Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 33)

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3.3 Instrument Exposure
To obtain data on the radiative field with respect to the surroundings, it is necessary to m ap the
horizon of the instrum ent. W ith few exceptions this actual horizon will be different from the
theoretical horizon because of buildings, trees or landform s. In som e cases other instrum ents will
create reflecting surfaces from which additional radiation will be incident on the receiver of the
sensor of interest.
The archive requires that the elevation be catalogued at 10° intervals beginning at 0° N and
ending at 350°. All prom inent features are also to be catalogued and inserted as ordered pairs in
the increasing sequence of azim uth angles. This accuracy is increased to a 5° interval for the
published station description (see below).
The two m ost com m on m eans of accom plishing horizon m apping are by a survey cam era, which
exposes azim uth and elevation grid lines on the negative, or by theodolite. The advantage of the
form er is that it also provides evidence of various reflecting surfaces. In cases where a theodolite
is used, either panoram ic photographs or an all-sky im age from the location of the instrum ent
should also be obtained.
Surveys should be carried out before installation of the equipm ent and then at a m inim um once
every five years. If significant changes in the horizon occur, they should be docum ented
im m ediately and a new site survey perform ed.
If buildings or other objects are in the near field of view, separate surveys should be m ade fro m
the location of each instrum ent if they are affected differently.
In cases where the obstructions are highly reflective, a separate m easurem ent of the reflected
radiation should be attem pted. This is of particular im portance if the object is m an-m ade and
constant (e.g., a white building). This inform ation should be subm itted to the Archive as part of the
m etadata.
Corrections to the data to elim inate the effe ct of obstructions (e.g., assum ing an isotropic radiance
distribution and adding the difference between the actual and the theoretical horizon to the signal)
should not be used. In cases where an object blocks the direct beam radiation during all or part of
the year, the data during these periods should be appropriately flagged.
3.4. Additional Station Requirements
The installation of the radiation instrum ents at a given location is dependent on a num ber of
factors beyond the sighting of the instrum ents (Sec.3.1.1). This section is m eant to provide a
guide to ensure that these other factors are considered.
3.4.1 Ease of Access
Sensors m ust be easily accessible for daily m aintenance. If the sensors are distant from the
workplace of the suppo rt personnel, the quality of m aintenance will be reduced, particularly
following significant weather events. If the pyranom eters are located on a building roof, access to
the roof m ust be such that a technician will not be hesitant in inspecting or working on the
instrum ents several tim es per day if required. If the instrum ents are m ounted above the surface on
a pole, a perm anent platform or a ladder m ay be required so that the technician will be able to
visually inspect the top of the instrum ent without difficulty. Safety factors m ust also be considered
if instrum ents are to be located on towers or on the top of buildings. Hum an nature is such that
instrum ents that are in areas that are inaccessible or can only be checked at som e personal risk
will be poorly m aintained.
3.4.2 Electrical Power
The instrum entation used for the accurate m easurem ent and storage of radiation fluxes and
related m eteorological variables requires reliable and stable electrical power over long periods of
tim e. Depending upon the location of the site, to obtain and/or m aintain such requirem ents m ay
require devices as sim ple as surge protectors or as sophisticated as back-up generators. During
the initial design phase of a BSRN station it is crucial to determ ine the quality of the electrical