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Dr. Darrell R. Dean, Jr., P.S. 1

GPS:  The Basics

Darrell R. Dean, Jr.

Civil and Environmental Engineering

West Virginia University

Expected Learning Outcomes for GPS

• Explain the acronym GPS

N 3 i t t d t i hi t f GPS• Name 3 important dates in history of GPS

• Name the 3 components of GPS system

• Explain geometric basis for position calc.

• Explain how signal travel time is meas.

• Name 2 codes used to measure rangeName 2 codes used to measure range

• List the general requirements for GPS work

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Dr. Darrell R. Dean, Jr., P.S. 2

Expected Outcomes (Cont.)

• State the required number of satellites to map i ta point.

• Name two types of GPS receivers

• List 3 attributes of GPS receivers

• Explain positional accuracy for different receivers and different techniquesreceivers and different techniques

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What Is GPS?

• Global Positioning System

• NAVSTAR ‐ NAVigation System using Timing and Ranging

• Radio‐based Navigation System Using Satellites Developed by the U.S. Department of Defenseof Defense

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Dr. Darrell R. Dean, Jr., P.S. 3

GPS Attributes

• Output can include position (latitude, longitude, height), velocity, and time (PVT)longitude, height), velocity, and time (PVT)

• All‐weather, any place (?), and any time

• One‐way ranging system

• Requires precisely synchronized clocks on each end

• Atomic clocks in the satellite and a quartz clock in the receiver

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GPS History

• Developmental satellites began launch in 19781978

• Operational satellite launches start in 1989

• Full Operational Capability (24 satellites) in 1995

• Selective availability turned off May 1,

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y y ,2000

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GPS System Components

UserEquipment

Space

User

Satellites --Block II/IIA/IIR/IIF

Master Control Station Monitor

GroundAntenna

GroundStation

Master ControlStation

UserEquipment

From: Moore & Parisi, “Master GPS WRC-2000 Modular Briefing,” MITRE, Dec. 1999

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Space Segment

•24 - 30 Satellites24 30 Satellites• Altitude: 20,200 km

• Orbital Period: 12 hrs

• Orbital Plane: 55 deg

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• Number of Planes: 6

• Vehicles per plane: 4From: Moore & Parisi, “Master GPS WRC-2000 Modular Briefing,” MITRE, Dec. 1999

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Dr. Darrell R. Dean, Jr., P.S. 5

Ground Control Segment

• Five  monitoring stationsk (d )• Precise position known in WGS‐84 (datum)

• Passively tracks satellites in view

• Accumulates range or distance data to the satellites

• Data transmitted to the master control station

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Ground Control Segment (Cont.)

• Three  ground antennas at 3 of the monitoring t tistations

• Transmits updated information to each satellite

• One  master control station• Processes range data to precisely determine satellite orbits and positions

• Updates each satellite’s navigation message

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Ground Control Segment

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User Segment

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From: Moore & Parisi, “Master GPS WRC-2000 Modular Briefing,” MITRE, Dec. 1999

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How Does GPS Work?

In addition to knowing the distance to a satellite, a userneeds to know the satellite’s4

To measure travel time, GPS needs veryaccurate clocks.

needs to know the satellite slocation.

As the GPS signal travels throughthe ionosphere and earth’s atmosphere,it gets delayed.

3

5

X

Trilateration fromsatellites is thebasis of the system.

To trilaterate, GPS measuresdistance using the travel time of a radio signal. 12

From: Moore & Parisi, “Master GPS WRC-2000 Modular Briefing,” MITRE, Dec. 1999

Feb. 2010 13Darrell.Dean@mail.wvu.edu

How Does GPS Work? (Cont.) 

• Uses basic equation: qdistance = rate x time

• Approximate rate for radio waves = 186,000 miles per second

• Satellite transmits a signal with a time codewith a time code

• Receiver locks onto the  signal

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From Mar. ’96 P.O.B

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How Does GPS Work?(Cont.)• Signal used to calculate pseudo range or distancedistance.

• With distances to 3 satellites known, trilateration can be  used to calculate a  position (latitudeposition (latitude, longitude, and height)

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•Actually, four satellites are required because a calculation for time is also needed

From Mar. ’96 P.O.B.

System Signal Characteristics

• Pseudo random noise code (PRN) modulated on two carrier frequency waves.

• Navigation message‐satellite ephemeris and time corrections l d l t dalso modulated on carrier frequency

Feb. 2010 Darrell.Dean@mail.wvu.edu 16From Feb. ’96 P.O.B.

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System Codes (PRN Codes)

• C/A code:  course acquisition coded h d d ( )• Associated with standard positioning system(SPS)

• SPS available to all users

• P(y)‐code:  precision code• Associated with precise positioning system (PPS)

• PPS for military users

• L2C code: for civilian use – recent update• Provides improved accuracy

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System Protection Procedures

• Selective availability (S/A) ‐ reduces accuracy of positions from systemaccuracy of positions from system

• Introduce error in  satellite clock (dither)

• Introduce error in orbit data (epsilon)

• Turned off since May 1, 2000

• Anti‐spoofing (A‐S) ‐ guards against fake transmissions of satellite data

• Encryption of the p‐code to form the y‐code or P(Y)

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GPS Receivers• Signal tracked

• Code ‐ “mapping grade” & “consumer grade”grade

• Carrier frequency ‐ “survey grade”

• Code + Carrier frequency 

• Number of frequencies

• Single or dual

• No. of channels (no. of satellites tracked at one time) ‐ 6 to many.

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GPS Receivers Cont.• External or internal antenna

• Accuracy vs. ease of signal acquisition

• Data logging ‐integrated field computer• Data logging ‐integrated field computer• Enter and store attribute data (data dictionary)

• Point averaging

• Point nesting ‐ e.g. change from line feature to point feature w/o interrupting lineto point feature w/o interrupting line 

• Offset capability‐ compass/range finder for remote point location

• Storage ability for large projects

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GPS Positioning Methods

• Autonomous  vs Relative positioning• Static positioningp g• Real‐time correction capability

• Wide area augmentation system (WAAS)• Satellite Based Augmentation System(SBAS)

• Radio (RTCM) broadcasts from beacons or communication satellites, some free some require a subscription fee (DGPS)

• RTK – Single base• RTK Network connection• RTK Network connection 

• Wireless (cell phone) data connection• Commercial networks• Public entity (DOT, e.g.) networks

• Post Processing

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Positional Accuracy (in general)

Accuracy Receiver Method/Processing

• 3‐5 mm SurveyG d

Relative positioning /static , RTK• 1 cm Grade

Relative positioning /static , RTK

• Submeter• 1 – 3 m

Mapping Grade

Autonomous/DGPS, real‐time or post processing

• 2 – 5 mMappingGrade

Autonomous / WAAS/DGPS

• 2 – 5 mConsumer Grade

Autonomous /WAAS

• > 5 mConsumer Grade, others

Autonomous No DGPSNo WAAS

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Factors Affecting Accuracy

• Type of receiver• Code based/carrier phaseCode based/carrier phase

• Dual frequency (L1 & L2)

• Methodology• Autonomous positioning

• Relative positioning (static vs. RTK)

P i R l ti t i• Processing: Real‐time or post processing

• Signal interruption (trees, bldg., topo.)

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Accuracy Factors (Cont.)

• Atmospheric factors • Ionosphere significant error factor

• dual freq. receivers min. effect due to ionosphere

• Multipath reflections of the signal

• Geometric configuration of satellites• Position dilution of precision (PDOP) 

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From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000

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From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000

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From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000

What’s Required to Use GPS?

• Receiver

• Mission planningMission planning

• Free download available

• See reference sheet

• Check number of satellites and location

• Check satellite geometry ‐position dilution of precision (PDOP)of precision (PDOP)

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Enter Survey Location

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Number of Satellites

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Number of Satellites (Cont.)

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Elev. and Azimuth of Satellite

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Elev. and Azimuth (Cont.)

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PDOPs for Survey Date

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PDOPs (Cont.)

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PDOPs (Cont.)

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Number of Satellites by Interval

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Number by Interval(Cont.)

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Number by Interval(Cont.)

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What’s Required (Cont.)?

• Clear view of sky

• Log positions in field enter attribute• Log positions in field , enter attribute data

• Download spatial data and attribute data

• Post process spatial data as necessary• Post processing software

• Base station files

• Create required products and services

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Dr. Darrell R. Dean, Jr., P.S. 21

Conclusions

• Proper equipment and methods are necessary f i i ith GPSfor precise surveying with GPS

• Open conditions and a clear view of the sky are required for GPS surveying

• Relatively inexpensive GPS receivers may be used for collecting survey data and featureused for collecting survey data and feature attributes for management purposes

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