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CE 211 – SURVEYING ENGINEERING CLASS 27: INTRODUCTION OF GPS

Ahmed Abdel-Rahim, Ph.D, P.E.

Associate Professor, Civil Engineering

Class Objectives

• Be familiar with with basic GPS principals and definitions

What is GPS

• Global Positioning System

• Formal name: Navigation Satellite Timing and Ranging (NAVSTAR)

CE 211 – Surveying Engineering

What is GPS

• The Navstar Global Positioning System (GPS) is an all-weather, space-based navigation system developed by the Department of Defense (DoD) to satisfy the requirements for the military forces to accurately determine their position, velocity, and time in a common reference system, anywhere on or near the earth on a continuous basis.

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• GPS has many advantages over Traditional Surveying Techniques

– Traditional surveying using total stations rely on the visibility between the survey instrument and a target

– If an obstructions exists, it must be traversed around

– Typically distance measurement is limited to 5 Km

– Weather can limit operations, e.g. fog, rain etc

GPS-Based Surveying

Line of sight is not necessary

•Weather Independent

•Does not require line of sights

•Gives high Geodetic Accuracy

•Can be operated day and night

•Quicker and requires less Manpower

•Common Coordinate System

•Wide Range of Applications

•Competitively Priced

Why GPS-Based Surveying?

GPS-Based Surveying

• The use of GPS does have limitations.

– In environments where the view of the sky is limited, such as urban areas, the traditional survey techniques must still be used.

– While GPS surveying does not require a line-of-sight between points within a survey, it does require unobstructed lines-of-sight to at least four satellites

• Developed by the US Department of Defense

• Provides

– Accurate Navigation (5 - 15 m)

– Worldwide Coverage

– 24 hour access

– Common Coordinate System

• Designed to replace existing

navigation systems

• Accessible by Civil and Military

GPS General Characteristics

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Control Segment 1 Master Station

5 Monitoring Stations

Space Segment NAVSTAR : NAVigation

Satellite Time and Ranging

24 Satellites

20200 Km

User Segment Receive Satellite Signal `

GPS System Components

• Master Control Station

– Responsible for collecting tracking data from

the monitoring stations and calculating

satellite orbits and clock parameters

• 5 Monitoring Stations

– Responsible for measuring pseudorange

data. This orbital tracking network is used to

determine the broadcast ephemeris and

satellite clock modeling

– Ground Control Stations

– Responsible for upload of information to the

satellites

GPS System Components – Control Segment

• Initially 24 Satellites (31 as of

2008)

– 4 satellites in 6 Orbital

Planes inclined at 55

Degrees

• 20200 Km above the Earth

• 12 Hourly orbits

– In view for 4-5 hours

• Designed to last 7.5 years

• Different Classifications

– Block 1, 2, 2A, 2R & 2 F

Equator

55

GPS System Components – Space Segment

• The most visible segment

• GPS receivers are found in

many locations and applications

GPS System Components - Users

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Monitor stations • Diego Garcia • Ascension Island • Kwajalein • Hawaii •Colorado Springs, and Cape Canaveral,

Current ephemeris is transmitted to users

Space Segment

GPS Control Colorado Springs

• By measuring distance from several satellites you can calculate your

position

Trilateration From Satellites

Satellite Ranging

Measuring the distance from a satellite

• Done by measuring travel time of radio signals

• Done by measuring the phase of radio signals

Measure how long it takes the GPS signal to get to us

• Multiply that time by 300,000 km/sec

– Time (sec) x 300,000 = km

• Multiply that phase by the carrier wavelength.

Outline Principle : Range

Xll

Vl

Xll

Vl

Outline Principle : Range

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Xll

Vl

Outline Principle : Range

Xll

Vl

Range = Time Taken x Speed of Light

Outline Principle : Range

We are somewhere on a sphere of radius,

R1

R1

Outline Principle : Position

2 Spheres intersect as a circle

R1

R2

Outline Principle : Position

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3 Spheres intersect at a point

3 Ranges to resolve for Latitude, Longitude and Height

R1

R2

R3

Outline Principle : Position

• The satellites are like “Orbiting Control Stations” • Ranges (distances) are measured to each satellite using time

dependent codes • Typically GPS receivers use inexpensive clocks. They are much

less accurate than the clocks on board the satellites • A radio wave travels at the speed of light

• (Distance = Velocity x Time) – Consider an error in the receiver clock

• 1/10 second error = 30,000 Km error • 1/1,000,000 second error = 300 m error

Outline Principle : Position

4 Ranges to resolve for Latitude, Longitude, Height & Time

It is similar in principle to a resection problem

Point Positioning Autonomous Navigation

Accuracy 5 - 50 m

A receiver in autonomous mode provides navigation and positioning accuracy of about 5 to 50m

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Heighting

• Heights determined using GPS are

referenced to the WGS 84 Ellipsoid

– Ellipsoid Heights are heights above the

ellipsoid

Ellipsoidal height = h

Ellipsoid

P Topography

h

The Continuously Operating Reference Stations (CORS) system is a network that continually collects GPS signals and provides these data to GPS users via the Internet for precise positioning activities

Exam-3 Review

•Horizontal Curves • Circular curve relationships [R, D, L, LC, E, M, T, …] • Stationing (PI, PC, PT) • Setting up a curve (deflection angle and chord method • Curves through fixed points

•Vertical Curves • Elevations (VPI, VPC, VPT) and elevation of different points given x • High and Low Points • Curve through fixed points

Exam-3 Review

•Earthwork • Cross section areas by coordinate method • Cut and Fill computations • Mass Diagrams

• Datum, coordinates, and GPS • Basic definitions