Re: [Flightgear-devel] Typical NAV system errors

[John D. <js...@av...>]

On 01/05/2017 05:39 AM, Thorsten Renk wrote:

> * for TACAN, I found as figures of merit quoted 0.5 deg in azimuth, 0.2 %  
> in range with a minimum of 0.1 miles range error

In practice, it's quite a bit worse than that, for a number of
reasons.

For starters, there is VOR twist:
    https://www.av8n.com/how/htm/xc.html#sec-VOR-twist

You could model this quantitatively, on a VOR-by-VOR basis, by
looking at the VOR alignment in the A/FD and comparing it to
up-to-date information about the magnetic declination.

Also ... the VOR /receiver/ is part of the "system" and it could
be off by 4 degrees, or maybe even 6 degrees.
   http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/92b0831bac15c193852566cf0061999e!OpenDocument


The transmitter might be good to 0.5 degrees under absolutely
ideal conditions, e.g. sited in the middle of an infinitely
large flat uniform field.  However, under other conditions --
with messy geography, geology, and hydrology -- it's going to
be worse.

If things get too messy, they will build a humongous ground
plane under the transmitter
   https://www.flickr.com/photos/toms_3d/4946837712
but usually they don't bother.

Also, if the A/FD says the VOR is unusable beyond XX miles
in certain sectors, you'd better believe it.