What is GNSS?
The European Global Navigation Satellite Systems Agency defines GNSS on their website as follows:
"Global Navigation Satellite System (GNSS) refers to a constellation of satellites providing signals from space that transmit positioning and timing data to GNSS receivers. The receivers then use this data to determine location. By definition, GNSS provides global coverage. Examples of GNSS include Europe’s Galileo, the USA’s NAVSTAR Global Positioning System (GPS), Russia’s Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) and China’s BeiDou Navigation Satellite System."
The performance of GNSS can be improved by regional satellite-based augmentation systems (SBAS), such as the European Geostationary Navigation Overlay Service (EGNOS). EGNOS improves the accuracy and reliability of GPS information by correcting signal measurement errors and by providing information about the integrity of its signals.
Note that as GPS was the first GNSS, and is the most widely used system, the term GPS is still commonly used when referring to all types of GNSS. However, the correct term is GNSS, which is an umbrella term that covers all global satellite positioning systems, including GPS.
Positioning performance in wearables
When having a closer look at what factors affect positioning performance in the wearables category, there are a few variables that stand out. These are:
From an industrial design standpoint, a thin and small device is desirable. However, these elements cause challenges for the GNSS antenna.
Thinness and small design: To perform well the antenna requires as much volume as possible, so a thin and small device is not optimal for antenna performance. In addition, the antenna needs to be as far as possible from the wrist and body (which are lossy material that have an adverse effect on performance.)
The GNSS chipset is also a factor in positioning performance. Different chip suppliers have different incentives related to power consumption, performance in different use cases, and target groups. These steer their chip development and the focus areas of their chips.
The dynamics and characteristics of the use environment have an impact on data accuracy and performance. Some factors affecting common sports are listed below:
When walking your body can block the GNSS signal, and constant arm swinging also causes challenges for GNSS performance. In general, the watch is in a poor signal reception position making it prone to data inaccuracies. Changes in your direction of travel have a high impact on accuracy.
When running your body can block the GNSS signal, and constant arm swinging also causes challenges for GNSS performance. In general, the watch is in a medium signal reception position.
When cycling your body often blocks the GNSS signal to the watch as you lean forward when riding. In general, the device is in a medium to good signal reception position watch facing up. When worn on your arm or the handlebars the device is still, which minimizes dynamics and allows better data accuracy.
When swimming there is no body block. In general, the watch is in a poor to good signal reception position. Constant arm movement and the fact that there is no signal reception when the watch is under water cause challenges for GNSS performance. Your swimming style also affects GNSS accuracy (style, arm swing). Optimally, the watch should be above water for over a second to acquire signals.
For example, a street surrounded by tall buildings from both sides forming as canyon-like environment that is called an urban canyon.
In urban canyons, the device is prone to multipath (a propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths due to reflections). Multipath can cause erroneous GNSS readings.
The visible satellite constellation also changes a lot when navigating in an urban canyon which also causes challenges for GNSS.
When training in wooded areas such as forests signal attenuation (the reduction of signal strength during transmission) hinders GNSS performance.
Swimming in open water causes problems with signal visibility. There is limited time to acquire a signal as your wrist is above water for just a short time, and the water also causes signal reflection. Open water swimming is probably the most challenging use environment for a wearable GNSS device such as a Polar device.
Software algorithms are used to enhance performance. The software ties everything together and adapts to:
- Arm swing
- Body block
- Dynamically changing environment
- Interval training
- Multipath environment
- Low signal level environment
Algorithms are developed by both the GNSS receiver chipset supplier and the wearable product companies.
GNSS and Polar watches
In Polar watches, GNSS is utilized for tracking speed, distance, and location. Speed and distance values are used by the parameters of many Polar features (such as Running Index). Note that if satellite navigation is not used in a training session, speed and distance can be measured from your wrist movements with a built-in accelerometer. See Speed and distance from the wrist with GPS calibration and What affects the accuracy of speed and distance measurement from the wrist? for more information.
For GNSS accuracy/GPS accuracy details of your Polar watch, see section Technical specification in the User Manual.
Positioning in Polar watches
The satellite fix requires four or more satellites. After the fix, the number of satellites will increase as more are found. The maximum number of satellites that can be used is 12. This number is easier to achieve when using A-GPS. When the fix is complete and the training session has been started, the watch attempts to add more satellites to the calculation on the go.
Some Polar watches have a barometer (pressure sensor) for accurate altitude data. Initial altitude is calibrated with GNSS data. Polar watches that do not have a barometer (but do have built-in GNSS), altitude is only based on GNSS data. GNSS based altitude data can occasionally be inaccurate. Note that Altitude calculation requires 6 or more satellites, and that altitude data is not taken into account when measuring distance.
You can change the satellite navigation system your watch uses in addition to GPS. The setting is located on your watch in General settings > Positioning satellites. You can choose GPS + GLONASS, GPS + Galileo or GPS + QZSS. The default setting is GPS + GLONASS. These options give you the possibility to test different satellite navigation systems and find out if they can give you improved performance in areas covered by them.
GLONASS is a Russian global satellite navigation system. It is the default setting as its global satellite visibility and reliability is the best of these three, and in general we recommend using it.
Galileo is a global navigation satellite system created by the European Union.
QZSS is a four-satellite regional time transfer system and a satellite-based augmentation system developed to enhance GPS in the Asia-Oceania regions, with a focus on Japan.
A-GPS – Assisted GPS (Ephemeris prediction)
A-GPS predicts the positions of the satellites and their orbits, eliminating major position calculations which equals faster fix times. 14 days of satellite prediction data is downloaded to the watch once per day via the Flow app when connected to your mobile or when you sync your watch with FlowSync.
A-GPS should always be used because it enables faster fix times. The more challenging the conditions are, the more important and noticeable the benefits of A-GPS become. It also enables better overall route, speed and distance accuracy through better dynamic satellite selection during training sessions. This difference also is highlighted during challenging conditions.
A-GPS needs to know your approximate position, so if your training location changes over 100 kilometers/60 miles from your last session, acquiring the first fix is takes a little bit longer.