Presentation On Gps
- 1. AN INTRODUCTION TOGPS FOR GEOCACHERSPRESENTED BY :-TUHIN DAS
- 2. WHAT IS GEOCACHING?“Geocaching is a sport where you use a multi billion dollar satellite system to locate rubbermaid containers in the middle of the woods.”- Anonymous
- 3. GLOBAL POSITIONING SYSTEM (GPS)GPS was developed by US DoD and is managed by the US Air Force$750 million annual cost to operateNow widely used in the civilian sector as well as military usersSurveyingGeologyHikingMaritimeAirlinesAnd on and on…
- 4. THREE SEGMENTSSpace SegmentMinimum of 24 satellites (currently 30) in orbit around Earth at altitude 20,000 kmControl SegmentSatellites are tracked by ground stationsNavigation updates sent to satellitesUpdates ephemeris data (position) and atomic clocksUser SegmentGPS ReceiverUses data from satellites to calculate user’s position, altitude and other data
- 5. TRANSMITTED SIGNALSEach satellite transmits:It’s approximate positionExact orbit and locationNavigation signalCivilianMilitary (more precise)
- 6. Your positionThe Simple ViewKnowing how far you are from at least three fixed positions, you can determine your position (triangulation)GPS Technology actually uses trilaterilization
- 7. EXACT POSITIONUsing three satellites, you should be able to determine exact position but other factors prevent “prefect” overlapAtmospheric interferenceBuildingsTrees, etcUsing additional satellites, the GPSr attempts to reconcile position by adjusting the offsetsSame “interference” for all signals
- 8. POSITION ERRORPosition (Uncertainty) ErrorGPSr will display uncertainty values
- 9. OTHER DATAWith multiple satellites can get altitudeAs you move, a GPSr will calculateVelocityCourseTrack (“bread crumbs”)Distance & Heading to marked positionElapsed time, average, speed, etc
- 10. SELECTIVE AVAILABILITY & GEOCACHINGPrior to May 2000, civilian GPS signal included a random error (Selective Availability)Civilian accuracy >30mPres. Clinton turned off SA at 0000, 1 May 2000Resulted in accuracy < 10mBenefit sportsmen, FAA, etc3 May 2000 - Dave Ulmer placed first “cache”Now over 400,000 caches worldwide!
- 11. TRANSMITTED SIGNALSEach satellite transmits ephemeris, almanac and navigation dataAlmanac data – coarse orbit & status info for all satellitesEphemeris – specific orbit and position dataNavigation – Unique code every millisecondC/A (Coarse/Acquisition) – CivilianEncrypted P(Y) (Precision) - Military
- 12. EXACT POSITIONUsing three satellites, you should be able to determine exact position but other factors prevent “prefect” overlapAtmospheric interferenceBuildingsTrees, etcUsing additional satellites, the GPSr attempts to reconcile position by adjusting the offsetsSame “interference” for all signals
- 13. BUT HOW DO YOU GET DISTANCE?Satellites have atomic clockGPSr has accurate internal clockEach satellite’s C/A code is a unique stringAllows for reception of signals from multiple satellites on same frequencyGPSr reproduces each C/A code internallyGPSr “lines up” the two sequences, and based on the offset, you can determine distance to satellite
- 14. COMPARING SEQUENCESGPSr SequenceSatellite SequenceTime Offset x Speed Of Light = Distance From SatelliteSince you know the position of the satellite, and the distance from that satellite, you now have a sphere, centered on the satellite, you can be on