Design and performances of a radar-based orbit determinationconcept aimed at Enceladus

As a Saturnian satellite featuring a subsurface ocean, Enceladus stands as a primary target for

upcoming exploratory missions aimed at identifying habitable environments and biological signatures.

Building upon Cassini-Huygens, future missions are targeting the Saturnian moon, notably the L4 ESA

mission expected to be launched in the early 2040s.

Within DLR’s Enceladus Explorer (EnEx) initiatives, the RaTNOS (Radar Transponder based Navigation

and Orbit Determination for Satellites) project is aimed at assessing a mission concept based on an

orbiter with a radar instrument as payload and one or multiple small landers with radar transponder

functionalities. Leveraging high-accuracy ranging and Doppler measurements between the orbital radar

and the small landers on the surface, the system is expected to provide improved orbit determination

compared to Earth-based range and Doppler measurements, while offering the option for radar

mapping of the moon’s surface and subsurface. For a mission to Saturn and its moons, the expected

accuracy achievable with standard Earth-based measurements is in the range of tens to a few hundreds

of meters, which greatly limits the potential for geophysical analysis and science. Specifically, the

proposed radar link between orbiter and lander may enhance navigation precision and mission

capabilities, directly improving the accuracy of scientific data products. Beyond the direct impact on

gravity measurements, the improved orbit determination may significantly improve coherent radar

imaging modes such as Synthetic Aperture Radar (SAR) imaging, repeat-pass interferometry, and tomography [1, 2, 3]. In particular, repeat-pass radar interferometry and tomography, enabling high-

resolution Digital Elevation Model (DEM) generation, volumetric imaging of Enceladus’s upper ice shell, alongside precise deformation mapping of the surface, may greatly benefit from improved orbit determination.

The proposed concept builds upon previous EnEx initiatives that analyzed the use of transponder

systems on melting probes for precise localization. The employment of ice-buried radar transponders

enables a novel suite of measurements, including refined characterization of ice and snow dielectric

properties, bulk density, grain morphology, and three-dimensional scanning of the surface ice and snow

layers [4, 5]. These advantages are not mission-specific; they apply to any orbiter-lander platform

equipped with radar payloads.

The prototype transponder system is currently being developed and tested using DLR’s airborne radar

sensor F-SAR and the German radar satellite TerraSAR-X. To facilitate potential surface deployment on

Enceladus, the design of the radar transponders targets a highly compact form factor. The transponder

prototype is capable of operating at different frequencies, specifically within the L- to X-band range [6],

to allow performance assessments for different radar systems. The system employs a bistatic acquisition

geometry in which the transponder receives, records, amplifies, and retransmits signals from the radar

sensor (spaceborne or airborne). By configuring the retransmission delay, the system enables active

clutter mitigation in the processed radar data, thereby enhancing tracking accuracy. Beyond supporting

orbit determination functionalities, the transponder signatures can potentially be used for radiometric

and geometric calibration of the radar system, improving, for example, backscatter measurements.

The potential performance of the proposed system consisting of an orbital radar system and

transponders on the Enceladus surface is further evaluated by end-to-end radar data simulations,

utilizing a 30 m-resolution Digital Elevation Model (DEM) of Enceladus and a suitable backscatter model

as clutter background, as well as the radar carried by an orbiter on a stable repeat-pass orbit around

the moon [3, 7, 8]. The potential for inverting snow and ice volume parameters for a transponder located within the ice (e.g., on an ice-penetrating melting probe) was demonstrated using airborne L-

band radar data collected over the Aletsch Glacier in Switzerland as an Earth analog [9, 10]. Analysis of these datasets already demonstrates that centimeter-level ranging accuracy is achievable, alongside snow and ice parameter estimation.

Figure 1. Schematic illustration of the EnEx-RaTNOS mission concept

References

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deformation derived from repeat-pass interferometric SAR at Enceladus – why and how?”,

Bulletin of the AAS, vol. 55, no. 8, 2023.

[2] P. A. Rosen et al., “Repeat Pass InSAR at Enceladus: A Geophysics Mission Concept to

Understand Dynamics and Habitability”, 15th European Conference on Synthetic Aperture

Radar (EUSAR), Munich, Germany, 2024, pp. 1318–1323.

[3] A. Benedikter et al., “Periodic orbits for interferometric and tomographic radar imaging of

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[4] A. Benedikter, C. Huber, L. Gambacorta, M. Rodriguez-Cassola, and G. Krieger, “Travel Time

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[7] M. Rodriguez-Cassola et al., “End-to-end Level-0 Data Simulation Tool for Future

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[8] R. S. Park, N. Mastrodemos, R. A. Jacobson, A. Berne, A. T. Vaughan, D. J. Hemingway, and

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[9] R. Horn et al., “F-SAR – Recent upgrades and campaign activities”, 18th International Radar

Symposium (IRS), Prague, Czech Republic, 2017, pp. 1–10, doi:10.23919/IRS.2017.8008092.

[10] M. Stelzig et al., “Virtual Synthetic Aperture Radar Target Based on a Miniaturized

Monostatic Digital Delay Transponder”, IEEE Microwave and Wireless Components Letters, vol.

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