The Lunar Volatile and Mineralogy Mapping Orbiter (VMMO) is a 16U CubeSat mission for ESA currently undergoing Phase B1. The study is carried out by an international consortium led by MPB Communications and targets high-resolution mapping of water ice and other volatiles in shadowed lunar south-polar regions including near Shackleton, Faustini, and Cabeus craters.  Accurate orbit determination and precise spacecraft state knowledge are essential to achieving VMMO’s scientific and operational objectives, ensuring proper instrument pointing, surface geolocation, and data georeferencing.
 
This study investigates the mission’s orbit determination performances across multiple tracking architectures, quantifying the benefits of integrating complementary navigation sources. For the purposes of the present paper and within the consortium, the University of Surrey provides the nominal orbital design, while Indra Space leads the orbit-determination analysis.
 
VMMO’s reference orbit adopted for this study is a 41x200 km altitude polar lunar frozen orbit. High-fidelity dynamical propagation indicates that the periapsis altitude remains within 41 to 57 km throughout the planned two-year baseline mission, while the argument of periapsis stays close to 270 degrees. This orbital configuration is well suited for studying water-ice deposits in permanently shadowed regions.
 
VMMO’s orbit determination performances are assessed under three different observation architectures via the ESA flight dynamics software GODOT: (1) ground-station radiometric tracking, (2) Earth GNSS-based tracking, and (3) tracking from future lunar navigation support provided by Moonlight Lunar Pathfinder and Lunar Communication and Navigation Service (LCNS). All scenarios are based on high-fidelity dynamical model, accounting for high-order lunar gravity harmonics, third-body perturbations, solar radiation pressure, and representative spacecraft physical parameters. 
 
Orbit determination performances are first evaluated using two-way range and Doppler measurements from Earth ground stations, processed with an Extended Kalman Filter (EKF). Tracking is modelled with Goonhilly station (United Kingdom), currently considered as the nominal scenario for VMMO. Following current ESA guidelines, a 15 degrees minimum elevation mask is imposed, alongside a 2-hour daily tracking limit and Moon as main occulting body to reflect realistic operational constraints. Under these conditions, the system achieves orbit knowledge better than 500 m in position and 0.5 m/s in velocity (3σ) within 25 hours after the VMMO in-orbit release. 
 
The second scenario considers Earth GNSS-based tracking using GALILEO signals at lunar orbital altitudes. Although terrestrial GNSS constellations are designed for users below ~3,000 km altitude, spillovers and side-lobe signals remain trackable in cislunar space for pseudo-range and pseudo-range rate observables. A carrier-to-noise ratio threshold of 20 dB-Hz is adopted to determine GALILEO signal availability, consistent with deep-space GNSS link budget assessments. As operational constraints, Earth and Moon act as occulting bodies. Preliminary results show that the position knowledge drops below 100 m (3σ) in less than 10 hours. 
 
A third scenario focuses on tracking performances from Moonlight Lunar Pathfinder and LCNS. Such constellation is designed to provide communication and navigation services around the Moon offering up to 15 hours of daily coverage in the south polar region. A 2-hour maximum tracking duration and lunar occultations are imposed as operational constraints. Navigation performance is evaluated using an EKF processing pseudo-range and pseudo-range rate observables. Preliminary results show that the orbit knowledge (3σ) converges below 50 m and 0.1 m/s after only a few measurement updates (approximately 10 minutes of tracking). 
 
As a final scenario, a mixed architecture is assessed combining measurements from only one LCNS satellite with daily tracking from Goonhilly. This configuration is particularly relevant, as only one satellite from Lunar constellation is likely to be available by the time VMMO flies. The combined geometry notably enhances observability, with the orbit knowledge (3σ) converging to better than 100 m and 0.1 m/s after approximately one orbital period of VMMO.  
 
Overall, the study indicates that VMMO can reliably attain sub-100 m (3σ) orbit knowledge in relatively short time when combining ground-based and lunar-based tracking assets. These findings thus establish a clear navigation performance trade-off for the VMMO mission and provide valuable basis for future low-cost lunar CubeSat missions operating in similar environment.

Acknowledgements
The authors would like to acknowledge the support of the European Space Agency, UK Space, Portugal Space, the Spanish Space Agency and the Canadian Space Agency for the VMMO Phase B1 study.