Distribution of the GPS signal interruption for the spacebore receiver on board Swarm satellites

This paper focuses on the Global Positioning System (GPS) signal interruption at middle latitudes for the spaceborne receiver on board the ESA's Swarm satellites. Using nearly 9 years data from 2014 January to 2022 September, the spatiotemporal distributions of these GPS signal interruption events as well as their relation with the fluctuation of ionospheric plasma density along Swarm orbits have been investigated. Taking Swarm C as example, we found that the GPS signal interruptions show totally different spatiotemporal characteristics before and after the phase-locked-loop (PLL) bandwidth of Swarm receiver changed from 0. 75 Hz to 1. 0 Hz. During the first period from 1 January 2014 to 11 August 2016, GPS signal interruptions are found mainly located in the vicinity of low ( forming two bands along the magnetic equator) and polar latitudes, indicating the ionospheric sources for causing such GPS signal interruptions as reported by Xiong et al. (2018). However, during the latter period from 11 August 2016 to 30 September 2022, the GPS signal interruptions occur randomly at all latitudes, but show prominent dependence on the longitude. In addition, the GPS signal interruption events exhibit obvious seasonal distributions during the first period, rather than the second period. The different distribution of GPS signals during two period is associated with the enlarge of PLL bandwidth of Swarm receiver from 0. 75 Hz to 1. 0 Hz, which promotes the PLL dynamic range, and increase also the thermal noise of carrier phase, which further causes the random transient disruption of signals. Apart from the discrepancy mentioned above, there are also common features during the two periods. For instance, interruptions mainly accrue for the signal rays from GPS satellites to Swarm at low elevation angle (less than 20). Moreover, interuptions are mostly observed for GPS satellites of types Block IIR and Block IIA, which were launched earlier than the other types. This result also suggests that the stability of GPS satellites becomes worse, with the increasing years after launched and reduced performance of on-board instrument. By a further comparison associated with the in-situ electron density measurements of Swarm, GPS signal interruptions was found to be easier to happen when the fluctuation of electron density is more severe from low to middle latitude, suggesting that the gradient intensity of background plasma density is an important criteria to cause the GPS signal interruption. Our results are helpful for the optimization of the spaceborne receiver, in particularly to promote the stability of receiver under ionospheric scintillation.