AFC2 sets up a photo shoot for Tango

The second DLR's Autonomous Formation Control (AFC) experimental slot is dedicated to the fully autonomous formation keeping and reconfiguration based on GPS relative navigation. Several constellations were acquired autonomously by AFC and maintained over prescribed time intervals.

After 12 days of the AFC2 experiment, on March 27th 2011, the on-going fly-around and inspection phase reached the closest planned relative orbit. Referring to the plan, it is mentioned as geometry "I" and it is characterized by a minimum relative distance of 30m.

This scenario, together with the "Target pointing" attitude mode, set up an appealing chance for performing some activities with the Digital Video System (DVS): a photo shoot of the Tango satellite was realized!

As part of the operational activities, a video sequence starting at 22:40:0.0 UTC was planned. It lasted 2850s and shooting pictures every 75s. At the starting epoch Mango was occupying the green spot of Fig. 1. Each dot on the nominal relative orbit identifies a scheduled picture. In particular, the ones marked in red correspond to the pictures shown in Figures 2, 4, 6, respectively named pictures A, B and C. Our strategy can be easily understood by focusing on the Radial/Cross-track view of Fig. 1, where Mango moves in clockwise direction. Picture-A was meant to show Tango at almost the minimum relative distance with no Earth in the background. Pictures-B and -C were scheduled some time later in order to catch the re-approach to Tango with the Earth entering the camera field of view.

Fig. 1 Nominal relative trajectory of Mango (blue line) w.r.t. Tango (origin)

Afterwards, the real photos were analyzed to improve our understanding of the obtained images. To this aim, they were compared with the corresponding simulated-images produced by a specific simulation tool that models the camera behavior. The relative orbit geometry is rebuilt replaying the orbit estimated on-board. Whereas the attitude is given by the nominal guidance computed starting from such orbital information.

This post-processing campaign mainly addressed two objectives: improving our insight of the real images and tuning our DVS planning tool. Both aspects can be fruitful in view of future operational campaigns, also involving images taken at a larger distance which may be of difficult interpretation.

AFC 2
AFC 2

Fig. 2 Tango at 22:45:00 UTC (Picture A)

Fig. 3 Simulated image of picture A

AFC-2
AFC-2

Fig. 4 Tango at 23:17:30 UTC (Picture B)

Fig. 5 Simulated image of picture B

Fig. 6 Tango at 23:23:45 UTC (Picture C)

Fig. 7 Simulated image of picture C

The difference in the timings between real and simulated images is a consequence of having replayed the on-board estimated orbit. Such information is in fact available every 10 s with some drift with respect to the GPS alignment.

In all the simulated images, the dashed-black square defines the camera field of view. The red and green segments in the center of the plot respectively indicate the x and y direction of the camera-fixed frame. Thanks to this information one can visualize to where the bore-sight (aligned to z) points. The layout of the Tango spacecraft is simplified as a box whose darkest face stands for the solar panel. The body-frame is marked with dashed red-green-blue segments, respectively referring to x-y-z axis. This helps in completing the rough representation of the spacecraft configuration: the +y axis lies between the two tilted FFRF antennas, the +z axis coincides with the normal to panel. The last two segments dashed yellow and black mark the Sun and Nadir directions in Tango body-frame. The angle between Sun and +z directions is due to the particular setting of the attitude guidance which currently favors the Zenith bearing of the Sun/Zenith pointing configuration. As expected, in Fig. 5 and 7 it can be noted that the Earth appears in the field of view.