GPS: added velocity and acceleration components

used by AHRS for acceleration correction

libraries/AP_GPS/GPS.cpp

@@ -11,6 +11,8 @@
 # define Debug(fmt, args...)
 #endif
 
+#include <AP_Common.h>
+#include <AP_Math.h>
 #include "GPS.h"
 #if defined(ARDUINO) && ARDUINO >= 100
 	#include "Arduino.h"
@@ -53,17 +55,33 @@ GPS::update(void)
 			// update our acceleration
 			float deltat = 1.0e-3 * (_idleTimer - last_fix_time);
 			float deltav = 1.0e-2 * ((float)ground_speed - (float)_last_ground_speed);
+			float gps_heading = ToRad(ground_course * 0.01);
+			float gps_speed   = ground_speed * 0.01;
+			float sin_heading, cos_heading;
+
+			cos_heading = cos(gps_heading);
+			sin_heading = sin(gps_heading);
+
 			last_fix_time = _idleTimer;
 			_last_ground_speed = ground_speed;
 
+			_velocity_north = gps_speed * cos_heading;
+			_velocity_east  = gps_speed * sin_heading;
+
 			if (deltat > 2.0 || deltat == 0) {
 				// we didn't get a fix for 2 seconds - set
 				// acceleration to zero, as the estimate will be too
 				// far out
 				_acceleration = 0;
+				_acceleration_north = 0;
+				_acceleration_east = 0;
 			} else {
 				// calculate a mildly smoothed acceleration value
 				_acceleration = (0.7 * _acceleration) + (0.3 * (deltav/deltat));
+
+				// calculate the components, to save time in the AHRS code
+				_acceleration_north = _acceleration * cos_heading;
+				_acceleration_east  = _acceleration * sin_heading;
 			}
 		}
     }

libraries/AP_GPS/GPS.h

@@ -126,6 +126,14 @@ public:
 	// our approximate linear acceleration in m/s/s
 	float acceleration(void) { return _acceleration; }
 
+	// components of acceleration in 2D, in m/s/s
+	float acceleration_north(void) { return _status == GPS_OK? _acceleration_north : 0; }
+	float acceleration_east(void)  { return _status == GPS_OK? _acceleration_east  : 0; }
+
+	// components of velocity in 2D, in m/s
+	float velocity_north(void) { return _status == GPS_OK? _velocity_north : 0; }
+	float velocity_east(void)  { return _status == GPS_OK? _velocity_east  : 0; }
+
 	// the time we got our last fix in system milliseconds
 	uint32_t last_fix_time;
 
@@ -196,8 +204,15 @@ private:
 	// previous ground speed in cm/s
     uint32_t _last_ground_speed;
 
-	// smoothed estimate of our acceleration
+	// smoothed estimate of our acceleration, in m/s/s
 	float _acceleration;
+	float _acceleration_north;
+	float _acceleration_east;
+
+	// components of the velocity, in m/s
+	float _velocity_north;
+	float _velocity_east;
+
 };
 
 inline long