Apparatus and method for a continuous replenishing of the lift gas and maintaining the proper pressure and lift of a tethered medium altitude aerostat utilizing a novel feed tube running the entire length of the tether. A first end of the feed tube is connected to the aerostat while a second end is connected through a novel slip ring means to a pressure controlled helium ballast chamber and scrubber on the ground to maintain pressure and lift by a reversible compressing pump. A plurality of pressure and temperature sensors and tension gauges strategically placed inside and around the airship continuously monitor the temperature and pressure changes in the aerostat. A data retrieval and communication unit mounted on the aerostat collects measurement data from the pressure, temperature and tension sensors which is relayed to the ground station and the data is used to regulate lifting gas pressure inside the aerostat.