May 25, 2023
Until June 2, 2023, expert researchers from ITS are in Geneva, Switzerland, where they are collaborating with colleagues from all over the world to reach consensus about improvements to the International Telecommunication Union (ITU) technical standards for radio propagation modeling. Updated standards manifest a one-hundred-year-old tradition of U.S. Department of Commerce engineers and scientists promoting international collaboration in the scientific study of radio and the establishment and use of common methods and standards.
Paul McKenna, William Kozma, Erik Hill, and Adam Hicks are attending this year’s final ITU Radiocommunication Sector (ITU-R) Study Group 3 (SG3, Radiowave Propagation) meetings, which began May 22. At SG3’s final plenary meeting on June 2, some of the contributions will be approved by the full SG3 for updated international standardization.
At the ITU-R Study Group 3 Working Party meetings, Paul McKenna presides as International Chair of Working Party 3K and Adam Hicks participates as U.S. Chair of WP 3L.
The meetings occur in advance of the 2023 World Radio Conference (WRC) that will convene in November. WRCs are international forums for world agreement on the use of the radio-frequency spectrum and the geostationary-satellite and non-geostationary-satellite orbits. Preparations for WRCs involve extensive studies and preparatory discussions among all stakeholders at the national, regional, and worldwide levels. ITU-R Study Groups undertake technical and scientific studies and develop the technical bases that inform decision-making at WRCs, and also develop international standards (ITU-R Recommendations) on radiocommunication matters. ITU Working Parties (WPs) consider dozens of technical contributions from Member States.
Radio propagation models are sets of mathematical formulas that describe how far a radio wave will travel (propagate) as a function of frequency, distance, and many other conditions. The models are used to plan wireless telecommunications systems and to harmonize spectrum allocations nationally and internationally—radio waves, after all, are not limited to geo-political boundaries.
Within both U.S. SG3 and ITU-R SG3, ITS experts hold numerous leadership positions, including one of the four international chairs, head of the U.S. delegation to SG3, and three of the four U.S. Working Party chairs. Heading into the current meetings, ITS had authored two technical inputs to SG3 and also chaired several SG3 correspondence groups, which meet between SG3 WP meetings to continue technical discussion on modeling updates.
ITS’s predecessor laboratory, the Central Radio Propagation Laboratory (CRPL)—a direct descendent of the Radio Section of the National Bureau of Standards (NBS)—was headed by John H. Dellinger. Dellinger was a founding delegate to the International Radio Consultative Committee (CCIR, for Comité Consultatif International pour la Radio), which later became the ITU-R. As Director of first the NBS Radio Section and then the CRPL until his retirement in 1948, Dr. Dellinger led the development of seminal studies in Radio Propagation and was widely recognized and respected for his international leadership in conferences contributing to worldwide cooperation in telecommunications.
In 1967, Philip Rice, Anita Longley, Kenneth Norton, and Albrecht Barsis authored the final revision of NBS Technical Note 101, Transmission Loss Predictions for Tropospheric Communication Circuits: Volumes I and II. This Technical Note built on more than a decade of work to develop comprehensive methods for calculating how well proposed radio systems would meet requirements for satisfactory service, free from harmful interference. Then as now, the goal of widely publishing these methods was to promote more efficient use of the radio spectrum.
The model that Rice, Longley, and their colleagues developed and then made available for use by others as a FORTRAN program is now known as the Irregular Terrain Model (ITM), or the Longley-Rice Model. It was designed to be broadly applicable to frequencies from 20 MHz to 20 GHz and over distances between 1 and 2000 km. Significantly, it also was designed to take into account the impacts on radio propagation of terrain, such as mountains and valleys, and climate conditions.
The model was validated by an extensive collection of measured data that had accumulated at the NBS since the earliest decades of the 20th century. By comparing theoretical calculations to ground-truth measurement data, the researchers could develop and incorporate mathematical algorithms that took into account a wide variety of climates, terrains, and ground conditions. An important feature of Technical Note 101 was the presentation of both empirical graphs, from which propagation distances could be interpolated, and calculations, which were adaptable to computer programming.
In 1968, Anita Longley and Philip Rice followed up with the publication of Technical Report ERL 79-ITS 67, Prediction of Tropospheric Radio Transmission Loss Over Irregular Terrain: A Computer Method-1968. This report, published by the Institute for Telecommunication Sciences when it was housed within a different component of the Department of Commerce, presented one of the first computerized implementations of a radio propagation model. The FORTRAN code was printed in full—i.e., open sourced—with schematics of the subroutines as Section 3-5 of Annex 3 in ERL 79-ITS 67.
Portions of ITM still provide the foundation for a number of the P (Propagation) series of ITU-R international standards. The ITM also remains integral to certain Federal Communications Commission (FCC) rulemakings. As the Institute improves the accuracy and utility of the ITM, so does ITS contribute to improving the international standards.
Consistent with the Technology Transfer mission of federally funded laboratories, ITS provides open source software implementations of validated and authoritative propagation models. These promote agreement on the use of common trusted modeling tools to advance more efficient spectrum use and—of critical importance in the wireless era—effective spectrum sharing to optimize the use of spectrum.
The ITS Propagation Library (PropLib), open-sourced on github.com/NTIA, currently is composed of four repositories: 1) a C++ implementation of Longley and Rice’s ITM; 2) the U.S. Reference Software Implementation of ITU-R Recommendation P.528: A propagation prediction method for aeronautical mobile and radionavigation services using the VHF, UHF and SHF bands; 3) the U.S. Reference Software Implementation of ITU-R Recommendation P.2108: Prediction of clutter loss; and and 4) the NTIA/ITS implementation of the Low Frequency / Medium Frequency Propagation Model, LFMF. Software developed by ITS is freely available for use, is not subject to copyright protection within the United States, and to the extent that NTIA holds rights in other countries, is offered on a royalty-free basis throughout the world.