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original filters and from the same manufacturing lot. In this manner, the waveband characteristics can
be maintained over longer periods of time.
7.3 Data Acquisition
7.3.1 Sampling
Observations are to be made at a frequency of once per minute. Unlike other irradiance observations
that obtain one-minute averages, the measurement of direct spectral irradiance is to be an instantaneous
observation at a given time (t). When multiple measurements are made with a single-detector instrument
or as part of a large set of observations, the difference in time (Mt) between the first and last observation
must also be known. If multiple instruments are to be used, both t and Mt must be known for both
instruments, and the observation times must by synchronized.
The dark (or zero) signal should be measured for each transducer as part of the observation sequence
when possible. For instruments that are not capable of obtaining a dark signal with each observation,
dark signals should be measured at night, or a sampling scheme should be designed so that a dark
signal is measured at least daily. This may be as simple as blocking the entrance aperture of the instrument
during an observation time.
Measurements should continue throughout the period that the sun is above the local horizon (see Section
3.1.2). The sampling program must not automatically screen observations because of cloud or other
forms of contamination.
The use of sam pling frequencies of less than once per minute will be accepted into the archive until
2004 for automated instruments that meet all other BSRN specifications.
Measurements obtained from handheld sunphotometers will not be accepted by the archive.
7.3.2 Data Acquisition
Depending upon the radiometer, the data acquisition system may be part of the instrument package.
Other radiometers require separate instrumentation for data collection. In all cases, the collection system
must be capable of providing:
(1) A signal-to-noise ratio no less than 5000:1 for the smallest output signal of the spectral irradiance
channels measured when the sun is at its highest elevation for the station. In high turbidity
500
cases (Ja > 0.750 occurs more than 20% of the time) the signal-to-noise ration of the data
collection system should be at least 12000:1. For example, if the highest signal output by
the least sensitive channel was 500 mV, the collection system would have to measure accurately
a signal of 100 :V or less (5000:1) or ~40 :V or less (12000:1). In contrast, the complete
system signal-to-noise ratio for irradiance measurements is 10000:1.
(2) An ability to maintain timing accuracy to better than one second (see Section 2.3.1).
(3) A time stamp so that Mt can be given or calculated.
7.4 Calibration
7.4.1 General
The calibration of spectral radiometers has yet to be fully resolved. At present there is no hierarchy
such as the WRR for spectral radiation. Until such time as a calibration hierarchy becomes available,
the BSRN Working Group on Aerosol Optical Depth suggests the following calibration methods:
(1) An independent detector-based standard. This method uses a fully characterized silicon
photodiode trap detector for the measurement of a stable irradiance source that illuminates
both the detector to be calibrated and the trap detector simultaneously. This methodology
is very successful for calibrating instruments using specific laser wavelengths. The use of
this method for the calibration of wider bandwidths remains under development.
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