RF Propagation Concepts

This following sections describe phenomena that arise from various propagation mechanisms and are basic concepts in radar.

Topic covered include:

Line-of-Sight

The radar line-of-sight is the unobstructed straight-line, geometric path between the radar and the target. The line-of-sight is limited by the curvature of the Earth, terrain, and other obstacles. The length of the line-of-site path is defined as the slant range.

In the figure below, the radar line-of-sight is shown in black. The line-of-sight tangent point, which is sometimes referred to as the geometric or optical horizon, is also indicated.

The figure below shows line-of-sight visibility considering terrain.

Radar Horizon

The radar horizon is the farthest distance from a radar to a point on the Earth's surface at which the radar beam remains unobstructed, factoring in the large scale curvature of the Earth surface and effects of atmospheric refraction on the path geometry. Refracted path curvature increases the horizon of the radar beyond the geometric line-of-sight distance.

In the figure below, the refracted propagation path is shown in blue and the corresponding line-of-sight path is in black. The radar horizon is located further away than the line-of-sight tangent point, which is sometimes referred to as the geometric or optical horizon. Over the horizon diffraction can enable the detection of targets beyond the radar horizon in the shadow, sometimes termed the diffraction region. See the radar propagation factor discussion in RF Propagation Models for more information.

Multipath Ghosts

Ghost targets occur when the path length of the multipath returns is significantly longer than the path length of the direct path returns, causing apparent erroneous target detections or false alarms. These multipath returns can come from ground surfaces or objects such as buildings or guardrails. The measured location of these multipath returns will appear at longer ranges than the true target range and may also appear to come from a different direction such as beneath the ground surface or behind a wall, like a mirror image about the reflection surface.

Multipath Fading

Fading occurs when the difference between the direct path and bounce path length is small (less than the range resolution of the radar), causing multipath reflections to combine constructively and destructively, which results in lobing in elevation coverage and fluctuations in detection range. When the difference between these path lengths becomes greater than the range resolution, the paths become resolved and no longer combine coherently, resulting in ghosting instead of fading.

The figure below shows that fluctuations or fading is evident when multipath reflections combine coherently.

Angle (Height) and Range Error From Refraction

Ignoring refracted path curvature causes targets to appear at higher altitudes and closer ranges than their actual positions.

  • Refraction bends the propagation path geometry and therefore affects measured target positions.

  • If refracted path curvature is ignored, targets will appear at higher altitudes and closer ranges than their actual positions, especially over longer distances.

In the figure below, the black line shows the pointing geometry or line-of-sight of the radar, the blue curve is the actual refracted propagation path, and the orange line is the corresponding slant range distance from the radar to the target. Notable differences between the ideal free-space line-of-sight path and the actual refracted propagation path becomes apparent for distances greater than ~10 km. The angle between the black and blue paths is large at long range. You can see that there is a 0.15 km height error at a distance of 84 km. Similarly, the reported target range will be longer than the geometric slant range because radar range is derived from round trip time. You can see in the figure below that the actual curved propagation path (blue) is longer than the geometric slant range distance (orange), which results in range error.

The figure below shows target error relative to predicted error for measurement-level (radarDataGenerator) and waveform-level (radarTransceiver) simulations.

References

[1] Richards, M. A., & Melvin, W. L. (Eds.). Principles of Modern Radar: Basic Principles (2nd ed.). SciTech Publishing, 2023. ISBN 978‑1839533815.

[2] J. S. Seybold, Introduction to RF Propagation. Hoboken, NJ: John Wiley & Sons, 2005.

See Also

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