Since the start of the FFRF commissioning one week ago, the subsystem has been put through a series of functional tests and has proven quite resistant to different contingency scenarios (link breakdown, power switching, etc.). Thanks to the AFF variable trajectory and high reactivity from the SSC operations team, we have also been able to test the instrument for different inter satellite configurations (various relative distances, velocities, pointing and antenna selection on TANGO). This also worked very well.

The last five days have been spent in a stable navigation mode, as to allow testing of the long-term stability and performance of the FFRF as well as the CNES navigation filter which is part of the PRISMA onboard software. The results can be seen in the enclosed figures.

The first figure below shows the FFRF fine distance measurement over a 14 hour period starting a little after midnight on September 3^rd, when MANGO moves away from TANGO on a corkscrew trajectory. The results are compared with the GPS POD (Precise Orbit Determination) data provided by DLR., and show good stability and a very good performance with a mean bias of about 10 cm, and a noise level smaller than 1 cm (STD) outside the SAA (South Atlantic Anomaly where the GPS is OFF – marked in red). The bias variations are due to internal instrument artifacts (temperature and AGC variations) and multi path effects that will hopefully be corrected through instrument calibrations, which are yet to be performed.

The next figure gives the precision for the same time interval of the line of sight (LOS) measurements, which represent the direction of TANGO with respect to the MANGO satellite. In this particular case, with MANGO flying behind and pointing towards TANGO, the XLOS measurement match with the in plane component and the YLOS measurement coincide with the across plane component of the vector pointing towards TARGET. The results are encouraging with less than 0,01 m ( < 0,6 °) bias for XLOS (orbit plane angle) and less than 0,025 m (< 1,5 °) for YLOS (across plane angle). These biases are well estimated by the CNES on board navigation filter (green curve), and can thus be corrected for, leaving only the noise which is less than 0,2° on both axes!

The good performance of the CNES onboard navigation filter is confirmed in the last figure, which shows the first real-time estimation of the relative position of MANGO with respect to TANGO. The position noise is less than 3 cm along track (X) and less than 12 cm in nadir direction (Z), compared with the GPS. The relatively poor cross-track (Y) precision of 6,4 m is unsatisfactory, but is expected to improve significantly once the filter parameters have been tuned.   

Note that the position error is decreasing on the along track (X) and cross track (Y) coordinates as a result of the convergence of the filter following a delta-v maneuver (engine burn) before the start of the given time period. 

The FFRF and CNES navigation filter commissioning continues this week, with several new interesting scenarios and configurations (close range, attitude variations, etc.).

Since passage 1125, 17:35 UTC Wednesday 1/9, Mango is fully authorized to make its own decisions on when, how long and in which direction it needs to fire a thruster in order to establish and maintain the constellation geometry commanded from ground. While it is still too early to say anything about the long term performance, during the first 17 orbits (~28 hours) everything was nominal. Two reorientations of the constellation have been performed where the later one involved increasing the distance from about 800m up to 5km. This took nine orbits and required three short correction pulses of about 0.6 seconds burn time in total (about 3 mm/s correction in total). This is inline with the validation simulations which have been performed for this scenario.

Enabling formation autonomy on Prisma is a major step for the project. To enable autonomous formation control requires all other subsystems to perform nominally, pointing and momentum offloading for two spacecrafts, electrical and thermal stability, the onboard GPS navigation very precise and stable, thrusters that deliver accurate and repeatable impulses. These are just a few of the subsystems that need to deliver for AFF to be possible.

Full autonomy was nominally scheduled for Monday 6/9, however all requirements where fulfilled already on Wednesday which allowed an early start of the closed loop tests.

While the distance is large it is hard to visualize this in a nice way, later next week we will command Mango to move much closer to Tango. This will allow taking pictures of Tango from Mango as well as generating nice videos of the relative motion. Until then the best I can offer is one more AOCS analysis figure of the relative motion. The figure shows the two reorientations of the constellation done autonomously by Mango:

In parallel with the AFF Early Harvest, the FFRF subsystem commissioning started today with the first power-up of the FFRF instruments on Mango and Tango at 18:33 and 18:39 UTC. During the pass that followed ten minutes later, CNES was able to follow in real time as both receivers locked successfully onto the RF signals transmitted by the two FFRF transmitters, thus establishing the first RF metrology link in orbit.

Before the end of the twelve minute pass, successful checkout had been declared for both instruments, with normal temperatures, power consumptions and successful transmission of other telemetry. CNES could even provide the first coarse distance measurements which with an accuracy better than 50 cm exceeded expectations.

Instrument commissioning will continue until Wednesday and will be followed by calibration and thorough assessment of the in-orbit performance of the subsystem.

The FFRF instrument validation on PRISMA is part of the FFIORD experiment provided by CNES.

Tonight’s activities have been to verify the fine tuning of the GPS navigation for the coming experiments, final checkout of the Safe Orbit Guidance and initializing the relative orbit to a good relative state for the start of the AFF Early Harvest.

The results of the GPS calibration campaign have been applied in orbit and we can already verify improved in-orbit navigation accuracy. With the new settings provided by Simone d'Amico (DLR) the navigation filter is now much better balanced and provides better accuracy during attitude maneuvers and data gaps.

The final checkout of the Safe Orbit Guidance involved enabling the orbit control in closed loop, Mango has in this mode authority to command orbit maneuvers autonomously to adjust the orbit if it finds it necessary. Safe Orbit Guidance performed completely nominal by doing nothing at all, the current relative orbit was correctly considered to be safe.

For details of the Safe Orbit Guidance:

Orbit Constellation Safety on the Prisma In-orbit Formation Flying Test Bed (Robin Larsson, Joseph Mueller, Stephanie Thomas, Björn Jakobsson, Per Bodin, Proc. 3rd Int. Symp. on Formation Flying, Missions and Technologies, The Netherlands, April 2008) (ESA SP-654, June 2008)

Next week the first satellite formation flying experiments on PRISMA is starting, the AFF. One of the main objectives of AFF is to realize all other experiments initial relative orbits. To realize the initial conditions of the first experiment for next week the bigger on-ground version of AFF, the AFF toolbox (developed by SSC), was used to plan the optimal transfer trajectory. The resulting delta-Vs have then been commanded manually to Mango to perform the transfer from the current relative state to the initial conditions of the first AFF experiment.

The planed transferee is visualized in the following two figures. For both figures Tango is the reference and is thus located at origo. 

First figure shows the relative motion of Mango in the cross-track and radial plane. The starting state is indicated by a green star. This part of the trajectory involved removing a lot of the cross track motion that was introduced during the HPGP experiment. The cross-track motion was reduced and also phased to the desired orbit by two 20s thrusts with the HPGP thrusters. The thrusts where executed nominally and the resulting orbit was very close to the predicted one.

The second figure shows relative state plotted versus time (number of orbits since start of the transferee). The reduction of the cross-track motion is visible here as well in Y (green). Z is the motion in radial direction (red). The blue line (X) shows the distance in along-track. We have tonight started to approach Tango again, reducing the distance from 3 km down to 500m. The thin lines shows contingency trajectories, if a scheduled thrust is not performed.

When writing this Mango is about 5 orbits through the transferee. Arrival at the initial conditions for the first experiment of AFF will be already this night.