Project 3 Charles Hunter AME 30381 03/06/2012
Introduction The goal of this project is to determine which attitude control method is best for various satellite mission types. Significant perturbative effects for each orbital altitude are researched along with common satellite mission types. The assumptions include:
The Earth is oblate
The information to be determined is:
Orbital perturbations in LEO, MEO, and GEO/HEO Mission types in LEO, MEO, and GEO/HEO Attitude control methods for mission types Attitude control type for the company to invest in
Outside information required:
Information on LEO, MEO, and GEO/HEO perturbations affecting satellites Information on mission types in LEO, MEO, and GEO/HEO Information on attitude control methods
Procedure Task 1: Orbital Altitude Perturbative Effects Significant Perturbations in LEO:
Atmospheric Drag Earth Gravity Effects
In LEO atmospheric drag and Earth gravity effects (J2 Effects) are the most significant perturbations on satellite orbits. Satellites in LEO are less than 2000 km from the Earth which leads to the effect of other bodies such as the Sun and moon neglected from significant perturbations, but is the reason why the J2 Effects and atmospheric drag are significant. The oblateness of the Earth causes the J2 Effects which alter the acceleration of the satellite along its orbit. The Earth gravity effects alter the semi-major axis, inclination, and eccentricity of the satellite’s orbit. It is necessary to account for this effect especially if the satellite is meant to pass over locations at the same time every day. The satellite experiences atmospheric drag due to the gases, oxygen, nitrogen, helium, etc., which are significant in the Earth’s atmosphere below 1000 km. The density of the gas-filled atmosphere coincides with how much drag the satellite will experience. This is a significant perturbation because it will decrease the acceleration of the satellite along the orbital path and if not overcome may cause the satellite to fall out of orbit by lowering the semi-major axis and decreasing the eccentricity of the orbit.
Significant Perturbations in MEO:
Solar Radiation Pressure Third Bodies
In MEO solar radiation pressure and third bodies are the most significant perturbations on satellite orbits. Solar radiation pressure affects the velocity of the satellites. Electromagnetic radiation exerts a pressure force on the satellite which is proportional to its area to mass ratio. This is significant because it changes the orbit’s eccentricity, perigee, and apogee which may lead to the satellite crashing down to Earth. The most significant third body effects on satellites are caused by the Sun and the moon. The large planetary bodies add a gravitational pull on the satellite. This will affect the satellite along its orbital path especially in areas when the satellite is at locations along the orbit nearest to the moon or Sun. This is significant because the force provided by the Sun and moon may accelerate the satellite farther and farther away from the Earth. Significant Perturbations in GEO/HEO:
Solar Radiation Pressure Third Bodies
In GEO/HEO solar radiation pressure and third bodies are the most significant perturbations on satellite orbits. These perturbations are significant for the same reasons mentioned for MEO orbits; however, GEO/HEO satellites are even more affected by the perturbations. See Also Appendix Sources 1-3 Task 2: Most Common Satellite Mission Types Common Mission Types in LEO:
Cartography Reconnaissance/Observation Meteorological/Environmental Monitoring Science
[4],[6],[7] Common Mission Types in MEO:
Navigation Geodynamics
[6],[7] Common Mission Types in GEO/HEO:
Communication/Broadcast Atmospheric Sensing/Meteorological
[5],[6],[7]
Task 3: Attitude Control Methods for Mission Types There are several varying methods which satellites use in order to take the perturbations from earlier into account. Current attitude control methods for satellites in each mission type are researched to help determine which attitude method is best to invest in. The CARTOSAT-1 and CARTOSAT-2 satellites are intended for cartographic applications in LEO. The CARTOSAT-1 satellite was used to capture stereo images of the Earth while in orbit and the CARTOSAT-2 satellite is used for remote sensing. Both of these satellites use high torque reaction wheels, magnetic torquers, and thrusters for three-axis attitude control. See Also Appendix Sources 9 and 10 The meteorological satellite CloudSat studies the formation of clouds and aerosols in the atmosphere in LEO. The CloudSat satellite uses a chemical blow-down monopropellant system for attitude control. This system allows for three-axis control of the satellite during its mission. See Also Appendix Source 8 The satellite CBERS-4 is used to monitor the environment and study the Earth resources in LEO. This satellite utilizes attitude control software which uses GPS and Star Sensor information for stabilization. See Also Appendix Source 11 GOSAT is an observation satellite in LEO used to observe greenhouse gases around the Earth. The satellite attitude control system consists of monopropellant thrusters. Similar to CloudSat these allow for three-axis control of the satellite. See Also Appendix Source 12 GRACE is a science satellite in LEO used to measure gravity with high precision for gravity field models. The attitude control system in GRACE consists of cold gas propulsion with thrusters which are supported by magnetic torque rods. See Also Appendix Source 13 The GLONASS set of satellites in MEO are used to provide global navigation positioning. The attitude control system on the set of satellites includes momentum wheels and Sun and Earth sensors for stabilization. See Also Appendix Source 14 The LAGEOS 2 satellite in MEO is used to study the geodynamics and crustal motion of the Earth. This satellite was designed to have a spherical body and to minimize the effects of the Earth’s magnetic field in MEO. This satellite has no sensors and was not attitude controlled because the spherical body was covered in retroreflectors and thus could perform its mission no matter what its attitude. See Also Appendix Source 15 The Japanese BSE satellite was used to provide direct television broadcasting to Japan. This satellite was placed into a GEO orbit. The satellite utilized a three-axis zero-momentum attitude control system. This system consists of reaction wheels, gyroscope, Sun and Earth sensors, and thrusters. See Also Appendix Source 16 The satellite DSCOVR (Triana) is placed at the L1 lagrangian point between the Earth and the Sun. DSCOVR is a meteorological satellite which studies how solar radiation affects the Earth’s
climate. The attitude control system on this satellite includes four reaction wheels, ten thrusters, six Sun sensors, a three-axis inertial measuring unit, and a star tracker. See Also Appendix Source 17
Results Satellites use a variety of attitude actuators in the attitude control systems. In the satellites discussed previously the most common sensors are the Sun and Earth sensors and the most common actuators are the monopropellant thrusters and momentum wheels. The attitude control method which is recommended to invest in is the momentum wheels.
Discussion The attitude controller which the company should invest in is the momentum wheels. The monopropellant thrusters are the most common attitude actuator of the satellites for the mission types above, but only are a small sampling of the entire catalogue of satellites. The thrusters are able to provide three-axis control which can be used to remove precessions and stabilization of the satellite. Other attitude control systems such as momentum wheels need at least two to maintain orientation in three dimensions. The thrusters often comprise of a pressurized gas which can be fired at specific impulses to apply a torque onto the satellite. A drawback from thrusters is the fuel limitation dependent upon the size of the fuel tank fitted into the satellite which is an advantage for momentum wheels because they are not limited by fuel. In this regard momentum wheels eliminate the need for the satellite to carry extra fuel and lower the weight of the satellite. Instead of fuel they use onboard batteries which allow for a longer life versus that of thrusters using fuel. This means the satellite can stay in orbit longer and the correct attitude which translates to a longer mission life. Momentum wheels are also very light compared to the satellite. All of the attitude control methods are computer controlled so a large factor is precision. The precision of one momentum wheel is comparable to the precision a satellite receives through the use of several thrusters placed around the satellite body. Several satellites used Sun and Earth sensors to determine their direction and orientation based relative to the Sun or Earth. These are extremely useful if the satellite has a communications array which must always be pointed toward the Earth or a solar array pointed toward the Sun. Momentum wheels are the recommended option but not the only option for the company to invest into; they may also invest in thrusters, control moment gyroscopes, solar sails, and magnetic torquers. See Also Appendix Source 18
Appendix 1) Perturbation Effects on Satellites http://www.rfcafe.com/references/articles/Satellite%20Comm%20Lectures/SatelliteComms-Orbital-Perturbation.pdf 2) Perturbation Analysis on Spacecraft http://en.wikipedia.org/wiki/Orbital_perturbation_analysis_(spacecraft) 3) Perturbation Effects on LEO Satellites http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CC8QFj AC&url=http%3A%2F%2Fwww.agi.com%2Fdownloads%2Fpartners%2Fedu%2FOrbita l%2520Mechanics%2520and%2520Design_Rev17.ppt&ei=iaNLT4XaDYfn0QGXzeWR Dg&usg=AFQjCNHYW9LYh3v1IhvHtJixGXgCeKwWcQ
4) LEO Satellite Information – http://en.wikipedia.org/wiki/Low_Earth_orbit 5) GEO Satellite Information – http://en.wikipedia.org/wiki/Geosynchronous_orbit 6) General Satellite Information – http://en.wikipedia.org/wiki/Satellite 7) Alphabetic List of Satellites http://www.eohandbook.com/eohb05/pdfs/missions_alphabetical.pdf 8) CloudSat Satellite Information – http://www.nasa.gov/pdf/133608main_cloudsat-calipso-launch.pdf 9) CARTOSAT-1 Satellite Information – http://www.isro.org/satellites/cartosat-1.aspx 10) CARTOSAT-2 Satellite Information – http://www.isro.org/satellites/cartosat-2.aspx 11) CBERS-4 Satellite Information – http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedAM/920_Liporace_JA CIE_11.025.pdf 12) GOSAT Satellite Information – http://www.ihi.co.jp/ia/en/product/satellite.html 13) GRACE Satellite Information – http://articles.adsabs.harvard.edu//full/2000ESASP.465..769S/0000769.000.html 14) GLONASS Satellite Information – http://acc.igs.org/orbits/glonass-attitude-model_ASR10.pdf 15) LAGEOS 2 Satellite Information – http://www.astronautix.com/craft/lageos.htm 16) BSE Satellite Information – http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=04044294 17) DSCOVR Satellite Information – http://airex.tksc.jaxa.jp/pl/dr/20010084979/en 18) Attitude Control Information – http://en.wikipedia.org/wiki/Attitude_control_(spacecraft)