AP_NavEKF: Add public method reporting horizontal speed limit

This is required if using optical flow, as depending on height, the speed must be limited to prevent the sensor saturating

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -3622,6 +3622,17 @@ uint8_t NavEKF::setInhibitGPS(void)
     }
 }
 
+// return the horizontal speed limit in m/s set by optical flow limitations
+// allow 1.0 rad/sec margin for angular motion
+float NavEKF::getSpeedLimit(void) const
+{
+    if (useOptFlow()) {
+        return max((_maxFlowRate - 1.0f), 0.0f) * (flowStates[1] - state.position[2]);
+    } else {
+        return 400.0f; //return 80% of max filter speed
+    }
+}
+
 // return weighting of first IMU in blending function
 void NavEKF::getIMU1Weighting(float &ret) const
 {

libraries/AP_NavEKF/AP_NavEKF.h

@@ -119,6 +119,9 @@ public:
     // This command is forgotten by the EKF each time it goes back into static mode (eg the vehicle disarms)
     uint8_t setInhibitGPS(void);
 
+    // return the horizontal speed limit in m/s set by optical flow limitations
+    float getSpeedLimit(void) const;
+
     // return weighting of first IMU in blending function
     void getIMU1Weighting(float &ret) const;