AxSys Blog

GPS Basics: GPS Errors - What causes them?

GPS signals are affected by several environmental factors leading to the absolute positioning error discussed in the last article.

The key factors affecting GPS accuracy include:

  1. Atmospheric effects (the signal needs to pass through the entire atmosphere and this can result in errors in the signal timing and strength). This factor is the largest in creating the absolute errors that we see at the Earth surface.
    1. Cloud and Fog cover ‘DO NOT AFFECT GPS ACCURACY’.
  2. Multi-path – this is when the GPS is being used in a built-up environment (eg tall building areas). The GPS signal from the satellites can bounce off a building wall and be seen by the GPS antenna resulting in a timing error that results in positioning, distance and speed errors.
  3. Altitude spikes – Occasionally GPS will incorrectly calculate the altitude of the GPS device – this can have a major effect upon data accuracy and associated distance/speed calculations.
  4. Dilution of Precision – this is the geometric positioning of all the GPS satellites in range at the time of use of your GPS device. There is nothing you can do about this (except for maybe determining when the geometry is at its best and training then). If the geometry is poor this can have a negative impact upon your absolute positioning.

The key things you can do to maximise the quality of the data being captured by your GPS device include:

  1. Ensure you turn your GPS unit on outside so that the moment the G.PS units starts looking for satellites it can see their signals (some GPS devices if turned on inside will freeze as they cannot see any GPS signals and will then need to be reset).
  2. Ensure you are using your GPS device in an open field environment where possible.
  3. Ensure you give your GPS device a few seconds after it has acquired GPS satellites to pick up as many satellites as it can within range.

AxSys Performance has added additional software checks to ensure that any speed or altitude spike is identified and removed prior to generating final results for the client. This ensures high quality data being captured and exported from the AxSys GPS unit.


GPS technology will occasionally generate poor results – this can be caused by a range of issues – if you are aware of these you can typically avoid poor GPS performance by training in the right environment.


In the next article in this series we will explain what core and secondary metrics are able to be captured/calculated using GPS technology.