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Ground wave

Ground wave propagation refers to the transmission of electromagnetic radio waves that travel along the Earth's surface, following its curvature through diffraction and surface guiding, enabling reliable communication over distances beyond the optical horizon without dependence on ionospheric reflection.^[1] This mode of propagation is most effective for frequencies between 10 kHz and 30 MHz, where the waves interact closely with the ground's conductivity and dielectric properties, resulting in relatively low attenuation over both land and sea paths compared to higher frequencies.^[2] The ground wave signal comprises three primary components: the direct wave, which follows a straight line-of-sight path from transmitter to receiver; the ground-reflected wave, which bounces off the Earth's surface; and the surface wave (also known as the Norton or Sommerfeld wave), which diffracts around obstacles and is guided parallel to the terrain.^[3] Key factors influencing ground wave propagation include the electrical characteristics of the ground—such as conductivity (higher over seawater, around 5 S/m, versus poor soil at 0.001 S/m) and permittivity—as well as terrain irregularities, atmospheric refractivity in the troposphere, and the heights of the transmitting and receiving antennas.^[1] Signal strength decreases with distance due to absorption and spreading losses, often following an inverse distance relationship for short ranges and steeper attenuation over longer paths, with vertical polarization being predominant to minimize ground losses.^[4] Unlike skywave propagation, ground waves are available continuously day and night, unaffected by solar activity, but their range is limited by increasing frequency and poor ground conditions.^[2] The theoretical foundations of ground wave propagation were established in the early 20th century, with Johann Zenneck proposing a surface wave model in 1907 and Arnold Sommerfeld developing a rigorous mathematical solution for wave attenuation over a flat Earth in 1909, later refined by Kenneth Norton in the 1930s and James Wait in the mid-20th century to account for spherical Earth geometry and mixed terrain paths.^[3] International standards, such as ITU-R Recommendation P.368, provide prediction methods and curves for field strength calculations, incorporating numerical models like the GRWAVE program to support planning for broadcasting and other services.^[1] Practically, ground wave propagation underpins medium-frequency (MF) amplitude modulation (AM) radio broadcasting, which reaches hundreds of kilometers for regional coverage; maritime mobile communications in the high-frequency (HF) band; and specialized applications like differential global positioning systems (DGPS) at 285–325 kHz for precision navigation, as well as intelligent transportation systems (ITS) in the 535–1605 kHz range for traveler information dissemination.^[4] Its reliability over irregular terrain makes it essential for non-line-of-sight scenarios, though modern implementations often integrate it with digital modulation techniques to enhance data rates and robustness.^[2]

Fundamentals

Definition and Characteristics

Ground wave propagation refers to a mode of radio wave transmission in which electromagnetic waves travel along the surface of the Earth, closely following its curvature. Unlike sky waves that rely on ionospheric reflection or direct waves limited by line-of-sight, ground waves are primarily induced by currents flowing through the ground itself, enabling propagation without significant atmospheric refraction or reflection.^[5]^[6] This mechanism is particularly effective for medium frequency (MF, 300 kHz to 3 MHz) and low frequency (LF, 30 kHz to 300 kHz) bands, where wavelengths are sufficiently long—related to frequency by the inverse proportion λ ∝ 1/f, with c as the speed of light—to interact strongly with the Earth's surface.^[5]^[3] Key characteristics of ground waves include their ability to diffract around the Earth's curvature, allowing signals to extend beyond the optical horizon, and a preference for vertical polarization, which minimizes attenuation compared to horizontal polarization.^[6]^[5] The field strength of ground waves decays approximately inversely with distance, further modified by an attenuation factor that includes exponential losses dependent on ground conductivity and surface properties, resulting in stronger signals over conductive terrains like seawater.^[6]^[3] Propagation distances typically reach hundreds of kilometers over highly conductive surfaces, though they are limited by increasing frequency, terrain irregularities, and atmospheric conditions, often dropping to tens of kilometers over poor soil.^[5]^[3] These properties make ground waves a reliable mode for short- to medium-range communication in the lower frequency spectrum, distinct from higher-frequency propagations that suffer greater surface losses.^[6]

Historical Development

The concept of ground wave propagation emerged from early experiments demonstrating the behavior of electromagnetic waves near the Earth's surface. In the 1880s, Heinrich Hertz conducted pioneering laboratory experiments that confirmed the existence of electromagnetic waves and observed their diffraction around obstacles, laying the groundwork for understanding wave guidance along boundaries like the ground.^[7] These findings, building on James Clerk Maxwell's theoretical predictions, highlighted how waves could bend over curved surfaces, though practical applications in radio were yet to come. Marconi's early experiments in the late 1890s and his successful 1901 transatlantic transmission, received in Newfoundland from England, further underscored the distinction between surface-following ground paths, which offered reliable but limited range, and ionospheric-reflected sky paths that enabled longer distances but were less predictable.^[8] Theoretical foundations were laid in the early 1900s. In 1907, Johann Zenneck proposed a surface wave model for propagation along the Earth-air interface. Arnold Sommerfeld developed a rigorous mathematical solution in 1909 for wave attenuation over a flat Earth with finite conductivity.^[3] In the 1920s, as amplitude modulation (AM) radio broadcasting expanded, ground waves gained recognition for providing stable daytime coverage over hundreds of kilometers, unaffected by ionospheric interference that plagued nighttime sky wave signals.^[9] This reliability made ground waves essential for local and regional AM stations, which proliferated following the first commercial broadcasts around 1920, enabling consistent service without the fading common in higher-angle propagation. By the 1930s, international efforts through the International Telecommunication Union (ITU)'s predecessor, the International Radio Consultative Committee (CCIR), began systematic studies on ground conductivity's impact on propagation, compiling data on soil types and their effects on signal attenuation to standardize predictions for broadcasting and maritime communication.^[10] These investigations, informed by empirical measurements across diverse terrains, emphasized how conductive surfaces like seawater enhanced range compared to dry land. Key theoretical milestones advanced ground wave understanding in the mid-20th century. In 1936, Kenneth A. Norton introduced a practical flat-Earth model in the Proceedings of the Institute of Radio Engineers, providing simplified formulas and graphs for calculating ground wave field strengths over uniform paths, which became a cornerstone for engineers. Post-World War II, ground waves found critical application in navigation systems like LORAN (Long Range Navigation), developed in the early 1940s by the U.S. military and deployed widely by 1944, using medium-frequency pulses to achieve hyperbolic positioning over 1,000 miles via surface propagation for Allied ships and aircraft. By the 1950s, the field shifted from empirical observations to rigorous theoretical frameworks, incorporating Sommerfeld's 1909 boundary solutions and Norton's refinements, though further updates diminished after the 1970s as higher-frequency VHF and UHF bands dominated broadcasting and communication, relegating ground waves primarily to low-frequency uses.

Propagation Physics

Diffraction and Surface Wave Formation

Ground waves enable propagation beyond the line-of-sight horizon by diffracting around the curvature of the Earth, allowing radio signals to follow the terrestrial surface over distances that exceed direct geometric visibility.^[11] This diffraction phenomenon is fundamentally described by the Huygens-Fresnel principle, which posits that every point on a wavefront serves as a source of secondary spherical wavelets whose superposition forms the subsequent wavefront.^[11] In the context of radio waves, this principle accounts for the bending of electromagnetic fields over irregular terrain, where phase shifts and amplitude variations arise from interactions with the Earth's surface, facilitating ground wave coverage particularly at medium and high frequencies.^[11] The formation of surface waves in ground propagation occurs through the induction of electric currents within the conductive layers of the Earth's surface, effectively turning the ground into a guiding medium akin to a waveguide.^[12] These induced currents, driven by the tangential components of the incident electric field, support a non-radiating mode that propagates parallel to the interface between air and ground.^[13] Zenneck's theory, developed in 1907 for an idealized flat, homogeneous surface, models this as a polarized plane wave solution to Maxwell's equations at the boundary, where the wave is bound to the surface and attenuates both laterally and perpendicularly without significant radiation into free space.^[12] This theoretical framework, later refined by Sommerfeld in 1909, highlights the surface wave's dependence on the dielectric and conductive properties of the ground to maintain guided propagation.^[14] At very low frequencies, the Earth-ionosphere system provides partial guidance for ground waves by forming a natural waveguide, where the ionosphere acts as a reflective upper boundary that channels energy alongside the ground interaction.^[15] However, the primary mechanism remains the direct coupling with the Earth's surface, as the ionospheric contribution diminishes at higher frequencies within the ground wave regime, with attenuation rates more closely tied to ground conductivity than to cavity resonances.^[16] The characteristic field pattern of a ground surface wave features a tangential electric field component that decays exponentially in the vertical direction away from the surface, ensuring the energy remains confined near the ground.^[17] This vertical attenuation, often modeled as evanescent in the air medium, contrasts with the more uniform propagation along the surface, where the induced ground currents sustain the wave's forward momentum without substantial upward leakage.^[17]

Polarization and Frequency Effects

Vertical polarization is preferred for ground wave propagation due to its significantly lower attenuation compared to horizontal polarization, as the latter experiences substantial losses from the image cancellation effect at the conductive ground surface.^[18] This preference arises because the vertical electric field aligns with the induced currents in the ground, minimizing energy dissipation, whereas the horizontal component induces opposing image currents that nearly cancel the signal.^[18] In practical scenarios, the polarization often exhibits ellipticity due to terrain variations and multipath effects, though the dominant vertical component preserves the overall efficiency.^[19] A polarization mismatch between transmitter and receiver, such as a horizontal receiving antenna for a vertically polarized ground wave, introduces substantial signal loss, typically ranging from 10 to 20 dB depending on the degree of orthogonality.^[18] Ground wave performance is highly frequency-dependent, with optimal propagation occurring below 30 MHz where ionospheric interference is negligible and surface wave binding to the Earth is effective.^[5] In the low-frequency (LF: 30 kHz to 300 kHz) and medium-frequency (MF: 300 kHz to 3 MHz) bands, ground waves achieve the longest ranges, often exceeding thousands of kilometers over conductive paths like seawater, due to reduced absorption.^[5] At higher frequencies in the high-frequency (HF: 3 MHz to 30 MHz) band, attenuation escalates rapidly, confining reliable propagation to shorter distances of tens to hundreds of kilometers.^[5] The quadratic frequency dependence of attenuation is captured in the basic formula for the attenuation constant:

\alpha = k \frac{f^2}{\sigma}

where \alpha is the attenuation (in nepers per unit distance), f is the frequency, \sigma is the ground conductivity, and k is a propagation constant incorporating Earth radius and permittivity effects.^[20] This relationship highlights how attenuation intensifies with the square of frequency for a given conductivity, explaining the preference for lower frequencies to maintain signal strength over distance.^[20] Additionally, the diffraction angle for ground waves scales inversely with frequency, enabling lower frequencies to curve more effectively around the Earth's curvature and obstacles.^[5]

Influencing Factors

Ground Conductivity and Terrain

Ground conductivity, a measure of the soil's ability to conduct electrical current, plays a critical role in determining the efficiency of ground wave propagation, as it influences the attenuation of radio signals traveling along the Earth's surface. Higher conductivity values reduce signal loss by allowing better coupling between the electromagnetic wave and the ground, thereby extending propagation range. Seawater exhibits the highest conductivity, typically around 5 S/m, which minimizes absorption and supports the longest transmission distances. In contrast, average land soils have conductivities ranging from 0.001 to 0.01 S/m, leading to moderate attenuation suitable for regional coverage. Poor conductors, such as ice or desert sands with values below 0.001 S/m, result in rapid signal decay due to high absorption.^[21]^[22] The dielectric constant (ε_r), which describes the soil's permittivity relative to free space, further modulates wave propagation by affecting the refractive index and phase velocity at the ground interface. For most soils, ε_r varies between 10 and 30, with lower values (around 7) for dry, poor-conductivity earth and higher values (up to 30) for moist, fertile soils; this parameter is particularly influential at lower frequencies where displacement currents dominate.^[20] Terrain features significantly alter ground wave paths by introducing diffraction, scattering, and shadowing effects that deviate from ideal flat-earth assumptions. Over flat or gently rolling surfaces, waves follow the curvature of the Earth with minimal disruption, preserving signal strength. However, irregular terrain, such as hills or mountains, can cause shadowing, where obstacles block the direct surface wave, leading to diffraction losses of 10 dB or more in shadowed regions behind the feature. Urban environments exacerbate these effects at higher frequencies (above 1 MHz), with building clutter inducing additional scattering and absorption, further reducing signal penetration.^[6]^[23]^[24] Global ground conductivity is mapped and classified by the International Telecommunication Union (ITU) in Recommendation P.832, providing contour-based data for regions worldwide to inform propagation predictions; these maps derive from empirical measurements and account for variations like seasonal moisture changes.^[21] Representative examples illustrate these impacts: at 100 kHz, ground waves over seawater can achieve ranges exceeding 1000 km with field strengths around 200 mV/m at 100 km, enabling transoceanic coverage. Over average land, ranges typically extend to 100-200 km, while over dry soils or ice, effective distances drop below 50 km due to excessive attenuation.^[5]

Attenuation Mechanisms and Range Prediction

Ground wave propagation experiences several key attenuation mechanisms that determine signal strength over distance. The primary components include free-space spreading loss, where the electric field strength decreases inversely with distance as E \propto 1/r, corresponding to a power loss of $1/r^2. This is the dominant mechanism at short ranges before ground effects become significant. Additionally, absorption occurs in the ground due to its finite conductivity, converting electromagnetic energy into heat, with the extent of penetration governed by the skin depth \delta = \sqrt{2 / (\omega \mu \sigma)}, where \omega = 2\pi f, \mu is permeability, and \sigma is conductivity.^[25] The surface wave component, induced by diffraction along the earth's surface, undergoes exponential decay characterized by e^{-\alpha d}, where \alpha is the attenuation constant derived from ground conductivity \sigma and frequency f, typically increasing with frequency and decreasing with higher \sigma. Range prediction for ground waves relies on integrating these attenuation mechanisms through theoretical and empirical models. Seminal work by Sommerfeld established the foundational integral for the attenuation factor F(p), where p is the numerical distance incorporating distance d, wavelength \lambda, and complex ground permittivity \epsilon_c = \epsilon_r - j \sigma / (\omega \epsilon_0).^[26] Practical predictions often use empirical curves, such as those in ITU-R Recommendation P.368, which plot median field strength versus distance for frequencies between 10 kHz and 30 MHz, accounting for various ground conductivities and accounting for earth curvature via numerical methods. The combined effects of these mechanisms yield typical reliable ranges of 50-100 km for medium frequency (MF, 300-3000 kHz) signals over average land terrain with conductivities around 0.001-0.01 S/m, beyond which field strength drops below usable levels for most applications.^[4] Unlike skywave propagation, ground wave attenuation shows minimal diurnal or nocturnal variations, with seasonal changes limited to 5-15 dB at MF due to temperature and moisture effects on ground parameters. Limitations include negligible contribution beyond 2000 km, where diffraction losses exceed 100 dB relative to free-space, and practical ranges are further constrained by atmospheric and man-made noise floors, often setting the minimum detectable signal threshold around -100 dBm for MF broadcasting.

Practical Applications

Broadcasting and Communication

Ground waves serve as the primary propagation mode for daytime amplitude modulation (AM) radio broadcasting in the medium wave (MW) band, spanning 540 to 1700 kHz, enabling reliable regional coverage free from skywave interference. This method supports service radii typically ranging from 100 to 500 km, influenced by transmitter power, antenna efficiency, and ground conductivity, making it ideal for local and semi-local stations delivering news, talk, and music to urban and suburban audiences.^[27]^[28] In the longwave band, particularly around 198 kHz in Europe, ground waves facilitate national-scale broadcasting, as exemplified by BBC Radio 4's transmissions from sites like Droitwich, which provide consistent coverage across most of the United Kingdom and extend to adjacent regions such as Ireland and parts of northwestern continental Europe. These lower frequencies enhance propagation over diverse terrains, including water and forested areas, offering superior mobile reception in vehicles compared to higher-frequency bands, where signals degrade more rapidly.^[29] Beyond broadcasting, ground waves underpin fixed-service communication links in rural and remote areas, where they enable point-to-point radio connections over distances of tens to hundreds of kilometers without requiring line-of-sight paths, supporting utilities, emergency services, and agricultural networks in regions lacking fiber optic infrastructure. In amateur radio, the 160 m band (1.8-2.0 MHz) relies on ground wave paths for regional contacts, often achieving 50 to several hundred kilometers under favorable conditions, providing a stable alternative to skywave-dependent modes for local emergency and hobbyist communications.^[30]^[31] Contemporary developments have integrated digital technologies with ground wave propagation to revitalize medium-frequency broadcasting. Digital Radio Mondiale (DRM), operating in the MW band, exploits the inherent stability of ground waves to deliver CD-quality stereo audio and multimedia data services over extensive areas with reduced transmitter power, achieving field strengths as low as 33-44 dB(μV/m) for robust reception. Despite the dominance of FM and VHF digital services since the early 2010s, ground wave-based AM and longwave systems endure for their resilience in mobile and rural scenarios, continuing to serve niche audiences in automotive and portable receivers.^[32] Ground waves have played a pivotal role in navigation systems, particularly through the Long Range Navigation (LORAN-C) system, which operated at 100 kHz and utilized ground wave propagation for hyperbolic positioning based on time differences of arrival from multiple land-based transmitters. This low-frequency approach ensured reliable signal coverage over long distances, especially over seawater where ground conductivity is high, enabling accurate positioning for maritime and aeronautical applications until its decommissioning. The U.S. Department of Homeland Security initiated the termination of LORAN-C signal transmission in February 2010, completing the phased decommissioning by October 2010, as GPS had become the primary navigation aid.^[33]^[34] In response to GPS vulnerabilities, enhanced LORAN (eLORAN) proposals have gained traction in the 2020s as a terrestrial backup, modernizing the original system with improved timing and data capabilities while retaining 100 kHz ground wave signals for robust, wide-area coverage. eLORAN's low-frequency, high-power transmissions make it highly resistant to jamming and spoofing, penetrating challenging environments like urban canyons or foliage where satellite signals falter, and providing positioning accuracy better than 20 meters. Governments, including the UK Ministry of Defence, are investigating deployable eLORAN systems to ensure resilient positioning, navigation, and timing (PNT) independent of space-based assets. As of November 2025, South Korea led an international meeting with the UK and France to advance eLoran deployment for enhanced PNT resilience.^[35]^[36]^[37] In military applications, very low frequency (VLF) ground waves in the 3-30 kHz band are essential for submarine communication, allowing one-way transmission from shore stations to submerged vessels over oceanic paths where high seawater conductivity supports efficient propagation to depths of tens of meters. Stations like the U.S. Navy's VLF facilities enable strategic messaging without requiring submarines to surface, leveraging ground wave stability for global reach. For ground forces, tactical medium frequency (MF) radios operating in the 300 kHz to 3 MHz range utilize ground waves—particularly surface waves that follow Earth's curvature—for reliable, non-line-of-sight voice and data links over hundreds of kilometers in varied terrain. Systems such as the AN/GRC-106 exemplify this, providing secure communications for maneuver units in environments where higher frequencies are disrupted.^[38]^[39]^[40] Ground waves also support precise time synchronization in military and civilian contexts, as demonstrated by the National Institute of Standards and Technology's WWVB station broadcasting at 60 kHz. This low-frequency signal propagates primarily via ground waves, offering stable dissemination of Coordinated Universal Time (UTC) with accuracy to within 1 part in 10^12, enabling synchronization for radio-controlled clocks, scientific instruments, and defense systems requiring exact timing.^[41] Recent developments in the 2020s emphasize low-frequency (LF) ground wave systems for enhanced resilience against jamming, with eLORAN positioned as a key element in hybrid PNT architectures integrating terrestrial signals with satellite communications (satcom). These hybrid approaches provide path diversity, ensuring continuous operations during GNSS outages or electronic warfare, as explored in international collaborations like UK-France initiatives for jamming-resistant infrastructure.^[36]^[42]^[43]

Theoretical Modeling

Classical Models

The Sommerfeld-Zenneck solution represents the foundational exact mathematical framework for ground wave propagation, developed in the early 1900s to describe electromagnetic waves traveling along the interface between two homogeneous media, such as air and the Earth's surface. Jonathan Zenneck first proposed the concept of a surface wave supported by the air-earth boundary in 1907, characterizing it as a vertically polarized plane wave with fields decaying exponentially away from the interface. Arnold Sommerfeld extended this in 1909 by solving the problem of radiation from a vertical electric dipole over a finitely conducting half-space, expressing the electric field in terms of improper integrals involving Bessel functions, which capture both the direct, reflected, and surface wave components. Although exact, this formulation proved computationally impractical for engineering applications due to the complexity of evaluating the integrals numerically, particularly for radio frequencies. To address these challenges, Kenneth A. Norton introduced a practical approximation in 1936, known as Norton's model, under a flat-Earth assumption suitable for short-range predictions. Norton's approach incorporates surface impedance Z_s = \sqrt{\frac{j \omega \mu}{\sigma (1 + j \omega \epsilon / \sigma)}}, where \sigma is ground conductivity, \epsilon is permittivity, \omega is angular frequency, and \mu is permeability, to model the interaction between the wave and the lossy ground. The vertical electric field strength is approximated as E \approx \frac{E_0}{d} e^{-p d / \sqrt{\lambda}}, with E_0 the reference field, d the distance, \lambda the wavelength, and p a numerical factor depending on ground parameters (typically around 0.1 for good conductors like seawater). This model simplifies the Sommerfeld integrals using asymptotic expansions and image theory for the reflected wave, enabling graphical and tabular predictions of field attenuation. Classical models, including both the Sommerfeld-Zenneck solution and Norton's approximation, rely on key assumptions such as a homogeneous, flat or gently curved Earth with uniform conductivity and permittivity, vertical dipole antennas, and low grazing angles typical of ground wave paths. These frameworks are primarily valid for vertically polarized waves at frequencies below 10 MHz, where surface wave effects dominate over direct or skywave propagation. Limitations include neglect of terrain irregularities, atmospheric refraction, and variations in ground properties, which can introduce errors in heterogeneous environments; the exact Sommerfeld solution, while rigorous, requires numerical evaluation that was infeasible before computers, while Norton's flat-Earth version underestimates curvature effects beyond a few hundred kilometers. Experimental validations in the 1930s through 1950s, including field measurements over land paths, confirmed the models' predictions with typical range accuracies of 10-20% for smooth terrains at low and medium frequencies, aligning well with observed field strengths and phase shifts in LF/MF broadcasting scenarios.

Advanced and Numerical Methods

Post-1960s advancements in ground wave modeling have focused on computational extensions to account for Earth's sphericity and real-world terrain variations, building on foundational theories like the Van der Pol-Bremmer residue series solution from the late 1930s. This approach, which treats ground wave propagation as an eigenvalue problem using normal mode theory, has been numerically enhanced through residue series summation and integration techniques to handle spherical Earth curvature effects more accurately than flat-Earth approximations. For instance, modern implementations employ numerical integration over propagation paths to compute field strengths, incorporating exponential tropospheric refractive index gradients as per ITU-R P.453, achieving errors below 1 dB for distances where the curvature parameter k \cdot r > 10. These methods, detailed in ITU-R Recommendation P.368-10 (2022), provide curves and algorithms for frequencies from 10 kHz to 30 MHz over homogeneous spherical Earth, enabling precise predictions for both smooth and mixed-path scenarios via the reciprocity-satisfying Millington method. The 2022 update includes refinements to numerical models like the GRWAVE program and improved conductivity maps for global path predictions.^[1] Contemporary numerical tools have further refined these models for irregular terrain, particularly in medium frequency (MF) applications. The Numerical Electromagnetics Code (NEC), originally developed in the 1970s and evolved into NEC-3 and later versions, applies the method of moments to simulate antenna performance and ground wave propagation over complex surfaces, including forests and buildings modeled as segmented slabs. In MF broadcast systems, NEC integrates with the irregular-Earth mixed-path (IEMP) propagation model to calculate equivalent antenna gains relative to a short dipole over lossy ground, supporting up to 50 terrain segments and frequencies from 0.5 to 30 MHz; finer terrain resolution (e.g., 1 km spacing) yields accuracies within 1-2 dB compared to direct NEC computations.^[28] Hybrid approaches combining ray-tracing with mode theory address diffraction in transitional regions, as in the 1998 algorithm by Sevgi and Felsen, which efficiently couples ray-based geometrical optics for far-field propagation with modal expansions near the surface, reducing computational demands while maintaining field strength predictions for curved, inhomogeneous paths. Irregular terrain corrections have seen specific improvements for MF ground waves through dedicated empirical-statistical models. The Longley-Rice irregular terrain model targets higher frequencies from 20 MHz to 20 GHz and is not directly applicable to MF scenarios. Instead, MF predictions rely on methods like IEMP in NEC-based tools, which apply single-knife-edge diffraction and mixed-path attenuation corrections, accounting for terrain roughness and conductivity variations to predict median path losses over distances up to 2000 km. Emerging developments in the 2020s incorporate machine learning (ML) for radio propagation path loss forecasting in urban and cluttered environments at sub-6 GHz frequencies, where neural networks and ensemble methods (e.g., XGBoost) train on measurement data to integrate terrain, building density, and multipath effects, outperforming traditional models with root-mean-square errors reduced by up to 5 dB; while primarily applied to higher sub-6 GHz bands, such techniques show potential for extension to lower frequencies including MF ground waves.^[45] These advancements, aligned with ITU-R P.368-10 guidelines, have collectively lowered prediction errors to under 5 dB in validated cases—compared to 10-15 dB in early classical implementations—enhancing reliability for broadcasting and navigation over diverse terrains.

Versus Skywave Propagation

Ground wave propagation relies on the diffraction of radio waves along the Earth's surface, allowing signals to follow the curvature of the terrain without significant reflection from the atmosphere. In contrast, skywave propagation involves the reflection and refraction of radio waves by the ionized layers of the ionosphere, primarily the E and F layers (with the D layer causing daytime absorption), enabling signals to bounce back to Earth after traveling upward. This fundamental difference in mechanisms—surface-guided diffraction for ground waves versus ionospheric bounce for skywaves—determines their respective propagation characteristics and applications.^[46]^[47]^[48] In terms of range and variability, ground waves provide a stable coverage area typically extending 100 to 1000 kilometers, depending on terrain conductivity and frequency, with minimal fluctuations between day and night or across seasons. Skywaves, however, achieve longer distances of 1000 to 3000 kilometers for single-hop paths, but their reliability is compromised by high variability influenced by solar activity, time of day, and ionospheric conditions, often resulting in signal fading or skip zones. Ground wave signals remain consistent and frequency-independent in terms of diurnal effects, while skywave performance degrades during the day due to D-layer absorption and improves at night when higher ionospheric layers dominate.^[49]^[48]^[49] Both propagation modes overlap in the high-frequency (HF) band of 3 to 30 MHz, where ground waves are preferred for reliable short- to medium-range communication during daylight hours to minimize interference from dominant skywave signals. At lower HF frequencies (e.g., around 5 MHz), ground waves are more effective due to reduced absorption, whereas skywaves excel at higher HF frequencies (e.g., 25 MHz) for extended reach when ionospheric conditions permit. This overlap necessitates careful frequency selection in HF systems to balance the two modes.^[49]^[48] The trade-offs between ground and skywave propagation highlight their complementary roles: ground waves offer high reliability for regional coverage but are constrained by terrain irregularities and ground conductivity, limiting their utility over varied landscapes. Skywaves enable global or transcontinental communication but suffer from unpredictability due to ionospheric disturbances, making them unsuitable for applications requiring consistent signal strength. These differences make ground waves ideal for stable, local HF operations, while skywaves support long-haul links under favorable conditions.^[49]^[48]

Versus Direct and Tropospheric Waves

Ground wave propagation differs fundamentally from direct and tropospheric modes in its ability to follow the Earth's curvature, enabling reliable communication beyond line-of-sight (LOS) at lower frequencies. Direct waves, also known as space waves in LOS contexts, travel in straight lines from transmitter to receiver, adhering to free-space path loss that scales with the inverse square of distance (1/r²). This mode is constrained by the radio horizon, typically limiting effective ranges to approximately 50 km for medium-frequency (MF) ground-based antennas at heights of around 30 meters, without benefiting from Earth's curvature to extend coverage.^[50] In contrast, tropospheric propagation occurs primarily at very high frequencies (VHF, above 30 MHz) and ultra-high frequencies (UHF), where signals can extend beyond LOS through mechanisms like ducting and scattering. Ducting traps waves within atmospheric layers of varying refractive index, often over water or in stable weather conditions, achieving ranges of 100 to 500 km or more, while tropospheric scatter involves forward scattering by atmospheric irregularities, supporting links up to several hundred kilometers but with higher attenuation. These modes are highly sensitive to meteorological factors, such as temperature inversions and humidity gradients, leading to intermittent and unpredictable performance. Ground waves excel in non-LOS scenarios at frequencies below 30 MHz, where diffraction allows the surface wave to bend over the Earth's horizon, maintaining signal strength over distances of hundreds of kilometers with minimal atmospheric interference. Unlike direct waves, which fail beyond the visual horizon, or tropospheric modes, which require specific weather for extension, ground waves provide stable regional coverage in MF bands (e.g., 300 kHz to 3 MHz) due to lower sensitivity to tropospheric variations and effective energy replenishment via diffraction.^[51]^[20] Practically, direct waves suit short-range, high-reliability links like local VHF voice communications within LOS limits. Tropospheric propagation enables erratic but potentially longer VHF/UHF extensions for applications such as over-water broadcasting during favorable conditions. Ground waves, however, dominate for consistent MF regional broadcasting and navigation, offering diffraction-mediated reliability over terrain without the variability of tropospheric effects.^[51]

References

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Ground-wave propagation is an electromagnetic wave propagation between points on or above the earth, affected by the troposphere and ground, dominated by ...
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None
### Summary of Ground Wave Propagation from ITU-R P.368-10
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To compound the confusion, it turned out that Zenneck's surface wave wa very imilar to Sommerfeld's solution in his 1909 paper. Since that time a large number ...
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None
### Summary of Ground Conductivity Values and Classifications
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[PDF] OT Report 78-144: Radio Propagation in Urban Areas
However, as terrain irregularity increases, or as buildings and trees are added to the surface, the signal is reduced by shadowing, absorption, and scattering ...
[25]
Clutter and terrain effects on path loss in the VHF/UHF bands
Dec 8, 2017 · This study investigates the effects of terrain and clutter on frequency-dependent path loss models in the very high frequency (VHF) and ultra high frequency ( ...
[26]
[PDF] FCC/OET R86-1, Modern Methods for Calculating Ground-Wave ...
Sep 9, 2011 · This report provides a computer program for new FCC ground-wave propagation prediction curves, calculating ground-wave radio field intensity ...Missing: studies | Show results with:studies
[27]
[PDF] The Ground-Wave Attenuation Function for Propagation over ... - DTIC
A less than TT/2 is considered in some detail. The numerical distance, p, over such a surface is characterized by 0 f arg p £ n/2 . The ground wave behaves ...
[28]
Why AM Stations Must Reduce Power, Change Operations, or ...
Dec 11, 2015 · This is known as "groundwave" propagation. Useful daytime AM service is generally limited to a radius of no more than about 100 miles (162 km), ...
[29]
[PDF] Ground-wave Analysis Model for MF Broadcast Systems
The performance of telecommunication systems that rely on ground-wave signals is dependent on transmitter power, signal frequency, transmitter and receiver.
[30]
What is Medium Wave (MW) & Long Wave (LW) Radio? - BBC
Radio 4 LW can be received across most of the UK on 198kHz. To see what MW services are available where you live, you will need to use our Transmitter Checker ...
[31]
https://docs.fcc.gov/public/attachments/FCC-87-245A1.pdf
[32]
[PDF] FCC 87-245
Groundwave signals travel along the surface of the earth and are thus affected by the characteristics of soil conductivity along the propagation path. During ...
[33]
[PDF] DRM Handbook - Digital Radio Mondiale
May 5, 2020 · DRM enables digital radio transmissions in the former AM bands (short, medium and long wave, i.e. broadcasting bands up to 30 MHz), providing ...
[34]
[PDF] Complementary PNT and GPS Backup Technologies Demonstration ...
eLORAN is an evolution of LORAN-C, which was a TDOA positioning system, and which itself evolved from the initial World War II LORAN system. The main ...
[35]
Terminate Long Range Aids to Navigation (Loran-C) Signal
Jan 7, 2010 · DATES: Transmission of the Loran-C signal and phased decommissioning of the Loran-C infrastructure will commence on or about February 8, 2010.
[36]
[PDF] A Holistic Approach to Trusted, Resilient PNT: GNSS, STL and eLoran
Jul 20, 2023 · eLoran uses pulsed signals at a center frequency of 100 kHz. The pulses are designed to allow receivers to distinguish between the groundwave ...
[37]
eLoran: Part of the solution to GNSS vulnerability - GPS World
Nov 3, 2021 · eLoran, which has no common failure modes with GNSS, could provide continuity of essential timing and navigation services in a crisis.Missing: 2020s | Show results with:2020s
[38]
Underwater Radio, Anyone? - DARPA
Dec 16, 2016 · A nearby band of very-low-frequency (VLF) signals (3 KHz to 30 KHz) opens additional communications possibilities because for these wavelengths ...
[39]
Very Low Frequency Electromagnetic (VLF) | US EPA
Dec 12, 2024 · VLF-EM is a passive method using radio transmitters (3-30 kHz) to assess subsurface conductors, operating in a very low radio frequency band.
[40]
[PDF] INTRODUCTION TO TACTICAL RADIO COMMUNICATIONS
In the case of radio waves, the basic paths of transmission are the ground wave and the sky-wave. Figure 1-3 illustrates these transmission paths. a.
[41]
WWVB: A Half Century of Delivering Accurate Frequency and Time ...
This paper provides a history of the National Institute of Standards and Technology (NIST) radio station WWVB.
[42]
UK and France Renew Ties, Resilient PNT, eLoran a Key Part
Jul 10, 2025 · “UK and French experts will work more closely to increase the resilience of both countries' critical infrastructure to the signal-jamming seen ...Missing: 2020s | Show results with:2020s
[43]
Why GPS Isn't Enough: The Rise of Hybrid Navigation Systems
Oct 10, 2025 · GPS dependence is starting to look fragile. The future of resilient navigation is hybrid - multiple GNSS, A-PNT, eLORAN and quantum sensors.
[44]
[PDF] Machine Learning for Radio Propagation Modeling
Mar 18, 2024 · This survey explores machine learning (ML) for radio propagation modeling, which is used to estimate Path Loss (PL) or Received Signal Strength ...
[45]
What is Ground Wave: Radio Signal Propagation - Electronics Notes
Ground wave propagation is a form of signal propagation where the signal travels over the surface of the ground, and as a result it is used to provide regional ...Missing: formation | Show results with:formation
[46]
Ionospheric Layers: D, E, F, F1, F2, Regions - Electronics Notes
When a sky wave leaves the Earth's surface and travels upwards, the first region of interest that it reaches in the ionosphere is called the D layer or D region ...
[47]
[PDF] IRPL RADIO PROPAGATION HANDBOOK
Nov 15, 2024 · Fig. 9: The Tsluel of the gyro-frequency giTen are roughly 0.06 Me too low, and should accordingly be ral zed wh.m calcula-.
[48]
[PDF] Report ITU-R M.2234
Whatever the antenna type, the antenna gain for ground-wave propagation is typically 0-2 dBi. The maximum antenna gain for sky-wave propagation varies from. 6 ...
[49]
6.5: Line-of-Sight Transmission - Engineering LibreTexts
May 22, 2022 · Using a 100 m antenna would provide line-of-sight transmission over a distance of 71.4 km. Using such very tall antennas would provide ...
[50]
https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Introductory_Electrical_Engineering/Electrical_Engineering_%28Johnson%29/06%253A_Information_Communication/6.05%253A_Line-of-Sight_Transmission