The Satellite

UARS/ACRIM II Total Solar Irradiance Measurements

October 1991 - December 1993

Note -- for more current data, please visit the ACRIM website or see the NGDC Total Solar Irradiance webpage.

R.C. Willson
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, California 91109 USA

The second Active Cavity Radiometer Irradiance Monitor (ACRIM) satellite solar monitoring experiment (ACRIM II) has been providing total solar irradiance observations since its launch as part of the Upper Atmospheric Research Satellite (UARS) in late 1991. The UARS is a three-axis stabilized, Earth-oriented spacecraft with an orbit at inclination of 57 degrees and altitude 585 km. The UARS orbit provides about 60 minutes of sunlight in each orbit of which about 35 minutes are available for solar viewing. During this period the Solar/Stellar Pointing Platform points the instrument to the center of the Sun (Reber, 1990).

The UARS/ACRIM II instrument consists of three Active Cavity Radiometers (ACR's) (Type V). The ACR's are electrically self-calibrated pyrheliometers, which are uniformly sensitive from the extreme UV to the far infrared. The principle of measuring total solar irradiance is that the heating effect of irradiant flux on a detector is compared with that of electrical power dissipated in a heating element in intimate thermal contact with the detector. An accurate knowledge of the effective absorptance of the detector for the irradiant flux, the area over which the detector is illuminated and the electrical heating power facilitates the accurate measurement of irradiant fluxes on an absolute basis in the International System of Units. The total solar irradiance data, expressed in Watt per square meter at the instrument, are calculated based on the equation:

               Hsol = K(Pref-Pobs)+E

where Hsol is the calculated irradiance, Pref and Pobs are the cavity electrical heating powers during the reference and observational phase of the measurements. K is the standard detector constant of proportionality which contains instrument parameters, such as the area of the primary aperture, effective cavity absorptance for solar irradiance, cavity reflectance for solar irradiance, and reflectance of solar radiation by the cavity field of view. E summarizes small terms due to small departures from instrument equilibrium.

Nominal observations are comprised of multiple cycles of an open-shutter/closed-shutter 131 seconds cycle. The observational sequence is symmetrical to minimize systematic error. Settling time for the ACRs is less than 30 seconds. The first half of each 65.536 seconds shutter open cycle is disregarded in the computation of the shutter-cycle irradiance result. A single shutter cycle solar observation uses the average value of the electrical self-calibration during adjacent shutter closed periods to provide an average electrical self-calibration reference. Further details for the instrument operation are given by Willson (1979, 1984).

Corrections for temperature dependence, solar viewing angle, Sun-satellite distance and relative velocity, and sensor degradation are applied to the calculated Hsol values to obtain the total solar irradiance data reduced to the mean Sun-Earth distance. The ACRIM's Sun position sensor signals and instrument temperature data are used to correct solar pointing errors and temperature dependencies. Astronomical ephemeris data supplied by JPL's Navigation System section are used to reduce the irradiance data to the mean Sun-Earth distance. These corrections include (1) the annual Sun-Earth distance variation, (2) the orbital Sun-satellite distance variation, and (3) the relativistic effect of the Earth's and satellite's orbital velocities relative to the Sun. Corrections for sensor degradation are made by the inflight intercomparison of Channels A, B and C. In case of ACRIM II, Channel B is the primary solar detector, while A is operated once a month, and C is operated once in every second month. These corrections have been applied to each shutter cycle irradiance results. The daily averages of the ACRIM II total solar irradiance are calculated from about 240 daily shutter irradiance values. The uncertainty of the daily average ACRIM II total irradiance values is about 5 ppm.

The viability of the long term total solar irradiance database depends on the precision with which successive satellite solar monitors' results can be related. Since the absolute uncertainty of current solar monitoring instrumentation is unable to provide the required precision (about 0.25 %/century), the Total Solar Irradiance database relies upon an 'overlap strategy' in which successive satellite experiments are directly compared, thereby transferring their operational precision to the database. Since the UARS/ACRIM II experiment began two years after the end of SMM/ACRIM I, this strategy could not be implemented. The next best option was derivation of a relationship between their results based on mutual comparisons of the ACRIM I and II experiments and Nimbus7/ERB. The results reported for ACRIM II here are based on the instrumentation parameters of the sensors. To relate these observations to those of the SMM/ACRIM I, a multiplicative factor of 1.002069 can be used (see figure). While the 1 sigma standard error of this factor is < 20 ppm, it is recognized that significant uncertainty has been introduced by uncalibrated degradation of Nimbus7/ERB during its last years of operation. Current efforts are being expended to ascertain the nature and impact of this degradation on the long term database.

The data files contain the daily average UARS/ACRIM II total solar irradiance in units of W/m2. The data are available on-line at the ACRIM website. They are also available on the NGDC on-line ftp anonymous (ftp If these data are used in publication please acknowledge as follows: UARS/ACRIM II total solar irradiance used here are produced at the Solar Irradiance Monitoring Group of the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration.


  1. Reber, C.: 1990, EOS Trans. AGU 71, 1867.

  2. Willson, R.C.: 1979, Applied Opt. 19, 3256.

  3. Willson, R.C.: 1984, Space Sci. Rev. 38, p. 203.

  4. Willson, R.C.: 1994, In "The Sun as a Variable Star: Solar and Stellar Irradiance Variations" (eds. Pap, J.M., Frohlich, C., Hudson, H.S, Solanki, S.K.), Cambridge Univ. Press, 1994.

Click here to go back to the solar irradiance introduction page and see a plot of the data.