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 WCRP-
54, Mar 1991
** from WCRP-64,
Nov 1991
estimates based
†
on current
research
Table 2.1. Uncertainty requirements for the Baseline Surface Radiation Network radiation
fluxes. Where values are given in percent and absolute, the latter are the minimum deviation
from the “true” value measured by the instrument for any irradiance.
Experiments have shown that for many nip instruments the uncertainty associated with the noise
of these instruments exceeds the uncertainty requirements for direct solar irradiance
measurements. Therefore, an absolute cavity radiometer (ACR) should be used in parallel to
"calibrate" the normal incident pyrheliometer quasi-continuously (every 5-60 minutes, if the
normal direct beam intensity (I) > 400 W m ).
-2
Pyrheliometers normally operate with a window that blocks part of the solar infrared signal.
Similarly, many ACRs when used as all-weather instruments also have a window to protect the
instrument from the elements. These windows must be made of the same material to ensure that
differences in window transmittance are not ‘calibrated’ into the measured irradiance and thus
increase the uncertainty of the measurement. To obtain higher quality measurements that include
the signal from the infrared portion of the solar spectrum, the instrument can be operated without
a window or with a window made of a material that has flat transmission characteristics from
approximately 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 systems have reduced the dependence on
simple thermopile pyrheliometers that require frequent comparison with fair-weather ACR
instruments. It is recommended that an all-weather ACR be used continuously with a standard
pyrheliometer used to fill ‘data gaps’ during the period when the ACR is in calibration mode.
Caution must be exercised if a windowless ACR is to be operated continuously. The minimum
protection required is to house the instrument in a ventilated housing. The opening aperture of the
housing should be a minimum of 10 radiometer-opening-aperture diameters distant from the
entrance aperture of the enclosed ACR and have a diameter no greater than twice the field of
view of the ACR. Care must be taken when ventilating the instrument so that no venturi effects
are created that might alter the thermal equilibrium of the instrument. In areas where severe
weather conditions are prevalent, systems that include a means of closing the opening aperture
are required.
When using a calcium fluoride window, yearly inspections are recommended to ensure the
integrity of the flat because of the anhydrous nature of the material. In very humid or wet
environments, inspections of the flat should be made monthly. To protect the instrument from
precipitation, an automatic cover triggered by a rain sensor can be installed.
Experiments have also shown that maintaining the temperature of the thermopile on certain ACR
instruments, when used in an all-weather mode, further enhance performance.
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