On-orbit servicing and debris-removal missions are currently drawing the attention of national and international space agencies due to the versatile and strategic applications they could enable.
These complex missions require the raising of the technology readiness level in several involved key technological fields (e.g., guidance navigation and control algorithms, robotics and clamping devices, communication architectures). In this framework, the AVANTI experiment represents the in-flight technological demonstrator of one of the on-orbit servicing essential enabling technologies: the capability to approach a noncooperative passive Client from large distances in an autonomous, fuel efficient, and safe manner. AVANTI exploits a fully vision-based navigation approach: an appealing solution that simplifies an aspect of the Servicer spacecraft design by employing simple passive low-cost sensors (e.g., optical or infrared cameras).
AVANTI aims at demonstrating autonomous rendezvous and receding maneuvers of a Servicer vehicle with respect to the passive uncooperative Client picosatellite making use of only optical or vision-based navigation at far- to mid-range separations (few hundreds of meters to 10 km). To this aim, the star-tracker embarked on the Client satellite is employed as far-range camera to take images of portions of the sky and the following chain of activities is performed autonomously onboard. First image processing and target identification provide the angles measurements of the line-of-sight to the target spacecraft. Secondly the real-time relative navigation filter estimates the current relative state. Finally, the so obtained information is used by a maneuver planner to produce the required impulsive maneuvers’ profile to perform a rendezvous in a safe, fuel efficient manner.
AVANTI is one of the scientific experiments hosted by the DLR BIROS (Bi-spectral InfraRed Optical System) satellite, launched on the 22nd of June this year. Slightly later, on the 9th of September, BIROS released in-orbit the BEESAT-4 picosatellite of the Technische Universität (TU) Berlin University.
AVANTI is a whole Guidance Navigation & Control (GNC) module implemented on the spacecraft bus computer of BIROS. Its activation will occur soon, and the total experimental activity is intended to occupy an overall time frame of 30 days.
During the experiment, BIROS plays the role of the active Servicer satellite which performs some proximity operations with respect to the BEESAT-4 picosatellite.
In order to meet its objectives, AVANTI implements onboard the following flight dynamics functionalities.
Image Processing: After extracting the object centroids comprised in a picture, this unit performs the identification of the target spacecraft among all objects imaged by the camera. The core algorithm relies on the kinematic identification of the target trajectory throughout a sequence of pictures, robustly with respect to variable inter-satellite ranges and illumination conditions.
Relative Orbit Determination: Its objective is to estimate the current relative state by processing the time-tagged line-of-sight measurements output by the image processing unit. It implements an extended Kalman filter which uses an analytical model for the relative motion that includes the mean effects of both Earth’s equatorial bulge (J2) and differential aerodynamic drag perturbations.
Maneuver Planning: The guidance algorithm computes the maneuvers required for a rendezvous to stepwise modify the relative orbit, so that passive safety and delta-v consumption minimization are pursued. Moreover, it supports the definition of time slots where no maneuver activities shall occur, to be compliant with the operational constraints that arise during the experiment execution.
Onboard Safety Monitoring: This module supervises the safety of the BEESAT-4 – BIROS formation during the execution of the AVANTI experiment. Its algorithm does not rely on the continuous availability of tracking data of the picosatellite, but rather exploits the passive safety of relative orbits characterized by a certain inter-satellite separation in the radial-normal plane. OSM preventively checks each maneuver generated by AVANTI and allows their execution only if the safety of the post-maneuver orbit is verified.