Flight Dynamics – how to pilot a satellite

How does a satellite remain in its orbit? Which forces work on a satellite in space, and how do these forces act at different orbital positions?

It takes several years to ready a satellite for launch, and once the satellite detaches from the rocket that brought it out into space, the job is far from over. Space Norway’s two highly elliptical satellites will be launched by a Falcon 9 up to about ten thousand kilometers. From there starts a long journey into space, ending at 43 thousand kilometers where it establishes its 15 years orbit. This journey is called orbit raising, and it is predicted to last for ten days. On board are two fuel tanks, and all around the body of the satellite are small thrusters used for correcting the course. Approximately 90% of all the fuel will be used while orbit raising. To reach its correct trajectory, engineers must determine how much force to use for exact course adjustment. Flight dynamics is the art of calculating the effect of all the forces that work on the satellites and all the flight path corrections consequently needed in its entire life span. The remaining ten per cent of fuel on board is used to uphold an exact path for 15 years.

Satellites in orbit are subject to continuous forces. The strongest effect comes from earth’s gravity. Additionally, both the sun and the moon’s gravities influence the satellites. The position of the sun, moon and earth are predictable, meaning that the effect of these forces can be calculated and simulated in advance. But then you never know. Maybe the weather is bad on launch day and a delay is inevitable. All calculations would have to be made over.

The satellite itself does not know exactly where it is, so this information is uploaded from the ground. The exact position is determined by triangulating signals pointed at it, and all path corrections are made based on these calculations. The engineers on the ground compute the exact location on each 16 hours orbit, giving the exact amount of thrust needed from each on board engine to correct the course, all the while keeping an eye on how much fuel is remaining. These flight maneuvers are done every fortnight for the entire satellite life span, and two major path corrections are planned in year 10 and 12.

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