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GPS in Land Surveying. Evergreen Valley College Engineering and Engineering Technology H. Johnston, T. Redd, A. Tabrizi July 12, 2005. Today’s Topics . Part I Background Information Accuracy and Precision What is GPS? Why and who uses it? How does it work? GPS Surveying – The Basics

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GPS in Land Surveying

Evergreen Valley College

Engineering and Engineering Technology

H. Johnston, T. Redd, A. Tabrizi

July 12, 2005


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Today’s Topics

  • Part I

    • Background Information

    • Accuracy and Precision

    • What is GPS?

    • Why and who uses it?

    • How does it work?

    • GPS Surveying – The Basics

  • Part II

    • GPS Surveying Techniques

    • Mission Planning and Design

  • Part III

    • Field Exercises

    • Post Processing Field Data

    • Advanced Topics


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GPS Course Information

  • Lecture component:

    • Accuracy, precision, & error

    • Oral & written communication

    • Introduction to GPS

  • Laboratory component:

    • Field activities

    • Data processing

  • Expected learning competencies

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  • HHistory of Measurement &

  • DDevices

    • Mechanical

    • Opto-Mechanical

    • Electronic

      • Electronic Distance Measuring

      • Total Station

      • Satellite Assisted Systems


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Mechanical & OpticalDevices

  • Simple to use

  • Usually cheap

  • Poor accuracy

  • Simple applications

  • Poor productivity

  • Poor practicality

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Some Oldies

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Some Oldies

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Some Oldies

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Some Oldies

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Some Oldies

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Some Oldies

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Some Oldies

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Some Oldies

Wireless Communication Technology

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Two-way ranging by EDM

D

one clock used to measure

t

d

D

Two way travel time:

t=2d/c.

D

Distance: d=c

t/2

Electronics Devices

  • Electronic Distance Meter

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  • Total Station

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  • Satellite Assisted

    • Celestial satellites (not electronic)

    • Navy Navigation Satellite System (NNSS)

      or TRANSIT (5 to 7 satellites at 1100 km

      polar orbits. Provided navigational help to

      the US Navy's Polaris submarine fleet.

    • NAVigation Satellite Timing and Ranging (NAVSTAR)

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Accuracy and Precision

  • Accuracy: Degree of perfection obtained in any measurement, i.e. closeness to the actual value

  • Precision: Degree of refinement of measurement, i.e. degree of repeatability or consistency of a group of observations

  • Both are important in Surveying

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Accuracy & Precision

Good Precision Poor Precision Good Precision

Good Accuracy Good Accuracy Poor Accuracy

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Can Hi-Tech Equipmentbe Trusted?

  • Accuracy and precision may be improved:

    • If we follow directions

    • If we stay within the operating limits of the equipment

    • If we use the equipment properly

    • If we use the right equipment for the job

    • If we use care and preplanning

    • If we build redundancies into the measurement

    • If we can trust the people who are using the equipment!

      So nothing is new here! Hi-tech or not, we still need to use caution.

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What is a GPS?

Definitions

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What is GPS?

  • A system capable of providing position

    information anywhere on earth –

    Global Positioning System

  • A constellation of orbiting satellites

  • Various orbits around the earth

  • NAVSTAR GPS

  • User receivers acquire signal and determines its position

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GPS

  • Global Positioning System

  • Developed by DOD

  • Cost $10 billion

  • Triangulation-based technology

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Why use it?

  • AAA (who can resist it!)

    • All weather operation

    • Always available (24/7 operation)

    • Anywhere available

  • Economical

  • Increased Productivity

  • Improved Customer service

  • Accuracy (3-D data, Velocity and timing)

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Who Uses it?

  • Land, sea, and airborne navigation, surveying, geophysical exploration, mapping and geodesy, vehicle location systems, farming, transportation systems

  • Telecommunication infrastructure applications include network timing and enhanced 911 for cellular users

  • Global delivery of precise and common time to fixed and mobile users

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Some Applications

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Some Applications

Could be used to track mail

if properly used!

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Some Applications

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Some Applications

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Some Applications

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Some Applications

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Some Applications

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Some Applications

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Some Applications

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Some Applications

Mapping

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How does The GPS work?

  • The GPS System Components

    • The User Segment

    • The Control Segment

    • The Space Segment

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The GPS System Components

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The User Segment

  • GPS user equipment – portable and fixed

    • Military

    • Civilian

      • Navigation

      • Surveying

      • GIS

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The Control Segment

  • Ground facilities responsible for

    • satellite tracking

    • telemetry

    • orbit & ephemeris computations

    • uplinking of the computed data

    • supervising the daily management of the space segment

    • Five ground control stations (Monitor Stations)

    • One Master Control Station

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Master Control Station

  • Receive tracking data from the monitor stations

  • Calculates satellites ephemeris

  • Adjusts satellite clocks

  • Maneuvers satellites, if needed

  • Encrypts signals

  • Maintains GPS reference system (WGS84)

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The Space Segment

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The Space Segment

  • Constellation of 24 Satellites

  • In six orbital planes around the equator (60 degrees apart)

  • Four satellite per orbit

  • Orbital planes inclined 55 degrees from the equator

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Satellite Constellation

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GPS Satellite

  • Seven satellites are typically visible 10 degrees or more above the horizon

  • Each satellite is about 2 to 3K lbs

  • Satellites orbit the earth every 12 hours

  • Time can be figured to within 100 nanosecs

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GPS Satellite

  • BLOCK IIA SATELLITE CHARACTERISTICS

    • Weight (in orbit): 2,175 pounds

    • Orbit altitude: 10,988 nautical miles

    • Power source: solar panels generating 700 watts

    • Dimensions: 5 feet wide, 17.5 feet long (including wing span)

    • Design life: 7.5 years

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GPS Satellite

  • BLOCK IIR SATELLITE CHARACTERISTICS

    • Weight (in orbit): 2370 pounds

    • Orbit altitude: 10,988 nautical miles

    • Power source: solar panels generating 1136 watts

    • Dimensions: 5 feet wide, 6.33 feet in diameter, 6.25 feet high (38.025 feet wide including wing span)

    • Design life: 10 years

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GPS Satellite

  • BLOCK IIF SATELLITE CHARACTERISTICS

    • Weight (in orbit): 3758 pounds

    • Orbit altitude: 10,988 nautical miles

    • Power source: solar panels generating up to 2900 watts

    • Dimensions: 8 ft x 6.47 ft (stowed) 70.42 ft (deployed 4 panel solar arrays) x 12 ft

    • Design life: 15 years

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GPS Satellite

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What is so special about an 11,000 mile orbit?

  • Mathematically perfect orbit

  • ‘Orbits’ twice per day

  • Large ‘viewable’ area

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Basic Concept

  • Satellites are reference points to locations on earth (their location are known)

  • A location of a point on earth is identified by “triangulation”

  • Signals from three satellites are used

  • Travel time of each signal is determined

  • Signals travel at Speed of light

  • Distance = Travel Time * Speed of Light

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Triangulation (2-D)

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Triangulation (3-D)

  • 1 satellite

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Triangulation (3-D)

  • 2 satellites

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Triangulation (3-D)

  • 3 satellites

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The Triangulation Equation

  • 3 variables

    • Where, exactly, are the satellites

    • How long it takes the radio signal to travel that distance

    • How far is the point from the satellite

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Where are the satellites?

  • From orbital mechanics, the location of satellites are determined

  • An almanac of orbital information for all satellites are stored in each satellite

  • Ground control-stations continuously update location information of each satellite and transmit it to them (i.e. ephemeris)

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Satellite Mechanics

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Functions of a Satellite

  • Maintain an accurate time using onboard atomic clocks

  • Receive and store data transmitted by the control stations such as constellation almanac and individual ephemeris

  • Transmit signal containing time and orbital information to the user receiver

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Satellite Signals

  • A GPS satellite transmits continuously at two frequencies in L band:

    • 1575.42 MHz (L1, civilian & military use)

    • 1227.6 MHz (L2, military use only)

  • These signals are modulated by a pseudorandom noise (PRN)

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GPS Codes & Carriers

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Makeup of a Signal

Each signal contains:

  • A carrier (L1 or L2)

  • A unique PRN code

    • C/A code (Coarse/Acquisition for L1)

    • P code (Precise or Private)

  • A binary data message

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Elements of a GPS Signal

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L1 Carrier Signal

  • Has a unique 1023-bit-long C/A code

  • C/A code repeats every 1-millisecond

  • Has 50 bits/s navigation message containing:

    • Data on satellite orbit

    • Clock

    • Health

    • Etc.

  • The chipping rate is 1.023 MHz

  • Length of each chip (wavelength) is 293 m

  • Each satellite transmits a different set of C/A code

  • This is the basis for the Standard Positioning Service (SPS)

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Signals for Military Use

  • L1 & L2 signals with PRN codes encrypted

  • The chipping rate for these is 10.23 MHz

  • The length of each chip is 29.3 m

  • The non-encrypted version called P code

  • The encrypted version called Y code

  • P code is long – repeats every 37 weeks

  • Each satellite transmits a portion of P code

  • These signals are basis for Precise Positioning Service (PPS)

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Comparison of Signals on L1& Their PRN

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Satellite Ranging

  • Calculation of distance

  • One-way ranging

  • Needs two clocks

    • On-board satellite (accurate)

    • On-board user receiver (not as accurate)

  • Called pseudoranging due errors present

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Pseudoranging

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Signal Travel Time

  • GPS satellite and GPS receiver generate the same signal at the same time

  • Satellite transmits the generated signal

  • Receiver acquires the satellite signal

  • The receiver generated signal and the acquired satellite signals are compared

  • The difference between these is the travel time of the satellite signal (about 0.07 sec)

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Time Lag of Signals

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What is the Distance?

  • Range (distance) = Time * Speed of Light

  • Three satellites will provide

    • Latitude

    • Longitude

    • Height

  • Fourth satellite is needed to account for clock time difference – solve for time

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Pseudoranging to four Satellites

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Accuracy of Pseudoranging

  • P code – 10 meters

  • C/A code – 20 to 30 meters

  • With Selective Availability (SA) – 100 meters

  • SA was turned off since 2000

  • Other techniques are needed to improve accuracy:

    • Carrier Phase measurement (Surveying)

    • Differential GPS

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Sources of Error

  • Atmospheric scattering

  • Clock errors

  • Receiver errors

  • Multi-Path Interference

  • Ephemeris

  • GDOP

  • SA

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Multi-path Error

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GDOP(Geometric Dilution of Precision)

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Carrier Phase Method

  • Measure the end segment of the carrier signal that is not a complete cycle

  • Determine the number of whole cycles

  • Note that the difference with the code comparison technique (binary comparison)

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Differential GPS (DGPS)

  • Two receivers used simultaneously

  • One located at a control station (or a monument) where the coordinates are precisely known (base station)

  • One is located at a survey point where coordinates are desired

  • Both stations measure distance

  • Base station calculates error and transmits it to the survey station

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DGPS

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Distance Adjustment

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Errors Compensated

  • The adjustments made by DGPS technique represents a net sum of various errors present in the process.

  • This correction doesn’t address problems with the receiver clocks

  • This correction may not be sufficient when the receiver and the base station are too far from each other

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GPS Surveying – The Basics

  • Carrier phase measuring employed

  • Reference system: World Geodetic System (WGS84)

  • WGS84 – a geocentric 3-D Cartesian coordinate system

    • Primary parameters: define the shape of an ellipsoid for the earth, angular velocity and mass of earth

    • Secondary Parameters: define detailed gravity model of the earth which are used to define the orbits of satellites

    • Defined and maintained by the U.S. Defense Mapping Agency

  • Relative positioning method is used for increased accuracy


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Selection of Ranging Method

  • According the clients expected accuracy, select a ranging method

    • Static

    • Rapid Static

    • Pseudo-Kinematic

    • Kinematic

    • Real Time Kinematic

  • Some methods require dual frequency systems or multiple receivers

  • Size of the crew depends on the method used

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Survey Points

  • Identify points to be surveyed, i.e. stations

  • Organize stations into groups

  • First group should contain a control station

  • Each group should include at least one station from another group – pivoting station

  • All stations in a group should be observed during the same session

  • Pivoting stations are observed twice

  • Collect GPS data at each station

  • Process the data in the office using corrections at the control station

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Grouping of Survey Points

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