The various groups involved in the preparation and operation of the ISO mission receive guaranteed observing time. These groups are: the four Principal Investigators and their teams, who built the ISO instruments; the five Mission Scientists; the Science Operations Team; the National Aeronautics and Space Administration (NASA), USA; and the Institute of Space and Astronautical Science (ISAS), Japan. Guaranteed time holders are also able to apply for Open Time.
A ``Central Programme'' was drawn up by the Instrument Groups, the Mission Scientists and the Science Operations Team. The requirement upon this programme was that it be a ``balanced and coherent programme ......providing a solid scientific core for the ISO observing programme''. Its publication is intended to ``give an overview of the observing potential of the observatory to enable the submission of top quality, complementary and feasible observing proposals leading to the maximum output of the observation programme''. The programme was unanimously endorsed by the Observing Time Allocation Committee in early 1994.
ISAS and NASA contribute to the ISO programme by providing a second ground station and associated resources. In return, they each receive 30 minutes observing time per day. The US guaranteed time has been allocated to 4 ``Key Projects''. The Japanese guaranteed time has been allocated to 7 proposals.
The guaranteed time programme was published to the community with the pre-launch Call. Some updates have been made due to in-orbit experience and to the additional guaranteed time made available because of the extended lifetime. All observations currently planned for the guaranteed time are contained in the ``Reserved Observations List'' (see section 4.3.2).