Study of the Effect of Ionosphere Scintillation-Induced Deep Fades on Multi-Frequency GNSS Carrier Phase
Brian Breitsch and Jade Morton, University of Colorado, Boulder
It is important to characterize ionosphere scintillation-inducing irregularities for error-mitigation, error-prediction, and scientfic efforts. TEC-like phase structure associated with refractive effects is a phenomenon of weak scintillation, while strong scintillation can cause uncorrelated fluctuations between phase measurements (diffractive effects). In particular, during deep-fade conditions, ionosphere-induced rapid phase changes can occur for individual phase measurements. Previous studies have investigated and simulated the decorrelation between GPS L1 and L2 carrier phase measurements as well as the occurrence of half and full-cycle changes during scintillation-induced deep fades. (Carrano et al., "Scintillation Characteristics across the GPS Frequency Band," 2012) extends the comparison to the GPS L5 frequency.
In (Carrano et. al., "Direct Measurement of the Residual in the Ionosphere-Free Linear Combination during Scintillation," 2013), they investigate phase residuals due to scintillation diffraction in the ionosphere-free combination of GPS carrier phase observables. Using the ionosphere-free combination allows observation of phase scintillations without the added fluctuations of ionosphere TEC structures, although slowly-varying geometric doppler effects must still be detrended. The authors demonstrate and validate a correlation between scintillation $S_4$ index and ionosphere-free combination RMS residual phase by comparing the dual-frequency ionosphere-free combination from simulation and real data. They also study full-cycle phase transitions that occur during deep ionosphere fades.
Our work will continue to investigate the diffraction effects of scintillation by looking at carrier phase behavior during deep fades. By using detrended phase and signal intensity, and the geometry-ionosphere-free combination (GIFC) of triple-frequency GPS phase observables, we are able to identify and differentiate the occurrence of canonical fades that contain full-cycle phase transitions and those that do not. The GIFC observable removes both geometric doppler and ionosphere TEC signal components, allowing for a more direct observation of the strong diffractive effects and rapid phase changes associated with scintillation. This means that aggressive detrending is not needed in order to observe residual phase, although small effects such as multipath and inter-frequency bias variations do remain and must be accounted for. In addition, we discuss the disadvantage this approach poses, in that it may be difficult to determine on which signal rapid phase changes originate when looking at the GIFC without referencing un-combined phase measurements.