Remeasuring the extraterrestrial solar spectral irradiance

We will measure the solar spectral irradiance by deploying a CCD-array solar spectrograph to a high altitude favorable site, as part of a self-contained autonomous system with a calibration system using a monochromator and absolute photodiode "trap detectors." Data will be reduced using Langley extrapolation (and in the stronger absorption bands methods similar to Reagan-Brugge fitting), to yield the solar output free of atmospheric absorption. This measurement system will substantially improve the accuracy of the field measurements by making the instrument continuously self-calibrating against a local absolute standard in the range 400 - 900 nm. In the ranges 360 - 400 nm and also 900 - 1100 the trap detectors are not an absolute standard, but serve as a very reproducible transfer standard from an irradiance scale to be taken from either NIST lamps, or more recently-introduced detectors with calibrated efficiencies. We expect an absolute accuracy of 0.3% for solar-spectrum determination in the range 400 to 900 nm, not including the O 2 band at 760 nm, and the H2O bands at 820 and 940 nm. In the 360 - 400 nm domain we may be able to extend trap-detector quantum efficiency to allow an accuracy better than a secondary irradiance transfer, otherwise this domain and the range 900 to 1100 nm will have an accuracy of ≈ 1 %. The extrapolations in the strong-absorption bands will have an increased uncertainty which can be estimated from the statistics of the data. We describe the instrument and self-calibration methadologies and design.