The Global Positioning System.

GPS antenna

GPS receiver and batteries!

Windows CE handheld computer

Paper Map with targets

Matt Evans of Abe591a and Aaron Pierce of the Biology Department; mapping the Purdue University Ross Reserve.

Sample Location Flag/Pin

ASM 215 April 2009

How does GPS work

• In a “nutshell”, time equals distance.• If the time signal is delayed by

bounces, then the receiver will think the satellite is in the wrong place, and the location is calculated wrong.

• Our discussion is VERY simplified

GPS Basics• GPS = Global Positioning

System (uses satellites)• GPS has significantly

changed surveying, navigation, shipping, airline, transportation

• GPS has become the most common method for field data collection in GIS

http://classic.mountainzone.com/everest/98/photos/4-27/gps-receiver.html

Three components of the Global Positioning System

US Satellite Component

4 to 8 satellites are typically visible from any unobstructed viewing location on earth

Control component• Master control station in Colorado Springs

Collects: • Satellite health and status information• Satellite tracking information from each tracking

station• Timing data from the U.S. Naval Observatory• Earth data from the U.S. Defense Mapping

Agency

Signals course corrections, changes in operation, etc.

User component

• Individuals with GPS receivers

GPS systems around the world

• NAVSTAR – U.S. (Department of Defense)

• GLONASS – Russia

• Galileo – Consortium of European governments and industries

http://europa.eu.int/comm/dgs/energy_transport/galileo/video/index_en.htm

GPS Range Distance

• Range = speed of light * travel time

Combining range measurements

GPS Signals – Coded and Carrier

• Two carrier signals

• Modulated to produce two coded signals– Also called pseudo-random code, because it

appears similar to random noise

• Positions based on carrier signal measurement are more accurate than those based on the code signal measurements

Range measurement from coded signal

Uncertainty in position

• Atmospheric and ionospheric delays– (speed of light is only constant in a vaccuum)– No analytical method to remove errors

• System operation and delays (smaller error)

• Receiver errors– Clocks may use algorithms that do not

precisely calculate position– Multipath signals from reflections off buildings

Satellite geometry and Positional Dilution of Precision

Time sat 1

Time sat 2

Time sat 3

Your location

Accuracy without differential correction

• Code phase receivers typically provide – 3 to 30 meter accuracy for single reading– 2 to 15 meters for multiple fixes

• Carrier phase: a few centimeters but need differential

Time sat 1Time sat 2

Trees

Time sat 3

Pavement

Multipath error.

These represent locationscollected by a fixed GPS antenna on our roof. In 24 hours the points scatter around a 4.0 meter circle. In Tippecanoe county a second of a degree is about 30 meters.

Reducing positional error

1. Collect many position fixes while remaining stationary– Also provides an estimate of variation

(standard deviation)– Cannot be used in moving vehicle, for example

2. Use differential correction

Differential Correction

• Two receivers are used to greatly improve the accuracy of GPS positional measurements

• Base station at known location

Differential correction

• Post-processed

• Real-time

Sources of real-time differential correction

• Radio transmitter from a private base station

• U.S. Coast Guard has established GPS radio beacons– Concentrated near the oceans, Great Lakes,

and Mississippi River– Need to purchase beacon receiver package

that supports real time correction using the Coast Guard beacon signal

Sources of real-time differential correction - Wide Area Augmentation System (WAAS)

• Administered by the U.S. Federal Aviation Administration for dependable aircraft navigation

• Based on a network of ground reference stations scattered about North America. Correction transmitted to a satellite.

• Individual errors are less than 7 m 95% of the time; errors for multiple readings (30 minutes) are 1 to 3 meters

Source of real-time differential correction – Commercial satellites

• Omnistar and Landstar have a set of base stations distributed across a region

• Available on a subscription or license basis

Carrier phase GPS (RTK)

• Real Time Kinematic GPS, or RTK can accuracy needed for surveying

• Uses the carrier phase of the GPS signal

• Accuracies of around ½ inch horizontal and 1 inch in the vertical direction.

Topographic assessment at Davis Purdue Ag Center

Applications of GPS

Lab Tomorrow

We will use low precision Garmin 12XLGPS units from Forestry and Natural Resources.

You may use your own if you are familiar with it.

Push the “Page” button until you get the screen which displays location.They will be set to display degrees, minutes and decimal seconds.

Lab ExerciseCollect readings at six locations.

Each location you collect 5 readings over 5 to 10 minutes.

Do some calculations

GIS lab will use this data

Latitude - Longitude

• GPS measures where you are on the planet, not where you are on a map, so it uses DEGREES:

• 400 25 ‘ 19.1”

• 40 degrees 25 minutes 19.1 seconds – 60 seconds in a minute, 60 minutes in a degree

Latitude Longitude

Meets at the poles Goes on around the world

Geographic coordinate system

• Latitude varies from north to south

• Longitude varies from east to west

• Measurements in degrees minutes and seconds, or “decimal degrees”

Not sphere but spheroid

• Newton and others in the 17th and 18th century proposed that the Earth is flattened due to rotational forces.

• Complex, repeated, highly accurate measurements established that the curvature of the Earth was greater at the equator than the poles

Image from ESRI online course

Projected coordinate systems

• Map projections: The transformation of coordinate locations from the curved Earth surface onto flat maps

• Point to remember: Distortions are unavoidable when making flat maps

Image from ESRI online course

Representing locations with coordinates: Two types of coordinate

systems1. Geographic coordinate system

– Locates objects on the curved surface of the earth

2. Projected coordinate system– Locates objects on a flat surface such as a

paper map or a computer screen

• Each has advantages and disadvantages for various applications.

Some map projections

Understanding distortion: Four spatial properties

• Shape– Conformal maps:

shapes are the same as they are on earth

• Area– Equal area map: Sizes

are the same relative to earth, and if you move a shape around on the map its size will be the same

• Distance– Equidistant map

preserves true scale for all straight lines passing through a specified location

• Direction– Azimuthal map:

Directions from one location to all other points on themap are shown correctly

Universal Transverse Mercator Coordinate System

• Divides the Earth into 6°longitude zones.

• Extends from 80° S to 80° N

Universal Transverse Mercator Coordinate System

• Indiana is UTM Zone 16 in meters

State Plane Coordinate System

• Defined for each state in the US.

• Often used by local governments

• In Indiana, based on Transverse Mercator projection and Feet

Coordinate systems in Indiana

• Federal and state Government use UTM Meters,

• Indiana county government uses State Plane Feet

• Google uses degrees, mins, secs

Good tutorials on GPS

• http://www.aero.org/education/primers/gps/GPS-Primer.pdf

• http://www.trimble.com/gps/

• To really learn more, take the GPS course in Civil Engineering