GPS in Land Surveying

1
GPS in Land Surveying

• Evergreen Valley College
• Engineering and Engineering Technology
• H. Johnston, T. Redd, A. Tabrizi
• July 12, 2005

2
Todays 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

3
GPS Course Information

• Lecture component
• Accuracy, precision, error
• Oral written communication
• Introduction to GPS
• Laboratory component
• Field activities
• Data processing
• Expected learning competencies

4

•  
• HHistory of Measurement
• DDevices
• Mechanical
• Opto-Mechanical
• Electronic
• Electronic Distance Measuring
• Total Station
• Satellite Assisted Systems

5
Mechanical OpticalDevices

• Simple to use
• Usually cheap
• Poor accuracy
• Simple applications
• Poor productivity
• Poor practicality

6
Some Oldies
7
Some Oldies
8
Some Oldies
9
Some Oldies
10
Some Oldies
11
Some Oldies
12
Some Oldies
13
Some Oldies
Wireless Communication Technology
14
Electronics Devices

• Electronic Distance Meter

15

• Total Station

16

• 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)

17
(No Transcript)
18
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

19
Accuracy Precision
Good Precision Poor Precision Good
Precision Good Accuracy Good Accuracy
Poor Accuracy
20
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.

21
What is a GPS?
Definitions
22
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

23
GPS

• Global Positioning System
• Developed by DOD
• Cost 10 billion
• Triangulation-based technology

24
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)

25
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

26
Some Applications
27
Some Applications
Could be used to track mail if properly used!
28
Some Applications
29
Some Applications
30
Some Applications
31
Some Applications
32
Some Applications
33
Some Applications
34
Some Applications
35
Some Applications
Mapping
36
How does The GPS work?

• The GPS System Components
• The User Segment
• The Control Segment
• The Space Segment

37
The GPS System Components
38
The User Segment

• GPS user equipment portable and fixed
• Military
• Civilian
• Navigation
• Surveying
• GIS

39
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

40
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)

41
The Space Segment
42
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

43
Satellite Constellation
44
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

45
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

46
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

47
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

48
GPS Satellite
49
What is so special about an 11,000 mile orbit?

• Mathematically perfect orbit
• Orbits twice per day
• Large viewable area

50
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

51
Triangulation (2-D)
52
Triangulation (3-D)

• 1 satellite

53
Triangulation (3-D)

• 2 satellites

54
Triangulation (3-D)

• 3 satellites

55
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

56
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)

57
Satellite Mechanics
58
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

59
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)

60
GPS Codes Carriers
61
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

62
Elements of a GPS Signal
63
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)

64
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)

65
Comparison of Signals on L1 Their PRN
66
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

67
Pseudoranging
68
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)

69
Time Lag of Signals
70
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

71
Pseudoranging to four Satellites
72
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

73
Sources of Error

• Atmospheric scattering
• Clock errors
• Receiver errors
• Multi-Path Interference
• Ephemeris
• GDOP
• SA

74
Multi-path Error
75
GDOP(Geometric Dilution of Precision)
76
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)

77
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

78
DGPS
79
Distance Adjustment
80
Errors Compensated

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

81
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

82
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

83
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

84
Grouping of Survey Points