WELCOME TO THE SMARTLOC WIKI :)
There are several different GNSS, to start this WIKI it is interesting to understand the history, the stakes and the organisation of the different GNSS:
PART.1 : GNSS ARCHITECTURE
There are several different GNSS, but GNSS is often confused with GPS (the American GNSS), which is the best known GNSS today. However, there are also GLONASS, Galileo, Beidou. In GeolocPVT, we use Galileo, GPS and Beidou.
Regional satellite positioning systems
IRNSS and QZSS are regional satellite positioning systems.
In a second step, let's look at how an individual's overall positioning works:
PART.2 : HOW IT WORKS
Few things we need before the calculation process:
First of all, a ground segment is needed to provide additional information and services that are not available from the space segment alone. Overall, the ground segment plays a critical role in ensuring the overall performance and functionality of the GNSS system.
Then, the user segment plays a critical role in enabling GNSS applications and services, it is the end-user's device that makes the position, velocity and time information available to the user.
Moreover, the space segment is the backbone of the GNSS system, it provides the signals that are used to determine the user's position, velocity, and time, and it also provides the necessary information to correct errors in the user's receiver clock.
Eventually, we need 4 satellites to locate:
First, we need to understand how de we use Doppler effect in geolocation.
For the PVT calculation in static mode the SPP (Single Point Positioning) method is used. This is the most basic positioning technique, using a trilateration method with observations from a single receiver.
Here we will discuss the PVT calculation in dynamic mode with GPS/GALILEO/BEIDOU. This calculation is more complicated than its static counterpart because the element to be geolocated is in motion.
The two most commonly used methods of calculation are
Now that we have the overall operation of the localization process, let's look at how the signal is received and how the signal is processed:
PART.3 : SIGNAL PROCESSING
First, the receiver receives the signal from the satellite.
After the acquisition step which aims to detect the presence of a satellite and to estimate the initial delay and Doppler frequency of the signal at reception, the receiver switches to tracking mode.
Once the tracking has been carried out, monitoring will make it possible to determine the quality of the measurements.
However, as you have seen there are some inaccuracies that lead to errors, so let's look at what those errors are:
PART.4 : ERRORS
When processing the signal received from satellites, there are so-called "natural" factors that limit the accuracy of GNSS. We can mention:
- refraction in the ionosphere
- refraction in the troposphere
- positioning accuracy of GNSS satellites
- multipath phenomena
The ionosphere extends from 60 km altitude to 1000 km altitude. The carrier wave of the GNSS signal must penetrate this layer on its way. The fact that this layer is not neutral in terms of its charge causes a disturbance in the speed of the propagating electromagnetic wave. The time taken by the GNSS wave is modified by an unknown amount of time, called the ionospheric delay. The evaluation of the distance between the satellite and the station will be distorted. Similarly, the propagation time of the GNSS wave is affected by the water vapour content of the lower layer of the atmosphere (0 to 10 km altitude): the troposphere.
The position of the satellites is calculated on the ground and is made public by the constellation control centres several times a day. This information is called the ephemeris. They will be used for position calculations. But it is obvious that if there is an error in the position of the transmitting satellite, this error will be reflected directly in the position displayed by the receiver.
This phenomenon is problematic because it is difficult to model and eliminate. Any reflective object placed in the vicinity of the GPS station's antenna can reflect part of the signal from the satellite back to the antenna. The signal will therefore be deteriorated, more difficult to interpret and therefore a source of large errors. Moreover, this kind of phenomenon is very frequent in the life of any user, especially in the city where the path that the signal makes is rarely clear.
The satellites of the GNSS system are equipped with several atomic clocks; however, their high accuracy has a limit. The corrections for the offset of the clocks are transmitted in the "navigation message". These corrections are calculated by reference ground stations and are used to compensate for the continuous deviation of the clocks. The uncertainty in the receiver clock, which is much less accurate since it is not an atomic clock, is considered an additional unknown, hence the need for at least four satellites.
You now have a global overview of how a person's location works, and why some locations are inaccurate. So here are some videos that may answer some of your questions
PART 5 : OUTREACH VIDEOS
- GPS Satellites are the only one used to locate people?
- When my geolocation is on, satellites can track me?
- It's easier to locate people in urban areas?
If you want to think about this further, here is some information that may be useful
PART 6 : TO GO FURTHER...
The Allan variance is a statistical method used to measure the stability of a frequency or time standard over time. It is a measure of the frequency stability of a clock or oscillator, and is commonly used in the field of time and frequency metrology.
A coordinate system for GNSS is a reference framework used to define the position and orientation of objects in space.
A decoy is a false signal that is designed to mimic the signals emitted by real GNSS satellites. It is used as a countermeasure against GNSS jamming and spoofing attacks, where an attacker transmits false signals to disrupt or manipulate the receiver's navigation solutions.
Dilution of Precision (DOP) is a measure of the geometric quality of the satellite configuration in the sky, as seen from a receiver. It is used to determine how well a set of satellites are positioned to provide an accurate navigation solution.
An ephemeris is a set of data that provides information on the precise location and velocity of a satellite at a specific point in time. It is used by a GNSS receiver to calculate the position and velocity of the receiver by determining the distance between the receiver and the satellites
Inertial Navigation is a method of navigation that uses accelerometers and gyroscopes to track the motion of an object and determine its position and velocity. It is commonly used in applications where GNQSS signals are not available or unreliable, such as in deep space or underwater navigation.
Jamming refers to the deliberate transmission of radio frequency signals in the same frequency band as the GNSS signals, with the intent of disrupting or overpowering the legitimate GNSS signals.
Map matching in the context of GNSS is the process of comparing the estimated position of a vehicle or device with a digital map to determine the most likely path of travel.
Pedestrian navigation in the context of GNSS is a type of navigation that allows users to navigate through urban environments while walking. It is designed to be used indoors and outdoors, and to provide a more accurate navigation experience for users on foot.
Orbits of satellites in GNSS are the paths that the satellites follow as they revolve around the Earth.
In the context of GNSS, time reference refers to the standard time used by the satellites and the receivers to synchronize their clocks and determine the time of arrival of the signals.