Institute for Telecommunication Sciences / July 1968
In July 1968 one of ITS’s most famous publications was released. “Prediction of Tropospheric Radio Transmission Loss Over Irregular Terrain: A Computer Model” described a software product based on the groundbreaking work done for Tech Note 101 three years earlier. The FORTRAN program created by ITS engineers Anita Longley and Phil Rice is better known as the Irregular Terrain Model (ITM) or simply Longley-Rice. This was one of the first computerized models of radio propagation that could be used effectively in the real world. It incorporated general terrain features, climate, antenna characteristics, and wavelength into a general propagation model. The computerized model could be used to predict the propagation between two points or to predict the full propagation from a transmitter. To verify their work the team compared the model's predictions to over 800 measured communications paths from around the world. ITM predated digital elevation databases, and was used heavily for decades after its publication. The FCC used the Longley-Rice model for expansion of both satellite and digital television allocations. ITM remains an important propagation model almost 50 years later, and the software implementations in FORTRAN and C++, dated as the languages are, are still among the most downloaded ITS software. ITS continues to improve and update the model, which remains a fundamental tool in the effort to maximize spectrum utilization.