Generation of Langley Voltage Intercepts

Last Updated: July 2012

Langley Offsets

Every month new Langley voltage intercept data are generated using the program named la (Langley Analyzer). The program originated at ASRC, Atmospheric Solar Radiation Group at SUNY-Albany NY, and uses the methodology described in Harrison and Michalsky (1994). The method performs regressions of bias- and cosine-corrected direct normal voltages, expressed as ln(mV), on airmass and subjects them to a sequence of statistical tests.

A Langley analysis is attempted for each morning and afternoon period corresponding to the time of a selected airmass range. From each successful final regression, comprised of points that pass all of the tests, a Langley voltage intercept value (V0) is taken as the intercept of the line at airmass 0; the slope of the line is the average optical depth. The results of analysis with 5 or more points remaining in the final regression are kept. A second pass is then done, using a 12 count running mean of the normalized intercept. The new intercepts have to be within 2 standard deviations of the running mean, or they will be discarded, unless there are 3 in a row, then all three are kept and added to the running mean. Upon examination, if it is a significant change which continues, a new running mean is created (usually caused by cleanliness of the instrumentation). This process is used to remove variations from the Langley Analyser from clouds or atmosphere anomalies.

Finally, a third pass uses the running mean values to calculate a daily Langley voltage offset. An inverse time weighting scheme is used to calculate a value for each day.

These Langley Voltage intercepts are needed to convert measured voltages from a given instrument to irradiances using the Langley calibration method and to compute aerosol optical depths and column ozone. Ideally, Langley analysis of the MFRSR data would produce two intercept values (one for morning, another for afternoon) for each channel for each day of instrument operation. Also ideally, since it represents the voltage that would be measured by the instrument if it were placed at the top of the atmosphere, the intercept would be constant. In reality, neither of these ideals is met. The density of the intercept points varies greatly from site to site, being quite sparse at the more turbid sites; and changes in both atmospheric conditions and instrument function can result in significant variation and drift.