diff --git a/libraries/AP_Mount/AP_Mount_Backend.cpp b/libraries/AP_Mount/AP_Mount_Backend.cpp
index 483abf6337..d941f849c1 100644
--- a/libraries/AP_Mount/AP_Mount_Backend.cpp
+++ b/libraries/AP_Mount/AP_Mount_Backend.cpp
@@ -185,7 +185,7 @@ bool AP_Mount_Backend::calc_angle_to_location(const struct Location &target, Vec
     if (!AP::ahrs().get_position(current_loc)) {
         return false;
     }
-    const float GPS_vector_x = (target.lng-current_loc.lng)*cosf(ToRad((current_loc.lat+target.lat)*0.00000005f))*0.01113195f;
+    const float GPS_vector_x = Location::diff_longitude(target.lng,current_loc.lng)*cosf(ToRad((current_loc.lat+target.lat)*0.00000005f))*0.01113195f;
     const float GPS_vector_y = (target.lat-current_loc.lat)*0.01113195f;
     int32_t target_alt_cm = 0;
     if (!target.get_alt_cm(Location::AltFrame::ABOVE_HOME, target_alt_cm)) {
@@ -217,4 +217,4 @@ bool AP_Mount_Backend::calc_angle_to_location(const struct Location &target, Vec
     return true;
 }
 
-#endif // HAL_MOUNT_ENABLED
\ No newline at end of file
+#endif // HAL_MOUNT_ENABLED