Skyplots with the distribution of GPS satellites and colour-coded L2 signal strength during the SRB on 24 September 2011 at 13:05:30 UTC for IGS stations VILL (left panels) and YEBE (right panels). The colour code is: green (for signal strengths higher than 36 dBHz), yellow (in the range 30–36 dBHz), orange (18–30 dBHz), red (lower than 18 dBHz) and white if the satellite is not available. The thresholds follow the standardized signal strength indicators scale. The top panels indicate the expected undisturbed conditions, while the bottom panels show the conditions observed during the SRB.