Introduction to Satellites and Orbits

Velocity

As a satellite orbits the Earth in an elliptical orbit, its distance from the Earth's surface changes. The point in its orbit at which the satellite is closest to the Earth is called perigee. The point opposite to perigee, when the satellite is at its furthest point from Earth, is called apogee. As a satellite approaches perigee, its orbital velocity increases. At perigee, the satellites velocity is at its maximum. As it approaches apogee, its orbital velocity decreases. At apogee, the satellites velocity is at its minimum. Thus, satellites with a nearly circular orbit maintain a nearly constant orbital velocity, while satellites with highly elliptical orbits have a wider range of orbital velocities, speeding up as they get closer to Earth and slowing down as they move further away.

Period

The period of an orbit is the amount of time it takes for a satellite to complete one full orbit around its primary body. A general rule of orbital mechanics states that the closer an orbiting object is to its primary body, the higher its velocity. In addition, the closer a satellite is to the Earth, the less distance it must travel to complete a single orbit. The result is a general relationship between a satellite's altitude and its period: the lower the altitude, the shorter its period. The lowest satellites orbit the earth with a period of approximately 87 minutes per orbit (if a satellite were placed any lower in the orbit the atmosphere would interfere so much that it could not maintain its orbit). Other satellites at higher altitudes have orbital periods that are longer than a full 24 hour day.

Inclination

Inclination of a satellite orbit describes the tilt of the orbit plane with respect to the equatorial plane. An orbit with inclination angle of 0º would orbit the Earth in the same plane as the Equator. This is known as an equatorial orbit, and a satellite in this type of orbit follows the Earth's equator. An orbit with an inclination angle of 90º would orbit the Earth crossing the North and South Poles in a plane that is perpendicular to the equatorial plane. This type of orbit is known as a polar orbit. Other satellites are in orbits with inclinations between 0 and 90º.

Polar orbiting satellites

While a true polar orbit has an inclination of 90º, many satellites orbit the earth with inclinations that are close to 90º. These form a class of satellites known as polar orbiting satellites. These satellites orbit the Earth in an orbital plane that goes nearly from pole to pole. They are considered Low Earth Orbiters (LEO), which orbit the Earth at an altitude of approximately 300 km. Polar orbits are usually nearly circular and the satellites have a constant height above the planet. They generally have a period around 90 minutes maintain a constant orbital velocity.

As a polar orbiter circles the planet, and as the Earth rotates underneath, the satellite crosses a different strip of the Earth with each orbit. The effect is that a polar orbiting satellite can scan the Earth in strips, and over the course of several orbits, it can collect data over a significant portion of the planet. The lower altitude of the polar orbits can allow the sensors to study the Earth in greater detail than a higher altitude craft, and it is far less expensive to build, launch, and maintain than a higher altitude satellite.

This type of orbit is primarily used for surveillance, environmental monitoring, and space related research. Examples of polar orbiting satellites are the Landsat satellites, the TIROS-class meteorological satellites, the space shuttle, and the Mir space station.