VLBI Tracking of the JUICE Mission: Two Years of Cruise Phase Operations and Performance Analysis

Abstract: The JUpiter ICy moons Explorer (JUICE) mission, launched by the European Space Agency (ESA) in April 2023, represents one of the most ambitious deep space exploration endeavours targeting Jupiter's icy moons. This paper presents results from the Very Long Baseline Interferometry (VLBI) radio telescope tracking conducted by the University of Tasmania during the first two years of JUICE's cruise phase operations. We have conducted over 100 tracking sessions capturing the spacecraft across different orbital regimes as JUICE progresses through its complex cruise trajectory towards Jupiter. Our analysis focuses on three key areas: Doppler residual characterisation, mission performance indicator extraction, and solar wind scintillation pattern analysis (including space weather forecasting). UTAS measurements demonstrate the enhanced capabilities that VLBI networks provide for deep space mission support, particularly for precision orbit determination and spacecraft health diagnosis. The results showcase the UTAS VLBI array as a valuable complement to traditional tracking infrastructure, offering Southern Hemisphere coverage and enhanced geometric diversity for deep space missions.

• The paper demonstrates that combining phase-referenced VLBI with Doppler tracking achieves sub-milliarcsecond orbit determination for precise spacecraft navigation.
• The study employs a heterogeneous telescope array with up to 32 Gbps bandwidth to overcome challenges in real-time tracking and space weather diagnostics.
• Results validate VLBI as a reliable supplement to traditional deep space networks for monitoring spacecraft health and analyzing interplanetary plasma turbulence.

VLBI Tracking of the JUICE Mission During Cruise: Performance and Scientific Yield

Introduction

This work articulates the outcomes of two years of VLBI-based tracking for the ESA JUICE mission during its cruise to the Jovian system, executed predominantly by the University of Tasmania (UTAS) radio network under the PRIDE experiment framework. The experiment extends the methodology lineage from the Huygens, Venus Express, and Mars Express missions, combining phase-referenced VLBI and Doppler tracking to address the evolving challenges of real-time orbit determination, interplanetary navigation, and in situ space weather analysis.

Methodology and Operational Framework

UTAS deployed a heterogeneous telescope array—including Ceduna (30 m), Hobart (26/12 m), Katherine (12 m), and Yarragadee (12 m)—integrated with both DBBC2/3 back-ends and new SDR-based pipelines, to achieve wide bandwidth acquisition up to 32 Gbps with robust redundancy. Phase-referencing VLBI sessions used near-field delay models (Duev et al.) and frequent calibrator switching, leveraging the DiFX correlator stack and AIPS for astrometric imaging.

Single-dish Doppler observations supplied high-cadence radial velocity residuals, exploiting improved SATRK pointing cadence and SCtracker/PLL-based tone extraction for sub-mHz frequency precision. The data pipeline explicitly separated space weather-induced fluctuations from system noise, with power spectral analysis employed for solar wind scintillation diagnostics. Model-based predictions for phase scintillation incorporated updated TEC integrals along the JUICE–Earth line of sight, informed by both legacy (Mariner II) and recent solar wind electron density scalings.

Key Results and Technical Performance

Orbit Determination and Tracking

The project executed over 100 distinct JUICE tracking sessions, with nine achieving phase-referenced imaging. Astrometric precision at the milli-arcsecond level was demonstrated, particularly during the 2024 Lunar-Earth gravity assist. The improved SATRK-driven pointing and frequent calibrator switching minimized systematic baseline errors and increased SNR, especially during critical flybys. The experiment demonstrated that two-way Doppler modes produce frequency and phase residuals more than two orders of magnitude less variable than one-way operation, which is essential for both precise orbit determination and meaningful scintillation analysis.

Significant coverage flexibility was demonstrated, with the Southern Hemisphere array filling geometric gaps in global networks, providing essential redundancy when European or northern assets were unavailable or unfavorable due to geometry or regulatory restrictions.

Space Weather and Scintillation Analysis

Extensive Doppler residual series enabled statistical characterization of phase scintillation due to the solar wind and Earth's ionosphere. Measured TEC-based phase scintillation values tracked theoretical predictions to within model and systematics-dominated scatter. Discrepancies were traced to ionospheric and tropospheric propagation effects, which are planned to be progressively decorrelated using vTEC products and advanced atmospheric models.

Robust metrics for interplanetary plasma turbulence, including Kolmogorov power-law slopes of the solar wind spectrum, are being extracted, facilitating both spacecraft operational forecasting and comparative studies with prior missions (e.g., Mars/Venus Express). The documented SNR and residual performance for various polarisation modes also provide engineering data for future mission downlink design and ground station calibration.

Event-Based Observational Campaigns

The network captured transitions through the Lunar-Earth gravity assists, providing both Doppler and VLBI solutions prior to and after the events. The necessity of antenna and power state cycling (switching to low-gain antennas, regulatory communications blackout zones) imposed data gaps at closest approach, yet post-facto frequency and phase continuity were maintained. The Venus flyby encountered technical correlation issues in VLBI but delivered high-fidelity Doppler measurement continuity.

Implications and Theoretical Significance

The results robustly validate VLBI as a critical adjunct to DSN/ESTRACK for interplanetary operations, especially for missions with intricate gravitational assist trajectories, high dynamic range, or Southern sky constraints. The experiment consolidates the PRIDE protocol as mature for routine integration in planetary navigation, enabling $\ll$10 m lateral accuracy in the B-plane and facilitating real-time spacecraft health diagnostics via radio systematics.

Further, the Doppler-based plasma diagnostics produce high-cadence, line-of-sight resolved measures of heliospheric state, with clear potential for space weather forecasting supporting both science payload planning and autonomous navigation.

Future Prospects

Forthcoming analysis will implement multi-layer ionosphere/troposphere decorrelation, refined event-based orbital parameter estimation with TUDAT, and solar wind/CME event tagging based on phase scintillation outliers. The substantial dataset enables comparative modeling across solar cycle phases and geometric tracks with extended baseline coverage. Upcoming PRIDE campaigns intend to cover additional flybys, refine the electron density scaling factors empirically for the outer solar system, and enhance the operational playbook for next-generation deep space missions.

Conclusion

VLBI and Doppler tracking by the UTAS network during JUICE's cruise phase demonstrates high-fidelity, operationally relevant, and scientifically productive augmentation of deep space navigation and science-support infrastructure. The campaign’s extensive session cadence, hybrid hardware architectures, flexible software pipelines, and application to both orbit determination and interplanetary plasma diagnostics set a strong precedent for future planetary missions, especially in context of increasing demands for autonomous spacecraft navigation and rapid-response space weather characterization.

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Reference: "VLBI Tracking of the JUICE Mission: Two Years of Cruise Phase Operations and Performance Analysis" (2607.00414)