Updating GND_ALT_OFFSET during flight


I have a Tarrot 650 sport that I have equipped with a Pixhawk 2.1 flight controller and a Here+ RKT GNSS system. I was curious if anyone knows how to continually update the GND_ALT_OFFSET parameter via another pixhawk baro at a base station? In other words, what is the path of least resistance to having the GND_ALT_OFFSET parameter updated automatically every 30 seconds via Mavlink?