The characteristics of the 11-year solar cycle were of mere academic curiosity; but in the present satellite age, the strength of the solar cycle makes a huge difference to satellite operators, who plan their launches many years in advance. Each solar peak heats and expands the outer atmosphere, which in turn increases the drag on satellites, especially those in low-Earth orbits. Hence, satellite planners decide their missions and adjust orbital heights to take advantage of weak solar activity, if possible. Occurrence of a solar peak earlier or later or of unexpectedly large magnitude could alter the expected useful life of the satellite. Predictions of the solar activity are also useful for other purposes, such as operation of power grids on Earth and satellite communication systems. Solar activity forecasting is an important topic for various scientific and technological areas, like space activities related to operations of low-Earth orbiting satellites, electric power transmission lines, high frequency radio communications and geophysical applications. Therefore, the prediction of sunspot cycle is one of the important activities of Regional Warning Center (RWC-India), operated by NPL since last more than 30 years, as part of International Space Environment Services (ISES) which run 12 RWCs all over the globe.

Based on cycles 17-23, linear correlations are obtained between 12-month moving averages of the number of disturbed days when Ap is greater than or equal to 25, called the disturbance index, DI, at thirteen selected times (called variate blocks 1, 2, …each of them in six-month duration) during the declining portion of the ongoing sunspot cycle and the maximum amplitude of the following sunspot cycle. In particular, variate block 9, which occurs just prior to subsequent cycle minimum, gives the best correlation (0.94) with a minimum standard error of estimation of ±13, and hind casting shows agreement between predicted and observed maximum amplitudes to about 10 percent. As applied to cycle 24, the modified precursor technique yields maximum amplitude of about 124 ± 23 occurring about 45 ± 4 months after its minimum amplitude occurrence, probably in mid- to- late 2011 as shown in Fig. 6.1.