Satellite Communication System

Satellite Communication System
that satellite technology has a key role to play in disaster relief efforts due to the vulnerability of terrestrial communications infrastructure.

Topics Introduction Orbit characteristics Earth station technology
Satellite sub-systems Launching and positioning

Introduction to Satellite Communication
History and Overview

Satellite A satellite is any object that orbits or revolves around another object. For example, the Moon is a satellite of Earth, and Earth is a satellite of the Sun.

Artificial Satellite Artificial Satellites are man-made machines that orbit Earth and the Sun. These are highly specialized and complex machines and perform thousands of tasks. These satellites have many sub-systems. There are hundreds of satellites currently in operation.

Earth’s atmosphere Source: All about GPS [

Elements of a Satellite
Payload Bus

Payload Antennas and electronics.
The payload is different for every satellite. The payload for a weather satellite includes cameras to take pictures of cloud formations, while the payload for a communications satellite includes large antennas to transmit TV or telephone signals to Earth.

The Payload The payload is the part of the satellite that performs the required mission mission describes the purpose for which a satellite is put in space. The mission of a communications satellite is to receive, process, amplify and retransmit signals or effectively just repeaters. They receive the signals that are transmitted to them and then retransmit them at a different frequency back to earth.

Bus Carries the payload and all its equipment into space.
The bus also contains equipment that allows the satellite to communicate with Earth. Holds all the satellite's parts together and provides electrical power,computers, and propulsion to the spacecraft.

Spectrum

Why Satellite Communication?
The Earth is a sphere & The microwave frequencies travel in straight line but to connect two regions very far away on the two side of the sphere, the link requires lot of repeaters because of Earth’s curvature. A single satellite can do the magic linking the continents with one repeater.

Satellite It is a repeater which receives signal from Earth at one frequency, amplify it & transmit it back to Earth at other frequency. Active Satellite: Power amplification on board Passive Satellite: Only a reflector

EARTH STATION There are two earth station in a simple Satellite communication link. One transmits the signal to satellite called transmitting Earth station. The other receives the signal from satellite called receiving Earth Station.

UPLINK & DOWN LINK The communication link from Transmitting earth station to satellite is called Up-link. The communication link from satellite To receiving earth station is called Down-link.

Introduction to Satellite Communication
In the 1950s and early 1960s – attempts for communication systems by bouncing signals off metalized weather balloons received signals - too weak to be of any practical use Communication satellites can be regarded as big microwave repeaters in the sky

Communications Satellites
“All these problems can be solved by the use of a chain of space-stations with an orbital period of 24 hours, which would require them to be at a distance of 42,000 Km from the center of the Earth. There are a number of possible arrangements for such a chain. The stations would lie in the Earth’s equatorial plane and would thus always remain fixed in the same spots in the sky, from the point of view of terrestrial observers. Unlike all other heavenly bodies they would never rise nor set. This would greatly simplify the use of directive receivers installed on the Earth.” science fiction author, Arthur C. Clarke "Wireless World" British Magazine May 1945

Arthur Clark’s View of a Global Communications System Following longitudes were suggested for the stations to provide the best service to the inhabited portions of the globe 30° E - Africa and Europe 150° E - China and Oceania 90° W - The Americas Each station would broadcast programs over about a third of the planet.

History of Satellite Communication
1945 Arthur C. Clarke publishes an essay about „Extra Terrestrial Relays“ first satellite SPUTNIK 1960 first reflecting communication satellite ECHO first geostationary satellite SYNCOM 1965 first commercial geostationary satellite Satellit „Early Bird“ (INTELSAT I): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime 1976 three MARISAT satellites for maritime communication 1982 first mobile satellite telephone system INMARSAT-A 1988 first satellite system for mobile phones and data communication INMARSAT-C 1993 first digital satellite telephone system 1998 global satellite systems for small mobile phones

replaced by fiber optics
Applications Traditionally weather satellites radio and TV broadcast satellites military satellites satellites for navigation and localization (e.g., GPS) Telecommunication global telephone connections backbone for global networks connections for communication in remote places or underdeveloped areas global mobile communication  satellite systems to extend cellular phone systems (e.g., GSM or AMPS) replaced by fiber optics

Classical satellite system
Universität Karlsruhe Institut für Telematik Classical satellite system Mobilkommunikation SS 1998 Inter Satellite Link (ISL) Mobile User Link (MUL) MUL Gateway Link (GWL) GWL small cells (spotbeams) base station or gateway footprint ISDN PSTN GSM User data PSTN: Public Switched Telephone Network Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Critical Elements of the Satellite Link

Satellite Network Configurations

Why do satellites stay moving and in orbit?
v (velocity) F2 (Inertial-Centrifugal Force) F1 (Gravitational Force)

Once a ship is in orbit, do we have to do anything to keep it there?
Yes. Satellite orbits will degrade (slow and fall slightly) over time. As a ship travels for a long time, it goes through space, which is almost but not completely empty. The collective force from millions of tiny collisions with floating matter in space will decrease an orbiting object's speed. This force is very small since the floating matter is usually nothing more than dust and occasional clouds of gas. Overall, the effect of all these tiny forces hitting the satellite will act like drag or resistance on a plane flying in our atmosphere. Drag is the slowing or resistance force caused by air. Try swinging your arm around your body with your palm facing the direction your hand is moving. Do you feel the wind on your palm? That's drag. As you might guess, the faster your hand moves, the harder the air molecules hit your hand, and the more drag force will be exerted on your hand. If your hand is not moving at all compared to the air, then there isn't any drag force acting on it. We say that the drag force on an object like your hand is "directly proportional" to its speed. That just means that as the speed goes up the force goes up, and as the speed goes down, the force goes down. Now, back to satellites in space. If space were a perfect vacuum, meaning there was absolutely nothing in it, we probably wouldn't have to worry about any of this, and stuff would stay in orbit as long as we liked because of inertia. But space isn't a perfect vacuum. Even though that dust and dirt and gas that hits the satellite is very thin and spread out, its effect is like an extremely thin atmosphere. Even though any orbiting object is moving at thousands of miles per hour (speeds which would cause an object to break apart and burn up if it was in the atmosphere being hit by bazillions of air molecules) too few particles are hitting it to cause a significant drag force like the one you felt on your hand. However, over long periods of time, the effect of the particles colliding with the orbiting object are significant and slow the ship. For instance, NASA scientists estimate that the space shuttle, about the size of a passenger plane, can stay in orbit for about a month before this force causes it to slow enough that it falls out of its orbit. Sometimes NASA scientists dip a satellite into the atmosphere of a planet on purpose so that drag will slow it. This is called aerobraking.

Is there gravity in space?
There is gravity everywhere. It gives shape to the orbits of the planets, the solar system, and even galaxies. Gravity from the Sun reaches throughout the solar system and beyond, keeping the planets in their orbits. Gravity from Earth keeps the Moon and human-made satellites in orbit. It is true that gravity decreases with distance, so it is possible to be far away from a planet or star and feel less gravity. But that doesn't account for the weightless feeling that astronauts experience in space. The reason that astronauts feel weightless actually has to do with their position compared to their spaceship. We feel weight on Earth because gravity is pulling us down, while the floor or ground stop us from falling. We are pressed against it. Any ship in orbit around the Earth is falling slowly to Earth. Since the ship and the astronauts are falling at the same speed, the astronauts don't press against anything, so they feel weightless. You can feel something very like what the astronauts feel for a moment in a fast-moving elevator going down or in a roller coaster, when you start going down a big hill. You are going down rapidly, but so is the roller coaster or the elevator so for a second you feel weightless.

Here’s the Math… Gravity depends on the mass of the earth, the mass of the satellite, and the distance between the center of the earth and the satellite For a satellite traveling in a circle, the speed of the satellite and the radius of the circle determine the force (of gravity) needed to maintain the orbit The radius of the orbit is also the distance from the center of the earth. For each orbit the amount of gravity available is therefore fixed That in turn means that the speed at which the satellite travels is determined by the orbit

Let’s look in a Physics Book…
From what we have deduced so far, there has to be an equation that relates the orbit and the speed of the satellite: T is the time for one full revolution around the orbit, in seconds r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

The Most Common Example
“Height” of the orbit = 22,300 mile That is 36,000km = 3.6x107m The radius of the orbit is 3.6x107m x106m = 4.2x107m Put that into the formula and …

The Geosynchronous Orbit
The answer is T = 86,000 sec (rounded) 86,000 sec = 1,433 min = 24hours (rounded) The satellite needs 1 day to complete an orbit Since the earth turns once per day, the satellite moves with the surface of the earth.

Basics Satellites in circular orbits Stable orbit
attractive force Fg = m g (R/r)² centrifugal force Fc = m r ² m: mass of the satellite R: radius of the earth (R = 6370 km) r: distance to the center of the earth g: acceleration of gravity (g = 9.81 m/s²) : angular velocity ( = 2  f, f: rotation frequency) Stable orbit Fg = Fc

Ways to Categorize Communications Satellites
Coverage area Global, regional, national Service type Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS) General usage Commercial, military, amateur, experimental

Classification of Satellite Orbits
Circular or elliptical orbit Circular with center at earth’s center Elliptical with one foci at earth’s center Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)

Satellite Orbits Equatorial Inclined Polar

Basics elliptical or circular orbits
complete rotation time depends on distance satellite-earth inclination: angle between orbit and equator elevation: angle between satellite and horizon LOS (Line of Sight) to the satellite necessary for connection  high elevation needed, less absorption due to e.g. buildings Uplink: connection base station - satellite Downlink: connection satellite - base station typically separated frequencies for uplink and downlink transponder used for sending/receiving and shifting of frequencies Transponder – electronics in the satellite that convert uplink signals to downlink signals transparent transponder: only shift of frequencies regenerative transponder: additionally signal regeneration

Inclination plane of satellite orbit satellite orbit perigee d
inclination d d satellite orbit perigee plane of satellite orbit equatorial plane

Elevation Elevation: angle e between center of satellite beam
and surface e minimal elevation: elevation needed at least to communicate with the satellite footprint

Minimum Elevation Angle
Reasons affecting minimum elevation angle of earth station’s antenna (>0o) Buildings, trees, and other terrestrial objects block the line of sight Atmospheric attenuation is greater at low elevation angles Electrical noise generated by the earth's heat near its surface adversely affects reception

Communication Satellites
Contain several transponders – device which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal The downward beams can be broad, covering a substantial fraction of the earth's surface, or narrow, covering an area only hundreds of kilometers in diameter - bent pipe mode

Another issue is the presence of the Van Allen belts - layers of highly charged particles trapped by the earth's magnetic field Any satellite flying within them would be destroyed fairly quickly by the highly-energetic charged particles trapped there by the earth's magnetic field Hence there are three regions in which satellites can be placed safely - illustrated in the following figure

Orbits Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit: GEO: geostationary orbit, ca km above earth surface LEO (Low Earth Orbit): ca km MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca km HEO (Highly Elliptical Orbit) elliptical orbits

Orbits HEO Equatorial LEO Polar LEO GEO

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

Satellite Orbits – 2A Orbit ranges (altitudes) LEO 250 to 1,500 km
MEO 2, to 15,000 km GEO 35, km HEO examples: 500 to 39,152 km Molniya (“Flash of lightning”)  16,000 to  133,000 km Chandra Mean earth radius is 6, km Period of one-half sidereal day Period of 64 hours and 18 minutes

Orbits Defining the altitude where the satellite will operate.
Determining the right orbit depends on proposed service characteristics such as coverage, applications, delay.

Orbits Van-Allen-Belts: ionized particles 2000 - 6000 km and
inner and outer Van Allen belts earth km 35768 10000 1000 LEO (Globalstar, Irdium) HEO MEO (ICO) GEO (Inmarsat) Van-Allen-Belts: ionized particles km and km above earth surface

 Orbits (cont.) GEO (33786 km) GEO: Geosynchronous Earth Orbit
MEO: Medium Earth Orbit LEO: Low Earth Orbit Outer Van Allen Belt ( km) MEO ( < 13K km) LEO ( < 2K km)  Inner Van Allen Belt ( km)

Satellite Orbits Type LEO MEO GEO Description Low Earth Orbit
Medium Earth Orbit Geostationary Earth Orbit Height miles miles 22,300 miles Time in LOS 15 min 2-4 hrs 24 hrs Merits Lower launch costs Very short round trip delays Small path loss Moderate launch cost Small roundtrip delays Covers 42.2% of the earth's surface Constant view No problems due to Doppler Demerits Short life Encounters radiation belts Short LOS Round trip delays Greater path loss Larger round trip delays Expensive equipment due to weak signal

MCCS - Satellites MEO GEO LEO

Main orbit types: GEO 36,000 km MEO 5,000 – 15,000 km LEO 500 -1000 km
Geosynchronous & Geostationary Orbits A geosynchronous orbit is defined as an orbit with a period of one sidereal day ( minutes). A geostationary orbit is a special case of a geosynchronous orbit with zero inclination and zero eccentricity, i.e., an equatorial, circular orbit. A satellite in a geostationary orbit appears fixed above a location on the surface of the Earth. In practice, a geosynchronous orbit typically has small non-zero values for inclination and eccentricity, causing the satellite to trace out a small figure eight in the sky. The footprint or service area of a geosynchronous satellite covers almost one-third of the Earth's surface (from about 75 deg South to about 75 deg North latitude), so that near-global coverage can be achieved with as few as three satellites in orbit. A disadvantage of a geosynchronous satellite in a voice communication system is the round-trip delay of approximately 250 milliseconds. A Polar Orbit The plane of a polar orbit is inclined at about 90 deg to the equatorial plane, intersecting the North and South poles. The orbit is fixed in space, and the Earth rotates underneath. Thus, in principle, the coverage of a single satellite in a polar orbit encompasses the entire globe, although there are long periods during which the satellite is out of view of a particular ground station. This gap in coverage may be acceptable for a store-and-forward communications system. Accessibility can, of course, be improved through the deployment of two or more satellites in different polar orbits. Most small LEO systems employ polar or near-polar orbits. An example is the COSPAS-SARSAT Maritime Search and Rescue system, which uses eight satellites in near polar orbits: four SARSAT satellites moving in 860 km orbits inclined at 99 deg (which makes them Sun-synchronous) and four COSPAS satellites moving in 1000 km orbits inclined at 82 deg. A Sun-Synchronous Orbit In a Sun-synchronous or helio-synchronous orbit, the angle between the orbital plane and Sun remains constant, resulting in consistent light conditions for the satellite. This can be achieved by careful selection of orbital altitude, eccentricity and inclination, producing a precession of the orbit (node rotation) of approximately 1 deg eastward each day, equal to the apparent motion of the Sun. This condition can be achieved only for a satellite in a retrograde orbit. A satellite in Sun-synchronous orbit crosses the equator and each latitude at the same time each day. This type of orbit is therefore advantageous for an Earth observation satellite, since it provides constant lighting conditions.

Altitudes of orbits above the earth
There are 3 common types of satellite based on altitude, i.e. GEO, MEO & LEO Orbit Altitude Missions possibles Low-Earth orbit LEO 250 to 1,500 km Earth observation, meteorology, telecommunications (constellations) Medium-Earth orbit MEO 10,000 to 30,000 km Telecommunications (constellations), positioning, science Geostationary Earth orbit GEO 35,786 km Telecommunications, positioning, science Elliptical orbit Between 800 and 27,000 km Telecommunications Hyperbolic orbit Up to several million km Interplanetary missions

GEO, MEO, LEO

Orbital Period The time taken by a satellite to complete one rotation in its orbit is called its period. The GEO satellite takes 23 hrs & 56 minutes & 4.1 Seconds to complete its rotation which is approximately equal to the period of rotation of earth around its axis. This is why it appears to be stationary by the observer on Earth moving with the same speed as that of satellite. So one GEO stationary satellite can serve a ground user round the clock.

Orbital Period Satellite System Orbital Height (Km)
Orbital Velocity (Km/Sec) Orbital Period (H M S) Intelsat (GEO) 35,786 3.0747 New ICO (MEO) 10,255 4.8954 Iridium (LEO) 1,469 7.1272 Notice as altitude decreases, the velocity must be increased to minimize the gravitational effect.

Satellite Orbits Some special orbits Molniya (contd.)
Apogee: moving slowly Perigee: moving quickly

Satellite Orbits – 2E Some special orbits Molniya (contd.)
Satellite Orbits – 2E Some special orbits Molniya (contd.) Fall 2010 TCOM 707 Advanced Link Design Lecture No © Jeremy Allnutt August 2010

MOLNIYA APOGEE VIEW Fall 2010 TCOM 707 Advanced Link Design Lecture No © Jeremy Allnutt August 2010

Satellite Orbits – 2F Some special orbits Chandra
Satellite Orbits – 2F Some special orbits Chandra

Satellite Orbits – 3 Orbit period, T (seconds) 2a
 is Kepler’s constant T2 = (42a3)/  =  105 km3/s2

Satellite Orbits – 4 Orbit period, T – examples
Orbital height Orbital period 500 km h 34.6 min ,000 km h 45.1 min ,000 km h 21.3 min ,000 km h 47.6 min ,786 km h min ,000 km 28 days 148,800,000 km days Typical LEO Typical MEO GEO Moon’s orbit Earth’s orbit

One-Way Delay Times – 1 GEO satellite: 35,786 km
One-way delay: ms MEO satellite: 10,355 km One-way delay: 34.5 ms LEO satellite: 800 km One-way delay: 2.7 ms

One-Way Delay Times – 2 GEO satellite: 35,786 km
Path Loss Issues: LEO = MEO – 22dB LEO = GEO – 33dB One-way delay: ms MEO satellite: 10,355 km One-way delay: 34.5 ms Beware: Propagation delay does not tell the whole story! LEO satellite: 800 km One-way delay: 2.7 ms

v = (/r)1/2 where r = radius from center of earth
Satellite Orbits – 5 Orbit velocities (m/s) LEO  7 km/s MEO  5 km/s GEO = km/s v = (/r)1/2 where r = radius from center of earth GEO Orbital radius = 35, , km Orbital circumference = 2  42, km Orbital period = (2  42, )/ seconds = 86, seconds = 23 hours minutes

FREQUENCIES For Uplink & Down link
Uplink uses higher frequency than the down link. Frequency of satellite is always specified as UPLINK frequency/ Down link Frequency e.g. C band 6/4 GHz Ku band14/11 GHz Ka band30/20 GHz presentation by SANIA GUL

Effect of rain on signal
Rain heavily effects the wireless communication above 10 GHz. So Ku band & Ka band will be effected by rain & specially above 20 GHz the Ka Band link can fail during heavy rain fall. presentation by SANIA GUL

Why fup is always Higher than fdown?
The beam of higher frequency is narrow & that of lower is broad. As the earth station has to target the signal to a small point (satellite) in space so it does it by using narrow beam produced by higher frequency. While the Satellite has to cover a large area on earth to provide services to many Earth station so it does it by using broad beam produced by lower frequency. As the rain effects higher frequencies more than lower one so they need to be boosted up more to overcome the propagation losses. The Energy can be given to signal much more easily on earth than on satellite because the satellite has limited power resources like solar cells & batteries so we use higher frequencies on Earth & amplify them with enough power supply resources we have on Earth presentation by SANIA GUL

Signal Propagation DELAY
Using c= 3*10 ^ 8 m/s & time= distance(altitude)/ speed Uplink delay from earth station to Satellite. Round trip delay 4* uplink delay. All other delays in signal coding, compression, & processing on Satellite & earth Station are neglected. orbit Average altitude of Orbit Uplink Delay Round trip delay LEO 800 Km 2.7 ms 10.8 ms MEO 10,355 Km 34.5 ms 138 ms GEO 35,786 Km 119.3 ms 480 ms = ½ Second presentation by SANIA GUL

Round trip delay of GEO signal
presentation by SANIA GUL

Coverage Area of Satellite
The Earth surface covered by satellite radiations is called FOOT PRINT. The coverage area is inversely proportional to frequency. The foot print will be large if the frequency of down link is low.

SATELLITE FOOTPRINT The geographical representation of a satellite antenna radiating pattern is called the footprint. The footprint of a communications satellite is the ground area that its transponders offer coverage, and determines the satellite dish diameter required to receive each transponder's signal.

Satellite Beams

presentation by SANIA GUL
Satellite Beams presentation by SANIA GUL

GEO satellite Coverage
One GEO can cover 1/3 of earth surface so the earth is divided in 3 regions. AOR (Atlantic Ocean Region) POR (Pacific Ocean region) IOR (Indian Ocean region) presentation by SANIA GUL

Satellite Orbits – Coverages – 1
LEO Track of the sub-satellite point along the surface of the Earth Movement of the coverage area under the satellite Orbital path of satellite Fig Pratt et al. The Earth

Multiple beams Spectrum A Spectrum B Spectrum C Fig Pratt et al. Instantaneous Coverage

Iridium Fig (a) Pratt et al.

New ICO Fig (b) Pratt et al.

Satellite coverages – 1 Determined by two principal factors Height of satellite above the Earth Beamwidth of satellite antenna Same beamwidth, different altitudes Same altitude, different beamwidths

Satellite coverages – 2 Orbital plane usually optimized Equatorial orbits – simplest, equal N-S coverage Inclined orbits – cover most of populated Earth Polar orbits – cover all of the Earth at some point Retrograde orbits – gives sun synchronized orbit LEO chosen for two reasons usually Low link power needed good optical resolution

Satellite coverages – 3 MEO chosen for a variety of reasons GPS half sidereal orbit covers same tracks alternately Compromise between LEO and GEO delay Compromise between LEO and GEO total number GEO chosen for two reasons mainly Optimizes broadcast capabilities Simplest earth terminal implementation

Parameters Determining Orbit Size and Shape
Definition Semimajor Axis Half the distance between the two points in the orbit that are farthest apart Apogee/Perigee Radius Measured from the center of the Earth to the points of maximum and minimum radius in the orbit Apogee/Perigee Altitude Measured from the "surface" of the Earth (a theoretical sphere with a radius equal to the equatorial radius of the Earth) to the points of maximum and minimum radius in the orbit Period The duration of one orbit, based on assumed two-body motion Mean Motion The number of orbits per solar day (86,400 sec/24 hour), based on assumed two-body motion Eccentricity The shape of the ellipse comprising the orbit, ranging between a perfect circle (eccentricity = 0) and a parabola (eccentricity = 1)

Satellite Communications Lecture 3
Look Angle

Look Angle Determination

Azimuth and elevation Angles
When your dish is pointed low down near the horizon the elevation angle is only a few degrees. At low elevation angles, below 5 deg at C band and 10 deg at Ku band, the path through the atmosphere is longer and the signals are degraded by rain attenuation and rain thermal noise. When your dish is pointed almost straight up the elevation angle is nearly 90 degrees. Sites near the equator may require you to point to almost 90 deg elevation angle when the longitude of the satellite is similar to the longitude of the site location. In high elevation cases watch out for the possibility of rain water collecting in the dish. Azimuth refers to the rotation of the whole antenna around a vertical axis. It is the side to side angle. By definition North is 0 deg, East is 90 deg, South is 180 deg and west is 270 deg. Elevation refers to the angle between the beam pointing direction, directly towards the satellite, and the local horizontal plane. It is the up-down angle.

Look Angle Definition

Calculating Look Angle

Coordinate System

Satellite Coordinates

Review of Geometry

Geometry of Elevation Angle

Central Angle

Elevation Angle Calculation

Example: Elevation Angle for GEO Satellite
Using rs = 42,164 km and re = 6, km gives d = 42,164 [ cos(γ)]1/2 km Which finally gives the elevation angle

Azimuth Angle Calculation
More complex approach for non-geo satellites. Different formulas and corrections apply depending on the combination of positions of the earth station and subsatellite point with relation to each of the four quadrants (NW, NE, SW, SE). Its calculation is simple for GEO satellites

Azimuth Angle Calculation for GEO Satellites
SUB-SATELLITE POINT Equatorial plane, Latitude Ls = 0o Longitude ls EARTH STATION LOCATION Latitude Le Longitude le

Azimuth Angle for GEO sat.

Example for Look Angle Calculation of a GEO satellite

Example (Contd.) El=5.85o

Example (Contd.)

Definitions (Contd.)

 Types of Satellites GEO: 35786 km LEO: < 2K km
HEO: var. (Molniya, Ellipso) LEO: < 2K km MEO: < 13K km (Odyssey, Inmarsat-P) GEO: km (Globalstar, Iridium, Teledesic) Geostationary/Geosynchronous Earth Orbit Satellites (GSOs) (Propagation Delay: ms) Medium Earth Orbit Satellites (MEOs) (Propagation Delay: ms) Highly Elliptical Satellites (HEOs) (Propagation Delay: Variable) Low Earth Orbit Satellite (LEOs) (Propagation Delay: ms)

Geostationary/Geosynchronous Earth Orbit Satellites (GSOs)
35786 km equatorial orbit Rotation speed equals Earth rotation speed (Satellite seems fixed above the Earth) Wide coverage area Applications (Broadcast/Fixed Satellites, Direct Broadcast, Mobile Services)

Advantages of GSOs Wide coverage, Fixed and continuous service
High quality and Wideband communications Economic Efficiency Tracking process is easier because of its synchronization to Earth Doppler effect is minimum for GSO satellites

Doppler Effect

Disadvantages of GSOs Long propagation delays ( ms). (e.g., Typical Intern. Tel. Call  540 ms round-trip delay. Echo cancelers needed. Expensive!) (e.g., Delay may cause errors in data; Error correction /detection techniques are needed.) Large propagation loss. Requirement for high power level. (e.g., Future hand-held mobile terminals have limited power supply.) Currently: smallest terminal for a GSO is as large as an A4 paper and as heavy as 2.5 Kg.

Disadvantages of GSOs (cont.)
Lack of coverage at Northern and Southern latitudes. High cost of launching a satellite. Enough spacing between the satellites to avoid collisions. Existence of hundreds of GSOs belonging to different countries. Available frequency spectrum assigned to GSOs is limited. Requires heavy propulsion devices on board to keep the satellite on the orbit Artificial satellites must be launched into orbit and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of alttitude control so that they are correctly pointed with respect to the Earth, the Sun, and possibly some astronomical object of interest. They are also subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital stationkeeping). Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion. A satellite's useful life is over once it has exhausted its ability to adjust its orbit. Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earth's atmosphere just as satellites do. Once there, they need to leave orbit and move around. For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments. In between these adjustments, the spacecraft simply falls freely along its trajectory. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination. Special methods such as aerobraking or aerocapture are sometimes used for this final orbital adjustment. Artist's concept of a solar sail Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrust;[7] an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun or constantly thrusting along its direction of motion to increase its distance from the Sun. The concept has been successfully tested by the Japanese IKAROS solar sail spacecraft.

Medium Earth Orbit Satellites (MEOs)
Positioned in 10-13K km range. Delay is ms. Will orbit the Earth at less than 1 km/s. Applications Mobile Services/Voice (Intermediate Circular Orbit (ICO) Project) Fixed Multimedia (Expressway)

Highly Elliptical Orbit Satellites (HEOs)
From a few hundreds of km to 10s of thousands  allows to maximize the coverage of specific Earth regions. Variable field of view and delay. Examples: MOLNIYA, ARCHIMEDES (Direct Audio Broadcast), ELLIPSO. Geostationary satellites must operate above the equator and therefore appear lower on the horizon as the receiver gets the farther from the equator. This will cause problems for extreme northerly latitudes, affecting connectivity and causing multipath (interference caused by signals reflecting off the ground and into the ground antenna). For areas close to the North (and South) Pole, a geostationary satellite may appear below the horizon. Therefore Molniya orbit satellite have been launched, mainly in Russia, to alleviate this problem. The first satellite of the Molniya series was launched on April 23, 1965 and was used for experimental transmission of TV signal from a Moscow uplink station to downlink stations located in Siberia and the Russian Far East, in Norilsk, Khabarovsk, Magadan and Vladivostok. In November 1967 Soviet engineers created a unique system of national TV network of satellite television, called Orbita, that was based on Molniya satellites. Molniya orbits can be an appealing alternative in such cases. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite's position above the horizon. Thus, a satellite at the horizon has zero elevation and a satellite directly overhead has elevation of 90 degrees). The Molniya orbit is designed so that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint moves only slightly. Its period is one half day, so that the satellite is available for operation over the targeted region for six to nine hours every second revolution. In this way a constellation of three Molniya satellites (plus in-orbit spares) can provide uninterrupted coverage.

Low Earth Orbit Satellites (LEOs)
Usually less than 2000 km ( km are favored). Few ms of delay (20-25 ms). They must move quickly to avoid falling into Earth  LEOs circle Earth in 100 minutes at 24K km/hour. (5-10 km per second). Examples: Earth resource management (Landsat, Spot, Radarsat) Paging (Orbcomm) Mobile (Iridium) Fixed broadband (Teledesic, Celestri, Skybridge)

Low Earth Orbit Satellites (LEOs) (cont.)
Little LEOs: 800 MHz range Big LEOs: > 2 GHz Mega LEOs: GHz

Comparison of Different Satellite Systems

Comparison of Satellite Systems According to their Altitudes (cont.)

Why Hybrids? GSO + LEO LEO or GSO + Terrestrial Infrastructure
GSO for broadcast and management information LEO for real-time, interactive LEO or GSO + Terrestrial Infrastructure Take advantage of the ground infrastructure

Frequency Bands NarrowBand Systems
L-Band  GHz DL; GHz UL S-Band  GHz DL; GHz UL C-Band  GHz DL; GHz UL X-Band  GHz DL; GHz UL

Frequency Bands (cont.)
WideBand/Broadband Systems Ku-Band  GHz DL; GHz UL (36 MHz of channel bandwidth; enough for typical Mbps applications) Ka-Band  GHz DL; GHz UL (500 MHz of channel bandwidth; enough for Gigabit applications)

Next Generation Systems: Mostly Ka-band
Ka band usage driven by: Higher bit rates - 2Mbps to 155 Mbps Lack of existing slots in the Ku band Features Spot beams and smaller terminals Switching capabilities on certain systems Bandwidth-on-demand Drawbacks Higher fading Manufacturing and availability of Ka band devices Little heritage from existing systems (except ACTS and Italsat)

Frequency Bands (cont.)
New Open Bands (not licensed yet) GHz of bandwidth Q-Band  in the 40 GHz V-Band  60 GHz DL; GHz UL

FREQUENCIES for Satellite Communication
Letter Designation Frequency range USE L band 1 to 2 GHz Satellite phone, GPS S band 2 to 4 GHz Satellite phone C band 4 to 8 GHz TV transmission X band 8 to 12 GHz Ku band 12 to 18 GHz TV transmission, Communication K band 18 to 26.5 GHz Ka band 26.5 to 40 GHz Satellite Internet Q band 30 to 50 GHz Experimental U band 40 to 60 GHz presentation by SANIA GUL

Frequency Bands Available for Satellite Communications

Space Environment Issues
Harsh  hard on materials and electronics (faster aging) Radiation is high (Solar flares and other solar events; Van Allen Belts) Reduction of lifes of space systems (12-15 years maximum).

Space Environment Issues (cont.)
Debris (specially for LEO systems) (At 7 Km/s impact damage can be important. Debris is going to be regulated). Atomic oxygen can be a threat to materials and electronics at LEO orbits. Gravitation pulls the satellite towards earth. Limited propulsion to maintain orbit (Limits the life of satellites; Drags an issue for LEOs). Thermal Environment again limits material and electronics life.

Done By:- Zakie Mohamed

Satellite Communication System

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