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data. While it is impossible to have complete defence against loss, the need
for security must be balanced against the cost of its implementation.
Distance: The physical distance between the data collection location and the data
archive location will often determine the methods of communication that are
available. For example, the only viable solutions for long distances may be
common carriers or even satellite communications, while shorter distances
(e.g., within a complex of buildings) can utilize hardwired local area
networks.
Environment: Physical or legal constraints may affect the type of medium that can be
used. Local ordinances m ay prohibit the use of certain types of radio
communications or the laying or stringing of cable. Climatic variables, such
as wind, temperature, rain and icing conditions also need to be considered
when selecting a com m unication medium. For example, overhead wires in
areas where icing is a common problem may not provide a reliable means of
communication.
Application: Peculiarities in particular applications will dictate significant portions of the
selection of a media. This is especially true at remote sites where power
conservation requires only limited communication access. The slowest
portion of the communication chain will also dictate the overall requirements
of the entire chain. For example, if communication between a data logger
and a computer, through common carrier lines, is constrained by the baud
rate at which the data logger can transfer the data to the modem, there is no
reason to purchase modems with a higher baud rate.
Maintenance: All media are subject to failure. The system design should take into account
the probability of a failure, the cost of such a failure to the user in money and
inconvenience, and the ability of the user to obtain alternate communication
media for the duration of the failure. Routine m aintenance m ust also be
considered in terms of down time and overall cost of the system.
3.4.4 Security
Depending upon the location, security may be a significant consideration. Security is both for the
protection of the site against vandalism and theft, and for the protection against harm of would-be
intruders (the concept of being responsible for a thief’s well-being while on the victim’s property
may well be found only in North America).
At a minium the measurement site should be well-fenced against intruders, both human and
animal. Further security measures may include alarm systems, security lights (on buildings, but
away from the instrumentation) and video camera systems.
In some locales special security should be considered against burrowing and gnawing rodents.
3.5 Site Preparation
The preparation of the site before measurements begin consists of designing the installation to
reduce interference of the sensors from buildings and other sensors, ensuring that the instrument
platforms are appropriate for the climate and soil conditions, and designing a signal cable grid that
is efficient and easy to maintain. While general principles can be applied to each of these aspects
of the site, individual stations will require special adaptations to the following procedures.
3.5.1 Instrument siting
Care must be taken so that the instruments do not interfere with each other. Ideally, instruments
should be far enough apart that they become insignificant objects in the field of view of adjacent
instruments. Space limitations, however, often restrict the distance apart instruments can be
placed. To reduce such interference, the instruments should be lined up in a poleward direction
with slightly increasing elevation. In cases where the measurement of diffuse radiation and direct
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