AHRS: added AHRS_GPS_GAIN parameter

this allows control of how much GPS velocity information is used in
doing centripetal correction in DCM

ArduCopter/Parameters.pde

@@ -314,6 +314,9 @@ static void load_parameters(void)
 		}
 	}
 
+	// change the default for the AHRS_GPS_GAIN for ArduCopter
+	ahrs.gps_gain.set(0.0);
+
 	if (!g.format_version.load() ||
 	    g.format_version != Parameters::k_format_version) {
 

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -31,13 +31,26 @@
 
 // table of user settable parameters
 const AP_Param::GroupInfo AP_AHRS::var_info[] PROGMEM = {
-	// @Param: YAW_P
-	// @DisplayName: Yaw P
-	// @Description: This controls the weight the compass has on the overall heading
-	// @Range: 0 .4
-	// @Increment: .01
+    // @Param: AHRS_YAW_P
+    // @DisplayName: Yaw P
+    // @Description: This controls the weight the compass has on the overall heading
+    // @Range: 0 .4
+    // @Increment: .01
     AP_GROUPINFO("YAW_P", 0, AP_AHRS_DCM, _kp_yaw),
+
+    // @Param: AHRS_RP_P
+    // @DisplayName: AHRS RP_P
+    // @Description: This controls how fast the accelerometers correct the attitude
+    // @Range: 0 .4
+    // @Increment: .01
     AP_GROUPINFO("RP_P",  1, AP_AHRS_DCM, _kp),
+
+    // @Param: AHRS_GPS_GAIN
+    // @DisplayName: AHRS GPS gain
+    // @Description: This controls how how much to use the GPS to correct the attitude
+    // @Range: 0.0 1.0
+    // @Increment: .01
+    AP_GROUPINFO("GPS_GAIN",  2, AP_AHRS_DCM, gps_gain),
     AP_GROUPEND
 };
 
@@ -437,6 +450,7 @@ AP_AHRS_DCM::drift_correction(float deltat)
     // are m/s/s
     Vector3f GA_e;
     float v_scale = 1.0/(_ra_deltat*_gravity);
+    v_scale *= gps_gain;
     GA_e = Vector3f(0, 0, -1.0) + ((velocity - _last_velocity) * v_scale);
     GA_e.normalize();
     if (GA_e.is_inf()) {

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -25,6 +25,7 @@ public:
 
                 _kp.set(0.4);
 		_kp_yaw.set(0.4);
+		gps_gain.set(1.0);
 	}
 
 	// return the smoothed gyro vector corrected for drift
@@ -45,6 +46,7 @@ public:
 	// settable parameters
 	AP_Float	_kp_yaw;
 	AP_Float	_kp;
+	AP_Float	gps_gain;
 
 private:
 	float		_ki;