Research Supported by NSF Alliance for GrResearch Supported by NSF Alliance for Graduate Education and Professiorate (AGEPaduate Education and Professiorate (AGEP))

Coverage Patterns for the Coverage Patterns for the Molniya Orbit and Iridium Molniya Orbit and Iridium

Constellation in Real Time with Constellation in Real Time with the STKthe STK

Pedro A. Capó-Lugo, Peter M. BainumPedro A. Capó-Lugo, Peter M. BainumHoward University, Department of Mechanical EngineeringHoward University, Department of Mechanical Engineering

Washington, D.C. 20059, USAWashington, D.C. 20059, USA

Ijar FonsecaIjar FonsecaNational Institute for Space Research—INPENational Institute for Space Research—INPESpace Mechanics and Control Division—DMCSpace Mechanics and Control Division—DMC

Sao Jose dos Campos, S.P. BrazilSao Jose dos Campos, S.P. Brazil

IntroductionIntroduction

Introduce the theory of Introduce the theory of communication architecture for a communication architecture for a single satellite and constellation.single satellite and constellation.

Explain the communication Explain the communication architecture design for an satellite-architecture design for an satellite-Earth link.Earth link.

Introduction and simulation with the Introduction and simulation with the Satellite Tool Kit (STK) software.Satellite Tool Kit (STK) software.

Communication ArchitectureCommunication Architecture

Arrangement or configuration of Arrangement or configuration of different subsystems communicating different subsystems communicating from the Earth to the Satellite.from the Earth to the Satellite.

Architecture depends on three main Architecture depends on three main objectives: objectives: • Mission ObjectivesMission Objectives• Data RatesData Rates• Earth-Satellite LinkEarth-Satellite Link

Communication SubsystemsCommunication Subsystems

Satellite link:Satellite link:• SensorSensor• TransponderTransponder• TransmitterTransmitter

Earth link:Earth link:• Ground StationGround Station• Amplifier and Amplifier and

ReceiverReceiver• DemodulationDemodulation• DisplayDisplay

Communication LinksCommunication Links

Ground Station to SatelliteGround Station to Satellite• UplinkUplink• DownlinkDownlink

Satellite to SatelliteSatellite to Satellite• CrosslinkCrosslink• Intersatellite LinkIntersatellite Link• Forward LinkForward Link• Return LinkReturn Link

Store and ForwardStore and Forward

Has an altitude under 1000 km.Has an altitude under 1000 km. Receives and stores information from Receives and stores information from

a group of ground stations.a group of ground stations. Has a small link access time to Has a small link access time to

download the information.download the information. Has a low-cost launch, wider antenna Has a low-cost launch, wider antenna

bandwidth, minimum stabilization, bandwidth, minimum stabilization, and covers polar areas. and covers polar areas.

Molniya OrbitMolniya Orbit

Covers northern polar areas for long Covers northern polar areas for long periods of time.periods of time.

Has a low launch-cost.Has a low launch-cost. Requires continuous changing of Requires continuous changing of

antenna pointing angles.antenna pointing angles. Requires satellite station keeping.Requires satellite station keeping.

Low AltitudeLow 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 satellitesHas greater lifetime than other satellites Depends on the inclination angle for Depends on the inclination angle for

coverage pattern.coverage pattern. Has complex dynamic controls, link Has complex dynamic controls, link

acquisition, and high link quality.acquisition, and high link quality. The information moves for different paths.The information moves for different paths.

Criteria for the Communication Criteria for the Communication ArchitectureArchitecture

OrbitOrbit• The satellite coverage depends on the The satellite coverage depends on the

altitude and inclination.altitude and inclination.• The transmitter and receiver power The transmitter and receiver power

depends on the altitude of the satellite.depends on the altitude of the satellite.• The satellite orbit dictates whether The satellite orbit dictates whether

crosslink or intersatellite link is required.crosslink or intersatellite link is required.

RF SpectrumRF Spectrum• The radio frequency is chosen for the The radio frequency is chosen for the

communication.communication.• The selection of the frequency changes The selection of the frequency changes

the size, mass, and complexity.the size, mass, and complexity.• The frequency is allocated depending on The frequency is allocated depending on

the mission objectives.the mission objectives. Data RateData Rate

• The size of the transmitter depends on The size of the transmitter depends on the amount of data.the amount of data.

• The information can be compressed in The information can be compressed in the satellite.the satellite.

Duty FactorDuty Factor• Is the time needed to communicate Is the time needed to communicate

between a ground station and satellite.between a ground station and satellite.• Is a function of the mission and the time Is a function of the mission and the time

the satellite takes to orbit around the the satellite takes to orbit around the Earth.Earth.

• Is low when the ground station serves Is low when the ground station serves different satellites.different satellites.

Link AvailabilityLink Availability• Is defined as the time the link is available Is defined as the time the link is available

for the user divided by the total time the for the user divided by the total time the satellite covers all the area.satellite covers all the area.

• Depends on the reliability of the Depends on the reliability of the equipment.equipment.

Link Access TimeLink Access Time• Is the maximum time to access the Is the maximum time to access the

satellite information.satellite information.• Depends on the selection of the orbit.Depends on the selection of the orbit.

ThreatThreat• Depends on the perturbations due to the Depends on the perturbations due to the

Moon, Sun, atmosphere, and weather.Moon, Sun, atmosphere, and weather.• Depends on the noise created by human Depends on the noise created by human

beings.beings.

STK SoftwareSTK Software

Used to simulate satellite orbits in Used to simulate satellite orbits in real time.real time.

Synthesizes the positioning of the Synthesizes the positioning of the ground antenna once the orbit has ground antenna once the orbit has been defined.been defined.

Used to establish communication Used to establish communication time intervals with the satellite.time intervals with the satellite.

Has a three dimensional window to Has a three dimensional window to visualize the satellite orbit.visualize the satellite orbit.

Iridium ConstellationIridium Constellation

Has an altitude of 700 km.Has an altitude of 700 km. Has 6 orbital planes with 11 satellites Has 6 orbital planes with 11 satellites

per orbital plane.per orbital plane. Used for phone satellite Used for phone satellite

communication.communication. Can communicate to any place on Can communicate to any place on

the Earth. the Earth.

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

Molniya OrbitMolniya Orbit

Has a period of 12 hours.Has a period of 12 hours. Has an altitude of 40,000 km at Has an altitude of 40,000 km at

apogee and an altitude of 500 km at apogee and an altitude of 500 km at perigee.perigee.

Covers the northern polar areas for a Covers the northern polar areas for a period of 8 hours. period of 8 hours.

Has an inclination angle of 63.14 Has an inclination angle of 63.14 degrees.degrees.

Graph 2. Link Access Time for the Molniya Satellite

ConclusionConclusion

Simulations with the STK software Simulations with the STK software are very accurate.are very accurate.

Predicts the position and availability Predicts the position and availability of the satellite.of the satellite.

Has a better visualization in real time Has a better visualization in real time for a single satellite or constellation.for a single satellite or constellation.

Has different perturbations like J2, Has different perturbations like J2, drag, models for clouds and drag, models for clouds and precipitation, etc.precipitation, etc.