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.

SUMMARY

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.

GPS Basics: Absolute v's Relative Positioning Accuracy

When discussion the accuracy of a GPS system it is important to distinguish b/w absolute positioning v’s relative positioning.

Absolute positioning is your exact location at any given time on Earth (indicated by a Latitude and Longitude).

Relative positioning is the actual distance travelled as indicated by the GPS from a start point to a known end point.

The image to the left shows that on any given day your absolute positioning will be +/- 2-5m of the true location BUT if you travel a known distance (regardless of your starting point), the accuracy of a system like the AxSys system will be around 99% of true distance.

It is for this reason that being able to accurately display player to player positioning is difficult due to the absolute positioning error, but the distances covered by the athlete (relative positioning) are very accurate and reliable.

There are methods to improve on the absolute accuracy of a GPS system, but this typically involves additional technologies and increased cost.

If you are interested in a reasonable estimation of the positioning of your player – then the absolute positioning error of systems like the AxSys system is likely to be acceptable.

As with all things GPS, the clearer the area that the data is being collected (eg open field), the better chance of capturing accurate performance data.

SUMMARY

We are regularly asked “How accurate is the GPS product we have” – Before we can answer this question appropriately we need to explain the difference between Absolute and Relative Positioning and the associated accuracies when applied to a sporting environment.
 

In the next article in this series we will explain why GPS is not cm level accurate and highlight how you can use the technology to ensure the most accurate signal possible.

GPS Basics: Overview of GPS

 

There are many myths around the use of GPS in sport – This series aims to highlight the strengths and weaknesses of GPS technology and associated sensors (Accelerometers/Gyroscopes) for use in a sport environment.
 

Firstly, there is no doubt that Sport GPS units have changed the team sport landscape for ever. Gone are the days of guestimating how much work has been completed by each individual player. With the advent of accurate, small GPS units, coaching staff now have access to all key metrics needed to make informed decisions on training loads and intensities.
 

 

 

 

 

GPS BASICS:

  1. GPS is a free technology (No fees to be paid to access the signal).

  2. GPS does not need to be calibrated, it self-calibrates.

  3. GPS only works outdoors (requires a direct line of sight from the satellite to the GPS receiver in your device/phone).

  4. GPS has a position error (where you are located on the Earth) of between 2-10m depending on the environment and quality of GPS module.

  5. GPS doesn’t like built up environments (city centres, forests) – the accuracy will decrease in these environments.

  6. GPS uses two methods to calculate distance/speed of a user

    1. Latitude/Longitude/time, or

    2. Doppler (rate of change of the GPS signal reaching a GPS module).

  7. There are 20+ GPS satellites in the sky in most regions of the world.

  8. There are multiple GPS companies with GPS satellites in the sky – Most modern GPS units will have GPS modules that can see 2 or more of these systems (ensuring very good satellite coverage anywhere on Earth).

  9. Only 4 satellites are required to generate an accurate and reliable position.

  10. GPS devices have different update rates – Typically your phone or GPS watch will only sample at 1Hz (once per second) – this is not suitable for rapid change of direction activities as would happen in a team sport environment.

  11. The team sport specific GPS devices have sampling rates 10Hz or more.

    1. AxSys GPS has a sampling rate of 18hz – leading to a very high accuracy of all GPS metrics.

  12. GPS can be used to capture the following performance metrics from an athlete:

    1. Position

    2. Distance

    3. Speed

    4. Acceleration

    5. Deceleration
       

Summary Video: How Does GPS Work? by sciBRIGHT - https://www.youtube.com/watch?v=FU_pY2sTwTA

 

SUMMARY

GPS technology is a free platform that can allow for accurate measurement of sport performance.

There are some limitations to the technology, that if you understand and avoid will maximise your use of GPS technology in your training.

In the next article in this series we will explain the difference in accuracy using GPS (Absolute versus Relative positioning accuracy).