AP_GPS: added last_ground_speed() method

used for dead-reckoning in AHRS

libraries/AP_GPS/GPS.cpp

@@ -54,7 +54,7 @@ GPS::update(void)
 		if (_status == GPS_OK) {
 			// 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 deltav = 1.0e-2 * ((float)ground_speed - (float)_last_ground_speed_cm);
 			float gps_heading = ToRad(ground_course * 0.01);
 			float gps_speed   = ground_speed * 0.01;
 			float sin_heading, cos_heading;
@@ -63,7 +63,7 @@ GPS::update(void)
 			sin_heading = sin(gps_heading);
 
 			last_fix_time = _idleTimer;
-			_last_ground_speed = ground_speed;
+			_last_ground_speed_cm = ground_speed;
 
 			_velocity_north = gps_speed * cos_heading;
 			_velocity_east  = gps_speed * sin_heading;

libraries/AP_GPS/GPS.h

@@ -134,6 +134,11 @@ public:
 	float velocity_north(void) { return _status == GPS_OK? _velocity_north : 0; }
 	float velocity_east(void)  { return _status == GPS_OK? _velocity_east  : 0; }
 
+	// last ground speed in m/s. This can be used when we have no GPS
+	// lock to return the last ground speed we had with lock
+	float last_ground_speed(void) { return _last_ground_speed_cm * 0.01; }
+	
+
 	// the time we got our last fix in system milliseconds
 	uint32_t last_fix_time;
 
@@ -202,7 +207,7 @@ private:
     GPS_Status					_status;
 
 	// previous ground speed in cm/s
-    uint32_t _last_ground_speed;
+    uint32_t _last_ground_speed_cm;
 
 	// smoothed estimate of our acceleration, in m/s/s
 	float _acceleration;