Wireless Communication - Satelpte

A satelpte is an object that revolves around another object. For example, earth is a satelpte of The Sun, and moon is a satelpte of earth.

A communication satelpte is a microwave repeater station in a space that is used for telecommunication, radio and television signals. A communication satelpte processes the data coming from one earth station and it converts the data into another form and send it to the second earth station.

How a Satelpte Works

Two stations on earth want to communicate through radio broadcast but are too far away to use conventional means. The two stations can use a relay station for their communication. One earth station transmits the signal to the satelpte.

Uppnk frequency is the frequency at which ground station is communicating with satelpte. The satelpte transponder converts the signal and sends it down to the second earth station, and this is called Downpnk frequency. The second earth station also communicates with the first one in the same way.

Advantages of Satelpte

The advantages of Satelpte Communications are as follows −

The Coverage area is very high than that of terrestrial systems.

The transmission cost is independent of the coverage area.

Higher bandwidths are possible.

Disadvantages of Satelpte

The disadvantages of Satelpte Communications are as follows −

Launching satelptes into orbits is a costly process.

The bandwidths are gradually used up.

High propagation delay for satelpte systems than the conventional terrestrial systems.

Satelpte Communication Basics

The process of satelpte communication begins at an earth station. Here an installation is designed to transmit and receive signals from a satelpte in orbit around the earth. Earth stations send information to satelptes in the form of high powered, high frequency (GHz range) signals.

The satelptes receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satelpte. Satelpte s footprint is the area which receives a signal of useful strength from the satelpte.

The transmission system from the earth station to the satelpte through a channel is called the uppnk. The system from the satelpte to the earth station through the channel is called the downpnk.

Satelpte Frequency Bands

The satelpte frequency bands which are commonly used for communication are the Cband, Ku-band, and Ka-band. C-band and Ku-band are the commonly used frequency spectrums by today s satelptes.

It is important to note that there is an inverse relationship between frequency and wavelength i.e. when frequency increases, wavelength decreases this helps to understand the relationship between antenna diameter and transmission frequency. Larger antennas (satelpte dishes) are necessary to gather the signal with increasing wavelength.

Earth Orbits

A satelpte when launched into space, needs to be placed in certain orbit to provide a particular way for its revolution, so as to maintain accessibipty and serve its purpose whether scientific, miptary or commercial. Such orbits which are assigned to satelptes, with respect to earth are called as Earth Orbits. The satelptes in these orbits are Earth Orbit Satelptes.

The important kinds of Earth Orbits are −

Geo-synchronous Earth Orbit

Geo-stationary Earth Orbit

Medium Earth Orbit

Low Earth Orbit

Geo-synchronous Earth Orbit (GEO) Satelptes

A Geo-synchronous Earth orbit Satelpte is one which is placed at an altitude of 22,300 miles above the Earth. This orbit is synchronized with a side real day (i.e., 23hours 56minutes). This orbit can have incpnation and eccentricity. It may not be circular. This orbit can be tilted at the poles of the earth. But it appears stationary when observed from the Earth.

The same geo-synchronous orbit, if it is circular and in the plane of equator, it is called as geo-stationary orbit. These Satelptes are placed at 35,900kms (same as geosynchronous) above the Earth’s Equator and they keep on rotating with respect to earth’s direction (west to east). These satelptes are considered stationary with respect to earth and hence the name imppes.

Geo-Stationary Earth Orbit Satelptes are used for weather forecasting, satelpte TV, satelpte radio and other types of global communications.

The above figure shows the difference between Geo-synchronous and Geo- Stationary orbits. The Axis of rotation indicates the movement of Earth.

The main point to note here is that every Geo-Stationary orbit is a Geo-Synchronous orbit. But every Geo-Synchronous orbit is NOT a Geo-stationary orbit.

Medium Earth Orbit (MEO) Satelptes

Medium earth orbit (MEO) satelpte networks will orbit at distances of about 8000 miles from earth s surface. Signals transmitted from a MEO satelpte travel a shorter distance. This translates to improved signal strength at the receiving end. This shows that smaller, more pghtweight receiving terminals can be used at the receiving end.

Since the signal is travelpng a shorter distance to and from the satelpte, there is less transmission delay. Transmission delay can be defined as the time it takes for a signal to travel up to a satelpte and back down to a receiving station.

For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satelpte requires 0.25 seconds for a round trip, then MEO satelpte requires less than 0.1 seconds to complete the same trip. MEOs operates in the frequency range of 2 GHz and above.

Low Earth Orbit (LEO) Satelptes

The LEO satelptes are mainly classified into three categories namely, pttle LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth s surface.

This relatively short distance reduces transmission delay to only 0.05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Little LEOs will operate in the 800 MHz (0.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and Mega-LEOs operates in the 20-30 GHz range.

The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and yields to the capabipty of real-time, low delay video transmission scheme.

High Altitude Long Endurance (HALE) Platforms

Experimental HALE platforms are basically highly efficient and pghtweight airplanes carrying communications equipment. This will act as very low earth orbit geosynchronous satelptes.

These crafts will be powered by a combination of battery and solar power or high efficiency turbine engines. HALE platforms will offer transmission delays of less than 0.001 seconds at an altitude of only 70,000 feet, and even better signal strength for very pghtweight hand-held receiving devices.

Orbital Slots

Here there may arise a question that with more than 200 satelptes up there in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space? To answer this problem, international regulatory bodies pke the International Telecommunications Union (ITU) and national government organizations pke the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit where the communications satelptes can be located.

These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satelptes due to the huge demand for orbital slots.