Global Positioning System (GPS)

1. Global Positioning System (GPS)

2. GPS Basics • GPS stands for Global 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)

3. GPS 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

4. 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 Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

5. Satellites cont. • 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 Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.

6. 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

7. 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.

8. 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

9. 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

10. 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 1sec Satellite signal Receiver signal

11. How GPS Works: Trilateration • Start by determining distance between a GPS satellite and your position

12. How GPS Works: Trilateration • Adding more distance measurements to satellites narrows down your possible positions

13. How GPS Works: Trilateration

14. How GPS Works: Trilateration • The 4th satellite in trilateration is to resolve any signal timing error • Unlike GPS 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

15. GPS Error Sources • 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 • 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 Error • 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 Error • Atmospheric Delays/Bending • +/- 5 meters or error

18. Sources of Error • Multi Path Interference (signal bouncing off of buildings, trees, etc.) • +/- 1 meter of error

19. Sources of Error • Receiver Timing/Rounding Errors • +/- 1 meter of error (depends on the quality of the GPS receiver) Quadruple Redundant Atomic Clocks Accurate to Nanoseconds $800,000 in clocks on each satellite 2:02:01.23456789012 Powered by 4 AA Batteries ~$2.99 2:02:01.2345

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 Percision • 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. SatelliteClocks 0 6 12 18 24 30 Orbit Error Receivers Atmosphere Meters GPS - Error Budget Example of typically observed error from a consumer GPS 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

23. GPS - Error Correction • 2 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. Averaged Location GPS - Point Averaging This figure shows a successive series of 3-D positions taken using a receiver kept at the same location, and then averaged

26. ~ 300 miles (~ 480 km) or less Base station (known location) Rover receiver GPS - 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

27. GPS - 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 Example: Base Station File 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

28. GPS - 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. GPS Applications • Generating mapped data for GIS databases • Collecting field data - travel to the field and capture location & attribute information • Other uses (many in real time): • 911/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. Strengths of GPS • Easy To Incorporate into Project • Once trained, just about anyone can use it • Cheap • Widely Available

31. Weaknesses • Does require a training component • Accuracy Issues • Differential Correction may not be an option in many parts of the world

32. Planning a GPS Project • GPS point collection can be an easy way to build your database but planning is essential

33. Planning a GPS Project • Identify Your Accuracy Needs • Identify Error Correction Methodology • Point Averaging • How long will points be collected? • Differential Correction • Find a base station • Identify Point Collection Methodology • Where will points be collected? • Contingency plans • Data backups

34. Bottom Line • Cost depends on Project • Hardware • GPS -- $200 to $100,000 or more • Differential Correction (yes? no? real-time?) • Additional hardware • Computers, cables, batteries, antennas, etc. • People • Field Teams -- depends on length of field work • Simple projects in a day or two • More Complex projects can last months or years

35. Garmin GPS Introduction

36. Garmin GPS Antenna Control Buttons Display Screen

37. Garmin GPS • Step 1: Turn on Unit After a couple of seconds, the GPS unit will start looking for satellites. Off/On Button

38. Search for Satellites • Step 2: Wait for Satellites North Indicator Satellite Located, but not locked in Battery Gauge Satellite Located, and locked in Signal Strength for satellite Outer circle represents horizon, inner circle represents 45 degrees above horizon Satellite Acquisition Page

39. Acquire Position • Step 3: Once enough satellites have been located, the GPS unit will provide you with a position Direction Indicator Speed Altitude Position Time Position Page

40. Collect a Point • Step 4: Press the Mark button and begin collecting data. Mark Button To collect a single point, highlight Save and press ENTER. To collect an averaged point, highlight Average, press the Enter button and wait for a few minutes, then highlight Save and stop point collection Waypoint Page

41. Record Point • Step 5: Go to Menu Princ. Page and view waypoint list and record coordinate Waypoint ID Averaged Position Main Menu Page Waypoint Page

42. Other Pages Map Page Compass Page

43. On Friday • Meet outside at the Old Well • Don’t be late! • Be prepared to walk short distance • You will need a pencil or pen • Read your worksheet BEFORE class and remember to bring it on Friday • Rain Date – Monday, October 29