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Heliograph

The heliograph is a portable optical signaling device that transmits messages by reflecting sunlight through a mirror to create flashes of light, typically encoding information in Morse code for long-distance communication without wires.^[1] Invented in 1869 by British engineer Henry Christopher Mance for the British-Indian Army, it combined elements of earlier heliotropes—surveying tools using mirrors to reflect sunlight for alignment—with Samuel Morse's telegraph code to enable rapid, line-of-sight signaling.^[1] The device consists of a silvered mirror (often 5 to 8 inches in diameter) mounted on a tripod or surveying instrument, operated via a simple key that tilts the mirror to interrupt the beam, producing short flashes for dots and longer ones for dashes, with skilled operators achieving speeds of 12 to 15 words per minute.^[2]^[1] Early concepts of heliographic signaling date back to antiquity, with historical accounts describing its use by ancient Greeks around 405 BC during military campaigns, where polished bronze shields reflected sunlight to convey warnings, and by Romans under Emperor Tiberius in the 1st century AD for similar purposes.^[2] The first detailed written description appeared in 1292 from English philosopher Roger Bacon in his work Opus Majus, outlining a method to use mirrors for transmitting signals via sunlight.^[2] Modern development accelerated in the 19th century; German mathematician Carl Friedrich Gauss created a heliotrope in 1821 for geodetic surveying, which Mance adapted by adding a shutter mechanism and integrating it with electrical telegraph codes.^[2] By the 1870s, the heliograph had evolved into a military tool, featuring adjustable mirrors and tripods for precise aiming, capable of ranges up to 50 miles with a 5-inch mirror and over 100 miles in optimal conditions like clear mountain air.^[2]^[3] The heliograph proved invaluable in 19th- and early 20th-century warfare due to its portability, low cost, and effectiveness in open terrain, seeing widespread use in conflicts such as the Second Anglo-Afghan War (1878–1880), the Anglo-Zulu War (1879), and the Second Boer War (1899–1902) by British forces.^[1] In the United States, the U.S. Army Signal Corps adopted a variant in 1878 under General Nelson A. Miles, establishing networks like the 140-mile chain in Montana and a 27-station, 800-mile system during the 1886 Apache campaign against Geronimo, where it relayed over 2,200 messages, including critical intelligence that helped end the last major Apache raids on June 5, 1886.^[3]^[1] Other nations, including the Ottoman Empire, employed it for Morse code transmission in World War I theaters.^[4] Its advantages included rapid deployment without infrastructure and secure, narrow-beam signals resistant to broad interception, though limitations such as dependence on direct sunlight, clear visibility, and daytime conditions restricted its reliability in overcast or foggy weather.^[2]^[1] By the early 20th century, the heliograph's role diminished with the advent of radio telegraphy, which offered all-weather, non-line-of-sight communication, though it remained in limited reserve use during World War I and even into World War II in some remote areas.^[2] Today, it stands as a historical precursor to modern optical and wireless technologies, exemplifying early innovations in visual telecommunications.^[1]

Fundamentals

Principle of Operation

The heliograph functions as a wireless optical telegraph by utilizing the reflection of sunlight from a mirror to produce a concentrated beam of light directed toward a distant receiver. This beam is generated when the operator aligns the mirror to capture direct solar rays and focuses them into a narrow, intense flash visible over extended distances, enabling line-of-sight communication without intermediate relays.^[3]^[5] To encode messages, the beam is intermittently interrupted using a mechanical key or shutter mechanism, creating short and long flashes that correspond to the dots and dashes of Morse code. This modulation allows for the transmission of textual information at rates of approximately 8 to 16 words per minute under favorable conditions, with the operator adjusting the mirror's orientation to maintain alignment during signaling.^[6]^[2]^[7] The effective range of a heliograph is influenced by several key factors, including the size of the mirror, atmospheric conditions such as clarity and humidity, and the elevation of the sun relative to the horizon, which affects beam intensity. In standard military applications with mirrors typically 5 to 8 inches in diameter, operational ranges commonly spanned 20 to 50 miles (32 to 80 km), though under exceptional clear conditions, a record distance of 183 miles (295 km) was achieved between Mount Ellen in Utah and Uncompahgre Peak in Colorado on September 17, 1894.^[2]^[7]^[8] Among its advantages, the heliograph offers high portability due to its lightweight construction, requiring no external power sources beyond sunlight, and provides inherent security through line-of-sight transmission that is difficult to intercept without physical elevation. These attributes made it particularly suitable for field operations in remote or rugged terrains where electrical alternatives were impractical.^[6]^[3]

Basic Components

The traditional heliograph consists of several key physical elements designed for portability and precise sunlight manipulation. At its core is the primary mirror, typically a flat, silvered glass disc measuring about 5 inches in diameter, which captures and reflects sunlight toward a distant receiver. In dual-mirror configurations, such as the duplex setup of the Mance heliograph, a secondary or rear mirror redirects the beam when the sun is positioned behind or overhead relative to the sender, ensuring the light path aligns correctly with the primary mirror. These mirrors are mounted on a lightweight frame, often with an unsilvered central spot to facilitate direct sighting through the glass.^[9]^[2]^[10] The shutter mechanism, integral to signal modulation, is usually a hand-operated lever or key that pivots the primary mirror slightly to interrupt the continuous beam of light. This device, positioned directly behind the mirror, allows for quick tilting—often just a few degrees—to create on-off flashes, and in some designs, it incorporates a slotted plate for beam occlusion. The mechanism is connected via simple linkages to ensure responsive manual control, emphasizing the heliograph's reliance on mechanical simplicity for field use.^[9]^[2]^[10] For accurate alignment, sighting devices are attached to the mirror assembly, including a V-shaped vane or rod equipped with crosshairs or a notched sight that allows the operator to aim the reflected beam at the receiving station. These are adjustable via tangent and elevation screws on the frame, enabling fine-tuning of the mirror's orientation relative to the target. In duplex modes, the secondary mirror can double as a sighting aid when solar angles exceed 120 degrees.^[9]^[2]^[10] The entire assembly is supported by a portable tripod mount, typically constructed from sturdy wood with telescoping legs for height and angle adjustments, providing stability on uneven terrain. A central hook beneath the tripod head accommodates a stabilizing weight, such as a sandbag, to prevent vibration during operation. The design prioritizes mobility, with the complete unit folding into a protective case for transport.^[9]^[2]^[10] A representative example is the British Army's Mance Mark V heliograph, featuring a 5-inch primary mirror and weighing approximately 7 pounds, which assembles by securing the mirror frame to the tripod and attaching the auxiliary arm for the secondary mirror as needed. Together, these components facilitate the reflection and interruption of sunlight to transmit signals like Morse code over long distances.^[9]^[10]

Historical Development

Origins and Early Inventions

The earliest recorded instances of heliographic signaling trace back to ancient civilizations, where sunlight reflected from polished metal surfaces served as a rudimentary communication method during military engagements. In ancient Greece, soldiers reportedly used burnished shields to flash sunlight signals across distances, with one possible example occurring at the Battle of Marathon in 490 BC, as alluded to in Herodotus's account of a shield raised as a signal to Persian forces after the Athenian victory.^[1] Similarly, around 405 BC, Athenian forces employed polished shields for battlefield signaling, as interpreted from Xenophon's Hellenica, enabling quick transmission of basic alerts from elevated positions.^[2] These primitive techniques relied on the reflective properties of bronze or silvered surfaces to create visible flashes, marking the conceptual origins of solar-based optical communication. Subsequent ancient uses included Roman Emperor Tiberius (reigned 14–37 AD), who reportedly signaled from Capri to the mainland using mirrors, as described by Suetonius in his Lives of the Caesars.^[11] In the medieval period, the first detailed written description of heliographic signaling appeared in 1292 in English philosopher Roger Bacon's Opus Majus, where he outlined methods to transmit signals via sunlight reflected by mirrors.^[12] In the early 19th century, advancements in surveying instruments laid further groundwork for heliographic development. German mathematician Carl Friedrich Gauss invented the heliotrope in 1821, a device consisting of a mirror mounted on a theodolite to reflect sunlight toward distant observers, facilitating precise geodetic measurements over distances exceeding 50 kilometers under clear conditions.^[13] Primarily designed for land triangulation in hazy daylight—where traditional flags or lamps proved inadequate—the heliotrope produced a bright, intermittent beam visible from afar, though it was intended solely as a positional marker rather than a messaging tool.^[1] This innovation, tested extensively in German surveys, demonstrated the potential for long-range solar reflection but remained limited to non-communicative applications. Building on such precursors, early experimental signaling systems emerged in the mid-19th century. British officer Sir Henry Mance, stationed in India, proposed and developed a heliograph prototype in the 1860s, drawing direct inspiration from Gauss's heliotrope to create a portable device for military use.^[2] Mance's design featured a five-inch silvered mirror adjustable via a simple mechanism, allowing operators to direct sunlight flashes toward receivers up to 50 miles away.^[1] However, these initial inventions suffered from significant constraints: they lacked standardized encoding schemes, restricting signals to binary or prearranged patterns rather than complex messages, and were entirely dependent on favorable weather, rendering them useless during overcast skies or at night without moonlight.^[2]

19th Century Advancements

The heliograph evolved from earlier heliotrope devices used in surveying, which reflected sunlight steadily for alignment rather than for signaling.^[1] In 1869, British engineer Henry Christopher Mance developed the first practical heliograph while working in Karachi for the British-Indian Army's telegraph department, featuring a 5-inch silvered mirror with an unsilvered sighting spot and a tilting mechanism operated by a telegraph key to create intermittent flashes.^[2]^[1] This design addressed the limitations of prior reflectors by enabling controlled signaling over distances, making it suitable for military field operations.^[9] By the mid-1870s, the Mance heliograph gained widespread adoption and standardization among armies globally, including the British and U.S. forces, which integrated it into their signal corps for reliable long-range communication.^[2]^[1] The U.S. Army, for instance, officially adopted it in 1877 and deployed networks of up to 34 stations across regions like Arizona by 1886 to enhance coordination in expansive terrains.^[1] Throughout the late 19th century, innovations focused on enhancing portability and extending range, such as mounting mirrors on lightweight tripods and enclosing them in compact leather cases for easy transport by troops.^[2] Larger mirrors, up to 12 inches in diameter, and refined sighting systems with telescopic aids were tested during colonial operations in areas like India and Afghanistan, achieving effective ranges of 50 to 100 miles under clear conditions.^[9]^[2] These upgrades prioritized mobility without sacrificing signal clarity, allowing operators to set up and transmit in minutes across rugged landscapes.^[1] The heliograph's effectiveness was further advanced by integrating Morse code for messaging, where short and long flashes represented dots and dashes, respectively, standardized across adopting militaries.^[2] Skilled operators could achieve transmission rates of 10 to 15 words per minute, sufficient for tactical updates over vast distances when electrical telegraphs were unavailable.^[1]^[2]

Applications

Military Uses

The heliograph played a pivotal role in British military communications during colonial conflicts in the late 19th century. In the Anglo-Zulu War of 1879, British forces deployed heliographs during the second invasion starting in June, establishing a chain of 15-16 stations covering up to 35 miles, such as from Doornberg to Itelezi Mountain and Ibabanango, to link with Landman's Drift.^[14] These devices, including 3-inch, 6-inch, and 10-inch mirrors supplemented by Roorkee 5-inch models from India, enabled transmission rates of 14-15 words per minute in Morse code, surpassing the inventor's claimed 10 words per minute, though initial effectiveness was limited by unfamiliarity among troops.^[14] Similarly, during the Second Anglo-Afghan War (1878-1880), British troops utilized heliographs for long-distance signaling in rugged Afghan terrain, as illustrated in contemporary accounts from operations in the Kurram Valley.^[2] In the Boer War (1899-1902), the heliograph served as a critical mobile communication tool for British forces, facilitating coordination across vast South African landscapes and proving even more vital for Boer commandos lacking fixed infrastructure.^[15] The United States Army also employed heliographs extensively in frontier warfare. In the 1880s campaign against Apache leader Geronimo, Brigadier General Nelson A. Miles established a network of 23 stations across Arizona and New Mexico territories, spaced approximately 25 miles apart, with key sites at Fort Bowie and Bowie Peak connected to telegraph offices.^[3] This system, staffed by teams of three operators and two guards per station and supplied via pack mules, transmitted real-time intelligence on Apache movements—for instance, a June 5, 1886, report from Antelope Springs—and deterred nighttime raids by illuminating potential escape routes, ultimately contributing to Geronimo's surrender.^[3] Heliographs offered several tactical advantages in military operations, particularly in challenging environments. Their lightweight design—typically around 7 pounds for portable models—allowed a single operator to carry and deploy them rapidly, often in under five minutes on tripods for stability, enabling quick establishment of ad hoc networks without extensive infrastructure.^[7]^[2] The narrow beam of reflected sunlight provided inherent security, as signals were nearly impossible to intercept outside the direct line of sight, unlike radio transmissions vulnerable to eavesdropping.^[2] In rugged terrain, such as mountains or deserts, heliographs excelled with ranges of 40-50 miles under clear conditions—up to 183 miles recorded in optimal tests—outperforming alternatives like flags in mobility and reliability for coordinating dispersed units.^[3]^[1] Despite these strengths, heliographs had notable limitations that curtailed their battlefield utility. Operations required direct sunlight and clear atmospheric conditions, rendering them ineffective during overcast weather, fog, or at night without moonlight adaptations, which restricted continuous use in variable climates.^[1]^[2] By the early 20th century, the advent of wireless radio technology rendered heliographs obsolete for most applications, as radios offered all-weather, non-line-of-sight communication.^[1] The British Army phased out heliographs in the 1960s, though they persisted in limited roles until then, while Australian forces continued using them for signaling and surveying into the mid-1960s.^[16]^[17] Other nations adopted heliographs for military purposes. During World War I, the Ottoman Empire employed them for Morse code transmission in various theaters, leveraging their portability in regions with limited infrastructure.^[4] Military adoption of heliographs was supported by formalized training and doctrinal protocols within signal corps. In the British Army, instruction began at the Signal School established in Chatham in 1865, with manuals like the 1907 Training Manual—Signalling detailing setup procedures, mirror alignment for beam control, and Morse code protocols, emphasizing one-operator efficiency and atmospheric adjustments.^[2]^[18] U.S. Army Signal Corps training, outlined in the 1905 Manual of Visual Signaling, covered heliograph operation alongside flags and lamps, including care instructions and range testing, with regimental signallers drilled in on-the-job proficiency during campaigns like the Anglo-Zulu War.^[19] These doctrines prioritized heliographs for emergency or backup roles in visual signaling hierarchies, integrating them into broader communication strategies for colonial and frontier operations.^[14]

Non-Military Uses

The heliograph, particularly in the form of the heliotrope, played a significant role in non-military surveying efforts, enabling precise triangulation for land mapping over vast distances. Surveyors used mirrors to reflect sunlight toward distant observation points, serving as visible targets for angular measurements in geodetic networks. In the United States, the U.S. Coast and Geodetic Survey adopted heliotropes starting in the mid-19th century, with continued use into the 1880s for coastal and inland triangulation projects that mapped national boundaries and topographical features.^[20]^[13] These instruments allowed accurate positioning without physical connections, essential for establishing control points in rugged terrains where direct measurement was impractical. In forestry, the heliograph facilitated communication and fire detection across expansive, inaccessible woodlands. The U.S. Forest Service began adopting portable heliographs in 1909 to supplement limited telephone lines, enabling rangers to signal fire sightings or coordinate responses using Morse code flashes visible up to 50 miles on clear days.^[7]^[21] This system proved vital for early wildfire management in remote national forests, where rangers on mountaintops could alert crews to smoke plumes without relying on foot messengers or incomplete wire networks. Heliographs also supported exploration and mountaineering by providing reliable signaling in line-of-sight conditions during expeditions and infrastructure projects. In Arctic and high-altitude ventures, explorers employed them for inter-party coordination in areas lacking other communication means, while railway construction teams used heliotropes for surveying alignments across prairies and mountains, ensuring precise route planning in the late 19th and early 20th centuries.^[13]^[22] These applications highlighted the device's civilian benefits: its low cost—often under $10 per unit—simplicity in construction from basic mirrors and mounts, and minimal maintenance needs, requiring no batteries or wires, made it ideal for remote operations without established infrastructure.^[2] The heliograph's prominence waned in the early 20th century as electrical telegraphs and radios offered all-weather, non-line-of-sight alternatives, but it persisted in isolated regions for surveying and emergency signaling into the mid-20th century, particularly where sunlight was abundant and modern electronics unavailable.^[1]

Automated and Modern Variants

Early Automated Systems

The transition from manual heliographs to early automated systems in the late 19th and early 20th centuries focused on mechanical innovations to compensate for the sun's apparent motion, thereby minimizing constant operator adjustments for beam alignment. British mathematician Charles Babbage proposed one of the first such concepts around 1860, suggesting a clockwork mechanism to automatically rotate the mirror and track the sun, building on earlier manual designs like Henry Mance's 1869 heliograph. However, Babbage's idea was rejected for military adoption due to its mechanical complexity and potential unreliability in rugged field environments.^[2] French engineers advanced these ideas with clockwork-driven heliostats, which used geared mechanisms to steer mirrors and maintain a steady reflected beam without ongoing manual input. By the 1880s, such heliostats were used in military signaling. These setups allowed for more reliable long-distance communication in colonial campaigns, though the core flashing modulation remained manual via a pivoting mirror or shutter.^[2] During World War I, British forces used the heliograph for visual signaling. The Mk V heliograph, standard issue since 1904, saw active use in this period for ranges up to 50 miles under clear conditions, often bridging gaps in radio reliability.^[23]^[2] Despite these advances, early automated systems faced significant limitations, including mechanical unreliability from gear wear, dust ingress, and temperature extremes in field conditions, which often necessitated manual overrides. The persistent requirement for operators to encode messages in Morse code via physical controls limited full autonomy, while dependence on sunlight restricted use to daytime and clear weather, foreshadowing the shift toward electrical and radio alternatives that overcame optical constraints.^[2]^[1]

Contemporary Developments

In contemporary contexts, simplified heliograph variants known as signal mirrors continue to serve as essential components in military and aviation survival kits. These compact, reflective devices enable emergency signaling over distances of up to 10 miles or more by flashing sunlight, requiring no power source. The U.S. military has included such mirrors in standard issue since World War II, with models like the MIL-M-18371E glass signal mirror designed specifically for downed pilots and still in active use as of 2025 for their durability and precision aiming grids.^[24] Similarly, they feature in broader rescue survival kits for remote emergencies, providing a reliable optical backup when radios fail.^[25] Prototypes integrating digital elements with traditional heliograph principles have emerged since 2015, aiming to enhance functionality in off-grid environments. One notable example is the world's first digital heliograph, developed by an eighth-grade student for a science fair project using Autodesk Inventor software; this design automated aspects of sunlight reflection for messaging without relying on electronic transmission.^[26] Such innovations combine mirrors with basic sensors to facilitate targeted signaling, though they remain experimental and focused on low-tech reliability rather than widespread deployment. Heliographs find niche roles in disaster response for remote areas, where electronic systems may be disrupted, and as alternatives to radio during events like solar flares that ionize the ionosphere and cause communication blackouts.^[27] Their inherent EMP resistance—stemming from the absence of circuits—and zero-power operation make them valuable for scenarios involving electromagnetic threats or power shortages, such as amateur radio backups.^[28] Despite these advantages, heliographs have seen no widespread military adoption since the 1960s, relegated to supplementary status amid advances in radio and satellite technology.^[25]

References

[1]
Light Conversation: The Heliograph - HistoryNet
Feb 6, 2017 · The heliograph, a mirrored instrument that could blink messages across great distances, was the latest in military communications technology.
[2]
Heliograph - Royal Signals - Index
A first clear description of a heliographic signalling method was published in 1292 by the English author Roger Bako (often spelled Bacon, 1214-1292), in a work ...
[3]
The Heliograph - Fort Bowie National Historic Site (U.S. National ...
Feb 24, 2015 · The heliograph was the invention of a British engineer who attached a mirror to surveying equipment in order to redirect a beam of light on distant points.
[4]
Ottoman Army heliograph - NZ History
Apr 2, 2015 · The heliograph was a wireless solar telegraph instrument that relayed Morse code using flashes of sunlight reflected by a mirror.<|control11|><|separator|>
[5]
Heliograph | National Museum of American History
A heliograph is a simple but effective solar telegraph that signals by flashes of sunlight reflected by a mirror.
[6]
Signal Corps harnesses the sun | Article | The United States Army
Jul 27, 2020 · The US Army Signal Corps began experimenting with the heliograph, a method of visual signaling using reflections of sunlight bounced off of mirrors, around ...
[7]
[PDF] The heliograph was a communications device used to transmit ...
DESCRIPTION: The heliograph was a communications device used to transmit messages over long distances. It uses a mirror attached to a surveying device to ...
[8]
Observation Programs - National Weather Service
... 183 miles apart. This unprecedented feat of long distance intercommunication by visual signals was made on Sept 17, 1894, with Signal Corps heliographs ...Missing: maximum | Show results with:maximum
[9]
Heliographs. - Douglas Self
May 1, 2003 · The heliograph was a simple but highly effective instrument for instantaneous optical communication over 50 miles or more in the 19th century.
[10]
Heliograph Mk5 - Collections WA
Feb 12, 2025 · In 1869 he invented the heliograph, a wireless solar telegraph that signals by flashes of sunlight using Morse code reflected by a mirror. The ...
[11]
The Surveyors' Heliotrope: Its Rise and Demise - LIDAR Magazine
Oct 31, 2004 · Carl Friedrich Gauss The solution proved to be the heliotrope instrument, an invention that was conceived early in the nineteenth century by ...
[12]
[PDF] Signalling in the Anglo Zulu War, 1879
Apr 2, 2020 · The heliograph had recently been invented and used in military operations, but mostly in India, and British troops were not familiar with it.
[13]
Signals communication in the South African War 1899-1902
Jun 1, 1998 · In heliography, a mirror is used to flash light in Morse code. Using this method, eight to sixteen words could be transmitted per minute and ...
[14]
Military - Museum of Technology, The History of Gadgets and Gizmos
Heliograph's were standard issue in the British and Australian armies until the 1960s, and were used by the Pakistani army as late as 1975. This item can be ...
[15]
Instrument - Heliograph Mk V Training Aid - circa 1912, 1912
Heliograph ... Army and other military forces from around 1906 until the mid 1960s. It remained in use with the Australian Army Survey Corps until the mid 1980s.
[16]
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Contents ; To set up the heliograph. 279 ; To lay the heliograph. 280 ; To signal with the heliograph. 283.
[17]
Manual Of Visual Signaling of the U.S. Signal Corps - Internet Archive
Apr 17, 2016 · The heliograph 63 Ranges accomplished 66 Setting up and operating the heliograph 75 Suggestions for operating and caring for the heliograph 80 ...Missing: protocols training
[18]
Heliotrope | National Museum of American History
417" inscriptions indicated that this heliotrope was made after 1878 when the U.S. Coast Survey became U. S. Coast and Geodetic Survey, and before Saegmuller's ...Missing: 1880s | Show results with:1880s
[19]
The Heliograph: Spring 2024 (U.S. National Park Service)
May 31, 2024 · In the early 20th century, the US Forest Service used the heliograph to aid in forest management. Using Morse Code to communicate through dots ...Missing: mountaineering | Show results with:mountaineering
[20]
The Survey of the Earth and The White Tank Mountains
Dec 2, 2014 · Working with acetylene signal lamps at night and heliographs by day, the men sent signals from mountain top to mountain top, taking measurements ...<|control11|><|separator|>
[21]
Heliostat by Jules Duboscq, 1850-1880
This heliostat was made by the Parisian instrument maker, Jules Duboscq. It consists of a mirror attached to a clockwork drive that will track the Sun over ...Missing: 1884 | Show results with:1884
[22]
Early communications – pre Corps | Royal Signals Museum
Victory signals and heliographs C Telegraph Troop saw active service in the Zulu War of 1879 providing telegraph communications for the field army.
[23]
https://www.royalsignalsmuseum.co.uk/early-communications-pre-corps/
[24]
Heliographs: From Sun Signals to Early Telecommunications
Jul 7, 2023 · The first recorded use of a heliograph dates back to 405 BC when the Ancient Greeks ingeniously employed polished shields to signal during ...
[25]
World's First Digital Heliograph designed in Autodesk
Sep 17, 2015 · Before building the world's first Digital Heliograph for a science fair project, an 8th grade student designed it in Autodesk Inventor.Missing: GPS prototype
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Understanding how solar flares affect radio communications
Jul 20, 2022 · Solar flares affect radio communications by interfering with the active frequencies of communication systems, particularly HF radios and transceivers.
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Radio blackouts are classified using a five-level NOAA Space Weather Scale, directly related to the flare's max peak in soft X-rays reached or expected.