Satellite Connection

Satellite communications is one of the types of radio communications based on the use of artificial earth satellites as repeaters.

Satellite communications are carried out between earth stations, which can be both stationary and mobile. Satellite communication is the development of traditional radio relay communication by placing the repeater at a very high altitude (from hundreds to tens of thousands of kilometers). The zone of its visibility in this case is almost half of the globe, then there is no need for a chain of repeaters. For transmission via satellite, the signal must be modulated. The modulation is done at the earth station. The modulated signal is amplified, transferred to the desired frequency and fed to the transmitting antenna.

- A non-regenerative satellite, having received a signal from one earth station, transfers it to another frequency, amplifies and transmits it to another earth station. A satellite may use several independent channels to perform these operations, each operating on a specific part of the spectrum (these processing channels are called transponders).

- The regenerative satellite demodulates the received signal and modulates it again. Due to this, error correction is performed twice: at the satellite and at the receiving earth station. The disadvantage of this method is the complexity (and hence the much higher cost of the satellite), as well as the increased signal transmission delay.

The orbits in which satellite transponders are located are divided into three classes:

1 - equatorial, 2 - oblique, 3 - polar

- An important variation of the equatorial orbit is the geostationary orbit, in which the satellite rotates with an angular velocity equal to the angular velocity of the Earth, in a direction coinciding with the direction of the Earth's rotation. The obvious advantage of the geostationary orbit is that the receiver in the service area "sees" the satellite all the time. However, there is only one geostationary orbit, and it is impossible to put all the satellites into it. Another disadvantage is its high altitude, and hence the high cost of launching a satellite into orbit. In addition, a satellite in geostationary orbit is unable to serve earth stations in the circumpolar region.

- An inclined orbit solves these problems, however, due to the movement of the satellite relative to the ground observer, it is necessary to launch at least three satellites into one orbit to ensure round-the-clock access to communications.

- A polar orbit is the limiting case of an oblique one.

When using inclined orbits, earth stations are equipped with tracking systems that point the antenna at the satellite. Stations operating satellites in geostationary orbit are also typically equipped with such systems to compensate for deviations from the ideal geostationary orbit. The exception is small antennas used to receive satellite television. Their radiation pattern is wide enough that they don't feel the vibrations of the satellite near the ideal point. A feature of most mobile satellite communications systems is the small size of the terminal antenna, which makes signal reception difficult.

Backbone satellite communications. Initially, the emergence of satellite communications was dictated by the need to transmit large amounts of information. Over time, the share of voice transmission in the total volume of backbone traffic has been constantly decreasing, giving way to data transmission. With the development of fiber-optic networks, the latter began to displace satellite communications from the backbone communications market.

VSAT systems: VSAT (Very Small Aperture Terminal) systems provide satellite communication services to customers (usually small organizations) that do not require high bandwidth. The data transfer rate for a VSAT is typically less than 2048 kbps. The words "very small aperture" refer to the size of the terminal antennas compared to older backbone antennas. VSAT terminals operating in the C-band usually use antennas with a diameter of 1.8-2.4 m, in Ku-band - 0.75-1.8 m. VSAT systems use on-demand channeling technology.

Mobile satellite systems: A feature of most mobile satellite systems is the small size of the terminal antenna, which makes signal reception difficult.

The main element of the VSAT satellite network is the NCC (Network Control Center). It is the Network Control Center that provides access to client equipment from the Internet, the public telephone network, other terminals of the VSAT network, and implements traffic exchange within the client's corporate network. The NCC has a broadband connection to backbone communication channels provided by backbone operators and provides information transfer from a remote VSAT terminal to the outside world.

In order for the signal strength reaching the mobile satellite receiver to be sufficient, one of two solutions is applied.

Satellites are located in geostationary orbit. Since this orbit is 35,786 km away from the Earth, a powerful transmitter must be installed on the satellite.

Many satellites are located in inclined or polar orbits. At the same time, the required transmitter power is not so high, and the cost of launching a satellite into orbit is lower. However, this approach requires not only a large number of satellites, but also an extensive network of terrestrial switches.

The client's equipment (mobile satellite terminals, satellite phones) interacts with the outside world or with each other through a relay satellite and gateways of the operator of mobile satellite communications services, providing connection to external terrestrial communication channels (public telephone network, Internet)

Frequency reuse in satellite communications. Radio frequencies are a limited resource, it is necessary to ensure that the same frequencies can be used by different earth stations. You can do this in two ways:

- spatial separation - each satellite antenna receives a signal only from a certain area, while different areas can use the same frequencies.

- polarization separation - different antennas receive and transmit a signal in mutually perpendicular polarization planes, while the same frequencies can be applied twice (for each of the planes).

frequency ranges.

The choice of frequency for transmitting data from an earth station to a satellite and from a satellite to an earth station is not arbitrary. The frequency depends on the absorption of radio waves in the atmosphere, as well as the required dimensions of the transmitting and receiving antennas. The frequencies used for transmission from an earth station to a satellite are different from those used for transmission from a satellite to an earth station. The frequencies used in satellite communications are divided into ranges, denoted by letters:

Ku-band allows reception with relatively small antennas, and therefore is used in satellite television (DVB), despite the fact that weather conditions have a significant impact on transmission quality in this band. For data transmission by large users (organizations), the C-band is often used. This provides better reception quality, but requires a rather large antenna.

Modulation and noise-immune coding.

A feature of satellite communication systems is the need to work in conditions of a relatively low signal-to-noise ratio caused by several factors:

- significant remoteness of the receiver from the transmitter,

- limited satellite power.

Satellite communications are not well suited for transmitting analog signals. Therefore, to transmit speech, it is pre-digitized using pulse-code modulation.

To transmit digital data over a satellite communication channel, they must first be converted into a radio signal occupying a certain frequency range. To do this, modulation is used (digital modulation is also called keying).

Due to the low signal strength, there is a need for error correction systems. For this, various noise-correcting coding schemes are used, most often various variants of ultra-precise codes, as well as turbo codes.

The modern organization is characterized by a large amount of information, mainly electronic and telecommunications, that passes through them every day. Therefore, it is important to have a high output to the switching nodes, which have access to all important communication lines.