AP_NavEKF2: Change assumed gyro calibration accuracy

If an external gyro calibration has been performed, we should assume that it has been done under static conditions
Otherwise it is pointless and we should allow the EKF to find its own gyro bias offsets.

libraries/AP_NavEKF2/AP_NavEKF_GyroBias.cpp

@@ -19,12 +19,15 @@
 extern const AP_HAL::HAL& hal;
 
 // reset the body axis gyro bias states to zero and re-initialise the corresponding covariances
+// Assume that the calibration is performed to an accuracy of 0.5 deg/sec which will require averaging under static conditions
+// WARNING - a non-blocking calibration method must be used
 void NavEKF2_core::resetGyroBias(void)
 {
     stateStruct.gyro_bias.zero();
     zeroRows(P,9,11);
     zeroCols(P,9,11);
-    P[9][9] = sq(radians(InitialGyroBiasUncertainty() * dtIMUavg));
+
+    P[9][9] = sq(radians(0.5f * dtIMUavg));
     P[10][10] = P[9][9];
     P[11][11] = P[9][9];
 }
@@ -38,4 +41,4 @@ float NavEKF2_core::InitialGyroBiasUncertainty(void) const
 }
 
 
-#endif // HAL_CPU_CLASS
+#endif // HAL_CPU_CLASS