As any mother when their offspring eventually leave the nest, Mango felt a tinge of cold feet and aloneness when Tango separated and left on her journey in to dark space.  This was seen as an expected sinking of temperatures on Mango right after Tango was let go.  Some of Mango's components are plotted below...

Tango, out on her own now in the cold black space, felt much the same and her temperatures dropped as well upon leaving Mango.  Some of Tango's hardware temperatures are plotted below...

The more rational engineering explanation is that while mated, there were large radiators on both Mango and Tango that were blocked by each other (on the surface areas that were between the two spacecraft), and hence these radiators did not have much cooling effect as their "view" to cold space was poor.  Upon separation, these radiator surfaces were opened up, exposing both the Mango and Tango radiators to cold space, hence cooling the spacecraft down considerably.  This was anticipated and both Mango and Tango responded accordingly by turning on internal heaters to regulate their temperatures.  Now, a number of days later, they are happy and warm flying together through space! 

Yesterday, TANGO successfully performed a transition to Sun Zenith pointing mode. As TANGO is only controlled by Magnetotorquers (like electro-magnets interacting with the Earth magnetic field) it has to be handled with extra care, and all manoeuvres require time and patience. To give you an idea, the typical time constant for any attitude manoeuvre on TANGO is in the order of a few hours.

So TANGO slowly reduced its initial Safe Mode rate (about 0.5 deg/s) and accomplished its manoeuvre to enter its nominal working mode: Sun / Zenith. This means that it points its Solar Array towards the Sun, and its GPS antenna towards Zenith.

The manoeuvre as well as the steady state reached by TANGO afterwards were as expected.

DVS_0856_Image_00010.jpg

As Tango is making its way towards Sun Zenith pointing we took the opportunity to also test one of the attitude pointing modes of Mango: GPS based pointing towards Tango. This mode will eventually be used by several of the experiments, among them both FFRF and VBS. As the telemetry showed us that the attitude manoeuvre was complete we took the opportunity to take some new images with the DVS camera. The result was a stunning image of Tango flying solo, about 100 meters away. Zooming in you can clearly make out details such as the FFRF and ISL antennas.

As Prisma entered this new phase of the mission the operations team have to deal with some new challenges. There are now two spacecrafts to monitor and control, and as they are very close to each other it becomes crucial to have a good knowledge about the relative position between them at all times. The system has of course been designed with this in mind. Almost all functions can be handled autonomously on-board, but all this is experimental and needs a careful examination in-orbit and that the complexity is increased in steps.

One key function is the GPS navigation, for Prisma this function is developed by DLR and then integrated into the on-board software by SSC. The function provides precise position and velocity for both spacecrafts at any time, necessary to determined if a maneuver is needed to maintain the constellation.

The separation event is a challenging task for the GPS navigation as it is the only time in the mission where an impulse will be applied to Tango. The preliminary analysis done during the night has shown a nominal and well behaved GPS navigation. The accuracy is expected to be very good almost directly after the separation. As you can see in the figure below the motion is smooth with only a few jumps, these are related to periods where the GPS receivers where off as Mango was flying through the south Atlantic anomaly.

Figure showing the relative motion before and after the separation event. In these plots Tango is fixed at [0 0] and the relative motion done by Mango is drawn out. Tango's location is shown with a "+" and the last position of Mango is shown with a "o".

The three dimensional relative motion is shown from three different sides, the first plot is looking at the motion from above, second one from the side (cross-track) and the third one from the direction where both satellites are flying (along-track). The last plot shows the relative motion on a timescale. All lines start close together, before separation, and then drift apart until the drift cancelation maneuver is performed around midnight.

Tonight has been a very special night. The Prisma satellites has been developed, built and tested with the primary objective to demonstrate satellite formation flying. This night we have started to demonstrate satellite formation flying. Two small spacecrafts orbiting earth with a speed of 7km/s, maintained in a constellation controlled at meter level.

Have a look at this video showing the separation event and also a little later the first maneuver performed by Mango to cancel the drift away from Tango. This is the first small step to enable an actively controlled satellite constellation. The video is generated from on-board navigation data.

We followed the entire separation from Mango with the video camera developed by TechnoSystems and got the first few images down to ground during the first passage after separation. Fully lit by the sun against the blackness of space we can still clearly make out the FFRF antennas and can that the separation worked flawlessly. We will continue to download more images throughout the night, so we are hoping for further images of Tango starting its own journey in space.

All systems look to be working as expected, sun pointing was achieved very quickly so the battery aboard Tango was rapidly starting charging and the platform looks very stable.

Wednesday august 11, 2010