There are many types of propagation models for various wireless network configurations. There is something for everything: WiFi connectivity inside of multistory residences. Cellphone network coverage across cities/towns. Broadcast services coverage. And of course, two way communications modes as in amateur radio.
Because of the wide range of frequencies allocated to the amateur service, there isn't one good model that can cover them all. Some frequency bands allow for radio waves to reflect off of the ionosphere, which is constantly changing. Others occasionally take advantage of sporadic-E conditions to allow for long-distance links. In some cases radio waves are reflected off of things we might not expect, such as in EME communications. We won't look at these specialized cases here, other than acknowledge that they exist.
|Unofficial KUOI-FM Model by KE7PHG. No warranties implied by the model.|
Our interest will be in the world within 20 MHz to 20 GHz, as covered by the Longley-Rice Irregular Terrain Model. This code was incorporated by VE2DBE in his application Radio Mobile, which is free but copyright software. This application links the ITM model with various data sources such as GPS/APRS, SRTM v2 elevation data, various publicly available maps online, receiver/transmitter characteristics, and a powerful bit of code to optimize the location of things such as repeater locations based on your criteria.
Terrain Data Sources
First we need to discuss one of the most important pieces of the irregular terrain model - the terrain data. In February 2000 the Space Shuttle Endeavour flew a mission where it collected elevation data of nearly 80% of all land surfaces. This collected data forms the SRTM topography data, which is currently processed into a V2 release where extensive edits were made by the National Geospatial Intelligence Agency to better define bodies of water and coastal areas as well as remove single-pixel spike and well errors. Due to the use of different collection bands and extensive processing techniques, data for most of North America is available in 1-arc second resolution. 3-arc second resolution is available for the rest of the world. These correspond to 30 meter and 90 meter resolutions.
This data is used in current Longley-Rice ITM models to ensure that they model actual propagation given the terrain at the area of interest. In order to use it, the GPS coordinates of the station locations needs to be known. Also, the FTP server directory for SRTM data needs to be configured, or a local copy of this data needs to be used for faster access. For locations in the USA, the above figure shows region definitions which are used to organize the 1-arc second resolution data. These represent the folder structure found in the NASA SRTM FTP server @ ftp://e0srp01u.ecs.nasa.gov/srtm/