During the first two weeks of experiment we approached BEESAT-4 very closely already three times. Particular interesting for us was to explore the phase below 1 km of relative distance, since here many phenomena take place: the cubesat starts appearing very bright and large in the pictures, and the differential aerodynamic drag perturbation drastically changes, due to the tracking observation attitude profile that is required to keep the cubesat in the camera field of view.
Relative trajectory of BIROS with respect to BEESAT-4 during the first 2 weeks of AVANTI
While reducing the inter-satellite separation, we changed also the shape of the relative orbit that BIROS describes with respect to BEESAT-4. The size of the trajectory, in fact, determines the closest 3D distance that we can achieve. Our relative trajectory appears as a shifting-spiral with different amplitudes. This is driven by the fact that we are performing rendezvous compliant with the “passive-safety” concept. In simpler words, we are always keeping a certain margin of distance in the plane perpendicular to the BEESAT-4 trajectory, to avoid any risk of collision despite navigation and control errors.
Maneuvers performed over the first 2 weeks of experiment
To obtain such relative motion, the onboard maneuver planner computed and commanded some maneuvers, every time it received the command from ground of a new target point of the trajectory. The thruster burned only in tangential (black dots) and normal (blue dots) directions, to guarantee that each target point is reached with the minimum possible usage of fuel. At the end of the first week we reduced a lot the size of the spiral, as one can see from the big blue-dot values around the 5th of November.
A spiraling relative trajectory produces one-orbit period oscillations in the 3D distance over time (at BIROS height, the satellite covers 15 orbits every day). Given eclipses, Sun-blinding of the camera, intervals of “cooling-down” attitude mode, occurrence of maneuvers and, phases of Earth pointing attitude to dump the data to ground, we actually get pictures only in limited moments of the orbit (red stars in the plot). During the approach performed from the 12th to the 14th of November, Beesat-4 looked like this.