A Novel Quasi-Open Loop Architecture for GNSS Carrier Recovery Systems

The problem of designing robust systems to track global navigation satellite system (GNSS) signals in harsh environments has gained high attention. The classical closed loop architectures, such as phase locked loops, have been used for many years for tracking, but in challenging applications their design procedure becomes intricate. This paper proposes and demonstrates the use of a quasi-open loop architecture to estimate the time varying carrier frequency of GNSS signals. Simulation results show that this scheme provides an additional degree of freedom to the design of the whole architecture. In particular, this additional degree of freedom eases the design of the loop filter in harsh environments. 1. Introduction In global navigation satellite systems (GNSSs) the relative motion of both GNSS satellites and the user causes a Doppler effect, which results in a large frequency shift in the carrier and in the code of the received signal [1]. Precise estimation of this frequency shift is one of the most demanding requirements for GNSS receivers, because only an accurate tracking of the carrier frequency and Doppler shift allows the receiver to work properly, enabling reliable estimates of position velocity and timing (PVT). In any GNSS receiver, the acquisition stage provides an initial coarse estimation of the frequency shift, which is subsequently refined by the tracking systems. They are generally implemented in the form of closed loops, that is, phase lock loops (PLLs) and frequency lock loops (FLL), which track respectively the phase and the frequency of the incoming carrier, [2]. The main building block of a closed loop architecture is the loop filter. The design of a loop filter has been extensively addressed in the literature regarding the continuous-time PLLs, and many results and methods exist for different scenarios. However, modern receivers work in the discrete-time domain, and so PLLs and FLLs are digital systems, whose loop filters are often designed starting from some equivalent analog prototypes, by adopting transformation techniques from the analog to the digital domain. These tracking loops are therefore de facto digital approximations of analog loops, whose quality breaks down as the integration time increases. A valid assumption for this approximation is that the product between the loop noise bandwidth , and the integration time remains close to zero. As this product increases, the loop becomes unstable, as discussed in [3]. However, in high dynamic and weak signal applications, it is necessary to work with large values. In these