Satellite Communication (Part Two)

Read through the first part of this article.

Applications:

The applications where satellite services are most appropriate are those in which:

• We want to spread the same information, the link of descent, by a very large geographical region, for example, for TV and the Internet.

• We want to reach remote locations, for example, mining camps, logging, the owner of edades rural and suburban stations and on highways.

• It is hoped that the deployment time is very fast, or occasional use, such as, for concerts, rodeos, car racing.

The earth station is more popular than the VSAT is an abbreviation for Very Small Aperture Terminal. They are usually stations with antennas ranging from 80 cm to 2 meters in diameter and little.

Architecture

A VSAT network consists of a number of VSAT stations and a main station (hub station ").
The main station has larger antenna and communicates with all remote VSAT stations, coordinating traffic between them. The station hub also provides a point of interconnection to other communications networks.

Gateways and Base Stations are earth stations.

• GOCC (Ground Operations Control Center): plan and control the use of satellites by gateway terminals and for coordinating this use with soccer.

• Soccer (Satellite Operations Control Center): Scans satellites, controls their orbits and provides Telemetry and Command (T & C) for the satellite constellation. Also oversee the launch of satellites. Each station is owned and operated by each operator. Receives transmissions from satellites in order to process the calls and forward them to the destination network land. A gateway can serve more than one country. Effect the integration with fixed networks or mobile land.

Category satellites

There are three types of satellites which have no teams on three different orbits:

LEO (LowEarthOrbit) is approximately 500-1500 km

MEO (MediumEarthOrbit): approximately 6000-15000 km

HEO (HighEarthOrbit): from 20000 km (which includes GEO: Geostationaryorbit Apro-mately 36000 km)

Satellites in low earth orbit (LEO) and medium earth orbit (MEO)

LEO orbits are those in which the satellites travel at an altitude between 500 and 3000 km, approximately. In orbit MEO satellites are traveling between 13,000 and 20,000 km altitude. As these orbits are close to Earth, the satellite must travel at an angular velocity greater than that of pla-granddaughter, otherwise, the force of gravity will pull it to the ground, destroying it.

'S LEO satellites are usually divided in two categories: "LEO'S BIG" and "LEO'S SMALL. The difference between them is that LEO'S BIG use the frequency range above 1GHz and LEO'S SMALL below 1GHz.

The average velocity of a LEO satellite is around 25000km / h, making one complete revolution around the Earth in about 90 to 100 minutes. The orbits can be both circular and elliptical, depending on the need for the project to be executed in, for elliptical orbits cause the satellite to spend more time on a particular region, facilitating and expanding the communication time between the satellite and ground station .

Communication satellites use these orbits, because they are closer to Earth, fazen-do with the equipment used can be smaller, even portable, since the need for low-power transmission.

Geo-stationary.

They are so called because they are placed in the orbit over the equator so that the satellite has a rotation period equal to that of our planet Earth, or 24 hours. Thus the angular velocity of rotation of the satellite equals the Earth and everything happens as if the satellite was stationary in space for an observer on Earth.

For a satellite into orbit is need-ed to reach a speed of at least 28,800 km / h. With this speed, if we position the satellite at 36,000 km altitude above the equator, it will be in geostationary orbit.

The International Telecommunication Union (ITU) has divided the space into 180 Geostationary orbital positions, each separated from one another at an angle of 2 °. Brazil has pleaded 19 orbital positions with the ITU. Of these, seven are currently designated for use by Brazilian operators (Star One, Loral and Hispasat).

A comparative view between orbit satellite LEO, MEO and GEO

The satellites that operate in a Geo-stationary orbit rocket launchers require more complex due to its high altitude, which pro-yourself cost of a higher release than the release of satellite technology (MEO and LEO). This cost is up to 4 times more expensive than the satellite itself. This launch operation, about 10 years ago, it was a risky operation because there was a reasonable amount of explosions and failures. Today, every year that passes the reliability in launch vehicles has grown rapidly.

The GEO satellites have an advantage of covering a footprint much larger than those of LEO and MEO satellites. By contrast, the high altitude causes a delay of approximately 0.5 s the signal. This delay causes problems in masm complex protocols of verification and error correction data, where at any moment a system transmitter interrupts the communication of data to await the response of the receiver that there is error data you send. Only then the data is relaying. This interaction between the two systems (Tx and Rx) would normally be instant if they had to wait 0.5 s for a confirmation message.

For this reason the communication protocol via GEO satellite systems, are different from other telecommunication systems, making the technology viable.

Advantages and disadvantages

Disadvantages

Certainly capable of transmitting informa-tion for multiple users separated from miles away as quickly is and will be an advantage over any other medium. Applications for this military-tech abound, as wars always take place and, by recent events, there continued to spread throughout the globe. But the use of electromagnetic waves in a transmission, even in a transmission involving huge areas brings a point of discussion: security. Even in systems that have the focus of the antenna off the satellite, which often are the transmissions that use encryption to ensure that only authorized persons containing up-access to the information provided.

Besides the problem of rain that affects differently the different bandwidths, there is the problem not commented on when the satellite eclipses the sun. This effect interferes in the Communication interrupted (once a year in the period for a few minutes early to forecast) to be a source of electromagnetic waves enough power-sa. Thus, critical systems that can not get minutes without communication should not use VSAT.

One factor that ought to be highlighted is that its BER is variable, as the weather (rain) interfere directly, since the electromagnetic waves are the first few miles of his journey into the atmosphere.

Problems of communication satellites

The problems of communication satellites can be grouped into 3 fields:

Physical:

- The atmosphere causes reflection of wave Provocan-the delays and errors
- Change in signal intensity due to the spread of multipath
- Interruptions in the signal due to shadowing
- It is necessary to take into account that the greater the dis-tance, the greater the energy cost of the equipment must (both land and satellite)
- You need good equipment to ensure greater efficiency in communication.

Monetary:

- There is big money involved in transmission

Software:

- Problems in the transport layer, such as e-EXAMPLE, error in the transmission of bits.

Advantages

After all the problems presented, one can see the advantages. The next point to raise is speed. Reports of practice facilities show that VSAT networks can be implemented and begin in a few days. This is due to an important feature that the systems must have the maturity and not use a fixed physical environment. In addition to several problems have already been detected and resolved, the maturing of a technology brings the advantage of lower cost.

As cost, time and knowledge of the problems are important factors in commercial applications, comparing, analyzing these topi-cos, should always be made between technologies.
Taking into account the loca-des farther away are always left to the background with regard to communications, factories and rural villages can always count on this means of transmission. Places island, poor infrastructure and vehicles, mo-bility intercontinental (aircraft and ships) are made possible candidates to possess VSAT.

Conclusion

Every system of communication shall, prior to their deployment, have an assessment as-relating to its cost, its deployment time and the service it offers. The satellite, when it wishes to communicate to remote locations, or when you want to spread the reception in very large geographical areas, is the system used in the moment. But in some cases, this techno-logy finds himself clashing with some others that for-They provide the same service with greater reliability, faster speed and lower cost of deployment.

So this system will be evaluated according to the needs of the project, because at times issues such as signal delay, signal outages due to rain, can not exist for the proper functioning of a certain service, for example.

References

Sverzut, José Umberto
GSM, GPRS, EDGE and UMTS - Evolution Path to Third Generation
Publisher Erika, first edition

Medeiros, Julio Cesar de Oliveira
Principles of Telecommunications - Theory and Practice
Publisher Erika, First Edition

Soares Neto, Vicente
Telecommunications - Modulation Systems
Publisher Erika, First Edition

Soares Neto, Vicente
Telecommunications - Convergence of Networks and Services
Publisher Eika, First Edition

Sites:

http://www.starone.com.br

http://www.teleco.com.br

http://www.wikipedia.org

Authors

André Mendes LC
Erwin C. Blank
Roger G. Dutra
Steve Hsu