[Flightgear-devel] ATC and radio signal attenuation (original) (raw)

Hi,

This message is mostly meant for Durk, but I expect suggestions from anyone who knows the scenery subsystem of Flightgear.

I have started to implement radio signal attenuation into the ATC subsystem, with the goal to later move this to it's own location. I have chosen the Irregular Terrain Model (ITM) developed by Longley-Rice in the 70's for several reasons:

    1. the software was developed by an US government agency and is 

therefore public domain. Original code and documentation: http://flattop.its.bldrdoc.gov/itm.html

    2. a more recent version of the software is available from the 

developers of Splat!, a GPL radio-propagation tool used by professionals and amateurs. This version is just a rewrite of the original C++ code that removes DLL-isms and adds useful comments. More information on Splat! and example use-cases can be found at: http://www.qsl.net/kd2bd/splat.html

    3. this propagation model is extremely configurable and I have verified 

via experimental results it's precision (when fed with appropriate data). It is capable of computing line-of-sight atmospheric loss, troposcatter loss for non LOS endpoints, simple and double diffraction loss when terrain is a factor between the transceiver and the receiver. It does not calculate reflection on terrain features, and it's diffraction routines, especially for large knife-edge angles, have been verified to be inaccurate. Nevertheless, important improvements have been added by Sid Shumate et al. in the last few years, that increase the precision by a large number. These improvements, however, are limited by a non-commercial clause, so if there is a need for great accuracy one must compile his own ITM. His progress and detailed explanations on limitations and advantages of ITM can be tracked at the IEEE Broadcast Technology Society Newsletter. http://bts.ieee.org/ieee-broadcast-technology-society-newsletter.html

This radio propagation model ensures that terrain between the station transmitting and the station receiving (the pilot) will block communication under the right circumstances, close to reality. The earth curvature is also taken into account and has an effect on signal quality. Apart from cutting off the transmission, I am also mangling the radio text if the signal-to-noise ratio is below a certain value. Coupled with festival voice, the results are interesting, but more could be done.

The key point of the implementation is sampling of terrain elevation between the two radio terminals. This may require some scenery tile loading, but I leave that to the experts.

So far, only ground to AI aircraft and AI aircraft to ground is implemented, as part of the FGATCController class. This system, if proven functional, should probably be split into a separate module and applied to all comunication, including player-to-ground and player- to-player. All code is available in my clone, branch radio-att.

73, Adrian, YO8RZZ

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