Objective
The TIM will provide a measurement of total solar irradiance (TSI)
with an absolute accuracy of 0.01% and a relative stability of 0.001%
per year. Imperative for climate modeling, this instrument will report
the total radiative input at the top of the Earth's atmosphere four
times each day.
Above : TIM Irradiance Measurements
Measurement Concept
This new instrument uses the best heritage of previous radiometers, but is enhanced with modern, state-of-the-art technologies including phase-sensitive detection, metallic absorptive materials, and digital electronics. Any one of four electrical substitution radiometers (ESRs) can measure the incident solar power. A precision aperture defines the area through which sunlight is collected, and the ratio of sunlight power to area gives the solar irradiance (Watts per square meter - W/m2). The TIM achieves extremely low noise using phase sensitive detection at the shutter frequency. This detection method reduces sensitivity to noise, thermal drifts, and thermal emission from the instrument itself, improving the signal/noise ratio compared to traditional time domain analysis. Using this technique, the TIM has demonstrated noise levels less than 0.0002%.
Above : The Electrical Substitution Radiometer
What is an Electrical Substitution Radiometer, or ESR?
The TIM measurement of solar radiation is a thermal measurement and the radiation sensor is a gold-coated silver cone. Each of the four identical cones has a precision temperature sensor and heater wire on the outside. The cones are used in pairs - one cone is active and the other is the reference. When a shutter in front of the active cone opens, solar radiation passes through a precision aperture and is absorbed by the active cone's black interior. Sophisticated electronics sense a temperature rise and quickly reduce electrical power to the active cone in order to maintain thermal balance to the reference, which remains shuttered. The amount of electrical power removed from the active cone is "equivalent" to the power of the incident solar radiation.
How Do the TIM ESRs Work?
Each sensor is an electrodeposited silver cone with an interior coating
of nickel phosphorus (NiP), a metallic black absorbing layer stable under exposure
to solar ultraviolet light. Several temperature sensors in the TIM allow
corrections for thermal changes. The thermal background from the instrument
itself is established from measurements of dark space during the night portion
of each orbit.
Four ESRs provide redundancy and calibration comparisons. To monitor the stability of the ESR absorptance,
the TIM will use the four ESRs to characterize instrument degradation with varying duty cycles. Comparisons
between nearly simultaneous measurements of the TSI from the different ESRs indicate absorptance changes
attributable to solar exposure.
Above: SORCE's TIM Instrument Illustration
Irradiance: The TIM Measures Power Over a Known Area
Sunlight entering any one of the four ESRs is modulated by a shutter (the rightmost one is shown in its open position). A precision aperture behind each shutter accurately determines the area over which sunlight is collected. This radiant power is then measured by a black absorptive ESR. The ratio of power to area gives the total solar irradiance.
Above : Inside the TIM Instrument
What Is Inside the TIM?
Sunlight enters through vacuum doors that open on orbit and maintain cleanliness during ground tests and instrument integration.
Above : Inside the TIM Instrument
