Details of the SORCE/TIM On-Orbit Calibrations
SORCE Science Meeting
4-6 Dec. 2003
Greg Kopp, George Lawrence, and Gary Rottman
Laboratory for Atmospheric and Space Physics, Univ. of Colorado, 1234 Innovation Dr., Boulder, CO 80303, USA, Greg.Kopp@LASP.Colorado.edu
The SORCE Total Irradiance Monitor (TIM) has been acquiring total solar irradiance (TSI) measurements since early March of 2003. As with any instrument exposed to unfiltered
sunlight for extended periods of time, on-orbit changes must be estimated to provide corrections for data processing. We provide details of the post-launch TIM calibration tracking
methods, including radiometer degradation measurements, cavity reflectance monitoring, power linearity correction, dark signal estimates, and servo gain calibrations.
The TIM Measurement Equation
NiP black electrical substitution radiometer cavities measure radiant power
absorbed. Precision apertures determine area AP through which shuttered
sunlight passes. Total solar irradiance (TSI) is fundamentally E = P / AP. Several
corrections, including a dark subtraction, are applied.
E Sun = Re( ESun _ obs − EDark )
Sun
Observation
Standard
Watt V2/R
€
Dark
Observation
V2
1 1
1
1
ESun _ obs =
⋅
⋅ ⋅
⋅
64000 ⋅ Reff AP α f AU f Doppler f point f degrade Ψ
Aperture
Gain


1
1  ZH
⋅ −DNSun 1+  + DNFF ⋅  ⋅

G
G  ZR

Efficiency Factors
€
Absorptance
Bold type
denotes phasors.
Shutter Waveform
Feedforward Data
Numbers
Equivalence Ratio
Uncertainty Budget
Correction
1. Distance to Sun
2. Doppler Velocity
3. Shutter Waveform
4. Aperture
5. Cone Reflectance
6. Equivalence ratio, ZH/ZR
7. Servo Gain
8. Standard Volt + DAC
9. Standard Ohm, Leads
10. Dark Signal
11. Scattered Light & IR
Total RSS
Value [ppm]
±33,444/yr., 93/orbit
±57
100
1,000,000
200 (169,139,307,360)
100, AC (280)
16,000
1,000,000
1,000,000
2,700
100
δ [ppm]
0.1
0.7
1
55
54 (?)
21 (30)
1
9 (~150)
17
1.7 (8)
14
83 (~170)
Post-launch changes to the TIM uncertainty budget are shown in red. The largest uncertain currently is the application of
power linearity measurements (Standard Volt) from ground-based units to the flight unit. Determination of possible
reflectance variations since final ground calibrations is underway.
Ground-Based Calibrations
Results – TIM TSI Data
Aperture area, standard watt (voltage reference and resistive cavity heater), and
cavity reflectances are ground calibrations.
TIM TSI data are available via http://lasp.colorado.edu/sorce/tsi_data.html
(4) NIST measured
the geometric aperture
areas. Corrections are
applied for diffraction,
scatter, temperature,
and pressure.
(8) Flight voltage references show -0.2 ppm/°C temperature stability (left).
Ground-based voltage units are stable with time to ~1 ppm/yr (right).
(9) Cavity heater
resistance R is
calibrated vs.
temperature to
ppm accuracies.
(5) Cavity reflectance is calibrated across the solar spectrum
(right). Metallic NiP black (left) is robust against radiation.
Changes are monitored on-orbit.
A:
B:
C:
D:
543.82054
538.29771
546.18576
537.76194
±
±
±
±
Resistances at 30.8 C:
0.00095 (±
1.75 ppm)
0.00095 (±
1.77 ppm)
0.00095 (±
1.74 ppm)
0.00095 (±
1.77 ppm)
+
+
+
+
8.59ppm/C
9.78ppm/C
9.09ppm/C
11.41ppm/C
The TIM TSI record since launch shows typical 0.1% fluctuations due to solar activity (top left). The passage of two
exceptionally large sunspot groups across the disk in late October created the largest short-term TSI decrease (right) in
the 25-year space-borne TSI record (bottom left).
Power Linearity Measurements (8)
Cavity Inter-Comparisons (5)
Cavity Reflectance Monitoring (5)
Results – A Flare in TSI
Linearity of the applied cavity power is estimated from ground calibrations of a
nearly identical TIM. The variance in these measurements limits the accuracy to
which we know the SORCE TIM’s measured sunlight power to >110 ppm.
Each of the TIM’s four electrical substitution radiometer cavities is duty cycled
to track on-orbit changes using the lesser exposed cavities as references.
A photodiode monitors changes in the light reflected from each cavity.
The X17 flare on 28 Oct. 2003 was observed in TSI.
P = εG*R*TSI
PA = 0.128e-t[days]/305 + 0.869
x20
x100
x100
Non-linearities in the applied pulse-width modulated power are measured from several circuits made from the same lot
of electrical components as the SORCE TIM (left plot). The average of these measurements (red curve, right plot) is the
estimated non-linearity for the flight unit. The variance in these measurements (blue curve, right plot) and the lack of
direct measurements of the flight unit increase the TIM’s combined standard uncertainty to no better than 150 ppm. This
is the largest uncertainty in the instrument’s accuracy.
The primary cavity (B) measures irradiance almost continuously. Simultaneous inter-comparisons with Cavity A are
done for one orbit each week. Cavities A and C have been compared every 10 weeks. The simultaneous intercomparisons (left) show two effects: 1) Annual variations (faint lines) are likely due to improper linearity corrections.
2) The primary cavity (B) is degrading with exposure, and currently has changed ~60 ppm relative to A (red curve). At
current exposure rates (right), no other cavity will reach the present exposure levels of Cavity B, and thus should not
reach its current level of degradation. More frequent comparisons are planned since degradation is so small.
Dark Signal Correction (10)
Servo Gain Calibrations (7)
Measurements of dark space during orbit eclipses give the thermal background
signal from the instrument. Fits to instrument thermistors allow extrapolation to
actual orbit daytime temperatures. This extrapolation is a ~100 ppm correction.
Servo gain is calibrated on-orbit by applying a known step function power
change to a cavity and recording the response as the servo system brings the
cavity back into equilibrium.
A singular value decomposition method fits measured “dark” signals (blue ‘+’) of empty space to four instrument
temperatures to determine the instrument’s thermal background. Extrapolation of the fit to relevant daytime temperatures
estimates the thermal background relevant during observations (red curve).
Gain is measured on-orbit by analyzing the servo system response to a DSP-applied step function (top left). The complex
gain remained reasonably constant in both magnitude (center) and phase (right) early in the mission. Newer calibrations
have been done, but are not yet analyzed. A feed-forward system (lower left) that anticipates shutter transitions greatly
reduces the effect of any gain uncertainty on the TSI accuracy.
The noise of the instrument has been measured on-orbit, and is ~2 ppm. Phase sensitive detection at the shutter
fundamental is partly responsible for this low noise.
Normalized photodiode signals (top left) are sensitive to changes in on-orbit cavity reflectance. We infer ~20 ppm
variations from ground-based absolute reflectances (bottom left). The photodiodes of the lesser-used cavities show
nearly identical signal decreases with a 300-day time constant, independent of solar exposure. The primary cavity, B
(blue curves), initially followed a similar decrease, but more recently has been increasing. This brightening (right) may
account for about 20 ppm of the difference between the Cavity A and B inter-comparisons.
Summary
On-orbit calibrations track changes in the TIM since launch.
Primary cavity degradation is ~60 ppm, and tracking will
facilitate correction in data processing. Phase sensitive
detection reduces sensitivity to noise and thermal fluctuations.
The TIM calibrations are meeting design specifications with
the exception of the linearity of applied power, limited by
ground measurement variations in similar units.
References
• Kopp, G., Lawrence, G., and Rottman, G., “Total Irradiance Monitor Design
and On-Orbit Functionality,” SPIE Proc. 5171-4, 2003 (available via
http://spot.colorado.edu/~koppg/TSI/index.html).
• Lawrence, G., Rottman, G., Harder, J., and Woods, T., “Solar Total
Irradiance Monitor (TIM),” Metrologia 37, 2000, pp. 407-410.
• SORCE web page: http://lasp.colorado.edu/sorce/
The TIM TSI (red) shows an increase of 270 ppm at the time of the X17 flare. The abruptness of this increase and its
timing with the GOES X-ray flare observation (blue) indicate this is the flare’s signature in TSI.
Results – TSI Comparisons
On a relative scale, TIM, ACRIM III, and VIRGO detect very similar changes in
the TSI. The TIM measures lower TSI values than other currently operating
instruments.
The TIM measurements are very similar, on a relative scale, to both the ACRIM III and VIRGO TSI instruments (left).
The TIM is measuring TSI values on an absolute scale that are ~4 W/m2 (0.3%) lower than the currently flying ACRIM
III, VIRGO, and ERBE instruments (right).
Greg Kopp, 4-6 Dec. 2003