The Total Irradiance Monitor (TIP) for the EOS SORCE mission

The Total Irradiance Monitor (TIM), to be launched in 2002 on the NASA Earth Observing System (EOS) SOlar Radiation and Climate Experiment (SORCE), will stare at the Sun for five years, and measure the absolute total solar irradiance (TSI). The TIM is an active cavity radiometer with a relative standard uncertainty 100 ppm and a fractional stability of less than or equal to 10 ppm/year. The estimated uncertainties are "type B" determined from the parametric uncertainties in a model of the instrument; and the dominant uncertainty will be in the effective aperture area. To obtain such low uncertainty, we: 1. Use metallic NiP as the cavity (diffuse) black. 2. Retrieve the irradiance in the frequency domain. 3. Use phase sensitive detection. 4. Use four separate, duty-cycled cavities. 5. Measure the aperture transmission integral over area. 6. Use diamond thermal/electrical nodes. 7. Use 400 seconds for each completely independent data point for low noise. 8. Use a pulse-width-modulated "standard digital watt" as the onboard standard. 9. Take advantage of the 1 ppm noise level to discover systematic effects. 10. Measure IR shutter radiation from in-flight measurements of dark space. We compare with other TSI measurements on orbit, and as separate shuttle experiments.