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By John Haystead

lthough the use of the HF band (3-30 MHz) for tactical communications by near-peer adversaries has been, and very much still is, alive and well, today, its

use by other military and para-military organizations is becoming increasingly prevalent – and increasingly problem-atic. This is particularly true of insur-gent/terrorist groups, as well as criminal organizations. The reasons for this trend are many, but two particularly impor-tant ones are the fact that HF commu-nications systems can serve in place of expensive satellite links, together with their ability to avoid the interception dangers posed by the use of cellular com-munications networks.

As described by Assaf Zelinger, Director of Research and Development for Israel Aerospace Industries subsidiary, ELTA North America (Annapolis Junction, MD), “Initially, with the development and emer-gence of cell phones, these quickly became the communication devices of choice for terrorists. But, it then became obvious that intelligence organizations could detect, monitor, and eavesdrop on cell phones so, as a result, they started to go back to older technologies that had them-selves also benefitted from technological advances driven by cell phone technology.”

Today, Zelinger says, HF is being heav-ily used by terrorist factions, pirates, ille-gal immigration organizations and other criminal groups, “for the sole reason that it’s better than SATCOM and cell phones because it’s untraceable, and the signals are very hard to geolocate because of the reflection/bounce characteristics of HF signals off of the ionosphere. They’re also basically taking advantage of the fact that modern HF communication devices have

Tactical HF SIGINT/Geolocation Systems Tackle Elusive Challenges

become so simple to use and carry. Today, the HF transmitters don’t have to look like a big radio that you carry on your back. They’re almost the size of a regular phone.”

Greg Patschke, Director, Special Mission Solutions, L3 Technologies Communications Systems – East (Camden, NJ), agrees. “L3 has been in the HF SIGINT business for many years, and we defi-nitely still see HF playing an important, in fact increasingly important, role, espe-cially for adversary communication. The technology is easy to get, low cost and relatively easy to use. They not only allow you to communicate over long distances, but, we’re also seeing these systems being used to communicate over short distances in very mountainous terrain, taking advantage of some of the features of the waveforms. And, potential adversaries are also now using HF to connect to and communicate and coordinate over the Internet, which increases the problem.”

In fact, the development of, and advantages provided by cell phone advances, and digital signal processing technology overall have also had a sig-nificant impact on the state of HF voice communications. The improved security, filtering, interference-suppression, etc., provided by digital processing have also all contributed to major advances in com-munication over HF links.

CHALLENGES OF TACTICAL HF GEOLOCATION

The task of detecting and geolocat-ing HF signals has always posed unique challenges, not the least of which being that, unlike other RF signals such as VHF and UHF, HF signals not only fol-low line-of-sight paths, but also bounce off of the Earth’s ionosphere. Thus,

collection antennas can receive the same signal from different directions (multiple angles of arrival) and at different times. The high noise environment of the band together with ionospheric variations, and multipath-induced signal fading add to the challenge.

At the strategic level, very large, HF/DF SIGINT collection arrays employing com-plex mathematical algorithms and compu-tational processing have been in use for years to intercept and locate HF signals over long distances. And, while this capa-bility has served national intelligence com-munities quite well, and is currently being dramatically improved by developments made through the Intelligence Advanced Research Projects Activity’s (IARPA’s) HFGeo program (see “IARPA Tackles HF Emitters” on p. 41), at the tactical level, real- or near-real-time intercept and geo-location is the most critical and driving requirement, with low size, weight, and power (SWAP) characteristics close behind.

TACTICAL MEANS MOBILEIn order to perform effectively at the

tactical level of battlefield operations, HF geolocation systems need to be able to rapidly move and deploy over large areas of interest. To address this requirement, a new breed of tactical-level HF geoloca-tion systems and technologies is being brought to the battlefield. One such sys-tem is the “SandDust” HF DF/Geolocation System from Leonardo DRS, Airborne & Intelligence Systems – EWISR & Border Security (Melbourne, FL).

Able to be put into operation by two operators in under 30 minutes, the com-plete SandDust system, including power generator, multi-channel HF receiver, and antenna array can be carried in a single SUV-sized vehicle.

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Operating over the 2- to 30-MHz fre-quency range (25 kHz bandwidth), the system is composed of an advanced multi-channel phase-coherent HF receiver and a five-element antenna array of one-meter diameter loops. A single system can gen-erate line-of-bearing (LOB) DF data on HF transmissions of multiple modulation type with 5 degrees RMS accuracy. Since it can be quickly relocated to multiple sites in a single day, it can then provide more tightly-defined geolocation solutions (ellipses) using recorded and archived LOB data.

TIME IS OF THE ESSENCEAlthough ground-transportable HF geo-

location systems are certainly useful and valuable, the requirement for immediate information over very large geographic areas requires a different approach. “For example, says ELTA’s Zelinger, “One of our customers was trying to locate some terror-ists working over a very large geographic area and only transmitting every few days. It was critical that we demonstrate that we could detect and begin the geolocation process from the first transmission.”

Able to be integrated onto both manned aircraft and UAVs, the ELK-7065 COMINT/DF 3D tactical HF COMINT and 3D geoloca-tion system is aimed at meeting this need. Developed by Israel Aerospace Industries (IAI) subsidiary ELTA North America (Annapolis Junction, MD), and announced in September of last year, the ELK-7065 tar-gets both land-based and naval applica-tions. Says ELTA’s Zelinger, “If you look back

just a few years, in order to detect HF com-munications, you needed to deploy a large array of antennas (about 300 feet apart), and if you wanted to go airborne, you had to go on a very large aircraft, implement the antennas inside the wings and hope that this would be accurate enough. Today, our solution is much smaller.”

In broad terms, Zelinger describes the system as “basically two antennas, with each antenna about 1 ft. in height and 1.5 ft. wide. By putting the two antennas on a mid-size UAV, for example, you get bet-ter control of electromagnetic interference, and those two antennas give you the same, if not much more, capability from systems of the past that required a large array. The obvious benefit is that you can now put it on a drone that is airborne for 36 hours at various ranges.”

As described in the company’s literature, the system “tags and identifies signals char-acteristics in a multi-dimensional domain, composed of signal identifiers such as power, center frequency, modulation, geolocation, polarization, etc.” Not surprisingly, Zelinger says their specific algorithms are proprietary, but he does acknowledge that a combination of azimuth and elevation data is used to provide geolocation. “Our secret sauce lies in two areas – one is the algorithm and the other is the unique design of the antenna.”

According to Zelinger, the system pro-vides two different levels of capability. “From very long range, we can provide basic DF information. And, while this is good enough for some requirements, it wasn’t good enough

for what we ultimately wanted to do, which was geolocation, so we developed a concept-of-operation where the user will initially operate the system from tens-of-miles away at very high altitude to obtain a general DF (LOB), and then, when we know this direction, we start flying toward the tar-get while also reducing altitude. As we get closer, we can get to geolocation capabili-ties. The system works in real time, and the more that the target uses their radio, and the more detections we have, we can do better and better with the estimation of their location.”

Zelinger references a test performed for a US customer about a year and a half ago where they demonstrated this capability. “In that test, I think we started with a kind of elliptically shaped area approximately one-mile-wide and approximately 1/2 mile high, and we finished with something about one-quarter of that as the user transmitted more and more times. With the increased data for integration, we were able to really close in on the location and identify the area within less than one square km.”

Currently, the company is flying the system on IAI’s “Heron” drone which is 27 feet in length with a wingspan of 54 feet, although other UAV options are also being explored. “The system itself is platform agnostic,” says Zeliner, “so any UAV that can carry a 60-lb. payload can use the system. We’ve also done analyses for integration on manned aircraft platforms as well, anything from a C-130J to a King Air 350.”

HOW FAST IS FAST ENOUGH?In combat, the ability to locate a deadly

enemy force before it detects you can mean the difference between mission success or failure – even the difference between who survives and who doesn’t. With that in mind, the time available to detect, tightly geolocate and neutralize known, high-threat tactical HF signals, may be measured not in hours, but in minutes.

This requirement, says L3 Technologies’ Patschke is the reason “we’ve developed technology that allows us to perform geo-location of the source of the emission using only a single pulse (short duration [one-sec-ond] emission), which is a major discrimina-tor of our system from others.” In addition, Patschke also sees the systems currently being used for tactical geolocation, “the traditional types of systems on the market

The SkyHawk HF Vector Geolocation System is integrated in a pod beneath the wing of this Cessna O-2 Skymaster (top). The operator display (bottom) shows the system’s single-pulse geolocation capability. (L-3 TECHNOLOGIES)

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today, as requiring very large antennas and not very flexible, whereas we’ve developed a capability than can perform the job with very low SWAP characteristics.”

Patschke is referring to the company’s “Skyhawk” single-pulse, single-platform, HF Vector Geo-Location (VGL) system. The pod-contained airborne system covers the 2- to 30-MHz range with 40 MHz instantaneous bandwidth, providing 2.5-degree RMS accuracy (azimuth and elevation). It is currently car-ried aboard a Cessna O-2 Skymaster aircraft, although the low-SWaP, 14-in. BRU-mount pod using standard aircraft power, is described as “an ideal configuration for both manned and unmanned aerial platforms.” The Skyhawk can interface and provide geolocation information to existing COMINT systems via an applica-tion program interface (API), with the type of control system used dependent on the API, but able to feed both ground stations or other external systems.

“Effectively what the airborne sensor allows us to do,” says Patschke, “is obtain an accurate measurement of the azimuth as well as the elevation of the target, and that allows us to create a vector pointing to the source of the emission. When there isn’t a relative elevation difference between the emitter and the sensor, the azimuth alone basically only gives you a line of bearing (LOB) to the target and not a geolocation. But, with the airborne system, we also have elevation data and, using our unique algorithms, can rapidly calculate where that vector intersects the ground map and provide geolocation. It’s the whole flash-light analogy, where up high you have the flashlight pointing down on the ground with a very accurately defined circle, but if you take it down to the ground, you get more of an oval shape.”

Although details of the antenna system are proprietary, it is described as “about the size of a volleyball,” utilizing small, fractional-wave-length antenna elements. “The technology,” says Patschke, “allows us to pick out signals in a very dense RF environment, because you can actually get that spatial separation on individual pulses. The other piece is that a lot of these signals are of short duration, so traditionally it has taken multiple aircraft to locate these emitters, but we can do it on a single aircraft on a single pulse. As soon as we intercept the signal, we have an accurate estimate of the location of the emitter.”

Patschke says the Skyhawk airborne capa-bility has evolved over time, and “it continues

The SandDust System can be carried in a single SUV. (LEONARDO DRS)

INTRODUCING THE LATEST SINGLE-PULSE, SINGLE-PLATFORM RF SIGINT SYSTEML3 Technologies introduces SkyHawk, our latest RF SIGINT direction-finding/geolocation system, which provides discriminating capabilities over traditional systems. The 10-inch sensor offers extremely low-SWaP HF-band geolocation. From a single aircraft, the system generates an instantaneous geolocation solution on a single received pulse, which eliminates the need for multiple lines-of-bearing or multiple aircraft. The technology provides 360 degrees in azimuth and elevation, removing the “cone of silence” below or in front of the aircraft. The sensor offers five-degree pointing accuracy, compared with 20 degrees or more for current HF DF systems. For single-pulse performance from a single platform, turn to L3 CS-East. L3T.com/cs-east

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SKYHAWK AIRBORNE HF GEOLOCATION SYSTEM.

Use of U.S. DoD visual information does not imply or constitute DoD endorsement.

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to mature especially as we work with cus-tomers and work toward their specific requirements. The long-range plan is to evolve this capability and integrate it into a larger overall electronic support measures (ESM) system role using the open standards, multi-source-intel fusion paradigm.”

WHAT MAY THE FUTURE HOLD?The need for low-SWAP, highly-mobile,

tactical HF SIGINT and geolocation is appar-ently not lost on US Special Operations Command (USSOCOM). In May of 2017, the Command issued a request for information (RFI) from the Program Manager for Joint Threat Warning Systems (PM-JTWS), the Command’s program of record for SIGINT col-lection systems, looking for industry input on SIGINT technology capable of advancing: “unique signals of interest, modular and scalable open architecture systems, and remote C2 and data viewing.” Hardware requirements were described as “solutions modular and scalable from a body-worn or small UAS form factor to a vehicle/maritime platform to an airborne chassis.”

Subsequently in July, the office issued another RFI detailing its particular inter-est in a “body-worn sensor with low Size,

HorizonTechnologies.com

898998_Horizon.indd 1 2018-01-31 4:13 PM

Weight and Power (SWaP) and a low-pro-file DF antenna.” This time the software defined radio (SDR) objective was specifi-cally defined as inclusive of HF coverage, having “a survey capability between the frequency range of 3 - 6,000 MHz” and a requirement to “conform to platform spe-cific requirements for use on a full range of platforms.”

The sensor is to weigh no more than 12 lb. with batteries, less antenna(s) and ancillary cabling. It is also to interface with RaptorX and display results in near-real time across the JTWS family of systems.

The Air Force is also showing particu-lar interest in HF communication technol-ogy. The Service released a Small Business Innovation Research (SBIR) solicitation last November for the High-Frequency Ionospheric Visualization Environment (High-FIVE) program whose objective is to “provide means to rapidly assess, predict, validate, and report HF signal propagation in both near-real-time and forecast situa-tions.” Although, the Air Force’s primary interest with High-FIVE is in advancing its own HF communications capabilities and efficiency, the implications beyond this for the SIGINT community are also clear.

As described in the solicitation, “With the advent of improved circuitry, software-defined radios (SDR), and our advancing understanding of high-frequency (HF), low-frequency, very-low-frequency (VLF) bands which are advantaged by favorable ionospheric conditions, communicators are able to take advantage of these well-established spectra through use of advanced antenna systems, improved discrimination and sensitivity in the receiver units, and employment of “wideband” bandwidths, among other things. Unlike the nearly 80-year history of traditional military use of HF, which utilizes narrow bandwidth channels, we may soon see 8-10+ times the bandwidth used in modern HF warfighter communications systems.”

ELTA’s Zelinger offers the following caution in that regard. “What history has taught us is that whenever you find a solu-tion to one problem, the enemy will find another approach. As such, we believe that once we provide adequate coverage and exploitation of adversaries currently using HF communication links, they will simply switch over to another band, so we are con-stantly looking out for what is coming next. This is critical.” a

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IARPA TACKLES HF EMITTERSAlthough, as recognized by the

Intelligence Advanced Research Projects Activity (IARPA), high frequency (HF) (3-30 MHz) communications systems and over-the-horizon radars are in widespread use around the world, the accurate stand-off-geolocation and characterization of these sources has proven difficult because of ionospheric variations, the high noise environment that exists at these frequen-cies, as well as factors such as ionospheric polarization rotation, multipath-induced signal fading and simultaneous multiple angles-of-arrival. Hence the launch of the HFGeo (HF Geolocation) program, with the overall goal, according to IARPA, of dra-matically improving the ability to detect and geolocate HF emitters through the “development of an agile, accurate, and highly-available geolocation capability against potential adversaries’ HF com-munications emitters.”

Conducted within the Office of the Director of National Intelligence (DNI), IARPA’s HFGeo program is perhaps one of the most closely-held signals intelligence research and development activities cur-rently underway. What is known, how-ever, is that, according to IARPA’s program description, the objectives of the HFGeo program include: the ability to accurately resolve multiple angles-of-arrival (AOA) and polarization states through novel antenna concepts, the ability to accurately determine the dynamic state of the iono-sphere, and the ability to enhance signal-to-noise ratio and signal detection through the use of multi-dimensional adaptive sig-nal processing. The program also seeks to “provide a clearly-defined application program interface (API) that will allow multiple users to provide data and obtain estimates from the developed propagation model(s) and geolocation engine(s).”

The HFGeo program entered Phase 3 of the effort in 2016, with Leidos S&R and Intelligence System Services (Reston, VA) awarded a contract through the Air Force Research Laboratory (AFRL). Phase 3 builds on work conducted in previous phases of the program dating back to 2011 which addressed key technology develop-ment areas.

Phase 1A addressed novel antenna concepts and multi-dimensional adap-tive signal processing algorithms. IARPA

chose Leidos with Strad Corp. (Chapel Hill, NC); Systems & Technology Research (STR) (Woburn, MA) with Innovative Adaptive Applications (IAA) (Gainesville, FL); Northrop Grumman Mission Systems (Linthicum, MD); Northwest Research Associates (NWRA) (Redmond, WA); and SoneSys LLC (Merrimack, NH) with MegaWave Corp. (Devens, MA) to conduct the Phase 1A effort.

The Leidos/Strad HFGeo technical approach comprised an integrated algo-rithm suite for geolocating HF signals using vector sensors. Its solution for noise

mitigation centered around two major techniques – Statistical Subspace Adaptive Processing (SSAP) and Spatial Clustering. Both techniques took advantage of spatial and temporal degrees of freedom provided by the vector sensors to reject noise or interference and isolate signals of interest.

NWRA’s Phase 1A work addressed the use of Cyclostationary Signal Processing (CSP) which exploits the statistical struc-ture of communication and radar signals for enhanced HF signal separation, as well as geolocation by extraction of temporal signatures. The Journal of Electronic Defense | M

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STR and IAA developed advanced signal processing algorithms for HF signal isola-tion, successfully isolating multi-mode sig-nals and ground waves, and demonstrating an initial capability for angle-of-arrival direction finding. An Electromagnetic Vector Sensor (EMVS) array provided direction-of-arrival and polarization infor-mation within a small physical footprint.

Northrop Grumman Mission Systems (Linthicum, MD) developed a number of key algorithms, including the application of calibration data and Compute Cross Ambiguity Function (CAF) for improved

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AOA estimation, as well as Backward Ray Tracing and Geo-Combining algorithms and data analysis tools.

The Sonesys/MegaWave team devel-oped and tested its Vector Sensor Antenna System (VSAS) incorporating Roke Manor’s (Romsey, UK) MUltiple SIgnal Classification (MUSIC) DF algorithm work.

Phase 1B, conducted solely by NWRA with Atmospheric and Space Technology Research Associates (ASTRA) (Boulder, CO), focused on ionospheric modeling, with the overall goal of improving the ability to resolve multiple AOA and polarization

states, enhancing signal-to-noise ratio and signal detection, and determining the dynamic state of the ionosphere.

Following the completion of Phase 1, HFGeo Phase 2 moved on to the system integration of Phase 1 innovations in non-real-time field tests. In March 2016, the US Air Force awarded Southwest Research Institute (San Antonio, TX) a $9.4 million contract to integrate a high-fidelity iono-spheric model with a geolocation system to achieve the level of precision sought. The 18-month effort included a non-real-time field test of a prototype system and off-line signal processing

The non-real-time field tests have now been completed, and as described by IARPA’s HFGeo Program Manager, Torreon Creekmore, Phase 2 provided the transi-tion from the key technologies of Phase 1 to the development of a prototype system of hardware and off-line signal processing for use in initial field tests with surrogate targets. “The program successfully dem-onstrated that electrically-small vector sensors do work, with good signal to noise ratios (SNR) that match performance of larger arrays, and offer angle-of arrivals accurate enough to meet the HFGeo pro-gram geolocation accuracy goals. After three field tests at the White Sands Missile Range in New Mexico, the program demon-strated that under Traveling Ionospheric Disturbance conditions, good signal to noise ratio (SNR) targets with as little as 8 km separation can be resolved in angle-of-arrival.”

SwRI continued to collaborate with NWRA, Lowell Digisonde International (Lowell, MA), and YarCom Inc. (Austin, TX) on phases two and three of the HFGeo effort.

In October of 2015, IARPA had also awarded a $7.2 million contract to STR for the second and third phases of the HFGeo program. Leidos (Reston, VA) had also been awarded an $18.7 million contract at that time for development work on HFGeo phases two and three. The 15-month third phase now tackles the real-time signal processing, and demonstration of HFGeo capabilities. Says Torreon, “Phase 3 will implement real-time signal processing, incorporate signal processing enhancements, and test against realistic targets in realistic environments.” – J. Haystead a