1GY 301: GeomorphologyGY 301: GeomorphologyLecture 9: GPS Surveying and Mapping TechniquesUNIVERSITY OF SOUTH ALABAMAhttp://www.gpsmagazine.com/assets/SPAC_GPS_NAVSTAR_IIA_IIR_IIF_Constellation_lg.jpgLast TimeCarol Sawyer Guest LectureToday’s AgendaGPS SurveyingIntroduction to GPS SurveyingSatellite characteristics GPS receiversLab preparationGPS Surveys• GPS = Global Positioning SystemGPS Surveys• GPS = Global Positioning System• Constellation of 24 satellites orbiting at 50,000 km altitudeGPS Surveys• GPS = Global Positioning System• Constellation of 24 satellites orbiting at 50,000 km altitude• At a given point on the earth at least 4 satellites can be tracked by receiver simultaneously2GPS Surveys• GPS = Global Positioning System• Constellation of 24 satellites orbiting at 50,000 km altitude• At a given point on the earth at least 4 satellites can be tracked by receiver simultaneously• 3 satellites plus the earth define a range of possible positions; the 4thsatellite timing is used to arrive at a consistent positionEarth- GPS Satellite GeometryEarthGPS Satellite Characteristics• Contain a very accurate atomic clock• Orbit at a high altitude so that no friction with the atmosphere is possible, resulting in a very predictable orbit• Broadcast signal contains the position of the satellite, and the time the signal was broadcast• Satellites are maintained by the Military and NASAGPS Error Sources• SA: selective availability +/- 50 m during a few minutes (turned off May 2, 2000)Plot of the position determination with and without SA (Diagram from http://www.igeb.gov/sa/diagram.shtml (page no longer available))GPS Error Sources• Satellite Geometryhttp://www.telematica.gr/Tech/GPS/images/accu.jpgGood Alignment Not-so-good alignment•Because of satellite geometry z accuracy is usually 1.5 to 2 times that of horizontal map accuracyGPS Error Sources• Atmospheric effects3GPS Error Sources• Multipath Error (largest source of error)GPS Error Sources• Clock Error (synchronization is only so good; +/- 2m) • PDOP: position dilution of precisionGPS Receiver Types• Autonomous– Hand held receivers with built-in antenna ($150 - $500)GPS Receiver Types• Autonomous– Hand held receivers with built-in antenna ($150 - $500)– Receiver and external antenna (usually as a backpack or harness) combo ($500 -$5,000)GPS Receiver Types• Autonomous– Hand held receivers with built-in antenna ($150 - $500)– Receiver and external antenna (usually as a backpack or harness) combo ($500 - $5,000)• Base Station (Survey Grade; Real-Time Kinetic) ($20,000 to $50,000)– Receiver and PDA data collector– Base station receiver with differential correction beacon broadcast Typical GPS Accuracy• Low-end autonomous: 5m (with differential beacon)• High-end autonomous: 2m (with differential beacon)• Real-Time Kinetic: 1cm4Differential Correction Beacons• A GPS receiver is permanently fixed at a known benchmark• A correction factor that accounts for the differential between the actual and calculated position is continually broadcast on the FM radio band from the benchmark• In theory any errors generated by PDOP or atmospheric conditions can be eliminated by a GPS receiver that applies the correction factor• Multipath errors are not eliminated by differential beacons (DGPS)GPS Accuracy & Precision• Precision is the reproducibility of the measurement• Accuracy is how close the measured position is to the actual location Concept of Precision & AccuracyGood precision,Poor accuracyPoor precision,Poor accuracyGood precision,Good accuracyPoor precision,Good accuracyAccuracy & Precision cont.Garmin Etrex Legend GPSUsing the Receiver for GPS Surveys(Lab Prep)Etrex Legend Buttons5Etrex Legend Buttons cont.Etrex Legend: Installing BatteriesEtrex Legend: Screen Pages Satellite PageMain Menu Page Main Menu Sub-Menus6Mark Waypoint Page Editing a Waypoint NameUpcoming StuffHomeworkNothing…..yetNext Lecture: More GPS and GIS Friday’s Lab: GIS