By Ron Whitelock

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

Under the Hood of GPS: Introduction

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*To Quote …

*"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

Under the Hood of GPS: Introduction

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*Acknowledgments

*GPS.gov website

*Wikipedia website

*http://aprs.gids.nl/nmea/

Under the Hood of GPS: Introduction

Under the Hood of GPS: Introduction 4

*Google Earth

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

*Current location

*ETA, ETE

*Navigation Systems

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*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

6Under the Hood of GPS: Introduction

*Then you should understand what is under-the-hood of these devices to obtain optimum use for the intended purpose

Under the Hood of GPS: Introduction 7

*Do you use any of these systems?

What does

this meanto me!

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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

Under the Hood of GPS: Introduction

*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

Under the Hood of GPS: Location Based Systems 9

*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

*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

Under the Hood of GPS: Location Based Systems 11

*Cartesian System

*Represents a point in the plane by a distance from the origin and an angle measured from a reference line intersecting the origin

Under the Hood of GPS: Location Based Systems 12

*Polar coordinate system

*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.

Under the Hood of GPS: Location Based Systems 13

*Universal Transverse Mercator (UTM)

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*UTM Zone MapZones

Under the Hood of GPS: Location Based Systems

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

*The CN Tower is zone 17, and the grid position is 630,084m east, 4,833,438m north

Under the Hood of GPS: Location Based Systems 15

*UTM

Click icon to add picture*LatitudeA position's distance north or south of the equator

*LongitudeThe distance east or west of the prime meridian (measured in degrees). [The prime meridian runs from the north to south pole through Greenwich, England]

Under the Hood of GPS: Location Based Systems 16

*Latitude and longitude

GreenwichEngland

*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.

Under the Hood of GPS: Understanding GPS 17

Greenwich

England

*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

Under the Hood of GPS: Understanding GPS 18

Greenwich

England

*Lat/Lng Conventions

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GreenwichEngland

W (-)

N(+)

Lat=43.xxxx (N)Lng=-79.yyyy(W)

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

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*GPS Calculations

*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

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 1983 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

Under the Hood of GPS: Understanding GPS

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*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

Under the Hood of GPS: Understanding GPS 25

*GPS Sentences

CSV format

Under the Hood of GPS: Understanding GPS 26

*GPS Sentence Detail

$GPRMC22051

6 A 5133.8 N 42.24 W 173.8 231.813069

4 4.2 W *701 2 3 4 5 6 7 8 9 10 11 12

1220516 Time Stamp

hhmmss GMT

2A validity - A-ok,

V-invalid

3 5133.82 current Latitude

4 N North/South

542.24 current

Longitude

6 W East/West

7173.8

Speed in knots

8 231.8 TRUE course

9130694 Date Stamp

ddmmyy

10 4.2 Variation11 W East/West12 *70 checksum

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*Time Keeping

*GPS satellites contain multiple cesium and rubidium clocks

*A cesium clock has an error of one second per million years

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

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

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

Under the Hood of GPS: Understanding GPS

* 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

Under the Hood of GPS: Understanding GPS 31

*Direction

*Speed

*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

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*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

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*DistanceACOS( SIN(lat1*PI()/180)*SIN(lat2*PI()/180) + COS(lat1*PI()/180)_*COS(lat2*PI()/180)*COS(lon2*PI()/180-lon1*PI()/180) ) * 6371

Click icon to add picture*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

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*Elevation

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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

Under the Hood of GPS: Tools 37

*Data Logger

Under the Hood of GPS: Tools 38

*Tru-Traffic

Under the Hood of GPS: Summary 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

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*Questions?