As soon as the commissioning phase of the AVANTI experiment could begin, in parallel to the vision-based navigation task, we started performing several activities to stepwise checkout and verify a certain number of functionalities. AVANTI, in fact, is a complete Guidance Navigation and Control system. Let us now focus on the “guidance” part which in our case means the capability to orchestrate different activities like: switching attitude mode, performing maneuvers, and logging pictures and our telemetry.
The commissioning phase has been a kind of trial stage, in which we could familiarize with the system and collect experience regarding all those aspects we could not fully simulate on-ground. Let us recall the major steps performed.
At first we activated the AVANTI software in a “passive” way, which means that it provided no commands to the other subsystems of BIROS. In this way we could test all input interfaces, especially the ones with the star-tracker sensor.
Second, we commissioned the Client Observation attitude Modes (COM). These are a family of orientations specially meant for supporting AVANTI. In these modes, in fact, the bore-sight of the camera head used for taking images is pointed into a prescribed direction. Different modes are obtained either letting the bore-sight to point constantly in flight direction or tracking the line-of-sight to the cubesat. This second option is needed to keep BEESAT-4 in the camera field of view while BIROS is spiraling towards it and decreasing the relative distance. In addition, COM modes vary from each other depending on how BIROS is rotated around the camera bore-sight direction, which determines the angle of incidence of the Sun on the solar panels. During this phase of the commissioning, we could get acquainted to the real optical effects of the camera baffle in space (Sun reflections, Earth and Sun blinding phases).
In parallel we started checking-out the experiment data handling features. Given the limited memory space of the onboard computer, the extended telemetry produced by AVANTI is forwarded to the payload computer and, afterwards, dumped to ground by means of high-rate data-link supported passes. Therefore, to download the complete telemetry set, BIROS has to be in an Earth pointing attitude mode throughout down-link passes. During the experiment, AVANTI autonomously commands the switches to Earth-pointing mode, since it has to synchronize it with the times when maneuvers occur, avoiding any conflict.
COM-fixed with camera head 0 COM-tracking with camera head 1
As suspected during the experiment preparation, the commissioning of the COM attitude modes confirmed that BIROS would not be able to remain continuously in an observation mode without encountering either thermal or power problems. Once constraining the camera to point towards BEESAT4, in fact, the Sun-angle with respect to the solar panel can only be partially controlled. In addition, the radiator is illuminated during a portion of the orbit, especially when the illumination conditions are the most favorable to image the cubesat. The star-tracker of BIROS presents two camera heads. It turned out that using the camera less affected by blinding issues put BIROS in a much critical thermal condition! In other words, it became clear that AVANTI should now and then select also a “cooling-down” attitude mode, in which the radiator looks in the deep space and the solar panels are perpendicular to the Sun direction (to recharge the batteries). To decide when switching into this auxiliary mode, AVANTI monitors the temperature of some critical components of the spacecraft. Nevertheless, it switches back to a COM mode, as soon as BIROS enters the part of the orbit where it is possible to get images of BEESAT-4.
Last we checked-out the capability to perform maneuvers. Given a target trajectory, AVANTI computes onboard time and duration of the maneuvers necessary to reach its goal. Therefore, a certain time before each burn, AVANTI sends to the thrusters an ON-OFF command and selects an attitude mode called Thruster Firing Mode. In this case, BIROS slews to direct its nozzle in the aimed delta-v direction.
Autonomous guidance: verified!