1. Characteristics of and Solutions for Maritime Wireless Communication

1.1. Impact of the Maritime Environment on Wireless Communication

a) Multipath Reflection
In terrestrial environments, although multipath effects exist (e.g., reflections from trees, buildings, mountains), the ground is typically rough and covered with absorbing objects. Reflected signals are relatively weak and scattered. Communication can rely on a direct path and a few reflected paths.
However, at sea, the surface acts like a giant mirror. When a drone transmits a signal to a vessel, strong specular reflection occurs. For the ground terminal on the ship, the signal arrives not only via the direct path but also via the sea surface reflection path. The antenna receives a superposition of the direct and reflected signals.
These signals travel different paths, leading to different phases upon arrival. When the reflected signal and the direct signal are out of phase, they cancel each other out, causing signal fading. When they are in phase, the signal is enhanced. However, since both the drone and the vessel are in motion, their relative positions and altitudes constantly change, causing the phase difference between the direct and reflected paths to change dynamically. This results in rapid, deep signal fluctuations.
Furthermore, the sea surface is not static. Due to waves, the reflection is not ideally specular. At certain moments, it becomes diffuse reflection, generating many more signal components with different phases. These components interfere with each other, causing the signal strength to fluctuate like waves, increasing the difficulty for the receiver to demodulate the signal correctly.

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b) Atmospheric Attenuation in High-Humidity, Salt-Spray Environments
Seawater evaporation leads to extremely high humidity and the presence of salt spray in the maritime air. Water vapor absorbs microwave signals, especially at higher frequencies (e.g., common bands like 2.4GHz, 5.8GHz), causing additional atmospheric attenuation. This attenuation, combined with multipath fading, further degrades the link margin.
Consequently, with rapid weather changes at sea, the attenuation of high-frequency signals by sea fog and rain is significantly greater than in dry terrestrial environments.

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c) Difficulty in Aligning Directional Antennas
On land, high-gain directional antennas are often used for high-speed data transmission like broadband video, also offering anti-interference capabilities. Generally, the beamwidth of directional antennas used for drone video transmission is narrow.
On land, the ground is stationary. Aligning a directional antenna with a drone only requires consideration of azimuth and elevation axes. However, at sea, a vessel experiences six degrees of freedom motion (heave, roll, pitch, yaw), making directional antenna alignment very difficult.
Vessel motion caused by waves can disrupt the communication link if the antenna deviates by even a few degrees. If directional antennas are required for long-range communication, a complex stabilization and tracking system is needed to keep the antenna constantly pointed at the drone.

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1.2 Specific Countermeasures

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1.2.1 Frequency Band Selection
High-frequency bands commonly used in terrestrial communication (2.4GHz, 5.8GHz) have short wavelengths, poor diffraction capability, and consequently, poor resilience to multipath effects. Additionally, signals at these higher frequencies are more susceptible to atmospheric absorption, especially by water vapor.
For maritime communication, the most preferable bands are the UHF bands (300MHz~900MHz). Signals in these bands offer strong resistance to multipath effects, good diffraction capability, and are less affected by atmospheric absorption. However, these bands have limited available bandwidth, making it difficult to support high-data-rate communications. Moreover, these bands are already very congested, used by land mobile communications, private networks, paging stations, etc., making them susceptible to interference from other devices.
For the C-band (e.g., 5.8GHz), due to its poor multipath resistance, weak diffraction, and particularly high susceptibility to absorption by water molecules in the high-humidity, salt-spray maritime atmosphere, signal attenuation is extremely rapid. Therefore, the use of the C-band is not recommended for maritime communication.
Considering the above factors, a more suitable band for maritime communication is the L-band (1.2GHz~1.6GHz). According to ITU-R Recommendation P.676, the L-band exhibits significantly lower atmospheric attenuation than the C-band, especially in high-humidity environments. Compared to the C-band, the L-band is less affected by absorption from maritime water vapor and sea fog, has stronger diffraction capabilities, and, given the same transmission power, achieves far greater transmission distances.

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1.2.2 Dual-Antenna Space Diversity to Counteract Multipath Interference
The signal strength variations caused by multipath effects fluctuate spatially. If two antennas are placed at a certain distance apart, the signal environment at antenna A becomes uncorrelated with that at antenna B. When the direct and reflected signals cancel each other out at antenna A's location, they might constructively interfere at antenna B's location.
By employing Maximal Ratio Combining (MRC) in the communication receiver, signals from both antennas are received simultaneously. These signals are then combined based on the principle of maximizing the post-combination Signal-to-Noise Ratio (SNR). Under ideal conditions (sufficient antenna spacing, uncorrelated channels following Rayleigh distribution), the theoretical gain of dual-branch MRC is approximately 3dB. In practical maritime environments, although channels are not perfectly independent, employing dual-antenna diversity reception typically improves the SNR by more than 2dB.

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1.2.3 Feasibility Assessment of Directional Antennas
In this project, based on discussions with the system integrator, the relative position between the drone and the vessel is uncertain. The drone could be anywhere around the vessel during its operation. Therefore, a directional antenna would require a servo-tracking mechanism. Maintaining the antenna's main beam pointed towards the drone via a stable and reliable servo mechanism, combined with a closed-loop tracking algorithm to scan for the target signal, is inherently difficult and costly.
Furthermore, the maritime environment presents challenges like high wind resistance and severe salt spray corrosion. The mechanical structure of the servo platform is susceptible to corrosion, leading to malfunction. Consequently, in a maritime environment, the directional antenna and servo mechanism must be housed within a sealed radome. To avoid dealing with mechanical wear and corrosion altogether, one would need to consider a phased array antenna, which presents even greater cost and complexity.

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2. Fresnel Zone Impact and Drone Flight Altitude

2.1 The Fresnel Zone
The Fresnel zone refers to a series of concentric ellipsoidal regions surrounding the direct line-of-sight path between transmitting and receiving antennas. According to the Huygens-Fresnel principle, radio waves do not travel solely along a single straight path from point A (transmitter) to point B (receiver) but propagate as waves spreading out in all directions. Waves emitted from the transmitting antenna interact with objects and the air, and these points of interaction themselves become new secondary wave sources. These secondary waves arriving at the receiver B from different points in space have traveled different path lengths and therefore have different phases. Some waves may arrive in phase, constructively interfering to enhance the signal; others may arrive out of phase, destructively interfering to weaken it.
The Fresnel zone is a conceptual tool used to describe this spatial region of interaction.
We are typically most concerned with the First Fresnel Zone. By definition, the First Fresnel Zone is the set of all points in space where the path length of any possible secondary wave is up to half a wavelength (i.e., a 180-degree phase shift) longer than the direct line-of-sight path. Obstacles within this region have the most significant impact on the signal.

Calculation of the First Fresnel Zone Radius:

F1=λ⋅d1⋅d2dF1​=dλ⋅d1​⋅d2​​​

Where:

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F1F1​: First Fresnel zone radius at a specific point (obstacle location) along the path (in meters).

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λλ: Wavelength (in meters).

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d1d1​: Distance from that point (obstacle) to transmitter A (in meters).

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d2d2​: Distance from that point (obstacle) to receiver B (in meters).

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d=d1+d2d=d1​+d2​: Total distance between A and B.

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The Fresnel zone radius is maximum at the midpoint of the path between A and B, where d1=d2d1​=d2​. The maximum First Fresnel zone radius F1(max)F1(max)​ is calculated using d1=d2=d/2d1​=d2​=d/2:

F1(max)=12λ⋅dF1(max)​=21​λ⋅d​

Given the wireless data link operating frequency of 1400MHz (λ≈0.214λ≈0.214 m) and a distance d=20d=20 km between the drone and the ground terminal, the calculated maximum First Fresnel zone radius is:

F1(max)=120.214⋅20000≈32.7 mF1(max)​=21​0.214⋅20000​≈32.7 m

In engineering practice, to achieve propagation conditions close to free space, it is necessary to ensure that at least 60% of the First Fresnel zone radius is clear of obstacles. This means that at any point along the communication path, obstacles (such as waves) must not encroach into the region defined by 60% of F1(max)F1(max)​. Thus, the minimum required clearance above an obstacle is:

Min. Clearance=0.6×F1(max)=0.6×32.7≈19.6 mMin. Clearance=0.6×F1(max)​=0.6×32.7≈19.6 m

Considering the Earth's curvature, the Earth's bulge height hearthhearth​ at the midpoint of the 20 km path between the drone and the ground terminal, relative to the chord connecting the two ends, is calculated as:

hearth≈d1⋅d22⋅Reff=r22⋅Reffhearth​≈2⋅Reff​d1​⋅d2​​=2⋅Reff​r2​

Where:

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rr: Distance from the midpoint to either endpoint (since d=20d=20 km, r=10r=10 km).

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ReffReff​: Effective Earth radius, depending on atmospheric refraction (K-factor). Under standard atmospheric refraction, K=4/3. Without refraction, K=1.

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Under poorer meteorological conditions, we take K=1, making ReffReff​ equal to the physical Earth radius of 6371 km. Substituting the values:

hearth≈(10000)22⋅6371000≈7.85 mhearth​≈2⋅6371000(10000)2​≈7.85 m

Considering both the Earth's curvature and the Fresnel zone clearance requirement, the line-of-sight path between the drone and the shipborne ground terminal must clear the sea surface by at least:

hearth+Min. Clearance=7.85+19.6=27.45 mhearth​+Min. Clearance=7.85+19.6=27.45 m

2.2 Required Altitudes for Drone and Shipborne Antenna

Assume the shipborne antenna is installed on the deck or side of the vessel, with its height above mean sea level approximately hship=10hship​=10 m. To ensure the midpoint of the link between the drone and the shipborne terminal clears the sea surface by 27.45 m, the required drone altitude hdronehdrone​ can be estimated using the following relationship for the midpoint height hmidhmid​:

hmid≈hship+hdrone2hmid​≈2hship​+hdrone​​27.45≈10+hdrone227.45≈210+hdrone​​hdrone≈2×27.45−10=44.9 mhdrone​≈2×27.45−10=44.9 m

Based on this calculation, the drone should fly at an altitude above sea level greater than approximately 45 m.

This calculation uses mean sea level. In practice, safety margins must be added to account for wave effects and vessel motion. Let's assume a significant wave height of 5 m.
The rolling of the vessel causes variations in the antenna height. This variation needs compensation, potentially by increasing the drone's altitude. A margin of 3 m can be reserved for this.

Considering these factors, the required line-of-sight clearance at the link midpoint becomes:

hmid, required=7.85 (Earth bulge)+5 (waves)+3 (vessel motion)+19.6 (Fresnel)=35.45 mhmid, required​=7.85 (Earth bulge)+5 (waves)+3 (vessel motion)+19.6 (Fresnel)=35.45 m

Applying the midpoint height formula again, with hship=10hship​=10 m:

35.45≈10+hdrone235.45≈210+hdrone​​hdrone≈2×35.45−10=60.9 mhdrone​≈2×35.45−10=60.9 m

Therefore, according to this engineering estimation, with a shipborne antenna height of 10 m and accounting for wave and vessel motion effects, the drone's flight altitude should be no less than 60 m above sea level.

Summary
This article comprehensively analyzes the primary challenges faced by drone networking communication in the maritime environment, including multipath effects, atmospheric attenuation in high-humidity and salt-spray conditions, and the difficulty of aligning directional antennas. To address these issues, effective solutions are proposed, such as frequency band selection strategies, dual-antenna space diversity techniques, and an assessment of directional antenna feasibility. Through the application of Fresnel zone theory and Earth curvature calculations, engineering requirements for drone flight altitude are derived, providing a theoretical basis for ensuring the stability and reliability of the communication link.