Guidance and NavigationDefinitionsSlide 3Process for Defining the Guidance and Navigation SubsystemAlternative Navigation MethodsAlternative Navigation Methods Advantages and DisadvantagesOrbit ControlSlide 8Slide 9Constellation StationkeepingGround-Based vs. Autonomous Orbit ControlSlide 12Things to Look ForGuidance and NavigationAERSP 401ADefinitions•Navigation = Orbit Determination = determining satellite’s position and velocity (or orbital elements) as a function of time–Real Time Orbit Determination estimates where the satellite is at the present time–Definitive Orbit Determination provides the best estimate of the satellite orbit at some earlier time–Ephemeris is a tabular listing of the state vector of the satellite as a function of time–Orbit Propagation refers to integrating the equations of motion to determine where the satellite is at some other timeDefinitions•Guidance = Orbit Control = adjusting the orbit to meet some predetermined conditions–Orbit Maintenance refers to maintaining the orbit elements but not the timing of when the satellite is at any location in its orbit•Altitude maintenance uses thruster firings to overcome drag–Stationkeeping refers to maintaining the satellite within a predefined box•Geosynchronous Stationkeeping maintains the satellite in a box over one place on the Earth•Constellation Maintenance maintains the satellite in a moving box defined relative to other satellites in the constellationProcess for Defining the Guidance and Navigation SubsystemStep Principal Issues Where Discussed1. Define navigation and orbit-related top-level functions and requirements•Mapping and pointing•Scheduling•Constellation or orbit maintenance•Rendezvous or destination requirements1.4, 2.1, 4.2, 7.12. Do pointing and mapping trades to determine preliminary navigation (position) accuracy requirements•What payload functions will the navigation data be used for?•Payload data processing (mapping)•Payload pointing5.43. Determine whether orbit control or maintenance is needed•Geosynchronous stationkeeping•Constellation stationkeeping•Altitude maintenance•Maintaining orbital elements•Mid-course correctionsChapter 7, Section 11.7.34. If yes, do trade on autonomous vs. ground-based orbit control•Is reduced operations cost and risk worth introducing a nontraditional approach? 2.1.2, 11.7.1, 11.7.35. Determine where navigation data is needed•Is it needed only at ground station for mission planning and data evaluation?•Is it needed on board (orbit maintenance, Sun vector determination, payload pointing, target selection)?•Is navigation (or target location) data needed by several end users who may get information directly from the spacecraft?2.1.16. Do autonomous vs. ground-based navigation trade•Does reduced operations cost and risk justify a nontraditional approach?•Is there a need for real-time navigation data?2.1.1, 11.7.17. Select navigation method •See section 11.7.2 for main options 11.7.28. Define G&N system requirements•Top-level requirements should be in terms of what is needed (mapping, pointing, constellation maintenance, level of autonomy) not how the mission is done11.7.1, 11.7.4Alternative Navigation MethodsSystem Basis Status Determines3 AccuracyOperating RangeCommentsGPS Network of navigation satellitesOperational Orbit 15 m – 100 m in LEOLEO only Semi-autonomousMANS Observations of Earth, Sun, and MoonFlight tested in 1993 Orbit, attitude, ground look point, Sun direction100 m - 400 m in LEO (using only Earth, Sun and MoonLEO to GEO, lunar and planetary orbitsCan use other instrumentation (GPS receiver, star sensor, IMUs to improve accuracySpace SextantAngle between stars and Moon’s limbFlight tested Orbit and attitude250 m LEO to GEONot being actively marketed for space at the present timeStellar RefractionRefraction of starlight passing through the atmosphereProposed, some ground tests doneOrbit and attitude150 m – 1 km Principally LEOCould use attitude sensor dataLandmark TrackingAngular measurements of landmarksProposed, observability conditions uncertainOrbit and attitudeSeveral kilometersPrincipally LEOCould, in principle , use observation payload dataSatellite CrosslinksRange and range rate or angle measurements to other satellites in a constellationProposed; may be used on communications constellationsOrbit Theoretically, as good as 50 mPrincipally LEOOperation with less than full constellation problematic; no absolute position referenceEarth and Star SensingObserve direction and distance to Earth in inertial frameProposed Orbit and attitude100 m-400 m in LEOLEO to GEO, planetary orbitsSimilar to MANS with higher accuracy and availabilityAlternative Navigation Methods Advantages and DisadvantagesSystem Advantages DisadvantagesGround tracking•Traditional approach•Methods and tools well established•Accuracy depends on ground-station coverage•Can be operations intensiveTDRS tracking •Standard method for NASA spacecraft•High accuracy•Same hardware for tracking and data links•Not autonomous•Available mostly for NASA missions•Requires TDRS tracking antenna•Usable for Earth orbiting spacecraft onlyGPS/GLONASS•High accuracy•Provides time signal as well as position•Semi-autonomous•Depends on long-term maintenance and structure of GOS•Orbit only•Must initialize some unitsMANS•Fully autonomous•Uses attitude-sensing hardware•Provides orbit, attitude, ground look-point, and direction to Sun•First flight test in 1993•Initialization and convergence speeds depend on geometrySpace Sextant •Could be fully autonomous •Flight tested prototype only – not a current production product•Relatively heavy and high powerStellar Refraction•Could be fully autonomous•Uses attitude-sending hardware•Still in concept and test stageLandmark Tracking•Can use data from observation payload sensor •Still in concept stage•Landmark identification may be difficult•May have geometrical singularitiesSatellite Crosslinks•Can use crosslink hardware already on the spacecraft for other purposes•Unique to each constellation•No absolute position reference •Potential problems with system deployment and spacecraft failuresEarth and Star Sensing•Earth and stars available nearly continuously in vicinity of Earth •Cost and complexity of star sensors•Potential difficulty identifying starsOrbit Control•The cheapest orbit control system is