The Satellite


by Robert B. Lee, III, NASA Langley, and Robert S. Wilson, SAIC

From 1984 to the present, total solar irradiance (TSI) values were obtained from the solar monitor on the Earth Radiation Budget Satellite (ERBS) nonscanner instrument. The ERBS solar monitor is an active cavity radiometer similar in design to the Active Cavity Radiometer Irradiance Monitors (ACRIM) which have flown on the NASA Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS), and Atmospheric Laboratory for Applications and Science (ATLAS) spacecraft missions. The ERBS satellite was placed into orbit on October 5, 1984 and the solar monitor is still operating properly, after 18 years. In the PowerPoint presentation is a graph of the entire time series. The ERBS solar monitor time series covers the period from October 25, 1984 through October 15, 2003. The measurement precision is approximately 0.01 percent while the accuracy is 0.2 percent. ERBS is still monitoring the TSI, but the data are no longer being reduced due to budget constraints.

The ERBS data reduction model is described in considerable detail in Reference 1. In Reference 2, analyses of the ERBS time series were presented as well as comparisons of the ERBS time series with those of the ACRIM Solar Maximum Mission (SMM) and Nimbus 7 Earth Radiation Budget (ERB) Channel 10c pyrheliometers. Recently, Lee and Wilson (1999) and Lee et al. (2002) presented analyses of the 1984-2002 ERBS TSI values, and comparisons among the ERBS values and those values from the Upper Atmosphere Research Satellite (UARS), Solar and Heliospheric Observatory (SOHO)/Variability of solar IRradiance and Gravity Oscillations (VIRGO), and Space Shuttle Atmospheric Laboratory for Applications and Science (ATLAS) Solar Constant (SOLCON) Spacecraft missions.

In the Table, the format for the total solar irradiance (TSI) values is:

  • Column 1 -- Calibration date - year/month/day
  • Column 2 -- Measurement time (universal) - hour:min:sec
  • Column 3 -- Total Solar Irradiance (Watts/meters squared) at 1 AU Corrected for Off-axis viewing and normalized to Astronomical Almanac Earth-Sun Distance tables
  • Column 4 -- Standard Deviation of averaged samples (Watts/meters squared) -- 0 indicates 1 sample or very close instantaneous samples

In Table 1, the individual total solar irradiance values represent orbital averages of the instantaneous measurements which are corrected for the angle between the instrument optical axis and the Sun and which are normalized to the mean Earth/Sun distance. At least once every 14 days, the Sun is observed by the monitor for several 64-second measurement intervals. Each interval is separated into two 32-second periods. During the first period, the Sun drifts across the 9.2-degree unocculted field of view, and its radiation field is measured. During the second period, a low-emittance shutter, representative of a near-zero irradiance source, is cycled into the field of view, and the low irradiance from the back of the shutter is measured. The resulting averaged measurements from the several 64-second time intervals are used to define the irradiance, using the algorithms which are described in Reference 1.

Typically, two to four values of the irradiance are determined during an orbit. Considering that these irradiance values are derived typically during a single orbit for a few minutes, the averaged irradiance values represent an almost instantaneous level, and not a daily average.

Between 1984 and 1993, the solar monitor was operated continuously with the exception of the July 2-3, 1987, September 4-9, 1992, and July 2-3, 1993 when spacecraft attitude control or battery cell failure problems caused the monitor to be turned off. Between July 18, 1993 and November 21, 1993, the monitor was turned off because the spacecraft battery system and the flight operations procedures could not provide sufficient power to all of the spacecraft sensors. Therefore, there are no data available for this period. The 14-day measurement schedule was resumed after November 22, 1993 when flight procedures were revised to provide sufficient power to the monitor. Beyond Feb. 28, 1998, we have processed with the exception of a few days, all of the 1998 ERBS TSI measurements. During March 1998, no ERBS TSI measurements were obtained because the ERBS/ERBE telemetry dedicated to earth and solar radiation measurement failed. The April through December 1998, TSI measurements were relayed in a different format using the remaining ERBS/SAGE telemetry system. ERBS TSI measurements are also not available for later December 1998 and early January 1999 because the ERBS spacecraft had another battery cell failure.

The TSI measurements started again on January 6, 1999. Also, between September 1999 and December 1999 no calibrations were done due to a failure in the elevation gimbal of the non-scanner instrument. Solar measurements were resumed for the solar monitor only December 22, 1999 using a different command sequence.

In Table 1, the measurement standard deviations (STD) increased significantly when the power was turned off for 1 to 8 day [an average of 4 days] every 22 to 40 days [an average of 30 days].

Specialized irradiance measurement missions were conducted during March 23, 1992 through April 2, 1992, January 16, 1993 through January 30, 1993, April 6, 1993 through April 22, 1993, November 4, 1994 through December 13, 1994, August 8, 1997 through August 25, 1997 and October 29, 1998 through November 11, 1998. The specialized missions included increased measurement opportunities over three to six orbits each day compared to the typical single orbit measurements. The missions were extended to as much as 10 consecutive days of measurements.


  1. R. B. Lee III, B. R. Barkstrom, and R. D. Cess, Characteristics of the Earth Radiation Budget Experiment Solar Monitors, Appl. Optics, 26-15 3090-3096, 1987
  2. R. B. Lee III, M. A. Gibson, R. S. Wilson, S. Thomas, Long-term Total Solar Irradiance Variability During Sunspot Cycle 22, Journal of Geophysical Research, Vol. 100, No. A2, pp. 1667-1675, February 1, 1995
  3. Robert B. Lee III and Robert S. Wilson, Validation of 1985-1997 Active Cavity Radiometer Spacecraft Measurements of Total Solar Irradiance Variability, Proceedings of SPIE [San Diego, CA; July 19-21, 1998, Vol. 3439, pp. 377-388, 1998
  4. Robert B. Lee III, Robert S. Wilson, K. J. Priestley, A. Al-hajjah, J. Paden, D. K. Pandey, and S.Thomas, 1978-1998 Total Solar Irradiance Variability Trends, Proceedings of 10th Conference on Atmospheric Radiation [American Meteorogical Society, Madison, WI, June 28 - July 2, 1999], paper No. 5.12.
  5. Robert B. Lee III and Robert S. Wilson: Total solar irradiance (TSI) variability: 1984-2002 Earth Radiation Budget Experiment (ERBE)spacecraft measurements. Presented at 2002 Spring AGU meeting, Washington, D. C., May 28-31, 2002, Eos Supplement, Vol. 81, No. 19, S133, May 9, 2002 S133, June 9, 2003

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