From 3c3f3a096eec57273c4585e948f13cc8e37c36e7 Mon Sep 17 00:00:00 2001
From: ThorstenB <brehmt@gmail.com>
Date: Wed, 16 Feb 2011 19:53:10 +0100
Subject: [PATCH] John Denker: move branch cut and apply clamps fixed
 un-normalized outputs

---
 src/Instrumentation/navradio.cxx | 33 +++++++++++++++++++++++++-------
 src/Instrumentation/navradio.hxx |  2 +-
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/src/Instrumentation/navradio.cxx b/src/Instrumentation/navradio.cxx
index 09e9c1dd0..5ae4e554f 100644
--- a/src/Instrumentation/navradio.cxx
+++ b/src/Instrumentation/navradio.cxx
@@ -608,9 +608,21 @@ void FGNavRadio::updateGlideSlope(double dt, const SGVec3d& aircraft, double sig
   SGVec3d pos = aircraft - _gsCart; // relative vector from gs antenna to aircraft
   // The positive GS axis points along the runway in the landing direction,
   // toward the far end, not toward the approach area, so we need a - sign here:
-  double dot_h = -dot(pos, _gsAxis);
-  double dot_v = dot(pos, _gsVertical);
-  _gsDirect = atan2(dot_v, dot_h) * SGD_RADIANS_TO_DEGREES;
+  double comp_h = -dot(pos, _gsAxis);      // component in horiz direction
+  double comp_v = dot(pos, _gsVertical);   // component in vertical direction
+  //double comp_b = dot(pos, _gsBaseline);   // component in baseline direction
+  //if (comp_b) {}                           // ... (useful for debugging)
+
+// _gsDirect represents the angle of elevation of the aircraft
+// as seen by the GS transmitter.
+  _gsDirect = atan2(comp_v, comp_h) * SGD_RADIANS_TO_DEGREES;
+// At this point, if the aircraft is centered on the glide slope,
+// _gsDirect will be a small positive number, e.g. 3.0 degrees
+
+// Aim the branch cut straight down 
+// into the ground below the GS transmitter:
+  if (_gsDirect < -90.0) _gsDirect += 360.0;
+
   double deflectionAngle = target_gs - _gsDirect;
   
   if (falseCoursesEnabledNode->getBoolValue()) {
@@ -631,10 +643,17 @@ void FGNavRadio::updateGlideSlope(double dt, const SGVec3d& aircraft, double sig
     }
   }
   
+// GS is documented to be 1.4 degrees thick, 
+// i.e. plus or minus 0.7 degrees from the midline:
+  SG_CLAMP_RANGE(deflectionAngle, -0.7, 0.7);
+
+// Many older instrument xml frontends depend on
+// the un-normalized gs-needle-deflection.
+// Apparently the interface standard is plus or minus 3.5 "volts"
+// for a full-scale deflection:
   _gsNeedleDeflection = deflectionAngle * 5.0;
   _gsNeedleDeflection *= signal_quality_norm;
   
-  SG_CLAMP_RANGE(deflectionAngle, -0.7, 0.7);
   _gsNeedleDeflectionNorm = (deflectionAngle / 0.7) * signal_quality_norm;
   
   //////////////////////////////////////////////////////////
@@ -957,9 +976,9 @@ void FGNavRadio::search()
         _gsAxis = tangentVector(_gs->geod(), gs_radial);
 
         // GS baseline unit tangent vector
-        // (perpendicular to the runay along the ground)
-        SGVec3d baseline = tangentVector(_gs->geod(), gs_radial + 90.0);
-        _gsVertical = cross(baseline, _gsAxis);
+        // (transverse to the runay along the ground)
+        _gsBaseline = tangentVector(_gs->geod(), gs_radial + 90.0);
+        _gsVertical = cross(_gsBaseline, _gsAxis);
       } // of have glideslope
     } // of found LOC or ILS
     
diff --git a/src/Instrumentation/navradio.hxx b/src/Instrumentation/navradio.hxx
index 5f81a8a08..96621b6d0 100644
--- a/src/Instrumentation/navradio.hxx
+++ b/src/Instrumentation/navradio.hxx
@@ -153,7 +153,7 @@ class FGNavRadio : public SGSubsystem, public SGPropertyChangeListener
     // internal periodic station search timer
     double _time_before_search_sec;
 
-    SGVec3d _gsCart, _gsAxis, _gsVertical;
+    SGVec3d _gsCart, _gsAxis, _gsVertical, _gsBaseline;
 
     FGNavRecordPtr _dme;
     bool _dmeInRange;
-- 
2.39.5