Greg Kopp, p. 1ASIC 3 – 16 May 2006TSI Accuracy The Accuracy of Total Solar Irradiance...

Greg Kopp, p. 1ASIC3 – 16 May 2006 TSI Accuracy

The Accuracy of Total Solar Irradiance Measurements

Greg Kopp, CU/LASP, Boulder, CO


Impacts of Solar Irradiance on Earth

Sun - Climate Correlations

.

.

.Across All Time

Scales

L. Hood, Univ. ArizonaSORCE Meeting 2003

Secular Trends 11-Year Solar Cycle

27-Day Solar RotationSultan Hameed, Stony Brook

SORCE Meeting 2004

(a proxy of TSI)


27-Year TSI Data RecordNone of these instruments is

calibrated end-to-end for irradiance.


Summary of Stated Instrument Accuracies

Uncertainties are 1-

Instrument

TSI Value [W/m^2]

Stated Uncertainty

[ppm]

ERB (NIMBUS 7) 1371.9 5000ACRIM I 1367.5 1000ACRIM II 1364.2 1000ACRIM III 1366.1 1000

ERBE 1365.2 833VIRGO 1365.7 1000

VIRGO-PMO 1365.7 1204VIRGO-DIARAD 1366.4 470

DIARAD-like 1366.4 600SORCE/TIM 1361.0 350


Climate Record Needs: 100 ppm Absolute Accuracy...

• Dick Willson: “...uncertainties of 100 ppm or less would be required to produce a TSI record with sufficient traceability over the multi-decadal to centennial time scales for climate change and solar physics investigations without employing and overlapping, redundant measurement...”

No TSI instrument has yet achieved this level of accuracy.


Two primary TSI composites differ by 40 ppm/yr. Caused by 2 years of marginal quality data – not even a gap!

Climate Record Needs: ... or 10 ppm/yr Stability and Overlap

?


27-Year TSI Record Relies on Continuity

Current climate record plan relies on continuity and mission overlap.

But why the offsets?


TSI Accuracy Workshop

• Organizer: Jim Butler, NASA/GSFC

• Location: NIST Gaithersburg, MD

• Dates: 18-20 July 2005

• Attendees– Representatives of several TSI instruments

• ACRIM I, II, and III

• ERBS/ERBE

• SORCE/TIM

• VIRGO/PMO

• VIRGO/DIARAD & SOLCON

– NIST, NASA

• Approach– Day 1: Accuracy (“the Day 1 Problem”)

– Day 2: Stability

– Day 3: Improved or current calibration facilities

• Dick Willson: “We haven’t had a meeting like this in 20 years!”

TSI Accuracy Workshop Questions

Multiple Radiometers Track Changes and Indicate

Consistency With Stated Uncertainties

• Review Instrument Designs– Are there systematic differences that could

cause TSI offsets?

• Review Calibrations & Uncertainties– How accurately is each instrument calibrated?

How well are uncertainties estimated?

– How well are degradation and stability understood?

– What were goals and actuals?

• Intra-instrument Consistency– Do intra-instrument cavity comparisons agree

with stated uncertainties?

– What ground calibrations or facilities would improve the future TSI record?

Agenda

Absolute Accuracy

Stability

Calibrations


Summary of Instruments

Instrument Cav #

Cavity Layout

Precision Aperture Position

V Mon

I/R Mon

Black Black Type

Active Therm Control

Dark Meas. Freq.

Comments

ERB 1 ? internal ? ? paint specular no every meas.

ACRIM I 3 cones, front to back

internal yes yes paint specular no low

ACRIM II 3 cones, front to back


ACRIM III 3 cones, front to back


ERBE 1 cones, front to back

internal yes yes paint specular no every meas.

one 3-min meas every 2 wks

VIRGO-PMO 2 inverted cones, front

to back

internal yes yes paint specular no

(at L1)

none low-freq. shutter

VIRGO-DIARAD

2 cylinders, side by side

internal yes yes paint diffuse no

(at L1)

none

TIM 4 cones, side by side

front of instrum.

no no NiP diffuse yes every orbit

pulse-width modulation


Reviewed Uncertainties

SORCE/TIM

Relative W/m2

Area 0.000425 0.58

Thermal efficiency 0.000130 0.18

Eectrical. Power 0.000150 0.20

Cavity absorption 0.000030 0.04

Total 0.000735 1.00

RSS 0.000470 0.64

VIRGO/DIARAD L

ACRIM

VIRGO/PMOComponent Value u c (u*c)^2Area N/APclosed 45 mW 0.0000045 5.00E+04 0.050625Popen 17 mW 0.0000017 5.00E+04 0.007225CNE 1 5.00E-04 1.40E+03 0.49CR N/A 7.00E-05 1.40E+03CSt N/A 1.00E-04 1.40E+03CLH N/A 3.00E-05 1.40E+03CApH N/A 5.00E-04 1.40E+03Cdiff N/A 1.00E-04 1.40E+03WRR-Factor 1 6.00E-04 1.40E+03 0.7056WRR/SI 1 9.00E-04 1.40E+03 1.5876

2.84105Uncertainty abs 1.6855 W/m2Uncertainty rel 1685.5 ppm95% Uncertainty 3371.1 ppm

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

Correction Value [ppm] SORCE Worst Case

Distance to Sun, Earth & S/C 33,537 0.1 0.1Doppler Velocity 57 0.7 0.7Shutter Waveform 100 1.0 1.0Aperture 1,000,000 30 651 Diffraction 452 47 47Cone Reflectance 250 54 108Non-Equivalence, ZH/ZR - 1 7, AC 23 46Servo Gain 16,129 0.0 0.0Standard Volt + DAC 1,000,000 7.0 100 Pulse Width Linearity 1,000,000 186 300Standard Ohm + Leads 1,000,000 17 25Dark Signal 2,693 10 25Scattered Light & IR 100 25 50Pointing 10 25Measurement Repeatability (Noise) 1.5 2.0Uncertainty due to Sampling 12 12Total RSS 206 738Cone Agreement Accuracy 301 301


Instrument

TSI Value [W/m^2]

Stated Uncertainty

[ppm]

ERB (NIMBUS 7) 1371.9 5000ACRIM I 1367.5 1000ACRIM II 1364.2 1000ACRIM III 1366.1 1000

ERBE 1365.2 833VIRGO 1365.7 1000

VIRGO-PMO 1365.7 1204VIRGO-DIARAD 1366.4 470

DIARAD-like 1366.4 600SORCE/TIM 1361.0 350

TSI Instrument Uncertainties & Intra-Instrument Variations

Uncertainties are 1-

InstrumentTSI Value [W/m^2]

Stated Uncertainty

[ppm]

Cavity Variations [ppm]

ERB (NIMBUS 7) 1371.9 5000 - ACRIM I 1367.5 1000 511ACRIM II 1364.2 1000 2046ACRIM III 1366.1 1000 1036

ERBE 1365.2 833 - VIRGO 1365.7 1000 2271

VIRGO-PMO 1365.7 1204 299VIRGO-DIARAD 1366.4 470 2858

DIARAD-like 1366.4 600 1612SORCE/TIM 1361.0 350 301


Possible Causes of Differences in Absolute Values

• Underestimated Uncertainties: Is this simply the state of the art in these radiometric measurements, with all uncertainties being underestimated?

• Apertures: Measurements from different facilities have greater variations than stated aperture measurement uncertainties.

– Does not account for 0.3% TSI differences

– Does not explain inter-cavity variations within single instrument

• Applied Power: The TIM uses DSP-controlled pulse width modulation while other radiometers apply DC power.

– Very unlikely to have 0.3% difference

• Optical – Scatter Prior to Limiting Aperture: Instruments with oversized (non-limiting) aperture near front of instrument allow much more sunlight into instrument.

– The TIM precision aperture and shutter are at the front of the instrument, so this is a difference.

– Scatter will erroneously increase the signal through the limiting aperture.

• Optical – Diffraction: This is a 0.12% effect in ACRIM and is not corrected

• Darks: Uncertainties in dark corrections are large. – These are large corrections, depend on FOV, and vary with temperature.

– Darks are not measured regularly on several instruments.

• Aperture Heating: Uncertainties in heating due to different aperture materials, conduction, mounting, emissivities


Roadmap for Future Improvements

• Complete aperture comparison measurements– Include ACRIM apertures in NIST aperture comparison

• Power comparison– NIST power comparison to trapped diode transfer standard

– NPL power trap comparison

– 0.05% accuracy

• Scatter/diffraction measurement– NIST to monitor changes in signal as beam expands to overfill entrance

aperture

• Would like an irradiance comparison against an absolute reference radiometer with 0.01% accuracy– JPL Table Mountain Observatory inter-comparisons are merely relative

– PMOD World Radiation Reference is linked to an absolute scale but without desired accuracy

– NASA’s Glory program is creating the TSI Radiometer Facility to compare TSI instruments on an absolute scale


Scatter Can Erroneously Increase Signal

Baffles

Cavity

Precision Aperture

View-Limiting Aperture

Precision Aperture


TIM Cutaway

ACRIM III Cutaway


Precision Aperture


Precision Aperture

SunlightSunlight

Additional light allowed into

instrument can scatter into cavity

• All instruments except TIM put primary aperture close to the cavity– Could cause systematically higher TSI values reported

Majority of light is blocked before

entering instrument


Diffraction Can Erroneously Change Signal


Precision Aperture


Precision Aperture

SunlightSunlight

Failure to correct for light diffracted

into cavity erroneously

increases signal

• All instruments except TIM put primary aperture close to the cavity

Failure to correct for light diffracted

out of cavity erroneously

decreases signal


TSI Instrument Uncertainties & Intra-Instrument Variations

NIST calculates diffraction should lower these results.


TSI Instrument Uncertainties – With Diffraction Correction

Correction not yet approved or applied by ACRIM or ERBE Teams


Address Applied Power: Trap Diode Power Comparison

• NIST and LASP are preparing to do optical power comparisons between a transfer standard and ground TSI instruments– NPL has done similar power comparisons before, but in air

stabilized laser

vacuum window

beamsplitter (1%)

TSI instrument

trap diode

vacuum chamber


“Benchmark observations of total solar irradiance and spectrally resolved solar irradiance to an accuracy of 0.03 percent referenced to NIST standards are required to elucidate the origin of climate change.”

[NRC Committee on Earth Science and Applications from Space]

Intend to Create Facility for TSI Calibrations

• Have strong community support for a NIST-traceable cryogenic radiometer facility to calibrate TSI instruments to ~100 ppm absolute accuracy (TSI Accuracy Workshop 2005, NEWRAD 2005)

• Glory Science will create this facility to enable TSI instrument comparisons against an absolute standard


TSI Instrument in Vacuum Tank

Cryogenic Radiometer

1-D Stage

Light Source

TSI Radiometer Facility Layout

Radiometer RequirementsParameter Requirement Nominal Measured Irradiance Level 1360 W/m2 Nominal Measured Power Level 68 mW (i.e. 8 mm diameter aperture) Accuracy (Relative Standard Uncertainty, k=1) for measurements of nominal inputs

100 ppm

Modes Irradiance, Power Noise, k=1 <2 ppm (0.14 µW) Measurement Time 10 min Electrical Power Linearity 10 ppm (0.68 µW) k=1 from 1 mW to 100

mW Spectral Range 532 nm nominal, but operational over 400-

1550 nm Optical Setup capable of sharing single optical window

with TSI instrument Vacuum Setup capable of sharing common vacuum with

TSI instrument Minimum Continuous Operational Time 13 hrs

Greg Kopp, p. 1ASIC 3 – 16 May 2006TSI Accuracy The Accuracy of Total Solar Irradiance...

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