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RTAnalysisTechnical / RTPPPMode

Introduction to PPP mode

PPP is an abbreviation of "Precision Point Positioning", as apposed to standard GPS which is referred to as SPP "Standard Point Positioning". PPP is a method of improving GPS accuracy by using high precision satellite and atmospheric information to process the GPS data. The high precision data is available from the internet in real time and results in improved positional accuracy, and theoretically improves speeds - although in the case of the speeds the performance improvement is very marginal.

In SPP mode the positions of the satellites, the estimated error of the satellites clock and atmospheric corrections are all based on the information transmitted by the GPS satellites themselves. In practice there are errors on all these transmitted measurements, which result in errors in the GPS position. Typically errors are of the following magnitude:

GPS satellite positioning error 2m
GPS satellite clock error 2m
Atmospheric errors (Ionosphere) 3m

In contrast PPP uses data downloaded from GPS reference stations to more accurately estimate the position of the satellites, the satellite clock errors and status of the atmosphere. There are 100's of these GPS reference stations all over the world that update a central database that can then be accessed over the internet. More details about this can be found at the International GNSS Service (IGS) based in the USA http://igscb.jpl.nasa.gov

In practice the Race Technology implementation of PPP corrects for the following errors:

  • Receiver clock errors
  • Satellite clock errors
  • Satellite positional error
  • Satellite antenna offset (orientation) error
  • Delay in the Ionosphere
  • Delay in the Troposphere
  • Realistic effects on the satellite clocks relative to earth
  • Rotation of the earth (Sagnac)

Because of the way that the information is calculated by the IGS, there are several standards of data available, depending on how quickly you require the information. For example, for the satellite clock error information (ephemerides) is available in 4 "qualities" depending on how long after the data recording you require the information:

Real time information (ultra rapid, predicted) 1.5m
After 3 hours (ultra rapid, observed) about 6cm
After 17 hours (rapid) about 3cm
After 13 days (final) under 3cm

So if you analyse the data immediately after the race you will get more error in position than if you wait the 3 hours (approximate) for the improved data to become available.

Page last modified on November 22, 2013, at 08:15 AM