Prisma Satellites - CNES Experiment: FFRF Initialize

(Durarion of 3 days)

The purpose of this experimental slot is to pursue FFRF sensor characterization that was started during the previous passenger mode sessions. It will include sensor calibration at medium and long range, functional tests and performance evaluation in different geometric and dynamic configurations.

This 3 days experiment will take Mango S/C from an initial point at 1000 m in front of Tango to a maximum separation of 10 km along track and then will take it back at 100 m on a stable relative orbit. Mango S/C will be Tango pointed most of the time but some attitude manoeuvering will be also applied at various intervals in order to perform FFRF sensor calibration.

Reports:

FFRF Init Synthesis

For the first time, CNES was defining the attitude of MANGO spacecraft as well as its relative trajectory relying on the fully operational AFF. The whole experiment has been designed to validate the FFRF initialization procedure over a wide range of distances and relative attitudes. This initialization involves two successive steps: (1) perform a successful signal acquisition, (2) achieve Integer Ambiguity Removal (IAR) that consists in removing the wavelength ambiguity affecting the Line of Sight (LOS) and distance measurements. LOS IAR involves a rotation of the spacecraft 50° around the antennas axis whereas distance IAR requires signal filtering over several minutes.

The intensive attitude manoeuvring is illustrated on the following Celestia® animations that show FFRF signal acquisition and IAR tests performed with initial off-pointing respectively as high as 90° and 30°. Some “quiet” periods have been also inserted to characterize the performance of the FFRF sensor and the related Navigation function.

After a shaky start, Day 1 run quite smoothly and showed the benefit of the FFRF parameter upload performed the day before: the YLOS bias level has been reduced to less than 1 cm (Figure 1) which represents the residual multi-path error when Target pointing is active.

Day 2 was unfortunately stopped after a few hours due to a platform problem that triggered a transition to Safe Celestial mode when the relative range was around 9 km. On top of that, FFRF data collection was also interrupted due to the unfavourable antenna pointing.

Thanks to SSC reactive operators that performed a quick recovery, Day 3 experiment was started on time at the right location (6 km from Tango) and allowed to complete a fruitful acquisition and IAR tests harvest.

Not surprisingly, signal acquisition has been quite satisfactory with a 100% success rate (32 tests) and a total duration reaching 50 s in the worst cases. IAR test results have been equally successful for medium elevations and azimuth (17 tests) but the tests at higher elevations (30°) have caused some unexpected failures that are currently under investigation.

Navigation results has benefited from the bias reduction and excellent results have been obtained in Target pointing phases with orbital plane errors in the centimetre range (Figure 2). Some manoeuvres sensitivity is still present along the cross-track component and the mitigation measures planned during Day 2 could not be tested due to the experiment interruption.

Early October, the next experiment relying on a new FFRF sensor software will pursue the sensor characterization and include multipath calibration. Navigation tuning will be then finalized prior to the closed loop experiment to follow.

Written by

Michel Delpech

2010-09-21 / 15:06:18

Unexpected Safe Celestial entry

During last night we had a a platform related problem onboard that caused an unscheduled entry of the Safe Celestial mode. The Safe Orbit Guidance was automatically enabled and requested delta-V to autonomously create a safe relative orbit. Before the first passage of the day, passage nr 1354, the operational team had a few different solutions prepared to restore nominal configuration.

Passage 1354, 15:12UTC: First solution solves the problem! Everyone is relieved and work starts to get back to the nominal schedule. For twelve hours Mango has not followed the nominal trajectory for the current experiment and is not at the correct location relative to Tango to start today’s experiment.
Passage 1355, 16:51UTC: Mango is commanded up in AFF mode and is set to perform a maneuver to establish the correct initial state for FFRF Initialize day 3. AFF is given 1.25 orbits to make the transfer.
Passage 1356, 18:30UTC: During passage it is verified that AFF has established the correct orbit already and FFRF Initialize day 3 experiment is given go ahead to be started from the time tagged queue at nominal starting time 18:55UTC.
Passage 1357, 20:09UTC: Everything looks nominal, experiment is proceeding well.

Even though an unfortunate night, Mango and Tango is flexible and responsive enough to fulfill today’s experiment initial conditions in a very limited timeframe. The scenario could even be started on the scheduled nominal starting time without any delay. The autonomy onboard and the support software on ground make it possible to handle unexpected events in a short time frame even with a very limited operations team.

Written by

Robin Larsson

2010-09-17 / 23:18:47