Blinding the Enemy – with Science

This is an article published in our Electronic Warfare June/July 2018 Compendium.

Royal Australian Air Force Boeing EA-18G Growler electronic attack aircraft.

Previously known as Electronic Counter Measures, Electronic Attack uses electromagnetic energy, directed energy, or anti-radiation weapons to attack enemy personnel, facilities, or equipment.

Electronic Attack (EA) uses the transmission of radio frequency energy to disrupt the operation of other radio frequency systems. This the essence of jamming, whether it is part of a radar or a communications system, and the purpose of which is to ensure that the receiver does not get the signal it needs. Like the other branches of electronic warfare, EA is evolving to counter smarter threats.

Classic applications include airborne escort jamming, a role in which aircraft accompany a strike package, carrying powerful jammers to blind the radars upon which enemy integrated air defence systems rely to mask the approach of the formation. An alternative approach is used in stand-off jamming, where a larger aircraft with greater endurance will target air defence radars and their communications links from outside the engagement envelopes of enemy air defence sysyems. For these applications, ESM systems with very accurate geolocation capabilities are essential because the jamming power must be concentrated in relatively narrow beams to ensure that they can put sufficient jamming power into the target receivers at long range.

Escort/support jammers advance

While the best known of these jammers is the Northrop Grumman AN/ALQ-99, a podded system that has been through many upgrades over several decades of service on platforms such as the EA-6B Prowler and subsequently the EA-18G Growler, it is due for replacement by the AN/ALQ-249 Next Generation Jammer (NGJ) that is designed to provide 10 times the power and handle four times the number of assignments. The NGJ is intended to provide EA capability that is more precise and powerful and with faster reactions and greater directivity, using Active Electronically Scanned Array (AESA) transmitters – akin to those in AESA radars – to create agile jamming beams.

In 2013, the US Navy chose Raytheon to lead the first increment of NGJ development – the mid-band system – beating the ALQ-99’s prime Northrop Grumman, which is reportedly targeting the low- and high-band increments that will be developed later to deal with emerging threats.

Investments planned for fiscal year 2019 are focused on the production of gallium nitride monolithic microwave integrated circuits and wideband circulator technologies. Both are essential for cutting edge AESA systems, the first in components such as transmit/receive elements, the second in all high-power microwave systems and antenna networks in which energy must be directed and isolated.

Saab is in the late stages of development of a podded escort and support jamming system for the Gripen E and other fast jet platforms. Developed with the same core technology as the Arexis EW system integrated internally into the Gripen E, the system is designed to screen the approach and departure of entire strike formations against low-frequency anti-stealth radars. The core technologies are ultra wideband digital receivers and DRFM (Digital Radio Frequency Memory) devices, gallium nitride solid-state AESA transmitters and interferometric direction finding systems.

EA against low-frequency radars requires very high power, and the equipment is too bulky and heavy to be integrated permanently into a tactical fast jet, so it has been podded to make it a role-specific solution, says Saab, with self-contained power generation in the pods. It is designed to defeat these radars with DRFM-based jamming techniques such as smart noise, coherent false targets and saturation techniques. Two pod-equipped strike fighters can protect an entire formation, said Petter Bedoire, sales and marketing VP for the company’s EW and surveillance systems business.

He told the author that to jam these surveillance surveillance radars effectively, attacking the main lobes is not sufficient and that the side lobes must also be jammed. “That means we need a lot of power, and that is why we have used two pods, because they are on different frequency bands, one covering the VHF and the other the UHF L-band.”

Petter Bedoire wouldn’t go into the sensitive subject of how to tell whether the aircraft is in the main beam or the side lobe of the radar it wants to target, as that’s where the company believes it has an edge, but he emphasised that the use of two aircraft equipped with the pods is important.

“You can use smart coordination of jamming techniques that can overcome the side lobe suppression algorithms in modern radars.”

Attacking frequency hoppers

One of the most demanding aspects of EA against communications systems is jamming the most modern equipment in dense EM environments. Such equipment includes software defined radios with Low Probability of Intercept (LPI) waveforms using high-speed Frequency Hopping Spread Spectrum (FHSS) techniques.

Guido Schwarzer, R&S product manager for COMINT/EW systems, explained that all communications jamming has three goals. The first is to interrupt or interfere with the enemy’s communications links to temporarily disable their C4ISTAR capabilities, the second is to screen or camouflage friendly C4ISTAR assets against enemy COMINT/CESM, with broadband noise, for example, while the third is to deceive the enemy, perhaps by re-transmitting or imitating their signals.

For jamming to be effective, the signal strength from the jammer must be higher at the target receiver than the signal from the transmitter that it wants to receive. The relative signal strength of the jamming signal and the desired signal at the target receiver is known as the jamming margin or J/S ratio, and depends on many parameters including the power of the two transmitters and their distances from the target receiver.

Achieving this against radio systems that use FHSS techniques is hard because they switch carrier frequencies very rapidly and they use pseudo-random hop sequences. Key parameters of these schemes are the range of frequencies over which they hop (the hop range), channel spacing, the number of channels they use, hop length and bandwidth and their modulation and error correction schemes. Additionally, they use self-organising networks and automatic back-up links.

The legacy approach to attacking them involves wideband noise or barrage jamming, but this cannot achieve a large enough J/S ratio because the jamming energy is spread over a broad spectrum, while the transmitters in the target system can concentrate their energy at specific frequencies. Target radios also have effective error correction capabilities and use anti-jam waveforms. Also, barrage jammers are easy to detect and inevitably affect friendly and neutral comms with so-called collateral jamming.

Today, the mainstream approach to attacking FHSS radio systems relies on narrowband follower jamming. In turn, this typically depends on a monitoring receiver and spectrum analyser, narrowband processing in a PC, which loses time in processing and format conversion. The computer then commands an exciter to generate jamming signals, with more latency and frequency settling time before sending the jamming signals to a power amplifier and an antenna for transmission. Additionally, these systems are limited in hop rate to about 300 to 500 hops per second and have no multi-target capability, says R&S.

Frequency Modulated Continuous Wave (FMCW) radars transmit very low power signals that are hard to distinguish from noise, but the latest ELINT systems are sensitive enough to look into the spectrum before processing the signal.

Targeted comms EA

The company calls its alternative approach targeted comms EA. This uses a combination of wideband monitoring and detection and narrowband jamming. It also incorporates ultra-fast wideband digital IQ baseband signal processing and parallel jamming signal generation, with an emphasis on a very fast jamming response. The system is designed to keep the processing time as short as possible. This is achieved by carrying out the target signal detection and identification, and the generation of the jamming signal, in a single unit, Guido Schwarzer said.

This is the integrated wideband detector and exciter unit, which uses Field Programmable Gate Array (FPGA) chips to carry out ultra-fast wideband digital IQ baseband signal processing and parallel generation of the jamming signal. Unpacking this a little, FPGAs can have their circuitry configured to carry out specific jobs, so they do those jobs faster than a general purpose processor would, IQ refers to the phase and amplitude characteristics of a signal, while the baseband is the original frequency range of a transmitted signal before modulation. Amplitude, frequency and phase are the three basic parameters of any RF signal that can be changed (modulated) to carry information, so being able to measure them all, practically instantaneously, is crucial to understanding and countering it.

R&S claims that there is virtually no time loss in this system, enabling the wideband monitoring receiver to find the frequency in use by the target radio, set the narrowband jammer onto it, stop the jamming at regular but tiny intervals to make sure the target signal is still there – this is look-through jamming – and if it has moved it starts the search-to-jam cycle again. This multi-pulse approach ensures that every target hop will be jammed multiple times, says the company, while friendly/neutral comms remain unaffected thanks to the use of the Joint Restricted Frequency List (JRFL)

These capabilities have been embodied in the company’s new ground-based Viper system, which is integrated into trucks and shelters.

Although major military forces, including the US Army, have established a philosophy of integrating cyber and EW operations, they are fundamentally different but potentially complementary. Jammers, as radio transmitters, are fully capable of transmitting cyber attacks, but this is not as reliable a means of attacking either a comms system or a radar as jamming because it relies on exploiting uncorrected vulnerabilities in enemy systems. Knowledge of these is hard to obtain and their value can be fleeting as they are quickly patched. In contrast, once the operating principles of target radars and communications systems are understood, Electronic Attack methods will remain effective for much longer.

by Peter Donaldson