1. Research Supported by NSF Alliance for Graduate Education and Professiorate (AGEP) 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

2. Introduction Introduce the theory of communication architecture for a single satellite and constellation. Introduce the theory of communication architecture for a single satellite and constellation. Explain the communication architecture design for an satellite- Earth link. Explain the communication architecture design for an satellite- Earth link. Introduction and simulation with the Satellite Tool Kit (STK) software. Introduction and simulation with the Satellite Tool Kit (STK) software.

3. Communication Architecture Arrangement or configuration of different subsystems communicating from the Earth to the Satellite. Arrangement or configuration of different subsystems communicating from the Earth to the Satellite. Architecture depends on three main objectives: Architecture depends on three main objectives: Mission ObjectivesMission Objectives Data RatesData Rates Earth-Satellite LinkEarth-Satellite Link

4. Communication Subsystems Communication Subsystems Satellite link: SensorSensor TransponderTransponder TransmitterTransmitter Earth link: Ground Station Amplifier and Receiver Demodulation Display

5. Communication Links Ground Station to Satellite Ground Station to Satellite UplinkUplink DownlinkDownlink Satellite to Satellite Satellite to Satellite CrosslinkCrosslink Intersatellite LinkIntersatellite Link Forward LinkForward Link Return LinkReturn Link

6. Store and Forward Has an altitude under 1000 km. Has an altitude under 1000 km. Receives and stores information from a group of ground stations. Receives and stores information from a group of ground stations. Has a small link access time to download the information. Has a small link access time to download the information. Has a low-cost launch, wider antenna bandwidth, minimum stabilization, and covers polar areas. Has a low-cost launch, wider antenna bandwidth, minimum stabilization, and covers polar areas.

7. Molniya Orbit Covers northern polar areas for long periods of time. Covers northern polar areas for long periods of time. Has a low launch-cost. Has a low launch-cost. Requires continuous changing of antenna pointing angles. Requires continuous changing of antenna pointing angles. Requires satellite station keeping. Requires satellite station keeping.

8. Low Altitude Has an altitude higher than 1000 km. Has an altitude higher than 1000 km. Connects via satellite crosslink. Connects via satellite crosslink. Has greater lifetime than other satellites Has greater lifetime than other satellites Depends on the inclination angle for coverage pattern. Depends on the inclination angle for coverage pattern. Has complex dynamic controls, link acquisition, and high link quality. Has complex dynamic controls, link acquisition, and high link quality. The information moves for different paths. The information moves for different paths.

9. Criteria for the Communication Architecture Orbit Orbit The satellite coverage depends on the altitude and inclination.The satellite coverage depends on the altitude and inclination. The transmitter and receiver power depends on the altitude of the satellite.The transmitter and receiver power depends on the altitude of the satellite. The satellite orbit dictates whether crosslink or intersatellite link is required.The satellite orbit dictates whether crosslink or intersatellite link is required.

10. RF Spectrum RF Spectrum The radio frequency is chosen for the communication.The radio frequency is chosen for the communication. The selection of the frequency changes the size, mass, and complexity.The selection of the frequency changes the size, mass, and complexity. The frequency is allocated depending on the mission objectives.The frequency is allocated depending on the mission objectives. Data Rate Data Rate The size of the transmitter depends on the amount of data.The size of the transmitter depends on the amount of data. The information can be compressed in the satellite.The information can be compressed in the satellite.

11. Duty Factor Duty Factor Is the time needed to communicate between a ground station and satellite.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 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.Is low when the ground station serves different satellites. Link Availability 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.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.Depends on the reliability of the equipment.

12. Link Access Time Link Access Time Is the maximum time to access the satellite information.Is the maximum time to access the satellite information. Depends on the selection of the orbit.Depends on the selection of the orbit. Threat Threat Depends on the perturbations due to the Moon, Sun, atmosphere, and weather.Depends on the perturbations due to the Moon, Sun, atmosphere, and weather. Depends on the noise created by human beings.Depends on the noise created by human beings.

13. STK Software Used to simulate satellite orbits in real time. Used to simulate satellite orbits in real time. Synthesizes the positioning of the ground antenna once the orbit has been defined. Synthesizes the positioning of the ground antenna once the orbit has been defined. Used to establish communication time intervals with the satellite. Used to establish communication time intervals with the satellite. Has a three dimensional window to visualize the satellite orbit. Has a three dimensional window to visualize the satellite orbit.

14. Iridium Constellation Has an altitude of 700 km. Has an altitude of 700 km. Has 6 orbital planes with 11 satellites per orbital plane. Has 6 orbital planes with 11 satellites per orbital plane. Used for phone satellite communication. Used for phone satellite communication. Can communicate to any place on the Earth. Can communicate to any place on the Earth.

18. Graph 1. Link Access Time for 11 Satellites of the Iridium Constellation

19. Molniya Orbit Has a period of 12 hours. Has a period of 12 hours. Has an altitude of 40,000 km at apogee and an altitude of 500 km at perigee. 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. Covers the northern polar areas for a period of 8 hours. Has an inclination angle of 63.14 degrees. Has an inclination angle of 63.14 degrees.

22. Graph 2. Link Access Time for the Molniya Satellite

23. Conclusion Simulations with the STK software are very accurate. Simulations with the STK software are very accurate. Predicts the position and availability of the satellite. Predicts the position and availability of the satellite. Has a better visualization in real time for a single satellite or constellation. 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. Has different perturbations like J2, drag, models for clouds and precipitation, etc.