Solar Total Irradiance Monitor (TIM)

The Total Irradiance Monitor (TIM), an active-cavity solar radiometer, is t o be launched in 2002 on the SORCE mission of the Earth Observing System (E OS). The relative uncertainty (1 sigma) will be better than 10(-4), i.e. 10 0 parts per million (ppm) (1 sigma) with a noise level of <1 ppm each 500 s . Sunlight passes through a shutter, a 50 mm(2) aperture, and is then absor bed into a silver cavity blackened inside with nickel phosphorus. High ther mal-conductivity diamond insulators at the electrical terminals help to loc alize thermal nodes. Four cavities are aligned side-by-side at the rear of a 2 kg heat sink. The flight "standard digital watt" is derived from a high -precision voltage and a pulse-width-modulator; this eliminates the need fo r a square root in the servo loop. We determine the irradiance from the in- phase sinusoidal component at the shutter frequency. This phase-sensitive d etection allows more accurate characterization than traditional time-domain methods. We calibrate the aperture transmission integral over area against a chrome-on-quartz ruling, transferring by charge-coupled device (CCD) ima ges. In flight, we recalibrate the servo-loop gain, pointing variations, an d dark signal. We determine the equivalence ratio from models and by laser measurements of the cavity parameters.