Whether in phones, watches, or cars, the use of GPS is becoming more and more widespread.
But even if people use it very often, the fact remains that its functioning sometimes seems strange to the sight of the data it transmits.
It is therefore important to better understand how it works to better understand the information transmitted. But before that, a little clarification.
GPS or GNSS ?
We commonly speak of GPS (Global Positioning System), but it is more accurate to speak of GNSS.
GNSS (global navigation satellite systems) is the term for satellite positioning technology. Indeed there are 6 different GNSS:
GPS (USA).
GLONASS (Russie).
Beidou (Chine).
Galileo (Europe).
QZSS (Japon).
IRNSS (Inde).
The latest GNSS chips found in our equipment make it possible to recover the signals from these 6 constellations of satellites. This has the effect of increasing the accuracy of our position. You can install an app like Statut GNSS or GPS TEST to see different GNSS based on their nationality.
The different satellites picked up by the GNSS Statut application.
Satellite detection problems.
1 - Environmental problem.
The first problem that can be encountered with a GNSS system is the detection of satellites.
To properly receive a signal, the GNSS must have a clear horizon. In other words, it is better to be outside than inside a building to pick up the signal (even the interior of a car is annoying).
For what reason would you tell me. Quite simply because the signal sent is weak. It is easily absorbed by walls and never reaches the receiver of your watch or smartphone.
But this problem can still be found when you are outdoors.
For example in the forest or when driving on a heavily wooded road.
Indeed, vegetation is known to absorb waves. Your GNSS receiver will still pick up signals, but less during the spring summer period because of the leaves.
Same problem in areas where the horizon is obstructed, such as urban areas. The surrounding buildings prevent you from receiving all the satellites present on the celestial vault. You are then satisfied with the satellites which are at the zenith. This reduces the number of satellites and therefore the precise calculation of your position.
2 - Mobile support problem.
If you do sports with a mobile application like Cyclope, whether for running or cycling for example, it is important to take into account smartphone support.
Indeed, the use of a shell can have a detrimental effect. It can simply absorb satellite signals preventing the receiver of the GNSS chip from picking up.
We are talking here about a shell with protection on the screen. Most of the time this will not prevent your smartphone from picking up signals, but over time this may change.
I speak from experience, because I had, for my bike mount (see blog on 4 tips for buying a smartphone mount), front and back phone protection. It worked for a long time without any problems. Until the day the GPS signal deteriorated.
The cause was simply the front side of the protection. The GNSS signal went from no satellites with the hull to 18 satellites without the hull. Stunning.
I imagine that the nature of the plastic had changed under the effect of UV and time, blocking the satellite signals. It is then necessary to think rather of a solution without protection of the screen to have a good reception.
3 - Ephemeris problem.
If you're used to constantly having your location enabled on your watch or smartphone, you probably wouldn't have this problem.
On the other hand, if you only activate it when you need it, you risk finding yourself without a satellite for a while.
Why ? Well, because your device won't have the ephemeris.
The ephemeris, what is it?
Satellites don't just transmit information to calculate your position.
Your mobile or your watch needs orbital parameters, clock corrections, data for ionospheric correction, and many other technical details.
This data is transmitted every 30 seconds. And it takes about more than 10 minutes to acquire the whole technical data of the system.
Now you see why it's important, when planning to use your device's location system, to enable it a bit in advance.
Accuracy issues in position calculation.
1- The number of satellites.
We must first understand what we are talking about when we talk about geolocation by satellite.
This involves locating your position by calculating the distance that separates you from the satellite whose signal you are picking up.
To have a first approximation of your position, you need at least 3 satellites. But as these 3 satellites are not perfectly synchronized, a fourth is needed which serves as a clock.
You will now understand that the more satellites you have above your head, the more precise your position will be.
The deviation in the measurement of your position generally does not exceed more than 3 meters.
2 - The echo.
A GNSS signal is a wave.
When you speak, you generate a wave. And if you shout in a deep valley, you will surely have a return of your voice after a lapse of time. In other words, an echo.
Well, for a signal sent by a satellite, it's the same.
If the signal encounters no obstructions between the space and your device, there is no problem.
But if it hits a building or a mountain and is sent back to your device, the signal acquisition time will be incorrect, because the signal path will be longer.
So the calculations will be less exact than they should be. This is especially true in the city with the facades of buildings that cause an echo.
3 - Energy saving mode.
Finally, there is the case where your watch or your smartphone will run out of battery.
This is when your operating system will go into a power saving mode.
It is generally when you approach 20%, or even 15% autonomy that your device will offer you, or automatically switch as the case may be, to an energy saving mode.
If you agree to consume less, the operating system will take a set of measures that will allow your device to consume less energy. And among these measurements, there is the frequency of calculation of your position.
Because acquiring a signal and calculating your geolocation requires energy. Your position will be refreshed less often, and therefore your data will be less accurate than before.
Conclusion
Satellite geolocation is a wonderful technology that has seen the number of applications grow over time.
But the improvement of GNSS could not erase all the imperfections.
It is good that the user remains aware of these failures.
But in the vast majority of the time, geolocation performance is highly accurate.