Home / Technology / Comm. & NW / Wideband High Frequency(HF) communications provide net-centric, high-speed beyond line of sight communications in Anti-Access/Area Denial (A2/AD) battlefield environments

Wideband High Frequency(HF) communications provide net-centric, high-speed beyond line of sight communications in Anti-Access/Area Denial (A2/AD) battlefield environments

Whether in the field of battle, search-and-rescue or humanitarian aid efforts, the ability to share real-time, networked information between ground, sea and airborne forces is rapidly becoming the defining factor in a mission’s success. However modern  satellite and other  communications systems in higher frequecies are under constant threat from adversaries like Russia and china have developed sophiticated Electronic warfare (EW) syatems through which they can jam them. NATO members and partner forces are vulnerable to disruption of satellite communications, particularly along the alliance’s eastern flank where Russian armed forces continue to conduct electronic warfare.


Electronic warfare elements deployed within theaters of operation threaten to degrade, disrupt or deny VHF, UHF and SATCOM communication. In this scenario, HF radio is a viable backup mode of communication.  Communication satellites are also easy targets for Russian and Chinese antisatellite systems both ground Ascent missiles and coorbital killer satellites .


Therefore Military is looking to High frequency communications a technology that  have been used since the 1930s as a means to communicate beyond line of sight to boost resilience. HF operates in the ~1.8 MHz  to 30 MHz frequency band by either reflecting off the ionosphere (called Skywave) or refracting off the surface of the earth (called Surface Wave).  It  can support Point-Point and P-Multipoint data rates up to ~10kbps w/o relays . HF is generally available, rapidly and readily deployable – requires very little infrastructure and can be made extremely reliable.


HF | frequency band | Britannica

HF’s single greatest value is its ability to provide reliable short and longrange Beyond Line Of Sight (BLOS) communications. Even in EMS-denied environments, HF radios can provide stable, beyond-line-of-sight communication permitting the ability to initiate a prompt global strike. While HF radio equipment is also vulnerable to electronic attack, it can be difficult to target due to near vertical incident skywave signal propagation. This propagation method provides the ability to reflect signals off the ionosphere in an EMS-contested environment, establishing communications beyond the line of sight. Due to the signal path, the ability to target an HF transmitter is much more difficult than transmissions from VHF and UHF radios that transmit line of sight ground waves.


What does the term LOS communication mean? Name the types of waves that are  used for this communication. Give typical examples, with the help of  suitable figure, of communication systems that use


Special operations commands across Europe are ramping up their capabilities with high-frequency communications to ensure connectivity on the battlefield. Leaders there are turning to high frequency communications as a way to optimize properties that provide a low probability of interception and detection.Germany’s Ministry of Defence, which is deciding whether to include an HF requirement as part of its wider Digital Land Based Operations communications program.


The US has a large installed base of HF radios for ground, maritime, and airborne operations. For example, there are more than 4000 fixed and rotary wing airborne platforms with HF radios installed. The US uses HF to communicate directly from platform to platform, such as air-to-ship communications. The US also has fixed and deployable HF entry station infrastructure for connectivity from deployed platforms to the Global Grid.

High Frequency Communication

HF operates BLOS in the 2 to 30 MHz frequency band by either reflecting off the ionosphere (called Skywave) or refracting off the surface of the earth (called Surface Wave). Different frequencies will reflect off the ionized layers depending upon their height and ionization density which varies depending upon time of day and solar activity.


Thus, multiple HF frequency assignments are required to ensure 24/7/365 reliable communications. It is possible to predict which frequencies will be propagating based upon time of day and solar conditions as well as physical measurement of the ionosphere layers via sounding stations.


However, actual propagation performance for frequency channels will often be better or worse than expected. Thus, automatic protocols are required to determine which frequencies are propagating and adapt the waveform modulation and coding to actual channel conditions, write James A. Stevens and others from  Rockwell Collins.


Automatic Link Establishment (ALE) protocols probe (sound) HF frequencies to identify good propagating channels to setup a link. In the past, successful HF communications required skilled operators to know when to manually switch frequencies to obtain or maintain good links to exchange voice or data. ALE enables radios to automatically determine which links are propagating and use those links.



Single sideband (up to 3 kHz bandwidths) channel modulations support data rates up to 9.6 kbps and independent sideband (6 kHz bandwidth) channel modulations support data rates up to 19.2 kbps. The serial tone data waveform modulations feature an “autobaud” capability that enables the receiver to automatically adapt the transmitter’s data rate and interleaver configuration without operator intervention. HF link layer protocols dynamically adapt data rate to channel conditions and improve reliability through acknowledgement and retransmissions. These link layer protocols support higher layer applications including email, chat, and file transfer.


According to Pätzold, so-called Skywave HF, which bounces signals off the ionosphere, enables beyond line-of-sight communications across “thousands of kilometers” without requirements. HF communications is also ideally suited to supporting local network coverage. “This offers advantages over SATCOM in urban areas, but also in mountainous areas or far north latitudes where no line of sight to existing satellites is possible,” Pätzold said


In an online presentation to the Association of Old Crows in Aug. 2020, Paul Denisowski, product management engineer at Rohde and Schwarz North America, Denisowski explained that HF is making a comeback in local and global communications. This renaissance comes as the result of improvements in a range of fields, including antenna design, digital modulation schemes and improved understanding of propagation. The market is also helped by reductions in size, weight and power requirements as well as the introduction of wideband data, enhanced encryption algorithms and interoperability with legacy HF sets, he said.


“This means end users are now benefiting from easier-to-use and cheaper solutions featuring improved data performance, audio quality, availability and operation. And because of a lack of infrastructure, HF is less expensive and relatively robust, although solar events may temporarily disrupt HF communications,” he said. Specific upgrades include “Adaptive HF,” which comprises automatic selection of frequency and the establishment of communication through automatic link establishment, or ALE, technology.


New HF Wideband Technology for A2AD enironment

Recent advances in HF radio and Digital Signal Processing (DSP) technology, along with new U.S. and international regulatory flexibility in spectrum allocation policies, have ushered in a new era for terrestrial-based, long-range communications capabilities. As a result, HF radio is now no longer limited to agonizingly slow 9,600 bps data transfer rates – slower than dial-up modems of the early 1990s. Advancements in HF technologies over the past two decades, especially automatic link establishment (ALE) and wideband HF (WBHF), improve the ease of use and capacity of HF communications. Today, modernized Wideband HF (WBHF) can deliver rates up to 240 kpbs on a 48 kHz wide channel.


The latest technology of its type — 4G ALE — is capable of supporting wideband HF communications, or WBHF for short, providing end users with the ability to “negotiate bandwidth, modulation type, error correction and the number of sub-carriers,” Denisowski explained. “ALE selects frequencies using link quality analysis, which allows it to listen and determine if a channel is in use and adapt if conditions change,” he said. He added that HF can now support data rates up to 240 kilobytes per second on a 48-kilohertz channel, particularly useful for more robust communications in hostile environments. “WBHF has already [been] used in military trials. It’s a technology which is most definitely here and now,” Denisowski said.


WBHF technology

The lowest data rate modulation for each WBHF channel bandwidth operates with a Signal to Noise Ratio (SNR) of about -9 dB for mid-latitude links. This provides data rates from 75 bps (3 kHz channel bandwidth) to 600 bps (24 kHz channel bandwidth). The latter is sufficient for the lowest data rate Mixed Excitation Linear Prediction – Enhanced (MELPe) vocoder and thus provides voice at significantly lower SNR than possible today, write James A. Stevens and others from Rockwell Collins.


The lowest data rate WBHF modulation is based upon Walsh codes that spread the transmitter power out in a wider channel than the information actually requires. This means that a transmitter has additional transmit power reserve. A WBHF solution leverages existing DoD worldwide HF infrastructure and platforms with installed HF. Only the HF radios/modems would have to be upgraded to run the latest military standards, including ALE and WBHF. This means that airborne platforms with existing HF only have to upgrade their radios and could reuse their existing HF antennas without having to make costly modifications to the outside of the aircraft , write James A. Stevens and others from Rockwell Collins.


WBHF, with its low SNR requirement, operates below the noise floor and can be used with lower transmit powers for lower detectability or with higher transmit powers for additional anti-jam margin. Anti-Access and Area Denial (A2AD) occurs when an enemy gains the ability to disrupt command and control to such an extent that friendly forces cannot control their assets nor risk sending assets to that region. Example A2AD communications threats include: loss of communications through satellite denial, loss of communications through jamming, platform detection and location from communications transmissions, and loss of communications due to nuclear effects. Nuclear events disturb the ionosphere by increasing its electron density which increases the absorption of lower HF frequencies and the reflection of higher HF and VHF frequencies .


WBHF mitigates many of the A2AD environment threats, including jamming, detection, and satellite denial . The HFGCS system can be used in an A2AD scenario to provide reachback connectivity if satellite communications are denied. HFGCS can also be used to heal jammed LOS links in the A2AD area by allowing forward deployed nodes to communicate via relay through the HFGCS.


WBHF’s voice service can heal A2AD jammed voice nets like UHF SATCOM voice; WBHF’s IP data service can heal A2AD jammed networks like SHF SATCOM; and WBHF, combined with JREAP, can heal A2AD jammed Link-16, Link-11, and VMF tactical data links.


WBHF technology applications

Eric E. Johnson Klipsch School of Electrical and Computer Engineering New Mexico State University discusses a few example applications in which the higher data rate of the wider-bandwidth waveforms could offer a qualitative improvement in mission performance.


Currently, most UAV video is sent via satellite or lineof-sight radio channels. WBHF offers the intriguing possibility of beyond line-of-sight communications to and from a UAV via HF radio. Of course, powering an HF transmitter and mounting an HF antenna on a UAV pose challenges. A link is established as soon as the aircraft comes within extended line of sight. At this extreme range, the path loss may be as much as 100 dB greater than when the aircraft passes near the ground transmitter, so the data rate must adapt to the varying SNR during the link.


A somewhat less challenging opportunity for delivering video over WBHF arises with ground forces in mountainous or dense urban terrain, where near-vertical incidence skywave (NVIS) is used to overcome the lack of line-of-sight between a video source and its users.


The authors consider their third example application, the one-to many communication involved in maintaining a common operating picture (COP) among vessels in a naval battle group. Propagation is via the (relatively benign) surface wave channel, but nodes in this HF LAN must share the channel. Often, a token passing channel access protocol is used.


HF radio security

Any party with access to an HF scanner has the potential to monitor radio activity. The sensitive nature of tactical operations makes encrypted radio transmissions vital in keeping data private.


Information privacy is of paramount importance in tactical conditions. When it comes to high frequency (HF) radio security, there are three elements that your radio communications equipment needs to be capable of for secure communication in a range of conditions.


Transmission Encryption.

Any party with access to an HF scanner has the potential to monitor radio activity. The sensitive nature of tactical operations makes encrypted radio transmissions vital in keeping data private.


The encryption process involves the transmitting radio scrambling a message in accordance with a specific algorithm that can only be decrypted by a receiving radio equipped with the same security key. People attempting to intercept the transmission without a valid security key will not be able to decipher this communication.


Audio communications broadcasted over Barrett Communications High Frequency radios are compatible with various encryption capabilities, including Data Encryption Standard 56 (DES) which uses a traditional symmetric-key algorithm for encryption, as well as the Advanced Encryption Standard 256 (AES), used worldwide.


Designed for a range of environmental conditions and temperatures, the Barrett PRC-2090 Tactical HF radio system provides reliable and secure communications. This tactical HF radio facilitates advanced calling features and interoperability with a range of security hardwares from different vendors, as well as accessible interfacing with external voice-crypto equipment.


Frequency Hopping.

Frequency hopping is a transmission option wherein a broadcast changes the carrier frequency in accordance with a particular pattern. This is an effective option for a variety of different tactical operations as it is incredibly difficult for an eavesdropping party to intercept. Only transmitter and receiver radios programmed to follow the corresponding hops of frequency can interpret the frequency hopping broadcast. Frequency hopping has been used for decades, and is an effective way of transmitting secure broadcasts to multiple points of contact.


Due to the constant hopping between radio bandwidths and changes in frequencies, the HF radio used needs to be of high quality and able to independently synchronise to a network from a range of control points. The Barrett PRC-2090 Tactical HF radio has advanced frequency hopping capabilities and no late entry time delays. Using a ten digit encryption key, the PRC-2090 can hop at rates of five or 25 hops per second, as well as providing advanced protection against interceptive electronic warfare attacks.


Secure Call.

Secure Call enables point-to-point and point-to-multipoint contact with radios in a locked network using an inband hopping technique on a frequency. These frequencies can change up to 15 items per second, depending on the operator’s preferences. This requires a four digit pin code for each radio, and if a transceiver is disconnected at any stage of contact, the operator will need to reestablish the connection using the coded method.


The Barrett PRC-2090 is host to a narrow band voice scrambler to provide secure communications, and has a number of internal and external encryption capabilities for both audio and data transmissions. This is interoperable with Barrett’s frequency hopping which ensures no late entry synchronisation delay.

Rockwell Collins demonstrated that its wideband high frequency (WBHF) communications

Rockwell Collins demonstrated that its wideband high frequency (WBHF) communications capability can stream video and digital audio, conduct real-time chat, and transfer files across a 5,000-mile distance from the east coast of the United States to Hawaii. The demonstration, which was done in partnership with the Air Force Research Lab and the Air Combat Command (ACC), took place in June 2017 aboard a C-17 transport aircraft. The goal is to provide the capability to aircraft that have long-haul missions and where there are gaps in those missions.


Ron Broden, account manager for high frequency systems at Rockwell Collins, said  it is mostly seen as a back-up system in today’s aircraft as most pilots use satellite links.  Legacy systems use three kilohertz wide channels, while the wideband system goes up to 48.  Wideband high frequency channels offer users four to seven times more capacity than the legacy high frequency channels,


“Whether it’s a real time conversation, streamed live video or the rapid transfer of large data files between an aviation platform, and support or command and control elements for a broad range of missions, this technology has the ability to deliver a true sovereign beyond line of sight communications capability for defence, and one that complements and hardens existing networks.”

Rockwell Collins’ modernized HF capabilities, coupled with the inherent anti-jam nature of the widely dispersed nodes in Automatic Link Establishment (ALE) based HF networks, create an ideal alternative to narrow-band SATCOM in Anti-Access/Area Denial (A2/AD) battlefield environments. “Being able to transfer secure data via the WBHF radio could provide greater operational resilience to the Australian Defence Force in the future, especially in satellite denied environments,” said Air Force’s Director of Plan Jericho Group Captain Carl Newman.


Rockwell Collins Asia-Pacific vice-president and managing director Jim Walker said: “WBHF technology is the only modernised HF solution that will deliver net-centric, high-speed communications at costs that are in line with today’s tighter military budgets.”


Since the ionosphere changes conditions, much like Earth’s weather, there are good days and bad days for using it as a means of communications. The experiment took place on average days — not the worst that a radio operator could encounter but not the best, either, he said.


HF has had its downside too, Dave Schreck, vice-president and general manager airborne solutions government systems for Rockwell Collins, acknowledged. For example, it could take time to establish a link. But today, that problem has been solved with Automatic Link Establishment (ALE) technology.


The  brand new ALE capability, known as 4G ALE, makes HF linking easier, faster and more reliable than ever before, while taking full advantage of wider channels. Its 4G ALE adds the spectral sensing parameter, as well as other signal enhancing characteristics, to create a new protocol that not only automatically determines the optimal bandwidth but also links much faster than legacy 2G or 3G ALE. Spectral sensing ensures that the established link not only has the best signal, but also the maximum available bandwidth. And it does it all without any operator intervention.


Raytheon-FlexRadio team to develop airborne high-frequency radio

Raytheon (NYSE: RTN) will develop and qualify a high-frequency radio under a $36 million Project Agreement through an Other Transaction Agreement with Consortium Management Group. The OTA is on behalf of Consortium for Command, Control and Communications in Cyberspace, in support of requirements from the U.S. Air Force Life Cycle Management Center.


The new radio will provide beyond line-of-sight, long distance communications for aircrews. “High-frequency radios provide the military with secure communications in an increasingly complex and congested threat environment,” said Barbara Borgonovi, vice president of Integrated Communication Systems. “Raytheon’s partnership with FlexRadio combines commercial innovation with advanced military hardening techniques to rapidly deliver a next-generation operational capability that supports strategic and tactical missions.”


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