From 29f84492a2cdc40a3275e6ff11b4b9520f8774d9 Mon Sep 17 00:00:00 2001
From: jmt <jmt>
Date: Wed, 16 Sep 2009 09:35:22 +0000
Subject: [PATCH] Different approach to handling the
 paired-ILS-on-opposite-runways situation. This uses additional information we
 now store (in FGRunway) to be more explicit about the behaviour (i.e only
 look at paired navids), and crucially, does not misbehave near either
 threshold. Instead it always switches about the runway midpoint (so
 back-course/missed-approaches are broken as before), leading to stable
 behaviour when touching down.

I am pretty confident this is (was, from FS2004) also the 'solution' taken by MSFS.
---
 src/Navaids/navlist.cxx | 76 +++++++++++++++++------------------------
 1 file changed, 31 insertions(+), 45 deletions(-)

diff --git a/src/Navaids/navlist.cxx b/src/Navaids/navlist.cxx
index f49052125..bbe3eb424 100644
--- a/src/Navaids/navlist.cxx
+++ b/src/Navaids/navlist.cxx
@@ -31,6 +31,8 @@
 
 #include "navlist.hxx"
 
+#include <Airports/runways.hxx>
+
 using std::string;
 
 // FGNavList ------------------------------------------------------------------
@@ -108,49 +110,38 @@ FGNavRecord *FGNavList::findByIdentAndFreq(const string& ident, const double fre
   return NULL;
 }
 
-// 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.)
-
-static bool penaltyForNav(FGNavRecord* aNav, const SGGeod &aGeod)
+// discount navids if they conflict with another on the same frequency
+// this only applies to navids associated with opposite ends of a runway,
+// with matching frequencies.
+static bool navidUsable(FGNavRecord* aNav, const SGGeod &aircraft)
 {
-  switch (aNav->type()) {
-  case FGPositioned::ILS:
-  case FGPositioned::LOC:
-  case FGPositioned::GS:
-// FIXME
-//  case FGPositioned::DME: we can't get the heading for a DME transmitter, oops
-    break;
-  default:
-    return false;
+  FGRunway* r(aNav->runway());
+  if (!r || !r->reciprocalRunway()) {
+    return true;
   }
   
-  double hdg_deg = 0.0;
-  if (aNav->type() == FGPositioned::GS) {
-    int tmp = (int)(aNav->get_multiuse() / 1000.0);
-    hdg_deg = aNav->get_multiuse() - (tmp * 1000);
-  } else {    
-    hdg_deg = aNav->get_multiuse();
+// check if the runway frequency is paired
+  FGNavRecord* locA = r->ILS();
+  FGNavRecord* locB = r->reciprocalRunway()->ILS();
+  
+  if (!locA || !locB || (locA->get_freq() != locB->get_freq())) {
+    return true; // not paired, ok
   }
   
-  double az1, az2, s;
-  SGGeodesy::inverse(aGeod, aNav->geod(), az1, az2, s);
-  az1 = az1 - hdg_deg;
-  SG_NORMALIZE_RANGE(az1, -180.0, 180.0);
-  return fabs(az1) > 90.0;
+// okay, both ends have an ILS, and they're paired. We need to select based on
+// aircraft position. What we're going to use is *runway* (not navid) position,
+// ie whichever runway end we are closer too. This makes back-course / missed
+// approach behaviour incorrect, but that's the price we accept.
+  double crs = SGGeodesy::courseDeg(aircraft, r->geod());
+  double hdgDiff = crs - r->headingDeg();
+  SG_NORMALIZE_RANGE(hdgDiff, -180.0, 180.0);  
+  return (fabs(hdgDiff) < 90.0);
 }
 
-// Given a point and a list of stations, return the closest one to the
-// specified point.
-FGNavRecord *FGNavList::findNavFromList( const SGGeod &aircraft,
-                                         const nav_list_type &stations )
+// Given a point and a list of stations, return the closest one to
+// the specified point.
+FGNavRecord* FGNavList::findNavFromList( const SGGeod &aircraft,
+                                  const nav_list_type &stations )
 {
     FGNavRecord *nav = NULL;
     double d2;                  // in meters squared
@@ -163,18 +154,13 @@ FGNavRecord *FGNavList::findNavFromList( const SGGeod &aircraft,
     // find the closest station within a sensible range (FG_NAV_MAX_RANGE)
     for ( it = stations.begin(); it != end; ++it ) {
         FGNavRecord *station = *it;
-        // cout << "testing " << current->get_ident() << endl;
         d2 = distSqr(station->cart(), aircraftCart);
-        if ( d2 < min_dist && penaltyForNav(station, aircraft))
-        {
-          double dist = sqrt(d2);
-          d2 = (dist + 5000) * (dist + 5000);
+        if ( d2 > min_dist || !navidUsable(station, aircraft)) {
+          continue;
         }
         
-        if ( d2 < min_dist ) {
-            min_dist = d2;
-            nav = station;
-        }
+        min_dist = d2;
+        nav = station;
     }
 
     return nav;