A commonality of all space missions is the need to receive, process, archive, and analyze on-board telemetry of the spacecrafts involved. For long-running missions, the amount of data that needs to be preserved can reach hundreds of gigabytes. At the German Space Operations Center (GSOC), the RootVis framework is under development; it shall allow to process the full telemetry dataset of the GSOC satellite missions for analysis of the long-term behavior of the spacecraft. Typically each mission has slightly different concepts of analysis, visualization and format of its telemetry, so RootVis was conceptualized as a modular telemetry visualization tool. It is built around the core telemetry archive in the file format of the ROOT data analysis library, which-thanks to its efficient serializationenables high performance data access. Thus, it is possible to handle large data sets with billions of data points in short time for all current and future GSOC missions.
The TET-1 satellite serves as a technology demonstrator within the On-Orbit Verification (OOV) program with the goal of providing German space companies and research institutions with the opportunity to test their equipment on actual spacecraft. It was launched on July 22, 2012, by a Russian Soyuz/Fregat into a Sun-synchronous orbit with an LTAN of 11:27 UTC. The satellite is powered by nickel-hydrogen batteries during eclipse. The operations team became aware of several issues shortly before and after the launch which created some challenges for battery operation. Some battery cells had undergone reversal prior to launch which seems to have created an imbalance between them. Also, the overall temperature within the satellite and that of the batteries turned out to be higher than predicted and the battery voltage slightly exceeded the limit for payload operations. Regulating the battery temperature was at first attempted by lowering the amount of charge put into the battery. However, this could not be done indefinitely due to worries of battery damage (memory effect) and the constricted charge available for payload operations. Hence, other approaches were employed. The Sun Pointing Rotate Mode (SPRM) was one of the methods used. The satellite was reoriented in such a way that the bus radiator was pointed away from the Earth and the Sun by more than 90 •. Consequently, its ability to radiate heat away into deep space was improved. Another method was a different charging scheme called "ratchet charging", known to have been successfully employed on the Hubble Space Telescope (HST). In this method the battery is charged more than once during a Sun phase and the End-of-Charge (EoC) value is raised every week. A software update during the latter stage provided more flexible control over the solar panel strings, allowing the satellite to fly without a pitch offset which was used earlier to lower the voltage. This paper describes in detail the circumstances in which these issues arose, wherever possible their causes and how they were dealt with. The capacity of the TET-1 spacecraft's batteries is found to have increased from 12 Ah to about 14 Ah without any significant increase in temperature. Data gained at a later point in time also suggest a positive effect of the reconditioning.
In this work we discuss about the two widely used energy storage systems in space applications namely, Lithium-Ion (Li-Ion) and Nickel Hydrogen (NiH2) batteries. A few of the satellite missions operated by the German Space Operations Centre (GSOC) is used in this work for case study.The NiH2 batteries have been a part of energy storage applications for over two decades and have a very good track record for their reliability and performance. These batteries are still one of the much preferred energy storage system for satellites. The TET-1 (called as "Technologie Erprobungs Träger" in German) was launched in 2012 is among the newer generation of satellites which is powered by NiH2 batteries. Even after having experienced some problems shortly before launch, the batteries are performing excellently. We have discussed some of those issues and their solutions.The use of Li-Ion chemistry in satellites is quite new but over the last decade has proved its reliability in several deep-space as well as low earth orbit missions operated by various space agencies worldwide. The operational experiences of the satellites TerraSAR-X (TSX, launched 2008) and TanDEM-X (TDX, launched 2010) are discussed to demonstrate the capabilities of the Li-Ion batteries. Although, the NiH2 chemistry is still widely used, our operational experience shows that the Li-Ion technology might power the majority of future spacecrafts.
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