Power spectral characteristics of topside GPS signal intensity and in-situ irregularities at low latitudes using high-sample-rate Swarm Echo observations

Low-latitude ionospheric irregularities disrupt radio signals like those from GNSS by causing rapid intensity and phase fluctuations, also known as scintillations. Using high-resolution, simultaneous measurements from the Swarm Echo (CASSIOPE/e-POP) satellite, we analyzed the spectral characteristics of topside (between 350 and 1280 km altitude) GPS signal intensity and in-situ irregularities at low latitudes. Signal intensity observations were obtained from the e-POP GPS Occultation (GAP-O) receiver after its antenna was re-oriented towards zenith. Relative variations in the satellite’s IRM surface current measurements are utilized as a proxy for in-situ electron density irregularities. Both GPS intensity and plasma density irregularity spectra exhibit a power-law behavior. Consistent with previous studies, the most frequent spectral index for density irregularities is found around 1.7, while all GPS intensity spectra show a single spectral index near 2. Our estimates of roll-off frequencies (0.4-2.5 Hz) in the GPS signal intensities are considerably higher than the 0.2-0.45 Hz range of Fresnel frequencies obtained from ground. This difference is largely due to satellite’s high orbital velocity and the dominant effect of the topside ionosphere on the Swarm Echo’s measurements compared to the relatively dense F-region plasma in the 250–350 km altitudinal range probed by ground-based GNSS signals. This research provides valuable insights into the spectral behavior of topside low-latitude ionospheric irregularities and their impact on GNSS signals.