## Re: [Flightgear-devel] Dots, degrees and magic '5's

 Re: [Flightgear-devel] Dots, degrees and magic '5's From: John Denker - 2008-12-31 17:29:44 ```On 12/31/2008 06:23 AM, James Turner wrote: > Reckons 5 degrees per-dot for a VOR, 1.25 for a LOC (yay, the 4x > factor is sane) and 'about a quarter of a degree per dot' for a GS > indicator, so the 0.32 term is plausible. Standard dogma in IFR training is that the VOR CDI indicates two degrees per dot, while the LOC CDI indicates half a degree per dot. These numbers are quite believable. Good practice is to check them as part of the 30-day IFR receiver check. Experience suggests that a better way to think about it is in terms of _full scale_. The pilot sees ±10 degrees full scale on a VOR and ±2.5 degrees full scale on a localizer. That's because talking about "degrees per dot" doesn't work too well for RNAV units, which display cross-track error with dimensions of distance, not dimensions of angle. Again the sensitivity is greater on approach than enroute. http://www.avweb.com/news/system/183179-1.html But even if we confine attention to VORs, the vast majority of CDIs have ± 5 dots full scale (or a ring and four dots, which comes to the same thing) ... so 10 degrees full scale is equivalent to 2 degrees per dot: http://www.psavionics.com/images/KI208hi.png http://www.psavionics.com/images/ki209hi.gif http://www.auf.asn.au/navimages/vor.jpg http://www.knappair.com/N4792CHSI.jpg http://www.aircraftspruce.com/catalog/graphics/11-05106.gif And meanwhile some instruments VERY EXCEPTIONALLY have fewer dots http://www.aircraftmech.com/pics/nd1.gif which would mean MORE degrees per dot. I suspect there may be some CDIs with 6 dots full scale, but I can't cite any examples at the moment. As to the "magic 5" fudge factor, I have no idea what purpose that serves. It is *not* meant to create 10 volts full scale or anything like that, because the industry standard is ±150 millivolts full scale. http://www.seaerospace.com/lc/cart.php?target=productDetails&model=GNS-530&substring=011-00550-10 I'd be happy to consider alternatives to the "magic 5" : One could make a case for *) ±1.0 full scale, for obvious reasons, or *) ±0.150 full scale i.e. ±0.150 _volts_ to model the real world *) ±10.0 full scale, just to avoid changing things that don't absolutely need to be changed. ============================================== Before diving into navradio.cxx, I suggest you take a look at http://www.av8n.com/fly/fgfs/servol.diff which fixes the worst of the service volume silliness. ```