The paper outlines the method developed at the Royal Observatory of Belgium in order to compute the TEC with a precision of about 2-3 TECU and to detect Travelling Ionospheric Disturbances using GPS measurements. We describe the solar cycle dependance of the TEC and TIDs. In 2000 and 2002, the ionospheric activity has reached two peaks correlated with solar activity. This ionospheric activity is characterized by larger TEC values (up to 90 TECU, in 2000 and 2002) and by a larger number of Travelling Ionospheric Disturbances. During period 2000-2002, the mean daily TEC was ranging from 10 to 45 TECU; at solar minimum the mean daily TEC was smaller than 12 TECU. In January 2000 and December 2001 more than 1300 “events” due to TID’s were detected: it is 6.5 more than in January 1996 (at solar minimum).