GPS Fundamentals

1
GPS Fundamentals
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What is a GIS?

• A GIS is a computerized database management
  system designed for the
• capture
• storage
• analysis
• and display
• of spatial (location-defined) data for the
  purposes of decision making and research

3
Forms of GIS Data Collection

• Paper records field notes
• Digitizing from paper maps
• Scanning
• Photogrammetry
• Remote sensing
• GPS

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GIS Data Storage
Feature (Point, Line, Area)
Non - Graphic (Attributes Attribute Values)
Unique ID
Point Feature Attributes ID
Address Condition 11. 123 Walnut Destroyed 12.
125 Walnut Standing 13. 127 Walnut Destroyed 14.
130 Walnut Damaged
Point Feature Coordinates ID
11. X Y Z
12. X Y Z
13. X Y Z 14.
X Y Z
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Global Positioning System
Your location is 37o 23.323 N 122o 02.162 W
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What is GPS?

• A constellation of satellites providing
  world-wide positionally accurate coordinates
• Developed and maintained by Department of
  Defense
• Congress sold on the idea that other applications
  would follow

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GPS Data Capture

• In a nutshell...
• GPS positions are translated into point, line,
  and area features.
• Attribute values (descriptive information) are
  tagged to each feature.
• Features and Attribute values are imported into a
  GIS.

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How GPS Positions Translate Into Graphic Features

• Point Features
• Multiple GPS positions are collected
• Average location of all GPS positions becomes the
  point feature

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How GPS Positions Translate Into Graphic Features
1100.10
1100.15
1100.05
1100.20

• Line Features
• Joins each GPS position together in TIME SEQUENCE
• Area Features
• Joins each GPS position together in TIME
  SEQUENCE
• Joins last recorded position to first

1100.25
1130.10
1130.05
1130.15
1130.25
1130.20
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The GPS System
Space Segment
User Segment
Monitor Stations

Diego Garcia

Ascension Is.

Kwajalein

Hawaii
Colorado Springs
Control Segment
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Control Segment
Transmits information
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Space Segment

• 24 Satellites
• Orbit with a 55 rotation
• 20,200 km orbit
• 1 revolution in 12 hours

• High Orbit For

Survivability
Copied from GPS Navstar Users Overview
prepared by GPS Joint Program Office, 1984
Coverage
Accuracy
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User Segment
Multiple Applications GIS Data
Collection Vehicle Tracking Marine/ Vehicle
Navigation Surveying Emergency ServicesAviation P
recise Positioning Agriculture Photogrammetry Recr
eation (hiking etc.) Many more ... and more to
follow
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Why We Use Satellites for Mapping

• Line of Sight

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GPS in 5 Steps
3
To measure distance, you need good clocks and
four satellites
4
Once you know the distance, you need to know
satellites position
2
To trilaterate, GPS measures distance from the
satellites
5
Differential Correction
1
Based on trilateration
Trilateration From SVs
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Trilateration from Satellites
1

• By knowing the location and distance of several
  satellites, the GPS receiver can calculate an
  accurate position

Trilateration
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Trilateration

• One measurement narrows down our position to the
  surface of a sphere

We are somewhere
19,000 Km
on the surface of
this sphere
Trilateration
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Trilateration

• Second measurement narrows it down to the
  intersection of two spheres

19,000

Km
20,000

Km
Intersection of two

spheres is a circle
Trilateration
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Trilateration

• Third measurement narrows it to just two points

Intersection of three
spheres is only two
points

Trilateration
20
Trilateration

• Fourth measurement decides between two points

Trilateration
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Trilateration

• In theory three measurements are enough
• One point will be a ridiculous answer
• Out in space
• Or moving at high speed
• Four measurements required to collect 3D
  positions
• GPS receiver must track at least four satellites
• Parallel
• Sequencing

SV Ranging
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Satellite Ranging
2

• Measuring the distance from a satellite
• Done by measuring travel time of radio signals

4 seconds
5 seconds
Speed of Light Measurement
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Speed-of-Light Measurement

• Measure how long it takes the GPS signal to get
  to us
• Multiply that time by 300,000 km/sec
• (186,000 mi/sec)
• Time (sec) x 300,000 (186,000) kilometers
  (miles)
• If you have precise clocks, all you need to know
  is exactly when the signal left the satellite

When the Signal Leaves the Satellite (B)
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How Do We Know When the Signal Left the Satellite?

• The receiver and satellites use the same code
• They are synchronized to generate the code at the
  same exact time
• Then, the receiver looks at the incoming code
  from the satellite and determines how long ago
  the receiver generated that code

from satellite
from ground receiver
Why a Code (B)
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Why a Code?

• Code lets many satellites operate on the same
  frequency
• Code gives us a way to increase the
  signal-to-noise ratio (SNR)
• Resistant to intentional or unintentional jamming
  by other radio signals

Accurate Clocks to Measure Travel Time
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Accurate Clocks are Necessary to
Measure Travel Time
3

• Receiver and satellite must be synchronized which
  requires precise clocks
• Satellites have atomic clocks
• Accurate but expensive
• Ground receivers have consistent clocks
• Quartz clocks are relatively inexpensive

Ideal Situation
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The Ideal Situation (Accurate Clocks)

• In 2 dimensions for the sake of the drawing

This is where we really are
4 secs
6 secs
Adding a Third Measurement
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Adding a Third Measurement (Accurate Clocks)

4 secs
6 secs
8 secs
Fast Clock
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With Fast Clock
7 secs
5 secs
wrong time
wrong time
Thrid Measurement
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Third Measurement (Fast Clocks)

• The three spheres will not intersect

5 sec (wrong)
7 sec (wrong)
Now the third
measurement
9 sec (wrong)
will not go through
the other two
Satellite Location
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Knowing Satellites Location
4

• After all, they are 20,200 km (12,600 miles) up
• Monitored by Department of Defense
• Four monitoring stations
• Sunsynchronus orbit
• 12 hour revolution
• Satellites transmit status information to the GPS
  users.
• SV location
• SV health

Almanac Message
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Almanac Message

• An Almanac is a set of parameters used to
  calculate the rough location of each satellite
• Almanacs are used for
• Rapid satellite acquisition
• Pre-mission planning

Ephemeris Message
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Ephemeris Message

• An Ephemeris is a set of parameters used to
  determine the exact location of a Satellite.
• Ephemeris used for
• Calculating a GPS position.

Differential Correction
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Differential Correction
5

• Without correction measures applied any GPS
  position will be up to 100 meters from truth.

lt100 m
GPS in 5 Steps
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Code Differential GPS
Two receivers track SAME signals and errors at
SAME time
Carrier Phase Differential
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GPS in 5 Steps
3
To measure distance, you need good clocks and
four satellites
4
Once you know the distance, you need to know
satellites position
2
To trilaterate, GPS measures distance from the
satellites
5
Differential Correction
1
Based on trilateration
Questions
37
Sources of Error in GPS

• Selective Availability
• Obstruction
• Multipath
• Atmospheric delay

Selective Availability
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GPS Error Types

• System-wide errors - DGPS Correctable

Receiver Noise
Ephemeris
Atmospheric
S/A
0
20
40
60
80
100
Meters
Autonomous Accuracy
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Selective Availability

• Government introduces artificial errors to reduce
  GPS position accuracy
• Discourages hostile forces from using GPS
• Largest source of error

Atmospheric Delay
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Atmospheric Delay

• GPS signals are delayed
• as they pass through
• the atmosphere

Ionosphere
Troposphere
Rover
Base
lt 10 km
gt 10 km
Obstruction
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Obstruction
Multipath
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Multipath
Review Questions
43
How Accurate is it?

• That depends
• Depends on some variables
• Time spent at a geographic location
• Design of receiver
• Relative positions of satellites
• Reicever onfiguration settings
• Correction method

How Accurate Is It? (B)
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How Accurate is It?

• Three major methods of positioning
• Autonomous
• No corrections applied
• Up to 100 meters
• Differential
• Corrections applied
• Submeter to 5 meters
• Phase Differential
• 10 - 100 Centimeters Phase Processing (mapping)
• 5 millimeter Wave Processing (surveying)

Questions
45
Autonomous Accuracy of Any GPS Receiver
95 (100m)
68 (50 m)
50 (40m)
Pro XR Accuracy
46
Vertical Accuracy

• Horizontal accuracy is 1 to 3 times better than
  vertical accuracy

Differential Correction
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Uncorrected vs. Corrected Data
Uncorrected
Corrected
Differential Correction (B)
48
Questions ?
Take a Break