orbital perturbations, Schemes and Mind Maps of Telecommunication electronics

Download orbital perturbations and more Schemes and Mind Maps Telecommunication electronics in PDF only on Docsity! ORBITAL PERTURBATIONS Theoretically, an orbit described by Kepler is ideal as Earth is considered to be a perfect sphere and the force acting around the Earth is the centrifugal force. This force is supposed to balance the gravitational pull of the earth. In reality, other forces also play an important role and affect the motion of the satellite. These forces are the gravitational forces of Sun and Moon along with the atmospheric drag. Effect of Sun and Moon is more pronounced on geostationary earth satellites where as the atmospheric drag effect is more pronounced for low earth orbit satellites. 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. 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 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. Atmospheric Drag  For Low Earth orbiting satellites, the effect of atmospheric drag is more pronounces. The impact of this drag is maximum at the point of perigee. Drag (pull towards the Earth) has an effect on velocity of Satellite (velocity reduces). This causes the satellite to not reach the apogee height successive revolutions. This leads to a change in value of semi-major axis and eccentricity. Satellites in service are maneuvered by the earth station back to their original orbital position. ORBIT DETERMINATION Orbit determination requires that sufficient measurements be made to determine uniquely the six orbital elements needed to calculate the future of the satellite, and hence calculate the required changes that need to be made to the orbit to keep it within the nominal orbital location. The control earth stations used to measure the angular position of the satellites also carryout range measurements using unique time stamps in the telemetry stream or communication carrier. These earth stations generally referred to as the TTC&M(telemetry tracking command and monitoring) stations of the satellite network. LAUNCHES AND LAUNCH VEHICLES A satellite cannot be placed into a stable orbit unless two parameters that are uniquely coupled together the velocity vector and the orbital height are simultaneously correct. There is little point in orbiting the correct height and not having the appropriate velocity component in the correct direction to achieve the desired orbit. A geostationary satellite for example must be in an orbit at height 35,786.03km above the surface of the earth with an inclination of zero degrees an ellipticity of zero, and a velocity of 3074.7m/s tangential to the earth in the plane of the orbit, which is the earths equatorial plane. The further out from the earth the orbit is greater the energy required from the launch vehicle to reach that orbit. In any earth satellite launch, the largest fraction of the energy expanded by the rocket is used to accelerate the vehicle from rest until it is about 20miles (32 km) above the earth. To make the most The rocket injects the satellite with the required thrust** into the transfer orbit. With the STS, the satellite carries a perigee kick motor*** which imparts the required thrust to inject the satellite in its transfer orbit. Similarly, an apogee kick motor (AKM) is used to inject the satellite in its destination orbit. Generally it takes 1-2 months for the satellite to become fully functional. The Earth Station performs the Telemetry Tracking and Command**** function to control the satellite transits and functionalities. (**Thrust: It is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system.) (***Kick Motor refers to a rocket motor that is regularly employed on artificial satellites destined for a geostationary orbit. As the vast majority of geostationary satellite launches are carried out from spaceports at a significant distance away from Earth's equator, the carrier rocket would only be able to launch the satellite into an elliptical orbit of maximum apogee 35,784-kilometres and with a non- zero inclination approximately equal to the latitude of the launch site.) (****TT&C: it‟s a sub-system where the functions performed by the satellite control network to maintain health and status, measure specific mission parameters and processing over time a sequence of these measurement to refine parameter knowledge, and transmit mission commands to the satellite. Detailed study of TT&C in the upcoming units.) It is better to launch rockets closer to the equator because the Earth rotates at a greater speed here than that at either pole. This extra speed at the equator means a rocket needs less thrust (and therefore less fuel) to launch into orbit. In addition, launching at the equator provides an additional 1,036 mph (1,667 km/h) of speed once the vehicle reaches orbit. This speed bonus means the vehicle needs less fuel, and that freed space can be used to carry more pay load. a Taman u Tramvtes =~" fT Om ese ; 2 ~ a) D corse ei So Dasacem hy @ wapnen mem Gh \ reaviencation T + 30 \ (18) it esit weceity Sen) Gee @) Fina ero = £-$59.0-6- (ene Sete ih cst a enue Sine Flrorlamation tar grit bat vality soerection + Figure : Launching stages of a GEO (example INTELSAT)