What is an Orbit? | Types of Orbit | Space Awareness

What is an Orbit?

An orbit is a path an object takes as it travels around another object. The satellite’s orbit is a crucial component of its function. The different satellite orbits are utilised depending on the function/application a particular satellite is expected to perform. The orbits of satellites are also dependent on its position relative to the surface of the Earth.

Types of Orbit:

Primarily, the orbit of the satellite is one of the two types: Circular satellite orbit and Elliptical satellite orbit. For a circular orbit the distance from the Earth remains the same at all times. However, the elliptical orbit keeps on changing the distance relative to the Earth.

The satellite orbits can also be categorised into various types like low-earth orbit (LEO), medium- earth orbit (MEO), highly elliptical orbits (HEO), geosynchronous orbit and geostationary orbits on the basis of altitude of satellite orbits.

Low-earth orbits (LEO)

As implied by its name, Low Earth Orbit is relatively low in altitude ranging between 200 and 1200 km above the Earth’s surface. However, LEO is still very close to the Earth in relative terms. It is especially so when it is compared to other forms of satellite orbit including geostationary orbit.

The low-earth orbits have the following features:

Orbit times are much less than for many other forms of orbit. The lower altitude means higher velocities are required to balance the earth’s gravitational field.
The lower orbit means the satellite and user are closer together and therefore path losses are comparatively lesser when compared to path losses experienced by other orbits such as geostationary orbits.
The round trip time (RTT) for the radio signals is considerably less than that experienced by geostationary orbit satellites. The actual time depends upon factors such as the orbit altitude and the position of the user relative to the satellite.
Radiation levels are lower when compared to what is experienced at higher altitudes.
Lesser energy is needed for placing the satellites in low-earth orbits (LEO) when compared to what is required for placing satellites in higher orbits.
The LEO satellites may experience speed reduction as a result of friction from the low (yet measurable levels of gases) especially at lower altitudes. An altitude of 300 km is basically regarded as the minimum for an orbit as a result of the increasing drag from the presence of gases at low altitudes.

Significance of LEO satellites

Different types of satellite use the LEO orbit levels. These involve multiple applications including:
The LEO orbits are used by some communications satellites with beneficial results.
Earth monitoring satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth.
The International Space Station is in a low-earth orbit (LEO) that varies between 320 km and 400 kms above the Earth’s surface. It can often be seen from the Earth’s surface with the help of a naked eye.

Merits of LEO orbit

As it is near to the earth, satellites launched in LEO orbit provide better signal strength. Hence less power (approximately 1watt) is needed for transmission.
It has least propagation delay compared to other orbits due to its proximity to the Earth. Due to lower latency, it can be used for realtime critical applications.
It eliminates need for bulky receiver equipments due to higher signal ratio.
Low price satellite equipments are sufficient for ground stations.
Better frequency reuse can be achieved due to smaller footprints.
It provides high elevation for Polar Regions of the Earth. Hence, relatively better global coverage can be achieved.

Demerits of LEO orbit

These LEO satellites move constantly and hence service is being handed off by each satellite to the next one in the constellation. Hence, a constellation of satellites are required to cover any region on the Earth.
Atmospheric effects are more pronounced in the lower altitudes of LEO satellites. Hence, a gradual orbital disorientation of satellites takes place. This requires regular maintenance of satellites to keep them on track in the LEO orbit.
As it is at lesser distance above the Earth, it covers less region of the earth. Hence, a large number of satellites are needed to cover the entire region of the Earth. Logically, the installation of such LEO based system is costly.
It is only visible for 15 to 20 minutes from particular area of the Earth. Hence, there is less time available for testing and troubleshooting activities.
Efficiency to serve populated region is less when compared to geostationary satellites.
The complete deployment of LEO constellation is essential to start the service to the customers. Hence it requires more time to provide the satellite service and mass adoption by the users when compared to geostationary orbits.
The LEO satellites have shorter life span of about 5 to 8 years when compared to geostationary satellites which have a life span of about 10 years.

Medium–earth orbits (MEO)

The medium earth orbits have medium range altitudes in the range of 5,000-10,000 km. They are generally used for navigation satellites. These orbits are also used for communication satellites that cover the North and South poles.

Merits of MEO orbits:

MEO satellites are launched at higher altitudes compare to LEO satellites. Hence, lesser number of satellites is actually required to cover entire area of the Earth.
MEO satellites are launched at lesser height as compared to geostationary satellites. Hence, time delay from earth to satellite and vice-versa is lesser as compared to geostationary satellites.
It requires slightly higher transmission power as compared to low-earth orbits (LEO).
The system is cheaper as compared to geostationary orbit satellites.

Demerits of MEO orbit:

The signals become weak when they reach earth from middle-earth orbit (MEO) as compared to LEO. This is due to higher altitude of MEO in comparison to LEO. Hence, more transmission power is needed to overcome path loss and other atmospheric factors diminishing signal strength.
It is visible for only two to eight hours from earth. Hence, satellites are required to be tracked from the Earth due to their rotation.
The system is more expensive as compared to low-earth orbits (LEO) satellites.
Multiple medium-earth orbits (MEO) satellites are needed to cover the region continuously.

Highly elliptical orbits (HEO)

The satellites in such orbits are placed at a high altitude of more than 35790 km. It means the elliptical HEO orbits are higher when compared to both geostationary and geosynchronous orbits. Technically, circular orbits provide optimal solution for many satellites but elliptical orbits have their own advantages in certain applications. The elliptical orbit is often called the Highly Elliptical Orbit (HEO).

Merits of Highly Elliptical Orbit (HEO)

Quite a significant number of satellites are placed in highly elliptical orbits because of certain specific applications associated with them. For instance, it does not require the orbits to be equatorial like the geostationary orbit. Logically, this means that polar and high latitude areas can be better covered with highly elliptical orbits (HEO).
The elliptical orbit gives a number of coverage options that are not available when circular orbits are used.
The highly elliptical orbit (HEO) follows the curve of an ellipse. However, one of the key features of an elliptical orbit is that the satellite in an elliptical orbit about Earth moves much faster when it is close to Earth than when it is further away. If the satellite orbit is very elliptical, the satellite will spend most of its time near apogee where it moves very slowly. This means that the satellite can be in view over its operational area for most of the time. However, the satellites fall out of the view when the satellite comes closer to the Earth and passes over the blind side of the Earth. But by placing a number of satellites in the same orbit (equally spaced apart) permanent coverage can be achieved.
The highly elliptical orbit (HEO) can be used to provide coverage over any point on the globe. It is not limited to equatorial orbits like the geostationary orbit and the resulting lack of high latitude and polar coverage.
The HEO satellites have enhanced ability to provide high latitude and polar coverage. Hence, countries such as Russia (which need coverage over polar and near polar areas) make significant use of highly elliptical orbits. With two satellites in any orbit, they are able to provide continuous coverage.

The main disadvantage is that the satellite position from a point on the Earth does not remain the same. It creates practical technical difficulties for HEO satellites.

Geostationary satellite orbits

Such satellites orbit once a day and move in the same direction as the Earth. So, such satellites appear stationary above the same point on the earth’s surface. They can only be above the Equator and are placed at an altitude of 35,790 km. They are one of the most popular orbit formats. They have diverse applications ranging from direct broadcast to communications or relay systems.

As the height of a satellite increases, so the time for the satellite to orbit increases. At a height of 35790 km, it takes 24 hours for the satellite to orbit. This type of orbit is known as a geosynchronous orbit, i.e. it is synchronized with the Earth. One particular form of geosynchronous orbit is known as a geostationary orbit. In this type of orbit the satellite rotates in the same direction as the rotation of the Earth and has an approximate 24 hour period. This means that it revolves at the same angular velocity as the Earth and in the same direction. So, it always remains in the same position relative to the Earth. Geostationary orbit can only be over the equator. For a satellite to be stationary, it must be above the Equator.

Merits of geostationary orbits

The geostationary orbit has the advantage that the satellite remains in the same position throughout the day. So, antennas can be directed towards the satellite and remain on track. This factor is of particular importance for applications such as direct broadcast TV where changing directions for the antenna would not be practicable.
A single geostationary satellite obviously cannot provide complete global coverage. However, a single geostationary satellite can see approximately 42% of the earth’s surface. But for a constellation of three satellites equally spaced around the globe, it is possible to provide complete coverage around the equator and up to latitudes of 81 degrees both north and south.
The geostationary satellites always remain in the same position relative to earth. So, the antennas do not need re-orientation and they can be fixed permanently.
The life span of a geostationary satellite is very high (around 15 years).
Weather monitoring satellites are in geostationary orbits because they have a constant view of the same area.
In a high Earth orbit, it’s also useful for search and rescue beacons.
As it is at greater height, it covers larger geographical area. Hence only 3 satellites are required to cover the entire Earth.
Satellites are visible for 24 hours continuously from single fixed location on the Earth.
It is ideal for broadcasting and multi-point distribution applications.
Ground station tracking is not required as it is continuously visible from earth all the time from fixed location. Additionally, inter-satellite handoff is not needed.
Almost there is no Doppler shift and hence less complex receivers can be used for the satellite communication.

Demerits of geostationary orbits

The geostationary orbits are one of the most popular orbits and are widely used for many satellite applications. But these orbits are not suitable for all situations.

They have the following drawbacks:

They have longer path length and so they experience more signal losses when compared to low-earth orbit (LEO) and medium-earth (MEO).
It is more costly to place satellites in geostationary orbits in view of greater altitude.
One of the vexed issues with satellites in a geostationary orbit is the delay introduced by the longer path length. The path length to any geostationary satellite is a minimum of 22300 miles. This assumes that the user is directly underneath the satellite to provide the shortest path length. In reality the user is unlikely to be in this position and the path length will be longer.
The satellites in geostationary orbits require high transmission power.
The larger antennas are required for northern/southern regions of the earth.
Geostationary satellite orbits can only be above the equator. Hence, the Polar Regions cannot be covered.
The signal requires considerable time to travel from Earth to satellite and vice versa. The signal travel delay is about 120m/s in one direction. Hence it is not suitable for point to point applications requiring time critical applications such as real time voice, video etc.
Due to longer transmission distance, the received signal is very weak. This requires advanced signal processing algorithms in the satellite modem. This increases cost of the ground station equipments.
It provides poor coverage at higher latitude places usually greater than 77 degrees.

Despite the disadvantages of using satellites in geostationary orbit, they are still widely used because of the overriding advantage of the satellite always being in the same position relative to a given place on the Earth.

Geosynchronous orbits

The satellites in these orbits are placed at an altitude of 35790 km. There is a specific spot above the Earth where a satellite can match the same rotation of the Earth. This special position in high Earth orbit is known as a geosynchronous orbit.

A geosynchronous orbit synchronizes with the rotation of the Earth. More specifically, the time it takes for the Earth to rotate on its axis is 23 hours, 56 minutes and 4.09 seconds, which is the same as a satellite in a geosynchronous orbit. The geosynchronous satellites are particularly useful for telecommunications and other remote sensing applications.

While geosynchronous satellites can have any inclination, the key difference to geostationary orbit is the fact that they lie on the same plane as the equator. Geostationary orbits fall in the same category as geosynchronous orbits, but it’s parked over the equator. This one special quality makes it unique from geosynchronous orbits.

Semi-Synchronous Orbits

Global Positioning System (GPS) satellites are in another specific spot known as semi-synchronous orbits. While geosynchronous orbits match the rotation of Earth (24 hours), semi-synchronous orbits take 12 hours to complete an orbit. The semi-synchronous orbits are placed approximately 20,200 kilometres above the surface. This puts them in the medium Earth orbit range out of the three classes of orbits based on altitude. These orbits are nearly circular.

Polar Orbits

There are different types of satellites. Some satellites orbit the equator while others orbit from pole-to-pole. For example, Landsat, Worldview and Sentinel-2 satellites are in a polar orbit (or near-polar orbit). Almost all the satellites that are in a polar orbit are at lower altitudes. They are often used for applications such as monitoring crops, forests and even global security. Polar orbits are used for reconnaissance and Earth observation. Polar orbit satellites are also used for photography and mapping.

A polar orbit travels north-south over the poles and takes approximately an hour and a half for a full rotation. As the satellite is in orbit, the Earth is rotating beneath it. As a result, a satellite can observe the entire Earth’s surface in the time span of 24 hours. Higher altitude satellites orbit more slowly because the circumference of the circular orbit is larger. In addition, the pull of gravity is weaker at higher altitudes.

Difference between polar orbits and geostationary orbits

Geostationary satellites are launched into orbit in the same direction the Earth is spinning. When the satellite is in orbit at a specific altitude, it will exactly match the rotation of the Earth. Weather, communication and global positioning satellites are often in a geostationary orbit. In the case of geostationary satellites, the Earth’s force of gravity is exactly enough to provide acceleration required for circular motion.
While polar orbits have an inclination of about 90 degrees to the equator, geostationary orbits match the rotation of the Earth.
Geostationary orbits are almost at a height of 36,000 km but polar orbits often fall into low Earth orbits in terms of its height.
Polar orbits pass over the Earth’s Polar Regions from north to south but geostationary orbits are placed above the equator.
Polar orbits mainly take place at low altitudes of between 200 to 1000 km. Satellites in polar orbit look down on the Earth’s entire surface and can pass over the North and South Poles several times a day.

Sun-synchronous orbits

These are polar orbits which are synchronous with the Sun. A satellite in a sun synchronous orbit would usually be at an altitude of 600 to 800 km. Generally these orbits are used for Earth observation, solar study, weather forecasting and reconnaissance. This happens because the ground observation is improved if the surface is always illuminated by the Sun at the same angle when viewed from the satellite.

When a satellite has a sun-synchronous orbit, it means that it has a constant sun illumination through inclination and altitude. For sun-synchronous orbits, it passes over any given point on Earth’s surface at the same local solar time. Because of the consistent lighting in sun-synchronous orbits, scientists leverage this in various remote sensing applications.

The orbit that is chosen for a satellite depends upon its application. Those used for direct broadcast television for example use a geostationary orbit. Many communications satellites similarly use a geostationary orbit. Other satellite systems such as those used for satellite phones may use Low Earth orbiting systems. Similarly satellite systems used for navigation like Navstar or Global Positioning (GPS) system occupy a relatively low Earth orbit. There are also many other types of satellite from weather satellites to research satellites and many others. Each will have its own type of orbit depending upon its application. The actual satellite orbit that is chosen will depend on factors including its function, and the area it is to serve.

There are many geostationary satellites located over the equator, which causes congestion of the area. To prevent signal interference, precise positioning of each satellite is ensured prior to launching. Geosynchronous satellites are often used to monitor weather events or transmit television and communications signals.