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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.