Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 72)

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7.4.2.3 Objective Algorithm
The objective algorithm described by Harrison and Michalsky provides a m eans to rem ove observations 5
that m ay contam inate the Langley calibration m ethod using a quantitative approach. The m ethodology
is used on airm ass between 2 and 6 where airm ass changes are rapid, but the problem of atm ospheric
refraction increasing the uncertainty of the analyses is avoided.
The m ethod for direct pointing instrum ents consists of four steps to rem ove observations that have
been contam inated:
(1) A forward finite-difference derivative filter is used to rem ove regions where the slope of the
dI/dm curve is positive indicating atm ospheric variability not consistent with uniform airm ass-
turbidity processes such as cloud contam ination. By determ ining the m inim um value of the
derivative, the process rem oves observations for a tim e period equal to the tim e between
the positive derivative and the m inim um derivative on both sides of the m inim um .
(2) A second forward finite-difference derivative filter is then used to determ ine regions of strong
second derivatives. In these regions, if the first derivative is negative and the value greater
than twice the m ean value of the first derivative for the observations, the regions are elim inated.
This m ethod elim inates observations that m ay have been contam inated by cloud, but m issed
using Step 1 and those regions where the data has been truncated.
(3) Perform a least-squares fit to the rem aining data. The standard deviation of the residuals
about the fit is calculated and all points that have a residual greater than 1.5 tim es the standard
deviation are elim inated. A second least-squares fit is computed on the rem aining observations.
(4) More than a of the original observations m ust be found valid in this m anner and the standard
deviation of the residuals about the regression line m ust be less than 0.006 before the Langley
calibration is accepted.
The m ethods described in Sections 7.4.2.2 and 7.4.2.3 can be com bined.
7.4.3 Lam p Calibrations
Standard Lam ps have not been used generally in calibrating spectral radiom eters used in the measurem ent
of AOD. This has arisen because of the lack of high quality top-of-the-atm osphere solar spectral data
that existed until recently. As AOD is a relative m easure, even a perfectly calibrated system , in absolute,
term s could not be used to obtain optical depths without a high solar spectrum with uncertainties lower
than those needed for the calculation of AOD. Top-of-the-atm osphere spectra are now available that
allows for absolute calibrations of spectral radiom eters.
Standard lam ps have been used successfully for the calibration of absolute spectral intensity. Most
national standards’ laboratories can provide calibrations of this type at the wavelengths used in standard
AOD m easurem ents. Lam p output uncertainty varies by wavelength, with the greater uncertainties
in the UV portion of the spectrum . This uncertainty is due to the decreased lam p output in this portion
of the spectrum and because sm all changes in the output current of the voltage source cause large
changes in lam p irradiance. A 0.1% variation in current changes the lam p output by 0.9% at 300 nm
and by 0.4% at 500 nm .
Standard uncertainties in calibrated-lam p output and transfer uncertainties rem ain large in com parison
with an AOD uncertainty of better than 1%. At 500 nm , the uncertainty associated with a lam p calibrated
by the N ational Institute of Standards and Technology (NIST) of the United States of Am erica is
approxim ately ± 1%. Transfer of this standard to a secondary standard (one used by certified calibration
laboratories) in the visible wavelength region is about ± 0.5%. Further uncertainties will be added with
the transfer from the secondary standards’ laboratory to the calibrated lam p used in the calibration
of the radiom eter. The increasing uncertainty with m ultiple transfers suggests that standard lam p
calibrations be used only when other m eans are unavailable.
The use of the sam e m ethodology described below, when used with a detector-based standard, provides
an instrum ent calibration based only on the uncertainty associated with the standard detector.