AP_OpticalFlow: Add noise to simulated sensor

libraries/AP_OpticalFlow/AP_OpticalFlow_SITL.cpp

@@ -91,8 +91,8 @@ void AP_OpticalFlow_SITL::update(void)
     // optical rates relative to X and Y sensor axes assuming no misalignment or scale
     // factor error. Note - these are instantaneous values. The sensor sums these values across the interval from the last
     // poll to provide a delta angle across the interface
-    state.flowRate.x =  -relVelSensor.y/range + gyro.x;
-    state.flowRate.y =   relVelSensor.x/range + gyro.y;
+    state.flowRate.x =  -relVelSensor.y/range + gyro.x + _sitl->flow_noise * rand_float();
+    state.flowRate.y =   relVelSensor.x/range + gyro.y + _sitl->flow_noise * rand_float();
 
     // The flow sensors body rates are assumed to be the same as the vehicle body rates (ie no misalignment)
     // Note - these are instantaneous values. The sensor sums these values across the interval from the last