Diurnal, monthly, and solar activity variations in L-band signal fluctuations are examined by ROTI (Rate Of Tec Index) derived from measurements of worldwide ground-based GNSS (Global Navigation Satellite System) receivers in 2000, 2009, and 2013. Routine observations of the ionospherically imposed propagation effects upon GNSS satellite signals are available online from IGS (International GNSS Service). With data over 2000 ground-based IGS stations of the globe, ionospheric TEC with the 30-sec time resolution can be derived. The standard deviation of TEC variations every minute in a 5-minute interval is further computed to obtain ROTI around each receiving station. Variations in diurnal, seasonal, solar activity, and geographic distribution of ROTI are examined during the solar maximum year of 2000, solar minimum year of 2009, and solar median year of 2013. ROTIs are proportional to the solar activity, those in the high-latitude ionosphere are much more intense than in the low- and mid-latitude ionosphere. In the low-latitude ionosphere, intense ROTIs frequently occur within ±30° magnetic dip, start at 1930 LT (Local Time); reach their maximum at 2000-2100 LT, and vanish by about 0200 LT; and appear prominently in March and September equinox seasons. The region experiencing the most intense ROTI is the low-latitude ionosphere in South America. The low-latitude ROTIs often exhibit a prominent double-peaked (or crest) structure centering at 20°N and 20°S dip latitude, especially in high/median solar activity years of 2000 and 2013. Monthly-longitude plots of low latitude ROTIs look like a masquerade mask with two eye holes (i.e., ROTI free).