An Out-of-Band Signal Jamming GNSS L1-Band in Observatoire de Paris

LNE-SYRTE in Observatoire de Paris (OP), located in Paris (France), is operating for time transfer an ensemble of calibrated GNSS stations, which stayed over years close to each other in the ns range. In addition, the typical daily average offsets between GPS Common-View (CV) and Two-Way Satellite Time and Frequency Transfer (TWSTFT) were remaining largely below 2 ns on the links between UTC(OP) and a selection of remote UTC(k) after independent relative calibrations. But from November 2018 on, a large discrepancy either between the different OP GNSS stations or between GPS CV and TWSTFT, together with an irregular loss of GNSS data, could be observed. The cause was identified as being an out-of-band powerful signal transmitted from nearby OP jamming GNSS L1-Band. The French Agence Nationale des Fréquences (ANFR), which is among other missions in charge of the use of RF spectrum over French territory, helped to identify the source and to establish a limited power transmission not having a significant effect anymore on OP GNSS L1-Band collected data. We describe the GNSS station ensemble located in OP at the time. We provide an analysis of the jamming signal, together with the effect of the jamming signal on time transfer activities. After agreement on the power transmission, the noise of GNSS data collected in OP became normal again, as can be seen on Allan Time Deviation (TDEV) analysis. We show the monitoring we put in place for the spectrum survey around the GNSS L1 carrier, with an example on a resurgence of this jamming signal after some maintenance activities at the transmission site. We emphasize the filtering of the jamming signal by different GNSS antennas as we could see from equipment already available in OP.