Study of high sensitivity GNSS receivers for space applications and lunar missions

Global Navigation Satellite System (GNSS) provides accurate Positioning, Navigation and Timing (PNT) information to the users. Originally, these have been designed to serve terrestrial users, but in the last years the interest in the use of GNSS for autonomous space navigation has significantly increased. These navigation systems, already used at low altitudes, are becoming attractive even for spacecraft navigation at larger distances, as in lunar missions. In fact, exploiting real-time, in-orbit GNSS-based navigation systems would make the spacecraft more autonomous, reducing the costs and the effort of ground operations. However, deploying GNSS receivers all the way to lunar distances is a challenging task due to multiple factors. Firstly, the received signals are characterized by low power levels, especially at altitudes above the GNSS constellation. Besides, signal availability is drastically impaired by poor geometry, leading to accuracy degradation in the positioning and navigation solutions. Additionally, GNSS space-born receivers could experience high relative dynamics w.r.t the GNSS satellites, which are responsible for both Doppler frequency and Doppler rate. This thesis work aims to study and implement techniques allowing to increase the sensitivity of GNSS space-born receivers. The targeted strategies involve the extension of the integration time in the acquisition stage, to accumulate signal energy and cope with low Carrier-to-Noise density ratio (C/N0). However, by increasing the integration time, the effect of the Doppler frequency on the signal gets enhanced and weakens the acquisition sensitivity improvement ensured by these techniques. Doppler shift impacts on acquisition performance by affecting both the carrier and the code. Therefore, a key requirement in space missions is to have an accurate estimation of the Doppler frequency so that to compensate for it. The Doppler estimate is of great importance to reduce the dimension of the Cross-Ambiguity Function (CAF) as well, in terms of Doppler frequency bins to be explored. In particular, not only the Doppler frequency shift must be taken into account, but also its change over time, namely the Doppler frequency rate. For this purpose, an adaptive Orbital Filter (OF), can supply signal frequency estimation, thus aiding both the acquisition and the tracking stages. Results about the impact of the Doppler on the acquisition performance are presented, together with the proposed compensation of these effects. Both Doppler compensation and high-sensitivity acquisition techniques have been validated through a GNSS software receiver. A real scenario has been analysed about a lunar mission in the framework of the Lunar GNSS Receiver Experiment (LuGRE). This project focuses on the development of a spaceborne GNSS receiver to support cis-lunar and lunar navigation. The addressed scenario foresees a constraint on the extension of the integration time. The latter is indeed bounded by the duration of the short-time snapshots of raw signal samples to be processed. Given the signal length, it has been investigated which is the best number of Doppler frequency bin and which is the best acquisition decision logic, in terms of false alarm probability, to acquire signal at different C/N0 levels, corresponding to different points of the lunar trajectory. Finally, a discussion follows about the CAF size and the false alarm probability, related to requirements on the accuracy of the Doppler frequency estimates provided by the OF.