ISO is operated in a highly-elliptical orbit with a perigee
at 1000km, an apogee at 70,000km and a period of 24 hours.
To provide visibility of the satellite from ground for the entire 
scientifically useful part of the orbit two ground stations are used: Villafranca/ESA in Spain and Goldstone/NASA in the United States. Science operations are carried out at the Villafranca ground station. Since there is no on-board data storage on the satellite, ISO needs to to be in continuous contact with a ground station for scientific use.
The orbit is illustrated in Fig. 6. The lower parts of
the orbit lie inside the Earth's van Allen belts of trapped electrons
and protons. Inside these regions, ISO's detectors are
scientifically unusable due to effects caused by radiation impacts.
Therefore, between about half an hour before perigee
passage and two hours
after, the instruments remain switched off. To switch the instruments on or
off, a number of procedures are being carried out which is called activation
and deactivation respectively. For the activation of all four instruments
two hours are needed, for their deactivation one and a half hours. Due to
the use of two ground stations a time period for the switch-over is needed.
During this time period additional calibrations are carried out.
For scientific use there are thus two time periods, one of approximately
8 hr 45 min, the other of about 7 hr 45 min. Fig. 5
illustrates the various time periods or windows.
Figure 5: Time periods - windows - of an ISO revolution. The time between two perigee passages is one revolution. The `X's denote periods when the instruments are switched off. The tildes denote periods when specific procedures or internal calibrations of the instruments are carried out.
Figure 6: The ISO orbit. The constraints imposed by the Sun, the Earth, the Moon and Jupiter are illustrated (not drawn to scale).
