1. 451-102 ERT 247 GEOMATICS ENGINEERING
Global Positioning System
(GPS)
Siti Kamariah Md Saat
School of Bioprocess Engineering
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2. Why GPS and RS in GIS?
GPS and remote sensing imagery are primary GIS data sources, and are very important GIS data sources.
GPS data creates points (positions), polylines, or polygons
Remote sensing imagery and airphotos are used as major basis map in GIS
Information digitized or classified from imagery are GIS layers

3. Overview Basics of GPS Applications and uses GPS Segments 451-102
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4. Globe Positioning System (GPS)
GPS is a Satellite Navigation System
GPS is funded and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military.
GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.
At least 4 satellites are used to estimate 4 quantities: position in 3-D (X, Y, Z) and GPSing time (T)
20,000 km

5. What does GPS provide? 3D position Navigation information Time
What does GPS provide?
3D position
Navigation information
position
heading
velocity
Time
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6. Parts of GPS
The three parts of GPS are:
Satellites
Receivers
Software

7. Satellites There are quite a number of satellites out there in space.
They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc.
Satellites can also be used by GPS receivers.

8. GPS Satellites
The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day.
GPS satellites emit continuous navigation signals.

9. Receivers and Satellites
GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things.

10.
GPS Signals
Each GPS satellite transmits data that indicates its location and the current time.
All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.
Physically the signal is just a complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.
Signal chosen because:
The complex pattern ensures that the receiver does not accidentally synchronize up to some other signal or so the receiver won’t accidentally pick up another satellite’s signal
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11.
What is GPS used for? ...
personal, vehicle, marine and aircraft Navigation
Surveying, deformation monitoring,geodesy, geodynamics
Precision farming, soil mapping, ionospheric modeling, asset management
Time transfer
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12.
In-car navigation
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13. Building/engineering set out
Building/engineering set out
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14.
Tectonic measurement
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15.
Machine guidance
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16.
Air navigation
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17.
Mapping
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18. Deformation monitoring
Deformation monitoring
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19.
Missile guidance
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20.
Three GPS segments
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21. Space segment Constellation of 27 satellites Six orbital planes
Space segment
Constellation of 27 satellites
Six orbital planes
Altitude of approx. 20,200km
Period of 11h 58m
Atomic clocks accurate to 1x10-13 s/day
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22.
Space segment …
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23.
Space segment …
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24. Signal structure

25.
Control segment
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26. Control segment ... Monitor and control the constellation
Control segment ...
Monitor and control the constellation
Establish and maintain GPS time
Provide satellite location (ephemeris)
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27. Control segment ... Master Control Station Ground Control Station
Control segment ...
UPLOAD NAVIGATION MESSAGE
TRACK
Master Control Station
Ground Control Station
Monitor Station
Ground Control Station
Monitor Station
Ground Control Station
Monitor Station
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28. User segment… Some examples already discussed…
Military applications include :
Navigation (land, sea, air)
Missile guidance
Search and rescue
Civilian applications include :
Recreational uses (hiking, 4WD…)
Professional uses (from metres to millimetres)
Position, dynamics, time

29. Position from satellites

30. Differential positioning

31.
Capabilities of GPS
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32. Factors Influencing Position Accuracy
Factors Influencing Position Accuracy
The number of satellites (channels) the receiver can track.
The number of channels a receiver has is part of it’s design.
The higher the number of channels---the greater the potential accuracy.
The higher the number of channels---the greater the cost.
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33. Factors Influencing Position Accuracy
Factors Influencing Position Accuracy
The number of satellites that are available at the time.
Because of the way the satellites orbit, the same number are not available at all times.
When planning precise GPS measurements it is important to check for satellite availability for the location and time of measurement.
If a larger number of channels are required (6-10), and at the time of measurement the number available was less than that, the data will be less accurate.
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34. Factors Influencing Position Accuracy
Factors Influencing Position Accuracy
The system errors that are occurring during the time the receiver is operating.
The GPS system has several errors that have the potential to reduce the accuracy.
To achieve high levels of precision, differential GPS must be used.
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35. Factors Influencing Position Accuracy
Factors Influencing Position Accuracy
Differential GPS uses one unit at a known location and a rover.
The stationary unit compares its calculated GPS location with the actual location and computes the error.
The rover data is adjusted for the error.
Real Time Kinematic (RTK)
Post processing
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36.
Location
Once the GPS receiver has located its position it is usually displayed in one of two common formats:
Latitude and longitude
Universal transverse mercator (UTM).
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37. Latitude and Longitude
Latitude and Longitude
Latitudes and longitudes are angles.
Both use the center of the earth as the vertex, and both utilize the equator, but they use a different zero reference.
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38.
UTM Zones
The world is divided into 60 zones of latitude, each 6o wide at the equator, that extend from 84o N to 80o s.
These zones begin at 180o longitude and are numbered consecutively eastward.
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39.
UTM Zones--cont.
The conterminous United States is covered by 10 UTM grid zones.
In the Northern Hemisphere each zone's northing coordinate begins at the equator as 0,000,000 and is numbered north in meters.
The easting coordinates are measured from an artificial reference line drawn perpendicular to the equator and centered in the zone at the equator.
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40. UTM--cont. The UTM system uses a different grid for the polar regions.
UTM--cont.
The UTM system uses a different grid for the polar regions.
These areas are covered by a different conformal projection called the Polar Stereographic.
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41.
UTM--cont.
Since compass directions have little meaning at the poles, one direction on the grid is arbitrarily designated "north-south" and the other "east-west" regardless of the actual compass direction.
The UTM coordinates are called "false northing" and "false easting.”
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42. Using Location Information
Using Location Information
Each system has its advantages and disadvantages.
Latitude and longitude
UTM
Advantages
With the proper instruments, a person can determine their position at the site without using GPS.
Used by most maps
Advantages
Best method for determining distances between two points.
Disadvantages
Difficult to determine distances between two or more points.
Disadvantages
Not very useful for finding a location.
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43. Time Difference
The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received.
The time difference tells the GPS receiver how far away the satellite is.

44. Velocity x Time = Distance
Calculating Distance
Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000 miles per second (mps)
If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly overhead, use this formula to find your distance from the satellite.
186,000 mps x 0.06 seconds = 11,160 miles

45. Triangulation Geometric Principle:
You can find one location if you know its distance from other, already-known locations.

46. Triangulation

47. Triangulation

48. 3-D Trilateration
1 Satellite
2 Satellites
3 Satellites

49.
Atomic Clocks
GPS satellites use Atomic Clocks for accuracy, but because of the expense, most GPS receivers do not.
the dashed lines show the actual intersection point, and the gray bands indicate the area of uncertainty.
the solid lines indicate where the GPS receiver "thinks" the spheres are located. Because of errors in the receiver's internal clock, these spheres do not intersect at one point.
The GPS receiver must change the size of the spheres until the intersection point is determined. The relative size of each sphere has already been calculated, so if the size of one sphere is changed, the other spheres must be adjusted by exactly the same amount.
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50. Line of Sight Transmissions
Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions.
Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

51. Light Refraction
Sometimes the GPS signal from the satellite doesn’t follow a straight line.
Refraction is the bending of light as it travels through one media to another.

52. Signal Refraction
Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

53. Signal Interference
Sometimes the signals bounce off things before they hit the receivers.

54. PDOP
PDOP = Positional Dilution of Precision
All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.”
11,000 miles
11,000 miles
11,000 miles
11,000 miles
A PDOP of <4 is excellent
A PDOP of 4-8 is good
A PDOP of >8 is poor

55. Satellite Distribution
When the satellites are all in the same part of the sky, readings will be less accurate.

56. Differential Correction
Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."
ISU Base Station -

57. Postprocessing / Real-time
Before
After

59. Waypoints
Waypoints are locations or landmarks that can be stored in your GPS. Waypoints may be defined and stored in the unit manually by inputting latitude and longitude from a map or other reference.
Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point.

60. Data Dictionary GPS units collect data in: These are called features.
Points
Lines
Areas
These are called features.
A data dictionary is a means by which we collect specific information about a data feature.

61.
GPS Errors
Noise
Biases
Blunder
Clock
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62.
Noise Error
Noise errors are the combined effect of code noise (around 1 meter) and noise within the receiver noise (around 1 meter).
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63. Bias Error Selective Availability (SA) Ephemeris data errors: 1 meter
Bias Error
Selective Availability (SA)
SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users.
Selective availability is turned off.
Ephemeris data errors: 1 meter
Satellite orbits are constantly changing. Any error in satellite position will result in an error for the receiver position.
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64.
Bias Error
SV clock errors uncorrected by Control Segment can result in one meter errors.
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65. Bias Error Tropospheric delays: 1 meter.
Bias Error
Tropospheric delays: 1 meter.
The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes.
Complex models of tropospheric delay require estimates or measurements of these parameters.
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66. Bias Error Unmodeled ionosphere delays: 10 meters.
Bias Error
Unmodeled ionosphere delays: 10 meters.
The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual.
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67. Bias Error Multipath: 0.5 meters.
Bias Error
Multipath: 0.5 meters.
Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite.
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68. Blunder Blunders can result in errors of hundred of kilometers.
Blunder
Blunders can result in errors of hundred of kilometers.
Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers.
User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters.
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69.
Blunder
Receiver errors from software or hardware failures can cause blunder errors of any size.
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70. Thank You...