Statistical study of ionospheric small-scale irregularities at mid-latitudes using GPS measurements

At the present time, Global Navigation Satellite Systems (GNSS) allow to obtain a precision of a few centimeters in real time using positioning techniques like the so-called Real Time Kinematics (RTK). Nowadays, the ionosphere remains the most important error source affecting such GNSS applications. Indeed, the presence of small-scale irregular structures in the Total Electron Content (TEC) can strongly affect the reliability and the precision of high precision real time positioning applications. Those small-scale structures can have several origins: Traveling Ionospheric Disturbances, "noise-like" variability in TEC, scintillations. The Royal Meteorological Institute of Belgium (RMI) has developed a software which allows to monitor the TEC using GPS measurements (at one station). The software can also be used to detect small-scale irregularities in TEC by monitoring TEC temporal gradients (i.e. rate of change). The RMI has applied this software to the GPS data collected at Brussels from April 1993 up to now what represents a period of more than a Solar Cycle. The results obtained from this GPS-TEC software will be presented. First, the paper analyzes the different types of small-scale structures which have been observed in TEC. Then, a statistical study of those ionospheric irregularities is presented; we shall analyze their probability of occurrence and their amplitude (in TECU/min) depending on daytime, season, solar activity, geomagnetic activity... The results of our study which is financed by the GNSS Supervising Authority will allow to have a better understanding of the usual case/worst case conditions encountered in high precision positioning applications based on GNSS.