diff --git a/libraries/AP_GPS/GPS_Backend.cpp b/libraries/AP_GPS/GPS_Backend.cpp
index 0af1b4ea47..95403f0090 100644
--- a/libraries/AP_GPS/GPS_Backend.cpp
+++ b/libraries/AP_GPS/GPS_Backend.cpp
@@ -367,7 +367,7 @@ bool AP_GPS_Backend::calculate_moving_base_yaw(AP_GPS::GPS_State &interim_state,
         }
 
 
-        if ((offset_dist - reported_distance) > (min_dist * permitted_error_length_pct)) {
+        if (fabsf(offset_dist - reported_distance) > (min_dist * permitted_error_length_pct)) {
             // the magnitude of the vector is much further then we were expecting
             Debug("Offset=%.2f vs reported-distance=%.2f (max-delta=%.2f)",
                   offset_dist, reported_distance, (double)(min_dist * permitted_error_length_pct));