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GPS Basics

GPS Basics. What is GPS? GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites • Created and Maintained by the US Dept. of Defense and the US Air Force • System as a whole consists of three segments

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GPS Basics

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  1. GPS Basics What is GPS? GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites •Created and Maintained by the US Dept. of Defense and the US Air Force • System as a whole consists of three segments satellites (space segment) receivers (user segment) ground stations (control segment)

  2. Satellites

  3. Satellites (space segment) • 24 NAVSTAR satellites (21 operational and 3 spares) • orbit the Earth every 12 hours • ~11,000 miles altitude • positioned in 6 orbital planes • orbital period/planes designed to keep 4-6 above the horizon at any time • controlled by five ground stations around the globe

  4. GPS – User Segment (Receivers) • Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once • Determine their position using the time it takes signals from the satellites to reach the hand-held unit • 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 system

  5. Ground stations (control segment) Ground Stations (control segment) Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. • Five control stations • master station at Falcon (Schriever) AFB, Colorado • monitor satellite orbits & clocks • broadcast orbital data and clock corrections to satellites

  6. How It Works (p. 1) GPS - 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 signals

  7. How It Works (p. 2) Start by determining distance between a GPS satellite and your position Adding more distance measurements to satellites narrows down your possible positions

  8. How It Works (p. 3) Three distances = two points • Intersection of Four spheres = one point • Note: • 4th measurement not needed • Used for timing purposes instead (discussed later)

  9. How It Works (p. 4) • Distance between satellites and receivers • determined by timing how long it takes the signal to travel from satellite to receiver. • How? • Radio signals travel at speed of light: 186,000 miles/second • Satellites and receivers generate exactly the same signal at exactly the same time • Signal travel time = delay of satellite signal relative to the receiver signal • Distance from satellite to receiver = • signal travel time * 186,000 miles/second 1sec Satellite signal Receiver signal

  10. How It Works (p. 5) • How do we know that satellites and receivers generate the same signal at the same time? • satellites have atomic clocks, so we know they are accurate • Receivers don't -- so can we ensure they are exactly accurate? No! • But if the receiver's timing is off, the location in 3-D space will be off slightly... • So: Use 4th satellite to resolve any signal timing error instead • determine a correction factor using 4th satellite • (like solving multiple equations...will only be one solution that satisfies all equations)

  11. Error Sources • Satellite errors • satellite position error • atomic clock, though very accurate, not perfect. • Atmosphere • Electro-magnetic waves travels at light speed only in vacuum. • The ionosphere and atmospheric molecules change the signal speed. • Multi-path distortion • signal may "bounce" off structures nearby before reaching receiver – the reflected signal arrives a little later. • Receiver error: Due to receiver clock or internal noise. • Selective Availability

  12. Poor Ideal GPS - Sources of Error • Satellite Coverage in Sky • Position Dilution of Precision (PDOP)

  13. 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 • Based on Clinton’s order, Selective Availability turned off early May 2, 2000

  14. SatelliteClocks 0 6 12 18 24 30 Orbit Error Receivers Atmosphere Meters GPS - Error Budget • Example of some typical observed using a consumer GPS receiver: • Typical observed errors • satellite clocks 0.6 meters • orbit error 0.6 meters • receiver errors 1.2 meters • atmosphere 3.7 meters • Total 6.1 meters • Multiplied by PDOP (1 - 6) 6.1 - 36.6 meters

  15. GPS - Error Correction • 2 Methods: • Point Averaging • Differential Correction

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

  17. ~ 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 the same among all receivers within 300 miles of each other

  18. 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

  19. 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 • Real-time differential correction • Done in real time by receiving a broadcasted correction signal (usually expensive), requiring other hardware (not just a consumer GPS receiver)

  20. GPS Applications • Generating mapped data for GIS databases • “traditional” GIS analysts & data developers • travel to field and capture location & attribute information cheaply (instead of surveying) • Other uses (many in real time): • 911/firefighter/police/ambulance dispatch • car navigation • roadside assistance • business vehicle/fleet management • mineral/resource exploration • wildlife tracking • boat navigation • Recreational • Ski patrol/medical staff location monitoring

  21. Garmin’s cheapest receivers Garmin’s Forerunner 201: A watch that uses GPS to determine current speed, average speed, exact distance traveled, etc. ( ) Basic features also available in the Forerunner 101 ($115). http://www.garmin.com/products/forerunner201/ Garmin’s iQue 3600 PDA: http://www.garmin.com/products/iQue3600/

  22. Garmin’s Outdoor GPS Receivers:etrex series Basic GPS Garmin makes a host of GPS receivers for outdoor sports enthusiasts. http://www.garmin.com/outdoor/products.html

  23. Garmin’s Outdoor GPS Receivers: Etrex Legend C ($375) “Along with the Etrex Vista C, is one of Garmin's smallest, least expensive products to combine a color TFT display and advanced GPS routing capabilities in a waterproof design.” --is WAAS enabled --has USB port for downloading maps from Garmin’s MapSource CD library Etrex Vista C ($430) --has a TFT (thin-film transistor, with 1-4 tranistors controlling each pixel; it is the highest-definition flat-panel technique) display --WAAS enabled --has USB port for downloading maps from Garmin’s MapSource library

  24. Bluetooth GPS Receivers Teletype’s Mini-bluetooth GPS receiver ($175) http://www.mightygps.com/Manufacturer/minibluetooth.htm Teletype’s USB GPS receiver for Laptops ($170) http://www.teletype.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=1250&Category_Code=

  25. HP’s Ipaqs and other PDAs with GPS software Hewlett-Packard’s new iPAQ h1945 PDA Now comes equipped with a hp GPS receiver and navigation system ($500) http://www.shopping.hp.com/cgi-bin/hpdirect/shopping/scripts/product_detail/product_detail_view.jsp?BV_SessionID=@@@@0280349227.1102102313@@@@&BV_EngineID=ccckadddfdjlkdgcfngcfkmdflldfgg.0&landing=null&category=handhelds&subcat1=classic_performance&product_code=PF527A%23ABA&catLevel=3 Garmin’s iQue 3600 PDA: http://www.garmin.com/products/iQue3600/

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