NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY

Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite

Assignment 1

SATELLITE COMMUNICATIONS

Olaño, Reymart September 05, 2011

Communications 1/BSECE 41A1 Score:

Engr. Grace Ramones

Instructor

INTRODUCTION TO SATELLITE TELECOMMUNICATION

A satellite communications (sometimes abbreviated to Comsat) is an artificial satellite stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits or near geostationary orbits, although some recent systems use low Earth-orbiting satellites.

Communications satellites provide a technology that is complementary to that of fibrotic submarine communication cables. Unlike fiber optic communication, satellite communication has a propagation delay (also called a path delay) of at least 270milliseconds, which is the time it takes the radio signal to travel 35,800 km from earth to a satellite and then back to earth. Satellite Internet connections average a 600-800millisecond delay, about ten times than that of a terrestrial Internet link. This delay is a challenge to deployment of Virtual private networks over satellite internet connections.

MAIN COMPONENTS:

Satellite communications are comprised of two basic elements:

1. THE SATELLITE - The satellite is also known as the space segment. It is composed of the following separateunits; i.e.

a) The satellite and telemetry controls

b) The transponder

The transponder comprised of the receiving antenna to catch-up signals from the ground station, a broad band receiver, an input multiplexer and a frequency converter that is used to reroute the received signals through a high powered amplifier for downlink.

The main function of satellite is to reflect signals. In case of a telecom satellite, the primary role is to pick up signals from a ground station, which is located, a considerable away from the first. This relay action can be two way, as in the case of a long distance phone call. Another use of satellite is the television broadcasts. Number of programs are first up-linked and then down-linked over wide region. The customer having appropriate devices can receive and watch the programs. One of the modern uses of satellite is getting information along with image (commonly known as space/satellite image) of any desired location on earth.

2. THE GROUND STATION - This is called the earth segment. A base band processor, an up-converter, high Powered amplifier and a parabolic dish antenna is involved to transmit the terrestrial data to an orbiting satellite. In the case of downlink, the ultimate reverse operation is being down and up-linked signals are recaptured through parabolic antenna.

DIFFERENT UTILITIES OF SATELLITE COMMUNICATION

1. TRADITIONAL TELECOMMUNICATIONS - People have been facing problems in case of long distance telephone network; there has-been a need to connect the telecommunications network of one country to another. Submarine cables played a major role to achieve the goal. Still some crucial and critical factors remained unachieved to connect transoceanic points, geographically remote areas or poor countries that have little communications infrastructure.

2. CELLULAR - Various schemes are invented to make the satellites feasible to increase the bandwidth available to ground based cellular networks. Every cell in a cellular network consists affixed range of channels which consist of either frequencies, as in the case of FDMA systems, or time slots, as in the case of TDMA. A particular cell that can only operate within those channels allocated to it, overloading can occur. With the help of satellite which operate at frequency outside those of the cell, extra channel can be provided on demand to an overloaded cell. Any of the overloaded cells can easily use these newly satellite-transmitted signals. In this case the cell is not bound by the bandwidth restrictions as compare to the previous scenario. In other words the satellite provides its own bandwidth for a network of cells that can be used by any cell (that need it) without being bound by terrestrial bandwidth and location restrictions.

3. TELEVISION SIGNALS - Satellites have been used since 1960 to transmit broadcast television signals between the network hubs of television companies and their network members. Sometime, a whole setoff programs is transmitted at once and recorded at the affiliate, and then broadcast to the local populace according to the appropriate time. In the 1970’s it became possible for private individuals to download the same signals that the network and cable companies were transmitting, using C-band reception dishes. This free viewing of the corporate contents by individuals let to scrambling and subsequent resale of the descrambling codes to individual customers, which started the direct-to-home industry. The direct-to-home industry has gathered even greater response since the introduction of digital direct broadcast service.

4. MARINE COMMUNICATIONS - In the marine community, satellite communication systems such as Immarsat provide good communication links to ships at sea. These links use a VSAT type device to connect to geosynchronous satellites, which in turn links the ship to a land based point having respective telecommunications system.

5.SPACE BOURNE LAND MOBILE - Along the same lines as the marine based service, there are VSAT devices which can be used to establish communication links even from the world’s most remote regions. Theses devices can be hand-held or fit into a briefcase or might be bigger. Digital data at 64KISDN is available with some (Immarsat).

6.SATELLITE MESSAGING FOR COMMERCIAL JETS - Another service provided by geosynchronous satellite is the ability for passenger on an airborne aircraft to connect directly to a land based telecom network.

7.GLOBAL POSITIONING SERVICES - Another VSAT oriented service, in which a small apparatus containing the ability to determine navigational coordinates by calculating a triangulating or the signals from multiple geosynchronous satellites.

1.Satellite systems:

There are three different types of satellite systems.

1. International satellite communication system INTELSAT.

2. Domestic satellite system DOMSAT.

3. Search and rescue system SARSAT.

INTELSAT:

    The INTELSAT Organization was established in 1964 to handle the myriad of technical and

administrative problems associated with a world wide telecommunication system. The international

regions served by INTELSAT are divided in to the Atlantic Ocean region (AOR), the Pacific Ocean Region 

(POR),  and the Indian Ocean region (IOR). For each region , satellites are positioned in geo-stationary orbit

above the particular Ocean, where they provide a transoceanic telecommunication route. In addition to

providing trans oceanic routes, the INTELSAT satellites are used for domestic services within any given

country and regional services between countries. Two such services are vista for telephony and Intelnet for

data exchange.

DOMSAT

    Domestic satellites are used to provide various telecommunication services, such as voice, data, and

video transmission (T.V channels), with in a country. Satellite cell phones allow global travelers and those

in remote areas to avoid landlines and terrestrial cell phone services entirely. Satellite cell phones relay

your call to a satellite and down through a hub to the end user. This means that most of the earth's

geographical area is now accessible by a satellite cell phone! Third party providers of  satellite cell include

Satcom Global, Roadpost Satcom, Online Satellite Communications, and others.

SARSAT

    SARSAT is one type of  Polar orbiting satellites.

    Polar-orbiting satellites orbit the earth in such a way as to cover the north and south polar-regions.

Infinite  number of polar polar satellite orbits are possible       

    Polar satellites are used to provide environmental data, and to help locate ships and aircrafts in  distress

.This service known as SARSAT, for search and rescue satellite.

This figure shows polar satellite path and earth rotation        

2.Kepler's Laws:

    Artificial  satellites  which orbit the earth follow the same laws that govern the motion of the planets

around the sun. Johannes Kepler (1571 -1630) was derived empirically three laws describing planetary

motion . In 1665, Newton was able to derive Kepler's laws from his own laws of mechanics and theory of

gravitation.

Kepler's laws:

    Kepler's first law: Kepler's first law states that the path followed by the satellite around the ptimary

will be an ellipse

    Kepler's second law: Kepler's second law states that for equal time intervals, the satellite will sweep

areas in its orbital plane.

    Kepler's third law: Kepler's third law states that the square of periodic time of orbit is proportional to

the cube of the mean distance between the two bodies. 

3.Definitions And Related  Terms  Of Earth-Orbiting Satellites   

  Apogee. The point farthest from earth.

  Perigee. The point of closest approach to earth.

  Line of apsides. The line joining the perigee and apogee through the center of the earth.

  Ascending node. The point where the orbit crosses the equatorial plane going from south to north.

  Descending node. The point where the orbit crosses the equatorial plane going from north to south.

  Line of nodes. The line line joining the ascending and descending nodes through the center of the earth.

  Inclination. The angle between the orbital plane and the earth's equatorial plane.

  Prograde orbit. An orbit in which the satellite moves in the same direction as the earths rotation. 

  Retrograte orbit. An orbit in which the satellite moves in a direction counter to the earth's rotation.

  Argument of perigee. The angle from ascending node to perigee, measured in the orbital plane at the

earth's center in direction of satellite motion. 

  Mean anomaly. Mean anomaly M gives an average value of the angular position of the satellite with

reference to the perigee

  True anomaly. The true anomaly is the angle from perigee to the satellite position, measured at the

earth's center. this gives the true angular position of the satellite in the orbit  as a function of time.

4.Satellite  system

  A satellite communication system can be broadly divided into two segments, a ground segment and a

space-segment. The space system includes Satellite.

  Satellite system consist of the following  systems.

Power supply:

   The primary electrical power for operating electronic equipment is obtained from solar cells. Individual

cells can generate small amounts of power, and therefore array of cells in series-parallel connection are

required .

  Cylindrical solar arrays are used with spinning satellites, (The gyroscopic effect of  the spin is used for 

mechanical orientational stability) Thus the array are only partially in sunshine at any given time. 

   Another type of solar panel is  the rectangular array or solar sail. solar sail must be folded  during the

launch phase and extended when in geo-stationary orbit. Since the full component of solar cells are

exposed to sun light ,and since the Sail rotate to track, the sun , they capable of greater power output than

cylindrical arrays having a comparable number of cells.

   To maintain service during an eclipse, storage batteries must be provided .

Attitude control:

  The attitude of a satellite refers to its Orientation in space. Much of equipment carried abroad a satellite

is there for the purpose of controlling its attitude. Attitude control is necessary, for example, to ensure that

directional antennas point in the proper directions. In the case of earth environmental satellites the earth-

sensing instrument must cover the required regions of the earth, which also requires attitude control. A

number of forces, referred to as disturbance forces can alter attitude, some examples being the

gravitational forces of earth and  moon, solar radiation, and meteorite impacts. 

Station keeping:

  A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing forces being the

gravitational  pull of the sun and the moon . the force cause the inclination to change at the rate of about

0.85 deg./year. if left uncorrected, the drift would result in a cycle change in the  inclination  going 0 to

14.67deg in 26.6 years and back to zero , when the cycle is repeated. To prevent the shift in inclination

from exceeding specified limits, jets may be pulled at the appropriate time to return the inclination to zero.

Counteracting jets must be pulsed when the inclination is at zero to halt that change in inclination.

Thermal control:

   Satellites are subject to large thermal gradients, receiving the sun radiation on one side while the other

side faces into space. In addition, thermal radiation from the earth, and the earth's abedo,  which is the

fraction on the radiation falling on the earth which is reflected can be sight for low altitude, earth-orbiting

satellites, although it is negligible for geo-stationary satellites. Equipment in the satellite also generates

heat which has to be removed. the most important consideration is that the satellite's equipment should

operate as near as  possible in a stable temperature environment. various steps are taken to achieve this.

Thermal  blankets and shields may be used to provide insulation. radiation mirrors are often used to

remove heat from communication payload. These mirrored drums surrounded the communication

equipment shelves in each case and provide good radiation paths for the generated heat to escape in to

surround space.

    To maintain constant-temperature conditions, heaters may be switched on to make up for the hearts

may be switched  on to make reduction that occurs when transponders are switched off. 

TT&C subsystem

Telemetry system

The telemetry, tracking, and command (TT&C) subsystem performs several routine functions abroad a

spacecraft. the telemetry or "telemetering"  function could be interpreted as "measurement at a distance".

specifically, it refers to the over all operation of generating an electrical signal proportional to the quantity

being measured, and encoding and transmitting this to a distant station, which for satellite is one of the

earth stations, which for the satellite is one of the earth stations. Data that are transmitted as telemetry

signals include attribute information such as obtained from sun earth sensors; environmental information

such as magnetic field  intensity and direction; the frequency of meteorite impact and so on ;and

spacecraft information such as temperatures and power supply voltages, and stored fuel pressure. 

Command systems

Command system receives instructions from ground system of satellite and decodes the instruction and

sends commends to other systems as per the instruction.

Tracking;

Tracking of the satellite is accomplished by having the satellite is accomplished by having the satellite

transmit beacon signals which are received at the TT&C earth stations. Tracking is obviously important

during the transmitter and drift orbital phases of the satellite launch. When on-station, a geo-stationary 

satellite will tend to shifted as a result of the various distributing forces, as described previously .

Therefore it is necessary to be able to track the satellites movements and send correction signals as

required. Satellite range is also required for time to time. This can be determined by measurement of

propagation delay of signals specially transmitted for ranging purposes.

Transponders:

A transponder is the series of interconnected units which forms a single communication channel between

the receive and transmit antennas in a communication satellite. Some of the units utilized by a

transponder in a given channel may be common to a number of transponders. Thus, although  reference

may be made to specific transponder, this must be thought of as an equipment channel rather than single

item of equipment.

Transponder consist of wideband receivers, input de-multiplexer, power amplifier components. 

Antenna sub system:

The Antennas carried abroad a satellite provide the dual functions of receiving the up link and transmitting

the down link signals. They range from dipole-type antennas, where omni directional characteristics are

required, to the highly directional antennas required for telecommunications purposes and TV relay and

broadcasting.