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Because of its low altitude, the ROSAT orbit will be affected
by atmospheric drag. The drag's magnitude
is proportional to atmospheric density which in turn depends
on solar activity. Consequently precise predictions of the exact ROSAT
position in its orbit are difficult to make on long time scales
because of the intrinsic unpredictability of solar activity.
Since ROSAT observations will be scheduled up to
six months (see § 8.3 
) ahead of time, guest observers wishing
to observe phenomena in absolute time (i.e., short duration
eclipses, observations at well-defined binary phases) are most affected because
it cannot be reliably predicted (in the long term planning process) whether
ROSAT is outside the radiation belts or a target is not earth-occulted
at some specified time.