Snapshot Acquisition of GNSS Signals in Space: a Case Study at Lunar Distances

Observation and characterization of GNSS signals in space are gaining momentum for the re-use of GNSS and its integration in Orbit Determination and Time Synchronization solutions, oriented towards more autonomous Guidance, Navigation, and Control systems. In the initial phase of this transient, Radio-Frequency signals observations from space-borne receivers allow supporting GNSS-based space navigation thanks the remote post-processing. This contributes to understand and compensate for unmodelled features of GNSS signals propagating at large distances, up to the Moon's surface. Such activities require the capture of Intermediate Frequency (IF) signal samples, and upcoming Lunar missions, such as the NASA/ASI Lunar GNSS Receiver Experiement (LuGRE) scientific payload, are going to support the collection of raw GNSS signal samples and the transmission of such data to the mission ground segment. The size of such data is the main bottleneck for the typical, narrowband communication channels dedicated to such payloads. Therefore a sufficient amount of signal samples must be defined for an effective post-processing at the ground segment. As an early investigation, this work sets as a minimum objective the acquisition of GPS signals at the low carrier-to-noise density ratio (C/N_0) throughout a sample Moon Transfer Orbit (MTO). The designed acquisition stage implements high-sensitivity techniques and Doppler compensation to guarantee successful signal acquisition at critical C/N_0 values. By investigating the main acquisition parameters, the proposed study identifies minimum chunks length to be imposed as mission requirements.