Effects of ionospheric small-scale structures on GNSS

Nowadays, Global Navigation Satellite Systems or GNSS allow to measure positions in real-time with an accuracy ranging from a few meters to a few centimeters mainly depending on the type of observable (code or phase measurements) and on the positioning mode used (absolute or differential). The best precisions can be reached in differential mode using phase measurements. In differential mode, mobile users improve their positioning precision thanks to so-called “differential corrections” provided by a fixed reference station. For example, the Real-Time Kinematic technique (RTK) allows to measure positions in real-time with a precision usually better than a decimeter. In practice, the ionospheric effects on GNSS radio signals remain the main factor which limits the precision and the reliability of real-time differential positioning. As differential applications are based on the assumption that the measurements made by the reference station and by the mobile user are affected in the same way by ionospheric effects, these applications are influenced by gradients in TEC between the reference station and the user. For this reason, local variability in the ionospheric plasma can be the origin of strong degradations in positioning precision. In this paper, we characterize local variability in the ionosphere which can pose a threat to high precision real-time differential positioning. GNSS carrier phase measurements can be used to monitor local TEC variability: small-scale ionospheric structures can be detected by monitoring TEC high frequency changes at a single station; as ionospheric disturbances are moving, we can expect that such structures will induce TEC temporal variability which can be detected at a single station. We applied this method (called the “one-station” method) to the GPS data collected at the permanent (mid-latitude) station of Brussels from 1994 to 2007 and performed a climatological study of the ionospheric structures on this period which covers more than one solar cycle. Two main types of structures have been observed: Traveling Ionospheric Disturbances (TID’s) and “noise-like” structures. TID’s have strong seasonal and solar cycle dependence when noise-like structures are “ionospheric variability” which is usually observed during geomagnetic storms. The largest Rate of TEC (RoTEC) detected at Brussels during the period considered in our study were observed during severe geomagnetic storms. Moreover, we found that strong irregularities occur even during solar minimum. This means that, even during periods where the probability of occurrence of ionospheric irregularities is very low, large RoTEC can occur. The one-station method allows to measure variability in time but GNSS differential applications are affected by variability in space between the user and the reference station. Therefore, in a second step, we measured TEC differential variability (using double differences of phase measurements) during few typical ionospheric conditions: quiet ionospheric activity, medium and large amplitude TID’s and noise-like variability due to a severe geomagnetic storm. We also analyzed the effects of the baseline length and orientation on the residual ionospheric term. As a last step, we developed a software which reproduces positioning conditions experienced by RTK users on the field. We used this software to assess positioning errors due to the different ionospheric conditions considered in the previous step. Again, the largest effects were observed during the occurrence of geomagnetic storms.