Published on January 9, 2008
Coverage Patterns for the Molniya Orbit and Iridium Constellation in Real Time with the STK: Coverage Patterns for the Molniya Orbit and Iridium Constellation in Real Time with the STK Pedro A. Capó-Lugo, Peter M. Bainum Howard University, Department of Mechanical Engineering Washington, D.C. 20059, USA Ijar Fonseca National Institute for Space Research—INPE Space Mechanics and Control Division—DMC Sao Jose dos Campos, S.P. Brazil Introduction: Introduction Introduce the theory of communication architecture for a single satellite and constellation. Explain the communication architecture design for an satellite-Earth link. Introduction and simulation with the Satellite Tool Kit (STK) software. Communication Architecture: Communication Architecture Arrangement or configuration of different subsystems communicating from the Earth to the Satellite. Architecture depends on three main objectives: Mission Objectives Data Rates Earth-Satellite Link Communication Subsystems: Communication Subsystems Satellite link: Sensor Transponder Transmitter Earth link: Ground Station Amplifier and Receiver Demodulation Display Communication Links: Communication Links Ground Station to Satellite Uplink Downlink Satellite to Satellite Crosslink Intersatellite Link Forward Link Return Link Store and Forward: Store and Forward Has an altitude under 1000 km. Receives and stores information from a group of ground stations. Has a small link access time to download the information. Has a low-cost launch, wider antenna bandwidth, minimum stabilization, and covers polar areas. Molniya Orbit: Molniya Orbit Covers northern polar areas for long periods of time. Has a low launch-cost. Requires continuous changing of antenna pointing angles. Requires satellite station keeping. Low Altitude: Low Altitude Has an altitude higher than 1000 km. Connects via satellite crosslink. Has greater lifetime than other satellites Depends on the inclination angle for coverage pattern. Has complex dynamic controls, link acquisition, and high link quality. The information moves for different paths. Criteria for the Communication Architecture: Criteria for the Communication Architecture Orbit The satellite coverage depends on the altitude and inclination. The transmitter and receiver power depends on the altitude of the satellite. The satellite orbit dictates whether crosslink or intersatellite link is required. Slide10: RF Spectrum The radio frequency is chosen for the communication. The selection of the frequency changes the size, mass, and complexity. The frequency is allocated depending on the mission objectives. Data Rate The size of the transmitter depends on the amount of data. The information can be compressed in the satellite. Slide11: Duty Factor Is the time needed to communicate between a ground station and satellite. Is a function of the mission and the time the satellite takes to orbit around the Earth. Is low when the ground station serves different satellites. Link Availability Is defined as the time the link is available for the user divided by the total time the satellite covers all the area. Depends on the reliability of the equipment. Slide12: Link Access Time Is the maximum time to access the satellite information. Depends on the selection of the orbit. Threat Depends on the perturbations due to the Moon, Sun, atmosphere, and weather. Depends on the noise created by human beings. STK Software: STK Software Used to simulate satellite orbits in real time. Synthesizes the positioning of the ground antenna once the orbit has been defined. Used to establish communication time intervals with the satellite. Has a three dimensional window to visualize the satellite orbit. Iridium Constellation: Iridium Constellation Has an altitude of 700 km. Has 6 orbital planes with 11 satellites per orbital plane. Used for phone satellite communication. Can communicate to any place on the Earth. Slide18: Graph 1. Link Access Time for 11 Satellites of the Iridium Constellation Molniya Orbit: Molniya Orbit Has a period of 12 hours. Has an altitude of 40,000 km at apogee and an altitude of 500 km at perigee. Covers the northern polar areas for a period of 8 hours. Has an inclination angle of 63.14 degrees. Slide22: Graph 2. Link Access Time for the Molniya Satellite Conclusion: Conclusion Simulations with the STK software are very accurate. Predicts the position and availability of the satellite. Has a better visualization in real time for a single satellite or constellation. Has different perturbations like J2, drag, models for clouds and precipitation, etc.