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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

GPS in Land Surveying

Evergreen Valley College

Engineering and Engineering Technology

H. Johnston, T. Redd, A. Tabrizi

July 12, 2005

today s topics
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
gps course information
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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slide4

HHistory of Measurement &

  • DDevices
    • Mechanical
    • Opto-Mechanical
    • Electronic
      • Electronic Distance Measuring
      • Total Station
      • Satellite Assisted Systems
mechanical optical devices
Mechanical & OpticalDevices
  • Simple to use
  • Usually cheap
  • Poor accuracy
  • Simple applications
  • Poor productivity
  • Poor practicality

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some oldies13
Some Oldies

Wireless Communication Technology

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electronics devices

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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slide16
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 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
Accuracy & Precision

Good Precision Poor Precision Good Precision

Good Accuracy Good Accuracy Poor Accuracy

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can hi tech equipment be trusted
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
What is a GPS?

Definitions

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what is gps
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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slide23
GPS
  • Global Positioning System
  • Developed by DOD
  • Cost $10 billion
  • Triangulation-based technology

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why use it
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
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 applications27
Some Applications

Could be used to track mail

if properly used!

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some applications35
Some Applications

Mapping

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how does the gps work
How does The GPS work?
  • The GPS System Components
    • The User Segment
    • The Control Segment
    • The Space Segment

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the user segment
The User Segment
  • GPS user equipment – portable and fixed
    • Military
    • Civilian
      • Navigation
      • Surveying
      • GIS

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the control segment
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
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 segment42
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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gps satellite
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 satellite45
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 satellite46
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 satellite47
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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what is so special about an 11 000 mile orbit
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
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 3 d
Triangulation (3-D)
  • 1 satellite

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triangulation 3 d53
Triangulation (3-D)
  • 2 satellites

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triangulation 3 d54
Triangulation (3-D)
  • 3 satellites

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the triangulation equation
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
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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functions of a satellite
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
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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makeup of a signal
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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l1 carrier signal
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
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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satellite ranging
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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signal travel time
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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what is the distance
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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accuracy of pseudoranging
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
Sources of Error
  • Atmospheric scattering
  • Clock errors
  • Receiver errors
  • Multi-Path Interference
  • Ephemeris
  • GDOP
  • SA

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carrier phase method
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
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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slide78
DGPS

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errors compensated
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
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
selection of ranging method
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
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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