Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Planetary and lunar navigation systems based on smallsat constellations

This thesis focuses on the design and performances of a Lunar and a Martian navigation system to support the human and robotic exploration planned in the next future. Concerning the Moon, a Lunar Communication and Navigation Service proposed in the context of the ESA Moonlight initiative has been examined. This navigation system is composed to 4 satellites in 2 ELFO orbits with optimized coverage on the South Pole. The Orbit Determination and Time Synchronization (ODTS) architecture relies on a network of ground stations located in 3 stations on Earth and equipped with small dish antennas able to perform a Multiple Spacecraft Per Aperture (MSPA) tracking in K-band thanks to the implementation of the Code Division Multiplexing with Majority Voting (CDM-M) modulation, exploiting ranging, Doppler and Single Beam Interferometry (SBI) measurements. After generating a realistic navigation message from the ODTS, with a maximum Signal-in-Space-Error (SISE) of 12 m with 2 hours refresh rate, this work demonstrates that for an orbital user the requirement of a position accuracy below 100m at 95%, velocity accuracy of 1m/s at 95% and a timing accuracy below 15 ms is satisfied for equatorial, polar and inclined orbits using a reduced dynamic Extended Kalman Filter (EKF). The best performances were achieved for the equatorial user, obtaining 16.43 m of mean error on the position reconstruction over one day of simulation. For a moving rover on the Lunar surface with a velocity of 0.36 km/h, starting from the same navigation message, the simulations showed that the achievable positioning performances when using 4 satellites and the DEM is around 5 meters in real-time, while when only 3 satellites and the DEM are used, performances are degraded but we are still able to position the rover with a rough accuracy of 15 meters at 3σ. Moving to Mars, the focus of this thesis is to support the navigation of users devoted to scientific investigations on the North Pole, to decipher the genesis and evolution of the Martian polar caps and provide crucial understanding of Mars’ climate system. In the context of the ARES4SC study, in this thesis is proposed a novel concept based on a constellation that can support autonomous navigation of different kind of users (rovers, orbiters) exploiting inter-satellite links (ISL) to provide both ephemerides and time synchronization for the broadcasting of the navigation message. Two constellations are examined, 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. Numerical simulations of the OD process were carried out to investigate the navigation performances of this novel concept. A perturbation analysis, carried out for an autonomous ISL-only case by introducing a 10% mismodeling error on the largest non-gravitational acceleration, shows a positioning accuracy of 10 m after a warm-up period of about 15 days, if the constellation is sufficiently close to Mars, as our simulations show in the case of the lower constellation (∼ 8000 km semi-major axis). With higher semi-major axes, a limited but periodic ground-tracking is required. It’s shown that just 2 h every 6 days can be sufficient to obtain 10 m positioning accuracy in case of ∼ 12000 km semi-major axis. The best performances are achieved by the lower constellation with 4 h every 3 days of ground-tracking, with accuracies down to 10 cm. To obtain the same results with the higher constellation, a configuration of 3 pseudolites equally spaced on the Mars equator is proposed. This solution, while quite more expensive, would greatly strengthen every orbital solution and provide a strong tie between the constellation and the Mars-centered, Mars-fixed reference frame. The pseudolites would also serve a future, global navigation system. 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; while a continuous synchronization between the spacecraft is enabled by ISL exploiting 2-way range coherent, with performances determined by the Line-of-Sight positioning error achieved with OD. It’s been also proved that this navigation system is capable of effectively supporting the positioning of different kind of users. Analyses presented in this thesis show that both a static (∼ 5 to 15 m positioning error assuming SISE of the constellation to the level of 10/20/30 m) and a moving rover (∼ 20 m positioning error for the higher constellation and ∼ 5 m for the lower constellation and with SISE = 10 m) can be positioned with satisfying accuracies (improved with respect to the state-of-the art). Concerning the low orbiter in polar orbit, only the higher constellation ensure a good positioning (∼ 36 m positioning error) due to longer visibility period, while the second one is not able to position the user with less than ∼ 200 m error.