I encountered the following code, in the middle of
FGNavList::findNavFromList:
======================================================
// LOC, ILS, GS, and DME antenna's could potentially be
// installed at the opposite end of the runway. So it's not
// enough to simply find the closest antenna with the right
// frequency. We need the closest antenna with the right
// frequency that is most oriented towards us. (We penalize
// stations that are facing away from us by adding 5000 meters
// which is further than matching stations would ever be
// placed from each other. (Do the expensive check only for
// directional atennas and only when there is a chance it is
// the closest station.)
int type = station->get_type();
if ( d2 < min_dist &&
(type == 4 || type == 5 || type == 6 || type == 12 ||
type == 13) )
{
double hdg_deg = 0.0;
if ( type == 4 || type == 5 ){
hdg_deg = station->get_multiuse();
} else if ( type == 6 ) {
int tmp = (int)(station->get_multiuse() / 1000.0);
hdg_deg = station->get_multiuse() - (tmp * 1000);
} else if ( type == 12 || type == 13 ) {
// oops, Robin's data format doesn't give us the
// needed information to compute a heading for a DME
// transmitter. FIXME Robin!
}
double az1 = 0.0, az2 = 0.0, s = 0.0;
SGGeod geod = SGGeod::fromCart(aircraft);
geo_inverse_wgs_84( geod, station->get_pos(), &az1, &az2,
&s);
az1 = az1 - station->get_multiuse();
if ( az1 > 180.0) az1 -= 360.0;
if ( az1 < -180.0) az1 += 360.0;
// penalize opposite facing stations by adding 5000 meters
// (squared) which is further than matching stations would
// ever be placed from each other.
if ( fabs(az1) > 90.0 ) {
double dist = sqrt(d2);
d2 = (dist + 5000) * (dist + 5000);
}
}
=========================================================
Now, there's two bad things here: (I think)
- in the DME case (type == 12 or 13), hdg is left 0.0
- hdg, having been computed, isn't then used: I assume it should
replace the 'station->get_multiuse' on this line:
az1 = az1 - station->get_multiuse();
It'd be great if someone who knows more about this code could comment
on how 'important' it is, since it's only doing the right thing in the
ILS/LOC case (the GS will be wrong because 'multiuse' contains other
data, and DME because multiuse is empty)
It feels like this ought to cause visible weirdness with nav-radios,
but I don't have a strong understanding for how 'bad' this might be.
Maybe we only actually encounter this code for ILS transmitters, so
the bug is masked?
Any light that can be shed, would be appreciated.
James
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