Altitude control of long-duration balloons

Altitude control enables balloons to perform atmospheric soundings, avoid restricted airspace, and exercise rudimentary navigational control. Reversible altitude control, which does not expend lift gas or ballast, is particularly attractive because long-duration flights can be achieved. Methods of reversible altitude control, including air ballast and mechanical compression, are compared. A new system, described here as differential expansion, is ideally suited to near-neutral density operation and has many advantages over existing altitude control systems including cost, ease of construction, and altitude range.