The objective of the study “Optimisation of NS, EW Station Keeping Manoeuvres for Geo Satellites Using Electric Propulsion” was to develop a design & analysis tool to optimise full electric station-keeping, bringing thus the EP-based station-keeping strategies from TRL 2-3 to TRL 4-5.
The developed STAKE (STAtion-Keeping with Electric propulsion ) tool is based on an optimiser from ASTOS Solutions, GESOP, which is used to compute the optimum manoeuvres according to several constraints and modular spacecraft and thrusters configurations.
The objective of this study was to develop a design & verification environment to support full electric station-keeping optimisation, the so-called STAKE (STAtion-Keeping with Electric propulsion) tool. It handles station-keeping in geostationary orbit with full electric or hybrid propulsion based on an optimiser from ASTOS Solutions, GESOP, which is used to compute the optimum manoeuvres.
STAKE tool has two complementary roles: the optimisation of the Station-Keeping strategies for a given thrusters configuration, and the optimisation of a thrusters configuration for a given station-keeping strategy by tuning the thrusters directions.
Several options for cost and constraints functions are available. It is especially possible to control the satellite angular momentum while fulfilling the station-keeping requirements in terms of station-keeping box. The satellite definition is representative and modular: the thrusters configuration can be defined as well as the reflectors & solar panels configuration.
Using STAKE tool, sensitivity to different on-station longitudes and spacecraft configurations has been analysed. Robustness of the station-keeping strategy has also been assessed with the tool.
The first challenge within STAKE tool conception and development phases were the mathematical definition of the Station-Keeping and of the Thrusters Optimization as optimization problems together with the integration of GESOP optimizer.
Then a major challenge in the tool development was to make available all the relevant cost and constraints functions to support system level analyses. This leaded to develop new constraints such as kinetic momentum with TOMs or modulated thrusts and power.
Finally after the development of STAKE tool on Linux platform, the portage of the tool on Mac and Windows platform were important and new issues. In particular on Windows platform on which all the Bourne Shell scripts had to be recoded in Python language.
The study “Optimisation of NS, EW Station Keeping Manoeuvres for Geo Satellites Using Electric Propulsion” lead to major outcomes both for obtained results and developed STAKE tool capabilities.
Regarding the obtained results, we showed that it is possible to perform optimal station-keeping using electrical propulsion controlling at the same time the orbit and the spacecraft attitude with a small over consumption.
- A defined Station-Keeping strategy,
- A given spacecraft configuration (solar arrays configuration, number and position of thrusters),
- Several system constraints.
Furthermore, with its second level of optimisation, it enables to optimize the thrusters configuration and can thus be used in A-B phases to define the most suitable thrusters configuration.
In the frame of this study, full electric station-keeping management has reached TRL 4-5 as targeted initially. Reaching TRL 7-8 will still require some work especially to take into account specific S/C platform constraints & operational concepts. However, STAKE core functional algorithms are likely to be a good basis towards an operational software.
STAKE software is composed of:
- A framework allowing the run of activities and the management of the database through dedicated GUIs, the plot edition, the management of archives,
- A database (ascii files),
- Flight Dynamics algorithms,
- An optimizer (GESOP),
- An interface between Flight Dynamics algorithms and the optimizer.
STAKE tool consists in four main activities:
- “SK Simulation” (Station-keeping simulations),
- “Optim Conf” (Optimisation of thrusters configuration),
- “Convert into working frame” (Orbit conversion),
- “Colocation Prediction” (Cluster geometry prediction).
STAKE includes two databases in which the satellite parameters are stored : the orbit database and the spacecraft constants database. Each of these databases includes an indefinite numbers of records. A database record is usually called a vector. Typically, the orbit vector includes orbital elements with the corresponding date. The spacecraft constant vector is dedicated to technological data (SK simulation parameters, electric thrusters configuration, optimisation parameters, mass properties).
The following figure presents STAKE software interfaces for SK Simulation Activity which is the main STAKE activity.
The GUI of SK Simulation Activity is run from the Main GUI. Available database files can be selected, edited or created from existing ones from the Main GUI or from the Activity GUI. Input data necessary for the module are edited and completed through the Module Input GUI. They may be initialised with Database inputs.
GESOP executable is run from Fortran Main SK Simulation executable thanks to a command line (call system). The link between GESOP Fortran model library and Fortran Main executable is done through input and output ascii interface files. Ouput files of GESOP and Main executable are located in current framework working directory and may be edited thanks to Activity GUI.
The study was based on a four steps standard logic:
- Review of the State-of-the-art EP technology & Station-Keeping strategies,
- STAKE software functional design and architecture,
- STAKE software development and validation,
- Analysis using STAKE software.