ECE411 Wireless Communications Lesson 7 Satellite Communications PDF

ECE 422 ECE ELEC 2 (WIRELESS COMMUNICATIONS) LESSON 7 SATELLITE COMMUNICATIONS PROFESSOR: ENGR. VICTOR – SOLITO DR. ISAAC OBJECTIVES : After completing this lesson, you will be able to: 1. Define Kepler’s Law, and satellite orbits 2. Describe Geosynchronous Satellit...

ECE 422 ECE ELEC 2 (WIRELESS COMMUNICATIONS) LESSON 7 SATELLITE COMMUNICATIONS PROFESSOR: ENGR. VICTOR – SOLITO DR. ISAAC OBJECTIVES : After completing this lesson, you will be able to: 1. Define Kepler’s Law, and satellite orbits 2. Describe Geosynchronous Satellites and Antenna Look Angle.  INTRODUCTION In astronomical terms, a satellite is a celestial body that orbits around a planet. In aerospace terms, however, a satellite is a space vehicle launched by humans and orbits Earth or another celestial body. 2 types of satellite 1. Natural Satellite – a celestial body that orbits around a planet. ©Google 2. Artificial Satellite – a space vehicle launched by humans and orbits earth or another celestial body.  BRIEF HISTORY OF SATELLITES 1954 – the U.S. Navy successfully transmitted the first message over the Earth-to-moon-to-Earth communication system making the moon the first passive satellite. 1957 – SPUTNIK 1, the first active satellite (capable of receiving, amplifying, reshaping, regenerating, and retransmitting information), was launched by Russia. 1957 – EXPLORER 1 (transmitted telemetry information) was launched by the United States. It was the first spacecraft to detect the Van Allen radiation Belt. 1958 – SCORE, a delayed repeater, was launched by NASA. SCORE rebroadcast President Eisenhower’s 1958 Christmas message. 1960 – ECHO was launched by NASA in conjunction with Bell Telephone Laboratories and the Jet Propulsion Laboratory. ©Google  BRIEF HISTORY OF SATELLITES 1960 – COURIER, the first transponder type satellite, was launched by the Department of Defense. It transmitted 3W of power. 1962 - TELSTAR 1, the first active satellite to simultaneously receive and transmit radio signal, was launched by AT&T. 1963 - TELSTAR II was successfully launched and was electronically identical to Telstar 1 except more radiation resistant. It was used for telephone, television, facsimile, and data transmission and it accomplished the first successful transatlantic video transmission. FEBRUARY 1963 – SYNCOM 1 was launched, and was the first attempt to place a geosynchronous satellite into orbit. FEBRUARY 1963 and AUGUST 1964 - SYNCOM 2 and SYNCOM 3 were successfully launched, respectively. SYNCOM 3 satellite was used to broadcast the 1964 Olympic games from Tokyo. ©Google  BRIEF HISTORY OF SATELLITES 1965 – INTELSAT 1 (Early Bird), the first commercial telecommunications satellite was launched by Intelsat. Between 1966 and 1987 - Intelsat launched a series of satellite designated INTELSAT II, III, IV, V and VI. 1966 – the former Soviet Union launched the first set of domestic satellites (domsat) called the MOLNIYA meaning “ lightning “. 1972 - Canada launched it’s first satellite designated ANIK, which is an Inuit word meaning “little brother “. 1974 – Western Union launched their first WESTAR satellite. Westar was America’s first domestic and commercially launched geostationary communication satellite. ©Google Communications satellites are man-made satellites that orbit Earth, providing a multitude of communication functions to a wide variety of consumers (i.e. military, governmental, private, and commercial subscribers). Satellite radio repeater is called a transponder, of which a satellite may have many. A satellite system consists of one or more satellite space vehicles, a ground-based station, and a user network of earth stations. Transmissions to and from satellites are categorized as either bus or payload. The bus includes control mechanisms that support the payload operation, while the payload is the actual user information conveyed through the ©Google system.  Communication Satellite receiver Microwave repeater waveguide transmitter in the sky antenna amplifier demultiplexer regenerator multiplexer filter onboard computer other electronic communications circuits ©Google  German astronomer Johannes Kepler (1571–1630) discovered the laws that govern satellite motion. The laws of planetary motion describe the shape of the orbit, the velocities of the planet, and the distance a planet is with respect to the sun. Kepler’s laws can be applied to any two bodies in space that interact through gravitation. The larger of the two bodies is called the primary, and the smaller is called the secondary or satellite. JOHANNES KEPLER ©Google ©Google Kepler’s laws may be simply stated in the following: (1) the planets move in ellipses with the sun at one focus ©Google (2) The line joining the sun and a planet sweeps out equal areas in equal intervals of time. ©Google (3) the square of the time of revolution of a planet divided by the cube of its mean distance from the sun gives a number that is the same for all planets. ©Google  Kepler’s first law states that a satellite will orbit a primary body (like Earth) following an elliptical path. The eccentricity of an ellipse is given by the formula: 𝜶𝟐 − 𝜷𝟐 𝝐= 𝜶 where: є = is the eccentricity α = semimajor axis β = the semiminor axis ©Tomasi Focal points 𝐹1 and 𝐹2 , semimajor axis α, and semiminor axis β Kepler’s second law, also known as the law of areas states that for equal intervals of time a satellite will sweep out equal areas in the orbital plane, focused at the barycenter. Kepler’s second law ©Tomasi Kepler’s third law, also known as the harmonic law, states that the square of the periodic time of orbit is proportional to the cube of the mean distance between the primary and the satellite. This mean distance is equal to the semi major axis; thus, Kepler’s third law can be stated mathematically written as: 2 Τ3 α= 𝐴𝑃 where: α = semimajor axis (kilometers) the distance from a satellite revolving in the geosynchronous orbit to the center of Earth A = constant (unitless) = 42,241.0979 P = mean solar earth days P = the ratio of the time of one sidereal day (𝑡𝑥 = 23 hours and 56 minutes) to the time of one revolution of earth on its own axis (𝑡𝑒 = 24 hours), thus, 𝐭𝐱 1436 minutes 𝐏= = = 0.9972, therefore, 𝐭𝐞 1440 minutes 𝜶 = 42,164 km Hence, geosynchronous earth-orbit satellites revolve around Earth in a circular pattern directly above the equator 42,164 km from the center of the Earth. Because Earth’s equatorial radius is approximately 6378 km, the height above mean sea level (h) of a satellite in a geosynchronous orbit around Earth is h = 42,164 km – 6,378 km h = 35,786 km ≈ 22,300 miles above Earth’s surface. Farthest Closest from sun to sun Slower Faster Velocity Velocity Greater Gravitational Pull ©Google The speed of the satellite is greater when it is close to earth than when it is farther away. PROGRADE (POSIGRADE) - if the satellite is orbiting in the same direction as earth’s rotation (counterclockwise) and at an angular velocity greater than that of earth. RETROGRADE - If the satellite is orbiting in the opposite direction as earth’s rotation or in the same direction with an angular velocity less than the earth. NOTE! Most synchronous satellites revolve around earth in a prograde orbit. Most of the satellites mentioned thus far are called orbital satellites, which are nonsynchronous. Nonsynchronous satellites rotate around Earth in an elliptical or circular pattern.  ©Tomasi ©Google Satellite orbits: a) Circular b) Elliptical  Satellites remain in orbit as a result of a balance between centrifugal and gravitational forces. Obviously, there is a delicate balance between acceleration, speed, and distance that will exactly balance the effects of centrifugal and gravitational forces. The closer to Earth a satellite rotates, the greater the gravitational pull and the greater the velocity Balance in centrifugal force and required to keep it from being pulled to the centripetal force or the Earth. gravity  LOW EARTH ORBIT (LEO)  MEDIUM EARTH ORBIT (MEO)  GEOSYNCHRONOUS EARTH ORBIT (GEO) ©Google Most LEO SATELLITES operate in the 1.0 GHz to 2.5 GHz frequency range The main advantage of LEO satellites is that the path loss between earth stations and space vehicles is much lower than for satellites revolving in medium- or high-altitude orbits. Less path loss equates to lower transmit powers, smaller antennas, and less weight. Advantages  Reduces transmission delay  Eliminates need for bulky receiving equipment Disadvantages  Smaller coverage area  Short life span (5-8 years) than GEO (10 years) ©Google MEO SATELLITES operate in the 1.2 GHz to 1.66 GHz frequency band. Altitude (8000 to 20,000 km) MEO satellite are above the LEO satellite and below the GEO satellite. It is visible for much longer period of time than LEO satellite, usually between 2 to 8 hours. MEO satellite have a large coverage ©Google area than LEO satellite. Disadvantage Due to the larger distance to the earth, delay increase to about 70-80 ms. The satellites need higher transmit power and special antennas for small footprints.  SATELLITE ORBITAL PATTERNS Apogee - the point in an orbit that is located farthest from Earth. Perigee - The point in an orbit that is located closest to Earth. Major Axis - the line joining the perigee and apogee through the center of earth (also called line of apsides). Minor Axis - The line perpendicular to the major axis and halfway between the perigee and apogee (half the distance of the minor axis is called the semiminor axis. ©Google  SATELLITE ORBITAL PATTERNS Equatorial orbit is when the satellite rotates in an orbit directly above the equator usually in circular path. ©Google Inclined orbits are virtually all orbits except those the travel directly above the equator or directly over the North and South poles. ©Google Polar orbit is when the satellite rotates in a path that takes it over the North and South Poles in an orbit perpendicular to the equatorial plane. ©Google  ©Tomasi Satellite orbital patterns  The angle of inclination is the angle between the Earth’s equatorial plane and the orbital plane of a satellite measured counterclockwise at the point in the orbit where it crosses the equatorial plane traveling from south to north. ©Tomasi Angle of Inclination  The point where a polar or inclined orbit crosses the equatorial plane traveling from north to south is called the descending node, and the line joining the ascending and descending nodes through the center of Earth is called the ©Tomasi line of nodes. Ascending Node, Descending Node GEOSYNCHRONOUS EARTH ORBITS (GEO) Altitude (35,786 km) Revolution time(24 hours) ©Google Geosynchronous orbit is a high Earth orbit that allows satellites to match Earth's rotation. Monitoring weather, Broadcast and communication satellites ©Google  Geosynchronous satellites  sometimes called stationary or geostationary satellites;  operating primarily in the 2 GHz to 18 GHz frequency spectrum;  orbits Earth above the equator with the same angular velocity as Earth;  travel at approximately 6,840 mph and complete one revolution of Earth in approximately 24 hours;  Geosynchronous orbits are circular, therefore, the speed of rotation is constant throughout the orbit;  appear to remain in a fixed location above one spot on Earth’s surface;  Geosynchronous satellites  Ideally, geosynchronous satellites should remain stationary above a chosen location over the equator in an equatorial orbit. However, the sun and the moon exert gravitational forces, solar winds sweep past Earth, and Earth is not perfectly spherical.  Ground controllers must periodically adjust satellite positions to counteract these forces. If not, the excursion above and below the equator would build up at a rate of between 0.6˚ and 0.9˚ per year. In addition, geosynchronous satellites in an elliptical orbit also drift in an east or west direction as viewed from Earth. The process of maneuvering a satellite within a preassigned window is called station keeping.  There are several requirements for satellites in geostationary orbits.  The first and most obvious is that geosynchronous satellites must have a 0˚ angle of inclination;  The satellite must also be orbiting in the same direction as Earth’s rotation with the same angular velocity at one revolution per day  Most commercial communications satellite are geosynchronous satellites or geostationary satellites that orbit in a circular pattern with an angular velocity equal to that of earth.  Near Synchronous Orbit – this are satellites in high elevation, non synchronous circular orbit between 19,000 miles and 25,000 miles above..  There is only one geosynchronous earth orbit, however, it is occupied by a large number of satellites. In fact, the geosynchronous orbit is the most widely used earth orbit; ©Tomasi Satellites in geosynchronous earth orbits GEOSYNCHRONOUS SATELLITE ORBITAL VELOCITY The circumference (C) of a geosynchronous orbit is C = 2π(42,164 km) C = 264,790 km Therefore, the velocity (v) of a geosynchronous satellite is v = 264,790 km 24 hr v = 11,033 km/hr v = 6,840 mph  ROUND-TRIP TIME DELAY OF GEOSYNCHRONOUS The round – trip propagation delay between a satellite and an earth station located directly below it is 𝑡 = 𝑑Τ𝑐 2 35,768 𝑘𝑚 𝑡= 5 3 𝑥 10 𝑘𝑚/𝑠 = 238 ms Including the time delay within the earth station and satellite equipment, it takes more than a quarter of a second for an electromagnetic wave to travel from an earth station to satellite and back when the earth station is located at more distant locations, the propagation delay is even more substantial and can be significant with two-way data transmissions.  CLARKE ORBIT A geosynchronous earth orbit is sometimes referred to as the Clarke orbit or Clarke belt, after Arthur C. Clarke, who first suggested its existence in 1945 and proposed its use for communication satellites. The Clarke orbit meets the concise set of specifications for geosynchronous satellite orbit: 1. be located directly above the equator; Arthur C. Clarke 2. travel in the same direction as Earth’s rotation at 6,840 mph; Arthur C. Clarke was an 3. have an altitude of 22,300 miles engineer, a scientist, and above Earth; a science fiction author. 4. complete one revolution in 24 hours. As shown in the picture, three satellites in Clarke orbits separated by 120˚ in longitude can provide communications over the entire globe except the polar region ©Tomasi Three geosynchronous satellite in Clarke orbits  ADVANTAGES AND DISADVANTAGES OF GEOSYNCHRONOUS SATELLITES Advantages: Disadvantages: 1. Does not require expensive a. Require sophisticated and heavy tracking equipment propulsion devices on board 2. Geosynchronous satellites are b. High-altitude geosynchronous available to all earth stations satellites introduce much within their shadow 100% of the propagation delays. time. c. Require higher transmit power 3. There is no need to switch from and more sensitive receivers. one geosynchronous satellite to another as they orbit overhead. d. Require high-precision spacemanship 4. The effects of Doppler shift are negligible.  ANTENNA LOOK ANGLE To optimize the performance of a satellite communications system, the direction of maximum gain of an earth station antenna (sometimes referred to as the boresight) must be pointed directly at the satellite. To ensure that the earth station antenna is aligned, two angles must be determined: the azimuth and the elevation angle. Since a satellite is orbiting many miles above Earth’s surface it has no latitude or longitude. Its location is identified by a point on the surface of earth directly below the satellite called the sub satellite point (SSP). For geosynchronous satellites the SSP must fall on the equator. Sub satellite points and earth station locations are specified using standard latitude and longitude coordinates.  ANTENNA LOOK ANGLE The standard convention specifies angles of longitude between 0˚ and 180˚ either east or west of the Greenwich prime median. Latitudes in the Northern Hemisphere are angles between 0˚ and 90˚N and latitudes in the Southern Hemisphere are angles between 0˚ and 90˚S. Since geosynchronous satellites are located directly above the equator, they all have a 0˚latitude. Hence, geosynchronous satellite locations are normally given in degrees longitude east or west of the Greenwich meridian.  The figure shows the position of a hypothetical geosynchronous satellite vehicle (GSV), its respective SSP, and an arbitrarily selected earth station all relative to Earth’s geocenter. The SSP for the satellite shown in the figure is 30˚E longitude and 0˚ latitude. The earth station has a location of ©Tomasi 30˚W longitude and 20˚N Geosynchronous satellite position, sub satellite point, and latitude. Earth longitude and latitude coordinate system  ANGLE OF ELEVATION Angle of elevation is the vertical angle formed between the direction of travel of an electromagnetic wave radiated from an earth station antenna pointing directly toward a satellite and the horizontal plane. The smaller the angle of elevation, the greater the distance a propagated wave must pass through Earth’s atmosphere. Generally, 5° is considered as the minimum acceptable angle of elevation. The figure shows how the angle of elevation affects the signal strength of a propagated electromagnetic wave due to normal atmospheric absorption, absorption due to thick fog, and absorption due to heavy rainfall. The figure also shows that at elevation angles less than 5°, the amount of signal power lost increases significantly.  ©Tomasi Attenuation due to atmospheric absorption a) 6/4 – GHz band b) 14/12 – GHz band  AZIMUTH ANGLE Azimuth is the horizontal angular distance from a reference direction, either the southern or northern most point of the horizon. Azimuth angle is defined as the horizontal pointing angle of an earth station antenna. For satellite earth stations in the Northern Hemisphere and satellite vehicles in geosynchronous orbits, azimuth angle is generally referenced to true south (i.e. 180°). Angle of elevation and azimuth angle both depend on the latitude of the earth station and the longitude of both the earth station and the orbiting satellite.  ©Tomasi ©Tomasi ©Tomasi A zi m u t h a n d A n g l e o f E l e va t i o n “Lookangles” For a geosynchronous satellite in an equatorial orbit, the procedure for determining angle of elevation and azimuth angle is as follows: a) From a good map, determine the longitude and latitude of the earth station. b) Determine the longitude of the satellite of interest (refer to Table 1 or Table 25 – 1). c) Calculate the difference, in degrees (∆L), between the longitude of the satellite and the longitude of the earth station. d) Determine the azimuth angle (from Figure 12 or Figure 25 – 12) and the angle of elevation angle (from Figure 13 or Figure 25 – 13). Note: The Table and Figures mentioned above are from Tomasi.  Table 1. Longitud i n a l Pos itio n of Several Cur r ent Sync hr o no us Satell i tes Par k ed in an Equator i a l Ar c 0 𝑜 𝑙𝑎𝑡𝑖 𝑡𝑢 𝑑𝑒  LIMITS OF VISIBILITY For an earth station in any given location, the Earth’s curvature establishes the limits of visibility (i.e., line-of-sight limits), which determine the farthest satellite away that can be seen looking east or west of the earth station’s longitude. Theoretically, the maximum line- of sight distance is achieved when the earth station’s antenna is pointing along the horizontal (zero elevation angle) plane. In practice, however, the noise picked up from Earth and the signal attenuation from Earth’s atmosphere at zero elevation angle is excessive. Therefore, an elevation angle of 5° is generally accepted as being the minimum usable elevation angle. The limits of visibility depend in part on the antenna’s elevation and the earth station’s longitude and latitude. STUDENT ACTIVITY Please answer the questions below and send to Google classroom in “pdf” or “jpg” format. File name format is “subjectcode_section_lastname_firstname”. 1. An earth station is located at Houston, Texas that has a longitude of 99.50 and a latitude of 29.50. The satellite of interest is Satcom V. Determine the look angle for the earth station antenna.