Supplementary material, in-flight results, submitted to Solar Physics.
NOT MANUSCRIPT.
Primary in-flight Results of Total Solar Irradiance Monitor on FY-3C Satellite, An Instrument with A Pointing System
Abstract Total Solar Irradiance (TSI) has been recorded daily by Total Solar Irradiance Monitor (TSIM) in a manner of accurate solar pointing onboard the nadir FY-3C satellite designed mainly for Earth observing, since Oct 2013. TSI measurement result of TSIM/FY-3C in October 2 2013 is 1364.88 W m-2 with uncertainty 1.08 W m-2. The instrument TSIM/FY-3C has a pointing system to perform solar tracking using visual servo control method. TSI is measured by two electrical substitution radiometers by the new TSIM with an absolute accuracy better than 900 ppm, with traceability to World Radiation Reference. The in-flight TSI data has proved that short term TSI variations recorded by TSIM/FY-3C have good agreements with VIRGO/SOHO or TIM/SORCE. TSI data quality and accuracy of TSIM/FY-3C is much better compared with the two previous TSIM operating in a scanning manner.
1 Introduction
Under normal conditions, most of external energy input to the Earth system is provided by the sun through solar radiation. The solar radiation, the Earth’s Albedo, and outgoing long-wave infrared radiation emitted from the Earth’s surface and the Earth’s atmosphere establishes a delicate thermal balance generally refereed as the earth radiation budget, defining the basic climate process on the Earth’s surface or above the Earth’s surface. Changes in solar irradiance may have delicate even uncertain impacts on the energy input to the Earth system, producing subtle effects on the climate system (Ermolli et al., 2013; Solanki et al., 2013). Knowing the variability of solar irradiance is essential to evaluate all climate forcing items, including both natural causes and anthropogenic causes (Rind, 2002). To establish continuous and accurate record of solar irradiance is vital to understand solar driving for Earth’s climate. The record of solar irradiance is also helpful to get some insight about the internal processes of the sun core.
The Total Solar Irradiance (TSI) is the radiative solar power flux at the top of the atmosphere, defined over the entire solar spectrum and at 1 Astronomical Unit (AU). Accurate and continuous measurements of TSI are essential quantitative record to find the sun’s signature in climate change and to understand the solar driving mechanism in current or historic record of the climate change. Spaceborne measurement of TSI had been continuous for about 38 years, based on contributions from a number of spaceborne instruments (Frohlich et al., 1997; Kopp et al., 2012; Meftah et al., 2014b). Variations of TSI on temporal scale have been detected over the solar cycles 21 to 23. However, current TSI observation is facing unprecedented challenges, with much less care and space missions than the past three decades. Few continuous spaceborne experiments with instrumentations using absolute radiometers of one kind have been approved now. FY-3 missions had been planned to record TSI using electrical substitution radiometers of one kind referred as the SIAR type radiometer (Yang et al., 2012), including FY-3A, FY-3B and FY-3C satellites. Observations of TSI will also be performed on the future FY-3 satellites in order to record the upcoming solar cycles (Thuillier et al., 2014). Continuous record of TSI will be recorded by FY-3 satellites using SIAR type radiometers, with overlapping measurements in time scale onboard FY-3 satellites. However, FY-3 satellites are spacecrafts mainly designed for Earth observing, not like solar-dedicated missions, such as the SDO mission (Hoeksema et al., 2014; Wieman et al., 2014), PICARD mission (Meftah et al., 2014a), and etc. A new TSIM with a pointing system was developed for the nadir FY-3C satellite in order to achieve accurate solar pointing. And the primary results of TSIM in the space flight will be given in this paper.
TSIM/FY-3C is developed by Changchun Institute of Optics, Fine Mechanics and Physics for China Meteorological Administration (CMA). TSIM is named as Solar Irradiance Monitor (SIM) in CMA documents and data sites.
2 Mission summary
The primary objective of TSIM/FY-3C experiment is to obtain accurate measurements of TSI at the top of the Earth’s atmosphere, the same as the previous two TSIMs. However, instrument requirements for TSIM/FY-3C are much stricter than the previous two. TSI should be measured under conditions of accurate solar pointing errors and sound thermal stabilities, in the FY-3C mission. The nominal lifetime of the TSIM experiment for the FY-3C satellite is five years, the same as the spacecraft.
The TSIM/FY-3C instrument is designed to be able to record TSI with nearly zero pointing errors and sound thermal stabilities of radiometers’ heatsink. As defined by its routine mode, the instrument is able to follow the sun in a limited range in the daytime portion of each orbit, near the north pole of the Earth. The routine observing mode provides observing of solar activities two times every orbit. This permits more accurate observations of incoming sunlight in a much smaller field of view (Girshovitz et al., 2014), compared with the previous instrument TSIM/FY-3B, without complex corrections of solar pointing errors anymore. Except the closed-loop mode of visual servo, some backup operation modes are designed for the pointing system to make the instrument operate still normally without image feedbacks provided by the digital sun sensor. The digital sun sensor is consisted of an image sensor (Cetin et al., 2014), an optical module with single aperture and etc.
After the pre-launch test, the FY-3C satellite with TSIM onboard was launched into the orbit. After the launch, science test of the TSIM instrument was began and the science test lasted for about three months. The major purpose of the science test is to verify that the TSIM instrument is compliant with the instrument requirements in the space flight. A variety of activities or instrument operations are performed for the post-launch validation of TSIM data products, including solar observation, solar tracking, temperature regulations and etc. The activities in the period of science test for the instrument are necessary to achieve the scientific objectives of the TSIM experiment.
When the period of the science test is over, the data quality of the instrument is still assessed routinely at all levels of data processing. Nearly all of the instrument data is analyzed routinely to ensure that the instrument is in a healthy state. System parameters of the instrument are generally compared to the acceptable ranges determined before the launch or from the historical space experiences. If any instrument parameter is out of the normal range, the instrument team will be informed to find what is affecting the instrument data.
3 Pre-launch tests
The system assembly and integration of the TSIM flight product was completed in 2012. Afterwards, the instrument was tested by a specific program of performance verification to ensure the TSIM performs to the required levels. The pointing system had been tested on the ground, using a solar simulator to get in-orbit representative conditions of sunlight.
Area of the primary apertures, reflectance of the cavity detector from ultraviolet to infrared wavelengths (Kopp et al., 2005; Witte et al., 2014) and output of the voltage reference had been measured in the pre-launch calibration. A comparison experiment was performed to obtain TSIM’s traceability to the World Radiometric Reference (Wang et al., 2014). The time duration left for the comparison experiment was not more than two weeks. The radiometer package of TSIM/FY-3C was pointed to sun by a solar tracker, with the standard radiometers SIAR-1a and SIAR-2c simultaneously. The radiometers SIAR-1a and SIAR-2c had been calibrated to World Radiometric Reference (WRR) already in the 11th International Pyrheliometer Comparison (IPC-XI), 2010. The on-ground comparison experiment was performed only in the air, not in vacuum, under conditions of ambient temperature and pressure. It is a pity that TSIM/FY-3C had not been calibrated through end-to-end calibration, like PREMOS, using the calibration device TRF. No cryogenic radiometer, high-stable laser and etc are available for the TSIM calibration.
After the comparison experiments for instrument calibration, TSIM was integrated with the FY-3C spacecraft. Performance of the TSIM instrument was validated further by extensive preflight tests on the spacecraft level, under ambient or thermal vacuum conditions.
4 Post launch tests
The meteorological satellite FY-3C was launched on 23 September 2013 and it was successfully placed in a sun-synchronous polar orbit. During the initial days in orbit, most of the TSIM instrument modules were not allowed to work except the thermal control system. The radiometer package was not enabled to work until the temperature of the heatsink was stabilized.
After the radiometer package was allowed to measure TSI, instrument science data was validated to test the performance of the instrument in flight. The science test lasted about three months. The commissioning phase was begun in October 2013 and the commissioning phase was ended in January 2014. Validation of TSIM science data is crucial for producing reliable science data about the solar activity.
Various experiments were performed during the commissioning phase, including electrical substitution of TSI, solar tracking control, temperature regulation and etc. Some functions of the instrument can only be validated when the instrument is located on the spacecraft in the space, such as the real performance of the pointing system. Results of the solar tracking experiments on the ground were not able to validate the real pointing performance completely, without the real sunlight in orbit.
In the commissioning phase, the radiometers in TSIM/FY-3C were commanded to operate in special test modes, slightly different from its routine mode. In the routine mode, the radiometers record TSI at 10-minute intervals in the sunlight portion. In the commissioning phase, measurement time of each radiometer for the reference phase is changed from 5 min to 3 min. The objective of the special test mode is to find the possibility to reduce measurement time for single observation.
The degradation effect of the cavity detector in the radiometers should be monitored in the space experiment and further taken into account for the correction of science data products. In the commissioning phase, the two radiometers in the radiometer package were enabled to measure TSI simultaneously for nearly two months in order to know whether their performance meet the instrument requirements or not. In the routine mode of the instrument, one radiometer is supposed to perform daily observation while another radiometer will only observe the sun occasionally and rarely as the backup channel. However, channel AR2 selected finally for occasional observation to investigate degradation was not shut off until December 2013, since its switch on in October 2013. The radiometer AR2 had been exposed to too much sunlight in the two months. This makes the investigation of degradation for the cavity detector somewhat complex. And the degradation will be studied in the future.
All function of the instrument in its routine mode was tested in the space experiments. The most critical test is to test the pointing system when the satellite leaves the eclipsed portion. The mission life of TSIM experiment is expected to be expanded and it is mainly determined by the in flight performance of the pointing system. As the solar pointing errors had puzzled data corrections of TSIM/FY-3A and TSIM/FY-3B, the solar tracking performance of the pointing system had been taken as a focus of the commissioning. The pointing system ran well in the mode of closed loop using visual servo control method. The backup modes of pointing system had not been tested in the commissioning phase. The performance of the thermal control system had been tested without human interferences, after the initial days in orbit. The thermal control system was tested in only a constrained small temperature range, not the full operating temperature range.
Knowing exact characteristic of the instrument, such as its precision and accuracy, is another goal of the validation activities. Uncertainty evaluation of the instrument was also performed during the test period, using experimental results of in flight experiments.
From the results of space experiments, it was found that the operation of the instrument TSIM/FY-3C was stable and the instrument’s response in the space flight was repeatable. The performance of TSIM/FY-3C was just expected as instrument requirements.
5 Thermal control experiments
Since the instrument TSIM is a thermal system, thermal stability of the radiometer system is critical to achieve accurate measurement of TSI. Before TSIM was able to measure TSI with solar pointing, thermal control system had to establish a stable thermal environment for the instrument. Temperature of radiometers’ heatsink is regulated through active thermal control of the instrument, with 13 separate heating regions including the pitch motor and the yaw motor for solar tracking, the gears, and etc. Temperature reference of each heating region is able to be changed by telemetry commands.
The thermal control system was tuned in order to achieve a stable thermal environment. Temperature reference of each heating region in the thermal control system was carefully changed from its design value to the operational value several times, in order to establish a proper thermal state for the instrument. The heatsink temperature was stabilized around 300 K as desired by the routine mode of the instrument before Oct 2013. Control errors of heatsink temperature had been nearly zero and very small. As soon as the expected instrument thermal stability was achieved, the pointing system was commanded to prepare to get the first light for the radiometer package, by the telemetry commands from the ground stations.
6 Solar pointing experiments
Before the pointing system began its solar pointing in the space, telemetry commands were sent to TSIM to free the locking unit. Response of the locking unit is correct as expected. In order to survive the launch vibration, the motors and the gears inside the pointing system were not allowed to move by using the locking unit. As soon as the instrument TSIM left the eclipsed portion, the pointing system tracked the sun in the closed loop mode using visual servo method, just as it was expected in the system design.
The solar pointing error for single solar measurement, as the angle between the incoming sunlight and the optical axis of the radiometer package, is shown in Figure 1. Feedbacks of the solar pointing errors are provided by the digital sun sensor.