DME units report a distance based on the assumption that the ground station

will delay it's reply by 50ms.  The ground station can change it's reply delay
to trick the airborn dme unit into reporting a distance that is offset from
the true distance by some constant value.  In FG we model this by subtracting
a fixed distance from the actual distance.

It is thus possible in our implimentation for the displayed distance to become
negative.  This patch clamp DME distance to a minimum value of 0.00 so it can
never go negative.

diff --git a/src/Cockpit/dme.cxx b/src/Cockpit/dme.cxx
index f8e40efd38af14c20a11285bf890e88001eb19e6..fb416fcaaa34e9d8d806e0f8b180389eb0359241 100644 (file)
--- a/src/Cockpit/dme.cxx
+++ b/src/Cockpit/dme.cxx
@@ -161,6 +161,9 @@ FGDME::update(double dt)
             station = Point3D( x, y, z );
             dist = aircraft.distance3D( station ) * SG_METER_TO_NM;
             dist -= bias;
+            if ( dist < 0.0 ) {
+                dist = 0.0;
+            }
 
             current_time.stamp();
             long dMs = (current_time - last_time) / 1000;

diff --git a/src/Instrumentation/dme.cxx b/src/Instrumentation/dme.cxx
index f9d186ca12c578853b7dc02ae58cb6bb6de59582..5aea2a8ba6ab74176c29cc7a0de7ec8633553d44 100644 (file)
--- a/src/Instrumentation/dme.cxx
+++ b/src/Instrumentation/dme.cxx
@@ -128,7 +128,11 @@ DME::update (double delta_time_sec)
         _last_distance_nm = distance_nm;
 
         _in_range_node->setBoolValue(true);
-        _distance_node->setDoubleValue(distance_nm - _transmitter_bias);
+        double tmp_dist = distance_nm - _transmitter_bias;
+        if ( tmp_dist < 0.0 ) {
+            tmp_dist = 0.0;
+        }
+        _distance_node->setDoubleValue( tmp_dist );
         _speed_node->setDoubleValue(speed_kt);
         _time_node->setDoubleValue(distance_nm/speed_kt*60.0);