HF radio facing the technical challenges and geostrategic challenges of the 21st century

The International Telecommunications Union (ITU) places the HF band between 3 and 30 MHz, technical usage extends its use from 2 to 30 MHz. This part of the spectrum, although limited with regard to the available bandwidth, deserves particular attention. Mastery of the HF domain is a major area in terms of Telecommunications and Electronic Warfare, like the maritime, cyber, air and space domains. The absence of physical borders and its ability to make intercontinental journeys gives it strategic depth.

This ability of waves to cover the entire world through the system of ionospheric rebounds reveals a range certainly known since the beginnings of radio but still very demanding in terms of the requirements for establishing a connection. 

The HF range: weaknesses and strengths.

Until the introduction of the first geostationary telecommunications satellites in the 60s, HF radio was the only solution for establishing a link between mobile stations. The know-how of the radio operator was then essential, he had to evaluate the environmental conditions, select the best frequency according to the hours of the day, the seasons, adapt the flow rate of his transmitter, retransmit his messages in the event of losses…

The arrival of satellite will simplify the operator's work, bring new and valuable services but also bring new constraints to the surface. Indeed, establishing a link on a geostationary satellite requires being able to aim the satellite, which is not always easy when moving in restricted spaces (buildings, relief, forests, etc.) and are geographically limited in coverage: For example, Inmarsat services are only possible up to 75 or 76 degrees North or South (with a loss of throughput from 65 degrees). In addition, this technology remains expensive, the price of the satellite link remains a barrier to its hegemony.

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The growing and constant need for communication in modern weapon systems offers a new lease of life to the HF band

Other factors could also have led to a decline in the use of HF, particularly during the 90s: V/UHF links with higher speeds, the advent of GSM, HF modems limited in speed, etc. At that time, faced with the emerging needs on the networks (image and video), the HF seems poorly equipped. But the interest of this frequency band lies in its flexibility with the possibility of receiving signals in the mountains, in the jungle, in a canyon, under a few meters of water, in short in environments where the other frequency ranges are often inoperable.

In order to be as accurate and up-to-date as possible, the efforts of satellite service providers should be noted. Indeed, the strategic aspect of the Arctic has led a company like Inmarsat to invest as part of its Global Express evolution on two high elliptical orbit satellites, the “Arctic Satellites Broadband”. Elon Musk's famous SpaceX launcher is expected to launch the two satellites on a Falcon 9 rocket at the end of 2022.

Why mention this polar region? Without (necessarily) knowing it, you consume the HF band when you take, for example, a Paris / New York flight. The great circle route takes you very north, the airliner will regularly send its position in HF via the HFDL system (High Frequency Data Link), on this data transmission frequency which has nothing military in it but which has its reason to be due to its very low operating cost and the need for redundancy of aeronautical resources. More than 3000 devices are equipped and traffic represents more than 4 million messages per month. 

The HF range: complex environment to master but constant technical improvements.

What makes HF connections special is that it requires perfect knowledge and understanding of the propagation conditions. Clearly, it is possible to go from (rare) conditions where it is practically impossible to establish a connection due to solar activity (blackout) to a situation of abnormal amplification of signals (the famous ducts and other very regular maritime surface conduits in case of high heat). Between its two extremes, propagation generates its share of reception of the same signal by multiple paths, and variation in power at reception (fading phenomenon), interference between signals coming from the three different regions of the International Telecommunications Union. 

In order to facilitate the work of operators, automatic link establishment (Automatic Link Establishment 2G then 3G) will revolutionize contact between stations with a capacity for automatic selection on the best frequency and convergence towards IP addressing. The man-machine interface will also evolve with software interface radios (Software Defined Radio). 

But it is the very clear improvement in throughput that places HF in a favorable position today. By simultaneously combining more than ten channels of around 3 kHz, speeds can exceed 100 kbps and projects like that of THALES HFXL/Salamander are giving new impetus to this range of respectable age.

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THALES HFXL modems offer a trans-horizon HF data connection solution without satellites at more than 100 kbps

It remains to solve the problem of antenna placement. France is not behind in the area of ​​propagation calculation and network planning. We must salute the work of the ATDI company with its particular work on this frequency range.

The HF range: classic users and new users

There are many users of this range: emergency services, radio amateurs, diplomatic services, tactical military data transmissions, civil aviation voice on transoceanic flights, maritime surveillance helicopter, aeronautical data links, military and civil navies, military air and land forces, security services, commercial analog or digital radios, etc.

But one user of the HF band stands out due to the size of its bandwidth: radar, with a very particular signature on the spectrum, present in coastal maritime surveillance as well as in strategic air surveillance.

The proliferation of HF radars

Within this already restricted band, the use of radar consolidates year after year. Faced with targets whose Radar Equivalent Surface Areas (SER) are very well studied, HF radar stands out. Losses in the atmosphere are much lower in HF and many nations have developed or installed radars operating on this frequency range. Two types of systems are mainly present: those dedicated to coastal surveillance and those dedicated to aerial surveillance (Over The Horizon Radar).

Coastal HF radars are multistatic, that is to say they transmit on several transmitters, and receive on several receivers, in vertical polarization to search for propagation by surface wave, at sea level. Initially used for measuring marine currents, they now make it possible to ensure surface monitoring over approximately 200 nautical miles, thus responding to the problem of monitoring the Exclusive Economic Zone. The USA (with the CODAR firm), France (Stradivarius project), Germany (WERA), Italy, Spain, Croatia, China, Russia, Saudi Arabia, Morocco (among others ) use these radars which can even monitor abnormal meteorological phenomena such as tsunamis.

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Presentation of the operation of the French Stradivarius by diginext

These surface wave radars often transmit in frequency modulated continuous wave (FMCW).

OTH radars, for their part, are oriented towards “air surveillance” more frequently in pulsed transmission, with a very long pulse repetition time, that is to say having a listening time calibrated for maximum theoretical surveillance of 6000 kilometers. Users of this type of radar are Australia with the JORN project, France with Nostradamus, the USA, the United Kingdom with the Pluto radar (in FMCW) located in Akrotiti (Cyprus), Iran with the Ghadir radar , Russia with the Rezonans and Kontayner radars, Israel, China, Romania, Ukraine… What constitutes the characteristic of these systems is their bandwidth of at least 25 kHz per transmitter, which can reach 160 kHz in China and even 1 MHz in Iran, around 28 MHz. For the record, a typical user occupies less than 3 kHz in single sideband.

Daily clashes on the HF spectrum

This range also sees states clashing through interference on commercial radio. Thus China regularly jams the broadcasts of the Taiwan radio “Sound Of Hope” directed towards its territory, South Korea jams the broadcasts of the official North Korean radio, Ethiopia interferes on the Eritrean radio with its HF jammers.

From the point of view of non-state actors, non-compliant uses are also present. On the surface of the Indian Ocean and more particularly the Arabian Sea, this area known for its illicit activities, pirates are also present in HF. Their lack of knowledge of frequency allocation limits regularly leads them to exchange on non-maritime frequencies. We see that their illegal activities also concern spectrum occupation, and in the radio world, this type of occupier is called an “intruder”.

As a more peaceful example, in the Atlantic Ocean, these “intruders” are made up of fishermen transmitting by voice, and beacons associated with drifting fishing nets. More generally across the globe, certain Russian military transmission systems are regularly out of their allocations and interfere with official users. 

Without playing Cassandra, it is possible to envisage future confrontations through the prism of combined attacks (physical and/or cyber) on submarine cables and satellite links, the power received of which remains low. The infrastructures, both for the cables and the ground/telemetry segment of the satellites could show their vulnerabilities, and in this context HF would return to a priority means of communications, secure, combining mobility, rusticity and advanced modulations to ensure competitive speeds.

Mikael Riot

Electronic Warfare Specialist

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