SITL: add parameter to simulate reversed pitot tube order

libraries/AP_HAL_SITL/sitl_airspeed.cpp

@@ -34,13 +34,17 @@ void SITL_State::_update_airspeed(float airspeed)
     airspeed = airspeed + (_sitl->arspd_noise * rand_float());
 
     if (!is_zero(_sitl->arspd_fail_pressure)) {
-        // compute a realistic pressure report given some level of trapper air pressure in the tube abd our current altitude
-        // algorithim taken from https://en.wikipedia.org/wiki/Calibrated_airspeed#Calculation_from_impact_pressure
+        // compute a realistic pressure report given some level of trapper air pressure in the tube and our current altitude
+        // algorithm taken from https://en.wikipedia.org/wiki/Calibrated_airspeed#Calculation_from_impact_pressure
         float tube_pressure = abs(_sitl->arspd_fail_pressure - _barometer->get_pressure() + _sitl->arspd_fail_pitot_pressure);
         airspeed = 340.29409348 * sqrt(5 * (pow((tube_pressure / 101325.01576 + 1), 2.0/7.0) - 1.0));
     }
 
-    const float airspeed_pressure = (airspeed * airspeed) / airspeed_ratio;
+    float airspeed_pressure = (airspeed * airspeed) / airspeed_ratio;
+
+    // flip sign here for simulating reversed pitot/static connections
+    if (_sitl->arspd_signflip) airspeed_pressure *= -1;
+
     float airspeed_raw = airspeed_pressure + airspeed_offset;
     if (airspeed_raw / 4 > 0xFFFF) {
         airspeed_pin_value = 0xFFFF;

libraries/SITL/SITL.cpp

@@ -107,6 +107,7 @@ const AP_Param::GroupInfo SITL::var_info2[] = {
     AP_GROUPINFO("ARSPD_FAIL_P", 6, SITL,  arspd_fail_pressure, 0),
     AP_GROUPINFO("ARSPD_PITOT",  7, SITL,  arspd_fail_pitot_pressure, 0),
     AP_GROUPINFO("GPS_ALT_OFS",  8, SITL,  gps_alt_offset, 0),
+    AP_GROUPINFO("ARSPD_SIGN",   9, SITL,  arspd_signflip, 0),
     AP_GROUPEND
 };
     

libraries/SITL/SITL.h

@@ -159,6 +159,9 @@ public:
     AP_Float temp_tconst;
     AP_Float temp_baro_factor;
     
+    // differential pressure sensor tube order
+    AP_Int8 arspd_signflip;
+
     uint16_t irlock_port;
 
     void simstate_send(mavlink_channel_t chan);