A CASE STUDY OF LANGLEY ACADEMY

A Case Study of Langley Academy

A Case Study of Langley Academy

1. Introduction

For vicarious calibration ground-truth measurements of direct solar irradiance spectra, sky-brightness spectra, incident global radiation spectra, and radiance spectra reflected from an underlying surface have to be carried out synchronously to measurements of the Modular Opto-electronic Scanner MOS (Posse and Zimmermann), an imaging spectrometer onboard the Indian Satellite IRS-P3. To do this, a HiRES spectrometer complex (grating spectrometers, weather station, central computer, etc.), with 512 spectral channels each, 16 bit radiometric resolution and different spectral resolution (0.3-10 nm) and wavelength range (400-800 nm) was designed and built ( Zimmermann, 1998).

Calibration of HiRES spectrometers for measurements of direct solar irradiance spectra can be realised using the classical refined Langley technique during highly stable atmospheric conditions found at a high mountain station. However, due to possible degradation of the instrument, calibration has to be repeated frequently to ensure correct ground-truth measurements. Regular access to high altitude clean air sites for frequent Langley-plot measurements is impracticable. Another possibility for calibration control is the use of laboratory measurements. However, problems result from the large uncertainties in the calibration of the lamps and possible degradation of the lamps themselves. The best way for frequent control of Sun calibration would result if near sea-level measurements during quite stable atmospheric conditions could be used for Sun calibration. Then measurements for Sun calibration control could be included in each ground-truth campaign.

Classical refined Langley analysis fails for near sea-level measurements, because the Langley-plot calibration requires a highly stable atmosphere and homogeneous layers during the minimal calibration phase of half a day. Even if receiving a straight Langley-plot line from measurements at near sea-level locations, the resulting calibration constant may have a serious error due to possible parabolic temporal drifts in atmospheric turbidity (Shaw, 1976). Tanaka; Tonna; Forgan, 1989 and Bannehr, 1990 proposed to add solar aureole measurements to Langley analysis for situations when requirements for the classical refined Langley-plot are not met. In this way time variations of atmospheric turbidity during the calibration period can be accounted for. Time variations of atmospheric turbidity are equivalent to variations of aerosol-loading or aerosol optical depth. In the present paper the method of this aureole-corrected Langley-plot for calibration control of the HiRES spectrometer using near sea-level measurements is investigated. After modifications of the HiRES spectrometer (Sun and sky measurements) and an adaptation to the Tanaka approach (point measurements), the potential of the method could be demonstrated by comparing classical plots from the Jungfraujoch (3580 m above sea level, a.s.l.) and aureole-corrected Langley-plots from Berlin (60 m a.s.l.).

2. Technical adaptation

Sun calibration using the aureole-corrected Langley-plot technique requires a half-day series with pairs of measurements of direct solar irradiance (sun signal CObsS), and sky brightness in the solar aureole (aureole signal CObsA. Measurements for the aureole-corrected Langley-plot thus include all the measurements necessary for classical Langley-plot analysis. Measurements of the circumsolar sky brightness in the solar aureole are made using a circular ring shading disk ...