mission block: fix incorrectly calculated ccw loiter exit (#19487)

* mission block: fix incorrectly calculated ccw loiter exit

* mission block: update comment on orbit exit location

src/modules/navigator/mission_block.cpp

@@ -466,9 +466,10 @@ MissionBlock::is_mission_item_reached()
 			     _mission_item.nav_cmd == NAV_CMD_LOITER_TO_ALT)) {
 
 				float bearing = get_bearing_to_next_waypoint(curr_sp.lat, curr_sp.lon, next_sp.lat, next_sp.lon);
-				// We should not use asinf outside of [-1..1].
-				const float ratio = math::constrain(_mission_item.loiter_radius / range, -1.0f, 1.0f);
-				float inner_angle = M_PI_2_F - asinf(ratio);
+
+				// calculate (positive) angle between current bearing vector (orbit center to next waypoint) and vector pointing to tangent exit location
+				const float ratio = math::min(fabsf(_mission_item.loiter_radius / range), 1.0f);
+				float inner_angle = acosf(ratio);
 
 				// Compute "ideal" tangent origin
 				if (curr_sp.loiter_direction > 0) {