Spaceborne radiometers for measuring total solar irradiance

WANG Hong-rui，WANG Yu-peng

(Changchun Institute of Optics，Fine Mechanics and Physics，Chinese Academy of Sciences，Changchun 130033，China)

*Corresponding author，E-mail:wang_hongrui@yahoo.com.cn

1 Introduction

At any time in history of the Earth，the Sun has been the dominant energy source of the Earth.Solar irradiance drives nearly every dynamic process on the Earth，from atmosphere circulation to ocean current circulation，from weather phenomenon to the Earth's climate system［1-2］.The climate system of the Earth is determined by the amount of incident radiation from the Sun，the interactions of solar irradiance with the atmosphere，oceans and the land mass of the Earth.Total Solar Irradiance(TSI)has been identified as the primary determining factor of the Earth's climate system［3-6］.The TSI is the quantity of the solar energy flux integrated over all wavelengths at the top of the Earth's atmosphere at the mean Sun-Earth distance(the Astronomical Unit，AU)，as a measure of electromagnetic radiation power supplied to the Earth by the Sun［7-8］.For solar radiometers，the TSI is power of the Sun's electromagnetic radiation received per unit area at the radiometer's entrance aperture over the entire spectrum of Sun light.The accurate measuring of TSI at the top of Earth's atmosphere is fundamental for understanding the Earth's climate system and climate change issues.However，the reliable，continuous and long-term record of TSI was not available until November in 1978，when spacecraft NIMBUS7 was launched with an electrical substitution radiometer Hickey-Frieden(HF)on board.The measurement of TSI on the Earth is influenced inevitably by the fluctuations of Earth's atmosphere.The clear evidence of solar irradiance variability is not able to be provided by an observation record on the Earth.It is a problem of atmosphere's impact，but not a problem of obtaining an accurate solar radiometer.This problem is solved by spaceborne experiments of monitoring TSI.Since TSI observations of HF in late 1978，the overlapping time series of TSI have been obtained by different radiometers from various space platforms for more than three decades.The variability of total solar irradiance have been detected on a wide range of time scales，from minutes to the 11 year solar cycle，according to the spaceborne observation records of TSI［7-9］.

2 Development history

Measurements of TSI defining in the International Systems of Units have been provided by spaceborne radiometers［9-10］， as summarized in Tab.1. In Tab.1， Sun-pointing manner refers to how the radiometer points to the Sun.The‘active，independent of spacecraft’in Tab.1，means that the radiometer has solar tracking device which is independent of the spacecraft.Measuring modes of solar radiometer are determined by the Sun-pointing manner.If the spacecrafts do not point to the Sun and the radiometer itself has no solar tracking device，TSI is generally measured via scanning mode［11-13］.In the scanning mode，the solar radiometer can not get measuring opportunities of TSI until sunlight passes its view-field and the observation time of TSI is limited.

Tab.1 Spaceborne radiometers for monitoring TSI

Overlapping measurements are essential to construct a reliable database of TSI，as shown in Fig.1.Variations of solar irradiance for three solar cycles are illustrated in Fig.1.Top part of Fig.1 is the daily average values of TSI from spaceborne solar radiometers since November in 1978.Bottom part of Fig.1 is the Sunspot number.Most of spaceborne radiometers measure TSI based on the principle of electrical substitution，by using cavity detectors as sensors［7，9-10］.Wavelength sensitivity of the cavity detectors is nearly uniform from the UV through the far IR.An open/close shutter unit and precision apertures are generally put in front of cavity detectors to control sunlight input to the cavity.A heater winding with low temperature coefficient is embedded into wall of the cavity detector.The cavity detector is generally connected to a heat sink to minimize change rate of cavity temperature and reduce impact of thermal noise.The principle of electrical substitution is illustrated in short below［7，9］.Electrical power is always applied to the cavity detector to maintain a constant temperature of the cavity.TSI is generally calibrated once in two subsequent phases，observation phase and reference phase.In the observation phase，the shutter is open and sunlight is allowed to pass the precision apertures and fall incident into the cavity.Electrical power is reduced due to the absorbed radiant energy of sunlight.In the reference phase，the shutter is shut and no sunlight is absorbed by the cavity.More electrical power is supplied again to maintain the same temperature of the observation phase.As the electrical power is able to be measured precisely，an accurate measure of the solar irradiance is obtained.

Fig.1 Overlapping measurements of TSI since 1978［9］

2.1 HF on NIMBUS7

The first long-term space mission for monitoring TSI is the Earth Radiation Budget(ERB)experiment on NIMBUS-7 spacecraft［14-16］. Hickey-Frieden(HF)radiometer，an electrical substitution radiometer，was chosen for measuring TSI as shown in Fig.2.Basic sensing element of the HF radiometer for TSI was a cavity like an inverted cone［16］.The cone interior had diffused black surfaces and the surfaces were coated with a reflecting black paint.A calibrated wire-wound resistor was embedded into walls of the inverted cone.The solar sensor was sampled once per second，with an integration period of 0.8 s.Observations for TSI were established with accuracy better than 0.5%.

Fig.2 Side view of HF radiometer［16］

2.2 ACRIM Ⅰ on SMM

The Active Cavity Radiometer Irradiance Monitor(ACRIM I)experiment on NASA Solar Maximum Mission(SMM)spacecraft，as shown in Fig.3，made regularobservationsoftotalsolarirradi-ance［17-19］.Three independent Active Cavity Radiometer(ACR)type IV sensors were introduced for measuring TSI in the experiment.Each ACR sensor was an electrical substitution radiometer.Its schematic is illustrated in Fig.4.Components of the ACR sensor were a shutter，primary aperture，secondary aperture， baffle，detection cavities， heat sink，outer case，electronic unit and etc.Internal surfaces of cavities in the ACR sensors were coated with a specularly reflecting black paint with absorptance of 0.999 5.The ACR sensors viewed the Sun through a circular 5°field of view.

Fig.3 Active cavity radiometer irradiance monitor on SMM

Fig.4 Schematic diagram of ACR type Ⅳ sensors

A spaceborne TSI database had been established by ACRIM I experiment with high precision during 9.75 years.Smaller measurement uncertainty was achieved by ACRIM I than ERB experiment on Nimbus 7.It was attributed to precise solar pointing of SMM satellite and self-calibration capability of ACR sensor.Large numbers of TSI daily observations were provided by full time solar pointing of the SMM satellite.Solar pointing system of SMM failed between November 1980 and early 1984.The SMM satellite ran into a spin mode due to failure of three axis stabilization.Number of daily TSI observations decreased and their measure uncertainty increased due to spacecraft failure of solar pointing.SMM pointed to the Sun again fortunately in April 1984 after reparation by NASA space shuttle.

2.3 ACRIM type radiometer on ERBS

An ACRIM type radiometer was aboard the NASA earth radiation budget satellite(ERBS)in Earth Radiation Budget Experiment(ERBE)，which was launched into a 57 inclination polar orbit in October 1984［20-21］.Primary objective of the ERBE was to detect long term trends in the Earth Radiation Budget(ERB)including earth-reflected shortwave solar irradiance(0.2 －5 μm)and earth-emitted outgoing long wave radiation(5 － 100 μm).TSI was recorded every two weeks to provide calibrations of the earth-viewing radiometers.The sensor for TSI is nearly identical to those in the ACRIM I experiment［20-21］.Solar observations in ERBE were limited the same as ERB on Nimbus 7.However，solar observations were much less than that in ERB experiment on Nimbus 7.Solar observation time was only on the order of minutes during a single orbit.

2.4 ACRIM Ⅱ on UARS

ACRIMⅡwas launched in September 1991 as a science payload of Upper Atmosphere Research Satellite(UARS)［22］.The launch of UARS mission had been delayed several years due to the Challenger accident.It was nearly two years after the end of ACRIMⅠexperiment on SMM.Planned direct onorbit comparisons between ACRIMⅠandⅡwas lost.ACR type sensors were employed for monitoring TSI too.Absolute uncertainty of ACR type sensors for TSI was about 0.1%in the laboratory.The absolute uncertainty of TSI measuring became several times larger in space solar observing.Both ACRIMⅠand ACRIMⅡexperiments were overlapped by ERB experiments on Nimbus 7 and ERBS observations for TSI.The overlapped observations are helpful to make comparisons between multi-databases for TSI on different spacecrafts.

2.5 SOHO on VIRGO

Solarand Heliospheric Observer(SOHO)spacecraft was launched in December 1995，toward the Sun-Earth Lagrange point L1，where continuous exposure to the Sun is possible［23-25］.One of payload on board SOHO satellite was the Variability of solar IRradiance and Gravity Oscillations(VIRGO)package.VIRGO package is shown in Fig.5［23］.SOHO had offered unique observation opportunity for VIRGO experiment to continue monitoring and research on the variability of total solar irradiance.The VIRGO experiment had two active cavity radiometers，DIARAD(Dual Irradiance Absolute RADiometer)and PMO6-V.Block diagrams of PMO6-V and DIARAD radiometer are presented in Fig.6 and Fig.7 respectively.PMO6-V and DIARAD were constructed based on the same concept of electrical substitution［24-25］.However，design and physical implementation of the two radiometers were different.One of the major differences between DIARAD and PMO6-V is configuration of cavities.Both primary cavities and compensation cavities of DIARAD faced to the Sun.Hence，thermal environment for primary cavities were the same with compensation cavities.One advantage of DIARAD is that the compensation cavity was also able to be used for TSI measurements.Compensation cavities of PMO6-V were located in the rear of primary cavities.And they were never exposed to the Sun.

Fig.5 VIRGO package on SOHO［23］

Fig.6 Block diagram of PMO6-V radiometer［23］

Fig.7 Block diagram of the DIARAD radiometer［23］

VIRGO was the first experiment with two radiometers to measure TSI simultaneously.It was not only for redundancy，but also for comparisons and investigation of long-term space behavior for radiom-eters.Initial rapid decrease of measured TSI had been detected in the experiment ACRIMⅠ.It was attributed to degradation of cavity sensors due to continuous exposure to the Sun.From the degradation experience of ACRIMⅠ，spare cavity sensors for TSI were added for two radiometers in the VIRGO experiment.The spare cavity sensors were only exposed to the Sun occasionally for degradation correction of TSI sensors.Initial observation results and findings of VIRGO were successful.However，shuttering system of PMO6-V lost some functions.VIRGO TSI database had several data gaps due to problems of SOHO spacecraft.

2.6 TIM on SORCE

Total Irradiance Monitor(TIM)recorded TSI with high accuracy and precision，resided on SOlar Radiation and Climate Experiment(SORCE)satellite which was launched on January 25，2003［26-28］.The instrument TIM was designed to achieve 100 parts per million(ppm)absolute uncertainty in TSI with a noise level of 10－6，and a long-term relative accuracy of 10－5per year，using state-of-the-art technologies of new materials and modern electronics.The high accuracy allows scientists to observe subtle changes of TSI impacted by the sunspot cycles.

TIM is an ambient temperature radiometer consisting of four side-by-side electrical substitution radiometers(ESRs)，as shown in Fig.8 and Fig.9.Redundancy is provided by the four identical ESRs，and it is helpful to detect changes of cavity sensors due to their exposure to solar radiation.Each pair of the four ESRs is thermally balanced in instrument design and it is able to be used for measuring TSI with any pair alternatively［26］.Cavity absorptivity of each ESR is high across the entire solar spectrum，allowing collection of nearly all the incoming sunlight.Response of servo system maintaining a constant temperature of the cavity is quite quick due to the thermal conductivity.Schematic diagram of the servo system is shown in Fig.10.Sunlight to each ESR radiometer is modulated by a 10-ms-oscillatin shutter［26］.A precision aperture over which sunlight is allowed to pass is located behind each shutter.Area of the precision aperture has been accurately determined.

Fig.8 Total irradiance monitor on SORCE

Fig.9 Cutaway schematic diagram of total irradiancemonitor［26］

Incoming radiant power of the sunlight is then measured based on the electrical substitution principle with the technique of phase sensitive detection.Technique of phase sensitive detection at the shutter fundamental period(10 mHz，100 s period)reduces the instrument's sensitivity to external noise，thermal background and time-varying temperatures.Frequency of the shutter had been carefully computed and selected near the minimum in the noise power density，determined by the thermistors of ESR at high frequencies and by the parameter variations in flight at low frequencies.At the shutter fundamental frequency，the measured on-orbit noise of TIM is much less than the signals of solar irradiance.In the space flight，TIM recorded TSI measurements four times daily.The SORCE spacecraft made TIM point to the Sun during the daylight portion of each orbit.

Fig.10 Servo system schematic diagram of TIM［26］

2.7 SOVIM on ISS

TSI was recorded by SOlar Variability and Irradiance Monitoring(SOVIM)package in SOLAR Experiment on International Space Station(ISS)［29-32］.The SOVIM package was launched in Columbus External Payload Facility by Space Shuttle on February 7，2008.SOVIM contains two type of radiometers.One type is Differential Absolute Radiometer(DIARAD)，developed at the Royal Meteorological Institute of Belgium.The other is PMO6 radiometer，including two PMO6V radiometers and one PMO6R radiometer.Different from DIARAD on SOHO，DIARAD in SOVIM package only has two cavities，not four cavities as that on SOHO.PMO6V in SOVIM shares the same design with that onboard SOHO.PMO6R radiometer is a heritage of SOVA in EURECA mission.

SOVIM was pointed towards the Sun by Coarse Pointing Device(CPD)in space，not by the spacecraft，as shown in Fig.11.It is an advantage of SOVIM compared with other experiments of measuring TSI in the space.CPD has two degree-of-freedom，used for compensation of the ISS orbital motion.CPD rotates around two perpendicular axes to provide a movable frame.CPD includes a digital sun sensor located on the moving frame，two brushless motors and two encoders mounted on each axis，etc.

As independent solar tracking device was introduced for the first time to record TSI，higher accuracy of TSI data had been expected.Solar activities are only observed and recorded by SOVIM when Beta angle is within 23°.The Beta angle refers to the angle between plane of the ISS's orbit and the line connecting centers of the Earth and the Sun.CPD keeps SOVIM package and other payloads pointed toward the Sun during solar observations，about 15 minutes for each 90 minutes orbit time.

Fig.11 Coarse pointing device for SOLAR package on ISS［29］

Since the first switch on of SOVIM in February 2008，CPD did not work well as expected.CPD often failed to track the Sun precisely due to the problems of hardware failure or software settings.Deviation angle between the Sun vector and optical-axis of radiometers was so large for quite a long time that TSI observation data could not be used for science purpose.Reasonable data were not obtained until May，2008，nearly 3 months after the first switch on，with complex tuning of CPD for precise solar tracking.Unfortunately，power unit of SOVIM failed finallyin October， 2008. SOVIM experiments ceased leaving un-continuous TSI record in a quite short term compared with other space mission，such as the VIRGO on SOHO.

2.8 TSIM on FY-3

Experiments of TSI monitoring have been conducted by Total Solar Irradiance Monitor(TSIM)on Feng Yun-3(FY-3)series satellites of China.Feng Yun-3A(FY-3A)launched on May 27，2008 is the first satellite for FY-3 series［33-35］.And Feng Yun-3B(FY-3B)launched on November 5，2010 is the second one.The objective of the experiment TSIM is to provide a long term，high precision TSI database for climate study.The instrument was designed to record TSI with±0.2%long term precision and an SI uncertainty less than ±0.5%.Total Solar Irradiance Monitor(TSIM)on FY-3A satellite have recorded TSI daily from June 2008 until now.FY-3 TSI database is further expanded by the overlapping observations by another TSIM on FY-3B satellite from November 2010 to the present.Radiometers TSIM on FY3-A and TSIM on FY-3B share the same design.TSIM is consisted of three absolute radiometers AR1，AR2 and AR3，as shown in Fig.12 and Fig.13.The three absolute radiometers are aligned with different inclinations to obtain a bigger field-of-view.Each absolute radiometer operates on the principle of electrical substitution.A cavity sensor is used for detecting TSI.In front of the inverted cavity there is a 0.5 cm2precision aperture，and its area had been calibrated.Resistors had been carefully encased into the walls of cavity detector to obtain the same heating effect due to the incoming sunshine.

Fig.12 Three absolute radiometers in the TSIM

Fig.13 Detector head of TSIM

Redundant absolute radiometers were employed to calibrate optical degradation of internal area for the cavities.The three radiometers share the same units of electrical substitution，data sample，communication，etc.FY-3A and FY-3B are Sun-synchronous polar orbit satellites，which do not point to the Sun accurately.As a result，TSIM works with scanning manner.TSIM are all fixed on the leading surface of the satellite.Solar observations were performed by the radiometers only when the satellite traverses the terminator near southern or northern extension of the orbit and the sun swept through 18.4°field-of-view of TSIM.Absolute radiometers AR1 and AR2 operate all the time for daily observation of TSI，and radiometer AR3 only works occasionally for degrade corrections of cone cavity due to ultra-violet radiation of the Sun.Shutters of absolute radiometers AR1 and AR2 were regulated to open or close every 120 s.

The experimental results have indicated that solar observation time was limited to about 6 minutes in each 110 min orbit time.While sunshine was not available，the TSIM radiometer views cold space as irradiance reference for compensation.The measuring uncertainties for TSI are derived from both offaxis angle calibrations and temperature dependent calibrations.

3 Conclusion and prospect

Precious time series of total solar irradiance have been obtained by spaceborne solar radiometers for the past three decades.Sound progresses have already been achieved for accurate recording TSI in the space.In addition，long-time TSI records of higher accuracy are still expected by societies of solar physics，Earth's climate，etc.Moreover，so far as the spaceborne missions of measuring TSI is concerned，it is more than just developing an accurate solar radiometer alone.From experience of the past space missions，major uncertainty sources for TSI measuring are the Sun pointing errors of radiometers，temperature fluctuation inside the radiometers，degradation of cavity sensors，etc.Corrections for these uncertainties are generally complex，time-consuming and hard for implementation.Even correction large than solar irradiance itself are probably introduced by the corrections.Angle between sun vector and optical axis of the radiometer known as the Sun pointing error or off-axis angle is generally not zero in the scanning mode.The Sun pointing error has been recognized as a major error resource of TSI record.Active solar tracking device which is independent of spacecraft has been introduced to make the Sun pointing error become zero.It is hard to track the Sun accurately and reliably in the space by experiment package of measuring TSI and little experience is available now，for example，the unexpected failure of CPD for SOVIM on ISS.

It is still challenging to maintain and construct continuous，reliable，accurate，long-term TSI database.It means less Sun pointing error，stable temperature of instrument and cavity sensors of lower degradation.Active solar tracking device which is independent of spacecrafts and the unit of precise temperature control should be developed and integrated into spaceborne radiometers for measuring TSI in the future to achieve higher measuring accuracy.Development of cavity sensors with lower degradation to solar irradiance remains a challenging problem in the future.

At least two independent experiments operating simultaneously in space are still desired to produce a reliable time series of TSI in the future.However，TSI measuring and its missions in space are affected by launch accidents，financial support and other unpredictable factors，such as the launch failure of GLORY with TIM onboard in 2010［36］，several years delay of ACRIM Ⅱ experiment on UARS.More space missions for monitoring TSI are expected and implemented to maintain a continuous，reliable，long-term TSI database and the precious climate record，for science and humankind in the future.

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