Quantifying the Impact of Ionospheric Irregularities on GPS Receiver Carrier Tracking Loop Performance

A number of environmental factors are known to affect the performance of the Global Positioning System (GPS) including electromagnetic interference from external sources, atmospheric delays, ionospheric scintillations, and multipath, among which the most significant factor can be the disturbed ionosphere and associated ionospheric scintillation. The main objectives of this paper are to investigate the impact of ionospheric scintillations on the GPS receiver carrier tracking loop performance, and to quantify these effects by defining receiver performance measures as a function of scintillation parameters for different tracking loop configurations. To accomplish this, two methodologies are employed (see Figure 1): a physics-based simulation of scintillations has been developed as a means of providing a variety of test cases for the simulated carrier tracking loop. In addition, a widely used stochastic model of scintillation activity is combined with a tracking model to produce a collection of receiver performance measures. This includes defining the phase tracking error variance and velocity error variance as a function of amplitude and phase scintillation parameters, defining the optimum tracking loop bandwidth for a minimum probability of losing lock, and determining the effect of ionospheric scintillations on the correlation of different L-band signals employed by the GPS under different ionospheric conditions. Simulation results are presented and compared with theoretical predictions. According to the simulation results, strong signal fading is almost always followed by a significant change in the phase of the carrier signal which, in turn, results in large carrier tracking errors and loss of lock. For the simulated second-order tracking loop, at C/N-0 = 42 dB-Hz, the optimum noise bandwidth (B-n1) for minimum probability of loss of lock is calculated as similar to 20 Hz. For B-n1 <= 20 Hz, the critical phase scintillation strength is calculated as T-scin = -10 dB. For higher magnitude scintillation, the tracking loop is expected to lose lock. The scintillation-induced velocity error variance is calculated to be of the order of a few tens of (cm/s)(2) which can be of concern for most (precision) applications. The correlation coefficient for pairs of GPS L-band frequencies (L-1/L-5) is fairly close to unity and reduces marginally as the variance of the electron density fluctuations increases.