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 $\sim$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.