Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 53)

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instructions for each of these devices. A broad overview, however, is im portant because of the significance
solar tracking plays in the m easurem ent of direct and diffuse radiation.
The following general characteristics are com m on to all solar pointing devices.
(1) The tracker location m ust be known. The m ore precisely the location can be determ ined,
the easier the setup of the tracker. W ith m odern GPS receiver system s the position of the
tracker can be determ ined (latitude and longitude) to within ±3 m . From a 1:50,000 topographic
m ap the location can be determ ined to within better than 50 m . Depending upon the size
of the installation care should be taken so that the actual location of the tracker is determ ined
and not sim ply the central location of the BSRN observatory.
(2) In all cases a reliable power supply is necessary. Synchronous trackers not only require a
constant power supply, as do other trackers, but also a constant and accurate electrical frequency
if the tracker is to m aintain accurate alignm ent on the solar disk at all tim es. Changes in power
line frequency will alter the speed at which the solar disk is tracked. Most utility com panies
are required by law to m aintain the power line frequency to som e stated accuracy within a
24-hour period with a m axim um excursion from that stated frequency at any given m om ent.
Stepper-m otor-controlled trackers are less susceptible to such changes because of their internal
conversion from AC to DC power. The use of UPS system s on synchronous m otor trackers
is also of lim ited utility because m any inverter system s output frequency as square waves
rather than the sinusoidal wave required by the tracker.
Figure 4.7. The contribution of the solar disk to the
irradiance of pyrheliom etric sensors depending on the
pointing error. (A) Case of m ountain aerosol and 60°
solar elevation. (B) Case of continental aerosol and a 20°
solar elevation. (Calculations and graph courtesy of G.
Major)