New Optimal Strategies for the Station Keeping of Communications Satellites in Geoestationary Orbits using Electric Propulsion

Problem raised by GMV Aerospace and Defence, S.A.

Coordinating teachers of the problem:

Pilar Romero (Universidad Complutense de Madrid)

Antonio Pérez-Cambriles (GMV Aerospace and Defence, S.A.)

Exposition of the problem:

Periodic orbital station-keeping manoeuvres for a geoestationary satellite are performed to compensate for the natural perturbations that tend to change its orbit to nongeoestationary, as well as to keep its orbital elements within the prescribed boundary according to the mission requeriments.

To this end normal and tangential orbital manoeuvres are applied. Whereas the aim of the (normal) north/south stationkeeping (NSSK) is the control of the oscillations in latitude due to the lunisolar perturbations on the inclination of the satellite´s orbit, the tangential east/west stationkeeping (EWSK) manoeuvres are peformed to control the longitude evolution, which is due both to the drift caused by the peturbing terms of the Earth potential and the nonzero eccentricity because of solar radiation presure effects.

Standard techniques uses analytical expressions to determine secular or long term variations in the orbit evolution as well as linearized equations to compute the correction manoeuvres, and optimal strategies to minimize the propellant consumption. The Secular Mean line (SML) strategy is usually chosen to plan the NSSK. This means that with this strategy the correction is applied in the direction of the secular drift for the log term evolution of the inclination vector. For the EWSK manoeuvres the optimal direction is obtained by pointing the perigeee of the satellite orbit towards the sun, i.e., according to the strategy called Sun Pointing Perige (SPP).

Different software package can be found as the Portable ESOC Package for Syncronous Orbit Control (ESA).

The propulsion systems presently used to correct the orbit are of chemical nature (usually, of hydrazine) although the new trends in spatial propulsion point towards the possibility of using electric propulsion systems (arc-jets, XIPS). This, which will lead to the reduction of an important amount of the satellite mass at launching time, makes necessary to implement new strategies because both of the limitations in the magnitude of the impulses provided by these systems and, given the high electric energy consumption, of the impossibility to perform any manoeuvre during a long period of time at the eclipse epochs.

Our main purpose is to make a planning as well as to analyze the implementation of optimal strategies for the station keeping (North/South) and East/West) to satisfy the constrains impossed by the use of electric propulsion systems

The use of these kind of systems introduce new important constrains:

§ Limitation in the magnitude of impulses.

§ Impossibility to perform manoeuvres during long time periods at eclipse epochs because of high electric energy consumption

To achieve this goal, we propose to modify the SML strategy for the inclination vector control and the SPP strategy for the control of longitud and eccentricity vector in order to keep orbital elements within the mission limits for the period without manoeuvres. This implies

§ To determine the period of time when manoeuvres are forbidden.

§ To determine optimal target values for the orbital elements before the eclipse epochs.

Scheme of the work to be done:

1) To determine the time and duration of eclipses for different inclination and node values.

2) To review the energy storage according to the mission type

3) From the conclusion, that If the node is corrected in a different way that one corresponding to MSL, eclipse duration can be reduced 20m a day and then batteries can be recharged more time, to evaluate if the increment of DV in the NSSK when the SML it is not applied is less than which is obtained from the additional solar exposition

4) To construct a simplified model for the analysis of orbital parameters evolution and the determination of optimal times for the manoeuvres