1-1 CF_NewRad2005.pdf - PMOD/WRC

Absolute Accuracy of Total Solar 

Irradiance Measurements from Space 

Claus Fröhlich 

Physikalisch-Meteorlogisches Observatorium Davos, World Radiation 

Center, CH 7260 Davos Dorf, Switzerland 

eMail: cfrohlich@pmodwrc.ch; http://www.pmodwrc.ch 

Report on Preliminary Results from the TSI Workshop at 

NIST, 18-20 July 2005 

10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 1

Participation in the TSI WS 

Barnes, Robert A., SAIC, Beltsville, MD 

Bloom, Hal, NIST, Gaithersburg, MD 

Bosworth, John M., Swales Aerospace, Beltsville, MD. 

Butler, James J., EOS NASA/GSFC, Greenbelt, MD. 

Cahalan, Robert F., NASA/GSFC, Greenbelt, MD 

Crommelynck, Dominique, IRMB, Brussels, Belgium 

Datla, Raju, NIST, Gaithersburg, MD 

Dewitte, Steven, IRMB, Brussels, Belgium 

Floyd, Linton, Interferometrics/NRL, Washington, DC 

Fowler, Joel, NIST, Gaithersburg, MD. 

Lawrence, George M., LASP, Boulder, CO 

Lee III, Robert Benjamin, Hampton Va 

Litorja, Maritoni, NIST, Gaithersburg, MD 

Lorentz, Steven R., Standards and Technology, Inc., 

Ijamsville, MD. 

Lykke, Keith, NIST, Gaithersburg, MD 

Morrill, Jeff, Naval Research Laboratory, Washington, DC 

Ohno, Yoshi, NIST, Gaithersburg, MD 

Pankratz, Christopher K., LASP, Boulder, CO 

Pap, Judit, NASA’s GSFC, Greenbelt, MD 

Parr, Albert, NIST, Gaithersburg, MD 

Rabin, Douglas M., NASA’s GSFC, Greenbelt, MD. 

Rager, Amy, Catholic University of America, Clarksville, MD 

Rice, Joe, NIST, Gaithersburg, MD 

Rottman, Gary, LASP, Boulder, CO. 

Shirley, Eric, NIST, Gaithersburg, MD 

Sparn, Thomas P., LASP, Boulder, CO 

Willson, Richard C., Columbia University, Coronado, CA. 

Yoon, Howard, NIST, Gaithersburg, MD 

Fox, Nigel, National Physical Laboratory, Teddington, Middlesex UK 

Fraser, Jerry, NIST, Gaithersburg, MD. 

Fröhlich, Claus, PMOD/WRC, Davos Dorf, Switzerland 

Helizon, Roger S., JPL, Pasadena, CA. 

Heuermann, Karl, LASP, Boulder, CO 

Johnson, B. Carol, NIST, Gaithersburg, MD 

Jordan, Stuart, NASA’s GSFC, Greenbelt, MD 

Kirk, Megan, Catholic University of America, Clarksville, MD 

Kopp, Greg, LASP, Boulder, CO 


Presentations at the TSI Workshop 

Monday, July 18 

Tuesday, July 19 

Satellite Instrument TSI Measurement Uncertainty: Chair G. Kopp 

Welcome and Meeting Charge J. Butler 

Session 1 Goals G. Kopp 

ACRIM I, II & III R. Willson 

ACRIM R. Helizon 

TIM on SORCE G. Kopp 

Wednesday, July 20 

Satellite and Ground-based TSI Instrument Comparison: Chair: R. Willson 

VIRGO/DIARAD flight performance and degradation S. Dewitte 

VIRGO/PMO6-V flight performance and degradation C. Fröhlich 

ACRIM 1, 2, & 3 flight comparisons, performance, and degradation 

R. Willson 

PMO6V on VIRGO/SoHO C. Fröhlich 

The DIARAD type instruments, principles and error estimates 

D. Crommelynck & S. Dewitte 

ERBE on ERBS R. Lee 

Disscussion and Session wrap-up G. Kopp 

Laboratory-based Comparison and Characterization 

Chair: J. Rice 

Aperture Area Comparison Results C. Johnson 

Diffraction Effects E. Shirley 

Applications of Cryogenic Radiometry S. Lorentz 

NIST Cryogenic Radiometer Intercomparison J. Rice 

Lab Solar Irradiance Scale Comparisons C. Fröhlich 

Discussion and Session wrap-up 

SORCE/TIM flight comparisons, performance, and degradation G. Kopp 

Shuttle TSI flight comparisons, performance, and degradation S. Dewitte 

ERBS/ERBE flight performance and degradation R. Lee 

Discussion and summary of flight performance and degradation 

JPL/TMO ground comparisons and significance for flight TSI results 

R. Willson 

Workshop closing comments J. Butler 

Discussion and summary of ground comparisons 


Why was the Work Shop Organized 

Until the launch of SORCE 

and the operation of TIM we 

all thought that we have 

improved the uncertainty from 

the early time of HF and 

ACRIM I. 

The composite shows how 

small the variation is 

compared to the spread of the 

data. It shows also that the 

precision is much higher than 

the absolute accuracy which 

allows to construct such 

composite with overlapping 

measurements. 


Operation of Room-Temperature 

Radiometers 

Operation 

The classical electrically calibrated radiometers are operated in either passive or active 

mode. Both modes have a measurement and reference phase which is determined by an 

open or closed shutter. In the passive mode the electrical heater is switched on when the 

shutter closes and its power is adjusted to produce about the same heat flux as the radiation 

seen during the measurement phase. In the active mode control electronics maintains the 

heat flux constant during both phases which are now alternated in a regular pace (e.g. 1-min 

open and 1-min closed). 

With the open and closed electrical power P dissipated in the cavity, the irradiance S equals 

with C Rad the radiometer constant 

For an ideal radiometer C Rad = 1/A with A the area of the precision aperture in front of the 

cavity. Deviations from the ideal behaviour are accounted for replacing 1/A by C corr /A with 

C corr determined for each individual radiometer by its characterization. 


Different Types of 

Room-Temperature Radiometers 

Characterization 

The determination of the deviations from ideal behaviour is called 

characterization. The following effects are determined by independent 

experiments: Non-equivalence between electrical and radiative heating 

determined from air-to-vacuum ratio, reflectivity of the cavity, scattered light 

and lead heating, 

and by numerical calculations: Diffraction 


VIRGO Radiometers 

Below is a block diagram of PMO6V (left) and a picture of DIARAD and PMO6V of 

VIRGO 


Characterization of 

Room-Temperature Radiometers 

Results of a typical characterization 

As an example the results for the characterization of PMO6 type radiometers is 

shown from Brusa & Fröhlich (1986). 


Aperture Measurements 

Area of the Precision 

Aperture 

An important part of the 

characterization is the 

determination of the area of the 

precision aperture. In order to 

keep the radiometer small with a 

reasonable time constant the 

aperture is nominally 5 mm or 8 

mm in diameter. The land of the 

aperture has to be as small as 

possible in order to prevent lightpipe 

effects due to the finite 

angular extent of the Sun and 

during slightly off-pointed 

conditions. Typically it is of the 

order of a couple 10 µm. This 

does not allow to measure the 

diameter by physically touching. 

Thus optical measuring methods 

have to be applied which are in 

general less accurate. 

Results from the presentation of C. Johnson 


Effect of Diffraction 

Diffraction has to be included as a correction and Eric 

Shirley from NIST did the calculations for all radiometers 

involved 


Uncertainty PMO6-9 and 11 


World Radiometric Reference and SI 

From a thorough discussion of the results of comparison of many ECRs with the sun as source 

at PMOD/WRC the Word Radiometric Reference has been defined and adopted by WMO in 

1975 and a group of radiometers identified to materialize it, the World Standard Group. The 

WWR is what is called in metrology a conventional reference as it has a much higher 

repeatability than its SI uncertainty of 0.3%. 


Comparison of WRR and SI, 1 of 2 

Comparison with cryogenic radiometers via trap detectors at NPL have shown 

that the WRR is within ±0.02% of the SI scale (Romero et al., 1991, 1996), which 

is fortuitous, but practical. These comparison are made with under-filled 

apertures and thus need some care about the performance of the under-filled 

radiometer relative to the one as measuring irradiance. Recently, such 

comparison were repeated with both rocket radiometers at NPL and at the Swiss 

Metrological Institute, METAS. The result, however, is still preliminary (see 

Poster 16). So we will only use the results of the 1991/96 comparison only. 

However,these need some corrections of the originally published values. The 

diffraction argument was incorrect and the new aperture areas as re-measured 

by NIST shall be used. Moreover, we need to apply a correction for the 

difference of the non-equivalence as determined from the air-to-vacuum ratio 

under-filled and normal. Then the results for 1991 and 1996 become 1.000557 

0.001326 and 0.999757 0.000948, respectively, with a weighted average and 

1 uncertainty of 

1.0000910. 0.000864 


Comparison of WRR and SI, 2 of 2 

Due to the small land of the precision apertures, the innermost part has a very 

short thermal time-constant and starts heating up when the shutter is opened. 

Thus the cavity receives some extra IR radiation and in air also some energy 

through air conduction. Although it is a rather small effect for the PMO6 type 

radiometers we need to correct for it. We estimate the effect from preliminary 

results of model calculations which indicate that the IR influence is of the same 

order as the one due to the exchange in air. So the IR effect is proportional to 

the air-to-vacuum ratio difference of 0820 ppm for 6-9 and 545 770 ppm for 6-11. 

Thus, a value of about 200~ppm is assumed and the final value of the WRR/SI 

becomes 

with a 95% uncertainty of 0.16%. 

0.999891 


Corrections for PMO6V Values 

The radiometric constants have to be changed due to a new 

evaluation of the air-to-vacuum ratio and the tracing to WRR and 

SI. 

The level-1 ratio A/B is now in space 633 ppm higher than on 

ground. This difference can be easily explained by the 

uncertainty of C NE , which is for each radiometer of the order 

1000 ppm (95% uncertainty). 

The new calibration factors change the values by 1314 ppm and 

1254 ppm for A and B, respectively. The difference is mainly due 

to the inclusion of the diffraction correction in the radiometric 

constants for the comparison with the WRR. The remaining 134 

and 74 ppm are due to changes of the re-evaluated air-tovaccum 

ratio. 

Finally the WRR/SI conversion has to be added and the 

presently adopted PMO6V values are increased by 1363 ppm to 

yield SI values 


Uncertainty of PMO6V WRR/SI 

Uncertainty of the PMO6V WRR/SI traceability @ 1400W/m2 

Component Value u c (u*c)^2 

Area N/A 

Pclosed 45 mW 0.0000045 5.00E+04 0.050625 

Popen 17 mW 0.0000017 5.00E+04 0.007225 

CNE 1 5.00E-04 1.40E+03 0.49 

CR N/A 7.00E-05 1.40E+03 

CSt N/A 1.00E-04 1.40E+03 

CLH N/A 3.00E-05 1.40E+03 

CApH N/A 5.00E-04 1.40E+03 

Cdiff N/A 1.00E-04 1.40E+03 

WRR-Factor 1 6.00E-04 1.40E+03 0.7056 

WRR/SI 1 9.00E-04 1.40E+03 1.5876 

2.84105 

Uncertainty abs 1.6855 W/m2 

Uncertainty rel 1685.5 ppm 

95% Uncertainty 3371.1 ppm 


Final values of VIRGO TSI at the 

Beginning of the Mission 1 of 2 

DIARAD is fully characterized and represents its own independent 

radiometric scale. However, some corrections due to time constant 

effects and the area have been determined since the first evaluation; 

they amount to an overall correction of + 447 ppm. With these 

corrections the final ‘first light’ data of VIRGO are very consistent 

(within about 300 pmm), although their tracing to SI is completely 

independent. 


Final values of VIRGO TSI at the 

Beginning of the Mission 2 of 2 

If we compare this result 

with the internal consistency 

of the IRMB radiometry it 

may be fortuitous. These 

comparisons may also give 

a hint that the uncertainty 

estimate of the IRMB may 

be too optimistic. On the 

other hand, the general 

picture indicates that the 

tracing to Si via WRR is 

within the stated 95% 

uncertainties of 0.3%. 


Uncertainties, Correction Factors and 

‘First Light’ Comparison of the Radiometers 

in Space 


Comparison of corrected TSI 


What’s next ? 

It is rather obvious that power comparisons of representative 

spares against calibrated traps would help to locate the reason 

for the still unresolved difference. In contrast to the WRR/SI 

comparison at NPL they will be done with the radiometers in 

vacuum. NIST is prepared to conduct them in early 2006. As the 

aperture area is no longer an issue, power comparisons are 

sufficient. However, the implications of under-filling the apertures 

should be studied in detail by each radiometer involved in the 

comparisons. 

There are also plans to conduct comparisons with the Sun as 

source at Table Mountain. 

We hopefully will know more in summer 2006. 


Conclusions 

The differences are still not resolved and the reason is unknown. 

It does not seem to be due to aperture areas, as it was most likely 

during the early measurements of TSI from space. 

The good agreement between PMO6V and DIARAD-L indicates that 

the WRR is most probably close to the SI as indicated by the WRR/SI 

comparisons. 

There may still be a problem with the WRR/SI comparison: the WRR 

factor of PMO6-9 and 11 are of the order of 1.004258 and 1.002751, 

indicating that the characterized PMO6 radiometers are about 0.35% 

below the WRR (similar to CROM-2L). 

Because the WRR/SI comparison is only based on the PM6-9 and 11 

radiometers, new tests can always be performed as e.g. the area of 

the precision aperture could be corrected. With radiometers launched 

to space this is obviously no longer possible and we have to rely on 

the similarity of the spares. 


You can find this presentation at 

ftp.pmodwrc.ch/pub/claus/newRad2005/CF_NewRad2005.ppt 

End 


Air-to-Vacuum Ratio (Non- 

Equivalence) 

5000 

4500 

4000 

3500 

3000 

2500 

2000 

Non-Equivalence PMO6_09 

5000 

4000 

3000 

Non-Equivalence [ppm] 

0 2 4 6 8 10 

2000 

1000 

0 

-1000 

Aperture Diameter [mm] 

Non - Equivalence PMO6_11 

3600 

3100 

2600 

2100 

1600 

1100 

Non-Equivalence [ppm] 

2 3 4 5 6 7 8 9 

Aperture Diameter [mm] 

The difference between the 

under-filled and illuminated 

aperture (8.5mm) sovim1 is 0820 ppm 

for 6-9 and 545 sovim2 770 ppm for 6- 

11. 

Non-equivalence [ppm] 

0 5 10 15 20 25 

Illuminated area [arbitrary units] 

r-111 

r-113 

sovim1 optik 

virgo2 

sovim1(gold mirror) 

sovim2(gold mirror) 

sovim1(g-plättli) 

sovim2(g-plättli) 

sovim13 


Aperture heating 

60/60 sec operation Case 4 

Case 2: Contact to heatsink only at nodes 1611,1711,1811 with 0.1 WK-1 each; 

Case 500 2: Contact to heatsink only at nodes 1611,1711,1811 with 0.1 WK-1 each; 

air conduction to cavity with 2.2 Wm-2K-1; 

Diff to model 

air conduction to cavity with 2.2 Wm-2K-1; 

450 

-0.08 % 

Diff 

total 

to 

input 

model 

flow: 22.080 

-0.08 

mW 

% 

400 

total 

total 

GF 

input 

to 

flow: 

99998 21.411 

22.080 

mW 

mW 

Total 

total 

350 

GF 

GR 

to 

to 

99998 

99998 &99999 

21.411 

0.652 mW 

mW 

300 

Flow 

Total 

to 

GR 

cavity 

to 99998 

by radiation 

&99999 

317.0 

0.652 

ppm 

mW 

Flow 

Flow 

to 

250 to 

cavity 

cavity 

by 

by 

radiation 

air conduction 500.6 

317.0 

ppm 

ppm 

Flow to cavity 

200 Total 

by air conduction 

817.6 

500.6 

ppm 

ppm 

Total 817.6 ppm 

150 

Averages for 8.5 mm and 60/60 s operation rad/air: 120.3 185.9 ppm 

Averages 

Averages 100 

for 

for 

8.5 

8.5 

mm 

mm 

and 

and 

660/360 

60/60 s operation 

s operation 

rad/air: 

rad/air: 278.1 

120.3 

438.9 

185.9 

ppm 

ppm 

Averages for 8.5 mm and 660/360 s operation rad/air: 278.1 438.9 ppm 

50 

Radiation 

AirCond 

0 

1 

29 

57 

85 

113 

141 

169 

197 

225 

253 

281 

309 

337 

365 

393 

421 

449 

477 

505 

533 

561 

589 

617 

645 

673 


PMO6V on VIRGO 

The main difference between the rocket PMO6-9, 10 and 11 and 

the space PMO6 is the heat sink. The former is made out of 

copper and the latter from aluminum. This seems to increases 

the aperture heating effect. 

The electrical power is determined with RI 2 instead of the 

normal VI with R determined during each closed phase 

Due to the failure of the shutter operation we replaced the 

shutters by the covers with the following open/closed periods: 

• 16-Jan-1996 shutter operation of A until 4-Feb-1996 

• 22-Feb-1996 change to PMO6V-A 8h open and 21 min closed; PMO6-B 

8H closed and during 39 min centered around open of A 

• 6-Jul-1996 change to PMO6-A 8h open and 6 min closed; PMO6-B once 

per week during 33 min open, still centered around open of A 


PMO6V on VIRGO: Electrical 

measurements 


PMO6V on VIRGO level-1 Data 

A major problem are 

changes of sensitivity of the 

radiometers due to the 

exposure to the strong UV 

radiation of the Sun and the 

exposure to the space 

environment. In the present 

context we are only 

interested in the early 

increase. 

Early increase 

of PMO6V-A 


PMO6V on VIRGO: Non-equivalence 


PMO6V on VIRGO: Radiometric 

constants 


Influence of new operation 


Changes of PMO6V final values 

The Feb/March 1996 irradiance become for PMO6V-A level-1.8 1366.785 

and for PMO6V-B 1366.782 with a mean of 1366.784 Wm -2 . These new 

radiometric constants need to be referred to SI by the WRR/SI factor. To 

convert the WRR value into SI we devide the values by 0.9998322 found 

from WRR/SI from 3 independent comparisons. This yields 1367.013 Wm -2 

which is 1408 ppm higher than the present PMO6V level 1.8 data. 


Uncertainty of Ground-based 

Calibrations

1-1 CF_NewRad2005.pdf - PMOD/WRC

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