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GPS

GPS. Philip Bunge, Andrew Malicowski, Kevin Wei. GPS. Global Positioning System Developed in 1973 Space/satellite based Provides: Location information Time information Works in any weather condition Works almost anywhere on earth With enough satellites. 2. What Uses GPS?. 3.

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GPS

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  1. GPS Philip Bunge, Andrew Malicowski, Kevin Wei

  2. GPS • Global Positioning System • Developed in 1973 • Space/satellite based • Provides: • Location information • Time information • Works in any weather condition • Works almost anywhere on earth • With enough satellites 2

  3. What Uses GPS? 3

  4. Why Use GPS? • Free to use • Accurate • Being outside is desirable • May work indoors, but not guaranteed • Small • Other systems • Not as accurate • Russian - GLONASS • European Union - Galileo 4

  5. GPS System Organization Space Segment • 24 satellites in orbit Control Segment • Global network of monitor and control stations User Segment • GPS receiver equipment 5

  6. How to Find Position • Uses location data of satellites to estimate distance • Creates spheres around each satellite, tries to find point where they all intersect 6

  7. How to Find Position • Distance = Transit Time * Speed of Light • Four Variables: • Longitude • Latitute • Height • Time Error 7

  8. Sources of Error • Atmospheric effects • Inconsistencies in atmosphere affect signal speed • Multipath effects • Reflected radio signals (buildings, canyons, etc) 8

  9. GPS Signal • Satellites transmit 1500 bits (frame) at 50bps • Subframes are 300 bits • Subframe 1 contains clock data • Subframes 2, 3 have the precise orbit of satellite (ephemeris) • Subframes 4 and 5 have almanac and configuration data 9

  10. GPS Signal • Each satellite has its own unique identifier signal • Pseudo Random Noise (PRN) • PRN and data are modulated by XOR operation • This signal modulates the L1 carrier frequency • To demodulate, perform steps in reverse 10 More information

  11. Startup Finding Satellites • Receiver isolates background noise by listening for a precisely timed pattern • Patterns are unique to each satellite • If a signal is found, needs to be listened to for the entire frame(30 seconds) • If signal lost within 30s, process must start over again. • If signal not found, this means: • satellite not nearby • path to satellite is obstructed Synchronizing Internal Clock • For positioning to be most accurate, needs synced clock • Usually requires 4 satellites 11

  12. Startup • A GPS receiver needs to get initial position when powered on • Three different kind of starts (slowest to fastest) Cold Start • No initial information • Searches for satellites and gets lock Warm Start • Remembers location • Searches for satellites Hot Start • Uses remembered satellites and tries to lock immediately 12

  13. Chipset Considerations • Antenna • Needs to see sky! • Size • Smaller may mean worse reception • Update rate • 1 Hz fine unless moving very quickly • Power requirements • Power hungry (30 mA at 3.3 V) • Number of channels • More means faster startup and greater accuracy (diminishing returns after 20) • Accuracy • Don't expect better than ± 3 m 13

  14. EM-406A 10m accuracy Small (30mmx30mmx10.5mm) 20 channels High sensitivity Multiple output formats $59.95 14

  15. EM-406A Default Serial Settings: • 4800 baud • 1 start bit • 8 bits data • no parity bit • 1 end bit Pin Assignments: 15

  16. NMEA 0183 Basic ASCII serial protocol predominantly used by marine electronics $GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M,,,,0000*18 Rules: • Message starts with dollar sign • First 2 characters identify talker • Next 3 identify type of message • Data fields separated by commas • Unavailable data fields left blank • Asterisk and checksum after last data field (optional) • Message ended with newline (<CR><LF>) 16

  17. Output (NMEA 0183 GGA) $GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M,,,,0000*18 17

  18. Questions? 18

  19. Sources http://gpsinformation.net/gpssignal.htm http://www8.garmin.com/aboutGPS/ http://www.gsmarena.com/glossary.php3?term=gps https://www.sparkfun.com/products/465 http://www.sparkfun.com/datasheets/GPS/EM-406A_User_Manual.PDF http://www.gps.gov/technical/icwg/IS-GPS-800B.pdf http://airandspace.si.edu/gps/work.html http://www.pcmag.com/article2/0,2817,2316534,00.asp http://www.losangeles.af.mil/library/factsheets/factsheet.asp?fsID=5311 http://geology.isu.edu/geostac/Field_Exercise/GPS/GPS_basics_u_blox_en.pdf http://www.gps-basics.com/ http://gpsinformation.net/gpssignal.htm http://www.physics.hmc.edu/research/geo/gps.html http://www.gps.gov 19

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