II.   The model : trajectory of a satellite.

        The model we have chosen to study and to correct is the trajectory of a satellite. S. Massart (thanks to him) has created a program with a graphic interface giving the position and the speed of a virtual satellite (the ray of earth is considered to be equal to one).
Here is the graphic interface designed by S. Massart :

It is possible to visualize the trajectory with xmgr :

With some models (like this one...), a small modification of one parameter (a change of speed of 0.01 m/s in this case) changes the whole behaviour of the system. It is important to control the values given by a model and check they do not drive to a non-sense that is why using observations is very important. The right balance between observation and prevision can approach more easily reality.

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