An Introduction to GPS John McGee Ph.D. Geospatial Extension Specialist Virginias Geospatial Extension Program A Partnership with VSGC and VCE

An Introduction to GPSAn Introduction to GPS

John McGee Ph.D.John McGee Ph.D.Geospatial Extension SpecialistGeospatial Extension Specialist

Virginia’s Geospatial Extension ProgramVirginia’s Geospatial Extension ProgramA Partnership with VSGC and VCEA Partnership with VSGC and VCE

www.cnr.vt.edu/gepwww.cnr.vt.edu/gep

OutlineOutline

Why GPS?Why GPS? What is GPS?What is GPS? How GPS Works.How GPS Works. What you need to know about GPS.What you need to know about GPS. What can you do with GPS?What can you do with GPS? Applications of GPS.Applications of GPS.

Why GPS?Why GPS?

Many features have addresses and Many features have addresses and landmarks that are associated with a landmarks that are associated with a destination.destination.

Why GPS?Why GPS?

Many features have addresses and Many features have addresses and landmarks to get you to a destination.landmarks to get you to a destination.

However, there are many features that do However, there are many features that do not have addresses…not have addresses…

Why GPS?Why GPS?

Many features have addresses and Many features have addresses and landmarks to get you to a destinationlandmarks to get you to a destination


There are many MAJOR cities that do not There are many MAJOR cities that do not even have STREET NAMES!even have STREET NAMES!

Why GPS?Why GPS?

Many features have addresses and Many features have addresses and landmarks to get you to a destinationlandmarks to get you to a destination


There are many cities that do not even have There are many cities that do not even have STREET NAMES!STREET NAMES!

And then there is the open ocean and sky…And then there is the open ocean and sky…

Why GPS?Why GPS?

Location, Location, Location Location, Location, Location

and and

INFORMATION!!!INFORMATION!!!

Pre-GPSPre-GPS

Navigation is critical!!!Navigation is critical!!! Historical Navigational tools have limits:Historical Navigational tools have limits:

-The Sextant – contingent on weather-The Sextant – contingent on weather

Pre-GPSPre-GPS

Navigation is criticalNavigation is critical Historical Navigational tools have limits:Historical Navigational tools have limits:

-The Sextant – contingent on weather-The Sextant – contingent on weather

-Radionavigation (Lowrance): only works -Radionavigation (Lowrance): only works near land…near land…

Pre-GPSPre-GPS

Navigation is criticalNavigation is critical Historical Navigational tools have limits:Historical Navigational tools have limits:

-The Sextant – doesn’t work if it is cloudy-The Sextant – doesn’t work if it is cloudy

--Lowrance – radionavigation: only worked near Lowrance – radionavigation: only worked near land…land…

The military had its own reasons for determining The military had its own reasons for determining location…location…

--Identify targetsIdentify targets

-Friendly fire issues-Friendly fire issues

-“smart bombs”-“smart bombs”

What is GPS?What is GPS?

GPS is not a single UNIT!GPS is not a single UNIT! GPS = Global Positioning GPS = Global Positioning

SYSTEMSYSTEM GPS was developed by GPS was developed by

the Department of the Department of DefenseDefense

Funding for the GPS was Funding for the GPS was contingent on making the contingent on making the system available to the system available to the public.public.

GPS is a SYSTEMGPS is a SYSTEM

There are three major There are three major components in this components in this system:system:

1.1. SatellitesSatellites

2.2. Ground Control StationsGround Control Stations

3.3. GPS Receivers (or units)GPS Receivers (or units)

SatellitesSatellites

There are 24 satellites (and 3 spares).There are 24 satellites (and 3 spares). The DOD knows the EXACT location of each The DOD knows the EXACT location of each

of the satellites at any given moment.of the satellites at any given moment. These satellites have VERY accurate clocks These satellites have VERY accurate clocks

on board.on board. The satellites continuously send radio The satellites continuously send radio

signals towards earth.signals towards earth. The radio signals contain several pieces of The radio signals contain several pieces of

information, including the satellite id#, a information, including the satellite id#, a time stamp, and the satellite’s true position time stamp, and the satellite’s true position in space. in space.

Control stations enable information on Earth Control stations enable information on Earth to be transmitted to the satellites (updates to be transmitted to the satellites (updates and fine turning).and fine turning).

Control stations continuously track satellites, Control stations continuously track satellites, and update the positions of each satellite.and update the positions of each satellite.

Without control stations, the accuracy of the Without control stations, the accuracy of the system would degrade in a matter of days.system would degrade in a matter of days.

Control StationsControl Stations There are five control There are five control

stations to monitor the stations to monitor the

satellites.satellites.

GPS ReceiversGPS Receivers

GPS units are referred to as “receivers”.GPS units are referred to as “receivers”. They They receivereceive information (radio signals) information (radio signals)

from satellites.from satellites. These radio signals containThese radio signals contain

important information…important information…

Signal containing:•Time stamp…•Satellite ID #•Exact position of satellite

GPS ReceiversGPS Receivers The Receiver knows exactly when the The Receiver knows exactly when the

signal leaves the satellite (time stamp) and signal leaves the satellite (time stamp) and when the signal arrives at the receiver.when the signal arrives at the receiver.

The Receiver is therefore able to calculate The Receiver is therefore able to calculate its distance from the satellite.its distance from the satellite.

-Distance = -Distance = time x velocitytime x velocity

-Distance = -Distance = timetime x 299,792,458 m/s x 299,792,458 m/s The receiver knows the exact position The receiver knows the exact position

(location) of the satellite (via the signal).(location) of the satellite (via the signal). The receiver is therefore able to determine The receiver is therefore able to determine

its exact distance from the satellite. its exact distance from the satellite.

How GPS WorksHow GPS Works

Signal with•Time stamp…•Satellite ID #•Satellite position information


If the GPS receiver only obtains signals from 1 Satellite, then it “knows” that it is located somewhere on this sphere…



If the GPS receiver only obtains signals from 2satellites, then it “knows” that it is locatedsomewhere where these 2 spheres intersect


How GPS WorksHow GPS WorksIf the GPS obtains signals from 3satellites, then it “knows” that it is located somewhere where these 3 spheres intersect (2 points)


How GPS WorksHow GPS Works A fourth GPS is required to determine the exact location and elevation.

What you need to know about GPSWhat you need to know about GPS

Signal Accuracy IssuesSignal Accuracy Issues Selective AvailabilitySelective Availability Tricks of the TradeTricks of the Trade Current Applications of GPSCurrent Applications of GPS Future applications of GPSFuture applications of GPS

Signal AccuracySignal Accuracy

There are 2 types of GPS Signals:There are 2 types of GPS Signals:

P-codeP-code: (“Precise” code) : (“Precise” code) This is only available to the military and This is only available to the military and

some selected public officials.some selected public officials. Very precise, not degraded.Very precise, not degraded.

C-codeC-code: (“Civilian” Code).: (“Civilian” Code). Less preciseLess precise Degraded (by scrambling the signal) Degraded (by scrambling the signal)

especially in times of conflict especially in times of conflict

Selective Availability (SA)Selective Availability (SA)

For national security reasons, the For national security reasons, the military sometimes degrades the C-military sometimes degrades the C-code signal.code signal.

These errors are randomThese errors are random Errors be as high as +300 feetErrors be as high as +300 feet

Selective AvailabilitySelective Availability SA errors can put you on the wrong SA errors can put you on the wrong

side of a stream, or even a different side of a stream, or even a different city block or street!city block or street!

300 feet is a lot of real estate!!!300 feet is a lot of real estate!!!

GPS Location

Real location

Selective AvailabilitySelective Availability

It is possible to correct for Selective It is possible to correct for Selective Availability.Availability.

This process is called This process is called Differential Differential CorrectionCorrection

Here’s how it works…Here’s how it works…

Differential CorrectionDifferential Correction There are already established base There are already established base

stations around Virginiastations around Virginia Surveyors have determined the precise Surveyors have determined the precise

location of these base stations already.location of these base stations already. Each base station has a GPS receiver, Each base station has a GPS receiver,

which collects incoming (scrambled) which collects incoming (scrambled) signals.signals.

The true (surveyed) location is then The true (surveyed) location is then compared to the GPS coordinates.compared to the GPS coordinates.

The correction values are then sent to The correction values are then sent to other GPS receivers in the field.other GPS receivers in the field.

Differential CorrectionDifferential Correction

Exact known coordinates differ significantly from GPS coordinates at this location = exact amount of error!

GPS receiver in the fieldcollecting points, routes, etc.

Differential Correction Signal

Base station w/ GPS receiver

at known location:

Other Tricks of the Trade:Other Tricks of the Trade:AveragingAveraging

Averaging: A GPS receiver can Averaging: A GPS receiver can collect points continuously for 15-30 collect points continuously for 15-30 seconds. The receiver can then seconds. The receiver can then average all these locations togetheraverage all these locations together

This only works when you are This only works when you are standing still!!standing still!!

GPS Collected Points

GPS Averaged Point

“True” location

Other Tricks of the Trade: Other Tricks of the Trade: Satellite DistributionSatellite Distribution

It is better for your receiver to get a It is better for your receiver to get a fix on “distributed” satellites, then fix on “distributed” satellites, then poorly distributed satellites.poorly distributed satellites.

Poor SatelliteDistribution

GoodSatellite Distribution

“Positional Dilution of Precision”

Other Tricks of the Trade: Other Tricks of the Trade: MultiPath ErrorsMultiPath Errors

Try and stay away from buildings and other Try and stay away from buildings and other structures when using a GPS receiverstructures when using a GPS receiver

Satellites may not be visible…Satellites may not be visible…

This can introduce error…This can introduce error…

Other Tricks of the Trade: Other Tricks of the Trade: Tracking SatellitesTracking Satellites

GPS has worldwide coverage…GPS has worldwide coverage…

HOWEVER…HOWEVER…You can lose satellite coverage (or received You can lose satellite coverage (or received

degraded signals) in areas with dense foliage, degraded signals) in areas with dense foliage, in downtown areas, etc.in downtown areas, etc.

You may also lose satellite coverage (or receive You may also lose satellite coverage (or receive degraded signals) in deep valleys or gorges.degraded signals) in deep valleys or gorges.

Other Tricks of the Trade: Other Tricks of the Trade: WAASWAAS

The Wide Area Augmentation System The Wide Area Augmentation System (WAAS) is a differential GPS system (WAAS) is a differential GPS system that is being constructed to support that is being constructed to support GPS accuracy in aircraft.GPS accuracy in aircraft.

WAAS is supported by a number of WAAS is supported by a number of satellites that emit signals to satellites that emit signals to standard GPS units. standard GPS units.

WAASWAAS

How accurate is a $100 GPS?How accurate is a $100 GPS?

That’s the million dollar question…That’s the million dollar question…

Brand “A” Day 1

Brand “A” Day 2

Brand “A” Day 3

Brand “A” Day 4

Brand “A” Day 5

Brand “B” Day 1

Brand “B” Day 2

Brand “B” Day 3

Brand “B” Day 4

Brand “B” Day 5

What can you do with a GPS?What can you do with a GPS?

Collect and store pointsCollect and store points (WAYPOINTS)(WAYPOINTS) Trail heads, creek crossings, Trail heads, creek crossings,

mountain tops, camping ground, mountain tops, camping ground, data collection points (for research), data collection points (for research), etc.etc.

Download the points onto your Download the points onto your computer and integrate them with computer and integrate them with other mapping programsother mapping programs


Collect and store routes (a series of Collect and store routes (a series of WAYPOINTS)WAYPOINTS)

A route is a path between two (or A route is a path between two (or more) Waypoints. The GPS “guides more) Waypoints. The GPS “guides you” from point “A” to point “B”.you” from point “A” to point “B”.


The GOTO functionThe GOTO function

Using the ‘GOTO’ function, the GPS will Using the ‘GOTO’ function, the GPS will guide you to a predefined Waypoint guide you to a predefined Waypoint (you choose which one…) using a (you choose which one…) using a compass and “pointer”compass and “pointer”

You can program the GPS to “beep” You can program the GPS to “beep” when you are within a certain when you are within a certain distance of the defined Waypointdistance of the defined Waypoint

Tide TablesTide Tables

Many of the marine GPS’s have built Many of the marine GPS’s have built in tide tables. They will provide tide in tide tables. They will provide tide information and ranges for any date information and ranges for any date and any place…and any place…


SpeedSpeed

GPS’s calculate your ground speed as GPS’s calculate your ground speed as you walk, run, drive or flyyou walk, run, drive or fly


ElevationElevation

In addition to providing you with your In addition to providing you with your latitude and longitude, GPS provides latitude and longitude, GPS provides you with altitude information.you with altitude information.


Current Applications of GPSCurrent Applications of GPS

Public SafetyPublic Safety Environmental Environmental

resource agentsresource agents AviationAviation MilitaryMilitary Local planningLocal planning SurveyingSurveying RecreationRecreation BusinessBusiness

The Future of GPSThe Future of GPS

Acknowledgements: Keith Clarke

The Future is here.The Future is here.The applications are endless…The applications are endless…

Questions?Questions?

John McGeeJohn McGee

[email protected]@vt.edu

www.cnr.vt.edu/gepwww.cnr.vt.edu/gep

Documents

An Introduction to GPS John McGee Ph.D. Geospatial Extension Specialist Virginias Geospatial Extension Program A Partnership with VSGC and VCE