Preliminary Navigation Analysis for the Flyby Tour of ESA’s JUICE mission: An investigation on the trajectory correction maneuvers design

The JUICE spacecraft is an ESA mission to the Jovian system that will be launched in 2022. It will collect scientific data about Jupiter and its Galilean moons thanks to a flyby tour of Europa, Ganymede, Callisto and a final Ganymede orbiting phase. A high number of flybys will be performed with minimumaltitudes of 200 km. The tracking of the spacecraft during the flybys allows the estimation of the position of the spacecraft and the moons; in particular the knowledge of the moons positions can improve noticeably. However, due to the uncertainties in the positions of the moons and the spacecraft itself, a robust trajectory control is required for precise flyby targeting, to ensure that the nominal mission plan is achieved with the required margins. A preliminary navigation analysis for the mission JUICE has been developed, with the objective of the studying the influence of the spacecraft and moons positions uncertainties on the trajectory correction maneuvers (TCMs). The navigation analysis is composed of an orbit determination covariance analysis and a guidance simulator. The covariance analysis determines the standard deviations of the parameters of interest, (spacecraft and moons initial states) from the observations computed along the trajectory of the spacecraft. Different parameters can be chosen for the covariance analysis in order to represent different mission conditions and tracking data performances. The guidance has the purpose of computing the necessary actions to bring back the spacecraft to the desired path after the nominal one has been perturbed with the spacecraft and moons positions uncertainties; these action are the TCMs, which are represented by a ¢V due to the high thrust engine of JUICE. Three different targeting algorithms have been implemented for the guidance, with different characteristics of accuracy and speed. Two statistical maneuvers per flybys have been implemented, the pre-flyby (targeting) maneuver and the post-flyby (cleanup) maneuver. The navigation analysis is performed using a Monte Carlo method, sampling with a normal distribution the results of the covariance analysis to obtain perturbed trajectories; statistical results are then computed. This is necessary because the results of the OD are to be interpreted in a statistical way; the real perturbed trajectory is always affected by an uncertainty. A sensitivity analysis with respect to the tracking data types has been performed for the covariance analysis. The results show that for JUICE an uncertainty of the level of a few hundred meters can be reached, if range and VLBI data are used. However these results are just indicative since Doppler data have not been included in the simulations. The uncertainty in the positions of the Galilean moons obtained thanks to the flybys depends highly from the tracking strategy used and the data types. With themost realistic configuration (tracking interrupted before the flyby and inclusion of all the past data), the level of uncertainty can arrive to a few tens of meters, with data accuracies of 0.2 m for the range and 0.1 nrad for the VLBI. These results are for the formal errors and it is known that they are usually overly optimistic; the true error will be higher. Moreover, the importance of optical data for a further uncertainty reduction has been proven. The improvement of the Galilean moons uncertainties along the Tour as data are continuously added is also observed. Sensitivity analysis with respect to the maneuver time and the maneuver execution errors have been performed for the Monte Carlo navigation analysis. The maneuver time has the highest influence on the size of a maneuver; each flyby presents a time for which the ¢V required to correct the trajectory is minimum. Very often this time is of 3 days before the flyby (for the targeting maneuver), as found often in literature; however many flybys have this minimum around 2 days before the flyby. The maneuver execution error has a limited impact on the total mission ¢V , but it can decrease the accuracy of the targeting, causing an higher miss distance at flyby. The total ¢V required to performthe flyby Tour varies considerably upon the different parameters chosen; using current level accuracies (1mfor the range and 1 nrad for VLBI) and a maneuver time of 3 days before (and after) the flyby, the result is ¢V Æ 13.5 m/s (around 0.67 m/s per flyby). This value includes the targeting and cleanup maneuvers; it is easily obtainable by the propulsion system of JUICE, concluding that it is possible to correct the trajectory for the uncertainties of JUICE and the moons with a small effort.Aerospace EngineeringSpace EngineeringAstrodynamics & Space Mission