounds before being deployed.
“It’s not uncommon for jamming systems to block out GPS. That’s a very common first mistake,” says Darren McCarthy, technical marketing manager for RF test at Tektronix. “Once you lose GPS, you lose not only position but a lot of timing for your communications system. Lots of them depend on sync with GPS.”
The Aegis Combat System, which is deployed on more than 100 ships in five navies worldwide, uses powerful computers and multiple radars to track and guide weapons to destroy enemy targets. Tektronix’s spectrum analyzers have been deployed to make spectral emissions measurements on specific radars to look for interoperability, jamming problems, and unintentional spectral emissions related to the radar.
For example, on board an Aegis-outfitted ship there might be a 2-GHz radar system generating a couple of kilowatts of power. Among the emissions from such a radar are pulses at unfortunate harmonics of other signals on the ship. The duration of these pulses is very short.
“To isolate one radar, we use two spectrum analyzers,” explains McCarthy. “One triggers on the events when the pulse occurs, while the other uses a gated sweep that’s on only when the pulse is on. Measurements are gated at different frequencies to measure spectral emissions from only the radar of interest. An analysis develops a profile of what the emissions would be, which reveals how the pulses are affecting other systems. These effects are mitigated by choosing different pulse rates and widths.”
Test and evaluation of radar systems is also “on the radar screen” at Agilent Technologies, which has recently introduced software and hardware aimed at shaping up advanced radar and electronic-warfare (EW) systems. The N7620A Signal Studio software for pulse building creates highly realistic test signals.
New options add the generation of pulse-width and pulse-repetition interval patterns, which enhance the realism of radar simulations. Impairments such as jitter and wobulation add even greater realism to simulated pulsed-RF signals. Options include a wide variety of antenna scan and radiation patterns, enabling accurate modeling of antenna behavior. Signals created with Signal Studio for pulse building can be downloaded into instruments such as the Agilent E8267D PSG vector signal generator.
Yet another area in which interoperability is an issue is that of intelligence, surveillance, and reconnaisance (ISR), says Scott Elson, Rohde and Schwarz’s business development manager for aerospace and defense.
“Over time, more investment has been made into unmanned aerial vehicles (UAVs) for ISR missions,” Elson says.
The importance of these platforms has driven more payload complexity, and that drives more need for instrumentation to sort out interoperability issues, as well as full-motion video and the quality of that video.
Ferreting Out IEDs
What makes IEDs so insidious is that they are typically detonated using commonly available wireless devices, such as the remote controllers for toys. Cellular handsets also are commonly used as detonators.
Early efforts to jam IEDs were simplistic. Jamming performance was gauged simply by the system’s ability to disrupt wireless activation from different distances and geometries between the jammer, the IED, and the detonator/transmitter. It was a basic pass/fail approach that did not help in understanding why jamming sometimes failed or how to improve it.
Working toward better IED jamming technologies required a more thoughtful approach with more questions being asked. What jamming-to-noise ratio is needed for different signals? What jamming waveforms are most effective? Where in the communication of the device being jammed is the signal being disrupted? And how can jammer effectiveness be maintained without causing interoperability issues with other mission-critical wireless systems?
Tektronix’s approach centers on its partnership with X-COM Systems and a combination of Tek’s RSA6000 series spectrum analyzers and X-COM’s IQC2110 long-duration RF signal storage system. The spectrum analyzers provide an acquisition bandwidth of 110 MHz with 75 dB of spurious-free dynamic range (SFDR), which is enough bandwidth to span the entire industrial-scientific-medical (ISM) band. Data flows from the spectrum analyzer to the signal storage system at a rate of 4.8 Gbits/s, or fast enough to fill up a terabyte redundant array of independent disks (RAID) system in less than an hour.
That’s an enormous amount of data regarding the ambient RF environment, and not all of it is helpful. The spectrum analyzers bring Tektronix’s real-time processing engine to bear, providing statistical density triggering, DPX Density triggering, or frequency-domain triggering (Fig. 1). By being able to set triggers and look at signal densities and statistics and the environment, you can time-correlate RF events.
“Our triggering capabilities allow you to flag things in real time, uninterrupted, while you collect data,” says McCarthy.
The RSA6000 spectrum analyzer offers Tektronix’s DPX Live RF display and DPX Density measurements. The real-time DSP engine in the instrument performs up to 292,000 spectrum updates per second and has a 100% probability of intercept for signals lasting at least 5.8 µs. The DPX display clearly shows complex time-varying signals, even in the presence of signals of much higher power (Fig. 2). This makes it possible to discern IED signals even if they are of short duration and are at frequencies close to those of powerful emitters.
On the X-COM side of the equation, the IQC2110/CPG system can store hours, even days, worth of uninterrupted signals captured by the Tektronix spectrum analyzer. It continuously records the full 16-bit I and Q data streams from the analyzer up to its full 110-MHz bandwidth. These long-duration recordings of IED jammer scenarios allow detailed analysis of all data. The ability to flag data with event triggers or provide accurate time stamps enhances the speed of analysis.
Together, these technologies enable forces in the field to sniff out and properly jam the signals from would-be IED detonators. In some cases, the idea is to jam the link just long enough so the explosion is delayed by the receiver continuing to poll for an incoming signal. In others, it’s a matter of forcing it to detonate early, before the convoy reaches it, for example.
Another option for real-time RF recording and analysis is Aeroflex’s CS9000 broadband signal system (BSS). The BSS, which comes in standard configurations for RF up to 6 GHz and 18 GHz, accurately records, demodulates, and analyzes advanced broadband and frequency-agile communications as well as radar signals.
Rohde and Schwarz’s FSVR spectrum analyzer also is applicable to the IED-jamming challenge (Fig. 3). The instrument offers a real-time analysis bandwidth of 40 MHz at frequencies of up to 40 GHz, which enables capture and display of RF spectra for quick characterization of signals. It seamlessly measures the signal spectrum in real time, and even with a time overlap. For visual evaluation, it offers a spectrogram in addition to the instantaneous spectrum in real-time. In persistence mode, it visualizes the real-time spectrum with color coding that indicates how often a signal occurs.
Frequency-dependent masks provide support when triggering on spectral events. This makes it possible to reliably detect signals that occur sporadically in the spectrum and to investigate them effectively.
Yet another piece of the IED-busting puzzle is found in instruments such as Rohde &Schwarz’s DDF255 digital direction finder (Fig. 4). The DDF255, which can be controlled through a PC or laptop, performs extremely fast spectrum monitoring (scan speeds reach 100 GHz/s). The instrument covers 20 MHz to 3.6 GHz or 9 kHz to 26.5 GHz (optional). Wideband direction finding comes with a real-time bandwidth of up to 20 MHz and selectable channel resolution. Hence, the instrument can simultaneously take the bearings of all broadcast, aeronautical, or maritime radio signals.
Military’s Own Test Changing
The complexity of military communications dictates that the test systems the military itself uses shields their users from it, says Sam Strang, vice president of Aeroflex’s military sector. Aeroflex’s test divisions all make at least two types of testers. One is for the ramp or field, typically called operational testing. The other is for intermediate or depot test, which is a more quantitative, robust test system for aligning, adjusting, and determining faults that might not be readily obvious.
For many years, the military has been tossing around a football called the Joint Tactical Radio Systems (JTRS). JTRS, a software-defined data and voice system, has long been planned as a universal communication system that would integrate and network various multiple weapons platforms and forward combat units. The program, unfortunately, has stalled amidst delays, technical challenges, and cost overruns.
JTRS became an overwhelmingly difficult technical challenge. “They originally thought they were building a system using 30 different types of waveforms and you just dial up the one you want,” says Strang. However, it got to a point where it was impossible to build a system that could handle all that functionality in a form factor that was portable enough. The current incarnation of JTRS is down to some five or six different protocols, but it has yet to be fully finalized.
JTRS has been in the background since the late 1990s, when it was initially proposed. Meanwhile, voice communications have been augmented by quite a bit of data traffic. About five to seven years ago, combat personnel began sending text messages over radios, which could be read and understood better in field conditions and were more immune to noise.
Satellite communications, such as the Link 16 protocol, were also added to the mix. Link 16, a tactical data exchange network used by NATO, brings situational data awareness, enabling units to exchange images, radar data, and other information.
Aeroflex’s equipment performs ramp testing of the communications systems on aircraft, tanks, and other military vehicles, providing support for virtually every type of common military communications system. Why is it important that systems be checked at the ramp or in the field? It’s because large amounts of hardware are pulled from aircraft, tanks, or ground vehicles for some kind of operational issue and returned to the depot, and subsequent testing at the depot is unable to duplicate the fault.
What the military terms “no fault found,” or NFF, costs the military a lot of money. It means warehouses full of spare equipment to support the necessity to send a radio pulled out of a jeep halfway around the world for repairs.
One example of how operational testing comes into play is in the testing of Identify Friend or Foe (IFF) transponders. IFF is an encrypted signal that aircraft, radar systems, and other military platforms continuously search for. If an aircraft picks up another aircraft in its vicinity, it will interrogate that aircraft’s IFF transponder. If it does not return the proper encrypted signal with that day’s code, it is automatically assumed to be a foe.
Aeroflex’s APM-424(V)5 IFF portable test set is used at the flight line for a simple “point-and-shoot” test of an aircraft’s IFF system (Fig. 5). The system supports the latest version of IFF, known as Mode 5, and ensures that the transponder in a given aircraft is “squawking” the right code in the right manner. This applies to both initial commissioning testing as well as routine pre-flight test. Mode 5 is a heavily encrypted signal that is updated every eight seconds using a GPS signal and encryption algorithms developed by the National Security Agency.
The IFF flight-line test is a deceptively simple matter of a crewman pointing the tester at the aircraft and getting either a green or red light. The tester checks some 40 to 50 items in about 20 seconds. But the test is literally a matter of life or death, because allowing an aircraft to go airborne with a malfunctioning IFF system can result in it becoming a target for friendly fire.
Testing The Whole System
No matter how complex a system may be, even those that employ extremely high rates of frequency hopping, many of the issues that crop up are related to outboard elements of the system, namely cabling and antennas.
“I can’t tell you where this occurred, but at one location we checked over 800 platforms, including Humvees, planes, and tanks,” says Strang. “We found four radios that were bad, but half of the platforms weren’t working right. The causes were broken cables and antenna problems.” Naturally, the military would like to avoid pulling a $30,000 radio out of an aircraft because of a short in a cable.
It’s also worthwhile to note that the military personnel who typically test and maintain military communications systems do not get as much training as they once did. “All of the services have decided, rightly or wrongly, to not invest as much in training as they used to,” says Strang. “They either expect the vendors of the equipment or civilians or the depots to take care of the problems. That leaves the platforms out there all by themselves.”
The answer is to automate a great deal of the testing, which Aeroflex has accomplished in its 3515A portable radio communications test set (Fig. 6). Weighing less than 8 lb, the set is designed specifically to reduce the number of radios needlessly removed from platforms only to end up in a “no-fault-found” situation. It supports AM, FM, data links, and satellite links, and it can also be used on the bench.
Not only does the 3515A test the functionality of the radio itself, it also employs frequency-domain reflectometry to test the cabling and antennas. “It’s one thing to check a radio, but it’s another to check the antenna that’s 65 feet behind the radio,” says Strang. The set measures the voltage standing-wave ratio (VSWR) or return loss of an antenna as well as the distance to a cable fault. Isolating problems to cables and/or antennas is one way in which the 3515A attacks the “no-fault-found” problem.
The instrument sweeps through its frequency range, finding the antennas’ sweet spots. If someone has managed to plug the wrong antenna into a particular ratio output port, the 3515A will determine the problem.
An optional advanced scripting language is available for the development and implementation of test procedures, including instrument settings, user prompts and instructions, and data collection and recording. Scripting is a powerful feature that helps to eliminate operator error, an unfortunate byproduct of the military’s trend toward less training for technicians.