gps basic concepts

7/28/2019 GPS Basic Concepts

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BASIC CONCEPTS OF

GLOBAL POSITIONINGSYSTEMS (GPS)

Introduction to GPS

Merritt College

Marc Epstein, Instructor


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History of Navigation

Early sailors navigated by sun and stars

Until the late 1700s sailors could onlydetermine latitude

Development of accurate seaworthychronometers by the British enabledcalculation of longitude


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Early Electronic Navigation

Radio based system

LORAN-C and DECCA

Triangulation of ground based radio signalsAccurate to 900 1000 feet


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What is GPS?

The Global Positioning System (GPS) is asatellite based navigation system made upof a network of 24 satellites placed into

orbit by the U.S. Department of Defensebeginning in 1976.


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How is GPS Used in Society?

GPS technology is used by any individual or groupwhich needs accurate location data.

Aeronautic Navigation First Responder Emergency ManagementGround Transportation Management Land Use Management Law Enforcement

Marine NavigationNatural Resource ManagementUtility Management


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Why GPS?

GPS was originally intended for militaryapplications, but in the late 1980s thegovernment made the system available for

civilian use.


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Is GPS Restricted?

GPS works in any weather conditions,anywhere in the world. 24 hours a day.There is no subscription fee or setup

charges to use GPS.


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The Satellites

The 24 GPS satellitescircle the earth twiceeach day in a veryprecise orbit andtransmit signalinformation to the

earth.


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About the Satellites

The GPS satellites are orbiting the earthabout 12,000 miles above the surface.

The GPS satellites are constantly moving,making two orbits in 24 hours at speedsroughly 7,000 miles per hour.

The GPS satellites are powered with solarenergy with backup batteries.


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The GPS Unit

The GPS Unitreceives the signalsfrom the satellites to

determine locationand conduct otherfunctions

programmed intothe unit.


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Determining LocationThe GPS Unit calculates location

based upon geometry and time.

GPS triangulation really istrilateration since it uses

geometry rather than angles.The GPS unit measures distance

through the time it takes for theradio signals to be received from

the satellites.The radio waves from the

satellites travel at the speed oflight (186,000 miles/second)


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Calculation of Location

The GPS unit is programmed withinformation on the orbits of the satellites.

The GPS unit calculates the time it takesfor each of the satellite signals to reachthe surface to determine distance.

The location on the surface is the pointwhere the GPS unit calculates where allthe distances intercept.


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The GPS Signal

GPS satellites transmit two low power radiosignals designated L1 and L2. Civilian GPSuses the L1 frequency of 1575.42 MHz in

the UHF band. The signals travel by line ofsight, meaning they pass through clouds,glass, and plastic but will not pass through

most solid objects such as buildings andmountains.


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Whats in the GPS Signal

Pseudorandom Code

Ephemeris Data

Almanac Data


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Pseudorandom Code

ID code that identifies which satellite istransmitting information. Many GPS unitswill provide information on which satellites it

is receiving.


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Ephemeris Data

Provides the GPS unit with the informationon where each GPS satellite should be atany time throughout the day. Each satellite

transmits ephemeris data showing orbitalinformation for that satellite and every othersatellite in the system.


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Almanac Data

Contains information about the status of thesatellite, current date and time. This part ofthe signal is essential for determining

position.


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Signal Error

Atmospheric conditions

Signal multi-path

Receiver clock errorsOrbital errors

Number of satellites visible

Satellite geometryIntentional signal degradation


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Atmospheric Conditions

The satellite signal slows as it passesthrough the ionosphere and tropospherelayers of the atmosphere. The GPS system

uses a built in model that calculates anaverage amount of delay to partially correctfor this type of error.


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Signal Multi-path

This occurs when the GPS signal is reflectedoff of objects such as tall buildings or largerock surfaces before it reaches the receiver.

This increases travel time of the signal,thereby causing errors.


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Receiver Clock Error

A receivers built-in clock is not as accurateas the atomic clocks onboard the GPSsatellites. Therefore, it may have very slight

timing errors.


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Orbital Errors

Also known as ephemeris errors, these areinaccuracies of the satellites reportedlocation.


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Number of Satellites Visible

The more satellites the GPS unit can receive,the better the accuracy. Buildings, terrain,electronic interference, or sometimes even

dense foliage can block signal reception,causing position errors or possibly no readingat all.


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Intentional Signal Degradation

Selective availability (SA) is an intentionaldegradation of the signal once imposed bythe Department of Defense. SA was

intended to prevent military adversariesfrom using the highly accurate GPS signals.The government turned off SA in May 2000.


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DGPS

Differential GPS

Ground stations which transmit true position

WAAS Wide Area Augmentation SystemDGPS increases accuracy

Non WAAS enabled units accurate to 30 feet

WAAS enabled units accurate to 15 feet


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The GPS Unit Functions

Display of Position

Marking Waypoints

TracksCompass


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Display of Position

The GPS Unit can display position in severalcoordinate systems

In order to make position data consistentwith GIS software set the units to display inlatitude and longitude

Use the degrees decimal option


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Waypoints

A waypoint is any point that you store inmemory

Waypoints are identified with unique

numbersPrior to recording a waypoint allow the unit

time to average the position for greater

accuracyThe time you use to average your position is

dependent on the estimated accuracy


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Tracks

The tracks feature creates an electronic trailas you move

It records and saves your movements

It allows you to retrace your path


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Compass

The compass page uses traditionalnavigation to display your current directionof travel

This is not a magnetic compass

It only provides readings with motion

Most GPS compass pages will providebearing and current speed


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Accuracy, Precision, andResolution

Accuracy Relationship to True Value

Precision Relationship Between DataPoints

Resolution - Smallest increment the GPSunit can measure or ability to distinguishbetween different points


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Recording GPS Data Points

When recording GPS data points you wantto use a spread sheet format


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The Data Sheet Columns

Waypoint or identification number

Latitude

LongitudeDescriptive data


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Improving Accuracy

Check satellite geometry

Further apart the satellites the greater theaccuracy

Increase height

If you are in a canyon or under canopycover increasing height may increase signalor increase number of satellites beingreceived

Use multiple GPS units and average readings


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Field Collection Tips

Object which you can not reach the centerCollect data points at corners and averagethe readings to get center point

When collecting data points where the dotswill be connected by GIS software Collectdata points for corners and in betweencorners

Pay attention to waypoint numbers if youare recording the waypoints in the GPS unit


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Support the Data Inputter

Write neatly

Sketch maps

Provide unit numberProvide names of data collectors

Make notes of any special features or

problems

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