Paper ID: 935 The 16th International Conference on Space Operations 2020

Flight Execution (FE) (3) FE - 2 ”Flight operations, lessons and plans” Oral Presentation (2)

Author: Mr. Stijn Ilsen QinetiQ Space nv, Belgium, [email protected]

Mr. Dennis Gerrits QinetiQ Space nv, Belgium, [email protected] Mr. Joris Naudet QinetiQ Space nv, Belgium, [email protected] Mr. Peter Holsters QinetiQ Space nv, Belgium, [email protected] Mr. Etienne Tilmans European Space Agency (ESA), Belgium, [email protected] Mr. Christian Baijot European Space Agency (ESA), Belgium, [email protected] Mr. Stefano Santandrea European Space Agency (ESA), The Netherlands, [email protected] Mr. Karim Mellab European Space Agency (ESA), The Netherlands, [email protected]

CHALLENGES AND CREATIVITY IN THE OPERATIONS OF THE THREE SENIOR ESA PROBA SATELLITES

Abstract 1 Introduction

In the total of 35 years of combined operations, the three ESA PROBA micro-satellites (100 to 150 kg) have redefined what can be done with a micro-satellite. By applying simple and powerful design solutions and autonomy concepts, they proved that micro-satellites provide ’big-mission’ return and reliability in a small package and a low-cost context. The three PROBA satellites are however becoming seriously ’senior’, with the oldest, PROBA-1, in orbit since 2001, thereby being the longest operations ESA Earth observation mission. With age comes wisdom, but also a variety of different challenges to keep the satellites operating nominally, fulfilling their scientific tasks within strict requirements. This abstract provides an anthology of the different challenges which were faced, how they were resolved and how this led to the development of the ’P200’ satellite platform. The paper will provide more details on each of these topics.

2 PROBA-1 2.1 Mission PROBA-1 is the first ESA micro-satellite, aiming at flight-testing of new space technologies. The platform and the payloads (mainly Earth observation) has a total mass of 94 kg. The satellite was designed for 1 year (+1 year extension), but is still functioning nominally (launched in 2001 from India).

1 2.2 Orbit drift and thermal impact At launch, the local time was 10:30 hours. As PROBA-1 has no propulsion, the orbit injection was crucial. The orbit was chosen so it would advance up to 10:46 and then gradually drift back earlier. This strategy worked and provided very good local times for about eight years. The drift however continued to the extent that PROBA-1 entered a dusk-dawn orbit. This orbit with no or very little eclipse, provided a completely different thermal environment for the platform and payloads. A true challenge for PROBA-1, having a passive thermal control system. It lead to a long period in safe mode during the winter months of 2011-2012. In safe mode attitude, the star tracker detectors would get too warm to make the transition back to geodetic mode. Clever analysis however allowed to resume nominal operations with smart changes in attitude, leading to colder star trackers. Currently PROBA-1 has drifted even further and now images the Earth on the upwards arc instead of downward arc. This required a software update, which is not straightforward with a more than 15 years old software development environment.

2.3 Eye surgery for the star trackers The increased temperature and the continuous bombardment by charged particles built up bright points on the star tracker CCDs. This phenomenon began to camouflage actual stars. DTU, manufacturer of the star tracker, resolved this issue by implementing a software modification, which can distinguish bright points from actual stars. Such hotspots correspond to single pixels, while stars extend beyond single pixels exhibiting lens effects. The improved algorithm, which extended PROBA-1 lifetime, was also included in subsequent missions, like ESA’s Smart-1 Moon mission, the Sun-monitoring Proba-2, gravity-mapping GOCE and the Swarm satellite constellation. It is now part of the standard functionalities of the star tracker.

3 PROBA-2 3.1 Mission The PROBA-2 satellite embarks 17 new technological developments and four scientific experiments. It was launched in a very stable dusk-dawn orbit in 2009. With so many new technologies, the mission, among the smallest ever flown by ESA, has made a big impact in space technology.

3.2 Time wrap-around The on-board computer of PROBA-2 provides a timer capable of counting up to 8.5 years. For a mission with a planned duration of 2 years, this is the least of the worries of the operators. But on May 6th 2018, PROBA-2 was 8.5 years in orbit and the on-board time reached its maximum value and restarted from 0. This has an important impact on the time-tagged telecommands, the GNC and the timestamping of housekeeping and science data. With the experience of similar events on PROBA-1, this was carefully prepared and corrective actions were executed. Similar events with the GPS week number were handled in the same way.

3.3 New technology demonstrator after 8 years in-orbit Recently, a new software was uploaded to PROBA-2. This software included an additional (software) technology demonstration to compress housekeeping packets based on the ESA POCKET technology. The in-orbit demonstration showed amazing results with compression factors of 3 to 5, with minimal impact on the CPU. This allows to reduce the bandwidth needs for housekeeping or allows the increase of the housekeeping rate without impact on the bandwidth.

3.4 Fixing the redundant star tracker In 2015, the redundant star tracker showed an issue, where it continuously rebooted after activation. While the mission continued nominally on the primary star tracker, investigations pinpointed the problem to the transfer of star tracker image data to RAM. An FGPA update resolved the issue completely and fully recovered the redundant star tracker.

2 4 PROBA-V 4.1 Mission PROBA-V, the third mission of ESA’s IOD Programme, is an operational mission based on a small, high performance satellite platform and a compact payload. Launched in 2013, the main objective of PROBA- V as an operational mission is to continue the data acquisition of the Vegetation Instruments on-board the CNES SPOT 4 and SPOT 5 satellites.

4.2 Magnetic mode On PROBA-2, a software technology demonstration was embarked to test a magnetic control mode for rough 3-axis stabilization. It showed very promising in-orbit results. This lead to its implementation as a nominal GNC mode for safe mode on PROBA-V.

4.3 Achieve 99.6% availability with a power-cycle every 55 hours A few days after launch, a latch-up was detected in an memory component within the on-board computer. The latch-up was non-destructive, but could only be removed, by power-cycling the on-board computer. This power-cycle can be performed by commanding a switch-over to the redundant computer and then a switch-back to the primary computer. Operators do not believe in coincidence and what was feared became true: The component latched again within a few days. From ground, the operator power-cycled the computer each time. As PROBA satellites are highly autonomous, an operator is only present during work-days and working hours. After several manual power-cycles, an automated flight operations proce- dure was built to automatically power-cycle the computer in case of the issue (e.g. providing unattended recovery during the night or weekend). Although this was an improvement, it still meant loss of avail- ability as a recovery was only possible at the next ground contact (about 3 per day). The solution was found in a smart software update. When a latch-up is detected, the full switch-over and switch back is performed autonomously on-board. This provides a full recovery of the issue, within 3 minutes and smooth continuation of all the mission and payload imaging activities. All without the need for neither ground contact nor an operator. More then 5 years later, this automatic recovery happened more than 1000 times! The impact on the mission is however neglectable, with an overall availability of 99.6%.

5 P200 satellite platform

The experience of 35 years of combined operations, of the three ESA PROBA satellites is priceless. QinetiQ space used this heritage for the development of a standardised platform, named ’P200’. This highly reliable satellite platform provides a cost-efficient solution for a multitude of different payloads. In times of rapid development of new satellite platforms, the P200 platform is an assurance for reliability and performance.

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