Calibration History Data

The table below shows the calibration history for sensors deployed at
the Saudi Network Station you have selected. The dates are the date of
installation of the sensor. The  application code is D= Diffuse sky
measurement, N=Normal Incidence (Direct Beam), and G= Global horizontal
measurement.  The Cal Factor column is the single, cosine weighted
composite responsivity, Rs. The reciprocal of Rs multiplied by 1000 is
the factor used during data collection. The following nine columns are
the "calibration vector" showing the responsivity as a function of
zenith angle, from Z= 0 to Z = 90 degrees, in 9 degree wide bins. These
are the factors used to post-process the data and correct for individual
radiometer cosine response variations. The figure shows typical cosine
response variations derived during our outdoor radioemeter calibration
and characterization.

   <<RCC.JPG>> 

Post Processed Saudi Network and BSRN Data

Until APRIL XX, 2000, the data available for the Saudi Network stations
was quality assessed and flagged based on the use of a single composite
calibration factor for the pyranometer deployed at each station.  As of
APRIL XX, 2000, the global horizontal data posted for all of 1998 to
date has been corrected for the cosine response of the individual
pyranometer deployed at each station.  During individual radiometer
calibration and characterization, a calibration factor for each of ten 9
degree wide zenith angle bins is constructed. To obtain the corrected
data, we applied a calibration factor interpolated from the two nearest
zenith angle bin responsivities.  The figure shows typical magnitudes of
corrections in Watts per square meter as a function of zenith angle for
a representative pyranometer (Gizan) in June and December. Keep in mind
that each pyranometer has a unique cosine response correction, and the
figure is only representative of corrections applied for a single unique
radiometer.
	 <<cosine.jpg>> 
The post processed data also contains a global horizontal irradiance
(derived global) computed from the direct normal irradiance and the
diffuse sky radiation. The global is computed as Idn * cos(z) + D  where
Idn is the measured direct normal irradiance (within 5 degree field of
view of the sun), z is the solar zenith angle (complement of the solar
elevation angle), and D is the diffuse sky radiation measured by a
pyranometer shaded with a tracking disk subtending a solid angle of five
degrees and obscuring the sun as seen from the pyranometer detector.
The BSRN data set also includes the derived global horizontal
irradiance. However the BSRN direct beam measurement is made by the
windowed all weather cavity radiometer, rather than a Normal Incidence
Pyrheliometer.

Diffuse Pyranometer Thermal Offsets

The Eppley Laboratory Precision Spectral Pyranometer (PSP) used to
measure the diffuse sky radiation is known to have a thermal offset
voltage which is a function of the net infrared radiation (or infrared
radiation balance between the radiometer and the sky).  Our study of the
correlations between the offsets seen in the Saudi  network radiometers
and the net infrared show that offsets range from 0 to -15 watts per
square meter, with a mean value of -4 Watts per square meter and
two-sigma standard deviation of 7 Watts per square meter. However, as
shown in the plot below, the standard error of an estimate based on a
linear correlation with net infrared is on the  order of +/- 8 Watts per
square meter ( the spread about the linear fit line).  Thus we have
chosen at this time NOT to apply any correction for Saudi Diffuse
Thermal offsets. Adding an additional 4 watts per square meter to the
DERIVED global horizontal is just as effective a correction, and much
simpler. However, the uncertainty in applying the four watt correction
is itself +/-4 Watt per square meter.  We are conducting co-operative
research to obtain more definitive correction algorithms. Watch for
updates!

    <<OFFSET.GIF>>