Line of Sight (LOS): Definition and Essential Concepts

Line of Sight (LOS) describes the direct, unobstructed path connecting two points—such as a transmitter and receiver, observer and target, or between antennas. Crucial across navigation, wireless communications, GNSS/GPS, radar, and positioning systems, LOS enables electromagnetic signals (radio, microwave, infrared, visible light, or ultrasound) to travel the shortest route without being blocked by physical obstacles.

When LOS is maintained, signal strength is maximized, interference is minimized, and high accuracy is possible. Any object—such as terrain, buildings, vegetation, or vehicles—entering the LOS path can cause signal attenuation, multipath effects, or complete loss of signal, resulting in degraded performance.

Why Line of Sight Matters

A clear LOS is indispensable for reliable operation in wireless networks, navigation, and safety-critical domains like aviation, maritime, or emergency services. LOS ensures:

• Maximum signal strength and data rates in wireless communication.
• Low error rates and minimal latency for digital systems.
• Accurate triangulation and positioning in GNSS/GPS.
• Effective detection and tracking in radar and surveillance.

In GNSS, for example, a receiver needs LOS to at least four satellites for accurate 3D positioning. In urban canyons or dense forests, LOS can be lost, leading to errors or loss of fix.

Wireless network designers conduct LOS analysis to plan the placement of cell towers, Wi-Fi access points, and other infrastructure, ensuring robust coverage and capacity.

The Physics of LOS

Electromagnetic Wave Propagation

Electromagnetic waves travel in straight lines unless deflected or blocked. Physical obstacles can absorb, reflect, diffract, or scatter these waves:

• Longer wavelengths (lower frequencies): Better at diffracting around objects, more robust in NLOS environments.
• Shorter wavelengths (higher frequencies): More easily blocked or reflected, requiring stricter LOS.

Earth’s Curvature and Terrain

On Earth, the planet’s curvature limits LOS distance—even tall towers eventually lose LOS. Terrain features like hills or valleys also impact LOS, especially in rural or mountainous regions. Urban environments add complexity with varied building heights and materials.

Atmospheric Refraction

Atmospheric refraction bends radio waves, effectively extending LOS slightly. Network designers use a “4/3 Earth radius” correction to account for this effect in coverage calculations.

Fresnel Zones

Beyond the simple straight line, electromagnetic energy occupies elliptical regions called Fresnel zones. The first Fresnel zone is most important: obstacles here can cause destructive interference, even if the direct LOS appears visually clear. For optimal performance, keep at least 60% of this zone free of obstructions.

Factors That Affect LOS

LOS in Key Application Domains

GNSS/GPS and Navigation

GNSS receivers depend on LOS to multiple satellites. Obstructions such as buildings, terrain, or trees cause multipath and errors. For best accuracy, antennas are placed high and clear of obstacles.

Wireless Communication

Cellular (4G/5G), Wi-Fi, and point-to-point microwave links need clear LOS—especially at higher frequencies. Antennas and access points are positioned to maximize LOS and coverage.

Radar and Surveillance

Radar systems require LOS to accurately detect and track targets. Terrain masking or urban obstacles create blind spots. Visual and sensor-based surveillance systems are similarly affected.

Indoor Positioning Systems

UWB, infrared, and ultrasound beacons require LOS for high-precision tracking indoors. Walls, machinery, or shelving create NLOS zones; redundancy and careful planning help maintain accuracy.

Transportation and Safety

LOS is key for safe stopping and passing sight distances in road design, and for navigation aids in aviation and maritime contexts.

Calculating LOS Distance

Basic LOS Distance Formula

The maximum LOS distance (considering Earth’s curvature):

D = √(2Rh₁) + √(2Rh₂)

• D: LOS distance (meters)
• R: Earth’s radius (~6,371,000 meters)
• h₁, h₂: Heights of the two points (meters)

Refraction-Corrected Formula

Accounting for atmospheric effects:

D = 4.12 × (√h₁ + √h₂)

• D: LOS distance (kilometers)
• h₁, h₂: Heights of the two points (meters)

Example:
A 50 m tower and a 2 m device:
D = 4.12 × (√50 + √2) ≈ 34.9 km

First Fresnel Zone Calculation

For radio links, keep at least 60% of the first Fresnel zone clear:

F₁ = 17.32 × √(d / 4f)

• F₁: Fresnel zone radius (meters)
• d: Path length (km)
• f: Frequency (GHz)

Practical Examples

• GNSS Installations: Place antennas above obstructions for clear sky view and minimal multipath.
• Wireless Network Planning: Mount access points/towers high and clear of metal structures or thick walls for optimal LOS.
• Radar Surveillance: Elevate radar towers for maximum LOS over water or terrain.
• Indoor Positioning: Strategically place beacons for LOS to tracked objects; use redundancy in complex environments.

Troubleshooting and Optimization

• Symptoms of LOS loss: Dropped connections, reduced throughput, increased errors, or lost GNSS fix.
• Troubleshooting steps: Inspect for obstructions, use signal analysis tools, adjust antenna placement, and perform regular site surveys.
• Optimization: Elevate antennas, maintain Fresnel zone clearance, deploy redundant paths, and reassess after environmental changes.

Best Practices

• Visual LOS is a good check, but RF LOS is what matters—some materials transparent to light block RF.
• Use lower frequencies for better diffraction if strict LOS is not possible.
• Regularly reassess LOS as environments evolve.

Summary Table: LOS Factors

Related Terms

Non-Line-of-Sight (NLOS):
When the direct LOS is blocked, signals reach the receiver via reflection, diffraction, or transmission through materials, resulting in higher signal loss and reduced accuracy.

Fresnel Zone:
Elliptical regions around the LOS path where obstacles can cause destructive interference. Keeping at least 60% of the first Fresnel zone clear is critical for reliable signal transmission.

Multipath:
A phenomenon where signals arrive at the receiver by multiple paths due to reflections, causing interference, fading, and errors.

Further Reading

• International Telecommunication Union – Radio propagation
• GNSS Planning Online Tool
• IEEE 802.11 Wireless LANs Technical Guidelines

Line of Sight is a foundational principle in wireless communication, navigation, and positioning systems. Ensuring clear LOS maximizes performance, reliability, and safety across a wide range of applications, from GNSS and cellular networks to radar and indoor tracking. Regular assessment, thoughtful planning, and strategic design are key to maintaining robust LOS in both static and dynamic environments.