Radio propagation

Using Flightgear as a general purpose, 3D, radio signal propagation and terrain analysis tool

The purpose of this document is to evaluate the possible usage of the Flightgear engine as a tool for RF signal loss prediction over a rough terrain, using geographical features embedded inside terrain data and well know propagation prediction algorithms.

There are currently a number of freeware and open-source applications that perform RF signal loss analysis using data available to the general public (elevation data, landcover etc.). Well known and used are Radiomobile , a Windows only application by Roger Coudé, VE2DBE and Splat! (multiple platform) by John A. Magliacane, KD2BD.

A common point of these applications is the usage of the Irregular Terrain Model (also known as Longley-Rice) which is a propagation model developed by the U.S. Department of Commerce NTIA/ITS - Institute for Telecommunication Sciences and improved by several others (notably ITWOM by Sidney E. Shumate, code available to the public on a limited usage license, copyright Givens-Bell ). It is a general purpose model that can be applied to a large variety of engineering problems. The model, which is based on electromagnetic theory and on statistical analyses of both terrain features and radio measurements, predicts the median attenuation of a radio signal as a function of distance and the variability of the signal in time and in space. It is currently used among others by NASA, Alcatel-Lucent, US Army, University of Massachusetts and amateur radio operators around the world, and is widely considered the best propagation model for frequencies between 50 - 5000 Mhz freely available to the public.

Rationale

The implementation of radio propagation analysis is currently underway inside Flightgear. Since the terrain used by this open-source flight simulator contains both elevation information and landcover data, it seems especially suited for a 3D tool to predict RF signal levels, both for aviation navigational aids and radio communications, and standalone as a point-to-point radio link analysis between two radio stations. Flightgear allows the user to place models on the terrain at specific locations, and to setup internal properties via a simple XML format. Simple radio receivers and transmitters could be implemented using the Nasal scripting language, together with an UI to perform signal level reading and modify receiver and transmitter physical characteristics. The 3D aspect is also interesting, since it would allow one to perform predicted signal level reading at different locations while inside the simulator, using visual cues to place the radio equipment on the terrain. Since this open-source flight simulator is capable of running on multiple platforms, including GNU/Linux, Microsoft Windows and MacOS, its availability is not limited to one operating system.

Data

Most of new generation Flightgear terrain uses NASA SRTM v3 elevation data, which has a distance of 90 meters between elevation points. This is considered accurate enough for most signal analysis, although more detailed elevation data from topographical maps, ASTER DEM or other sources can be acquired. Default landcover in Flightgear comes from VMAP0 layers, with more accurate geodata being available from national and international programs, such as the Corine landcover project. Landcover data could be used to analyse path loss and improve prediction algorithms over specific types of terrain, in combination with real-world RF signal readings took at the same locations. Also, data from Flightgear's realistic atmospheric and weather simulation could be taken into account when path loss analysis is perfomed.

Currently implemented features within the Flightgear engine

• terrain elevation data sampling
• point-to-point path loss calculations over rough terrain using the ITM library for air traffic control stations to pilot communication
• path loss over a smooth round earth using free space signal attenuation equations
• RF link budget calculations using transmitter power, receiver sensitivity, antenna gain and height above terrain, taking into account typical values from various specifications

Features which still need to be implemented

• point-to-point path loss calculations using the ITM library for VOR/Localizer and ILS/glideslope equipment
• placing radio transmitters and receivers with the model tool inside 3D space
• radio parameter configuration for radio stations
• signal level reading and statistics user interface
• usage of detailed landcover in RF propagation prediction
• usage of weather simulation and atmospheric conditions which affect radio propagation