Table of Contents

Orbital Mechanics

Changing Orbital Planes

Energy Gobblers

1. Circular Low Orbit

In order that a satellite appear to remain stationary over a single line of longitude above the Earth's surface it must be placed into a circular orbit in a plane approximately the same as the Earth's equator, and at an altitude such that the orbital period of the satellite is exactly 24 hours. This is called a geosynchronous orbit.

In most cases satellites are initially launched into low Earth orbit and subsequently manoeuvred into geosynchronous orbit by a series of carefully timed burns of the on-board rocket engines.

2. Burn One

Once the satellite has been placed in a stable low circular orbit a small rocket "burn" is applied. The burn is applied, (indicated at 1), so that the thrust from the rocket engine will accelerate the spacecraft in a direction tangent to the satellite's orbit and in the direction of its motion.

Point 1 is called the insertion point, because it is the point at which the satellite is inserted into the new orbit.

The effect of this burn is to change the satellite's orbit from a circular orbit to an elliptical orbit whose apogee (highest point), is at the altitude required for a geosynchronous orbit.

The perigee (lowest point), in the new elliptical orbit will always be at the insertion point.

3. Burn two

The next step in placing the spacecraft into a geosynchronous orbit requires a second rocket burn at point 2. Point 2 is the apogee point of its elliptical orbit.

Once again, the burn is applied (indicated at 2), so that the thrust from the rocket engine will accelerate the spacecraft in a direction tangent to the satellite's orbit and in the direction of its motion.

If exactly the right amount of energy is added to the satellite's orbital motion, it will assume a new orbit which is exactly circular and with an orbital period of 24 hours.

This circular orbit is called a geosynchronous orbit because the orbital period (24h) of a satellite in this orbit is synchronized with the rotational period (24h) of the Earth.

4. Burn Three

A satellite in geosynchronous orbit will not necessarily appear to "hover" over a single geographic location on the Earth's surface unless the orbital plane of the satellite is in the same plane as the Earth's equator.

To change the plane of the satellite's orbit requires a third rocket burn.

For reasons of fuel efficiency the changes in the orbital plane of a satellite are almost always made at the largest possible orbital distance from the Earth. Changes in the orbital plane (also called changes in orbital inclination), are extremely fuel "hungry". That is to say, they are very expensive in terms of fuel consumption.

As with all orbital manoeuvres, the insertion point of the "old orbit" will become a point in the "new orbit". Another way of saying this is that the insertion point will be common to both orbits, before and after the rocket burn.

Therefore, burn three must occur exactly "over" the geographic point on the Earth's equator over which the satellite is to remain in geosynchronous orbit. This new orbit is called a geostationary orbit.

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