Presentation on theme: "ECE 5233 Satellite Communications"— Presentation transcript:
1 ECE 5233 Satellite Communications Prepared by:Dr. Ivica KostanicLecture 5: Orbital perturbations, launch and orbital effects(Section )Spring 2014
2 Outline Orbital perturbations Launches and launch vehicles Placing a satellite in a geo-stationary orbitOrbital effectsExamplesImportant note: Slides present summary of the results. Detailed derivations are given in notes.
3 Sources of orbital perturbations Orbital perturbation – difference between real orbit and Keplerian orbit obtained from two body equations of motionMajor sources of orbital perturbationsPerturbations due to non-ideal EarthThird body perturbationsAtmospheric dragSolar radiation and solar windImportance of perturbation source depends on the satellite altitudeModeling of the real satellite orbitFind “osculating orbit” at some timeAssume orbit elements vary linearly with timeUse measured data to determine rate of change for orbital parameters
4 Effects of non-ideal Earth Earth is not a sphereEquatorial radius: ~ 6,378 kmPolar radius: ~ 6,356 km (about 22km smaller)Equatorial radius not constant (small variations ~ 100m)Earth mass distributionEarth mass distribution not uniformRegions of mass concentrations: masconsNon-ideal Earth causes non-ideal gravitational fieldFor LEO and MEO satellites this effect is not very significantGEO satellites are impacted the mostGEO satellites drift towards mascons in the “east-west” directionLongitudes of two stable equilibrium points: 75°E and 252°E (or 108°W)Longitudes of two unstable equilibrium points: 162°E and 348°E (or 12°W)
5 Third-body perturbation Motion of the satellite is not a “two-body” problemSatellite experiences gravitational pull from Sun and Moon as wellThe orbital relationship is complex and time dependentGravitational forces from Sun and Moon tend to move satellite out of the orbitUnder these condition the orbit will precess and its inclination will change (up to 1°/year)In practice:Some of these effects are planedMost of these effects are corrected using the on-board fuelFor GEO stationary satellite, the goal is to keep its apparent position within 0.05 degree boxOrbital position of a satellite the Earth, the Sun and the MoonNote: Orbital parameters are continuously measured and published as TLE data
6 Atmospheric drag Significant for satellites below 1000km Example: Consider ISS. From TLE dataMean motion: rev/dayDerivative of MM/2: rev/day2Significant for satellites below 1000kmDrag reduces the velocity of the satelliteSemi-major axis is reducedEccentricity is reducedApproximate equation for change of semi-major axiswherea0 – semi-major axis at t0n0 – mean motion (revs/day)n‘0 – first derivative of mean motion (rev/day2)Note: change is relatively small and it is long term. In practice it can be easily resolved through satellite maneuvering.Note: Derivative of mean motion is provided in TLE data
7 PropagatorsDetermine position of a satellite over timeThree types:AnalyticSemi-analyticNumericalAnalytic – approximate motion of the satellite through closed form equationsTwoBody – considers only forces of gravity from Earth, which is modeled as a point massJ2 Perturbation – models Earths oblateness, solar and lunar gravitational forcesJ4 Perturbation – second order improvements of J2 PerturbationSGP4 - Simplified General PerturbationsSemi-analytic – combination of analytical and numerical methodsLOP – Long-term Orbital Predictor. Uses same elements as analytic propagators. Accurate over many months.SGP4 for non LEO satellitesNumerical – Solve complex sets of differential equations. Accuracy is obtained at the expense of computational speedAstrogator – used for trajectory and maneuver planning and includes targeting capabilitiesHPOP – High Precision Orbital Propagator. Uses the same orbital elements as the analytic propagators.Note: Propagators are included in commercial software packages
8 Some of the most frequently used launch sites Satellite launchTwo categories of satellite launchesExpandable launch vehiclesAriane (EU)Atlas, Delta (US)Soyuz (Russia)Reusable launch vehiclesSpace Shuttle (STS) – up to 2011Dragon, Falcon 9 (Space X)Buran? Orion?Launches are usually done in several stagesLaunch vehicle is used to place satellite in one of the transfer LEO orbitsSatellite is maneuvered from a transfer orbit into the final orbitDue to Earth rotation – the launch is easiest from equatorVelocity boost from Earth rotation ~ 0.47km/secLEO orbits require velocities ~ 7.5km/secLaunching from equator ~ 6% fuel savingsLaunches from sites that are not on the equator place satellites in inclined orbitsIf the satellite is to be placed in GEO stationary orbit, the correction of the orbit inclination needs to be performedSome of the most frequently used launch sites
9 Two typical GEO satellite launch approaches Case 1: single elliptical orbitCase 2: two transfer orbitsCase 1: Launch vehicle puts the satellite into an elliptical transfer orbitThe orbit has high eccentricity with apogee at the geostationary (or geosynchronous) orbitWhen the satellite passes through apogee, additional velocity is given to the satellite by Apogee Kick Motor (AKM) and the satellite changes orbitsIf the orbit is not GEO stationary, additional adjustments are required to correct for the orbit inclinationCase 2: Lunch vehicle put the satellite into circular LEO orbitTwo maneuvers are requiredPerigee maneuver to transform circular orbit into elliptical orbit, andApogee maneuver to transform elliptical orbit into GEO stationary orbitNote: other launch approaches are possible
10 Example of GEO launches: French Guyana and KSC Launch from French GuyanaLaunch from Cape Carnival, FL
11 Orbital effectsMotion of the satellite has significant impact on its performanceSome of the orbital effects to take into accountDoppler shiftSolar eclipseSun transit outage
12 Note: the shift becomes larger with higher frequencies. Doppler shiftExample. Consider LEO satellite in circular polar orbit with altitude of 1000km. Transmit frequency is 2.65GHz.The velocity of satellite in orbitImportant in case of LEO and MEO satellitesNegligible for GEO satellitesCaused by relative motion between the satellite and earth stationThe magnitude of Doppler shift is given by:Largest component of velocity between satellite and Earth station occurs when the satellite is coming out of horizon directly over the stationWhere: fR – received frequency, fT – transmit frequency, VT – velocity between satellite and Earth station, l – wavelength, c – speed of lightMaximum Doppler shiftNote: the shift becomes larger with higher frequencies.Geometry used for calculation of maximum velocity
13 Geometry of solar eclipse Eclipse – sunlight fails to reach satellite due to obstruction from either Earth or MoonMore common type of eclipse – eclipse due to satellite coming in the shadow of the earthFor geostationary satellite the solar eclipse occurs for about 82 days every yearThe eclipse times vary from day to dayThe longest eclipses are on equinox days (days when the sun passes through equatorial plane) – Nominally: March 21st and September 23stWhen the satellite passes through eclipseIt loses its source of power (relies on battery power)It may need to shut off some of its transpondersExperiences cooling down in temperatureTransients due to switching of the equipment and temperature variations may cause equipment failureGeometry of solar eclipseNote: for non geostationary satellites orbital study necessary to determine eclipse timesDuration of solar eclipse of a geo stationary satellite relative to equinox time
14 Sun transit outageSun transit outage - occurs when the satellite passes in front of the SunSun is a source of electromagnetic radiationSun – noise source with equivalent temperature of 6000K – 11000KIn most cases this noise overcomes link design marginsDuring sun transit outage – communication lostDuration of the outage as much as 10 min/dayFor geo-stationary satellites outages occur 0.02% of the timeDuration problematic since it usually occurs in a traffic intensive part of the daySatellite providers try to re-route the traffic through other satellitesGeometry of sun transit eclipseRadiation spectrum of the Sun
Your consent to our cookies if you continue to use this website.