Tracking L1 C/A and L2C signals through ionospheric scintillations

Phase-lock loops are being developed and tested for robust tracking of the GPS L1 C/A and L2C CL signals through strong ionospheric scintillations. This work is part of an effort to design robust dual-frequency scintillation monitors that exploit the characteristics of the new civilian signals which are appearing on the GPS L2 frequency. Three new features increase L2 carrier tracking robustness in comparison to current civilian dual-frequency GPS receivers. The first feature is open access to the transmitted L2C PRN codes, which enables the tracking algorithm to eliminate the squaring loss of current semicodeless dual-frequency civilian receivers. The second feature is the use of the L2C CL pilot signal, which avoids a second source of squaring loss inherent in the removal of unknown navigation data bits. The third feature is a new PLL architecture that is based on a Kalman filter and that generalizes the notion of a discriminator in a way that tends to reduce cycle slipping. The new tracking loops have been tested on equatorial scintillation data that have been collected using a dualfrequency wide-band digital storage receiver. The new L2 tracking loop performs well, but the data did not provide a significant tracking challenge because the highest S4 index was 0.51. Additional tests have been performed on the L1 and L2 tracking loops using a simulation that includes a high-fidelity physics-based scintillation model. These tests demonstrate that the new phase-lock loops can track robustly, with only intermittent cycle slips, through scintillations with intensities up to S4 = 1.