Merge pull request #1266 from indy91/MCCLineOfSightFix

Fix bug in MCC line-of-sight to ground station calculation

Orbitersdk/samples/ProjectApollo/src_launch/mcc.cpp

@@ -950,11 +950,10 @@ void MCC::TimeStep(double simdt){
 
 void MCC::AutoUpdateXmitGroundStation(VESSEL* Ves, TrackingVesselType Type, TrackingSlot Slot)
 {
-	double Moonrelang;
 	double LOSRange;
 	VECTOR3 GSGlobalVector = _V(0, 0, 0);
 	VECTOR3 GSVector = _V(0, 0, 0);
-	bool MoonInTheWay, Sight;
+	bool Sight;
 
 	// Bail out if we failed to find either major body
 	if (Earth == NULL) { addMessage("Can't find Earth"); GT_Enabled = false; return; }
@@ -974,7 +973,7 @@ void MCC::AutoUpdateXmitGroundStation(VESSEL* Ves, TrackingVesselType Type, Trac
 				sin(GroundStations[StationIndex].Position[1] * RAD) * cos(GroundStations[StationIndex].Position[0] * RAD)) * R_E;
 
 			oapiLocalToGlobal(Earth, &GSVector, &GSGlobalVector);
-			MoonInTheWay = false;
+
 			if (GroundStations[StationIndex].StationPurpose & GSPT_LUNAR)
 			{
 				LOSRange = 5e8;
@@ -983,16 +982,11 @@ void MCC::AutoUpdateXmitGroundStation(VESSEL* Ves, TrackingVesselType Type, Trac
 			{
 				LOSRange = 2e7;
 			}
-			//Check if there is line-of-sight between vessel and station
-			Sight = OrbMech::sight(Vessel_Vector[Slot], GSVector, R_E);
-			//Moon in the way
-			Moonrelang = dotp(unit(MoonGlobalPos - VesselGlobalPos[Slot]), unit(GSGlobalVector - VesselGlobalPos[Slot]));
-			if (Moonrelang > cos(asin(R_M / length(MoonGlobalPos - VesselGlobalPos[Slot]))))
-			{
-				MoonInTheWay = true;
-			}
+			//Check if there is line-of-sight between vessel and station and that the Moon is not in the way either
+			Sight = OrbMech::sight(Vessel_Vector[Slot], GSVector, R_E) && OrbMech::sight(VesselGlobalPos[Slot] - MoonGlobalPos, GSGlobalVector - MoonGlobalPos, R_M);
+
 			if (Sight && GroundStations[StationIndex].AOS[Slot] == 0 && ((GroundStations[StationIndex].USBCaps & GSSC_VOICE) || (GroundStations[StationIndex].CommCaps & GSGC_VHFAG_VOICE))) {
-				if (length(Vessel_Vector[Slot] - GSVector) < LOSRange && !MoonInTheWay)
+				if (length(Vessel_Vector[Slot] - GSVector) < LOSRange)
 				{
 					//Dont switch to a new station if we're transmitting an uplink;
 					bool uplinking = false;
@@ -1022,7 +1016,7 @@ void MCC::AutoUpdateXmitGroundStation(VESSEL* Ves, TrackingVesselType Type, Trac
 
 				}
 			}
-			if ((!Sight || length(Vessel_Vector[Slot] - GSVector) > LOSRange || MoonInTheWay) && GroundStations[StationIndex].AOS[Slot] == 1) {
+			if ((!Sight || length(Vessel_Vector[Slot] - GSVector) > LOSRange) && GroundStations[StationIndex].AOS[Slot] == 1) {
 				GroundStations[StationIndex].AOS[Slot] = 0;
 
 				if (Type == TrackingVesselType::TypeCM && GT_Enabled == true) {

Orbitersdk/samples/ProjectApollo/src_rtccmfd/OrbMech.cpp

@@ -2398,21 +2398,34 @@ void umbra(VECTOR3 R, VECTOR3 V, VECTOR3 sun, OBJHANDLE planet, bool rise, doubl
 
 bool sight(VECTOR3 R1, VECTOR3 R2, double R_E)
 {
+	//Returns true if position vectors R1 and R2 have a line-of-sight, considering the body radius R_E
 	VECTOR3 R1n, R2n;
 	bool los;
-	double tau_min;
+	double r12, r22, tau_min, dot_r1r2;
 
+	//Preliminary calculations
+	//Normalized position vectors
 	R1n = R1 / R_E;
 	R2n = R2 / R_E;
-
-	tau_min = (length(R1n)*length(R1n) - dotp(R1n, R2n)) / (length(R1n)*length(R1n) + length(R2n)*length(R2n) - 2.0*dotp(R1n,R2n));
+	//Squares of radius magnitudes
+	r12 = dotp(R1n, R1n);
+	r22 = dotp(R2n, R2n);
+	//Dot product between normalized vectors
+	dot_r1r2 = dotp(R1n, R2n);
+
+	//Location on line connecting R1 and R2 where the distance from the center of the body to the line is minimized
+	tau_min = (r12 - dot_r1r2) / (r12 + r22 - 2.0*dot_r1r2);
+	//Default to no line-of-sight
 	los = false;
-	if (tau_min < 0 || tau_min>1)
+	//Is there a line of sight?
+	if (tau_min < 0.0 || tau_min > 1.0)
 	{
+		//Yes, because both vectors have to be in the same quadrant
 		los = true;
 	}
-	else if ((1.0 - tau_min)*length(R1n)*length(R1n) + dotp(R1n, R2n)*tau_min >= 1.0)
+	else if ((1.0 - tau_min)*r12 + dot_r1r2 * tau_min >= 1.0)
 	{
+		//Yes, because minimum distance from center of body to line connecting R1 and R2 is greater than R_E (R1 and R2 having been normalized)
 		los = true;
 	}
 	return los;