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Background Information:

The objective of this experiment was to measure the clear, cloudless sky
solar irradiance at the surface with the best instrumentation available,
and with the most appropriate procedures as followed by the solar 
measurement community. The irradiance measurements were carried out with  
extreme care, diligence and certainty with thorough documentation.  The 
ultimate goals of this experiment were 1) to validate similar measurements 
being taken at the ARM/CART Central Facility site on a routine basis and 
2) to provide a highly referenced and accurate measurement of clear sky
irradiances at the ARM/CART CF Site. 
All measurements were taken at the ARM/CART Site from 17 April to 26 
April.  The data used in the resultant clear day analysis were taken
on 18 and 19 April, 1996 (JD 109 and 110).

The instrumentation used in the experiment is as follows:

1) 2 Eppley Hickey-Frieden Absolute Cavity Radiometers, model AHF
   sensitive to 280-2800 nm wavelengths. Measures direct solar 
   radiation. (serial numbers 29222 and 30710) 
2) 4 Eppley Precision Spectral Pyranometers, model PSP sensitive to 
   280-2800 nm wavelengths.  2 were shaded to collect diffuse radiation
   and 2 were unshaded to collect global radiation (diffuse + direct).
   (serial numbers 29613, 29614, 29616, and 29912)
3) 2 Eppley Precision Infrared Radiometers (Pyrgeometers) model PIR 
   sensitive to 3500-50000 nm wavelengths.  The PIRs were shaded.
   (serial numbers 29143 and 29144)
4) 2 Sci-Tec 2AP Suntrackers.  They were used to point the cavities and
   to provide a platform for diffuse (shaded) measurements.
5) 1 Campbell CR10 datalogger and Campbell AM416 16 channel multiplexer
   was used for data collection.

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        Please Note:
        This data is preliminary.  In order to create the best data set
possible, the instruments used in collecting this data, especially the
Eppley PSPs (Global and Shaded) must be recalibrated to check for drift
during the experiment. After recalibration, the PSP data will most likely
not be changed by more than 2-3%.
 	The AHF Cavity 30710 was compared directly to the World Radiation
Reference Standards at Davos, Switzerland in October of 1995.  The average
comparison was within 0.2% of the WRR Standards.
	The Eppley PIRs were taken from a set of three reference ones used
by the Solar Radiation Research Branch of The National Oceanic and 
Atmospheric Administration. 

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  This file contains information essential to the understanding of the
data in cavity1.dat. 
The data file, cavity1.dat, contains the following data sets in comma
delineated lines:

Dec.Time,Avg_PIR,Avg_Cav,Avg_Shade,Avg_Global,Cos(sza)

Dec.Time   =  Hour Decimal Time (Local Standard Time,  add 6 for GMT)
Avg_PIR    =  Average of the two shaded Eppley PIRs for times corresponding
              to cavity data taken.
Avg_Cav    =  Average of the two Eppley AHF Cavites from days 109 and 110.
Avg_Shade  =  Average of the two shaded Eppley PSPs for times corresponding
              to cavity data taken.
Avg_Global =  Average of the two Global Eppley PSPs for times corresponding
              to cavity data taken.
Cos(sza)   =  Cosine of the solar zenith angle.

The PSP and PIR data were taken using 1 second spot samples and logging
1-minute averages and standard deviations.  The cavity data were taken as
1 second spot samples every 3 seconds and then averaged into 1 minute
data during the data reduction procedures.

Special Data Notes:
-------------------

1)  -9999.0 in the data set implies missing data for that sample.
2)  The constructed data set does not start at exactly sunrise nor end at
    exactly sunset due to limitations of the cavity radiometer at very high 
    solar zenith angles. 

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Note that this data set, representing 1 day, is actually comprised
of two days:

                            <9:30 am ---------- JD 109 ------------Sunset>
<Sunrise -- JD 110 -- 9:30 am> 

The day was created this way due to clouds during the morning of JD 109 and
the afternoon of JD 110.

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All inquries should be directed to:

Mike Rubes
NOAA/ARL/SRRB
R/E/AR*1
325 Broadway
Boulder, CO 80303
(303) 497-7316
rubes@srrb.noaa.gov

Note also that a WWW page has been created for this data set.  The raw data as
well as processed data along with more information about the experiment is
presented.

http://www.srrb.noaa.gov/apr96iop/hagsie.html

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