XTCE12 — Section: 7

CNES remarks and recommendations for the XTCE norm CNES (French Space Agency) has led a study to determine if and how the XTCE standard can be used in the context of the Myriade project. (Myriade is a microsatellites family, initiated by Demeter, Parasol …). Some files produced by the actual Myriade data base (called BDMS)for the CCC Control Center have been translated in XTCE. Here are the results of this study : About Telemetry : We have chosen the decommutation plan which is one of the most representative file among those who are exported from the system data base. In this file we have chosen a significant TM packet (called ECU) with a structure containing a discriminating element (part of the telemetry depends on the value of a parameter called a "selector"). The complexity is to separate the data from the processes in the BDMS to define the best implementation with theXTCE norm. We see that the data in the XTCE file must be processed in order to obtain the expected file (addition of ground calculated parameters, renaming of some parameters, calculation of the offset, ...). For the XTCE norm itself, we can notice that there are two ways to define conditional structure. We think that, at least, a way to use XTCE must be recommended in order to obtain a homogenous implementation in different projects. About Telecommands We encountered some difficulties in trying to implement the example of TC with the XTCE norm. We found a lot of similarities in the definitions of types included in the TelemetryMetaDataType and the CommandMetaDataType elements. In some cases, these similarities present slight differences, which are obviously not enough justified with sufficient explanations (ie the definition of the CommandContainer is different coming from CommandMetaData/MetaCommandSet/MetaCommand or coming from CommandMetaData/CommandContainerSet), and therefore resulted in some confusions. The structure of nested types is not easy to assimilate. Some information seem to be redundant inside a type. It would be useful to have more explanations of this case. At first, the information provided by the TC example and the elements defined in the XTCE norm didn’t fit. We finally carried out a way to proceed, nevertheless, there is no evidence that our solution is the best one. The point that is not resolved is how to implement the variable argument. Conclusion of the study Within the framework of this study, the scope of TM/TC implemented with XTCE clearly appeared as limited. We focused our study on a subset of one TM packet, and also on one specific TC. As a result, the first step can be consider positive. However, in the absence of a more exhaustive study which encompass more possibilities of XTCE, the overall feasibility is still pending. The most important problem of the norm is that it is very complex, and difficult to learn, even for engineers experimented in XML, and data base schemas. Its complexity comes from its genericity. We think that there are too many ways to define a TM packet with this norm to be sure that every project will define its telemetry using the same philosophy. In the SpaceSystem schema which describes the norm, the <Annotation> sections are very succinct and sometimes not sufficient. So, in order to help users to understand the schema, the chapter 6 (The Specification) of the XTCE norm should contains more in-depth explanations. Moreover, the standard is far from being user friendly due to excessive offered possibilities in terms of implementation. We did not find any example corresponding to our needs. The available documentation is surely not designed for an easy learning. Of course, a tutorial is to be issued. A solution to reduce the learning effort could be to define a smaller kernel of the norm, with some possible extensions for some specific use. One important lack of the norm seems to be that it does not enable a user to define its own tags and attributes in order to complete the description. This could be the solution to avoid changing the norm each time a particular need appears in a project. Another important remark we have to make is about the state of XTCE. The norm has changed a lot between the OMG version of 2004 and the referenced CCSDS red book. It seems not to be still defintly defined. This fact is very important because if we want to use the norm in an operational context, with have to be sure that there will not have big changes before starting the development of tools based upon the norm. Tools are mandatory to use this norm because it is quite impossible to create a full XTCE file with a simple XML editor. When the norm will be approved, the XML-oriented database can be the next step in the XTCE deployment. It could be interesting to have a generic code that enables the updates of a database by reading a XTCE file and enables the extractions from a database to produce some XTCE files. Xavier Passot CNES DCT/SB/CC With Erwann Poupart (CNES DCT/PS) and Frederic Berriri (CS/SI)

Future XTCE profile specifications and validation tools

Good examples would enhance the specification, but at this point the examples are emerging from the communities of use (CCSDS, US Government) and are probably most appropriate to be included in those specifications, since they will differ by community.