GPS Jamming and GPS Carrier-Phase Time Transfer

This paper studies the impact of GPS jamming on GPS carrier-phase time transfer. To study this issue, at NIST, we have installed a commercial GPS jamming detector since 2014 April. During 2014 April – 2015 April, the detector detected more than 100 jamming events, though there had been a few outages of jamming detection. The jamming events usually last for less than 2 min. We find that almost all jamming events lead to a significant drop in the L1 signal-to-noise ratio (SNR) for all observable GPS satellites. Another thing we notice is that the 3 GPS receivers which are closer to Broadway, a main street in Boulder, Colorado, are more likely to be jammed. This indicates that the jamming source may come from cars passing by. Although a jamming event causes a significant drop in L1 SNR, the GPS receiver can still track the GPS satellites properly for most cases. However, sometimes, the jamming can be too strong and then a GPS receiver may lose track of some GPS satellites. This leads to a GPS-data anomaly. Because of this anomaly, the carrier-phase time transfer processing re-estimates the phase ambiguities at the anomaly. Thus, there is often a time discontinuity at the anomaly. The discontinuity ranges from a few hundred picoseconds to a few nanoseconds. Then the next question is what we shall do when a jamming event occurs? Our earlier study [1] shows that the 9th-order polynomial curve fitting for the code and phase measurements can repair a short-term data anomaly (< 40 min). We apply this technique to repair the anomaly at jamming and it works well. Thus, we can eliminate the impact of a short-term jamming (< 40 min) on carrier-phase time transfer by repairing the GPS data.