Analysis of the PLL phase error in presence of ionospheric scintillation

The functioning of standard phase locked loops (PLL), including those used to track GNSS signals, is based on a linear approximation valid in case of small phase errors. Such an approximation represents a reasonable assumption in most of the propagation channels. However, in presence of a fading channel the phase error may become large and the PLL is expected to operate in a non-linear regime. As PLLs are generally designed and expected to operate in their linear regime, whenever the non-linear regime comes into play, they will experience a serious limitation in their capability to track the corresponding signals. The phase error and the performance of a PLL embedded into a commercial multi-constellation GNSS receiver was analysed in the presence of ionospheric scintillation. The limitation of the linear approximation becomes evident for a scintillation level given by S₄ ≥ 0.6. The limitations posed by a fading channel consist of an increased phase dynamics together with power fading. The presence of fading on the signal power causes the absorption of the propagating energy in such a way that the PLL is forced to lose lock, irrespective of the phase dynamics.