Global Positioning System
- 1. Global Positioning System Presented by: Prantik Chowdhury Dept. of Mechanical Engineering 3rdYear, B/1, 1457082.
- 2. Contents: ▪ Basics ▪ History ▪ Satellites ▪ Receivers ▪ Ground Stations ▪ Working ▪ Errors ▪ Error Correction ▪ Applications
- 3. Basics ▪ GPS stands forGlobal Positioning System which measures 3-D locations on Earth surface using satellites ▪ GPS operates using radio signals sent from satellites orbiting the earth ▪ Created and Maintained by the US Dept. of Defense ▪ System as a whole consists of three segments – Satellites (space segment) – Receivers (user segment) – Ground stations (control segment)
- 4. History ▪ Development began in 1973 ▪ First satellite became operational in 1978 ▪ Declared completely functional in 1995 ▪ A total of 52 satellites have been launched in 4 phases ▪ 30 satellites are currently functional ▪ Managed by the U.S. Department of Defense – Originally developed for submarines – Now part of modern “smart bombs” and highly accurate missiles
- 5. Satellites ▪ At least 4 satellites are above the horizon anytime anywhere ▪ GPS satellites are also known as “NAVSTAR satellites” ▪ The satellites transmit time according to very accurate atomic clocks onboard each one ▪ The precise positions of satellites are known to the GPS receivers from a GPS almanac A visual example of a 24 satellite GPS constellation in motion with the earth rotating. Notice how the number of satellites in view from a given point on the earth's surface, in this example in Golden CO (39.7469° N, 105.2108° W), changes with time.
- 6. Satellites(Contd.) ▪ The satellites are in motion around the earth ▪ Like the sun and moon satellites rise and set as they cross the sky ▪ Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected
- 7. Receivers ▪ Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once ▪ Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver ▪ Calculations result in varying degrees of accuracy that depend on: – Quality of the receiver – User operation of the receiver (e.g., skill of user and receiver settings) – Atmospheric conditions – Local conditions (i.e., objects that block or reflect the signals) – Current status of system A handheld GPS receiver.
- 8. Ground Stations ▪ Control stations – Master station at Falcon (Schriever)AFB, Colorado – 4 additional monitoring stations distributed around the world ▪ Responsibilities – Monitor satellite orbits & clocks – Broadcast orbital data and clock corrections to satellites Map from P. Dana,The Geographer's Craft Project, Dept. of Geography, U.Texas-Austin.
- 9. How GPS Works: Overview ▪ Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time – Think “synchronize your watches” ▪ The satellite signals contain information that includes – Satellite number – Time of transmission
- 10. How GPS Works: Overview ▪ 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 signals
- 11. How GPS Works: Signal Processing ▪ Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver – Radio signals travel at speed of light (186,000 miles/second) – All satellites send the identical time, which is also generated by the receivers – Signal travel time = offset between the satellite signal and the receiver signal – Distance from each satellite to receiver = signal travel time * 186,000 miles/second 1sec Receiver signal Satellite signal
- 12. How GPS Works: Trilateration
- 13. How GPS Works: Trilateration ▪ Start by determining distance between a GPS satellite and your position ▪ Adding more distance measurements to satellites narrows down your possible positions ▪ The 4th satellite in trilateration is to resolve any signal timing error – UnlikeGPS satellites, GPS receivers do not contain an atomic clock – To make sure the internal clock in the receiver is set correctly we use the signal from the 4th satellite
- 14. Errors ▪ Satellite errors: Satellite position error (i.e., satellite not exactly where it’s supposed to be) Atomic clocks, though very accurate, are not perfect ▪ Atmospheric Electro-magnetic waves travel at light speed only in a vacuum Atmospheric molecules, particularly those in the ionosphere, change the signal speed
- 15. Errors(Contd.) ▪ Multi-path distortion The signal may "bounce" off structures before reaching the GPS receiver – the reflected signal arrives a little later ▪ Receiver error Due to the receiver clock or internal noise ▪ Selective Availability No longer an issue
- 16. Sources of Errors ▪ Satellite Clock & Satellite Position • Atomic clock errors +/- 2 meters of error • Satellite is not in precise orbit +/- 2.5 meters of error
- 17. Sources of Errors(Contd.) ▪ Atmospheric Delays/Bending – +/- 5 meters or error
- 18. Sources of Errors(Contd.) ▪ Multi Path Interference (signal bouncing off of buildings, trees, etc.) – +/- 1 meter of error
- 19. Sources of Errors(Contd.) ▪ ReceiverTiming/Rounding Errors – +/- 1 meter of error (depends on the quality of the GPS receiver)
- 20. GPS - Selective Availability ▪ A former significant source of error – Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasons – Selective Availability turned off early May 2, 2000
- 21. GPS Error: Position Dilution of Precision ▪ Satellite Coverage: Position Dilution of Precision (PDOP) ▪ Remember that satellites are moving, causing the satellite constellation to change ▪ Some configurations of satellites are better than others ▪ PDOP values range from 1 to 50, with values < 6 considered “good” Poor PDOP Good PDOP
- 22. GPS: Error Budget ▪ Example of typically observed error from a consumerGPS receiver: • Typical Observed errors (meters) satellite clocks 0.6 orbit (position error) 0.6 receiver errors 1.2 atmosphere 3.7 • Total 6.1 • Multiplied by PDOP (1-6) • Total error ~ 6.1 - 36.6 meters Meters Atmosphere Receivers Orbit Error Satellite Clocks 0 6 12 18 24 30
- 23. GPS: Error Correction ▪ Methods: – Point Averaging – Differential Correction
- 24. Point Averaging ▪ Point Averaging is one of the simplest ways to correct GPS point locations – Collect many GPS measurements at the same location and then average them to get one point – The averaged point should have greater accuracy than a single point measurement – Accuracy varies with this method but you should have a position that is within 5 meters of its true location 95% of the time
- 25. Point Averaging Averaged Location This figure shows a successive series of 3-D positions taken using a receiver kept at the same location, and then averaged
- 26. Differential Correction ▪ Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver) ▪ Any errors in a GPS signal are likely to be almost the same among all receivers within ~ 300 miles of each other ~ 300 miles (~ 480 km) or less Base station (known location) Rover receiver
- 27. Differential Correction ▪ The base station knows its own location ▪ It compares this location with its location at that moment obtained using GPS satellites, and computes error ▪ This known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment Time GPS Lat GPS Long Lat. error Long. error 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3:15.0 35.50 35.05 34.95 36.00 35.35 35.20 79.05 78.65 79.55 80.45 79.30 79.35 .5 .05 -.05 1.0 .35 .20 .5 -.35 .55 1.45 .30 .35 Example: Base Station File
- 28. Differential Correction ▪ GPS error when using differential correction: 1 – 3 meters ▪ There are two ways that differential correction can be applied: – Post-processing differential correction ▪ Does the error calculations after the rover has collected the points ▪ Requires downloading a base-station file – Real-time differential correction ▪ Done in real time by receiving a broadcasted correction signal ▪ May require additional hardware
- 29. Applications • Emergency/firefighter/police/ambulance dispatch • Car & boat navigation • Roadside assistance • Business vehicle/fleet management • Mineral/resource exploration • Wildlife tracking • Recreational (fishing, hunting, hiking, etc.) • Ski patrol/medical staff location monitoring
- 30. Bibliography ▪ wikipedia.com ▪ gps.gov ▪ howstuffworks.com ▪ physics.org ▪ explainthatstuff.com
- 31. Thank you