Solar Radiation and Climate Experiment (SORCE)

Total Irradiance Monitor (TIM)
TIM will measure the total amount of radiation coming from the Sun. The sensor uses what is known as an absolute radiometer and houses four cone-shaped cavities. One of the cavities has an oscillating shutter that allows direct sunlight to shine into one of the cones. The material in the cone absorbs nearly all the Sun’s energy and heats up. By measuring the voltage needed to bring this heated cone back to the same temperature as one of the other “reference” cones, which are kept at a constant temperature, the instrument can obtain an extremely accurate reading of the TSI in watts.

Total Irradiance Monitor

The Total Irradiance Monitor (TIM) will measure the energy emitted by the Sun ten times more accurately than previous sensors. (Image courtesy Solar Radiation and Climate Experiment Project)

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. SORCE’s 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 TIM’s 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.

Photo of Hitchiker Cannisters aboard the Space Shuttle
Scientists will be able to confirm the accuracy of the TIM instrument on SORCE by comparing its measurements with those made by an identical instrument carried into space periodically in the bay of the Space Shuttle. (Photograph courtesy NASA Human Spaceflight)

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.

next: Spectral Irradiance Monitor (SIM)
back: The SORCE Satellite



Solar Radiation and Climate Experiment (SORCE)
Earth’s Energy Balance
Solar Variability
The Sun and Global Warming
Uncertainties in Solar Measurements

The SORCE Satellite
Total Irradiance Monitor (TIM)
Spectral Irradiance Monitor (SIM)
Solar Stellar Comparison Experiment (SOLSTICE)
Extreme Ultraviolet Photometer System (XPS)

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Reflected Solar Radiation
Outgoing Heat Radiation