SUMMARY OF THE EVENT:
This was an unexpected phenomenon! We monitor abnormal fades of GPS signal reception over the whole real-time EPN stations (www.epncb.oma.be) to warn about Solar Radio Burst (SRB) impacting the GNSS applications. To do so, we compute the carrier to noise density observations (C/N0) with respect to their quiet behaviour (C/N0 median of the 7 previous ground track) and output the median ‹ΔC/N0› of all satellites-receiver pair of the EPN. The use of a large number of stations distributed over the continent allows to avoid the detection of other sources of GPS signal reception fades (such as local Radio Frequency Interferences or scintillations)… But we did not anticipate this one.
The 13th of April 2018 around noon, we observed at the L1 frequency a general fade of -1dB-Hz, leading to a first warning of a potential small SRB detected… However, in the meantime, the signal reception at the L2 frequency was improved and hence dismissing the hypothesis of a SRB.
Looking closer, the signal reception was not fading for all satellites: 19 satellites belonging to the block IIF and IIR-M had their signal strength changed (see Figure for the station BRUX and the GPS satellite 25). The signal power was stronger on L2 with an average of +4.5dB-Hz but at the cost of the L1 signal with a fade of -1.5dB-Hz.
This signal strength remained until the 17th of April. Around 15:30 UTC, signal started to be re-established to the original situation to be totally back around 23:00 UTC.
We switched off the emails warnings during this period 14th – 19th April to avoid repetitive emails of false SRB warnings. The change of signal strength should not have had an impact on your GNSS applications, as a fade of 1dB-Hz should not disturb the GNSS applications.
Figure : Carrier to Noise Density (in red) and its expected behaviour (in blue) for a satellite track at the begining of the event (left) and the end (right)