 Several different tracking modes are used in operations, ranging from full Sun-tracking, to the drag-reduction mode ("night glider" and "Sun slicer" modes), to a drag-maximization mode used to lower the altitude.Since the station is often not in direct sunlight, it relies on rechargeable nickel-hydrogen batteries to provide continuous power during the "eclipse" part of the orbit (35 minutes of every 90 minute orbit).The batteries ensure that the station is never without power to sustain life-support systems and experiments.