First Multi-Constellation Observations of Navigation Satellite Signals in the Lunar Domain by Post-Processing L1/L5 IQ Snapshots

The use of Global Navigation Satellite Systems (GNSS) to increase spacecraft autonomy for orbit determination has gained renewed momentum following the Lunar GNSS Receiver Experiment (LuGRE), which demonstrated feasible onboard GPS and Galileo signal reception and tracking at lunar distances. This work processes in-phase and quadrature (IQ) snapshots collected by the LuGRE receiver in cis-lunar space and on the lunar surface to assess multi-frequency, multi-constellation signal availability. Signals from additional systems beyond GPS and Galileo, including RNSS and SBAS constellations, are observable and successfully acquired exclusively in the recorded IQ snapshots. These observations provide the first experimental evidence that signals from multiple constellations, including systems not supported by LuGRE realtime operations, are detectable at unprecedented distances from Earth. Useful observables can be extracted from the IQ snapshots, despite minimal sampling rates, 4-bit quantization, and short durations (200 ms-2 s), through a hybrid coherent/non-coherent acquisition stage compensating for code Doppler. These observations are exploited to tune simulation tools and to perform extended simulation campaigns, showing that the inclusion of additional constellations significantly improves availability; for a 26 dB-Hz acquisition threshold, the fraction of epochs with at least four visible satellites increases from 11% to 46% of the total epoch count. These findings indicate that BeiDou, RNSS, and SBAS signals can substantially enhance GNSS-based autonomy for lunar and cislunar missions.

💡 Research Summary

The paper presents the first experimental evidence of multi‑constellation GNSS signal availability in the lunar domain using post‑processed in‑phase and quadrature (IQ) snapshots collected by the Lunar GNSS Receiver Experiment (LuGRE) payload. LuGRE, a joint NASA‑ASI mission aboard the Blue Ghost Mission 1 (BGM1) lunar lander, operated a Qascom QN400‑SP ACE receiver equipped with a high‑gain antenna (15.35 dBi) that simultaneously observed the L1/E1 and L5/E5a bands. During its cruise to the Moon and surface operations, the receiver switched between real‑time processing (RTP) mode and a Sample Capture (SC) mode. In SC mode, raw IQ data were downlinked with sampling rates between 4 MHz and 24 MHz and quantized to 4‑bit (occasionally 8‑bit), resulting in snapshots lasting from 200 ms up to 2 s. Although the limited bandwidth constrained snapshot duration and resolution, more than 106 h of GNSS observables and roughly 12 s of IQ data were made publicly available.

The authors analyze these snapshots to assess the presence of signals from constellations beyond the GPS and Galileo bands that were used in real‑time operations. By applying a hybrid coherent‑non‑coherent acquisition scheme that compensates for code Doppler, they successfully detect signals from BeiDou, the Quasi‑Zenith Satellite System (QZSS), India’s NavIC (IRNSS), and a suite of Satellite‑Based Augmentation Systems (SBAS) such as EGNOS, WAAS, MSAS, and others. The acquisition algorithm uses a DFT‑based cross‑ambiguity function (CAF) with 1 ms coherent integration, up to 32 ms non‑coherent accumulation, and a frequency search step of 200 Hz. A detection threshold corresponding to a false‑alarm probability of 10⁻⁴ yields a 26 dB‑Hz C/N₀ acquisition limit.

To translate the experimental findings into a predictive tool, the authors calibrate an advanced Space Service Volume (SSV) simulator using the measured signal‑to‑noise ratios and detection statistics. Simulations show that when only GPS and Galileo are considered, the fraction of epochs with at least four visible satellites is about 11 % at the 26 dB‑Hz threshold. Adding the additional constellations raises this fraction to 46 %, a four‑fold improvement. This increase directly translates into higher autonomy for lunar and cislunar missions: more frequent position fixes, reduced reliance on ground‑based tracking networks (e.g., DSN), and potential fuel savings due to more accurate orbit determination.

The paper also discusses practical implications. The successful detection of weak, high‑Doppler signals despite 4‑bit quantization and short snapshot lengths demonstrates that low‑power, low‑bandwidth GNSS receivers can still provide valuable navigation data in deep‑space environments. The observed receiver clock drift, which introduces frequency offset variations within a snapshot, is mitigated by the code‑Doppler compensation in the acquisition stage. The authors suggest that future missions could exploit these findings by designing receivers that opportunistically capture raw IQ data for post‑processing, thereby extending navigation capability without increasing real‑time processing load.

In conclusion, the study validates that multi‑constellation GNSS—including regional systems and SBAS—are observable at lunar distances, and that their inclusion dramatically enhances signal availability for autonomous navigation. The publicly released LuGRE IQ dataset, together with the presented acquisition methodology, provides a valuable benchmark for future deep‑space GNSS research and for the development of next‑generation navigation architectures for lunar exploration.