GPS: added an acceleration estimate to the GPS driver

This uses the GPS ground speed to estimate acceleration

libraries/AP_GPS/GPS.cpp

@@ -33,6 +33,24 @@ GPS::update(void)
 
         // reset the idle timer
         _idleTimer = millis();
+
+		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);
+			_last_fix_time = _idleTimer;
+			_last_ground_speed = ground_speed;
+
+			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;
+			} else {
+				// calculate a mildly smoothed acceleration value
+				_acceleration = (0.7 * _acceleration) + (0.3 * (deltav/deltat));
+			}
+		}
     }
 }
 

libraries/AP_GPS/GPS.h

@@ -108,7 +108,10 @@ public:
     /// 1200ms allows a small amount of slack over the worst-case 1Hz update
     /// rate.
     ///
-    unsigned long	idleTimeout;
+    uint32_t	idleTimeout;
+
+	// our approximate linear acceleration in m/s/s
+	float acceleration(void) { return _acceleration; }
 
 protected:
     Stream	*_port;			///< port the GPS is attached to
@@ -165,11 +168,19 @@ private:
 
     /// Last time that the GPS driver got a good packet from the GPS
     ///
-    unsigned long 				_idleTimer;
+    uint32_t 				_idleTimer;
 
     /// Our current status
     GPS_Status					_status;
 
+	// the time we got our last fix in system milliseconds
+	uint32_t _last_fix_time;
+
+	// previous ground speed in cm/s
+    uint32_t _last_ground_speed;
+
+	// smoothed estimate of our acceleration
+	float _acceleration;
 };
 
 inline long