The timescales that LOFT will investigate range from sub-millisecond quasi-periodic oscillations (QPOs) to transient outbursts that last for years. The relevant objects include many that flare up and change state unpredictably. This makes relatively long observations, flexible scheduling and continuous monitoring of the X-ray sky essential elements for success.
During nominal operations, a typical selected target will be observed by the LAD for some tens of kiloseconds. Due to the Earth occultations in low Earth orbit, depending on the source direction, it might be visible for less than half of each orbit, making some slews between targets necessary during each orbit. Otherwise, targets will be selected nearer to the orbital poles for increased observing efficiency.
The WFM will simultaneously scan a contiguous, large area of sky for potential targets of opportunity, such as bright flares. After alerting the Science Operations Centre, the spacecraft may slew to the newly identified target within 8 hours.
Several orbital passes in a day will allow an X-band data downlink for several Gigabits per pass, allowing the 700 kilobits per second to be collected and stored for compression and downlink continuously.
LOFT will be operated as an observatory-type mission. Most of the observing time available for scientific investigations (Open Time) will be awarded to scientists (General Observers, GO) who will have submitted proposals selected by the Time Allocation Committee (TAC). This part of the programme will be called the general programme.
The observation plan will be designed to optimise science return. To this end, a core programme will be prepared by the LOFT Science Team and ESA to guarantee that the main scientific goals of the mission are achieved. The typical observations will range from a few to tens of kiloseconds, depending on the expected behaviour and variability of the target.
The LOFT ground segment would be composed of two major components: the Operations Ground Segment (OGS) and the Science Ground Segment (SGS). The OGS will be composed of the telemetry, tracking and command network ground stations, and the Mission Operations Centre (MOC).
The MOC handles all the classical spacecraft operations and maintenance tasks and will implement the observation plan as per the spacecraft system constraints into an operational command sequence. The MOC, which may be located at ESOC in Darmstadt, Germany, will be the sole interface with the satellite.
All commands to the spacecraft and instruments are generated at the MOC and all telemetry from the satellite is routed to the MOC, which is responsible for distributing it within the ground segment. The MOC is also responsible for in-orbit maintenance and on board software maintenance throughout the mission.
The Science Ground Segment would consist of the Science Operations Centre (SOC) and the LOFT science data centre (SDC). The SOC will process the accepted proposals into an optimised observation plan which consists of a timeline of targeted pointings as well as the corresponding instrument configuration. The MOC will provide the necessary inputs for the planning process, namely a skeleton plan of the essential spacecraft operations based on which the observation plan is to be created.
The SDC’s main role will be the monitoring of payload telemetry (including the instrument housekeeping data files and relevant ancillary spacecraft data from the MOC), and conversion of the raw data into physical units, taking into account the different instrument characteristics.
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