The Total Irradiance Monitor
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 Total Irradiance Monitor (TIM) is a total solar irradiance (TSI) instrument designed to achieve a relative standard uncertainty (1 σ accuracy) of 100 parts per million (ppm) and a
precision and long-term uncertainty of 10 ppm/year. This instrument is one of four on the Solar Radiation and Climate Experiment (SORCE), a NASA/EOS satellite mission scheduled for
launch in Nov. 2002. The TIM will report four TSI measurements d aily throughout SORCE’s goal 5-year mission life.
New Feature #1
Phase Sensitive Detection Improves Sensitivity
Phase sensitive detection
reduces sensitivity to
thermal drifts and noise
not at and in phase with
the shutter fundamental,
such as parasitic thermal
changes in the internal
instrument temperature.
New Feature #2
NiP Black Provides Robustness
Precision apertures
2. determine area over which
sunlight is collected.
3.
4. Highly absorptive and stable
Interior baffles reduce
stray light.
surfaces of NiP black absorb
sunlight, heating the active
Electrical Substitution
Radiometer (ESR).
1.
Apertures Are Measured to ~50 ppm
5.
Entering sunlight is
modulated by shutters.
Electrical power to thermally
stabilize paired active and
reference ESRs determines
the input radiative power.
Four ESRs provide
redundancy and degradation
tracking via duty cycling.
6.
NIST measured apertures geometrically.
Apply optical corrections to NIST
measurements to account for:
• Diffractive losses
• Scatter off edge imperfections
• Reflections off interior bevel
• Solar spatial and spectral distribution
Known very accurately from
VSOP87, JPL DE200 ephemeris,
and NORAD
Measurement
Uncertainty [ppm]
Aperture area (geometric) - NIST measurements
25
Diffraction correction (452 ppm ± 10%)
47
Aluminum thermal expansion (23 ± 1 ppm/K)
12
Baffle diffraction corrections - NIST calculated
1
Edge scatter measured optically w/ CCD system
10
Total (RSS)
55
Measured pre-launch;
low sensitivity to effect
Phase sensitive
detection at the shutter
frequency reduces
sensitivity to thermal
drifts and noise.
Nickel phosphorous (NiP)
black coats the inside of the
ESRs, providing a lightabsorbing surface across a
broad spectral range. This
metal
is
robust
against
degradation to UV and space
radiation, yet provides high
thermal conductivity.
Vacuum enclosure prevents contamination
through integration and launch.
a r e
n e e d e d
t o
u s e
t h i s
p i c t u r e
Correction
1. Distance to Sun
2. Doppler Velocity
Value [ppm] δ [ppm]
±33,444/yr.,
0.1
93/orbit
0.
±57
7
3. Shutter Waveform
4. Aperture
100
1
1,000,000
55
200
54
100, AC
16,000
21
1
5. Cone Reflectance
6. Equivalence ratio, ZH /ZR
7. Servo Gain
The TIM will continue a 24-year record of solar irradiance measurements from space.
The assembled TIM instrument prior to integration with the SORCE spacecraft.
ESR Reflectances Are Measured to Desired Accuracies
Laser maps of the ESR interiors give reflectances. Measurements
at multiple wavelengths give a solar-weighted spectral average.
TIM Achieves a 100 ppm Uncertainty Budget
8. Standard Volt + DAC
1,000,000
9
9. Standard Ohm, Leads
1,000,000
17
2,700
1.7
100
14
83
10. Dark Signal
11. Scattered Light & IR
Total RSS
Cone
A
B
C
D
R [ppm] 168.6 138.9 306.8 360.5
Modeled; measured on-orbit
using high order harmonics
Instrument Description
Electrical Substitution Radiometers (ESRs) measure incident radi ant power. The four ambienttemperature TIM ESRs operate in pairs, with a DSP-controlled servo system continually
balancing the temperature of an active ESR to its paired reference ESR. Incident sunlight
passing through an open shutter and a precision aperture radiatively heats the active ESR. The
replacement electrical heater power required by this ESR to maintain thermal balance as the
shutter is cycled determines the incident solar irradiance.
The relative standard uncertainty of this primary instrument is 100 ppm (1 σ) with 2 ppm noise.
References
• Lawrence, G., Rottman, G., Harder, J., and Woods, T., “Solar Total Irradiance Monitor (TIM),” Metrologia 37, 2000, pp.
407-410.
• Lawrence, G.M., Rottman, G., Kopp, G., Harder, J., McClintock, W., and Woods, T., “The Total Irradiance Monitor
(TIM) for the EOS SORCE Mission,” Earth Observing Systems V Proc. SPIE 4135-21, 2000, pp. 215-224.
• Woods, T., Rottman, G., Harder, G., Lawrence, G., McClintock, B., Kopp, G., and Pankratz, C., “Overview of the EOS
SORCE Mission,” SPIE 4135, 2000, pp. 192-203.
• SORCE web page: http://lasp.colorado.edu/sorce/
Measured pre-launch using
scattered light experiment
Reference voltage is driven by a
stable 7.1 V Linear Technology
LTZ1000A circuit
Measured on-orbit during eclipses
Noise Is <2 ppm
Measured on-orbit from response
to known input
TIM Parameters
ESRs Contain Precision Resistor
Parameter
QuickTime™ and a
Photo CD Decompressor
are needed to use this picture
The TIM achieves less than 2 ppm noise over
its 400-sec bandwidth at the 100-sec shutter
fundamental.
The standard resistor, encased in each ESR, is
wire wound, stress-controlled Evenohm alloy.
Wavelength Range
Relative Standard Uncertainty
Relative Stability
Sensitivity
Size (instrument)
Size (electronics)
Mass
Power
Design Lifetime
FOV
Pointing Requirement
Precision Aperture Size
Shutter Frequency
Cone Reflectance (1 −α)
Operating Temperature
Thermistor Sensitivity
Value
UV, visible, NIR
100 ppm (1σ)
10 ppm per year
2 ppm
34.732 long x 30.312 wide x 20.671 high [cm]
26.16 long x 21.84 wide x 6.10 high [cm]
7.869 kg instrument, 2.3398 kg electronics
14 W
5 yrs
sensitive to light in ±6° cone
±10 arcmin for ±100 ppm change
0.5 cm 2
0.01 Hz
200 ppm
5° - 20°C
0.001°C
Greg Kopp, 12 August 2002