1. GPS Philip Bunge, Andrew Malicowski, Kevin Wei

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

3. What Uses GPS? 3

4. Free to use Accurate o Being outside is desirable o May work indoors, but not guaranteed Small Other systems o Not as accurate o Russian - GLONASS o European Union - Galileo Why Use GPS? 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: o Longitude o Latitute o Height o Time Error 7

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

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

10. GPS Signal Each satellite has its own unique identifier signal o 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 o Patterns are unique to each satellite o If a signal is found, needs to be listened to for the entire frame(30 seconds) o If signal lost within 30s, process must start over again. o 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 o No initial information o Searches for satellites and gets lock Warm Start o Remembers location o Searches for satellites Hot Start o Uses remembered satellites and tries to lock immediately 12

13. Chipset Considerations Antenna o Needs to see sky! Size o Smaller may mean worse reception Update rate o 1 Hz fine unless moving very quickly Power requirements o Power hungry (30 mA at 3.3 V) Number of channels o More means faster startup and greater accuracy (diminishing returns after 20) Accuracy o 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: o Message starts with dollar sign o First 2 characters identify talker o Next 3 identify type of message o Data fields separated by commas o Unavailable data fields left blank o Asterisk and checksum after last data field (optional) o Message ended with newline ( ) 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