On Mon, 2009-09-14 at 10:38 +0100, James Turner wrote:
> Two (unrelated) glideslope things:
> 
> 1) I tested making the has-gs property be based upon the GS range  
> check (leaving aside any discussion of how GS reception range should  
> be calculated). This is good, because existing panels generally use  
> has-gs as a 'valid GS signal being received', to un-park / show the  
> needle/pointer.
>
> The catch is, most of the flightdirector / autopilot scripts have been  
> written assuming that when approach mode is *armed*, has-gs is already  
> true. I.e they don't arm GS holding, if no valid GS is being received  
> at that point. It feels like this is a genuine problem - depending on  
> how far out the localiser is captured, and given that you could be  
> intercepting the localiser from an angle, I *guess* the real-life  
> equipment wouldn't assume a valid GS signal was being received at that  
> time.
> 
> What do the real world pilots say? The 'fix' would be to remove the  
> 'has-gs' flag checks from the various autopilot/flight-director Nasal  
> scripts, which I'm not exactly thrilled at the prospect of - but it  
> would make the GS needle/pointer behaviour more correct 'for free'.

I have always assumed and used the "has-gs" to identify a valid ILS
frequency instead of a VOR frequency.  This flag is needed so every
instrument doesn't have to implement the rather arcane rules for
determining whether or not a tuned frequency is ILS or VOR.

"The localizer is a VHF radio transmitter and antenna system using the
same general range as VOR transmitters (between 108.10 MHz and 111.95
MHz). Localizer frequencies, however, are only on odd-tenths, with 50
kHz spacing between each frequency."
http://www.allstar.fiu.edu/aero/ILS.htm

Or the chart here:
http://en.wikipedia.org/wiki/Instrument_landing_system#Frequency_list

Thanks,
Ron



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