GPS Carrier Tracking Loop Performance in the presence of Ionospheric Scintillations

Abstract

The performance of several GPS carrier tracking loops
\nis evaluated using wideband GPS data recorded during
\nstrong ionospheric scintillations. The aim of this study is
\nto determine the loop structures and parameters that enable
\ngood phase tracking during the power fades and phase
\ndynamics induced by scintillations. Constant-bandwidth
\nand variable-bandwidth loops are studied using theoretical
\nmodels, simulation, and tests with actual GPS signals.
\nConstant-bandwidth loops with loop bandwidths near 15
\nHz are shown to lose phase lock during scintillations. Use
\nof the decision-directed discriminator reduces the carrier
\nlock threshold by ∼1 dB relative to the arctangent and conventional Costas discriminators. A proposed variablebandwidth
\nloop based on a Kalman filter reduces the carrier
\nlock threshold by more than 7 dB compared to a 15-Hz
\nconstant-bandwidth loop. The Kalman filter-based strategy
\nemploys a soft-decision discriminator, explicitly models
\nthe effects of receiver clock noise, and optimally adapts
\nthe loop bandwidth to the carrier-to-noise ratio. In extensive
\nsimulation and in tests using actual wideband GPS
\ndata, the Kalman filter PLL demonstrates improved cycle
\nslip immunity relative to constant bandwidth PLLs.