The major complication for the daily pre-processing is the ambition to process
a given time span before all the telemetry has arrived at the processing
centre, and without knowing for sure if data that appears to be missing will in
fact ever arrive. The driver is the wish to keep a close eye on the instrument
and to issue alerts on interesting new sources.
As a rule, housekeeping telemetry, and the so-called auxiliary science data
(ASD, see Section 2.2.2), is sent to ground first, followed by the actual observations for
selected magnitude ranges. This is meant to be the minimum required for the
monitoring tasks. Other magnitude ranges follow later, unless memory
becomes short on-board and the data in the down-link queue is overwritten (Section 1.3.3). Data can be received
at different ground stations (Section 1.1.5) and may also for that reason arrive unordered
at the processing centre.
The daily pre-processing is known as the Initial Data Treatment (IDT) and is
described below (see Section 2.4.2). It is followed
immediately by a quality assessment and validation known as First Look (FL, see
Section 2.5.1 and Section 2.5.2),
which takes care of the monitoring tasks and the daily calibrations.
The cyclic pre-processing, known as the Intermediate Data Updating (see
Section 2.4.2), runs over well-defined data sets and can
be executed in a much more orderly manner. It consists of three major tasks,
viz. calibrations, image parameter determination, and crossmatch, as well as
several minor tasks, as described below.