Time transfer and orbit determination for a Martian navigation system based on smallsats

We present a novel mission concept that can be used to support a near-autonomous navigation of different kind of users, such as rovers or landers in EDL (entry, descent and landing) phase, operating in the martian environment. We propose a constellation of 5 small satellites in polar orbits able to acquire their position in a Mars-fixed reference frame with minimal support from Earth thanks to a high accuracy Doppler system enabled by a novel inter-satellite link (ISL) communication architecture. The high quality of range rate measurements relies on radio link architectures able to suppress the adverse effects of on-board clock instabilities. Periodic synchronisation of the main spacecraft (mothercraft) with Earth UTC/TAI is enabled through a direct link to Earth, while the synchronization within the constellation is performed individually between each mothercraft-daughtercraft pair through the ISL, using a novel approach based on two-way coherent ranging measurements. The current orbital configuration provides mainly a regional coverage, but the navigation service could be easily expanded to cover the entire planet. In this work, we describe the overall system architecture and the time synchronization techniques, resulting from a tradeoff conducted both by analysis and numerical simulations, in terms of positional accuracy, reuse of the existing ground infrastructure, TRL of the onboard RF instrumentation and cost. We show that this navigation system, based on a simple and low-cost architecture, can autonomously reconstruct the trajectories of its nodes with ∼10 meters accuracy (3a, worst case) and can achieve a time synchronization accuracy at ns level, being on target to provide meter-level positioning service to a variety of end users

High Performance Orbit Determination and Time Synchronization for Lunar Radio Navigation Systems

This paper presents a concept for the orbit determination and time synchronization (ODTS) of a lunar radio navigation system. The proposed approach utilizes small ground antennas that simultaneously track all satellites in the constellation using K-band frequency links. The work describes the architecture and performance of the radio system, highlighting the implementation of the multiple spacecraft per aperture (MSPA) concept. This configuration ensures sufficient data rates and achieves high accuracy in Doppler and range observables, enabling precise orbit determination. One notable outcome of this configuration is the availability of a new observable quantity, the differential phase of the two-way signals received by the ground station from any pair of satellites, also known as single-beam interferometry (SBI). The paper examines the achieved time transfer accuracies with different onboard clocks and evaluates the standard asynchronous two-way satellite time and frequency transfer (TWSTFT) method. Additionally, a novel time transfer method is proposed, leveraging onboard code epoch timestamping and precise spacecraft range information. Through the analysis of realistic test cases, the study demonstrates that orbit determination accuracies consistently remain below 10 meters (root sum square). Furthermore, the findings indicate that ephemerides aging and clock drifts align well with a navigation message repeat time of approximately three hours (99% confidence level), while maintaining a signal-in-space error (SISE) of 25 meters