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1.3 Case studies 23

In Figure 1.7, I have used the UML package symbol to indicate that each system
is a collection of components and have identified the separate systems, using the
UML stereotype «system». The associations between the packages indicate there is
an exchange of information but, at this stage, there is no need to define them in any
more detail.
Each weather station includes a number of instruments that measure weather
parameters such as the wind speed and direction, the ground and air temperatures,
the barometric pressure, and the rainfall over a 24-hour period. Each of these instru-
ments is controlled by a software system that takes parameter readings periodically
and manages the data collected from the instruments.
The weather station system operates by collecting weather observations at fre-
quent intervals—for example, temperatures are measured every minute. However,
because the bandwidth to the satellite is relatively narrow, the weather station carries
out some local processing and aggregation of the data. It then transmits this aggre-
gated data when requested by the data collection system. If, for whatever reason, it is
impossible to make a connection, then the weather station maintains the data locally
until communication can be resumed.
Each weather station is battery-powered and must be entirely self-contained—there
are no external power or network cables available. All communications are through a rel-
atively slow-speed satellite link and the weather station must include some mechanism
(solar or wind power) to charge its batteries. As they are deployed in wilderness areas,
they are exposed to severe environmental conditions and may be damaged by animals.
The station software is therefore not just concerned with data collection. It must also:

1. Monitor the instruments, power, and communication hardware and report faults
to the management system.

2. Manage the system power, ensuring that batteries are charged whenever the
environmental conditions permit but also that generators are shut down in
potentially damaging weather conditions, such as high wind.
3. Allow for dynamic reconfiguration where parts of the software are replaced
with new versions and where backup instruments are switched into the system
in the event of system failure.

Because weather stations have to be self-contained and unattended, this means
that the software installed is complex, even though the data collection functionality
is fairly simple.