Satellite & Circular Motion & understanding of Geostationary Satellite

We will discuss here the circular motion of satellites, state of free fall of satellites, types of orbits, geostationary satellite and low Earth orbit satellites.

Satellites undergo circular motion

Satellites orbiting the Earth, including the Moon, are examples of objects undergoing circular motion. In this case of circular motion, the centripetal force is provided by the gravitational attraction force.

The circular motion described by satellites can be said to have two components: one that is constant tangential speed, the other being free fall with a constant acceleration of gravity.

Satellites orbiting the Earth are in a state of free fall

Satellites orbiting the Earth are in a state of free fall. As a satellite orbits the Earth, it is pulled downwards by the Earth’s gravitational field.

If the satellite was stationary, it would fall vertically down just as a ripe apple falls straight to the ground. The linear orbital velocity of the satellite doesn’t allow it to fall. That is, when the satellite is falling down, at the same time it is also moving away from the Earth. This results in its path being circular, as shown in Figure 1. Since the horizontal and vertical motions are independent, while the satellite is describing a circle, it can be considered to be in a state of free fall.

Indeed, if the orbital speed is not fast enough, the satellite will describe a parabolic path—a projectile motion—and fall back to the
Earth. But as the orbital speed increases, the length of the parabola increases. Eventually, when the orbital speed is sufficiently high so that the rate of falling can be matched by the rate of ‘moving away’, the satellite will describe a circular path (see Fig. 2).

Types of satellite orbits

Satellites orbits can be classified into different types. Two types of orbit that have specific characteristics and properties are geostationary and low Earth orbits.

Difference between geostationary satellite and low Earth orbits satellite (Tabular format)

	Geostationary satellite	Low Earth orbits satellite
	Geostationary satellites situated above the equator	Low Earth orbit satellites do not need to orbit above the equator.
	Geostationary satellites appear to be stationary in the sky when viewed from the surface of the Earth, hence the name: geo (earth) stationary.	No such feature is available with low Earth orbits satellites.
	Geostationary satellites have a limited view of the Earth’s surface (approximately one-third).	Low Earth orbits are often polar so that the satellite obtains a view of the entire surface of the Earth after several orbits.
	They are orbiting the Earth at the same rate as the Earth’s rotation. Thus, the geostationary satellites and the Earth have the same period, that is, they both complete one revolution or one rotation every 24 hours.	They may orbit the Earth many times per day.
	The orbital periods of geostationary satellites are more than those low Earth orbits satellites.	The orbital periods of low Earth orbits satellite are less than those that of geostationary satellites.
	Situated at a very high altitude, approximately 35,900 km above the Earth’s surface.	Much lower orbital altitude.

Geostationary satellite

Geostationary satellites appear to be stationary in the sky when viewed from the surface of the Earth, hence the name: geo (earth) stationary. However, like other satellites, they are also revolving around the Earth. The reason they appear to stay at one position above the Earth is because:

1. They are situated above the equator.
2. They are orbiting the Earth at the same rate as the Earth’s rotation. Thus, the satellites and the Earth have the same period, that is, they both complete one revolution or one rotation every 24 hours.

Definition of Geostationary Satellites

Definition: Geostationary satellites are satellites that are situated above the Earth’s equator and orbit the Earth with a period of 24 hours, remaining directly above a fixed point on the equator.

Low Earth orbit satellites

Low Earth orbit satellites are satellites with orbital radii altitude between 200–2000 km (their orbital periods are less than those that of geostationary satellites). They may orbit the Earth many times per day.

They do not need to orbit above the equator. Low Earth orbits are often polar so that the satellite obtains a view of the entire surface of the Earth after several orbits.

Definition of Low Earth orbit satellites

Low Earth orbit satellites are satellites with smaller orbital radii than those of geostationary satellites; their orbital periods are less than those of geostationary satellites. They orbit the Earth many times per day.

Advantages of Geostationary satellites

Easy to track since each satellite stays at one position at all times.
Do not experience orbital decay.

Advantages of Low Earth orbit satellites

Able to provide scans of different areas of the Earth many times a day. Geographical mappings are made possible.
Low altitudes enable a closer view of the surface of the Earth.
Low altitudes allow rapid information transmission with little delay.
Low altitudes mean the launchings of these satellites are easier and cheaper, as less fuel for the same satellite mass is required.

Disadvantages of Geostationary satellites

Delay in information transmission must be considered.

Each satellite has a limited view of the Earth as it only stays at one point above the Earth. (Each satellite can ‘see’ about one-third of the Earth’s surface.) Therefore many geostationary satellites are required to provide coverage of the entire surface of the Earth. Even then, polar regions may still not be properly covered.

Their high altitude makes launching processes more difficult and expensive as more fuel is needed.

They suffer more damage from incoming energetic cosmic rays due to their high altitude.

Disadvantages of Low Earth orbit satellites

Much effort is required to track these satellites, as they move rapidly above the Earth.

Atmospheric drag is quite significant and orbital decay is inevitable.

The orbital paths of the satellites have to be controlled carefully to avoid interference between one satellite and another.

They are more severely affected by the fluctuations in the Earth’s van Allen radiation belts.

Main uses of Geostationary satellites

Information relay: information is sent up to one satellite and is bounced off to another place on the Earth.

Communication satellites, e.g. Foxtel.

Weather monitoring.

Main uses of Low Earth orbit satellites

Geotopographic studies: including patterns of the growth of crops and spreading of deserts.

Remote sensing.

Geoscanning and geomapping.

Studying weather patterns.