[Flightgear-devel] Radio simulation of navigational aids (original) (raw)

Hi,

While working to add support for realistic radio capabilities of navradio equipment, I have transitioned the radio code to a subsystem, thanks to valuable advice from Torsten, which should remove performance issues and make the system more flexible and useful for other development, like integration with Fgcom.

Regarding the simulation of realistic ILS behaviour, I do have a couple of unanswered questions left. Right now, the code is able to simulate realistic coverage of a directional antenna system, like the ones used in localizer and glideslope systems. The navradio code can poll a property which is updated periodically, and obtain a radio signal strength value. This should be enough to simulate range coverage.

However, there is still the issue of how to address the simulation of directional features of the localizer and glideslope. Real systems use a rather complicated antenna and feed system, which generates two types of signals, with phase differences being used to modify the modulation depths of two tones (90 Hz and 150 Hz) which are transmitted either with a main carrier or as sideband only. This means that signal strength is not the main factor determining localizer course or glideslope angle.

Now, I can see a couple of ways to simulate ILS:

  1. Use the signal strength calculation only for available range (and coverage determined by terrain obstructions) and leave the directional calculations as they are now.
  2. Simulate a simplified system which uses two virtual antennas to determine localizer course and glideslope angle using only radio signal strength (not realistic but would work, this was tested by me).
  3. Go for something as close to real systems as possible.

Options 1 and partially 2 are available now. I'd like to know what you think about this, and what paths do you foresee in the future for Flightgear radio.

Cheers, Adrian

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