1. GPS Technology Jackie Van Ryzin, John Hinner, Ryan Maier, and Adam Kabat Global Positioning Systems

2. Technology involves a complex network of global satellites that utilizes radio signals and mathematic calculations to determine the location of the GPS receiver.

3. History and Development 1964, prior to GPS, there was the Transit system Transit had no timing devices and took 15 minutes to calculate a position US Department of Defense wanted a more precise method so they spent $12 million on what resulted in GPS

4. History and Development GPS was originally known as the Navstar Global Positioning System started in 1973 to reduce the need for other forms of navigational aid Overcame many navigational obstacles Assisted in many navigational applications

5. History and Development Original use of GPS was for military positioning, navigation, and weapons aiming system to replace Transit. It had higher accuracy and stable timing devices on board to achieve precise time transfer 1978-1 st GPS satellites we launched the first products for civilian use were available in the mid 80’s

6. History and Development met the requirement for Full Operational Capability (FOC) as of April 27, 1995 the first of the currently in-use satellites were launched in February 1989 the most recent satellite was launched on March 20, 2004

7. Satellites Satellite constellation ~ First of GPS satellites launched in 1978 ~ 16 years later, system up to full power ~ 24 Earth-orbiting satellites

8. Satellites Back-up Satellites ~ at least 3 at all times in case of failures ~ constantly developing and launching ~ too expensive to fix, just replace Satellites orbit so that at any time, 4 satellites are “visible” at any place on Earth

9. Satellites Solar-powered Rocket boosters keep them on track/orbit complete 2 full rotations within 24 hours 7,000 miles per hour 12,000 miles above the Earth

10. Radio Waves Transmit 2 low power signals (L1 & L2) frequency of 1575.42 MHz in the UHF band Travel at speed of light pseudorandom code, ephemeris data, and almanac data Ephemeris data tells satellite status (healthy/unhealthy, current date and time) Almanac tells where it should be

11. Measuring Distance Satellite transmits long digital pattern –Individual satellite ID Receiver begins running same pattern at same time When signal received, the lag or delay notes the time traveled Multiply by speed of light to get distance ~ this assumes straight line ~ no interference Pseudo-random Code

12. Measuring Distance Satellites and receiver must be in-sync Synchronize to the nanosecond Atomic clocks in satellites –$50,000-$100,000 Quartz clocks in receivers –Affordable –Constantly resetting to maintain accuracy –Look to four satellites to gauge own inaccuracy Synchronization

13. Trilateration / Triangulation 3-D / 2-D 2-D Example If you are unsure of your location but you know you are 25 miles from point A, you would know that you are somewhere on a circle with radius of 25 miles.

14. 2-D Example You learn from someone else that you are also 20 miles from point B. You now know that you are at one of two locations, the intersections of the two circles.

15. 2-D Example To determine your exact location, you ask a third person who responds that you are 15 miles from point C. This leaves you with one point at the intersection of the circles.

16. 3-D Translation Spheres in place of circles Two spheres give a perfect circle of possibilities Third sphere intersects this circle at two points –Only one of these will be on Earth (feasible) –Eliminate the one in space (infeasible)

17. Generally receivers will look at four satellites rather than the minimum three This offers greater accuracy Also helps in determining altitude

18. Differential GPS Four spheres will not intersect at same point if measured incorrectly Distances are proportionally incorrect GPS receiver adjusts –Adjusts distances proportionally to intersect –Resets clock to be in-sync based on proportional inaccuracy –Always adjusting so accuracy is near that of the atomic clocks in the satellites Department of Defense monitors any changes and transmits this to all receivers as part of satellite signal

19. Errors that Occur Earth’s atmosphere slows the signal –Receiver uses built-in model to partially adjust for this type of error Signal multi-path –Large objects such as skyscrapers cause signal to “bounce” and reflect, taking a longer path Receiver clock errors Orbital or ephemeris errors –Misreported location or orbital data Number of satellites visible

20. Error Budget SourceUncorrected Error Level Ionosphere0-30 meters Troposphere0-30 meters Measurement Noise 0-10 meters Ephemeris Data1-5 meters Clock Drift0-1.5 meters Multi-path0-1 meters Selective Availability 0-70 meters

21. To correct using differential GPS Gauge inaccuracy at a stationary receiver that knows its location Broadcast that inaccuracy to local DGPS receivers With DGPS accuracy up to 1-3 meters Provides accuracy to 10 meters

22. Protocol and PC Connections Basic link protocol –All data is transferred from the GPS unit (transmitter) to the PC (host) in byte-oriented packets –Each packet contains a three-byte header, a number of data bytes, and a three-byte trailer

23. Byte NumberByte DescriptionNotes 0Data Link EscapeASCII DLE Character (16 decimal) 1Packet IDIdentifies the type of packet 2Size of Packet DataNumber of bytes of packet data (bytes 3 to n – 4) 3 to n-4Packet Data0 to 255 bytes n – 3Checksum2’s complement of the sum of all bytes from byte 1 to byte n – 4 n – 2Data Link EscapeASCII DLE Character (16 decimal) n - 1End of TextASCII ETX Character (3 decimal) Packet Format

24. Protocol and PC Connections –Any device that receives a data packet must send an ACK or NAK packet in turn –The GPS unit recognizes an ACK from its packet ID of 6 and NAK packet ID of 21 –The checksum protocol in the trailer of every packet allows the receiver to send a NAK if the sum of the data it received is the same as the sum of the data that was sent –Additional basic packet ID’s are Product Request, which is sent from a host to request product information about the GPS unit that is being currently communicated with, and Product Data, which is sent to a host in return

25. Application Protocol Packets sent to the host are often grouped together during transfers to allow better surveillance of the transfer These transfers include standard beginning and end packets of type “records” and “Xfer Complete”

26. NDirectionPacket IDPacket Data Type 0Device 1 → Device 2Pid_RecordsRecords_Type 1Device 1 → Device 2Pid_Prx_Wpt_Data 2Device 1 → Device 2Pid_Prx_Wpt_Data ………… n-2Device 1 → Device 2Pid_Prx_Wpt_Data n-1Device 1 → Device 2Pid_Xfer_CmpltCommand_Id_Type

27. Byte NumberByte Description 0Data Link Escape 1Packet ID (Pid_Prx_Wpt_Data) 2Size of Packet Data 3 to n-4Packet Data n – 3Checksum n – 2Data Link Escape n - 1End of Text

28. typedef struct { charident[6];/* identifier */ semicircle_Typeposn;/* position */ longwordunused;/* should be set to zero */ charcmnt[40];/* comment */ bytesmbl;/* symbol id */ bytedspl;/* display option */ } D103_Wpt_Type;

29. Experimentations Accuracy –Recorded points at 10 minute intervals from one place to determine the change in the data due to error or inaccuracy –G:\PANKDC\cs225\projects\gps\Webpage\gps1.rtfG:\PANKDC\cs225\projects\gps\Webpage\gps1.rtf Unit comparison –We used two different units and compared both of their tracks to the road map to determine differences between them and accuracy to the actual map. –G:\PANKDC\cs225\projects\gps\Webpage\gps track.bmpG:\PANKDC\cs225\projects\gps\Webpage\gps track.bmp

30. Experimentations Number of Satellites –Recorded data from single place when more satellite signals were achieved to determine the difference in values. –G:\PANKDC\cs225\projects\gps\Webpage\gps4.bmpG:\PANKDC\cs225\projects\gps\Webpage\gps4.bmp –G:\PANKDC\cs225\projects\gps\Webpage\gps3.bmpG:\PANKDC\cs225\projects\gps\Webpage\gps3.bmp WAAS –WAAS is Wide Area Augmentation System –25 ground stations, 2 geostationary satellites –Still developing, not available in all areas –Unable to complete experiment, no WAAS satellite signal received

31. What GPS can tell you Odometer Time traveled Speedometer Average speed Trace your path Estimate time of arrival at current speed

32. How to use a GPS Each unit or model is different but the basics are the same Start at the main menu Some general options: –Mark –Find –Satellite –Routes –Tracks –Setup –Accessories

33. Mark Shows location: longitude and latitude Displays elevation in feet Shows the distance you have traveled Displays direction you are going (bearing)

34. Find Options to find are –waypoints (that you have marked) –Favorites (that you have saved or marked as favorite stops) –Cities (saved internally in map, detail depends on unit and map downloaded) –Exits (services, rest stops, etc.) Can find by –Nearest to current location –Name that was given to that point

35. Satellite Shows view of satellites looking up at the sky from location Satellites are numbered, outer ring is horizon center is directly above location middle circle is at a 45° angle from the vertical Bars above numbers denote signal strength ‘D’ means it has a WAAS signal Shows elevation and location

36. Other Options Routes & Tracks –Used to look up past paths –Allows to retrace old travels –Or return to original location by same path Some views that can be used for tracking and navigation

37. Setup –Time –Units –Display –interface (for PC communication) –system (GPS on/off, WAAS on/off, language) –Others dependent on the unit Accessories –Sun & Moon, Calendar, Hunt & Fish, Area Calculations, Calculator –Dependent on the model and its original purpose, these are just a few of the accessories Garmin offers

38. The End Thanks for listening For more information, view our website which will be available from the CompSci website at www.snc.edu/compsci/cs225F04/ProjectsF04.html www.snc.edu/compsci/cs225F04/ProjectsF04.html it is currently available at G:\PANKDC\cs225\projects\gps\Webpage\index.html Our webpage includes links to popular GPS websites throughout the explanations