The combination of GPS & GLONASS or GPS & GALILEO satellite systems has established itself as a standard for many outdoor & bicyle devices and wearables. Positioning with the help of two GNSS (= Global Navigation Satellite System) has its advantages.
The availability of two systems means that more satellites are available. This can have an impact on the time to first positioning and the positioning accuracy.
How many GPS, GLONASS & GALILEO satellites are available?
(status: January 2020)
- GPS: 29 operational satellites
- GLONASS: 23 operational satellites
- GALILEO: 22 operational satellites (the full operational constellation of 24 satellites is expected to be available in 2020)
Information on the current statuses can be found here:
What are the advantages of using two satellite systems simultaneously?
A good example is shown in the figure. Due to the availability of only four GPS satellites and an unfavourable satellite geometry (three out of four GPS satellites are in a row) in a mountain valley, the position determination is relatively inaccurate. The accuracy is shown as "25 m".
The same applies to large cities; high buildings ("urban canyons") can also cause shadows effects, there are fewer satellites in the direct field of view of the GPS receiver.
In addition, "multipathing" effects often occur in mountains and cities due to reflected satellite signals, also resulting in poorer accuracy.
What positive effect has a second GNSS?
More satellites distributed over the sky can lead to a more favourable distribution ("geometry") and thus to a better accuracy and a faster positioning.
Figure: GPS only, really bad satellite constellation and accuracy
Garmin defines GPS accuracy as follows: Accuracy of the position in m (CEP, Circular Error Probable: 50% of all measurements are within the specified radius (e.g. 25 m), 50% of all measurements are outside of this radius).
Another note: At least three satellites are required to determine the position, and at least four satellites must be in use to determine the GPS altitude.
What are the disadvantages of using GPS, GLONASS and GALILEO simultaneously?
The disadvantage is the slightly increased power consumption of the GPS receivers, in our experience it is about 10% (GPS handhelds like Garmin Oregon or eTrex). With wearables it can look different, because they rather have "power saving" chips.
Figures (screenshots Garmin eTrex 32x): Excellent satellite reception; GPS + GLONASS (left) does not lead to a better accuracy compared to GPS only. The accuracy is always 3 m. GPS: 02 – 30, GLONASS: 68 – 84.
Garmin – GPS only, GPS & GLONASS or GPS & GALILEO – What do you recommend?
Our recommendation is to deactivate GLONASS or GALILEO if possible (most handheld devices supports this) and to use GPS only; switch a second GNSS on when necessary (mountains, cities, forests with a high tree density).
Garmin devices allow these GNSS combinations:
- GPS + GLONASS (recommended for higher latitudes as GLONASS covers these regions better)
- GPS + GALILEO (GALILEO is recommended when fully operational in 2020)
A significant increase in accuracy can only be expected when chips supporting multiple GNSS and multiple frequencies are used in the future. An example is the Broadcom BCM47755 chip that can use GPS, GLONASS, BeiDou (the Chinese GNSS), QZSS (the Japanese GNSS) and GALILEO as well as two frequencies (L1+L5).
The situation is different for wearables
For example, when you run, the wearable is attached to another dynamic system: your moving arm. The GPS antenna usually points somehow to the side and not skywards, your body causes shadowing when receiving satellite signals. All these factors can lead to GPS inaccuracies.
Therefore, it is better to get the maximum out of it by using GPS + GLONASS – this setting is also the Garmin standard for wearables.
Must read: Garmin fenix GPS accuracy