AxSys Blog

GPS Basics: Key Metrics measured by GPS?

GPS technology provides the user with a range of core outputs that can be utilised to generate a whole range of additional performance information that can be useful to the end user to modify their training and performance over time.

These core metrics captured include Time, Distance, Speed & Position and these metrics allow GPS providers to calculate a range of other metrics that can be used to evaluate the performance of an athlete.

Additional Primary Metrics can include:

  1. m/min
    • How much work the player is doing per minute of training – a very good measure of training/game intensity)
  2. Number and distance of high-speed runs and sprints
    • A key factor in team sport injury is doing too much high-speed work within any training week/period.
  3. Number & volume of acceleration and deceleration efforts
    • Both Acceleration and Deceleration efforts load up specific muscle groups (hip flexors, hamstrings) that can be easily exposed to injury if the volume of this work isn’t carefully monitored over time.

Secondary Metrics:

  • Sports Load Rating (SLR)
  • Game Day Loading (GDL)
  • Metabolic Power
  • Fatigue Index
  • Dynamic Stress Load
  • High-Metabolic Power Distance

 

Many of the above secondary metrics typically have not been validated and as such are often estimates, or representations of the true metric as calculated within an Exercise Physiology laboratory. Or they are a combination of metrics to create a new more meaningful metric (eg SLR/GDL)

For the majority of users of this technology – if you can gain a strong understanding of the Primary metrics and how these are used to maximise performance and minimise injury then you will be in the top percentile of users of this GPS technology in the sporting environment.

 

SUMMARY

Core metrics captured using GPS include (Time, Distance, Speed & Position).

These core metrics allow GPS providers to calculate a range of other metrics that can be used to evaluate the performance of an athlete.

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).

 

Am I training specific to my sport?

When it comes to training, there isn’t a one size fits all solution. What works for some athletes may not work for all athletes, which is why it is necessary to make sport specific adjustments for different types of players.

 

If you have read our previous blogs you will know that training in your Sweet Spot will maximise performance and minimise your chance of injury. However, if you are not training with the right methods then it won’t matter that you are training in your Sweet Spot. 
Incorrect training will stagnate performance and potentially limit your maximum potential.  Spending too much time performing aerobic training when your sport is largely speed and endurance focused or excessive weight training in the gym when speed and power should be your focus are good examples of this.

The challenge with training specificity is that it changes depending on your training age (i.e. how many years of serious training you have completed to date), your chosen sport, and the time of season (in-season, pre-season, preparation phase).

Training effectively

The first step to training effectively is to monitor what you are actually completing during each training session. The AxSys GPS system captures training time, distance, high speed running and body load (the physical demand placed on the body through accelerations, decelerations, impacts, etc.).

Once this information is recorded the next step is to compare your training loads and intensities to well documented training data from different performance and age levels within your sport. At AxSys Performance, we have collated 1 000’s of hours of training data to provide each client with specific advice on their training load and effectiveness.

Understanding the data/results

AxSys understands that you’re most likely not a sports scientist and as such don’t want or need to be bombarded with hundreds of figures and metrics that mean very little. Instead we have identified three key questions to be answered at the end of every training session:

  1. Did you train hard enough today?
  2. Are you training specific to your sport?
  3. Are you at risk of injury through under or over training?

The AxSys GPS system has been designed to provide you with these answers at the end of every training session, week or cycle. If you want to look deeper into your performance data we have provided web access to your own personal training database allowing you to view session by session, second by second of all the most important performance metrics.

Let’s look at an example…

As a soccer player, you feel that your endurance isn’t up to scratch compared to that of your competitors. You then start adding additional long runs to your training program which results in rapidly increasing your SLR (potentially to an overtrained state) as well as beginning to reduce your overall speed and acceleration capacity which are key to your ultimate success in your sport.

Luckily, the AxSys GPS has recorded this information and identified the problem. Instead you should be completing field specific endurance training, such as repeated intervals with and without a ball with short recoveries to simulate the demands of the game.

Finally, to maximise your performance it is important that your training program is challenging you over time. The SLR and sports specific analysis tools have been designed to ensure you are following typical training load and performance increases over time. Comparisons of your performance with peers or even the most elite players in your sport will allow you to see where your strengths and, in particular, weaknesses are in order to target these areas for better long term performance.

If you would like to find out more about training specific to your sport, visit our website at www.axsysperformance.com or contact Adrian on 0417 290 854 to see how AxSys GPS can work for you.