RF Propagation Models
for Radar Coverage Analysis

Free-space path loss (FSPL) represents the theoretical minimum signal attenuation between two antennas with no obstacles, reflections, or atmospheric effects. Derived from the Friis transmission equation, it depends only on frequency and distance.

In radar coverage analysis, FSPL serves as the baseline against which all other propagation effects are measured. The actual path loss in any real scenario will always be equal to or greater than the free-space value. This makes it essential for calculating the theoretical maximum detection range of a radar system before terrain and environmental losses are considered.

The Two-Ray model accounts for two propagation paths between transmitter and receiver: the direct line-of-sight ray and a ray reflected off the ground surface. The interaction between these two rays creates constructive and destructive interference patterns that significantly affect received signal strength.

At distances beyond a crossover point (dependent on antenna heights and wavelength), the Two-Ray model predicts signal strength rolling off as the fourth power of distance (40 dB/decade), rather than the square law of free space. This makes it more realistic than FSPL for ground-level radar systems, particularly in flat coastal or maritime environments where ground reflection is significant.

The COST-231 Hata model extends the original Okumura-Hata model to frequencies up to 2000 MHz. It is an empirical model derived from extensive field measurements in European cities, making it the industry standard for cellular network planning in urban and suburban environments.

For radar and radio coverage analysis, COST-231 Hata provides environment-specific path loss predictions without requiring detailed terrain profiles. It classifies the environment as urban, suburban, or rural and applies correction factors accordingly. This makes it particularly valuable for planning radio networks, base station coverage, and VHF/UHF communication systems where the deployment environment is well characterized.

The Longley-Rice Irregular Terrain Model (ITM) is the most widely used terrain-based propagation model for frequencies between 20 MHz and 20 GHz. Developed by the U.S. National Telecommunications and Information Administration (NTIA), it predicts median path loss over irregular terrain using a combination of electromagnetic theory and statistical analysis.

ITM operates in two modes: point-to-point (using actual terrain profiles) and area prediction (using terrain statistics). In point-to-point mode, the model extracts terrain features from the elevation profile between transmitter and receiver, calculates diffraction losses over obstacles, and accounts for tropospheric scatter for beyond-horizon paths. It also models climate effects through refractivity parameters.

For radar coverage analysis, Longley-Rice ITM is the workhorse model for long-range systems operating over diverse terrain. Its ability to handle distances up to 2000 km with terrain-aware diffraction makes it essential for defense radar planning, border surveillance, and any scenario involving significant terrain obstruction.

ITU-R P.1812-6 is the most comprehensive terrain-based propagation model in the ITU-R series. It provides point-to-area predictions for terrestrial services in the 30 MHz to 6 GHz range, combining multiple propagation mechanisms into a single, unified framework.

The model calculates propagation loss by considering line-of-sight propagation with sub-path diffraction, complete diffraction over terrain obstacles using the Bullington method with correction terms, tropospheric scatter for beyond-horizon paths, and ducting / layer reflection for anomalous propagation conditions. It also accounts for clutter losses at the receiver location.

What makes P.1812-6 particularly powerful for radio network planning is its explicit handling of location variability and time variability. You can specify the percentage of locations and time for which coverage must be achieved, making it ideal for regulatory compliance and service-level planning in telecom networks.