Global Positioning Systems

1
Global Positioning Systems

• Three key topics
• Requirements

2
GPS Space Segment

• Constellation of NAVSTAR satellites
• Each satellite broadcasts signal

3
GPS Receiver Segment

• Receivers situated over unknown point receive
  signals from visible satellites
• Determine time for satellite signal to reach
  receiver
• Need multiple ranges from multiple satellites to
  derive location of receiver

4
GPS Control Segment

• Monitoring stations
• Application

5
GPS Reference SystemsSatellite Reference System

• Satellites fundamentally in an elliptical orbit
• Origin at ellipse foci
• Geocentric reference system

6
GPS Reference SystemsGeocentric Reference System

• Issue
• Origin at mass center of earth

7
GPS Reference SystemsGeodetic Reference System

• Geocentric coordinates are challenging to use
• Solution
• Conversions

8
Geographic to Geocentric Coordinates

• Where
• h ellipsoidal height e ellipsoid
  eccentricity
• ? latitude ? longitude
• RN radius of curvature a semi-major axis
  of prime vertical

9
GPS Timing Process

• Satellites broadcast signals
• Radio signals uniquely represent each satellite

10
GPS Timing Basic Procedure

• Receiver compares satellite signal to internal
  signal
• Calculate range from multiple satellites
• Application

11
Carrier Phase Measurement

• Process
• Application

12
Deliberate GPS Accuracy Reduction

• Selective availability
• Anti-spoofing

13
GPS Errors and Mistakes

• Instrument errors
• Natural errors

• Personal errors
• Mistakes

14
Code-based Differential Positioning

• Use two receivers
• Each unit determines position containing errors
• Processing differential corrections

15
Carrier Phase Relative Positioning

• Single difference

16
Carrier Phase Relative Positioning

• Double difference

17
Carrier Phase Relative Positioning
t2
t1
t1
t2

• Triple difference

18
GPS Considerations

• Accuracy
• Field situations

• Costs

19
Sample Applications

• Military
• Surveying
• Recreational

• Commercial

20
GLONASS

• Russian counterpart to GPS
• Emerging as a civilian measuring tool

21
Readings

• Chapter 13
• Chapter 14 sections 14.2.1, 14.6 14.7