1. Orbit Generator for the Next Generation SPENVIS INTRODUCTION TO SPENVIS-NG The Next Generation SPace ENViroment Information System is the latest revision and update to the current SPENVIS system, a web interface to models of the space environment and its effects, including the cosmic rays, natural radiation belts, solar energetic particles, plasmas, gases, and micro-particles. One of the core functionalities is the Orbit Generator: a module that reads the orbit definitions for a mission in a wide range of formats and computes a common-format orbit ephemeris to be used by the different models available to SPENVIS users. ©DEIMOS Space SLU All Rights Reserved. No part of this document may be reproduced, stored, or transmitted, without the prior written permission of DEIMOS Space SLU. ©DEIMOS Space SLU Ángela Rivera (1), Ignacio Grande (1), Esther Parrilla (1), Noelia Sánchez (1), Daniel Heynderickx (2), Michel Kruglanski (3) (1) DEIMOS Space S.L.U., Ronda de Poniente 19, 2°2, Tres Cantos, Madrid, 28760 Spain, Email: [angela.rivera, ignacio.grande, esther.parrilla, noelia.sanchez]@deimos-space.com (2) DH Consultancy, Bondgenotenlaan 148/0401, B-3000 Leuven, Belgium Email: DHConsultancy@skynet.be (3) Belgian Institute for Space Aeronomy, Ringlaan-3-Avenue Circulaire, B-1180 Brussels, Belgium Email: michel.kruglanski@aeronomie.be CURRENT ORBIT GENERATOR: SAPRE The current SPENVIS system uses SAPRE as orbit generator. SAPRE is a powerful and versatile orbit generator that suits SPENVIS needs, however, it lacks proper documentation and its capabilities exceed by far SPENVIS-NG requirements of a fast, not-so-accurate propagator. In addition, input format requirements of SPENVIS-NG would make necessary to modify SAPRE, a task that would require quite some time to accomplish. For these reasons, it has been decided to develop a new, easier to maintain Orbit Generator, with the required capabilities of input and output formatting. The Orbit Generator fulfils the requirements of the SPENVIS-NG space environment modules regarding the orbit ephemeris and also the user’s requirements regarding input file formats, and necessary graphic representation (actual graphic generation is done by a generic plotting module). Some of the capabilities of the Orbit Generator include: Segment-based propagation. A project may contain several segments with different orbit definitions. Multiple central bodies, including the Sun, Solar System planets and their large natural satellites. Keplerian propagation. This is a very fast analytical propagator with reduced accuracy, however, accuracy requirements are generally low for Space Weather applications, and propagations arcs are short. Versatile input formats: Keplerian elements, ESOC Long Term Orbit File (LTOF), CCSDS OEM format and current SPENVIS output ephemeris format. VOTables format used for ephemeris output. Output ephemeris contains a series of state vectors in J2000, coded in VOTables. TLE files can be uploaded from the user or downloaded from Internet providers (Space-Track, Celestrak, etc.) to be used as input format by the Orbit Propagator. DEIMOS SPACE S.L.U. an ELECNOR company Tres Cantos, Spain www.deimos-space.com 10th European Space Weather Week Solar-Terrestrial Centre of Excellence, November 2013 Antwerp, Belgium The work presented in this paper is done within ESA/ESTEC Contract No. 4000104812 A mission can be defined by different propagation segments, as the LEO-GTO-GEO case shown in the figure (not actual SPENVIS output) CAPABILITIES The design of the Orbit Generator will be based on the following principles: Simplicity: provide the simplest possible tool that covers the required functionality. Modularity/Flexibility: provide a solution that allows future improvements to be added easily, which is something difficult to do currently in SAPRE. Boost development process: trying to understand SAPRE architecture would probably take more time than implementing the necessary requirements. NEW ORBIT GENERATOR: DESIGN PRINCIPLES ARCHITECTURE The Orbit Generator will aim for short trajectories divided into segments instead of in creating the whole orbit of a spacecraft for the duration of the mission. This will provide a less cumbersome simulation model that will keep at a minimum the resources consumed by the users in terms of CPU time and disk space. A mission contains information about the starting date, mission duration and a set of segments. Each segment contains the following information: Segment start date and duration. Trajectory duration, used to compute the actual propagation. Central body and orbit information Independently of the input format of the orbit information, the propagation will be performed separately for every segment, and the output will contain the following information in VOTables format: Time interval computed. Ephemeris information with epoch in MJD UTC and state vector in J2000 coordinates for each ephemeris step. Both input and output is stored in a predefined directory structure, compatible with the SPENVIS-NG ecosystem. SUPPORTING LIBRARIES Two libraries will be used to support the development of the Orbit Generator: JPL SPICE Tool, a tool already known in other ESA missions (Huygens, Mars and Venus Express, Rosetta and currently in Ganymede). The tool provides subroutines to perform Keplerian and SGP4 propagation, coordinate transformation and state vector interpolation, among others. DH Consultancy tool currently used for GREET contract, that uses JPL SPICE itself to compute state vectors using Ganymede as central body. Source code of this tool is available and can be modified to suit SPENVIS- NG needs. IRBEM-LIB package will be also used for some Earth-centred coordinate transformations needed due to the versatile input format supported. All these libraries are available in Fortran, the chosen language for the Orbit Generator.