Thermal Background Is Measured Every Orbit
During the daytime side of the orbit, the active ESR alternately views the incident power from the Sun and that from its closed shutter. When the shutter is open, the instrument measures radiation from the Sun. When closed, the shutter both emits thermal radiation into the instrument and reflects the instrument’s own blackbody radiation back inside. This gives a non-zero “dark”, or thermal background, signal that must be corrected.
The instrument’s thermal background signal is measured by observing dark space during the eclipsed portion of each orbit. This signal is fitted to four instrument temperatures to model the contributions from different portions of the instrument, as these temperatures vary throughout an orbit. The instrument temperatures during solar measurements are used in this model to correct for the background thermal contribution appropriate at actual observing times.
Dark signals of roughly -3.15 W/m2 are measured. The negative value is due to the loss of energy from the cavities into space when the shutter is opened, so the dark correction increases the measured TSI. This value varies due to thermal fluctuations in the instrument by 0.1 to 0.2 W/m2 between orbit sunset and sunrise, giving a relative effect during the orbit of ~100 ppm that is corrected for via the instrument thermistor temperatures and the instrument’s dark model.
Detector Degradation Is Monitored By Duty Cycling ESRs
Simultaneous, pair-wise inter-comparisons of the four cavities allows monitoring of long-term changes in cavity absorption, with the exposure-dependent degradation lower in the lesser-used cavities.
The primary ESR is compared against the other, lesser-used ESRs regularly. Measurements with the primary ESR are acquired nearly continually. Simultaneous measurements of the Sun with both the primary and secondary ESR are done 1% of the time, or for one SORCE orbit per week. The secondary and tertiary ESRs acquire simultaneous measurements 0.5% of the time, and the fourth ESR is used simultaneously with the primary 0.2% of the time.
The primary ESR shows a slight decrease in its sensitivity due to a brightening of the nickel phosphorus (NiP) black cavity interiors. This is a very small change of only 0.11 ppm/day and indicates the TIM’s NiP black cavity interior is relatively robust. These sensitivity changes in the TIM are tracked to <10 ppm/yr. This degradation shows a classic exponential decrease, and is expected to eventually reach a net brightening of only 160 ppm (see Degradation Figure).
Several parameters, such as aperture area and shutter emission, change with temperature on time scales from several seconds to the length of the mission. These effects are well understood or are tracked on-orbit, and are corrected using the several thermistors and resistive thermal device (RTD) in TIM.
Electronic Gain Is Calibrated On-Orbit
In-flight calibrations include monthly electronic servo system gain measurements. These gain calibrations measure the thermal response of the ESRs to a known electrical heater input, and are used to track ESR response with temperature and aging. To date, the servo system gain has not changed noticeably (see Gain Figure below). Because a feedforward value is applied at each shutter transition, the servo system always remains well balanced, making the uncertainty due to servo system gain <1 ppm.
Orbital Corrections Are Very Accurately Known
Corrections to the data to account for the varying distance between the Earth and the Sun during the year are calculated based on JPL ephemeris VSOP87 data, which has very high precision and contributes <1 ppm to the TIM uncertainty. Doppler corrections due to the spacecraft’s line-of-sight velocity relative to the Sun are made based on spacecraft position and velocity data, which are very accurately tracked and propagated from NORAD TLE/SGP4 and similarly contribute <1 ppm to the TIM uncertainty. Corrections for both distance and radial velocity to the Sun are applied in ground data processing.
More details on the flight calibrations are described by Kopp, G., Heuerman, K., and Lawrence, G., “The Total Irradiance Monitor (TIM): Instrument Calibration,” Solar Physics, 230, 1, Aug. 2005, pp. 111-127.