|Instrument: ACRIM : Active Cavity Radiometer Irradiance Monitor|
Solar/Space Observing Instruments
Instrument Class: Visible/Infrared Instruments
Wavelength Keyword: Near Infrared
Number Channels: 3
Spectral/Frequency Coverage/Range: 180 nm to 3.000 nm
Wavelength Keyword: Visible
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The objective of the Active Cavity Radiometer (ACR) Irradiance Monitor (ACRIM) is to measure the total solar irradiance with state-of-the-art accuracy and precision. The ACRIM instrument was first flown on the Solar Maximum Mission (SMM/ACRIM I) satellite and was flown on all three Atmospheric Laboratory for Applications and Science (ATLAS) missions and the Spacelab 1 misson from the Space Shuttle. In addition, ACRIM was flown as a flight-of-opportunity instrument on the Upper Atmosphere Research Satellite (UARS/ACRIM II) and is scheduled to fly as a flight-of-opportunity mission during NASA's Earth Observation System (EOS) program (ACRIMSAT/ACRIM III) The principal role of the ACRIM is to support extended solar irradiance experiments on free-flying satellites and establishment of the radiation scale at the solar total flux level through direct intercomparison with other experiments. The total solar irradiance (TSI) from far ultraviolet through far infrared is measured by three Type V active-cavity radiometer sensors. These detectors are electrically self-calibrated, cavity pyrheliometers each capable of defining the absolute radiation scale with an uncertainty of +/- 0.1% SI units. The single sample irradiance precision is +/- 0.012 %. The three sensors are independently shuttered and their measurement cycles are different so that the three sensors can be used in various combinations to provide periodic cross references on the system's performance.
The ACRIM contains four cylindrical bays. Three of the bays house independent heat detectors, called pyrheliometers, which are independently shuttered, self calibrating, automatically controlled, and which are uniformly sensitive from the extreme UV to the far infrared. Each pyrheliometer consists of two cavities, and temperature differences between the two are used to determine the total solar flux. One cavity is maintained at a constant reference temperature, while the other is heated 0.5 K higher than the reference cavity and is exposed to the Sun periodically. When the shutter covering the second cavity is open, sunlight enters, creating an even greater difference in cavity temperatures. The power supplied to the second cavity by the ACRIM electronics decreases automatically to maintain the 0.5 K temperature difference between the two cavities. This decrease in the amount of electricity is proportional to the solar irradiance entering the cavity. Additional details about the individual sensors is given by Willson (1979 & 1980) and of the instrument by Willson (1981). The fourth bay holds a sensor that measures the relative angle between the instrument and the Sun.
To guarantee precision, the ACRIM cavities have mirror-like black surfaces that reflect light toward the apex of the cavity, where 99.99998 percent of the Sun's incoming energy in the 180 to 3,000 nm wavelength range is absorbed. In normal operation the ACRIM is on a platform which tracks the Sun. One of its detector channels makes regular measurements while the other two are kept shuttered to reduce possible degradation by solar UV radiation, atmospheric or satellite outgassed gases, etc. Readings are taken at 1.024 second intervals. About once a month the second channel, B, is opened for comparison measurements; while at longer intervals the third channel, C, is also compared. This triple detector arrangement proved valuable. On the SMM Satellite channel A degraded about 600 parts per million compared to channel C during the 9.75 year mission. Channel B, opened roughly once a month, also showed a slight degradation compared to channel C by 1989. This degradation was allowed for in the calibration equation (Willson and Hudson, 1991).
Note: ACRIM II was not officially part of the UARS project. It was included on the UARS platform as an instrument of opportunity.
Data Rate: 32 kbit/s
Instrument Start Date: 1991-04-10
Instrument Stop Date: 2001-11-01
Instrument Owner: NASA + Jet Propulsion Lab - JPL