Position drift and wrong takeoff altitude

Hello again,
Looking to the astech driver, I see that the only message that marvelmind transmits and can be parsed by the driver is $ GPGGA - essential fix data that provide 3D location and data accuracy.
So, PX4 receive 3D locations but not velocities. Is this possible to PX4 to control itself without velocity information?

Another thing I noticed is the variable gps_global_position-> alt is really different from vehicle_gps_position-> alt, while the vehicle_gps_position has the actual values ​​from altitude.