MAVLink: improve usefulness of logged GPS velocities

we need them to align with time to use them for accelerations

ArduCopter/GCS_Mavlink.pde

@@ -200,9 +200,20 @@ static void NOINLINE send_meminfo(mavlink_channel_t chan)
 
 static void NOINLINE send_location(mavlink_channel_t chan)
 {
+    uint32_t fix_time;
+    // if we have a GPS fix, take the time as the last fix time. That
+    // allows us to correctly calculate velocities and extrapolate
+    // positions.
+    // If we don't have a GPS fix then we are dead reckoning, and will
+    // use the current boot time as the fix time.    
+    if (g_gps->status() == GPS::GPS_OK) {
+        fix_time = g_gps->last_fix_time;
+    } else {
+        fix_time = millis();
+    }
     mavlink_msg_global_position_int_send(
         chan,
-        millis(),
+        fix_time,
         current_loc.lat,                // in 1E7 degrees
         current_loc.lng,                // in 1E7 degrees
         g_gps->altitude * 10,             // millimeters above sea level

ArduPlane/GCS_Mavlink.pde

@@ -237,9 +237,20 @@ static void NOINLINE send_meminfo(mavlink_channel_t chan)
 
 static void NOINLINE send_location(mavlink_channel_t chan)
 {
+    uint32_t fix_time;
+    // if we have a GPS fix, take the time as the last fix time. That
+    // allows us to correctly calculate velocities and extrapolate
+    // positions.
+    // If we don't have a GPS fix then we are dead reckoning, and will
+    // use the current boot time as the fix time.    
+    if (g_gps->status() == GPS::GPS_OK) {
+        fix_time = g_gps->last_fix_time;
+    } else {
+        fix_time = millis();
+    }
     mavlink_msg_global_position_int_send(
         chan,
-        millis(),
+        fix_time,
         current_loc.lat,                // in 1E7 degrees
         current_loc.lng,                // in 1E7 degrees
         g_gps->altitude * 10,             // millimeters above sea level