Cluster Master Science Plan - An Overview

The purpose of the Master Science Plan is to schedule the acquisition of science data by the four Cluster spacecraft in a way that is consistent with both the scientific objectives of the mission and the resources available for that data collection. Those resources (on-board data storage, telemetry bandwidth, spacecraft visibility from the Vilspa ground station and the capacity of the ground segment to collect and process Cluster data) are an important constraint on Cluster science operations. They are sufficient to allow acquisition of Cluster data for approximately 50% of the time that Cluster will be operational.

Thus it is essential to schedule Cluster data acquisition periods so that they are targetted on regions where we expect to observe phenomena that are the scientific objectives of Cluster (e.g. cusp, tail neutral sheet, etc). It is the Master Science Plan that specifies this targetting - in terms of the placement of data acquisition periods on orbits which cross the target regions - and in terms of data acquisition rates. Cluster supports two such rates: a normal mode and a burst mode. The latter yields much higher resolution data (factor of 6 in data volume) but at the cost of reducing the period over which data can be collected.

The Master Science Plan is represented graphically using the Bryant plot format. A low-resolution example of this format is shown in the figure below - for 13 orbits centred on the first Cusp crossing. (The actual plan contains a high-resolution version of the plot.) In this format, the horizontal axis is absolute time while the vertical axis is time since last perigee. Thus each orbit is represented by a sloping line as you can see in the figure. The weight of line indicates the type of data acquisition: dots for no acquisition, thin solid line for normal mode and thick solid line for burst mode. Predictions of various boundary crossings are indicated by coloured symbols. At the lower centre of the figure, you can see a series of burst mode acquisitions targetted on the northern cusp (on the outbound leg of the orbit). Some acquisitions targetted on the southern cusp can also be seen in the upper centre. The different impact of normal and burst mode on data acquisition can also be seen well. On orbits with burst mode acquisitions, data are taken only over small segments of the orbit. Whereas, for normal data acqusition, it is possible to collect data continuously up to a limit of two to three orbits. Bryant plot of the Master Science Plan for the first cusp crossing

It is planned that the Master Science Plan will be produced in segments, each of six months duration. Each segment will correspond to a Cluster constellation, which is a period in which the orbits of the four spacecraft will follow a particular pattern (which will be suitable for studying the major scientific targets to be encountered during that period). Each segment of the Plan must be delivered to ESOC in good time (at least six months before the start of execution) so that they can check that the Plan is consistent with the resource constraints described above.