Apogee and Perigee

At present Satellite communication is essential for making it possible to connect globally and share information worldwide. It is important to understand the fundamentals or basics of satellite communication to design and optimize satellite systems.

As we know, satellites revolve around the earth and make communication possible in this era, so while revolving in their orbits there are two critical points based on the position i.e., : Apogee point and Perigee point.

• Apogee point: If the distance between the satellite and the earth is very high(far) it is called an apogee point.
• Perigee point: If the distance between the satellite and Earth is the shortest point(near) it is called the perigee point.

These two points are defined based on the position of the satellites with respect to the Earth when they are moving in the elliptical order, satellites in an orbit path or an orbital path revolve around the Earth in an elliptical manner, when they revolve around the Earth few satellites will be nearer to the earth and few satellites will be far away from the earth if the satellite is far away from the earth that particular point is known as Apogee point and if the distance is between the earth and the satellite is very near or close we can consider it as Perigee point. As shown in the diagram below.

Apogee and Perigee in Satellite Communication

What is a Satellite ?

A satellite is a comparatively small object which revolves around a very large object in free space or air. It can be a natural or artificial satellite. Natural satellites are the naturally occurring objects in space which revolves in an orbit to a large object such as the Earth’s moon is an example of a natural satellite as we have not made it, and Artificial satellites are objects which are man-made to serve various functions, including communication, research, and weather monitoring depending on its mission.

In our daily life all the work we do mostly dependent on any satellites whether we are watching television or using GPS to navigate or seeing whether forecast and communicating through call etc. All these works are possible to us only because of any of the satellites revolving in the Earth’s Orbit.

Explanation of Orbital Parameters

Satellite orbits can be classified by many different types of parameters that define the path of satellite around a celestial body, such as Earth. These parameters are essential for understanding and some future predictions related to the satellite’s movement or behavior.

Some of the explanation of these orbital parameters are mentioned below:

• Inclination: The inclination is the angle between the orbital plane of satellite and the plane around the larger body it is orbiting (such as, Earth). with the help of inclination we can determine the orientation of the orbit relative to the equator and plane around it.
• Eccentricity: Eccentricity is a measure of how elliptical (that is, not circular) an orbit is in the field of astrodynamics. It determines the amount of deviation of the orbit from a perfect circle. Eccentricity can be shown as it affects the shape of the orbit.
• Orbital Period: Orbital period is defined as the total time taken by a satellite to complete one revolution around a celestial body, just as a man-made satellite revolves around the Earth. The orbital period can be represented as a fundamental parameter that determines the satellite’s orbital motion and affects its coverage area on the Earth’s surface.
• Right Ascension of Ascending Node (RAAN): RAAN stands for Right Ascension of the Ascending Node, it can be defined as the angle measured along the celestial equator from the reference direction to the ascending node, where the satellite crosses the celestial equator while moving from the southern to the northern hemisphere. Does. The importance of RAAN in satellite communications is that it is used to determine the orientation of the entire orbital plane in space with the help of inclination. For satellite communication systems, accurate knowledge of the satellite’s orbital parameters, including RAAN, is critical for such operations. In the form of orbital determination, tracking and orbital maneuvering planning. This allows satellite operators to ensure proper coverage, optimize communications links, and effectively coordinate satellite operations.
• Mean Anomaly: The mean anomaly can be defined as the angular distance of a satellite from its perigee, at any point in time, that is used to measure it along its orbital path. The importance of the mean anomaly is that it represents the position of the satellite in its orbit, with 0° corresponding to perigee and 180° corresponding to apogee. The mean anomaly varies uniformly with time in an elliptical orbit.

What is Apogee ?

Apogee is defined as the point in orbit where a satellite is farthest from the celestial body it is orbiting, which is our Earth around which satellites revolve.

The apogee can be represented as it represents the highest point in the satellite’s orbit. For satellites orbiting around the Earth, this is the point where the satellite is at its maximum distance from the Earth’s surface.

Apogee Equation

[Tex]r(apogee)=a \times (1+e)[/Tex]

Here in the above equation,

• r(apogee) is the distance from the center of the celestial body (e.g., Earth) to the apogee,
• a is the semi-major axis of the orbit,
• e is the eccentricity of the orbit.

The apogee of an orbit is when the satellite is at its farthest point from the celestial body it is orbiting. In the above equation for apogee, a represents half the major axis of the orbit, and a is the eccentricity which measures the deviation of the orbit from a perfect circle. The apogee distance is calculated by adding the semi-major axis to the product of the semi-major axis and the eccentricity.

Solved Example on Apogee

Consider an artificial satellite with a semi-major axis of 10,000 kilometers and an eccentricity of 0.2 and find the distances from the center of the celestial body to the apogee by using the apogee equation

r (apogee) = 10,000km × (1+0.2) = 12,000km

What is Perigee ?

Perigee, or the closest point to a celestial body that it usually orbits, is the angle measured along the orbital plane from the ascending node to the apogee.

The orientation of a satellite’s closest approach to a celestial body within its orbital plane is defined by this parameter.

Perigee’s equation

[Tex]r(perigee)=a \times (1−e) [/Tex]

here in the above equation,

• r(perigee) is the distance from the center of the celestial body to the perigee,
• a is the semi-major axis of the orbit,
• e is the eccentricity of the orbit.

The perigee of an orbit is the point where the satellite is closest to the celestial body. The equation of perigee is very similar to the equation of apogee, but here in the equation of perigee the eccentricity term is subtracted from 1 unlike the apogee’s equation where it was being added. This shows the fact that perigee is the distance from the center of the celestial body to the closest point in the orbit.

Solved Example 2

Let us take an artificial satellite with a semi-major axis of 5000 kilometers and an eccentricity of 0.8 and find the distances from the center of the celestial body to the perigee by using the perigee equation

r (perigee) = 5000km × (1-0.8) = 1000km

Comparison between Apogee and Perigee

Parameter	Apogee	Perigee
Definition	the farthest point from the celestial body it is orbiting.	the closest point to the celestial body it is orbiting
Altitude	r(apogee)=a×(1+e)	r(perigee)=a×(1−e)
Speed	v(apogee) = √G x r(apogee)/M	v(perigee) = G x r(perigee)/M
Coverage Area	As at apogee point the satellite is farthest from the celestial body it is revolving thus the coverage area tends to be larger because the satellite is higher in its orbit	As at perogee the satellite is nearest to the celestial body it is revolving thus the coverage area is less as compared to apogee point because the satellite is lower in its orbit.
Ground Track	The ground track tends to move more slowly at apogee compared to other points in the orbit, as the satellite covers a larger portion of its orbital path over the Earth’s surface.	The ground track moves more rapidly at perigee compared to other points in the orbit, as the satellite covers a smaller portion of its orbital path over the Earth’s surface.
Launch Considerations	Altitude Boost as satellite is launched to maximum distance. Velocity Adjustment to tune in to the orbit desired.	Injection Burn as satellite need to sent at nearest point so less fuel needed. Addittional burns required for Orbit Circularization
Station-Keeping Requirements	Altitude Maintenance and East-West Drift Correction	Altitude and Inclination Adjustments and Collision Avoidance
Payload Capacity	depends on the launch vehicle used for launching of satellite	depends on the launch vehicle used for launching of satellite

Applications of Apogee and Perigee

Let’s discuss some of the applications of Satellite communication, there an be many different types of applications for satellite but some of them are mentioned below

• Weather Forecasting: Everywhere we see the predictions or reports related to the weather or the state wise temperature and forecast information are done with the help of these satellites. LEO (low earth orbit) satellites are the nearest to the earth on tropospere of the earth’s atmosphere and they are used for the collection of data and telecasting to the earth stations.
• Telephone: These are used for conveying the information or the thoughts or communicating around the world, that can be done by the help of satellite communication which has many advantages over conventional ways of communication, it is very easy with the help of satellite.
• Navigation: GPS (Global positioning system) are generally used for the navigation purposes such as routing, navigating, information related to traffic or directions etc. This is also the application of satellite communication.
• Radio and TV broadcasting: We all have seen the Antenna on our roofs , the direct broadcasting is done there , we select our vendors or the companies provide these services, all these services are provided by a satellite and its a major application of satellites. such as D2H. Here in broadcasting as it is an one way satellite communication as we don’t transmit the data we just receive or collect information with the help of the parabolic reflectors or the antennas used. These services are provided by the GEO(Geostationary satellites) satellites.
• Access to remote areas: Some terrain such as hilly areas or far located villages or towns has many restrictions and not so affordable to use conventional ways of communication and connectivity and that’s why we use satellite communication to maintain the connectivity.
• Military: The Military Satellites are used for military and strategic applications like during war it is used to provide communications and navigations etc. It plays a huge role in the military.
• Global mobile communication: All the communication related to the mobile is made possible with the help of satellites, we can easily communicate to around the world with good connectivity.

Conclusion

The ability of satellite communication to pass through geographic boundaries sets it beyond land communication, represented by its around-the-world connection and need for satellites that orbit the earth. Orbital decomposition, crash risks, and risk to space-related environmental factors are major maintenance challenges for satellites. Resolving these problems and ensuring reliable and efficient global connectivity are crucial for the success of communication through satellites. In this article we have covered basics of satellite communication and some of its features in detail.

FAQs on Apogee and Perigee in Satellite Communication

How does satellite communication differ from terrestrial communication?

Since terrestrial communication depends on ground-based infrastructure with a very limited reachability and localized type of coverage, the satellite communication uses signals transmitted with help of the orbiting satellites to enable global coverage and better connectivity.

What are some common challenges faced in maintaining satellites in orbit?

The effects of solar radiation on satellite components, possible collisions with space debris, and orbital decay brought on by atmospheric drag are common maintenance challenges for satellites.

How does orbital inclination affect satellite communication?

The satellite communication coverage area is determined by orbital inclination; lower inclinations are better suited for regional coverage, while higher inclinations enable global coverage.