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.
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.
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