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Solar Physics (2004) 224: 237–246

Springer 2005

ABSOLUTE ACCURACY AND REPEATABILITY OF THE RMIB RADIOMETERS FOR TSI MEASUREMENTS SABRI MEKAOUI, STEVEN DEWITTE, DOMINIQUE CROMMELYNCK, ANDRE CHEVALIER, CHRISTIAN CONSCIENCE and ALEXANDRE JOUKOFF The Royal Meteorological Institute of Belgium, Ringlaan 3, B 1180 Brussel, Belgium (e-mail: [email protected])

(Received 2 September 2004; accepted 15 October 2004)

Abstract. Radiometers for the measurement of the total solar irradiance from space have been developed by D. Crommelynck at the Royal Meteorological Institute of Belgium (RMIB) since 1970. These radiometers, also used in the set of instruments defining the “World Radiometric Reference”, have successively flown on SPACELAB 1, ATLAS 1, EURECA, ATLAS 2, ATLAS 3, SOHO and HITCHHIKER, among them the SOLCON radiometer on the last STS107 flight. Other radiometers are under construction or integration for the upcoming missions: one as part of the SOVIM experiment on the International Space Station and SOVAP on the PICARD micro satellite. These radiometers have a symmetrical side by side cavity design. They are operated using the differential active cavity radiometric principle. In order to construct a long time series and to detect any variation between two cycles, it is important to know all the instrument parameters and make a clear distinction between what will lead to uncertainty on an absolute level and what guarantees the repeatability of the measurements. The purpose of this paper is to review these parameters; after a description of its principle, the instrumental equation will be established. The electrical parameters and the determination of remaining instrumental parameters are done during the characterisation phase at the RMIB. The results from the DIARAD/VIRGO radiometer on SOHO are presented. The repeatability of the measurements is found to be of the order of 100 ppm after correction of the in-flight aging. The accuracy on an absolute level is about 1.06 W m−2 for DIARAD left cavity. This accuracy is limited by the characterisation accuracy with a dominant role of the precision aperture area determination. The understanding of the behaviour of the DIARAD type instrument allows improving the design of the new radiometers on the upcoming missions.

1. Introduction To derive the most accurate total solar irradiance (TSI) measurements it is important to understand the error sources of the measuring instrument in order to try to apply appropriate corrections and to quantify the remaining uncertainty from the basic electrical power measurements. These corrections are used to deliver the final scientific TSI data set. Any misunderstanding or neglecting of an error source could lead to a trend or even a discontinuity in the TSI time series. In the present paper we proceed as follows: in Section 2 we describe the radiometers built at the Royal Meteorological Institute of Belgium (RMIB), in Section 3 we explain the measurement principle. In Section 4 we estimate the absolute accuracy from the instrumental equation whereas in Section 5 the repeatability is treated as

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a separate point. Finally we give the value of the accuracy and repeatability of the TSI data.

2. Description of the Radiometers The DIARAD/SOLCON type radiometers (Crommelynck, 1973, 1975, 1982; Crommelynck and Domingo, 1984; Crommelynck and Dewitte, 1999) is a differential absolute solar radiometer developed at the Royal Meteorological Institute of Belgium (RMIB). It is the first absolute radiometer operated in space which is based on a full symmetrical metrological design (two side by side cavities) and operation (successive opened and closed state of the measurement cavity). These radiometers are also the first to be completely characterized for operation at the ground in air. The RMIB maintains a set of these instruments for periodic comparisons at the ground as its “reference scale”. The instrument can be described by three parts: the detectors, the electrical measurement chains with their reference voltages and the servo loop system. – The detectors: the detectors are formed by two blackened cylindrical cavities mounted side by side on a common heat sink. Between each cavity and the heat sink a heat flux transducer and a heating resistor are mounted. Both cavity channels are equipped with a shutter in front to block or admit light to the cavity through a precision and a limiting aperture. Finally an optical baffle separates both channels from the other channel aperture, both channels see the same common angular limiting volume or baffle. It is imperative that the whole system be insulated from the outside and absolutely symmetric (Figure 1). – The electrical chains: every heating resistor is mounted in series with a current measuring resistor. The voltage over the four resistors is digitized through four electrical measurement chains. Every electrical chain is composed of a

Figure 1. Main features of the DIARAD radiometers.

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Figure 2. The electrical chains measurement for DIARAD/VIRGO, the counters are not represented.

differential amplifier, a voltage to frequency converter and a counter, see Figure 2. For the calibration of these electrical chains, DIARAD radiometers use highly accurate reference voltages derived with a precision resistor ladder fed by a highly accurate temperature-stabilized reference voltage. For the SOVIM (solar variability and irradiance monitoring) and SOVAP (solar variability on Picard) case, DIARAD uses six levels of the reference voltage and six for the current for the in-flight measurement of the function that relates digital count to voltage. – The servo loop system: equilibrium between the two cavity heat fluxes is maintained by regulating the electrical power in one of the two cavities using an analog proportional–integrator servo system.

3. Measurement Principle DIARAD operates similarly as a “household scale” and thus different radiometric states can be used. They enable to verify the coherence of the instrument. In the default measurement sequence a constant electrical power is fed into one cavity, the “reference” cavity, while its shutter remains closed. The electrical power in the other cavity, the “measurement” cavity, is regulated continuously, while its shutter sequentially opens and closes (both open and closed states take 90 s). When the instrument is pointed to the Sun, the equilibrium electrical power in the measurement

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Figure 3. Schematic representation of the different powers involved during the opened (right) and closed state (left).

cavity drops proportionally to the absorbed solar power when going from the closed to the open state. – Closed state: both shutters are closed, a reference power Pref is dissipated through the heating resistor of the reference cavity. Pref = V I , where V is the applied voltage and I is the current across the heating resistor. A heat flux is detected by the sensor. As a consequence, the servo loop system dissipates a power Pclosed in the measuring cavity (right) through its heating resistor. This state is represented in Figure 3 left. The servo system brings Pclosed to equilibrium with Pref with a systematic error Peq , Peq + Pref = Pclosed .

(1)

– Open state: the active shutter opens. If we suppose the cavity perfectly black and without any thermal loss, an absorbed solar power A · SI is detected by the heat flux sensor where SI is the solar irradiance and A the surface of the precision aperture. The optical baffle prevents scattered light from falling into the reference cavity. The servo system takes 90 s to reduce the dissipated electrical power in the active cavity Popen and to bring it to equilibrium with the reference power Pref , Peq + Pref = Popen + A × SI.

(2)

From the opened and closed state we have, if we assume the stability of Pref and Peq , a first approximation of the value of the solar irradiance, depending on the effective sensitivity of the exposed channel: SI ≈ (Pclosed − Popen )/A.

(3)

The equilibrium error Peq can be subject to changes. On the other hand the closed and opened state do not occur at the same time. It is therefore necessary to have an


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estimation of Pclosed during the open state. This is done by averaging the power of two closed states occurring before and after the open state. 4. Absolute Accuracy Equation (4) is modified to take the remaining instrumental parameters into account: SI =

1 (Pclosed − Popen + P)/ cos θ. kαabs αth A

(4)

These parameters and their estimation are described in the following: – A: the precision aperture area in front of the shutter. The area should be chosen as large as possible in order to be able to measure it with a high relative accuracy. For DIARAD left, the mean measured diameter of the precision aperture area is 8.00325 mm with a 95% level uncertainty of 0.5 µm, and with a departure from roundness1 of 0.6 µm (Chevalier, Conscience, and Lombaerts, 1995). Thus the resulting total absolute uncertainty on the diameter is 1.7 µm. Therefore the resulting relative uncertainty of A is 425 ppm. – αabs : the absorption factor of the cavity covered with diffusive black paint. From the measured black paint absorptivity, taking into account the known geometry of the cavity, we can calculate αabs = 0.999809 with an uncertainty of 40 ppm. αabs is systematicaly measured for each channel as well as αth that is measured in air and vacuum. – αth : the thermal efficiency factor of the cavity. αth is defined as the fraction of the total heat dissipated in the cavity that flows trough the heat flux sensor at the bottom of the cavity related to the incident radiation. The remaining part escapes trough the side walls of the cavity. This remaining part is proportional to the part of the incoming radiation that is reflected on the side walls, and hence with the reflection factor of the black paint. For the DIARAD left cavity, in vacuum, the measured αth = 0.997338 (Chevalier, Conscience, and Lombaerts, 1995; Crommelynck, Fichot, and Bauwens, 1995). For the DIARAD right cavity, in vacuum, the measured αth = 0.995918. Different values are used when operating instruments in air. The uncertainty of αth is dominated by the uncertainty of the black paint absorption factor. The latter is equal to 0.970 with an uncertainty of 0.005, leading to an uncertainty of 150 ppm of αth . – P: the internal thermal emission of the shutter. When the shutter is closed it irradiates some energy into the cavity. When the shutter is opened this energy is removed. P is the term which takes this effect into account. The relative magnitude of P is around 600 ppm for DIARAD/VIRGO but reduces to 60 ppm for DIARAD/SOVIM due to a low thermal emission coating. Moreover, 1 The departure from roundness is defined as the difference in radii of two coplanar concentric circles,

the annular space between which just contains the profile of the surface measured.

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the value of P is measured during deep space calibrations. During this phase, Equation (5) becomes: 0=

–

– –

–

1 (Pclosed − Popen + P)/ cos θ. kαabs αth A

(5)

We deduce P from the electrical power determination according to Equation (5). k: total correction factor for remaining effects. These effects are: the diffraction and scattering around the two apertures in front of the cavity and the precision of the servo system. The servo system does not reach its equilibrium value after 90 s. A systematic correction Peq is applied with an uncertainty of the order of 30 ppm. The experimental determination of the diffraction and scattering errors around the apertures is difficult because these effects are relatively small (Crommelynck, 1982). For the DIARAD left cavity, k = 0.998502. For the DIARAD right cavity, k = 0.998594 (Fichot, 1993). The uncertainty on k is at least 30 ppm without taking into account the diffraction and scattering effects. θ is the solar pointing angle. When θ = 0, the sunlight enters straight into the cavity. For θ = 0, the radiation received in the cavity varies as cos θ . Electrical power determination: accurate electrical power measurements are obtained by separate measurement of the voltage over and the current through both cavity heating resistors. The current I through the heating resistor is derived from the voltage measurement U I thanks to an accurate measurement resistor, Rmeasurement placed in series with the heating resistor. As a consequence, I = U I /Rmeasurement . The two voltages over the heating resistors UV , and the two voltages over the measurement resistors U I , are measured simultaneously with four electrical measurement chains, see Figure 2. Therefore, the electrical power is P = UV U I /Rmeasurement . The heating resistor wires contribute to some parasitic electrical power dissipation detected by the heat flux detectors which could give rise to some systematic error (Crommelynck, 1982). The uncertainty of P is then reduced to an uncertainty of two voltage measurements (UV , U I ), one measuring resistor determination (Rmeasurement ) and the uncertainty of the resistor wires effect. These values are detailed in the following. Electrical calibration: every three minutes, the electrical measurement chains are calibrated using a temperature-stabilized, high-accuracy reference voltage source. The electrical calibration allows the determination of the function that relates the counts to the voltages. This function is estimated with three (DIARAD/VIRGO) or six levels (SOVIM) of the reference voltages using a least squares fit. The residual error between the estimated function and the measurements is of the order of 60 ppm peak-to-peak for DIARAD/VIRGO. The resulting uncertainty of the electrical power is up to 131 ppm (60 ppm


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for UV an U I , 5 ppm for Rmeasurement and 6 ppm due to the heating wire effect.) The ‘solar constant’ or total solar irradiance (TSI) is defined as the solar irradiance at the mean Earth–Sun distance, i.e., 1 Astronomical Unit (A.U.). It is given by r 2 dr TSI = SI 1+2 c , (6) 1 A.U. dt where r is the distance between the satellite and the Sun and c the speed of light. /c corrects for the Doppler-shift effect due to the The factor (1− dr1 /c)2 ≈ 1 + 2 dr dt dt moving satellite (Fröhlich et al., 1997). The different parameters of Equation (4) are determined during the characterisation phase at the Royal Meteorological Institute of Belgium with a dominant uncertainty for the determination of the precision aperture. The resulting relative uncertainty of the TSI measurement is of the order of 776 ppm when adding the different contributions detailed previously and 471 ppm RMS. It corresponds to an absolute uncertainty of the irradiance of the order of 1.06 and 0.64 Wm−2 RMS for a nominal value of 1368 Wm−2 and without taking into account the diffraction and scattering uncertainties. 5. Repeatability of the Measurements The repeatability of the measurement is better than the absolute uncertainty. The repeatability is limited by two main elements : the stability of the reference voltage for the calibration of the electrical measuring chains and the correct monitoring of the in-flight aging effect. 5.1. STABILITY

OF THE REFERENCE VOLTAGE

Throughout the mission the ratio between the two references of VIRGO has remained stable within the measurement resolution of 24 ppm. For additional verification, 10 references of the same fabrication have been measured on the ground regularly since 1993. The largest measured peak to peak difference is 165 ppm over 8 years. The repeatability over that time period of the electrical measurements is then of the order of 100 ppm. Figure 4 represents the relative error that would be made on the measurement of the solar irradiance in the case that one would assume that the gains and offsets of the DIARAD/VIRGO amplifiers (i.e. the function relating the counts to the voltages) would remain equal to their initial value and without electrical calibration. In fact DIARAD is continuously corrected for this effect (160 ppm over 8 years). On the other hand, the separate measurement of the current and voltage allows the calculation of the heating resistor. This value

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Figure 4. Relative error on left channel irradiance without electrical calibrations.

Figure 5. The time variation of the daily mean value of the heating resistor of the DIARAD/VIRGO left cavity for left-side measurement days: “resistor.left.close”: resistor measured in closed state, corresponding to voltage level of 3.5 V; “resistor.left.open”: resistor measured in open state, corresponding to voltage level of 2 V. “resistor.left.model”; resistor modelled as a function of temperature, according to preflight measurements.

compared to a pre-flight model is used as quality control. This feature is represented in Figure 5. Moreover, a continuous measurement of resistors values after every SOLCON flight provides confidence in regard to an unlikely drift of resistors values.


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Figure 6. The total solar irradiance measured by the DIARAD right channel minus the one measured by the left channel. The difference is set to zero at the beginning of the mission. An unexplained difference of the order of 4 Wm−2 exists between both channels (Dewitte and Crommelynck, 2004).

5.2. AGING

EFFECT

DIARAD routinely measures the solar irradiance with its left channel. Exposure to the Sun may cause an aging of the black paint and affects the thermal efficiency of the cavity. To verify and correct this trend on the left cavity, measurements with the right cavity are made from time to time. As the exposure of the right cavity is lower, its solar induced aging is very small but at long duration a similar correction is applied. Figure 6 compares the right and left channel measurements of DIARAD/VIRGO. Without aging monitoring, a trend of 0.5 Wm−2 over 8 years would have occurred on the TSI measurements (Dewitte and Crommelynck, 2004).

6. Conclusion In this article we have presented the RMIB radiometer design. After a detailed analysis of the different uncertainties and their quantification, it is found that accurate TSI observations must be accompanied with accurate electrical power measurements and crosschecking capabilities. For DIARAD/VIRGO, the relative uncertainties of the electrical power are of the order of 131 ppm. For DIARAD, the absolute accuracy of the TSI measurements is of the order of 1.06 and 0.64 Wm−2 RMS without

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taking into account the diffraction and scattering uncertainties. The repeatability of the measurement is of the order of 100 ppm. The stability of the reference voltages is the main hypothesis. The aging effect is corrected by comparison between the channels of the radiometers. These values represent our current knowledge of the DIARAD type radiometer. Better achievement is foreseen for the upcoming missions.

Acknowledgements This work was supported in the beginning by the “Fonds de la Recherche Fondamentale d’initiative Ministerielle” followed by the support of the Belgian Science Policy office (DWTC-SSTC) through the ESA PRODEX program.

References Chevalier, A., Conscience, C., and Lombaerts, M.: 1995, Results from the Characterisation of the CR14 and CR15 Absolute Radiometer for the VIRGO Experiment, RMIB Radiometry Technical Note, pp. 107–108. Crommelynck, D.: 1973a, Considerations sur les différents types de zéros et la dissymétrie d’un pyrhéliométre a` compensation, IRMB. Pub. series. Pub. A, p. 3. Crommelynck, D.: 1973b, Instrumental Theory in Absolute Radiometry, IRMB. Publication Series Publ. A, p. 81. Crommelynck, D.: 1975, Considerations relatives au degré de comparabilité des radiomètres, methodes de mesure et contraintes de précision, IRMB. Publication series. Pub. A, p. 89. Crommelynck, D.: 1982, Fundamentals of Absolute Pyrheliometry and Objective Characterization, NASA. Conference. Pub. 2239, p. 53. Crommelynck, D. and Dewitte, S.: 1999, Adv. Space. Res. 195–204. Crommelynck, D. and Domingo, V.: 1984, Science 180–181. Crommelynck, D., Fichot, A., and Bauwens, F.: 1995, Determination of the Efficiency Factor of the Radiometric Cavities – Correlation with the Absorbtivity of the Paint and the Absorbtivity of the Cavities, RMIB Radiometry Technical Note, p. 119. Dewitte, S. and Crommelynck, D.: 2004, J. Geophys. Res. 109, A02102. Fichot, A.: 1993, Algorithm for Calculation of the Solar Constant Used for SOLCON and SOVA Instruments, RMIB Radiometry Technical Note, p. 100.