SITL: Add GPS noise parameter

libraries/SITL/SITL.cpp

@@ -88,6 +88,7 @@ const AP_Param::GroupInfo SITL::var_info[] = {
     AP_GROUPINFO("SONAR_POS",     55, SITL,  rngfnd_pos_offset, 0),
     AP_GROUPINFO("FLOW_POS",      56, SITL,  optflow_pos_offset, 0),
     AP_GROUPINFO("ACC2_BIAS",     57, SITL,  accel2_bias, 0),
+    AP_GROUPINFO("GPS_NOISE",     58, SITL,  gps_noise, 0),
     AP_GROUPEND
 };
 

libraries/SITL/SITL.h

@@ -76,6 +76,7 @@ public:
     AP_Vector3f accel2_bias; // in m/s/s
     AP_Float arspd_noise;  // in m/s
     AP_Float arspd_fail;   // pitot tube failure
+    AP_Float gps_noise; // amplitude of the gps altitude error
 
     AP_Float mag_noise;   // in mag units (earth field is 818)
     AP_Float mag_error;   // in degrees