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WHY GPS IS NOT PERFECT

Selective Availability: the intentional degradation of the timing and position information transmitted by a GPS satellite

PDOP: (the position dilution of precision); describes the amount of error due to the geographic position of the GPS satellites.

Multipath: an error caused by a delay in the signal due to reflections from the surfaces before reaching the receiver.

DGPS: (differential GPS); a method of using a ground based correction in addition to  the satellite signals to determine position.

  • In normal everyday use accuracy of GPS is around 10-15 meters.
  • This is because of different factors causing delays in the transmission.
  • The biggest source of error was intentional.
  • The united states introduced selective availability to make the signal less accurate so that no one could use the GPS against the united states.
  • Selective availability introduced two errors.  

TYPES OF ERRAS 

  • In normal everyday use accuracy of GPS is around 10-15 meters.
  • This is because of different factors causing delays in the transmission.
  • The biggest source of error was intentional.
  • The united states introduced selective availability to make the signal less accurate so that no one could use the GPS against the united states.
  • Selective availability introduced two errors.  

SELECTIVE AVAILABILITY

  • In 2000, the United States decided to turn the selective availability off.
  • They also announced that they do not plan to make future satellites with the feature.
  • However the public GPS will still be less accurate than the military's.
  • The arrangement of satellites can have an effect on determining position.
  • The further satellites are spread out the more accurate the measurements will be
  • On the other hand the less spread out satellites are the less accurate the measurements will be.
  • MORE INFORMATION

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Thoughts

  • Yep but Total Station has also lot of imperfection : mechanical axis errors, EDM calibration, verticality, instability of the tripod, refraction, etc, ... the question is not the error budget but well how surveyors can mitigate and compensate by appropriated methods these imperfection. GNSS is actually very much precise and accurate when you are mastering all the errors and use differential setup's.

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