Increased On-Orbit Autonomy of Large Platforms using Accelerometers

Objectives

In the frame of the development of the Alphabus telecommunication platform, a system study is being carried out to explore one way of increasing the spacecraft autonomy in orbit.





The main autonomy limitation of geostationary telecommunication spacecraft using electric propulsion for their North/South station keeping (orbit inclination and eccentricity control) comes from the manoeuvre triaxiality. Indeed, the diffuse plume of electric thrusters results in forces applied on the spacecraft in the direction tangential to the orbit, which is called triaxiality. Since tangential thrust creates longitude drift, the longitude has to be controlled frequently enough so that the spacecraft remains in its longitude window.


click for larger image


The purpose of this project is to assess the possibility to use an accelerometer during the electric manoeuvre in order to measure the tangential thrust. - This project is run in parallel with the development studies for an electrostatic accelerometer by ONERA, France, to suit the needs of Alphabus. - Such a measurement would allow the spacecraft to compensate for that parasitic thrust with the adequate East/West manoeuvre in an autonomous way. The autonomy target for Alphabus is 10 day, but this method may prove able to reach an autonomy of 28 days.

Challenges

The key issues to be addressed in this project are:



  • The technical convergence between the system needs assessed by this project and the accelerometer design performed by the parallel project carried out by ONERA. A management emphasis is put on the cooperation between Astrium and ONERA.



  • The feasibility of the precise accelerometer measurement in the spacecraft dynamic environment.



  • The introduction of a closed loop approach in the orbit control method.

Benefits

The main benefit expected from this project is to reach a higher level of autonomy for the future high power telecommunication platform Alphabus that will directly results in cost savings in the spacecraft operations.

Features

The system architecture is the current architecture of a telecommunication spacecraft (or a constellation of spacecraft) and its ground control segment.

The autonomy increase is achieved with the combined use of:



  • A new on board E/W control loop that measures the tangential thrust thanks to an on board electrostatic accelerometer, and commands East/West manoeuvres so as to compensate for the parasitic tangential thrust measured by the accelerometers.



  • A new ground control loop that makes use of longitude and longitude drift measurements at the chosen autonomy period to estimate the perturbing longitude drift and acceleration experienced by the spacecraft and to compensate for it over the next manoeuvre set. This ground control loop also permits to cancel out part of some accelerometer measurement errors such as misalignment and scale factor.

Plan

The project plan consists in two phases:



  • Phase 1 consists of assessing the feasibility of the tangential thrust measurement with an electrostatic accelerometer, in designing a control solution for the on-board and ground control loops, as well as in establishing an error budget of the longitude control.



  • Phase 2 consists of designing the on-board and ground control loops in detail, and carrying out a campaign of time simulations with representatives models of the accelerometer and the space mechanics phenomenon to validate and complement the results of Phase 1.

Current status

The first phase of the activity has been successfully completed, showing the feasibility of a new concept using accelerometers for increased on-orbit autonomy. For Alphabus an alternative option has been chosen to achieve on-orbit autonomy, and it has therefore been decided not to pursue the activity.

Contacts

Status date

Wednesday, November 16, 2011 - 12:42