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1
How Does GPS Work ?
2
Objectives

• To Describe
• The 3 components of the Global Positioning System
• How position is obtaining from a radio timing
  signal
• Obtaining instantaneous positions
• High precision GPS and how it works
• The method of operation
• Errors and data quality issues with GPS

3
The Global Positioning System

• 24 satellites
• 20,200 km altitude
• 55 degrees inclination
• 12 hour orbital period
• 5 ground control stations
• Each satellite passes over a ground monitoring
  station every 12 hours
• 3 Components a) Ground Control Segment b)
  Space Segment and c) User Segment

4
Obtaining a Position From a Radio Timing Signal

• Precise atomic clocks are needed to measure
  travel time
• Getting perfect timing means synchronizing the
  satellite with the receiver
• The travel time for a satellite directly overhead
  is about 0.06 seconds
• The difference in sync of the receiver time minus
  the satellite time is the travel time
• a GPS receiver determines the travel time of a
  signal from a satellite by comparing the "pseudo
  random code" it's generating, with an identical
  code in the signal from the satellite

5
Instantaneous positioning with GPS

• Accuracy of
• /- 10 m (30 ft) error (horizontal)
• /- 15 m (45 ft) error (vertical)

6
High-precision GPS

• -Use the carrier phase, not pseudorange
• Dual-frequency receivers (to remove ionosphere)
• High-precision orbital information
• Good monuments
• Multiple stations (to remove satellite clock
  variations)
• Sophisticated processing software
• Collect lots of data

7
High-Precision Systems for Field Surveying
Movable tripod with choke-ring antenna
Trimble 5700 geodetic receiver with flat antenna
Monument stability and centering and leveling the
antenna are critical !
8
Method of Operation

• Global positioning system (GPS)
• Receiver determines location, speed, direction,
  and time
• 3 satellite signals are necessary to locate the
  receiver in 3D space
• 4th satellite is used for time accuracy
• Position calculated within sub-centimeter scale

9
Atmospheric Delays

• Ionosphere (use dual frequency receivers)
• Troposphere (estimate troposphere)

10
Multipath echoes

• Similar issue to ghost images on TV
• Causes inaccurate measurements
• If we were outside right now, what would cause
  potential multipath errors?

11
Summary

• 3 components to GPS are 1) Ground Control
  Segment 2) Space Segment and 3) User Segment
• Radio signals are matched in satellite and
  receiver to obtain travel times used to calculate
  distance and position
• High Precision GPS can obtain millimeter level
  accuracy (but at a cost)
• Atmospheric delays and multipath can cause GPS
  inaccuracies