Monday, 11 June 2012

Satellite Communication Lecture Notes

SATELLITE COMMUNICATION

A satellite is an object that revolves around another object. For example, earth is a satellite of The Sun, and moon is a satellite of earth etc. All planets in the solar system can be considered as satellites of sun. An object in the solar system can have more than one satellite.

A communication satellite is a microwave repeater station in a space that is used for telecommunication, radio and television signals. A communication satellite process 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 SATELLITE 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 satellite. Uplink frequency is the frequency at which ground station is communicating with satellite.
The satellite transponder converts the signal and sends it down to the second earth station, this is called Downlink.
                                         
ADVANTAGES OF SATELLITE

1. The Coverage area is very high than that of terrestrial systems.
2. The transmission cost is independent of the coverage area.
3. Higher bandwidths are possible.

DIS- ADVANTAGES OF SATELLITE

1. Launching satellites into orbits is a costly process.
2. The bandwidths are gradually used up.
3. High propagation delay for satellite systems than the conventional terrestrial systems.
                       
PATH TRACED BY A SATELLITE

The path traced by a satellite around the earth is an elliptical path with two focuses.


How Do Satellites Work :

Satellite Communication Basics :

The process of satellite communication begins at an earth station. Here an installation designed to transmit and receive signals from a satellite in orbit around the earth. Earth stations send information to satellites in the form of high powered, high frequency (GHz range) signals. The satellites, which receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite's footprint is the area which receives a signal of useful strength from the satellite. The transmission system from the earth station to the satellite through a channel is called the uplink. The system from the satellite to the earth station through the channel  is called the downlink. Below Figure shows the basic elements of a satellite communications system.


Satellite Frequency Bands:

The satellite frequency bands which was commonly used for communication are the C-band, Ku-band, and Ka-band. C-band and Ku-band are the commonly used frequency spectrums by today's satellites. 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 (satellite dishes) are necessary to gather the signal with increasing wavelength.

From 4 to 8 GHz frequency range, C-band satellite transmissions occupy. Than the Ku-band or Ka-band, these relatively low frequencies translate to larger wavelengths. These larger wavelengths of the C-band mean that a larger satellite antenna is required to gather the minimum signal strength. Hence the minimum size of an average C-band antenna is approximately 2-3 meters in diameter. It is shown in Figure.

The frequency range from 11 to 17 GHz occupied by Ku-band satellite transmissions. These relatively high frequency transmissions correspond to shorter wavelengths. Thus a smaller antenna can be used to receive the minimum signal strength. Ku-band antennas can be as small as 18 inches in diameter. It can be commonly seen in the RCA DSS and Sony DSS systems. The Ku-band antenna of the Sony DSS system is shown in below figure.


The 20 to 30 GHz frequency range is occupied by the Ka-band satellite transmissions. These very high frequency transmissions means very small wavelengths and therefore very small diameter receiving antennas are used.

Geosynchronous Earth Orbit (GEO) Satellites

        The majority of satellites in orbit around the earth are positioned at a point 22,238 miles above the earth's equator in a special type of geosynchronous earth orbit (GSO) known as Geostationary earth orbit (GEO). it is also called as the Clarke orbit. This is in honour of Arthur C. Clarke. He is the man who first suggested in 1945 that satellites in geosynchronous orbits could be used for communications purposes. A satellite can maintain an orbit with a period of rotation around the earth exactly equal to 24 hours at the precise distance of 22,238 miles. satellites appear stationary from the earth’s surface, since the they revolve at the same rotational speed of the earth. Due to this reason, most earth station antennas (satellite dishes) don't need to move once they have been properly aimed at a target satellite in the sky. The mathematical derivation of the Clarke orbit can be obtained as a straight-forward calculus problem.


                                                        The Clarke Orbit

Medium Earth Orbit (MEO) Satellites

The technological innovations in space communications during the last few years, have given rise to new orbits and totally new systems designs. Medium earth orbit (MEO) satellite networks will orbit at distances of about 8000 miles from earth's surface. Signals transmitted from a MEO satellite travel a shorter distance. This translates to improved signal strength at the receiving end. This shows that smaller, more lightweight receiving terminals can be used at the receiving end. Also, since the signal is travelling a shorter distance to and from the satellite. Hence there is less transmission delay. Transmission delay can be defined as the time it takes for a signal to travel up to a satellite and back down to a receiving station. For real-time communications, the shorter the transmission delay, better the communication system. As an example, a GEO satellite requires .25 seconds for a round trip. A MEO satellite requires less than .1 seconds to complete a round trip. MEOs operates in the frequency range of 2 GHz and above.

Low Earth Orbit (LEO) Satellites

The LEO satellites are mainly classified into three categories: little 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 .05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Little LEOs will operate in the 800 MHz (.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 capability of real-time, low delay video transmission scheme. Microsoft Corporation and McCaw Cellular (now known as AT&T Wireless Services) have partnered to deploy 840 satellites to form Teledesic. It is a proposed Mega-LEO satellite network.

High Altitude Long Endurance (HALE) Platforms

Experimental HALE platforms are basically highly efficient and lightweight airplanes carrying communications equipments. This will act as very low earth orbit geosynchronous satellites. 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 .001 seconds at an altitude of only 70,000 feet, and even better signal strength for very lightweight hand-held receiving devices.

Orbital Slots

Here there may arise a question that with more than 200 satellites 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 like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit where the communications satellites 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 satellites due to the huge demand for orbital slots.