The SORCE/TIM measurement goals are a factor of 10 more demanding than demonstrated performance of prior satellite-borne radiometers. These improved accuracies are needed to maintain continuity of the 3-decade TSI record. The TIM achieves high absolute accuracy and good long-term stability because of its new-to-TSI technologies and thermally robust design: 1) phase sensitive detection at the shutter fundamental reduces sensitivity to noise, thermal drifts, and parasitic heating; 2) nickel-phosphorous black, an acid etched black metal, provides stability and good thermal conduction as the absorptive material in the ESRs; 3) direct and frequent thermal background measurements correct for the large source of background inherent in ambient temperature radiometers; and 4) thermal, optical, and electrical systems are regulated by a digital signal processor (DSP), allowing programmable control and on-orbit changes.
The TIM instrument consists of four Electrical Substitution Radiometers (ESRs), each a hollow 10° half-angle conical cavity with a cylindrical extension. These sensors are cantilevered from a single support for thermal stability. The mount provides thermal impedance between the ESRs and the DSP-controlled heat sink surrounding them. The sensors are made from 0.6 mm thick electrodeposited 99.99% pure silver. Their interiors are coated with a nickel phosphorous (NiP) black and etched to produce a diffuse black surface, providing a nominal photon-absorption efficiency of approximately 99.98%.
A resistive heater made of thermally stable wire is impregnated in each cone. Spinel chip thermistors sense cavity temperature for the electrical bridge circuit which regulates resistive heater power. Baffles, located behind each aperture, shield the ESRs from Earth albedo and off axis infrared radiation. Small silicon (Si) photodiodes, one located just in front of each ESR, view the measurement cavities at oblique angles to monitor changes in reflectance. The heat sink is actively controlled and isolated from the TIM case. A vacuum case protects the instrument from contamination prior to launch.