Solar Variability and Climatic Change:

Solar Irradiance Measurements


Personnel and instrumentation for pre-flight testing of the first Active Cavity Radiometer Irradiance Monitor (ACRIM I) experiment, Table Mountain Observatory Solar Test Facility of the Jet Propulsion Laboratory, February, 1978. The experiment flew on the Solar Maximum Mission (SMM) satellite, 1980 - 1989. The solar tracker on the left (white platform) contains international total solar irradiance (TSI) reference and sub-orbital rocket versions of instrumentation; the solar tracker on the right (black box) contains the SMM/ACRIM I flight instrument.

The test compared the TSI observations of the reference instrumentation and the ACRIM I in real-time. The rocket versions of the instrumentation were then flown to make real-time comparative observations with the ACRIM I instrument. This type of test was primarily a health and sanity check on the flight instrumentation although more was originally expected from such comparisons. The limitations of applying the information on absolute radiation scale from this type of testing became evident during the SMM/ACRIM I and subsequent Spacelab/ACRIM experiments. The absolute scale could not be reproduced with better than a few tenths percent accuracy, about the order of magnitude of the solar variability signal. The SMM/ACRIM I experiment showed that the flight observations, under optimum conditions, could be several orders of magnitude more precise. This led Willson to propose an 'overlap strategy' in which succesive solar monitoring experiments were directly compared in flight.


--Contributed by Dick Willson (kneeling, fourth from left)

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