A ``Fixed-Time'' observation (``Time Critical'' in RPS) is defined as an observation whose scheduling is not solely determined by its visibility. Examples of ``Fixed-Time'' observations are:
``Fixed-Time'' observations reduce the flexibility available to the mission planning system for scheduling of observations and can lead to larger than nominal slew overheads. Thus, all proposers are reminded that ``Fixed-Time'' observations should only be asked when there is a strong scientific requirement. This justification must be provided explicitly in the proposal justification. For operational reasons, the start time of ``Fixed Time'' observations should not be constrained below a margin of 15 ks. In addition, the scheduling constraints should be described in detail in the ``constraints'' sub-menu after entering an observation as ``Time critical''. ``Fixed-Time'' observations are approved by OTAC by allocating A or B observing priority. C priority observations cannot be ``Time-critical'' or ``Fixed-time''.
It is the responsibility of the PI to demonstrate that proposed ``Fixed-Time'' observations are visible, see section 5.8.1.