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A frequency lock loop (FLL) for Global Navigation SatelliteSystem (GNSS) applications is described here. The core element is the discrete time Fourier transform (DTFT), that leads to a maximum likelihood (ML) frequency estimation and can be conveniently implemented in a software defined radio (SDR) HW platform. The algorithm is based on the iterative evaluation of DTFTs in a single frequency until the incoming frequency is locked.

The objective of the work is the evaluation of the impact of new modulation strategies foreseen for the Galileo Signal In Space (SIS) on the carrier tracking operation within the Global Navigation Satellite Systems (GNSS) receiver (namely Frequency Locked Loop (FLL)/Phase Locke Loop (PLL)) in presence of multipath. Although multipath has more relevant effects in term of measurement errors on the code tracking, its consequences on the carrier tracking can not be neglected in high accuracy applications that rely on the carrier phase measurement such as, for example, Real Time Kinematics (RTK) survey. Particular attention is addressed to the Multiplexed Binary Offset Carrier (MBOC) signals and the Alternate BOC (AltBOC) signals. The investigation is conducted considering a two ray model with the aim of producing an error diagram showing the multipath effect in the carrier tracking: the Delay Locked Loop (DLL) PLL interaction is considered in a static -low dynamic environment. Closed loop simulations are conducted by means of specific software tools based on the GNSS fully software receiver developed by the NavSAS group. Performance verification is done for a noisy environment. Results show loops performance in a multipath affected environment and the effects of noise in the carrier tracking operations. The PLL performance evaluation under multipath conditions, considering as input MBOC and AltBOC modulated signals, is important to understand the behavior of the new modulations especially in case of carrier tracking based applications that are the more demanding in terms of final positioning performance (i.e. geodetic applications).