Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 22)

10
BSRN Measurement Uncertainty
Quantity 1991* 1997 Target** 2004 Target †
1. Direct Solar Irradiance 1% or 2 W m 0.5% or 1.5 W m -2 -2
2. Diffuse Radiation 10 W m 4% or 5 W m 2% or 3 W m -2 -2 -2
3. Global Radiation 15 W m 2% or 5 W m 2% or 5 W m -2 -2 -2
4. Reflected Solar
Radiation 15 W m 5% 3% -2
5. Downwelling Infrared
Radiation 30 W m 5% or 10 W m 2% or 3 W m -2 -2 -2
6. Upwelling Infrared
Radiation 30 W m 5% or 10 W m 2% or 3 W m -2 -2 -2
* from W CRP-
54, Mar 1991
** from W CRP-64,
Nov 1991
estim ates based †
on current
research
Table 2.1. Uncertainty requirem ents for the Baseline Surface Radiation Network radiation
fluxes. W here values are given in percent and absolute, the latter are the m inim um deviation
from the “true” value m easured by the instrum ent for any irradiance.
Experim ents have shown that for m any nip instrum ents the uncertainty associated with the noise
of these instrum ents exceeds the uncertainty requirem ents for direct solar irradiance
m easurem ents. Therefore, an absolute cavity radiom eter (ACR) should be used in parallel to
"calibrate" the norm al incident pyrheliom eter quasi-continuously (every 5-60 m inutes, if the
norm al direct beam intensity (I) > 400 W m ).
-2
Pyrheliom eters norm ally operate with a window that blocks part of the solar infrared signal.
Sim ilarly, m any ACRs when used as all-weather instrum ents also have a window to protect the
instrum ent from the elem ents. These windows m ust be m ade of the sam e m aterial to ensure that
differences in window transm ittance are not ‘calibrated’ into the m easured irradiance and thus
increase the uncertainty of the m easurem ent. To obtain higher quality m easurem ents that include
the signal from the infrared portion of the solar spectrum , the instrum ent can be operated without
a window or with a window m ade of a m aterial that has flat transm ission characteristics from
approxim ately 290 nm to 4000 nm (> 99% of the solar spectrum ). Recent advances in the
construction of all-weather enclosures, both windowless and those using sapphire or calcium
fluoride windows and special heating and ventilation system s have reduced the dependence on
sim ple therm opile pyrheliom eters that require frequent com parison with fair-weather ACR
instrum ents. It is recom m ended that an all-weather ACR be used continuously with a standard
pyrheliom eter used to fill ‘data gaps’ during the period when the ACR is in calibration m ode.
Caution m ust be exercised if a windowless ACR is to be operated continuously. The m inim um
protection required is to house the instrum ent in a ventilated housing. The opening aperture of the
housing should be a m inim um of 10 radiom eter-opening-aperture diam eters distant from the
entrance aperture of the enclosed ACR and have a diam eter no greater than twice the field of
view of the ACR. Care m ust be taken when ventilating the instrum ent so that no venturi effects
are created that m ight alter the therm al equilibrium of the instrum ent. In areas where severe
weather conditions are prevalent, system s that include a m eans of closing the opening aperture
are required.
W hen using a calcium fluoride window, yearly inspections are recom m ended to ensure the
integrity of the flat because of the anhydrous nature of the m aterial. In very hum id or wet
environm ents, inspections of the flat should be m ade m onthly. T o protect the instrum ent from
precipitation, an autom atic cover triggered by a rain sensor can be installed.
Experim ents have also shown that m aintaining the tem perature of the therm opile on certain ACR
instrum ents, when used in an all-weather m ode, further enhance perform ance.