The observatory schedule is defined by the database of accepted observations. The HSC carries out a careful study of the observation database to define a long-term mission plan that will accommodate all constraints and will maximise the scientific return. The Long Term Mission Planning tool developed at HSC is a very powerful aid to identifying potential future scheduling problems at a very early stage.
Following the agreed long term mission plan, short term observing schedules, together with the corresponding instrument commands, are produced with the Mission Planning System at the HSC, and transferred to the Mission Operations Centre (MOC), at ESOC. MOC adds the satellite commands and produces the final detailed mission timeline that is uplinked to the spacecraft.
The basic time unit for the mission planning is the Operational Day, or OD, defined as the interval of time between the start of two consecutive DTCPs. The DTCP, or Daily TeleCommunication Period, is the time interval when the spacecraft antenna will be pointed to the Earth to receive telecommands and send the recorded data. The duration of an OD will normally be about 24 hours, but depends on the availability and detailed schedule of the New Norcia Ground Station, which is shared with other ESA missions. The operational constraints on the Herschel instruments determine that only observations that use a particular sub-instrument are scheduled in a single OD. For sub-instruments that require cooler re-cycling, only observations with the cooled sub-instrument (e.g. PACS photometer) will be scheduled for the duration of the cooler hold time: two to two and a half consecutive ODs.
For a wide range of reasons, from safety to calibration needs, the instrument assignation for each OD is standardised. It consists of the repetition of 28 ODs, i.e. four weeks, during which the instruments follow one another and are used for a different number of consecutive ODs. The standard instrument distribution currently used currently during the routine phase is the one shown in Figure 7.1, which reflects the relative usage of each sub-instrument in the approved proposals so far, and can be revised according to the each instrument observing time preassure at different epochs of the year. This is what we call the "planning cycle", which is the foundation of most of the Ground Segment activities related to Mission Planning. This translates into an additional difficulty to accomodate observations that have been defined with timing or grouping constraints shorter than a few weeks.
The use of the different HIFI bands is an additional constraint on the optimisation of observatory time. Very limited HIFI band changes are allowed in a given OD, preferably no more than two. Therefore HIFI observations using different bands and with time constraints shorter than an OD will be very difficult to schedule.
As indicated when requesting the visibility window of an observation with HSpot, it is limited by the so-called "warm" attitudes, i.e. solar aspect angles between -30 and -20 degrees, in which the Sun warms the star trackers and the pointing accuracy may be de-graded (see Section 2.4). For this reason the scheduling of observations within this area is to be avoided where possible. Only in certain, justified occasions, are solar aspect angles between -25 and -30 degrees allowed for less than one hour, at user's own risk and only when it can be demonstrated that it will not affect later observations, while potential scheduling at solar aspect angles from -20 to -25 degrees is decided on a case-by-case basis.