Design and performance of a Martian autonomous navigation system based on a smallsat constellation

Highlights

• Polar constellation of small satellites on Mars.

• Provide navigation services for Martian landers, rovers, and orbiters.

• Quasi-autonomous constellation ephemeris reconstruction.

• Novel tracking technique for inter-satellite link.

Abstract

Deciphering the genesis and evolution of the Martian polar caps can provide crucial understanding of Mars' climate system and will be a big step forward for comparative climatology of the terrestrial planets. The growing scientific interest for the exploration of Mars at high latitudes, together with the need of minimizing the resources onboard landers and rovers, motivates the need for an adequate navigation support from orbit. In the context of the ARES4SC study, we propose a novel concept based on a constellation that can support autonomous navigation of different kind of users devoted to scientific investigations of those regions. We study two constellations, that differ mainly for the semi-major axis and the inclination of the orbits, composed of 5 small satellites (based on the SmallSats design being developed in Argotec), offering dedicated coverage of the Mars polar regions. We focus on the architecture of the inter-satellite links (ISL), the key elements providing both ephemerides and time synchronization for the broadcasting of the navigation message. Our concept is based on suitably configured coherent links, able to suppress the adverse effects of on-board clock instabilities and to provide excellent range-rate accuracies between the constellation's nodes. The data quality allows attaining good positioning performance for both constellations with a largely autonomous system. Indeed, we show that ground support can be heavily reduced by employing an ISL communication architecture. Periodic synchronization of the clocks on-board the constellation nodes with terrestrial time (TT) is enabled through the main spacecraft (the mother-craft), the only element of the constellation enabling radio communication with the Earth. We report on the results of numerical simulations in different operational scenarios and show that a very high-quality orbit reconstruction can be obtained for the constellation nodes using a batch-sequential filter or a batch filter with overlapping arcs, that could be implemented on board the mother-craft, thus enabling a high level of navigation autonomy. The assessment of the achievable positioning accuracy with this concept is fundamental to evaluate the feasibility of a future positioning system providing a global coverage of the red planet.

Introduction

The growing interest in the exploration of celestial bodies of the Solar System motivates the attention towards new navigation systems, capable of effectively supporting the new scientific and commercial missions planned in the coming years.

The objective of this work is to study a navigation system consisting of a constellation of small satellites that can handle, in almost complete autonomy, the relative and absolute positioning of its nodes around planetary bodies. This system must be able to provide the necessary tools to support the navigation of other probes (e.g., objects in Entry, Descent and Landing (EDL) phase, landers, rovers, or orbiters). Using small satellites leads to low development and launch costs, short development times, and offer good flexibility in mission implementation since multiple systems can be launched simultaneously with dedicated launches or as secondary payloads of larger missions.

The navigation system is based on radio observables obtained from the inter-satellite link (ISL) between the constellation satellites, heavily reducing the needs for ground support. The use of ISL technologies is widely recognized for its extraordinary results in terrestrial and planetary geodesy (GRACE and GRAIL missions) [1,2]. We demonstrate that the same concept of ISL, in the innovative configuration recently proposed [3,4] can provide observable quantities of high quality, enabling precise orbital determination, if applied to constellations with spacecraft on different orbital planes. Since the ISL provides information only on the relative motion between the spacecraft, the accurate knowledge of the Martian gravity field and rotational state acquired with previous mapping missions like MRO, MGS and Mars Odyssey [5,6], is fundamental, because it allows the conversion to absolute positioning.

We examine two different quasi-autonomous constellations of 5 SmallSats, in terms of orbital configuration and system requirements, and we compare their accuracy in terms of satellite positioning. Both constellations have a local coverage of the Mars polar regions, but this concept could be expanded to a global navigation system of the planet. The focus on the Martian polar caps origins from the need for a deep understanding of the connection between the polar deposits and the Martian climate to understand the Martian climate system [7].

The orbital period of the constellation has been selected equal to half of Mars' rotation period for the first constellation, and equal to a quarter of Mars rotation period for the second one, to have repeating ground tracks. The orbits are quasi-circular, resulting in an altitude of around 9500 km and of 4740 km, for the first and second constellation, respectively. The orbits are designed to provide navigation support during the EDL phase of a probe at high latitudes and to allow the positioning of rovers for the exploration of the Martian polar regions.

This study allows the demonstration of communication architectures potentially useful for subsequent applications, such as a future positioning system that provides global coverage of the planet.

This paper is organized as follows. In Sec. 2, we describe the design of the two constellations in terms of orbital configurations. In Sec. 3, we introduce the navigation system design based on the novel inter-satellite radio-tracking technique advised to precisely determine the orbits of the satellites, and then we focus on its implementation on the spacecraft platform. Starting from the architectural choices, in Sec. 4, we present the radio-tracking system in terms of error budgets, preparatory to the computation of the positioning performance for the two constellations. Then, in Sec. 5 we explain the orbit determination procedure, and in Sec. 6 we describe the numerical simulations in different mission scenarios for both constellations in orbit about Mars. Lastly, in Sec. 7 Autonomous system results, 8 Quasi-autonomous system results, we show the results of the numerical simulations in autonomous and quasi-autonomous configurations to demonstrate that excellent orbital accuracies can be obtained for a constellation of SmallSat on Mars. Sec 9 gives concluding remarks.