1. Under-the-Hood of Global Positioning Systems (GPS)
By Ron Whitelock
Under the Hood of GPS: Introduction

2. "When I'm playful I use the meridians of longitude and the parallels of latitude... and drag the Atlantic Ocean for whales." Mark Twain, Life on the Mississippi
To Quote …
Under the Hood of GPS: Introduction

3. Acknowledgments GPS.gov website Wikipedia website
Acknowledgments
Under the Hood of GPS: Introduction

4. Google Earth
Under the Hood of GPS: Introduction

5. Navigation Systems Shows turn-by-turn directions and map with route
Current location
ETA, ETE
Navigation Systems
Under the Hood of GPS: Introduction

6. Maps Set starting location, destination Get directions
Show turn-by-turn directions and map with route
Edit and updates
May even include traffic information, road closures, incidents, etc.
Maps
Under the Hood of GPS: Introduction

7. Do you use any of these systems?
Then you should understand what is under- the-hood of these devices to obtain optimum use for the intended purpose
What does
this mean
to me!
Do you use any of these systems?
Under the Hood of GPS: Introduction

8. Things you should know Under-the-Hood!
Become aware of geo location systems
Develop an understanding of GPS terminology
Develop an appreciation for GPS applications
Develop an understanding of GPS limitations
Things you should know Under-the-Hood!
Under the Hood of GPS: Introduction

9. What is a geo coordinate system
Typically, we identify locations by address – 123 Main Street, City, Prov/State, Country
However this approach is not specific nor accurate enough for many applications
What is a geo coordinate system
Under the Hood of GPS: Location Based Systems

10. What is a geo coordinate system
A system that uses one or more numbers, or coordinates, to uniquely determine the position of a point
There are many coordinate systems from which to choose
A couple common examples follow
What is a geo coordinate system
Under the Hood of GPS: Location Based Systems

11. Cartesian System Also called the "rectangular coordinate system”
Two and three- dimensional spaces, uses two (three) numbers representing distances from the origin in two (three) mutually perpendicular directions
Cartesian System
Under the Hood of GPS: Location Based Systems

12. Polar coordinate system
Represents a point in the plane by a distance from the origin and an angle measured from a reference line intersecting the origin
Polar coordinate system
Under the Hood of GPS: Location Based Systems

13. Universal Transverse Mercator (UTM)
UTM coordinate system was developed by the United States Army Corps of Engineers in the 1940s.
The system was based on an ellipsoidal model of Earth.
Universal Transverse Mercator (UTM)
Under the Hood of GPS: Location Based Systems

14. Divides the Earth into 60 zones, each 6° of longitude in width
UTM Zone Map
Under the Hood of GPS: Location Based Systems

15. The CN Tower is zone 17, and the grid position is 630,084m east, 4,833,438m north
UTM
Under the Hood of GPS: Location Based Systems

16. Latitude and longitude
Greenwich
England
Latitude
A position's distance north or south of the equator
Longitude
The distance east or west of the prime meridian (measured in degrees). [The prime meridian runs from the north to south pole through Greenwich, England]
Latitude and longitude
Under the Hood of GPS: Location Based Systems

17. Latitude-Lat for short
Lines of Latitude, or parallels of latitude, measure location north or south of the equator
Latitude lines are equidistant from each other and each degree of latitude is about 69 miles (110 km) apart
Zero degrees (0°) latitude is the equator, the widest circumference of the globe.  Latitude is measured from 0° to 90° north and 0° to 90° south
90° north is the North Pole and 90° south is the South Pole
Latitude is always listed before Longitude.
Latitude-Lat for short
Greenwich
England
Under the Hood of GPS: Understanding GPS

18. Longitude-lng for short
Lines of Longitude measure location east or west of the prime meridian and pass through Greenwich, England.  Zero degrees longitude (0°) is the prime meridian
Lines of Longitude run vertically around the globe, 180° east and 180° west from the prime meridian
For every 15° of longitude one travels eastward, the local time moves ahead one hour-  local time moves back one hour for every 15° travelling west
By knowing the local times at two points on Earth, we can calculate how far apart those places are in longitude, east or west
Longitude-lng for short
Greenwich
England
Under the Hood of GPS: Understanding GPS

19. Lat/Lng Conventions Lat=43.xxxx (N) Greenwich Lng=-79.yyyy(W) England
(+)
W (-)
Lat/Lng Conventions
Under the Hood of GPS: Understanding GPS

20. What is GPS Location system based on Latitude and Longitude Includes
Elevation above sea level
Speed (knots per hour)
Heading (degrees 0-259)
And error correction
What is GPS
Under the Hood of GPS: Understanding GPS

21. A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites
Each satellite continually transmits messages that include:
the time the message was transmitted and,
satellite position at time of message transmission
The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite
In typical GPS operation, four or more satellites must be visible to obtain an accurate result
GPS Calculations
Under the Hood of GPS: Understanding GPS

22. Short History of GPS Started as a military activity
After Korean Air Lines Flight 007 was shot down in after straying into the Russian airspace, President Reagan issued a directive making GPS available for non military use]
The first satellite was launched in 1989, and the 24th satellite was launched in 1994
As of December 2012, there are 32 satellites
President Clinton ordering Selective Availability (SA) to be turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 to 20 meters
Short History of GPS
Under the Hood of GPS: Understanding GPS

23. GPS Data Formats
Under the Hood of GPS: Understanding GPS

24. Common GPS (Data) Sentences
Data formats defined by NMEA
All sentences begin with ‘$GP’ then three characters
19 in total – not including proprietary ones
$GPRMC
Recommended minimum GPS/Transit data
$GPGGA
Global Positioning System Fix Data
$GPGSV
GPS Satellites in view
Common GPS (Data) Sentences
Under the Hood of GPS: Understanding GPS

25. CSV format
GPS Sentences
Under the Hood of GPS: Understanding GPS

26. GPS Sentence Detail $GPRMC 220516 A 5133.8 N 42.24 W 173.8 231.8
130694
4.2
*70 1 2 3 4 5 6 7 8 9 10 11 12
Time
Stamp
hhmmss
GMT
validity -
A-ok,
V-invalid
current
Latitude
North/South
Longitude
East/West
Speed in
knots
TRUE
course
Date
ddmmyy
Variation
checksum
GPS Sentence Detail
Under the Hood of GPS: Understanding GPS

27. GPS satellites contain multiple cesium and rubidium clocks
A cesium clock has an error of one second per million years
Time Keeping
Under the Hood of GPS: Understanding GPS

28. Differential GPS (DGPS)
An extension of the GPS system that uses land- based radio beacons to transmit position corrections to GPS receivers
DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to better than 10 meters
Differential GPS (DGPS)
Under the Hood of GPS: Understanding GPS

29. Dilution Of Precision (DOP)
A measure of the GPS receiver/satellite geometry
The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset)
A low DOP value indicates better relative geometry and higher corresponding accuracy
Dilution Of Precision (DOP)
Under the Hood of GPS: Understanding GPS

30. Wide Area Augmentation System (WAAS)
A system of satellites and ground stations that provide GPS signal corrections for better position accuracy
A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time
Wide Area Augmentation System (WAAS)
Under the Hood of GPS: Understanding GPS

31. Bearing is the direction to the destination or target regardless of your own direction of travel
Heading is the direction you are traveling or facing regardless of the bearing to the target
Expressed in degrees from either True North or Magnetic North
Direction
Under the Hood of GPS: Understanding GPS

32. GPS receivers display speed (knots) and calculate the speed using algorithms in the Kalman filter
Most receivers compute speed by a combination of movement per unit time and computing the Doppler shift in the pseudo range signals from the satellites
The speed is smoothed and not instantaneous speed
0.5 kph accuracy is common
Speed
Under the Hood of GPS: Understanding GPS

33. The length (in feet, meters, miles, etc
The length (in feet, meters, miles, etc.) between two waypoints or from your current position to a destination waypoint
This length can be measured in straight-line (rhumb line) or great- circle (the shortest course between two points on the surface of a sphere)
GPS normally use great-circle calculations for distance and desired track
Distance
ACOS( SIN(lat1*PI()/180)*SIN(lat2*PI()/180) + COS(lat1*PI()/180)_
*COS(lat2*PI()/180)*COS(lon2*PI()/180-lon1*PI()/180) ) * 6371
Under the Hood of GPS: Understanding GPS

34. Elevation Elevation is the distance above or below mean sea level
With most low cost GPS receivers, the horizontal error is within about +/- 15 meters (50 feet) 95% of the time
Elevation
Under the Hood of GPS: Understanding GPS

35. Tools
Under the Hood of GPS: Tools

36. UTM to Lat/Lng Redstart/Dundas W in Mississauga, ON
Under the Hood of GPS: Tools

37. Data Logger
Under the Hood of GPS: Tools

38. Tru-Traffic
Under the Hood of GPS: Tools

39. Summary GPS can be a very useful tool in our daily work
Understanding the creation of the data and its ‘first-principles’ can help in its use
Ignore the rumours that GPS satellite were going to fall to earth
Summary
Under the Hood of GPS: Summary

40. Questions?
Under the Hood of GPS: Summary