Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 33)

3.3 Instrument Exposure

To obtain data on the radiative field with respect to the surroundings, it is necessary to map the

horizon of the instrument. With few exceptions this actual horizon will be different from the

theoretical horizon because of buildings, trees or landforms. In some cases other instruments 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 azimuth angles. This accuracy is increased to a 5° interval for the

published station description (see below).

The two most common means of accomplishing horizon mapping are by a survey camera, which

exposes azimuth and elevation grid lines on the negative, or by theodolite. The advantage of the

former is that it also provides evidence of various reflecting surfaces. In cases where a theodolite

is used, either panoramic photographs or an all-sky image from the location of the instrument

should also be obtained.

Surveys should be carried out before installation of the equipment and then at a minimum once

every five years. If significant changes in the horizon occur, they should be documented

immediately and a new site survey performed.

If buildings or other objects are in the near field of view, separate surveys should be made from

the location of each instrument if they are affected differently.

In cases where the obstructions are highly reflective, a separate measurement of the reflected

radiation should be attempted. This is of particular importance if the object is man-made and

constant (e.g., a white building). This information should be submitted to the Archive as part of the

metadata.

Corrections to the data to eliminate the effect of obstructions (e.g., assuming 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 instruments at a given location is dependent on a number of

factors beyond the sighting of the instruments (Sec.3.1.1). This section is meant to provide a

guide to ensure that these other factors are considered.

3.4.1 Ease of Access

Sensors must be easily accessible for daily maintenance. If the sensors are distant from the

workplace of the support personnel, the quality of maintenance will be reduced, particularly

following significant weather events. If the pyranometers are located on a building roof, access to

the roof must be such that a technician will not be hesitant in inspecting or working on the

instruments several times per day if required. If the instruments are mounted above the surface on

a pole, a permanent platform or a ladder may be required so that the technician will be able to

visually inspect the top of the instrument without difficulty. Safety factors must also be considered

if instruments are to be located on towers or on the top of buildings. Human nature is such that

instruments that are in areas that are inaccessible or can only be checked at some personal risk

will be poorly maintained.

3.4.2 Electrical Power

The instrumentation used for the accurate measurement and storage of radiation fluxes and

related meteorological variables requires reliable and stable electrical power over long periods of

time. Depending upon the location of the site, to obtain and/or maintain such requirements may

require devices as simple as surge protectors or as sophisticated as back-up generators. During

the initial design phase of a BSRN station it is crucial to determine the quality of the electrical

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