Prisma Satellites - CNES Experiment: FFRF GNC 1

(Duration of 6 days)

This 6 days experimental slot constitutes the "core" part of the closed loop operations based on FFRF sensor. It is decomposed in three consecutive legs (2 days each).

Leg 1 includes first the commissioning of Stand-by (on a relative orbit), Collision Avoidance and Two Manoeuvres transfer algorithms. This is followed by two tests of the Rendezvous algorithm (first used in reverse to increase the distance to 10000 m then used in nominal conditions to approach Tango at 500 m). The final approach involves a transfer final approach.
Leg 2 includes the commissioning of Station-Keeping and Proximity operations at 100 m. A Collision avoidance test is performed next to create a separation of about 5000m It is followed by two consecutive rendezvous (first approach to 1000m distance, then reduction to 700m). The last part corresponds to the first long duration test of Stand-by (about 6 orbits)
Leg 3 starts with a Proximity Operations test at close distance (30 m). It is followed by a Collision Avoidance test that will drive Mango S/C on Tango opposite side up to a distance of about 5000 m. Rendezvous will be performed next to reduce the distance to 200 m in 8 orbits. It will be completed by another long duration test of Stand-by ( about 6 orbits).

Reports:

MANGO did rendezvous within a milk carton…

After a very nominal 1st day of Closed Loop activities, the FFIORD experiment keeps on running very well and fully as expected. Day 2 of the FFIORD GNC1 experimental slot was dedicated to an autonomous Rendezvous starting from 5000 meters away from Tango to reach an intermediate way point at 500 meters along track. Once arrived at this way point, a 2 Maneuvers Transfer was computed and executed to go down to 100 meters along track. One trim maneuver was required to improve accuracy and precisely reach the goal ([0.01 , 1 , 0.50] meters error respectively along track, cross track and radial).

Then it was time to activate the CNES Proximity module to do station keeping at 100 m from Tango. The accuracy of this station keeping phase was very good and it can be compared with the size of a milk carton (1 or 2 liter depending on the statistical point of view ;-) The RF navigation estimation shows a control error of 2.6 cm standard deviation along track, 12 cm cross track and 5.7 cm radial. The next phase consisted in going down to 70 m by following a forced trajectory and doing some more station keeping. Once again the behavior was very good. Within the scenario a collision avoidance triggering was planned so as to demonstrate the capability to safely go away in case anything goes wrong. This was performed according to plan, and now Mango is on a drifting relative orbit. Tonight FFIORD will again rely on its autonomous rendezvous module to come back for another day of experiment, the last of this first closed loop session.

After these very first days as Primary Closed Loop experiment onboard PRISMA, the FFIORD experiment has demonstrated full capability to maintain passively a safe relative orbit (Standby), to perform autonomous rendezvous from kilometers away down to a few hundred of meters, to perform proximity operations with a few centimeters accuracy and to trigger safe collision avoidance when necessary. All this was performed using the FFRF sensor measurements which may constitute the 1^st metrology stage for future Formation Flying missions.

The scenario described by the 1^st figure of this post is very typical of the formation acquisition and reconfiguration that will be performed by future formation flying missions (e.g. telescope distributed over 2 satellites): the chaser spacecraft (e.g. detector satellite) shall autonomously acquire the formation from the post launch initial conditions and progressively reach its nominal position at a specific distance from the target spacecraft (e.g. at the focal point behind the focusing satellite which is a few tenth of meters), then it shall reduce the control box within a few centimeters to allow for stabilized observation. If necessary a transition to a more accurate optical metrology stage shall be performed so as to offer 1 or 2 orders of magnitude improvement on the state estimation accuracy and on the control error. Reconfiguration of the formation baseline (e.g. focal length reduction or telescope reorientation) is to be performed with forced trajectory like movements.

Written by

CNES FFIORD GNC team

2011-03-11 / 22:20:44

CNES has invited PRISMA for a new dance

Yesterday on the 28th of October at 16:20:00 UTC the PRISMA Mango GNC mode switched from AFF to CNES mode. In other words, this means that the control of the Mango satellite trajectory is now given to CNES-FFIORD for new formation flying experiments. For the very first time delta-V maneuvers are to be computed based on the FFRF sensor measurements.

The SSC Autonomous Formation Flying module (AFF) delivered the Mango satellite right on its expected initial condition at 300 m from Tango. Then the CNES algorithms started to work with the objective of enabling and commissionning as much as possible the guidance and control algorithms within the closed loop mode (application of the computed maneuvers):

16:20:00 Initialization of the RF based navigation function within the Manual mode
17:10:05 Activation of the Standby mode to control the safe relative orbit around 300m
21:47:11 Triggering of a pseudo Collision Avoidance by setting an artificial offset on the relative position input of the guidance function
01:05:45 Perfoming a 2 Maneuvers Transfer (repeated 3 times) to reach a position at 600m from Tango
02:20:57 Starting an inverse Rendezvous to reach a far away position (5km)

The last data is from 05:19:00, and we are very confidently waiting for the 1st passage of this operational day (15:30:57) to check what happened during the end of the night. Tomorrow the majority of FFIORD guidance and control functions will be fully commissionned while in closed loop and Mango will be in a position to come back autonomously as well from 5km to 100m from Tango.

The experiment has started extremely smoothly and everything is fully in line with what was expected. As an illustration the following figure shows the comparison between the scenario simulation results (from SSC GNCViewer simulator) and the in flight data. The trajectory corresponds very well to what was simulated. The slight differences are nominal, and they are due to the RF navigation initialization process which is slighlty different between the 2 environments, which in turn affects the maneuvers computation.

In terms of performance it looks very promising. Since Mango is in an attitude mode favorable for the FRFF instrument (always Tango pointing except during delta-V maneuvers) the multipath errors are minimized. Moreover, FFIORD is benefiting from a very recent patch of some FFRF sensor parameters which greatly improves the quality of the RF measurements over the full measurement range (from a few meters up to 10km and more).

The following figures show comparisons between the FFRF sensor Line Of Sight measurements (angular value with 2 components X-LOS and Y-LOS) and the equivalent measurement issued from GPS data which is our LOS positioning reference. The FFRF sensor error consists in a bias of roughly 1 cm on XLOS (0.57 deg) and -1 cm on XLOS (-0.57 deg) which has not yet been calibrated, but which is fully estimated and compensated for by the RF navigation function. One can observe on this figure the convergence of LOS bias estimation (1 orbital period which is 1h40). The big LOS peaks correspond to the time when delta-V maneuvers are applied: Mango rotates and autonomously aligns along the thruster closest to the velocity vector to be applied. During this rotation maneuvers, the FFRF multipath errors which are attitude dependent get amplified.

Written by

CNES FFIORD team

2011-03-11 / 22:20:44