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Total Irradiance Monitor (TIM)
The electrical substitution radiometer of TIM is similar to that introduced in the ACRIM series, the most accurate being the current ACRIM III on ACRIMSAT. SORCEs TIM is expected to further increase the accuracy of TSI data by incorporating modern materials and electronics. In particular, it uses phase sensitive processing to achieve a major improvement in signal-to-noise. The goal of the TIM instrument team is to measure TSI with 0.01 percent relative standard uncertainty (relation of the measurement to SI units) and characterize sensitivity changes with a relative precision of 0.001 percent per year. Readers interested in procedures and terminology relating to the accuracy and precision of such instruments should refer to "Recommended Practice: Symbols, Term, Units, and Uncertainty Analysis for Radiometric Sensor Calibration", 1998, by Clair Wyatt, Victor Privalsky and Raju Datla, NIST Handbook 152, US Dept of Commerce, Technology Administration, NIST.
The accuracy of TIMs readings will allow scientists to observe the subtle changes in solar radiation brought on by the sunspot cycles. They will use these numbers to determine just how much the Sun varies on a day-to-day, a month-to-month, and a year-to-year basis and then compare any subtle oscillations to changes in the climate. The new readings will also help improve climate models.
To make sure the instrument continues to make accurate measurements (i.e., to calibrate the instrument), the researchers constructed an identical instrument that will remain on the ground. Once a year they plan to take this identical TIM into orbit on the Space Shuttle as part of the Solar Irradiance Hitchhiker program. By comparing these measurements with those of the instrument aboard SORCE, the scientists should be able to tell if the SORCE instrument has changed its properties. They can then make the compensating adjustments to the data they receive.