Satellite Communications

[kengloong]

apsidal precession earth artificial satellites

Anyone could explain to me what's the differential between LEO, MEO, HEO, GEO? Why/ how to choose which type of the satellite we need to use? Charaterictsics/ advantage/ disdvantage and some useful information that you think is helpful to understand of the said satellites.

Thanks in advance.

 

[toonafishy]

GEO appear to be fix in place in the sky so they cover the same earth footprint always. But they are very high altitude 22,300 miles so require high power satellite transmitter and sensitive receiver.

LEO orbit around the earth. I know about the weather satellites. Orbit is about 90 minutes and each time around, it passes 15 degrees (I think) further west. So in 24 hours, the satellite covers the entire earth almost twice. The orbit is low, a few hundred miles up so transmitter power can be much lower and the receiver less sensitive.

 

[ARTMehr]

When a satellite circles close to Earth we say it's in Low Earth Orbit (LEO). Satellites in LEO are just 200 - 500 miles (320 - 800 kilometers) high. Because they orbit so close to Earth, they must travel very fast so gravity won't pull them back into the atmosphere. Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)! They can circle Earth in about 90 minutes.


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A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction.

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(from geo = Earth + synchronous = moving at the same rate).

A satellite in geosychonous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth. (A geosynchronous orbit can also be called a GeoSTATIONARY Orbit.)

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A satellite in elliptical orbit follows an oval-shaped path. One part of the orbit is closest to the center of Earth (perigee) and the other part is farthest away (apogee). A satellite in this orbit takes about 12 hours to circle the planet. Like polar orbits, elliptical orbits move in a north-south direction.



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A Mid Earth Orbit (MEO) is one which has an average altitude between 6,000 and 38,000km. This corresponds to an orbital period of approximately 4 hours to just under 24 hours long, respectively. Many MEO satellites have 12-hour orbit periods in order to keep in synch with the Earth's rotation period. These 12-hour period satellite are sometimes called "Semi-Synchronous".
A new problem exists with MEO satellites. Although some MEO satellites also contend with atmospheric drag (especially ones with eccentric orbits), as LEOs do, MEOs also have to contend with increased radiation from the Van Allen belts (the radiation belts mainly responsible for the aurora borealis and aurora australis). This requires heavier shielding for these satellites, and therefore increases cost, along with higher launch fees.
Few MEO orbiting satellites can be seen with the naked eye. This is due to their larger distances from us, but some have eccentric enough orbits such that they can pass as close as the closest LEO orbiting satellite, thus providing visual access for several minutes from specific locations.

All GPS and Molniya satellites are in MEO orbits.


There are some satellites whose orbit periods are over one day long. These High Earth Orbit (HEO) satellites orbit the Earth at an average altitude of over 35,850 kilometers. You could say that our only natural satellite, the Moon, is also in a HEO orbit, but it is usually not categorized the same way as our artificial satellite community, for good reason!
All the Apollo missions were technically HEO orbits, especially some of the old spent Saturn V rocket stages and other debris from the mission, which is still orbiting us by the way! Other HEO satellites include those which measure the Earth's high altitude magnetic fields, solar radiation, and solar winds.
HEO orbiting satellites can be difficult to detect optically, thanks to their large distances from the Earth. If you are fortunate, you might get a good sun reflection off one and detect it. Its very low apparent angular velocity would certainly give it away!

The famous Chandra X-Ray Observatory (CXO) is in a HEO orbit.

The geostationary orbit is very practical for customers who are located South of the Arctic Circle. For those who reside in the extreme northern (or southern) latitudes, geostationary satellites are seen at a very low elevation and even below the horizon. As a result, their signals are heavily absorbed by the Earth's atmosphere, or non-existent, respectively.
In the Soviet Union during the early 1960's, aerospace engineers devised a very clever and practical type of orbit that would simulate the convenience of a geostationary orbit, while at the same time service the extreme northern regions of Russia. This orbit would have a high orbit inclination (from 60 to 70 degrees or more), a nearly 12-hour orbit, a very high apogee altitude at about 40,000km, and a very low perigee altitude (sometimes as low as 100km). This would make the eccentricity extremely high, thus making the satellite remain over the northern hemisphere for most of its orbit period.
Molniya orbits have another advantage over many other types of orbits. Their orbit inclinations are such that the effects of apsidal precession are virtually negligible, thus providing natural orbit preservation in that respect.
This type of orbit is called Molniya (Russian for "Lightning") because the first payload satellites that assumed this type of orbit were actually called Molniya.
Molniya satellite orbits have been used for European communications partnerships, and at one time was even used for the "Hot Line" between the Soviet Union and the United States.

More Info: h**p://w*w.artmehr.com/satellite/

:!: Good Luck

 

[Lupin]

Satellite mission

Hello people, can anyone tell me the satellite different features of Navigation by the Telecommunication mission?

In other words, I ask to anyone to support me in making a difference between a Payload designed for TLC mission and a Payload desinged for Navigation mission.

Thanks in advance.

If you need support in antenna frame, please contact me.

Best regads,

Lupin