SITL: added SIM_MAG_ERROR parameter

this is a compass yaw error in degrees, used for testing navigation
with a bad compass

libraries/AP_HAL_AVR_SITL/sitl_compass.cpp

@@ -31,6 +31,13 @@ void SITL_State::_update_compass(float rollDeg, float pitchDeg, float yawDeg)
 		// no compass in this sketch
 		return;
 	}
+	yawDeg += _sitl->mag_error;
+	if (yawDeg > 180.0f) {
+		yawDeg -= 360.0f;
+	}
+	if (yawDeg < -180.0f) {
+		yawDeg += 360.0f;
+	}
 	_compass->setHIL(radians(rollDeg), radians(pitchDeg), radians(yawDeg));
 	Vector3f noise = _rand_vec3f() * _sitl->mag_noise;
 	_compass->mag_x += noise.x;

libraries/SITL/SITL.cpp

@@ -29,6 +29,7 @@ const AP_Param::GroupInfo SITL::var_info[] PROGMEM = {
     AP_GROUPINFO("GPS_TYPE",  12, SITL,  gps_type,  SITL::GPS_TYPE_UBLOX),
     AP_GROUPINFO("GPS_BYTELOSS",  13, SITL,  gps_byteloss,  0),
     AP_GROUPINFO("GPS_NUMSATS",   14, SITL,  gps_numsats,   10),
+    AP_GROUPINFO("MAG_ERROR",     15, SITL,  mag_error,  0),
     AP_GROUPEND
 };
 

libraries/SITL/SITL.h

@@ -50,6 +50,7 @@ public:
 	AP_Float accel_noise; // in m/s/s
 	AP_Float mag_noise;   // in mag units (earth field is 818)
 	AP_Float aspd_noise;  // in m/s 
+	AP_Float mag_error;   // in degrees
 
 	AP_Float drift_speed; // degrees/second/minute
 	AP_Float drift_time;  // period in minutes