## 3 Observation of Scintillation in OEM7 Measurements
### Spatial decorrelation of phase under scintillation  
The primary concept of differential GNSS processing is to mitigate correlated error sources through the base-rover double differencing technique. Specifically, assuming a homogeneous ionospheric impact within a local region, it is assumed that implementing between-receiver differential processing can significantly reduce ionospheric-related errors. However, the effectiveness of this approach depends heavily on the spatial correlation as influenced by the baseline length under varying ionospheric conditions. The focus here is to investigate the spatial correlation of signal amplitude and phase as a function of the base-rover baseline length. If the signals are spatially correlated, it is expected that scintillation will affect both spatially separated antennas in the same manner. Consequently, in short baseline applications like heading determination, the impact of scintillation may be minimized since both antennas experience similar effects simultaneously. However, this correlation also implies that when signal fading occurs, both antennas are affected equally, offering no advantage in terms of spatial diversity. On the other hand, if the signals are spatially uncorrelated, combining signals from different antennas can be leveraged to enhance signal availability, especially in environments prone to signal disruptions or fading. Figure 7a shows the C/N\({}_{0}\) variation over time for GPS PRN 14 L1 CA using spatially separated antennas with a 7720 OEM7 dual-antenna card, with the antennas positioned less than 10 m apart (given that the 7720 cards share a single oscillator to track both antennas). Figure 6: Power spectrum for GPS PRN 18. The plot reveals a highly correlated amplitude fading effect between the antennas. Figure 7b illustrates single-difference carrier phase measurements between the two antennas (Ant1 and Ant2). Despite scintillation, the quality of the differenced carrier phase measurements remains unaffected. Therefore, it is anticipated that carrier phase-based applications operating with very short baselines are generally resilient to scintillation effects [9], unless disruptions like a loss of tracking lock occur, impacting measurement availability. To investigate the spatial correlation of ionospheric delay over longer baseline separations under scintillation conditions, data collection was conducted using a 6.5 km baseline between a base and a rover. The characteristics of carrier phase double-differenced (DD) measurements are essential for high-precision carrier phase-based positioning systems like RTK. Figure 8a shows carrier phase DD measurements for GPS L1 CA signals from several satellites, comparing measurements with and without scintillation impact. Figure 8b displays GPS PRN 23 carrier phase DD measurements alongside its elevation angle for reference. In Figure 8b, during the satellite's rise and fall, there are approximately two cycles of phase variations between the base and the rover. This variation is gradual, allowing the receiver to potentially model or estimate these ionospheric impacts. Furthermore, most positioning engines typically de-weight measurements from lower elevation angles. However, as demonstrated in Figure 8b, scintillation events occur at higher elevation angles and exhibit significant fluctuations, which could challenge the assumption of ionospheric correlation. Figure 8: Carrier phase double-differenced measurements. Figure 7: Short baseline decorrelation. Phase vs. Amplitude scintillation  
As discussed earlier, amplitude scintillation refers to rapid fluctuations in signal intensity (C/N0), while phase scintillation involves rapid fluctuations in carrier phase measurements. It was described previously that signal amplitude can change significantly over short periods (e.g., less than a couple of seconds). The focus of this section is to characterize the phase dynamics during episodes of scintillation.