Sub: update calibration if reading is above water level

ArduSub/sensors.cpp

@@ -4,6 +4,11 @@
 void Sub::read_barometer()
 {
     barometer.update();
+    // If we are reading a positive altitude, the sensor needs calibration
+    // Even a few meters above the water we should have no significant depth reading
+    if(!motors.armed() && barometer.get_altitude() > 0) {
+        barometer.update_calibration();
+    }
 
     if (ap.depth_sensor_present) {
         sensor_health.depth = barometer.healthy(depth_sensor_idx);