Features of observing for weak MSTIDs by GNSS satellites

To analyze the weak midlatitude medium-scale travelling ionospheric disturbances we use the sufficiently dense network of GNSS receivers (more than 150 ground-based stations). We show the reason that the monitoring for MSTIDs by GNSS satellites is fragmented. Comparing of satellites R18 and G18 data (at 9:40 UT on September 21, 2016) revealed that the LOS directed to ongoing MSTIDs front is preferred for MSTIDs observing. The characteristics of extremely weak MSTIDs with TECp values ~0.2 TECU, wavelength ~100 km and western direction of phase front moving are presented. We estimate these characteristics as a threshold for monitoring the MSTIDs by GNSS network we used.