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**Under-the-Hood of Global Positioning Systems (GPS)**

By Ron Whitelock Under the Hood of GPS: Introduction

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"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

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**Acknowledgments GPS.gov website Wikipedia website**

Acknowledgments Under the Hood of GPS: Introduction

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Google Earth Under the Hood of GPS: Introduction

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**Navigation Systems Shows turn-by-turn directions and map with route**

Current location ETA, ETE Navigation Systems Under the Hood of GPS: Introduction

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**Divides the Earth into 60 zones, each 6° of longitude in width**

UTM Zone Map Under the Hood of GPS: Location Based Systems

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**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

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**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

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**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

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**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

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**Lat/Lng Conventions Lat=43.xxxx (N) Greenwich Lng=-79.yyyy(W) England**

(+) W (-) Lat/Lng Conventions Under the Hood of GPS: Understanding GPS

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**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

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**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

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**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

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GPS Data Formats Under the Hood of GPS: Understanding GPS

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**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

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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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Tools Under the Hood of GPS: Tools

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**UTM to Lat/Lng Redstart/Dundas W in Mississauga, ON**

Under the Hood of GPS: Tools

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Data Logger Under the Hood of GPS: Tools

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Tru-Traffic Under the Hood of GPS: Tools

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**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

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Questions? Under the Hood of GPS: Summary

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