The AVANTI experiment has been designed based on the experience collected in 2012 during the so-called ARGON experiment. ARGON (Advanced Rendezvous demonstration using GPS and Optical Navigation) was the one of the first attempts for the German Aerospace Center to build up expertise in the field of vision-based approach to a non-cooperative object. It used the two satellites of the PRISMA formation-flying mission for the sake of the experiment: the main satellite Mango was used as chaser spacecraft while the Tango spacecraft was playing the role of the passive non-cooperative target to be approached.
The PRISMA formation-flying testbed
The experiment took place between the 23rd and 27th of May 2012, during which an approach from about 30 km to 3 km distance could be successfully achieved. This in-orbit adventure, rich in adrenaline and lessons learned about the behavior of the optical sensor system, motivated later the creation of the AVANTI experiment.
AVANTI aims at increasing the complexity level of ARGON to make it more realistic, in view of the future possible applications of such a technological know-how: rendezvous to space debris or to a non-cooperative satellite to be serviced. AVANTI will tackle the three major differences (with respect to the experimental conditions ARGON) that arise when approaching orbiting bodies:
The target object is now totally uncooperative. During ARGON, even if this information was not directly used during the experiment, the relative position of the Tango spacecraft was continuously monitored by another independent team to avoid any collision (in addition to the onboard GPS-based collision avoidance system). AVANTI is not so lucky: the target BEESAT-4 satellite embarks a GPS receiver, but there is no certainty that it will be available soon enough to support the AVANTI experiment. NORAD can of course track the picosatellite but there is as well no certainty that Two-Line-Elements (TLE) will be available during the whole experiment timeline (and the accuracy of TLEs in anyway not adapted for close range formation flying). In addition, it is also highly probable that ground-based radar-tracking methods are unable to differentiate the two satellites at close distance (below of few km).
The orbit plane is not perpendicular to the Sun anymore. During ARGON the satellites were flying on a particular orbit: a so-called dusk-dawn orbit. This kind of orbit is extremely popular among the space mission designers since they allow for continuous illumination. The AVANTI experiment is instead conducted on a more “standard” orbit, which is more representative of an on orbit servicing or debris removal scenario, in which the target object could fly on any orbit. This has dramatic influence on the visibility conditions: the target, which was always visible during ARGON, is now eclipsed by the Earth during a large part of the orbit and the camera following the object becomes blinded by the Sun during another large part of the orbit. As a consequence, the target is, according to simulation, visible during about only 15% of the time.
The approach is now done autonomously. During ARGON, the approach had been performed in a ground-in-the loop manner: the images were taken in orbit, downloaded and processed on ground. Based on the outcomes, the guidance and control profiles were computed by the experimenters and uploaded to the spacecraft. As a result, it was possible to check the consistency of the navigation solution before taking any decision and even to correct small software anomalies if necessary. In order to reduce the man power costs, AVANTI intends to perform all these tasks autonomously onboard, requiring a dramatic improvement of the overall robustness and safety concept: it has to be ensured that the onboard algorithms will always keep the formation in a safe state.
Optimal target visibility during the ARGON experiment
(April 23, 2012)
Summary of the new challenges faced
In addition, further constraints are making the problem even more difficult:
The effect of the atmosphere is much more pronounced. The BIROS satellite is flying on a much lower orbit (500km instead of 750km for PRISMA) and the difference of ballistic coefficients between a small satellite and a picosatellite is pretty large. This induces a tremendous differential drag, which is usually not modelled by the standard analytical model of the relative motion. Simulations forecast a decay of the picosatellite of about 2m per orbit more than BIROS. As a result, an improved analytical model for the relative motion needed to be developed to support the AVANTI experiment.
The BIROS satellite presents some technical constraints. The BIROS satellite is part of an Earth Observation mission and is thus not really designed for formation flying. In particular, it embarks a thruster system able to fire only in one single direction, while the PRISMA satellite had 3D maneuver capability. As a consequence, it is necessary to rotate the spacecraft every time a maneuver has to be executed, which can take up to 8 minutes, during which the camera does not point anymore to the desired direction. The camera pointing direction is also altered when the satellite needs to orient its antenna to the ground stations to download telemetry data. In addition, the bus of the spacecraft uses old components with limited computational resources and bandwidth, making image processing activities extremely challenging. As a matter of fact, AVANTI cannot process more than one single image every 30 seconds because of the limited bandwidth to transfer the images from the camera and because of the limited space onboard to store the data. Finally the misuse of the nominal spacecraft attitude to point the star tracker in the flight direction is likely to generate thermal and power issues, so that frequent interruptions of the optical tracking have to be foreseen to cool down the satellite and recharge the battery.
Will AVANTI be able to fulfill its mission despite the numerous new challenges and constraints? Well, we actually do not know, this still belongs to the glorious uncertainties of the space endeavor. Keep posted on this webpage, the answer will come in the next days!