Since the beginning of the space exploration age, the navigation of deep-space probes has been carried out through ground-based measurements of their relative distance and velocity with respect to the Earth's stations. The interplanetary spacecraft host sophisticated radio systems which enable the tracking from ground stations, allowing to acquire the navigation data. The data are collected in batches and then processed by the navigation team to provide the updated trajectory of the spacecraft. Therefore, the navigation of planetary probes completely relies on the ground-based tracking and processing. An alternative technique developed during the last decades is based on inter-satellite radio tracking. Multiple spacecraft equipped with radio subsystems establish the inter-satellite radio links that enable the acquisition of relative distance and velocity measurements. Initially conceived to perform gravity science, it has been successfully employed to obtain extremely accurate mapping of the Earth's and Moon's gravity fields by the missions GRACE and GRAIL, respectively. Besides the gravity field recovery, this configuration has another significant advantage compared to traditional tracking methods, because the inter-satellite measurements can be processed onboard the spacecraft by autonomous navigations systems. However, the processing of these data alone cannot provide the absolute position and velocity of the spacecraft. The purpose of this proposal is to simulate and assess the performances of an autonomous navigation system based on inter-satellite tracking and altimetry. The processing of simultaneously collected altitude and inter-satellite radio measurements would allow to solve the intrinsic deficiency of this latter data type, allowing to obtain the absolute position and velocity of the spacecraft. The successful combination of these two instruments would greatly enhance the capability of deep-space probes to operate autonomously.
Navigation of deep-space probes has been accomplished only through ground-based tracking so far. Autonomy would be a desired capability in deep-space operations for several reasons. Ground stations are able to track the probes for a limited time during the day (~8h/day) because their operations follow a tight schedule in order to provide coverage to the great number of probes that are operating across the Solar System. Conversely, the proposed navigation system would collect data for a longer time, not being limited by the station schedule. This would guarantee an extensive coverage of the spacecraft trajectory. Furthermore, deep-space probes usually face high-risk operations, such as aerobraking (i.e., spacecraft traversing the upper layers of a planet's atmosphere to lower the apoapsis of its orbit), which cannot be directly controlled from ground because of the communication delay between the ground station and the spacecraft. In this case, an autonomous navigation system would greatly help in mitigating the risk by providing a real-time update of the spacecraft trajectory. An additional factor of risk may be the loss of communications by the ground station, which occurs when the spacecraft is occulted by the planet. During occultations, the proposed system would still be able to navigate the spacecraft and keep its trajectory close to that planned by the navigation team.
The development of this research project will greatly help in advancing the knowledge in the autonomous navigation field using inter-satellite tracking by assessing the capabilities of a novel system. Since the inter-satellite system alone cannot provide autonomous navigation except in few special cases, augmenting its functionality with the altimeter is a key factor in enhancing the autonomy of the spacecraft.