Question:-
a) Please explain in full detail what are GEO, LEO and MEO Satellites??
b) Compare the three types of satellites (GEO, LEO and MEO Satellites) with respect to factors such as size and shape of orbits, signal power, frequency reuse, propagation delay, number of satellites for global coverage and handoff frequency ?? Explain your answer in full detail and provide any diagrams when possible
Answer:-
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Satellites that orbit in geostationary orbit,lower earth orbit and middle earth orbit are called Geo,LEO.MEO satellites respectively.
Geo satellite:-
A geostationary satellite is any satellite which is placed in a geostationary orbit. Satellites in geostationary orbit maintain a constant position relative to the surface of the earth.
Geostationary satellites do this by orbiting the earth approximately 22,300 miles above the equator. This orbital path is called the Clarke Belt, in honour of Arthur C. Clarke.
Geostationary satellites are used for weather forecasting, satellite TV, satellite radio and most other types of global communications.
In other words, if a satellite in a geostationary orbit is in a certain place above the earth, it will stay in that same spot above the earth. Its latitude stays at zero and its longitude remains constant.
In contrast to geostationary orbits, Medium Earth Orbit and Low Earth Orbit satellites constantly change their positions in relation to the surface of the earth.
A single geostationary satellite will provide coverage over about 40 percent of the planet.
Geostationary satellites are commonly used for communications and weather-observation.
The typical service life expectancy of a geostationary satellite is ten to fifteen years.
Because geostationary satellites circle the earth at the equator, they are not able to provide coverage at the Northernmost and Southernmost latitudes.
Geostationary orbits are often referred to as geosynchronous or just GEO
LEO Satellites:-
Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles.
Low Earth Orbit satellites must travel very quickly to resist the pull of gravity -- approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.
Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
Low Earth Orbit satellites typically operate in polar orbits.
Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS).
Low Earth Orbit satellites have a typical service life expectancy of five to seven years.
Low Earth Orbiting satellites travel in a circular orbit moving from pole to pole, collecting data in a swath beneath them as the earth rotates on its axis. In this way, a polar orbiting satellite can “see” the entire planet twice in a 24 hour period. The United States refers to their operational LEO satellites as Polar Operational Environ metal Satellites (POES) and maintains ascending (south to north) and descending (north to south) orbits, each circling the earth every 12 hours. POES are inserted into sun-synchronous orbits, placing the spacecraft in a constant relationship to the sun so that images and data can be downloaded by direct broadcast at the same time each day. POES are significantly closer to Earth than GOES, orbiting at an altitude of only 879 kilometers, (approximately 500 miles) so it only takes one hour and 42 minutes to complete a full orbit. This proximity results in high resolution images.
POES missions are managed by the National Oceanic and Atmospheric Administration (NOAA). The National Aeronautic and Space Administration (NASA) manages a series of low Earth orbiting research satellites called Earth Observing Satellites (EOS).
The next U. S. polar orbiting satellites program is called NPOESS (National Polar-orbiting Operational Environmental Satellite System) and will combine NOAA's missions with the Department of Defense (DOD) polar orbiting satellites, trimming the number of polar orbiting satellites that the United States supports. The NPOESS management team is a tri-agency program with contributions from NOAA, DOD, and NAS
MEO Satellites:-
Medium Earth Orbit (MEO) refers to a satellite which orbits the earth at an altitude below 22,300 miles (geostationary orbit) and above the altitude of Low Earth Orbit (LEO) satellites.
Medium Earth Orbit represents a series of tradeoffs between geostationary orbit (GEO) and Low Earth Orbit (LEO).
Medium Earth Orbit enables a satellite provider to cover the earth with fewer satellites than Low Earth Orbit, but requires more satellites to do so that geostationary orbit.
Medium Earth Orbit terrestrial terminals can be of lower power and use smaller antennas than the terrestrial terminals of geostationary orbit satellite systems. However, they cannot be as low power or have as small antennas as Low Earth Orbit terrestrial terminals.
Medium Earth Orbit satellite systems offer better Round Trip Time (RTT) than geosynchronous orbit systems, but not as low as Low Earth Orbit systems.
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Comparision between different satellites:-
GEO SATELLITE / MEO SATELLITE / LEO SATELLITEHeight:- 22,300 miles ,36278 kms to be exact from surface of earth / altitude below 22,300 miles and above 930 miles / 200 miles to 930 miles
Satellites Needed For Global Coverage:- 3(minimum) / 10 to 15 / Above 40(min)
Satellite Lifetime (years):-
10 to 15 years / 10 to 15 years / 3 to 7 years
Elevation Angle:-Low to medium / Medium to high / Low
Propagation Delay:- 0.25 s / 0.10 s / 0.05 s
Call Handover :- Never / Infrequent / Frequent
Building Penetration :- None / Poor / Poor
Space Segment Cost :-Medium / Low / High
Signal Power(varies for different satellites from band to band like l,s,x,Ka band)
EIRP:-30-40 dbw with losses of 192 db in free space. / EIRP is the highest with path loss in between / EIRP:-
45-61 dbw. Decrease of losses by 22-28 db in free space compared to geo.
Frequency reuse:-
Least (never) / In between Geo and Leo / Highest
Handoff frequency:-
Not present or (0)as there is no handoff required / Low / Very high
· The height gives the altitude from earth at which different satellites revolve around the earth surface.
· Satellites Needed For Global Coverage determine the minimum no. of satellites required.
· Satellite Lifetime (years) gives the life expectancy of satellite or determines how many years it can run without trouble.Life expectancy is determined by research of past values.
· Elevation Angle:- Elevation refers to the angle between the dish pointing direction, directly towards the satellite, and the local horizontal plane. It is the up-down angle.
· Propagation Delay gives the time taken for the signal to reach from the earth station to satellite and vice-versa.
· Call Handover:- it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service. The British term for transferring a cellular call is handover, which is the terminology standardised by 3GPP within such European originated technologies as GSM and UMTS.
· Building Penetration determines the ability of the signal of the satellite to penetrate a building.In other words it also is a measure signal strength.
· The Signal Power varies for different satellites and from band to band like l,s,x,Ka band with change in frequency.EIRP is the effective isotropic radiated power.With increase in frequency eirp and path loss increases.
· Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.
· Frequency reuse Frequency reuse is a technique of reusing frequencies and channels within a communications system to improve capacity and spectral efficiency. Frequency reuse is one of the fundamental concepts in satellites.
· Handoff frequency is the frequency at which hand off or hand over takes place.