Module 2 : Global Positioning System
  Lecture 9 : GPS Errors and Accuracy
On the basis of sources of errors in GPS observations
  • Satellite dependent
Originate from satellite or are found to be part of satellite transmitted signals
    • Orbit related: Ephemeris errors and orbit perturbations
    • Satellite clock
    • Selective Availability (SA)
  • Receiver Dependent
Originate from receiver
    • Receiver clock
    • Cycle slips
    • Antenna phase center movement
    • Receiver noise
  • Observation medium dependent
Originate during signal propagation from satellite to receiver
    • Ionospheric delay
    • Tropospheric delay
  • Station dependent
    • Multipath
  • Some of these errors may be systematic which change from one epoch to another whereas the random errors need to be distributed and adjusted within the set of GPS observations. Table 9.1 summariezs various types of errors in GPS measurements which will now be briefly explained.

Table 9.1 Error sources in GPS observations (Dhunta, 2001)

Sources of Errors
Effect
Satellite Gravitational
  • Non-sphericity of earth
  • General relativity
  • Tidal attraction
Orbital error
Non- Gravitational
  • Solar radiation pressure
  • Air drag
  • Solar wind
  • Magnetic field
Others
  • Satellite clock
  • Selective availability
Signal Propagation  
  • Ionospheric refraction
  • Tropospheric refraction
  • Multipath
Signal delays
  • Special relativity
Increase in received frequency
Receiver  
  • Antenna phase centre variation
  • Receiver clock bias
  • Receiver noise
Error in observed PRN
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