Merge pull request #36 from la1k/issue33

Add visibility status to struct predict_observation.

Closes #33.

include/predict/predict.h.in

@@ -289,6 +289,12 @@ struct predict_observation {
 	double range_x, range_y, range_z; 
 	///Range velocity (km/s) 
 	double range_rate;      
+	///Visibility status, whether satellite can be seen by optical means.
+	///The satellite is defined to be visible if:
+	//	- The satellite is in sunlight
+	//	- The satellite is above the horizon
+	//	- The sky is dark enough (sun elevation is below a fixed threshold)
+	bool visible;
 };
 
 /**

src/observer.c

@@ -29,6 +29,9 @@ void predict_destroy_observer(predict_observer_t *obs)
 	}
 }
 
+
+#define VISIBILITY_SUN_ELE_UPPER_THRESH -12.0
+#define VISIBILITY_ORBIT_ELE_LOWER_THRESH 0.0
 /**
  * \brief Calculates range, azimuth, elevation and relative velocity.
  *
@@ -43,6 +46,13 @@ void predict_observe_orbit(const predict_observer_t *observer, const predict_orb
 
 	observer_calculate(observer, julTime, orbit->position, orbit->velocity, obs);
 
+	// Calculate visibility status of the orbit: Orbit is visible if sun elevation is low enough and the orbit is above the horizon, but still in sunlight.
+	obs->visible = false;
+	struct predict_observation sun_obs;
+	predict_observe_sun(observer, orbit->time, &sun_obs);
+	if (!(orbit->eclipsed) && (sun_obs.elevation*180.0/M_PI < VISIBILITY_SUN_ELE_UPPER_THRESH) && (obs->elevation*180.0/M_PI > VISIBILITY_ORBIT_ELE_LOWER_THRESH)) {
+		obs->visible = true;
+	}
 }
 
 void observer_calculate(const predict_observer_t *observer, double time, const double pos[3], const double vel[3], struct predict_observation *result)