Sources of Geographic InformationGlobal Positioning System (GPS)GPS – Space Segment (Satellites)GPS – User Segment (Receivers)GPS - How Does it Work?GPS - Satellite SignalsGPS - TrilaterationGPS - Trilateration Cont.GPS - Using the 4th SignalGPS - Sources of ErrorGPS - Sources of ErrorGPS - Sources of ErrorGPS - Sources of ErrorGPS - Sources of ErrorGPS - Sources of ErrorGPS - Selective AvailabilityGPS - Error BudgetGPS - Error CorrectionGPS - Point AveragingGPS - Point AveragingGPS - Differential CorrectionGPS - Differential CorrectionGPS - Differential CorrectionGPS - Differential CorrectionGPS - Differential CorrectionGPS - Differential CorrectionGPS ApplicationsComparing Soil Moisture and TMIPond Branch Catchment – Control Color Infrared Digital OrthophotographyGlyndon Catchment – UrbanizingColor Infrared Digital OrthophotographyDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005•Data properties: Spatial data, i.e. data that are associated with geographic locations•Data format: digital (analog data for traditional paper maps)•Data Inputs:•sampled from the real world•digitizing from paper maps•produced by government agencies, e.g. census bureau, USGS, USFS, state government, etc.•space or airborne remote sensing (NASA, NOAA, commercial, etc.)Approximately 80% of the duration of many large scale GIS projects is concerned with data input and managementSources of Geographic InformationÆ Using GPS? DIY!David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Global Positioning System (GPS)• A space-based 3-dimensional measurement and positioning system that operates using radio signals from satellites orbiting the earth• Created and maintained by the US Dept. of Defense and the US Air Force• The system as a whole consists of three segments:– satellites (space segment)– receivers (user segment)– ground stations (control segment)• Note: Russia and a European consortium are implementing similar systems.David Tenenbaum – GEOG 070 – UNC-CH Spring 2005• 24 NAVSTAR satellites in the GPS constellation• orbit the Earth every 12 hours• ~11,000 miles altitude (a very high orbit)• positioned in 6 orbital planes (4 per plane)• orbital period & planes are designed to keep 4-6 satellites above the horizon at any time everywhere on the planet • controlled and monitored by five ground stations around the globe GPS – Space Segment (Satellites)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005David Tenenbaum – GEOG 070 – UNC-CH Spring 2005• Ground-based devices that can read and interpret the radio signals from several of the NAVSTAR satellites at once•Use timing of radio signals to calculate the receiver’s position on the Earth's surface • Calculations result in varying degrees of accuracy that depend on:• quality of the receiver • user operation of the receiver• local & atmospheric conditions• current status of systemGPS – User Segment (Receivers)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005• Five control stations• master station at Falcon (Schriever) AFB, Colorado• monitor satellite orbits & clocks• broadcast orbital data and clock corrections to satellitesGPS – Control Segment (Ground Stations)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005• GPS allows us to determine a position by calculating the distance between a receiver and multiple satellites• Distance is determined by timing how long it takes the signal to travel from satellite to receiver• Radio signals travel at speed of light: 186,000 mi / sec • Satellites and receivers generate exactly the same signal at exactly the same time • Signal travel time = delay of satellite signal relative to the receiver signal1µsecReceiver signalSatellite signalGPS - How Does it Work?David Tenenbaum – GEOG 070 – UNC-CH Spring 2005GPS - Satellite Signals• Satellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same time• The satellite signals contains information that includes– Satellite number– Time of transmission• Receivers use an almanac that includes– The position of all satellites every second– This is updated monthly from control stations• The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite• Trilateration (similar to triangulation) is used to determine location from multiple satellite signalsDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Start by determining the distance between a single GPS satellite and your position (a sphere)Adding a second distance measurement to another satellite narrows down your possible positions to a circlewhere the spheres intersectGPS - TrilaterationDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Adding a third satellite narrows down the position to two pointswhere the three sphere intersect, and usually only one point is a ‘reasonable’ answerThe intersection of four spheres occurs at one point, but the 4th measurement is not needed, and is used for timing purposes insteadGPS - Trilateration Cont.David Tenenbaum – GEOG 070 – UNC-CH Spring 2005• How do we know that satellites and receivers generate the same signal at the same time? • The satellites have atomic clocks, so we know they are accurate •But receivers do not -- so can we ensure they are exactly accurate? No! If the receiver's timing is off, the location in 3-D space will be off slightly...• So: We use the 4th satellite to resolve any signal timing error instead by• determining a correction factor using the 4th satellite• (like solving multiple equations ... there will only be one solution that satisfies all equations) GPS - Using the 4th SignalDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005• Satellite errors• satellite position / orbit error• satellite clock error • Atmospheric errors• Speed of electromagnetic waves in the atmosphere• Path taken by the signal• Multi-path distortion errors• Receiver errors• (Selective availability)GPS - Sources of ErrorDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005GPS - Sources of Error• Satellite Errors– Although the satellites are in high orbits to minimize their deviations, sometimes there is a slight ‘wobble’ due to local gravitational forces– While the atomic clocks used in the satellites are extremely accurate (and quadruple redundant), sometimes clock errors can occur• These can contribute up to