Guifre Molera Calves, HENSOLDT/University of Tasmania; Oliver White, University of Tasmania; Joe Wilson, HENSOLDT; Dominic Dirkx, Delft University of Technology; Pascal Christiaanse, Delft University of Technology
https://doi.org/10.64861/GCDT7296
Keywords: VLBI, cis-lunar, radio telescopes
Abstract:
The rapidly increasing space activity within the cis‑lunar domain is driven by renewed lunar exploration, increased commercial participation, and the emergence of distributed architectures in deep‑space operations. This growing operational footprint has intensified the need for reliable, continuous, and high‑accuracy tracking capabilities. As the number of spacecraft operating beyond geostationary orbit increases, it is essential to ensure robust space domain awareness and maintain safe space traffic management across the Earth-Moon corridor and beyond. Traditional tracking systems — particularly radar and optical sensors — remain valuable for near‑Earth and selected deep‑space applications, but they face fundamental limitations when applied to the broader cis‑lunar region. Radar systems encounter power and sensitivity constraints at lunar distances, while optical systems are significantly affected by weather, daylight, lunar phase, and geometric illumination. These constraints create critical coverage gaps during key operational phases such as deployment, orbit transfers, and landing operations.
Ground‑based radio astronomy infrastructure — specifically Very Long Baseline Interferometry (VLBI) networks — offers a compelling alternative for tracking spacecraft in lunar orbit and throughout cis‑lunar space. VLBI techniques exploit long baselines between geographically separated radio telescopes to perform highly precise differential timing and frequency measurements. When spacecraft transmit signals in S‑band or X‑band, these emissions can be captured simultaneously at multiple antennas, enabling passive and continuous tracking independent of atmospheric transparency or lighting conditions. The use of large‑aperture, high‑gain radio telescopes further enhances sensitivity, allowing the detection of low‑power onboard transmitters at lunar distances. Time‑ and frequency‑differential tracking systems can also be combined with traditional astronomical phase‑referencing techniques to achieve advanced tracking performance.
The key observables produced by VLBI for spacecraft tracking are Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA). TDOA exploits differences in signal arrival time across the network to constrain spacecraft position perpendicular to the line of sight, while FDOA uses differential Doppler shifts to provide complementary velocity information. When combined with baselines of thousands of kilometres, these observables offer extremely fine angular resolution and sensitivity to small trajectory deviations. This enables real‑time detection of spacecraft maneuvers, improved orbit‑determination accuracy, and resilience to unexpected trajectory changes or anomalies.
In this study, we present a comprehensive analysis of TDOA/FDOA‑based orbit determination for spacecraft operating in lunar and cis‑lunar regimes. Our approach integrates classical interferometric signal processing with phase‑referencing techniques, providing additional calibration and error mitigation by referencing spacecraft signals against stable celestial radio sources. We demonstrate this methodology using S‑band observations of active lunar missions, including the Korea Pathfinder Lunar Orbiter (KPLO), NASA’s CAPSTONE mission, the Lunar Reconnaissance Orbiter (LRO), and the Chinese probes DRO‑B and Tiandu‑1. These observations were collected using the University of Tasmania’s radio telescope network, composed of multiple >25‑m and 12‑m‑class antennas distributed across ~2000‑km‑scale baselines.
The University of Tasmania network, together with HENSOLDT Australia (also known as the Southern Guardian network), provides a unique combination of geographic spread, antenna size, and operational flexibility, enabling high‑quality passive RF measurements. Our processing pipeline incorporates delay‑rate estimation, baseline‑dependent calibration, and parameter‑estimation techniques to generate accurate orbit solutions from passive measurements alone. We combine our observational results with open‑source astrodynamics tools developed by the Technical University of Delft (TUDAT) to provide refined orbital solutions. The system achieves high‑precision orbit estimates, with sensitivity sufficient to detect small maneuvers and natural perturbations affecting lunar‑orbiting spacecraft. These results highlight the potential of radio‑astronomy‑derived techniques to augment existing tracking architectures, providing persistent coverage that complements radar and optical systems.
Looking forward, the planned integration of Australian facilities with international radio telescopes will enable even longer interferometric baselines, significantly improving angular resolution, increasing sky visibility, and further reducing orbit‑determination uncertainties. Such radio networks — when combined with optical and radar tracking methods already available within the Southern Guardian service — will provide a comprehensive approach to supporting future lunar exploration, space traffic management, and space domain awareness across the Earth-Moon system.
Date of Conference: September 15-18, 2026
Track: SDA Systems & Instrumentation