Orbit and Constellation DesignDr. Andrew KetsdeverMAE 5595Lesson 6

OutlineOrbit DesignOrbit SelectionOrbit Design ProcessV BudgetLaunchEarth CoverageConstellation DesignBasic FormationStationkeepingCollision Avoidance

Orbit DesignWhat orbit should the satellite be put in?Mission objectivesCostAvailable launch vehiclesOperational requirementsOrbit Design Process11 Step ProcessWide variety of mission types which will be unique in the orbit selection process

Orbit Design ProcessStep 1: Establish Orbit TypesEarth referenced orbitsGEO, LEOSpace referenced orbitsLagrange points, planetaryTransfer orbitGTO, InterplanetaryParking orbitTemporary orbit for satellite operational checks, EOL

Orbit Design Process

Orbit Design ProcessStep 2: Establish Orbit-Related Mission RequirementsAltitude Resolution Lower altitude is betterSwath width Higher altitude is betterInclination Ground station coverageOther orbital elementsJ2 effects on RAAN and AoPLifetimeSurvivability (ambient environment)Must be able to survive the entire orbital profile (e.g. transit through Van Allen Radiation Belts)

Orbit Design Process

Orbit Design ProcessStep 3: Assess Specialized OrbitsTypically set some orbital parameters (e.g. semi-major axis, inclination)Geosynchronous, GeostationarySemi-synchronousSun synchronousMolniyaLagrange points

GEO24 hr period co-rotation with EarthSolar/Lunar pertubationsN/S driftPointing requirementsJ2 perturbationsE/W drift10% of V required for N/SImportant due to neighboring slotsEarly 1990s50% of launches to GEOOvercrowding is a constant issue

Sun Synchronous OrbitJ2 causes orbit to rotate in inertial spaceRate is equal to average rate of Earths rotation around the sunPosition of Sun relative to orbital plane remains relatively constantSun Synchronous orbits can be achieved around other central bodiesUsually near 90 inclinations

Molniya OrbitHighly elliptical orbit with i=63.4 (zero rate of perigee rotation)Can have a large (up to 99%) fraction of the orbit period between

Any orbital period can be obtained change apogee altitudeSatellite constellations in these orbits can provide very efficient coverage of high (or low) latitudes

Orbit Design ProcessStep 4: Select Single Satellite or Constellation ArchitectureSingle satelliteAdvantagesReduced overhead (single system)More capability per copyDisadvantagesLimited coverage (potential)ReliabilityHigh costConstellationAdvantagesEnhanced coverageSurvivabilitySystem simplicityDisadvantagesHigher operational and launch costs (potential)Limited capability

Orbit Design ProcessStep 5: Mission Orbit Design TradesHow do orbital parameters affect the mission requirements?How are satellites in a constellation phased throughout the orbital plane(s)?ConstellationsTypically at the same altitude and inclinationDrift characteristicsAt different altitudes and inclinations, satellites in a constellation will drift apart

Orbit Design ProcessStep 6: Evaluate Constellation Growth and Replenishment or Single-Satellite Replacement StrategyConstellationGrowthTime consuming (several months to years)Operational without full constellationGraceful degradation (reduced level of service)ReplenishmentSingle SatelliteSingle point failure DegradationReplacement

Orbit Design ProcessStep 7: Assess Retrieval or Disposal OptionsRetrieval is typically not an option (Shuttle)On-orbit servicingDisposal (De-orbit) may be a requirement soonOrbital debrisLimited useful operational orbitsRe-enter (LEO)Disposal orbit (GEO)

Orbit Design ProcessStep 8: Create a V BudgetOrbital maneuversLaunch Vehicle may not get you directly to the desired orbitTransfer orbit VOrbit sizeOrbit inclinationRAANStationkeepingRephasingDe-orbit

The V BudgetManeuvers requiring VOrbital transferPlane changeDrag make-upAttitude controlStationkeepingRephasingRendezvousDe-Orbit

The V BudgetStart with the ideal rocket equation

Mpropellant for a particular burn is the difference in initial mass and final massHigh Isp is desirable, but it must be weighed versus the cost of the higher value (e.g. higher power, higher dry mass, etc.)Investigate concepts that reduce V requirementsAerobrakingSolar SailsTethers

Environment Interactions

Orbit Design ProcessStep 9: Assess Launch and Orbit Transfer CostAvailability of LVCostMass to particular orbit= $$$

Orbit Design ProcessStep 10 and 11: Document and Iterate

Earth CoverageEarth coverage refers to the part of the Earth that a spacecraft instrument can seeField of View: Actual area the instrument can see at any momentAccess Area: Total area on the ground that could potentially be seen at any moment.

Footprint

Hellas-Sat 2

FootprintASTRIUM Eurostar 2000+ PlatformPayload30 x 36 MHz transponders, onboard 8 x 36 MHz redundant12 on fixed beam F1, 6 on fixed beam F2, up to 12 on beam S1 and6 on beam S2.FootprintsFixed over EuropeSteerableover Southern Africa, Middle East, Indian subcontinent, South EastFrequenciesDownlink Ku-Band10.95-11.20 GHz (F2)11.45-11.70 GHz (S2)12.50-12.75 GHz (F1, S1)Uplink Ku-Band13.75-14.50 GHzServicesAudio/Video BroadcastsTelephone RelayInternet AccessBusiness TeleconferencingF1S1

FootprintGeostationary Operational Environmental Satellites (GOES)GOES 8: DecommissionedGOES 9: Operational (Japan)GOES 10: Operational, Standby, DriftingGOES 11: Operational, WestGOES 12: Operational, East

Sirius Radio

Earth CoverageEarth Coverage Figures of MeritPercent Coverage: Number of times that a point is covered by one or more satellites divided by a time periodMaximum Coverage Gap: Longest of the coverage gaps (no coverage) encountered for a particular pointMean Coverage Gap: Average of the coverage gaps (no coverage) for a particular pointMean Response Time: Average time from a random request to observe a particular point

Earth Coverage

A Different Kind of Gap?A U.S. Government Accountability Office report on a new polar-orbiting environmental satellite program has concluded that cost overruns and procedural difficulties could create a gap in important national weather data derived from the satellites that could last at least three years, beginning in late 2007. Polar-orbiting environmental satellites provide data and images used by weather forecasters, climatologists and the U.S. military to map and monitor changes in weather, climate, the oceans and the environment. The satellites are critical to long-term weather prediction, including advance forecasts of hurricane paths and intensity. The current U.S. program comprises two satellite systems - one operated by the National Oceanic and Atmospheric Administration, and one by the Department of Defense - as well as supporting ground stations and four central data processing centers. The new program, called the National Polar-orbiting Operational Environmental Satellite System, or NPOESS, is supposed to replace the two systems with a single, state-of-the-art environment-monitoring satellite network. NPOESS - to be managed jointly by NOAA, DOD and NASA - will be critical to maintaining the continual data required for weather-forecasting and global climate monitoring though 2020. The problem is the last NOAA polar-orbiting satellite in the existing program is scheduled to be launched in late 2007, while the first NPOESS launch will not be until at least late 2010. If the earlier satellite fails, its data capability would be difficult, if not impossible, to replace during the interim.

Swath Width

Earth Coverage

Earth Coverage(-) for subscript 1, (+) for subscript 2

Earth Coverage

Constellation DesignConstellation: Set of satellites distributed over space intended to work together to achieve a common objectiveSatellites that are in close proximity are called clusters or formationsConstellation architectures have been fueled by recent development of small, low cost satellites

Constellation DesignCoveragePrinciple performance parameterMinimize gap times for regions of interestEntire EarthNorth AmericaColoradoUS Air Force AcademyNumber of SatellitesPrinciple cost parameterAchieve desired coverage with the minimum satellitesExampleGPS requires continuous coverage of the entire world by a minimum of four non-coplanar satellites

Constellation DesignNumber of Orbital PlanesCan be a driver for coverageSatellites spread out (typically evenly) throughout planePlane changes require large amounts of propellantMeet requirements with the minimum number of orbital planesRephasing can be accomplished with less propellant in a single plane

Constellation DesignConstellation Build-Up, Replenishment, and End of LifeTypically a constellation is in a less-than-complete formBuild-up can be a several year process with multiple launchesRe-Configuration of the constellation is necessary when satellites failDead satellites need to be removed from the active constellationCollision avoidance

GlobalStarLEO Cellular Phone Constellation48 satellites in 8 planesh=1414kmi=52Latitude coverage: 707 Boeing Delta II Launches6 Russian Soyuz LaunchesEach launch vehicle carried 4 satellitesOn-orbit sparesTwo additional Deltas were purchased to ferry spares to the constellationGeneral Characteristics: Total weight - 450kg, Number of Spot beams - 16 Power - 1100W Lifetime - 7.5 years

Constellation Coverage

Street of CoverageSwath 2lstreet where coverage will be continuous

Street of CoverageAdjacent Planes

Iridium (Atomic No=77)66 active satellites, 6 planesReduced to 6 orbital planes (from a proposed 7) byIncreasing the orbital altitude slightly.

Iridium Satellite Constellation

Constellation Design

The Walker ConstellationSymmetricT = total number of satellitesS satellites evenly distributed in each of P orbital planesAscending Nodes of the P orbital planes are uniformly distributed about the equatorWithin each plane, the S satellites are uniformly distributed in the orbitRelative phase between satellites in adjacent planes to avoid collisions

StationkeepingApproaches to perturbationsLeave perturbation uncompensatedControl the perturbing force the same for all satellites in the constellationNegate the perturbing forceExample: h=700 km, i=30 and 70Node rotation rate of 2.62 /day and 6.63 /dayRelative plane movement of 4 /dayMakes construction of long term constellation difficultCoverage requirementsActive rephasing may be necessary

Collision Avoidance

Microsatellite Constellations