Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 21)

U ncertainty is defined as a param eter associated with the result of a m easurem ent, that characterizes the
1
dispersion of the values that could reasonably be attributed to the m easurand. Form er BSR N publications
have termed this accuracy. T he terminology has been changed to follow the ISO guidelines. Accuracy is a
m ore general term inology that expresses a variety of ideas, m any of which cannot be quantified.
ISO , 1993: G uidelines for the Ex press ion of U ncertainty M easurement. First Edition.
2
nip (lowercase) is used as an acronym and is not to be confused with N IP™ of Eppley Laboratories.
3
9
2.1.2 Sam pling Frequency of Ancillary Measurem ents
At stations where the ancillary m easurem ents are under the control of an independent agency,
such as a national weather service, the frequency of the various m easurem ents cannot often be
altered. The higher the frequency the greater the usefulness of the data, up to the sam pling rate
of the radiation m easurem ents. BSRN station scientists should encourage any independent
collection agency to sam ple and record data following standard W MO procedures at the very
m inim um .
W hen autom atic data logging is em ployed to record such variables as pressure, tem perature,
hum idity, wind speed and wind direction, providing these data at the sam e frequency as the
radiation data is beneficial. Stations are encouraged to obtain these observations coincide ntally
with the radiation m easurem ents using a one-m inute sam ple rate to aid in understanding the
energy balance of the radiation instrum ents and the infrared com ponent of the radiation balance.
At a very m inim um , all stations should record air tem perature at the sam e location and at the
sam e sam pling frequency as the radiation m easurem ents.
2.2 Uncertainty of Measurements 1
2.2.1 Uncertainty in Radiation Measurem ents
These accuracies are based upon state-of-the-art com m ercially available equipm ent. At the onset
of the BSRN program m e, a table listing the uncertainties about individual flux m easurem ents was
produced (Table 2.1) that included the uncertainties thought to be achievable by 1997. T hese
uncertainty values have been achieved using new sensors and m ethods of observation, som e
being surpassed. Nevertheless, new m ethods of observation are continuing to develop that will
continue to decrease the overall uncertainty associated with instantaneous m easurem ents.
Even as instrum entation and m ethods of observation have im proved over the decade since the
inception of the network, the estim ate of uncertainty has becom e m ore refined. The publication of
the International Organization for Standardization (ISO) Guide to the Expression of Uncertainty
Measurement (GUM) provides a standard m ethod for the determ ination of uncertainty in
2
m easurem ent. National Metrology Institutions (NMI) and industrial laboratories have adopted its
m ethodology and the BSRN recom m ends that all uncertainty calculations follow the procedures
of the guide.
To m eet and exceed these target accuracies, the m easurem ent of each quantity will require a
particular m ethodology of m easurem ent. W hile these m ethodologies are not absolute in nature,
they will ensure a given level of uncertainty in the m easurem ent if followed (assum ing appropriate
on-site m aintenance etc.). The BSRN is concerned m ore with m eeting the target m easurem ent
uncertainty however, than the m anner in which the uncertainty is m et. Methods of m easurem ent
associated with these uncertainties were first published in W CRP-64, 1991. W hile m any
m ethodologies have not changed significantly since the inception of the program m e, several
m easurem ent techniques have im proved. Those that have not changed are repeated verbatim in
this m anual.
2.2.1.1 Direct Solar Irradiance
The target uncertainty for m easurem ent of direct solar irradiance in the BSRN is 1% (or 2 W m -2
as the m inim um deviation from the "true" value as reflected in the uncertainty of the W orld
Radiom etric Reference). For the continuous m easurem ents used in providing the m ean value
over one-m inute, a norm al incidence pyrheliom eter (nip ) or sim ilar is recom m ended.
3