1-1 CF_NewRad2005.pdf - PMOD/WRC
1-1 CF_NewRad2005.pdf - PMOD/WRC
1-1 CF_NewRad2005.pdf - PMOD/WRC
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Absolute Accuracy of Total Solar<br />
Irradiance Measurements from Space<br />
Claus Fröhlich<br />
Physikalisch-Meteorlogisches Observatorium Davos, World Radiation<br />
Center, CH 7260 Davos Dorf, Switzerland<br />
eMail: cfrohlich@pmodwrc.ch; http://www.pmodwrc.ch<br />
Report on Preliminary Results from the TSI Workshop at<br />
NIST, 18-20 July 2005<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 1
Participation in the TSI WS<br />
Barnes, Robert A., SAIC, Beltsville, MD<br />
Bloom, Hal, NIST, Gaithersburg, MD<br />
Bosworth, John M., Swales Aerospace, Beltsville, MD.<br />
Butler, James J., EOS NASA/GSFC, Greenbelt, MD.<br />
Cahalan, Robert F., NASA/GSFC, Greenbelt, MD<br />
Crommelynck, Dominique, IRMB, Brussels, Belgium<br />
Datla, Raju, NIST, Gaithersburg, MD<br />
Dewitte, Steven, IRMB, Brussels, Belgium<br />
Floyd, Linton, Interferometrics/NRL, Washington, DC<br />
Fowler, Joel, NIST, Gaithersburg, MD.<br />
Lawrence, George M., LASP, Boulder, CO<br />
Lee III, Robert Benjamin, Hampton Va<br />
Litorja, Maritoni, NIST, Gaithersburg, MD<br />
Lorentz, Steven R., Standards and Technology, Inc.,<br />
Ijamsville, MD.<br />
Lykke, Keith, NIST, Gaithersburg, MD<br />
Morrill, Jeff, Naval Research Laboratory, Washington, DC<br />
Ohno, Yoshi, NIST, Gaithersburg, MD<br />
Pankratz, Christopher K., LASP, Boulder, CO<br />
Pap, Judit, NASA’s GSFC, Greenbelt, MD<br />
Parr, Albert, NIST, Gaithersburg, MD<br />
Rabin, Douglas M., NASA’s GSFC, Greenbelt, MD.<br />
Rager, Amy, Catholic University of America, Clarksville, MD<br />
Rice, Joe, NIST, Gaithersburg, MD<br />
Rottman, Gary, LASP, Boulder, CO.<br />
Shirley, Eric, NIST, Gaithersburg, MD<br />
Sparn, Thomas P., LASP, Boulder, CO<br />
Willson, Richard C., Columbia University, Coronado, CA.<br />
Yoon, Howard, NIST, Gaithersburg, MD<br />
Fox, Nigel, National Physical Laboratory, Teddington, Middlesex UK<br />
Fraser, Jerry, NIST, Gaithersburg, MD.<br />
Fröhlich, Claus, <strong>PMOD</strong>/<strong>WRC</strong>, Davos Dorf, Switzerland<br />
Helizon, Roger S., JPL, Pasadena, CA.<br />
Heuermann, Karl, LASP, Boulder, CO<br />
Johnson, B. Carol, NIST, Gaithersburg, MD<br />
Jordan, Stuart, NASA’s GSFC, Greenbelt, MD<br />
Kirk, Megan, Catholic University of America, Clarksville, MD<br />
Kopp, Greg, LASP, Boulder, CO<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 2
Presentations at the TSI Workshop<br />
Monday, July 18<br />
Tuesday, July 19<br />
Satellite Instrument TSI Measurement Uncertainty: Chair G. Kopp<br />
Welcome and Meeting Charge J. Butler<br />
Session 1 Goals G. Kopp<br />
ACRIM I, II & III R. Willson<br />
ACRIM R. Helizon<br />
TIM on SORCE G. Kopp<br />
Wednesday, July 20<br />
Satellite and Ground-based TSI Instrument Comparison: Chair: R. Willson<br />
VIRGO/DIARAD flight performance and degradation S. Dewitte<br />
VIRGO/PMO6-V flight performance and degradation C. Fröhlich<br />
ACRIM 1, 2, & 3 flight comparisons, performance, and degradation<br />
R. Willson<br />
PMO6V on VIRGO/SoHO C. Fröhlich<br />
The DIARAD type instruments, principles and error estimates<br />
D. Crommelynck & S. Dewitte<br />
ERBE on ERBS R. Lee<br />
Disscussion and Session wrap-up G. Kopp<br />
Laboratory-based Comparison and Characterization<br />
Chair: J. Rice<br />
Aperture Area Comparison Results C. Johnson<br />
Diffraction Effects E. Shirley<br />
Applications of Cryogenic Radiometry S. Lorentz<br />
NIST Cryogenic Radiometer Intercomparison J. Rice<br />
Lab Solar Irradiance Scale Comparisons C. Fröhlich<br />
Discussion and Session wrap-up<br />
SORCE/TIM flight comparisons, performance, and degradation G. Kopp<br />
Shuttle TSI flight comparisons, performance, and degradation S. Dewitte<br />
ERBS/ERBE flight performance and degradation R. Lee<br />
Discussion and summary of flight performance and degradation<br />
JPL/TMO ground comparisons and significance for flight TSI results<br />
R. Willson<br />
Workshop closing comments J. Butler<br />
Discussion and summary of ground comparisons<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 3
Why was the Work Shop Organized<br />
Until the launch of SORCE<br />
and the operation of TIM we<br />
all thought that we have<br />
improved the uncertainty from<br />
the early time of HF and<br />
ACRIM I.<br />
The composite shows how<br />
small the variation is<br />
compared to the spread of the<br />
data. It shows also that the<br />
precision is much higher than<br />
the absolute accuracy which<br />
allows to construct such<br />
composite with overlapping<br />
measurements.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 4
Operation of Room-Temperature<br />
Radiometers<br />
Operation<br />
The classical electrically calibrated radiometers are operated in either passive or active<br />
mode. Both modes have a measurement and reference phase which is determined by an<br />
open or closed shutter. In the passive mode the electrical heater is switched on when the<br />
shutter closes and its power is adjusted to produce about the same heat flux as the radiation<br />
seen during the measurement phase. In the active mode control electronics maintains the<br />
heat flux constant during both phases which are now alternated in a regular pace (e.g. 1-min<br />
open and 1-min closed).<br />
With the open and closed electrical power P dissipated in the cavity, the irradiance S equals<br />
with C Rad the radiometer constant<br />
For an ideal radiometer C Rad = 1/A with A the area of the precision aperture in front of the<br />
cavity. Deviations from the ideal behaviour are accounted for replacing 1/A by C corr /A with<br />
C corr determined for each individual radiometer by its characterization.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 5
Different Types of<br />
Room-Temperature Radiometers<br />
Characterization<br />
The determination of the deviations from ideal behaviour is called<br />
characterization. The following effects are determined by independent<br />
experiments: Non-equivalence between electrical and radiative heating<br />
determined from air-to-vacuum ratio, reflectivity of the cavity, scattered light<br />
and lead heating,<br />
and by numerical calculations: Diffraction<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 6
VIRGO Radiometers<br />
Below is a block diagram of PMO6V (left) and a picture of DIARAD and PMO6V of<br />
VIRGO<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 7
Characterization of<br />
Room-Temperature Radiometers<br />
Results of a typical characterization<br />
As an example the results for the characterization of PMO6 type radiometers is<br />
shown from Brusa & Fröhlich (1986).<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 8
Aperture Measurements<br />
Area of the Precision<br />
Aperture<br />
An important part of the<br />
characterization is the<br />
determination of the area of the<br />
precision aperture. In order to<br />
keep the radiometer small with a<br />
reasonable time constant the<br />
aperture is nominally 5 mm or 8<br />
mm in diameter. The land of the<br />
aperture has to be as small as<br />
possible in order to prevent lightpipe<br />
effects due to the finite<br />
angular extent of the Sun and<br />
during slightly off-pointed<br />
conditions. Typically it is of the<br />
order of a couple 10 µm. This<br />
does not allow to measure the<br />
diameter by physically touching.<br />
Thus optical measuring methods<br />
have to be applied which are in<br />
general less accurate.<br />
Results from the presentation of C. Johnson<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 9
Effect of Diffraction<br />
Diffraction has to be included as a correction and Eric<br />
Shirley from NIST did the calculations for all radiometers<br />
involved<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 10
Uncertainty PMO6-9 and 11<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 11
World Radiometric Reference and SI<br />
From a thorough discussion of the results of comparison of many ECRs with the sun as source<br />
at <strong>PMOD</strong>/<strong>WRC</strong> the Word Radiometric Reference has been defined and adopted by WMO in<br />
1975 and a group of radiometers identified to materialize it, the World Standard Group. The<br />
WWR is what is called in metrology a conventional reference as it has a much higher<br />
repeatability than its SI uncertainty of 0.3%.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 12
Comparison of WRR and SI, 1 of 2<br />
Comparison with cryogenic radiometers via trap detectors at NPL have shown<br />
that the WRR is within ±0.02% of the SI scale (Romero et al., 1991, 1996), which<br />
is fortuitous, but practical. These comparison are made with under-filled<br />
apertures and thus need some care about the performance of the under-filled<br />
radiometer relative to the one as measuring irradiance. Recently, such<br />
comparison were repeated with both rocket radiometers at NPL and at the Swiss<br />
Metrological Institute, METAS. The result, however, is still preliminary (see<br />
Poster 16). So we will only use the results of the 1991/96 comparison only.<br />
However,these need some corrections of the originally published values. The<br />
diffraction argument was incorrect and the new aperture areas as re-measured<br />
by NIST shall be used. Moreover, we need to apply a correction for the<br />
difference of the non-equivalence as determined from the air-to-vacuum ratio<br />
under-filled and normal. Then the results for 1991 and 1996 become 1.000557<br />
0.001326 and 0.999757 0.000948, respectively, with a weighted average and<br />
1 uncertainty of<br />
1.0000910. 0.000864<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 13
Comparison of WRR and SI, 2 of 2<br />
Due to the small land of the precision apertures, the innermost part has a very<br />
short thermal time-constant and starts heating up when the shutter is opened.<br />
Thus the cavity receives some extra IR radiation and in air also some energy<br />
through air conduction. Although it is a rather small effect for the PMO6 type<br />
radiometers we need to correct for it. We estimate the effect from preliminary<br />
results of model calculations which indicate that the IR influence is of the same<br />
order as the one due to the exchange in air. So the IR effect is proportional to<br />
the air-to-vacuum ratio difference of 0820 ppm for 6-9 and 545 770 ppm for 6-11.<br />
Thus, a value of about 200~ppm is assumed and the final value of the WRR/SI<br />
becomes<br />
with a 95% uncertainty of 0.16%.<br />
0.999891<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 14
Corrections for PMO6V Values<br />
The radiometric constants have to be changed due to a new<br />
evaluation of the air-to-vacuum ratio and the tracing to WRR and<br />
SI.<br />
The level-1 ratio A/B is now in space 633 ppm higher than on<br />
ground. This difference can be easily explained by the<br />
uncertainty of C NE , which is for each radiometer of the order<br />
1000 ppm (95% uncertainty).<br />
The new calibration factors change the values by 1314 ppm and<br />
1254 ppm for A and B, respectively. The difference is mainly due<br />
to the inclusion of the diffraction correction in the radiometric<br />
constants for the comparison with the WRR. The remaining 134<br />
and 74 ppm are due to changes of the re-evaluated air-tovaccum<br />
ratio.<br />
Finally the WRR/SI conversion has to be added and the<br />
presently adopted PMO6V values are increased by 1363 ppm to<br />
yield SI values<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 15
Uncertainty of PMO6V WRR/SI<br />
Uncertainty of the PMO6V WRR/SI traceability @ 1400W/m2<br />
Component Value u c (u*c)^2<br />
Area N/A<br />
Pclosed 45 mW 0.0000045 5.00E+04 0.050625<br />
Popen 17 mW 0.0000017 5.00E+04 0.007225<br />
CNE 1 5.00E-04 1.40E+03 0.49<br />
CR N/A 7.00E-05 1.40E+03<br />
CSt N/A 1.00E-04 1.40E+03<br />
CLH N/A 3.00E-05 1.40E+03<br />
CApH N/A 5.00E-04 1.40E+03<br />
Cdiff N/A 1.00E-04 1.40E+03<br />
WRR-Factor 1 6.00E-04 1.40E+03 0.7056<br />
WRR/SI 1 9.00E-04 1.40E+03 1.5876<br />
2.84105<br />
Uncertainty abs 1.6855 W/m2<br />
Uncertainty rel 1685.5 ppm<br />
95% Uncertainty 3371.1 ppm<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 16
Final values of VIRGO TSI at the<br />
Beginning of the Mission 1 of 2<br />
DIARAD is fully characterized and represents its own independent<br />
radiometric scale. However, some corrections due to time constant<br />
effects and the area have been determined since the first evaluation;<br />
they amount to an overall correction of + 447 ppm. With these<br />
corrections the final ‘first light’ data of VIRGO are very consistent<br />
(within about 300 pmm), although their tracing to SI is completely<br />
independent.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 17
Final values of VIRGO TSI at the<br />
Beginning of the Mission 2 of 2<br />
If we compare this result<br />
with the internal consistency<br />
of the IRMB radiometry it<br />
may be fortuitous. These<br />
comparisons may also give<br />
a hint that the uncertainty<br />
estimate of the IRMB may<br />
be too optimistic. On the<br />
other hand, the general<br />
picture indicates that the<br />
tracing to Si via WRR is<br />
within the stated 95%<br />
uncertainties of 0.3%.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 18
Uncertainties, Correction Factors and<br />
‘First Light’ Comparison of the Radiometers<br />
in Space<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 19
Comparison of corrected TSI<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 20
What’s next ?<br />
It is rather obvious that power comparisons of representative<br />
spares against calibrated traps would help to locate the reason<br />
for the still unresolved difference. In contrast to the WRR/SI<br />
comparison at NPL they will be done with the radiometers in<br />
vacuum. NIST is prepared to conduct them in early 2006. As the<br />
aperture area is no longer an issue, power comparisons are<br />
sufficient. However, the implications of under-filling the apertures<br />
should be studied in detail by each radiometer involved in the<br />
comparisons.<br />
There are also plans to conduct comparisons with the Sun as<br />
source at Table Mountain.<br />
We hopefully will know more in summer 2006.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 21
Conclusions<br />
The differences are still not resolved and the reason is unknown.<br />
It does not seem to be due to aperture areas, as it was most likely<br />
during the early measurements of TSI from space.<br />
The good agreement between PMO6V and DIARAD-L indicates that<br />
the WRR is most probably close to the SI as indicated by the WRR/SI<br />
comparisons.<br />
There may still be a problem with the WRR/SI comparison: the WRR<br />
factor of PMO6-9 and 11 are of the order of 1.004258 and 1.002751,<br />
indicating that the characterized PMO6 radiometers are about 0.35%<br />
below the WRR (similar to CROM-2L).<br />
Because the WRR/SI comparison is only based on the PM6-9 and 11<br />
radiometers, new tests can always be performed as e.g. the area of<br />
the precision aperture could be corrected. With radiometers launched<br />
to space this is obviously no longer possible and we have to rely on<br />
the similarity of the spares.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 22
You can find this presentation at<br />
ftp.pmodwrc.ch/pub/claus/newRad2005/<strong>CF</strong>_NewRad2005.ppt<br />
End<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 23
Air-to-Vacuum Ratio (Non-<br />
Equivalence)<br />
5000<br />
4500<br />
4000<br />
3500<br />
3000<br />
2500<br />
2000<br />
Non-Equivalence PMO6_09<br />
5000<br />
4000<br />
3000<br />
Non-Equivalence [ppm]<br />
0 2 4 6 8 10<br />
2000<br />
1000<br />
0<br />
-1000<br />
Aperture Diameter [mm]<br />
Non - Equivalence PMO6_11<br />
3600<br />
3100<br />
2600<br />
2100<br />
1600<br />
1100<br />
Non-Equivalence [ppm]<br />
2 3 4 5 6 7 8 9<br />
Aperture Diameter [mm]<br />
The difference between the<br />
under-filled and illuminated<br />
aperture (8.5mm) sovim1 is 0820 ppm<br />
for 6-9 and 545 sovim2 770 ppm for 6-<br />
11.<br />
Non-equivalence [ppm]<br />
0 5 10 15 20 25<br />
Illuminated area [arbitrary units]<br />
r-111<br />
r-113<br />
sovim1 optik<br />
virgo2<br />
sovim1(gold mirror)<br />
sovim2(gold mirror)<br />
sovim1(g-plättli)<br />
sovim2(g-plättli)<br />
sovim13<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 24
Aperture heating<br />
60/60 sec operation Case 4<br />
Case 2: Contact to heatsink only at nodes 1611,1711,1811 with 0.1 WK-1 each;<br />
Case 500 2: Contact to heatsink only at nodes 1611,1711,1811 with 0.1 WK-1 each;<br />
air conduction to cavity with 2.2 Wm-2K-1;<br />
Diff to model<br />
air conduction to cavity with 2.2 Wm-2K-1;<br />
450<br />
-0.08 %<br />
Diff<br />
total<br />
to<br />
input<br />
model<br />
flow: 22.080<br />
-0.08<br />
mW<br />
%<br />
400<br />
total<br />
total<br />
GF<br />
input<br />
to<br />
flow:<br />
99998 21.411<br />
22.080<br />
mW<br />
mW<br />
Total<br />
total<br />
350<br />
GF<br />
GR<br />
to<br />
to<br />
99998<br />
99998 &99999<br />
21.411<br />
0.652 mW<br />
mW<br />
300<br />
Flow<br />
Total<br />
to<br />
GR<br />
cavity<br />
to 99998<br />
by radiation<br />
&99999<br />
317.0<br />
0.652<br />
ppm<br />
mW<br />
Flow<br />
Flow<br />
to<br />
250 to<br />
cavity<br />
cavity<br />
by<br />
by<br />
radiation<br />
air conduction 500.6<br />
317.0<br />
ppm<br />
ppm<br />
Flow to cavity<br />
200 Total<br />
by air conduction<br />
817.6<br />
500.6<br />
ppm<br />
ppm<br />
Total 817.6 ppm<br />
150<br />
Averages for 8.5 mm and 60/60 s operation rad/air: 120.3 185.9 ppm<br />
Averages<br />
Averages 100<br />
for<br />
for<br />
8.5<br />
8.5<br />
mm<br />
mm<br />
and<br />
and<br />
660/360<br />
60/60 s operation<br />
s operation<br />
rad/air:<br />
rad/air: 278.1<br />
120.3<br />
438.9<br />
185.9<br />
ppm<br />
ppm<br />
Averages for 8.5 mm and 660/360 s operation rad/air: 278.1 438.9 ppm<br />
50<br />
Radiation<br />
AirCond<br />
0<br />
1<br />
29<br />
57<br />
85<br />
113<br />
141<br />
169<br />
197<br />
225<br />
253<br />
281<br />
309<br />
337<br />
365<br />
393<br />
421<br />
449<br />
477<br />
505<br />
533<br />
561<br />
589<br />
617<br />
645<br />
673<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 25
PMO6V on VIRGO<br />
The main difference between the rocket PMO6-9, 10 and 11 and<br />
the space PMO6 is the heat sink. The former is made out of<br />
copper and the latter from aluminum. This seems to increases<br />
the aperture heating effect.<br />
The electrical power is determined with RI 2 instead of the<br />
normal VI with R determined during each closed phase<br />
Due to the failure of the shutter operation we replaced the<br />
shutters by the covers with the following open/closed periods:<br />
• 16-Jan-1996 shutter operation of A until 4-Feb-1996<br />
• 22-Feb-1996 change to PMO6V-A 8h open and 21 min closed; PMO6-B<br />
8H closed and during 39 min centered around open of A<br />
• 6-Jul-1996 change to PMO6-A 8h open and 6 min closed; PMO6-B once<br />
per week during 33 min open, still centered around open of A<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 26
PMO6V on VIRGO: Electrical<br />
measurements<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 27
PMO6V on VIRGO level-1 Data<br />
A major problem are<br />
changes of sensitivity of the<br />
radiometers due to the<br />
exposure to the strong UV<br />
radiation of the Sun and the<br />
exposure to the space<br />
environment. In the present<br />
context we are only<br />
interested in the early<br />
increase.<br />
Early increase<br />
of PMO6V-A<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 28
PMO6V on VIRGO: Non-equivalence<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 29
PMO6V on VIRGO: Radiometric<br />
constants<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 30
Influence of new operation<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 31
Changes of PMO6V final values<br />
The Feb/March 1996 irradiance become for PMO6V-A level-1.8 1366.785<br />
and for PMO6V-B 1366.782 with a mean of 1366.784 Wm -2 . These new<br />
radiometric constants need to be referred to SI by the WRR/SI factor. To<br />
convert the WRR value into SI we devide the values by 0.9998322 found<br />
from WRR/SI from 3 independent comparisons. This yields 1367.013 Wm -2<br />
which is 1408 ppm higher than the present PMO6V level 1.8 data.<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 32
Uncertainty of Ground-based<br />
Calibrations<br />
10.11.2005 08:38:42 NewRad 2005, Davos 17-19 October 2005 33