Existing solar radiometers have sufficient short term precision to measure irradiance variations generated by solar rotation over time scales of days and weeks, but the precision of the measurements over the 11-year activity cycle is much less secure because of instabilities in radiometric sensitivity. Critical is the recognition that true solar irradiance variations cannot be reliably determined from successive measurements by different instruments unless they can be intercompared via overlapping flight epochs. This is because the systematic errors in current state-of-the-art solar radiometric metrology are of the order of of the solar cycle variability itself. Improvements in instrument precision and calibration accuracy are thus important. Furthermore, it must also be recognized that because satellite instruments can (and do) fail, concurrent measurements by at least two instruments are essential to ensure the continuity of the data base. Previous studies by the National Academy of Sciences (1988, 1991) have also emphasized the need for overlapping data bases.
The detection of solar luminosity variability during solar cycles 21 and 22, and the interpretation of this variability in terms of solar magnetic activity, thus far underscores the need to extend the solar irradiance data base indefinitely with maximum possible precision. The data are needed for the forseeable future to reduce the uncertainties in the detection of anthropogenic climate forcing. A careful measurement strategy will be required to sustain adequate precision ( < 50 ppm, or 0.005 percent). Due to the likelihood of instrument degradation, solar monitoring experiments using current radiometric technology can be expected to last no more than one decade. Drifts in sensitivity throughout a 10-year mission must also be anticipated and detected. Data gaps through instrument failure must also be prevented. Therefore, adequately overlapping experiments and intercomparison of successive experiments is crucial.
Continuation of the total solar irradiance data base that extends from November 1978 to the present is in serious jeopardy. The current and proposed total solar irradiance monitoring program shown in Figure 2.1 relies almost exclusively on one upcoming NASA mission, and as presently conceived will not satisfy the requirement for continuous overlapping experiments, nor even for third party comparisons between