GPSLecture Burgess

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ESSC 541-542 Lecture 4.14.05

INTRODUCTION TO GPS:

ESSC 541-542

4.14.05


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Presentation OutlineI. Learning Format

II. GPS Basics

III. GPS Under the Hood

IV.Datums and Coordinate Systems

V. Mobile Mapping Technology

VI.Hands on with GPS


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Learning Format Lecture

One hour presentation and in class

orientation to GeoXT

Lab

Field trip (when?) to practice using theGPS in the field


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What is GPS?The Global Positioning System (GPS)

A Constellation of Earth-OrbitingSatellites Maintained by the United

States Government for the Purpose ofDefining Geographic Positions On andAbove the Surface of the Earth. Itconsists of Three Segments:

Control Segment

Space Segment

User Segment


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GPS Satellites (Satellite Vehicles(SVs)) First GPS satellite launched in

1978

Full constellation achieved in 1994

Satellites built to last about 10years

Approximately 2,000 pounds,17feet across

Transmitter power is only 50 wattsor less


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GPS Lineage Phase 1: 1973-1979

CONCEPT VALIDATION

1978- First Launch of Block 1 SV

Phase 2: 1979-1985

FULL DEVELOPMENT AND TESTS

Phase 3: 1985-Present

PRODUCTION AND DEPLOYMENT


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Precise Positioning System (PPS) Authorized users ONLY

U. S. and Allied military

Requires cryptographicequipment, specially

equipped receivers

Accurate to 21 meters 95%of time


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Standard Positioning Service (SPS)

Available to all users

Accuracy degraded bySelective Availability until2 May 2000

Horizontal Accuracy: 100m

Now has roughly sameaccuracy as PPS


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Space Segment 24+ satellites

6 planes with 55

inclination Each plane has 4-5

satellites

Broadcasting position

and time info on 2frequencies

Constellation hasspares


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Space Segment Very high orbit

20,200 km

1 revolution inapproximately 12 hrs

Travel approx. 7,000mph

Considerations

Accuracy

Survivability

Coverage


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Control Segment

Master Control StationMonitor StationGround Antenna

Colorado

Springs

Hawaii AscensionIslands

DiegoGarcia

Kwajalein

Monitor and Control


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Control Segment: Maintaining the System

(5) Monitor Stations

Correct Orbitand clockerrors Create newnavigation message

Observeephemerisand clock

Falcon AFB

Upload Station


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User Segment

Over $19 Billion invested by DoD

Dual Use System Since 1985

(civil & military) Civilian community was quick to take

advantage of the system Hundreds of receivers on the market

3 billion in sales, double in 2 years 95% of current users

DoD/DoT Executive Board sets GPSpolicy


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Common Uses for GPS

Land, Sea and AirNavigation and

Tracking

Surveying/ Mapping

Military Applications

Recreational Uses


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How the system works

Space Segment24+ Satellites

The CurrentEphemeris is

Transmitted toUsersMonitor

Stations Diego Garcia Ascension Island Kwajalein

Hawaii Colorado Springs

GPS ControlColorado Springs

EndUser


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Triangulation

Satellite 1 Satellite 2

Satellite 3 Satellite 4


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Distance Measuring

Rate = 186,000 miles persecond (Speed of Light)

Time = time it takessignal to travel from theSV to GPS receiver

Distance = Rate x Time

Each satellite carriesaround four atomicclocks

Uses the oscillation of cesium andrubidium atoms to measure time

Accuracy?plus/minus a second over more

than 30,000 years!!

The whole systemrevolves around

time!!!


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SV and Receiver Clocks

SV Clocks 2 Cesium & 2 Rubidium in each SV $100,000-$500,000 each

Receiver Clocks Clocks similar to quartz watch Always an error between satellite

and receiver clocks ( t)

4 satellites required to solve forx, y, z, and t


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PROBLEM

Cant use atomic

clocks in receiver

SOLUTION

Receiver clocks

accurate over shortperiods of time

Reset often

4th

SV used torecalibrate receiverclock

Cesium Clock =$$$$$$$!!!

Size of PC

4


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Breaking the Code

Transmission Time

Receiver

The Carrier Signal...

combined with

The PRN code...

produces the

Modulated carrier signal

which is transmitted...demodulated...

And detected by receiver,Locked-on, butWith a time delay...

Time delay

Satellite


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Accuracy and Precision in GPS

Accuracy

The nearness of a measurement to the

standard or true value Precision

The degree to which several

measurements provide answers very closeto each other.

What affects accuracy andprecision in GPS?


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Sources of Error

Selective Availability

Intentional degradation of

GPS accuracy 100m in horizontal and

160m in vertical

Accounted for most error instandard GPS

Turned off May 2, 2000


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Sources of Error

Geometric Dilution ofPrecision (GDOP)

Describes sensitivity of receiver tochanges in the geometricpositioning of the SVs

The higher the DOP value, thepoorer the measurement

QUALITY DOP

Very Good 1-3Good 4-5Fair 6Suspect >6


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Sources of Error

Clock Error Differences between satellite

clock and receiver clock

Ionosphere Delays Delay of GPS signals as they

pass through the layer ofcharged ions and free electronsknown as the ionosphere.

Multipath Error Caused by local reflections ofthe GPS signal that mix withthe desired signal GPS

Antenna

Hard Surface

Satellite


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Differential GPS Method of removing errors that affect GPS

measurements

A base station receiver is set up on a locationwhere the coordinates are known

Signal time at reference location is compared totime at remote location

Time difference represents error in satellitessignal

Real-time corrections transmitted to remotereceiver Single frequency (1-5 m) Dual frequency (sub-meter)

Post-Processing DGPS involves correcting at a

later time

Referencelocation

Remotelocation

= Error

www.ngs.noaa.gov/OPUS

Online post-processing


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Wide Area Augmentation System (WAAS)

System of satellites and groundstations that provide GPS signalcorrections

25 ground reference stations acrossUS

Master stations create GPScorrection message

Corrected differential messagebroadcast through geostationarysatellites to receiver

5 Times the accuracy (3m) 95% oftime


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Datums and Coordinate Systems

Many datums and coordinatesystems in use today

Incorrect referencing ofcoordinates to the wrong datumcan result in position errors ofhundreds of meters

With, sub-meter accuracyavailable with todays GPS,careful datum selection andconversion is critical!

Why should I worry about datums andcoordinate systems when using GPS?


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Geodetic Datums: What are they?

Define the size and shapeof the earth

Used as basis forcoordinate systems

Variety of models:

Flat earth

Spherical

Ellipsoidal

WGS 84 defines geoid

heights for the entire earth


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Coordinate Systems: What are they?

Based on Geodetic Datums

Describe locations in two or three

dimensions (ie. X,Y,Z or X,Y)

Local and Global

Common systems Geodetic Lat, Long (global) UTM (local) State Plane (local)

Variety of transformation methodswww.uwgb.edu/dutchs/UsefulData/UTMFormulas.HTM

Online conversion tool


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World UTM Zones


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Geodetic Latitude, Longitude

Prime Meridian and Equator are referenceplanes used to define latitude and longitude


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Which is the correct location?

Same location can have many reference positions,

depending on coordinate system used


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Mobile Mapping

Integrates GPS technologyand GIS software

Makes GIS data directlyaccessible in the field

Can be augmented withwireless technology


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Mobile Mapping Pros and Cons

Pros More efficient data entry

Ready access to GIS data

Less transcription error

Possible real-timeupload/download throughwireless

Cons Cost

Data storage limit

Digital data can be

lost/corrupted


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Hands on with GPS


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Paul BurgessUniversity of Redlands

Redlands [email protected]

Questions?


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OBJECTID Northing Easting GPSDate GPSDateTime

1 3982854.745 656956.881 09-Apr-04 09-Apr-04

2 3982854.748 656956.885 09-Apr-04 09-Apr-043 3982854.84 656956.953 09-Apr-04 09-Apr-04

4 3982854.697 656957.42 09-Apr-04 09-Apr-04

5 3982854.55 656957.393 09-Apr-04 09-Apr-04

6 3982854.539 656957.365 09-Apr-04 09-Apr-04

7 3982854.443 656957.304 09-Apr-04 09-Apr-04

8 3982854.37 656957.461 09-Apr-04 09-Apr-04

9 3982854.303 656957.619 09-Apr-04 09-Apr-04

10 3982854.089 656957.571 09-Apr-04 09-Apr-04

11 3982854.062 656957.672 09-Apr-04 09-Apr-04

12 3982854.02 656957.581 09-Apr-04 09-Apr-04

13 3982853.882 656957.36 09-Apr-04 09-Apr-04

14 3982853.776 656957.314 09-Apr-04 09-Apr-04

15 3982853.821 656957.262 09-Apr-04 09-Apr-04

16 3982853.9 656957.222 09-Apr-04 09-Apr-04

17 3982853.865 656956.896 09-Apr-04 09-Apr-04

18 3982853.825 656956.811 09-Apr-04 09-Apr-04

19 3982854.382 656954.229 09-Apr-04 09-Apr-04

20 3982857.014 656958.041 09-Apr-04 09-Apr-04


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References

Bertorelli, Paul. GPS Explained. Downloaded April 2005: http://www.eaa1000.av.org/technicl/gps/gps.htm

Henstridge, Fred N and Bob Nelson. Introduction to GPS. Presentation for the ICAO/FAA WGS-84 Seminarand Workshop, November 9, 1999, San Salvador. Accessible on the International OceanographicCommissions website at : Http--ioc.unesco.org-oceanteacher-OceanTeacher2-02_InfTchSciCmm-01_CmpTch-10_enavsys-gps_0008.ppt.url

Peter H. Dana. Coordinate Systems Overview. Accessible at:

http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html Peter H. Dana. Geodetic Datum Overview. Accessible at:

http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html

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http://www.eaa1000.av.org/technicl/gps/gps.htmhttp://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/datum/datum_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/datum/datum_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.htmlhttp://www.eaa1000.av.org/technicl/gps/gps.htm

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