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Jhansirani Assoct.Prof ECE DeptUnit I – Satellite Orbits : Unit I – Satellite Orbits Kepler’s Laws Newton’s law Orbital parameters Orbital perturbations Station keeping Geo stationary and non Geo-stationary orbits Look Angle Determination Limits of visibility Eclipse-Sub satellite point Sun transit outage Launching Procedures Launch vehicles and propulsion . R. JHANSI RANI Assoct.prof/ECEWhat is a satellite? : What is a satellite? A Satellite is an object that goes around, or orbits, a larger object, such as a planet. While there are natural satellites, like the Moon, hundreds of man-made satellites also orbit the Earth. (Or) A satellite is a man-made object launched into space to orbit the Earth, moon, sun or other celestial body. Some examples are weather satellites and communications satellites. R. JHANSI RANI Assoct.prof/ECEHistory : History The first artificial satellite was the soviet Sputnik-1 , launched on October 4, 1957 , and equipped with an on-board transmitter that worked on two frequencies, 20.005 and 40.002 MHz The first American satellite to relay communications was project SCORE in 1958 , which used a tape recorder to store and forward voice messages. Telstar was the first active, direct relay communications satellite. Belonging to AT&T. R. JHANSI RANI Assoct.prof/ECESatellite communication : Satellite communication In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. So, all the signal transferring is happening in space. Thus this type of communication is known as space/satellite communication. R. JHANSI RANI Assoct.prof/ECETypes of satellite : Types of satellite R. JHANSI RANI Assoct.prof/ECEOrbit : Orbit Satellites move in a path around the Earth called an orbit. The orbit is a combination of the satellite's velocity - the speed it is travelling in a straight line - and the force of the Earth's gravitational pull on the satellite. These forces are similar to the forces that keep all the planets in their places in the solar system R. JHANSI RANI Assoct.prof/ECEClassification of orbits: Classification of orbits R. JHANSI RANI Assoct.prof/ECETypes of Orbits : Types of Orbits Geosynchronous Orbits (GEO) Also known as geostationary orbits, satellites in these orbits circle the Earth at the same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, this would put the satellite at approximately 35,790 km (22,236 mi) above the Earth. The satellites are located near the equator since at this latitude, there is a constant force of gravity from all directions. At other latitudes, the bulge at the center of the Earth would pull on the satellite. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Geosynchronous orbits allow the satellite to observe almost a full hemisphere of the Earth. These satellites are used to study large scale phenomenon such as hurricanes, or cyclones. These orbits are also used for communication satellites. The disadvantage of this type of orbit is that since these satellites are very far away, they have poor resolution. The other disadvantage is that these satellites have trouble monitoring activities near the poles. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Altitude classifications Low Earth Orbit (LEO) : Geocentric orbits ranging in altitude from 0–2000 km (0–1240 miles) Medium Earth Orbit (MEO) : Geocentric orbits ranging in altitude from 2,000 km (1,200 mi) to just below geosynchronous orbit. Also known as an intermediate circular orbit. High Earth Orbit (HEO) : Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,236 mi). R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEAdvantages of satellite communications: Advantages of satellite communications 1. Mobile/Wireless Communication, independent of location 2. Wide area coverage: country, continent, or globe 3. Wide bandwidth available throughout 4. Independence from terrestrial infrastructure 5. Rapid installation of ground network 6. Low cost per added site 7. Uniform service characteristics 8. Total service from a single provider 9. Small Fading margin (3dB) 10. Wide range of applications R. JHANSI RANI Assoct.prof/ECEDisadvantages of satellite communications: Disadvantages of satellite communications 1. High cost for satellite 2. Short life time maximum of 15 years 3. Redundancy in component 4. Noise and interference 5. Propagation delay R. JHANSI RANI Assoct.prof/ECEApplications : Applications Fixed-Satellite Services (FSS) Broadcasting satellite services (BSS)/ Direct-broadcast satellites (DBS) or Direct-to-home (DTH) Mobile satellite services Navigational satellite services Meteorological satellite services Broadband Digital Communications Environmental Monitoring R. JHANSI RANI Assoct.prof/ECEFrequency allocations for satellite services: Frequency allocations for satellite services R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Kepler’s laws R. JHANSI RANI Assoct.prof/ECEKepler’s 1st law: Kepler’s 1 st law The Law of Ellipses The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Kepler’s First Law Kepler’s first law states that the path followed by a satellite around the primary will be an ellipse. An ellipse has two focal points shown as F 1 and F 2. The center of mass of the two-body system, termed the barycenter, is always centered on one of the foci. The semimajor axis of the ellipse is denoted by a , and the semiminor axis, by b . The eccentricity e is given by For an el1iptical orbit, 0<e<1. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Kepler’s Second Law Kepler’s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter. Referring to Fig. assuming the satellite travels distances S 1 and S 2 meters in 1 s, then the areas A 1 and A 2 will be equal. Velocity at S2 is less than S1 R. JHANSI RANI Assoct.prof/ECEKepler’s 2nd law: Kepler’s 2 nd law The Law of Equal Areas : An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. R. JHANSI RANI Assoct.prof/ECEKepler’s 2nd law : Kepler’s 2 nd law R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Kepler’s Third Law Kepler’s third law states that the square of the periodic time of orbit is proportional to the cube of the mean distance between the two bodies. The mean distance is equal to the semimajor axis a . For the artificial satellites orbiting the earth, Kepler’s third law can be written in the form where n is the mean motion of the satellite in radians per second and µ is the earth’s geocentric gravitational constant. With n in radians per second, the orbital period in seconds is given by R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Example: Two planets are shown on orbits that are the same shape, but the larger orbit is 1.5874 times bigger than the smaller. Notice that the planet on the larger orbit takes twice as long to go around the star. This is because R 1 3 /R 2 3 = 1.5874 3 = 4 = 2 2 = P 1 2 /P 2 2 , as required by Kepler's Third Law . The Law of Harmonies: The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Born in England, Isaac Newton was a highly influential Physicist Astronomer Mathematician Philosopher Alchemist theologian. SIR ISAAC NEWTON R. JHANSI RANI Assoct.prof/ECENewton's Law of Motion : Newton's Law of Motion Newton's First Law (Law of Inertia): Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. (or) An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force . R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Newton's Second Law: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Newton's third law : For each and every action, there is an equal and opposite reaction. R. JHANSI RANI Assoct.prof/ECENewton's law of universal gravitation: Newton's law of universal gravitation It states that every particle in the universe exerts a force on every other particle along the line joining their centers. The magnitude of the force is directly proportional to the product of the masses of the two particles, and inversely proportional to the square of the distances between them . where m1 & m2 are the masses of the two particles r is the distance between the two masses F is the gravitational force between them G is the universal gravitational constant. G = 6.673 x 10 -11 N m 2 /kg 2 R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Definitions of Terms for Earth-Orbiting Satellites As mentioned previously, Kepler’s laws apply in general to satellite motion around a primary body. For the particular case of earth-orbiting satellites, certain terms are used to describe the position of the orbit with respect to the earth. Sub satellite path : This is the path traced out on the earth’s surface directly below the satellite. Apogee : The point farthest from earth. Perigee : The point of closest approach to earth. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: 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 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. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEAngle of inclination : Angle of inclination R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: An orbit in which the satellite moves in the same direction as the earth’s rotation. It is also known as a direct orbit. The inclination of a prograde orbit always lies between 0° and 90°. Most satellites are launched in a prograde orbit because the earth’s rotational velocity provides part of the orbital velocity with a consequent saving in launch energy. Prograde orbit R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Retrograde orbit: An orbit in which the satellite moves in a direction counter to the earth’s rotation. The inclination of a retrograde orbit always lies between 90° and 180°. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Argument of Perigee (Perihelion) : ( ω ) The angle between the ascending node and perigee, measured in the orbital plane at the earths center, in the direction of satellite motion. Vernal equinox (Spring Equinox/First point of Aries): Vernal equinox occurs when the sun crosses the equator going from south to north. It is nothing more than the ascending node of the Sun's orbit. Line of Aries( γ ): Imaginary line drawn from this equatorial crossing through the center of the sun points to the first point of Aries. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Right Ascension of the Ascending Node ( Ω): Another term for Longitude of the Ascending Node , It is the angle measured Eastward in the equatorial plane, from the γ line to the ascending node. (Or) The angle measured in the equatorial plane from a reference point in the sky where right ascension is defined to be zero. Astronomers call this point the vernal equinox . “ Right ascension of Ascending node " is an angle, measured at the center of the earth, from the vernal equinox to the ascending node. RAAN is a number in the range 0 to 360 degrees. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Mean anomaly(M): Mean anomaly gives an average value of the angular position of the satellite in its orbit with reference to the perigee, For a circular orbit. If the satellite were at the perigee, the mean anomaly would be 0. True anomaly: True anomaly is the angle measured in the direction of motion from perigee to the satellite's position, measured at the earth’s center. This gives the true angular position of the satellite as a function of time. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Orbital Elements Earth-orbiting artificial satellites are defined by six orbital elements referred to as the keplerian element set . Semi-major axis ( a): Fixes the size of orbit Eccentricity ( e): Give the shape of the ellipse. Mean anomaly (M0): Gives the position of the satellite in its orbit at a reference time known as the epoch. Argument of perigee (w): Gives the rotation of the orbit’s perigee point relative to the orbit’s line of nodes in the earth’s equatorial plane. Inclination (i): Fixes the plane’s position. Right Ascension of the Ascending Node ( Ω ): Relates the orbital planes position to the earth. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Apogee and Perigee Heights Although not specified as orbital elements, the apogee height and perigee height are often required, the length of the radius vectors at apogee and perigee can be obtained from the geometry of the ellipse: ha = r a – R hp = r p – R R. JHANSI RANI Assoct.prof/ECEOrbit Perturbations: Orbit Perturbations perturbations of the orbit are the results of various forces which are exerted on the satellite other than the forces of attraction of the central, spherical and homogeneous body. These forces mainly consist of Non – spherical nature of earth The attraction of other bodies like sun and moon Solar radiation pressure Atmospheric drag R. JHANSI RANI Assoct.prof/ECEEffects of a non - spherical earth : Effects of a non - spherical earth As the shape of Earth is not a perfect sphere, it causes some variations in the path followed by the satellites around the primary. As the Earth is bulging from the equatorial belt, and keeping in mind that an orbit is not a physical entity, and it is the forces resulting from an oblate Earth which act on the satellite produce a change in the orbital parameters. This causes the satellite to drift as a result of regression of the nodes and the latitude of the point of perigee (point closest to the Earth). This leads to rotation of the line of apsides. As the orbit itself is moving with respect to the Earth, the resultant changes are seen in the values of argument of perigee and right ascension of ascending node. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Due to the non-spherical shape of Earth, one more effect called as the “Satellite Graveyard” is seen. The non-spherical shape leads to the small value of eccentricity (10-5 ) at the equatorial plane. This causes a gravity gradient on GEO satellite and makes them drift to one of the two stable points which coincide with minor axis of the equatorial ellipse. Working satellites are made to drift back to their position but out-of-service satellites are eventually drifted to these points, and making that point a Satellite Graveyard. R. JHANSI RANI Assoct.prof/ECE: Effects of a non - spherical earth For a spherical earth of uniform mass, Kepler’s third law gives the nominal mean motion n 0 as R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEAtmospheric drag : Atmospheric drag An approximate expression for the change of major axis is where the “0” subscripts denote values at the reference time t 0, and n 0 is the first derivative of the mean motion. The mean anomaly is also changed, an approximate value for the change being: R. JHANSI RANI Assoct.prof/ECEStation keeping : Station keeping Even with a very good launch the satellite can drift some what from its orbit. This is called “ orbital drift ” The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from Sun/Moon will in general perturb the orbital plane. The process of firing the rockets under ground control to maintain or adjust the orbit is referred to as “ Station Keeping" R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Station keeping for drift due to ellipticity of earth: The equatorial ellipticity of the earth causes geostationary satellite to drift to one of the two stable points, at 75° E & 105 ° W To counter this drift, an oppositely directed velocity component is imparted to the satellite by means of jets, which are pulsed once every 2 to 3 weeks. This results in the satellite drifting back through, its nominal station position, coming to a stop and recommencing the drift along the orbit until the jets are pulsed once again. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: These maneuvers are termed east-west station keeping maneuvers. Satellites in the 6/4 GHz band must be kept within ±0.1 ° of the designated longitude, and in the 14/12 GHz band, within ±0.05 °. Station keeping for drift due to gravitational pull: The forces due to sun and moon causes the inclination to change at a rate of about 0.85 °/ year To prevent the shift in inclination from exceeding specified limits, jets may be pulsed at the appropriate time to return the inclination to zero. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Counteracting jets must be placed when the inclination is at zero to halt the change in inclination. These maneuvers are termed North-South station keeping maneuvers . They are more expensive in fuel than east-west station keeping maneuvers. North-South station keeping tolerance are the same as those for east-west station keeping ± 0.1° in the C band and ± 0.05° in Ku band. Orbital correction is carried out by command from the TT&C earth station, which monitors the satellite position. east-west and North-South station keeping maneuvers are usually carried out using the same thrusters as are used for attitude control. R. JHANSI RANI Assoct.prof/ECEGeostationary orbit : Geostationary orbit Satellite in a geostationary orbit appears to be stationary with respect to the earth, hence the name geostationary . Three conditions are required for an orbit to be geostationary : 1 . The satellite must travel eastward at the same rotational speed as the earth. 2 . The orbit must be circular. 3 . The inclination of the orbit must be zero . (orbit lies in earth’s equatorial plane) R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Kepler's 3 rd law is used to find the radius of the orbit R. JHANSI RANI Assoct.prof/ECEAntenna Look Angels : Antenna Look Angels The position of the satellite, as measured from the earth station, is usually given in terms of the azimuth and elevation angles and the range d. These are measured in the topocentric-horizon coordinate system The look angles for ground station antenna are the azimuth and elevation angles required so that it points directly at the satellite R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: In case of elliptical orbit , these angles had to change in order to track the satellite. In geostationary orbit , it is simple because the satellite is stationary with respect to the earth. In general no tracking should be necessary, with the large earth stations used for commercial communications. For the antenna beam width to be very narrow, tracking mechanism is required to compensate For the movement of the satellite about the nominal geostationary position For home reception, the antenna beam width is quite broad, and no tracking is necessary. This allows the antennas to be fixed in position. Eg : antennas used for reception of satellite TV R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: The 3 information's required to determine the look angles for the geostationary orbit are, The earth station latitude, denoted here by λ E The earth station longitude, denoted here by Φ E The longitude of the sub satellite point, denoted here by Φss (satellite longitude) Latitude north will be taken as positive angles Latitude south will be taken as negative angles Longitude east of the Greenwich meridian will be taken as positive angles and longitude west , as negative angles. Eg: Latitude of 40°S is taken as -40° Longitude of 35°W is taken as -35° R. JHANSI RANI Assoct.prof/ECE Latitude and Longitude : Latitude and Longitude R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: When calculating look angles for LEO satellites, variation in earths radius is taken into account For GEO, this variation is negligible effect on the look angles, and the average radius of earth used is R = 6371Km Geometry involving these quantities ES : position of earth station SS : sub satellite point S: satellite d: range from the earth station to the satellite σ: angle to be determined (look angle) R. JHANSI RANI Assoct.prof/ECEGeometry in determining look angles: Geometry in determining look angles R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Two types of triangle involved in the geometry spherical triangle (N, ES, Φ ss ) / (a,b,c) plane triangle (ES, a E : center of earth, S) R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Spherical triangle: These are sides of all arcs of great circles, and these sides are defined by the angles subtended by them at the center of the earth. a: angle between the radius to north pole N and radius to sub satellite point Φ ss , a= 90°, spherical triangle in which one side is 90° is called a quadrantal triangle . b: angle between the radius to the earth station ES and the radius to the sub-satellite point Φ ss c: angle between the radius to the earth station ES and the north pole N . c= 90° - λ E R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: 3 angles A, B, C are the angles between the planes A: Angle between the plane containing c and b B: Angle between the plane containing c and a, B = Φ E - Φ ss and B max = 81.3° C: Angle between the plane containing b and a R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Summary of spherical triangle to this point: a= 90° c= 90° - λ E B = Φ E - Φ ss Note: When ES is west of sub satellite point, B is (–)ve and when east, B is (+)ve. When ES latitude is north, c<90° and when south, c>90° By, Napier’s rules R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Two values will satisfy the above equation, A and 180° – A and is determined by inspection. A is acute (<90°), and the azimuth angle is A z = A A is acute, and the azimuth is, by inspection, A z =360° - A A c is obtuse and is given by A c = 180° - A where A is the acute value obtained from above equation, by inspection, A z = A c – 180° - A A d is obtuse and is given by A d = 180° - A where A is the acute value obtained from above equation, by inspection, A z = 360° - A d = 180° + A R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: plane triangle Applying the cosine rule for plane triangles to the triangle allows the range d to be found to a close approximation Applying the sine rule for plane triangles to the triangle allows the angle of elevation to be found to R. JHANSI RANI Assoct.prof/ECELimits of Visibility: Limits of Visibility Any geostationary satellite has an arc of visibility which can also be called area of coverage or foot print. This depends upon the height of satellite , elevation angle and area of coverage. There will be east and west limits on the geostationary arc visible from any given earth station. The limits will be set by the geographic coordinates of the earth station and the antenna elevation. The lowest elevation in theory is zero, when the antenna is pointing along the horizontal. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: A quick estimate of the longitudinal limits can be made by considering an earth station at the equator, with the antenna pointing either west or east along the horizontal, as shown in Fig. The limiting angle is given by R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Thus, for this situation, an earth station could see satellites over a geostationary arc bounded by ±81.3° about the earth station longitude. In practice, to avoid reception of excessive noise from the earth, some finite minimum value of elevation is used. Typically El min = 5° The limits of visibility will also depend on the earth-station latitude. let S represent the angle subtended at the satellite when the angle R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Applying the sine rule gives Assuming a spherical earth of mean radius 6371 km as was done previously. Once angle S is known, angle b is found from From equation, Once angle B is found, satellite longitude can be determined from . R. JHANSI RANI Assoct.prof/ECEEarth Eclipse of Satellite : Earth Eclipse of Satellite It occurs when earth’s equatorial plane coincides with the plane of the earths orbit around the sun (the ecliptic plane) near the time of spring and autumnal equinoxes. When the sun is crossing the equator, the satellite passes into earth’s shadow at certain period (10 min’s to 72 min’s) Geostationary satellites would be eclipsed by the earth once each day As, equatorial plane is tilted at an angle of 23.4° to the ecliptic plane, keeps the satellite in full view of the sun for most days of the year. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEEarth Eclipse of Satellite : Earth Eclipse of Satellite R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEEarth Eclipse of Satellite : Around the spring and autumnal equinoxes, when the sun is crossing the equator, the satellite passes into earth’s shadow at certain periods, these being period of eclipse. Spring equinox is the first day of spring Autumnal equinox is the first day of autumn Eclipse being 23 days before equinox and end 23 days after equinox. Earth Eclipse of Satellite R. JHANSI RANI Assoct.prof/ECECauses of eclipse: Causes of eclipse During full eclipse, satellite receives no power from solar array and relays only on batteries. Reduction in primary power reduces satellite life During eclipse it is necessary to shut down some transponders, and results in reduction of satellite performance. Solar fluctuation may cause failure of primary power system Severe thermal stress on a satellite. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: When the satellite longitude is east of ES, the satellite enters eclipse during daylight, this is undesirable if the satellite has to operate on reduced battery power. When the satellite longitude is west of ES, the eclipse does not occur until the earth station is in darkness, when the usage is low. Hence satellite west of ES is more desirable Probability of satellite failure is more during eclipse than at any other time. R. JHANSI RANI Assoct.prof/ECESub satellite point : Sub satellite point The point on the earth vertically under the satellite is referred to as sub satellite point. The latitude and longitude of the sub-satellite point and height of the satellite above the sub-satellite point can be determined from the radius vector r. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: The height of the terrain above the reference ellipsoid at the sub-satellite point is denoted by H ss The height of the satellite above this, by h ss . Thus total height above the reference ellipsoid is R. JHANSI RANI Assoct.prof/ECESun transit outage: Sun transit outage Another event which occurs during equinoxes is the transit of the satellite between earth and sun. Sun which is a powerful broadband microwave noise source passes directly behind the satellite (when viewed from Earth) and the receiver with the beam directed towards the satellite picks up both the satellite signal and the noise from the Sun which completely blanks out the signal from satellite. This effect is termed as sun transit outage. It lasts for short periods-each day for about 6 days around the equinoxes. Maximum outage time of 10 min being typical. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: The occurrence and duration of the sun transit outage depends on The latitude of the earth station Receiver location Location of the particular satellite Size, or more specifically the beam width of the antenna The apparent radius of the Sun as seen from the earth (about 0.25°). Accuracy of alignment of the antenna direction towards the satellite R. JHANSI RANI Assoct.prof/ECELaunching the satellite : Launching the satellite Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to perform their function. Built-in rockets called thrusters make these adjustments. Once a satellite is placed into a stable orbit, it can remain there for a long time without further adjustment. R. JHANSI RANI Assoct.prof/ECEHow does a satellite stay in it’s orbit?: How does a satellite stay in it’s orbit? R. JHANSI RANI Assoct.prof/ECELaunching of geostationary satellite: Launching of geostationary satellite Initially place spacecraft with the final rocket stage into LEO. After a couple of orbits, during which the orbital parameters are measured, the final stage is reignited and the spacecraft is launched into a geostationary transfer orbit( GTO ). Perigee of GTO is that of LEO altitude and apogee that of GEO altitude. After a few orbits in GTO, while the orbital parameters are measured, a rocket motor ( AKM ) is ignited at apogee and GTO is raised until it is circular geostationary orbit. AKM (Apogee Kick Motor) is used to circularize the orbit at GEO and to remove any inclination error so that the final orbit is very close to geostationary . R. JHANSI RANI Assoct.prof/ECEGeostationary Transfer Orbit: Geostationary Transfer Orbit BUT, if we fire a rocket motor when the satellite's at apogee, and speed it up to the required circular orbit speed, it will stay at that altitude in circular orbit. Firing a rocket motor at apogee is called "apogee kick", and the motor is called the " apogee kick motor". If we speed the satellite up while it's in low circular earth orbit it will go into elliptical orbit, heading up to apogee . If we do nothing else, it will stay in this elliptical orbit, going from apogee to perigee and back again. R. JHANSI RANI Assoct.prof/ECEORBITAL MANEUVERS Hohmann Transfer: ORBITAL MANEUVERS Hohmann Transfer Can be used to raise or lower altitude Most efficient method At minimum, requires completion of half revolution of transfer orbit R. JHANSI RANI Assoct.prof/ECEHohmann transfer: Hohmann transfer Most satellites launched today are initially placed into an low earth orbit . In the next phase the satellite is injected into an elliptical transfer orbit which has an apogee at the height of GEO and its apsides (line joining perigee-apogee) in the equatorial plane . Finally satellite is injected into GEO by imparting a velocity increment at the apogee equal to the difference between satellite velocity at GTO and velocity in GEO . A transfer between two coplanar circular orbits via elliptical transfer orbit requires the least velocity increment (and hence fuel ). This principle was recognized by Hohmann in 1925 and is referred as Hohmann transfer . R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: A Hohmann transfer is a fuel efficient way to transfer from one circular orbit to another circular orbit that is in the same plane (same inclination), but a different altitude . To change from a lower orbit (A) to a higher orbit (C), an engine is first fired in the opposite direction from the direction the vehicle is traveling. This will add velocity to the vehicle causing its trajectory to become an elliptic orbit (B). This elliptic orbit is carefully designed to reach the desired final altitude of the higher orbit (C ). In this way the elliptic orbit or transfer orbit is tangent to both the original orbit (A) and the final orbit (C). This is why a Hohmann transfer is fuel efficient. When the target altitude is reached the engine is fired in the same manner as before but this time the added velocity is planned such that the elliptic transfer orbit is circularized at the new altitude of orbit (C). R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Target Orbit Initial Orbit Transfer Orbit The orbital inclination is given by, cos i= sin ξ 1 cos θ 1 where i=inclination ξ 1 =azimuth of launch θ 1 =latitude of launching site R. JHANSI RANI Assoct.prof/ECELaunch Vehicles: Launch Vehicles Saturn V launch vehicle sends Apollo 15 on its way to the moon. A Russian Soyuz lifts off from the Baikonur Cosmodrome in Kazakhstan heading for the ISS Ukrainian launch Vehicle Zenit-2 is prepared for launch R. JHANSI RANI Assoct.prof/ECELaunch vehicles: Launch vehicles A launch vehicle is a rocket used to carry a payload from the earth station into outer space. Types of launch vehicles: 1. Expendable launch vehicle (ELV) 2. Reusable launch vehicles (RLV) Expendable Launcher: They are designed for one time use. They usually separate from their payload and crash back to earth. R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Payloads are government and commercial communication satellites, weather satellites, remote sensing satellites etc., ELV is made of one or more rocket stages After each stage has burned its propellant, it is jettisoned from the vehicle. R. JHANSI RANI Assoct.prof/ECE Eg: US Atlas-centaur & Delta rockets European space agency Airane rockets.PowerPoint Presentation: R. JHANSI RANI Assoct.prof/ECEPowerPoint Presentation: Reusable launcher They are designed to be recovered intact and used again for launches. Space shuttle also called the space transportation system (STS) are reusable The solid rocket boosters are recovered and refurbished for future mission and the shuttle vehicle itself is flown back to earth for reuse There are single stage to orbit (SSTO) launch and two stage to orbit (TSTO) launch. R. JHANSI RANI Assoct.prof/ECEPSLV: PSLV Polar satellite launch vehicle It is an expendable launch vehicle developed by ISRO It is used for remote sensing R. JHANSI RANI Assoct.prof/ECE PSLV-C8 (CA Variant) carrying the AGILE x-ray and γ-ray astronomical satellite of the ASI lifting off from Sriharikota You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.