What is the accuracy of GNSS location data?
There is no unambiguous and precise answer to the question how accurate the location data provided by GNSSs is. Uncertainty is generated by distortions caused by various atmospheric strata and challenges arising from the operating environment, among others. In other words, the accuracy of location data is affected by many different factors, only some of which can be influenced by the user.
GNSS system operators publish performance data for the systems to varying degrees, which makes it difficult to compare them. The performance data provided on the European system is fairly transparent and detailed, whereas information on other systems is somewhat more vague. Indicatively, the basic accuracy of the location data provided by the various GNSSs can be estimated to be:
- Galileo (EU): 1.5 – 2 metres
- GPS (USA): 2 – 4 metres
- GLONASS (Russia) 3 – 5 metres
- BeiDou (China) 1.5 – 10 metres
The above estimates on accuracy can be considered as the baseline at which each GNSS transmits the location data towards the end user. On the way to the end user, the data encounters various factors that add a dose of uncertainty, weakening the accuracy of the location data seen by the end user. In many cases, the accuracy of the location data experienced by the user is significantly lower than the basic accuracy of the system.
Factors causing uncertainty in GNSS location data
GNSS operation is based on time signals transmitted by satellites orbiting the Earth. The GNSS receiver can compute its position close to the ground when it receives time signals from at least four satellites at the same time.
The computing is based on two basic assumptions:
- The positions of satellites orbiting the Earth is known in centimetres.
- Time data between satellites is synchronized with an accuracy of nanoseconds.
Roughly speaking, these factors constitute the basic GNSS performance level at which the system provides location data to the user. The differences in the baseline performance of various GNSSs mainly derive from their ability to synchronise time data between satellites and to compute the position of satellites in orbit. For example, the position of Galileo satellites in Earth orbit is known with an accuracy of approximately 20 cm, and the inter-satellite timing data is synchronised with an accuracy of less than 15 nanoseconds. From these starting points, statistically speaking, Galileo is able to provide positioning information at an accuracy of 1.7 metres, the confidence rate being over 95%, in every corner of the globe.
GNSS satellites transmit radio signals from the altitude of approximately 23,000 km, and they travel to the user's receiver through various layers of the atmosphere.
The receiver examines the time difference between the signals received from the satellites, first computing the time it took for each signal to arrive from the satellite to the receiver. When the signal speed is constant (speed of light), the distance travelled by each signal, i.e. the distance of the satellite from the receiver, can be calculated based on the time of travel. The user's receiver knows the locations of satellites based on the content of the signals and computes its position based on the distances and satellite positions as mentioned above.
The activity of the layers of the atmosphere, especially the stratosphere and ionosphere, causes the radio signal to bend and change its speed, thus extending its time of travel from the satellite to the receiver. This causes inaccuracy in the satellite distances computed by the receiver, which manifests itself as uncertainty in the accuracy of positioning data.
The activity of different layers of the atmosphere varies depending on the time of day and year. Especially in the northern parts of Finland, these variations in ionospheric activity can even be seen in the form of spectacular northern lights.
When sending radio signals from remote orbit, satellites use a relatively low power output, which roughly corresponds to the output of a 100-watt LED lamp. In its simplest form, jamming means covering this weak GNSS signal with noise. This makes it difficult for the receiver to operate: if it cannot separate the signal sent by the satellites from noise, it cannot compute the location data either. Jamming and spoofing of GNSSs is always illegal and extremely dangerous.
Spoofing of GNSSs is more subtle than jamming. Instead of using noise, it mimics the radio signal from satellites deceiving the receiver to think that it is receiving a genuine satellite signal. However, the data content of the signal is changed so that the receiver provides the user with distorted location data.
As a solution for providing reliable location and time data, the EU's Galileo system provides all users with access to satellite signal authentication services OS-NMA and CAS, as well as to the Public Regulated Service (PRS) intended for public authorities.
GNSS receivers operate optimally in an open terrain without any materials obstructing or reflecting the radio signal from satellites.
Elements shadowing the view of the sky, such as tall buildings or dense trees, prevent some radio signals transmitted by satellites from reaching the user's receiver. At worst, this may completely prevent the provision of location data. Even partial obstruction of the view of the sky limits the direction from which the satellite signals can be received, which restricts the satellite geometry, i.e. all signals available to the receiver come from a narrow sector. Limited satellite geometry highlights the intrinsic uncertainties of GNSSs in computing location data.
Hard surfaces in the vicinity of the user, such as rocks or building walls, reflect radio signals from satellites so that the receiver may detect the signal twice or receive a signal that has travelled a longer distance than if it had arrived directly. This causes an error in the computed location seen by the user.
In addition, there may be faulty electrical equipment in the built environment that unintentionally interfere with GNSS signal reception in the same way as equipment used for intentional jamming.
Good-quality GNSS receivers feature a well-designed antenna the position of which in relation to the rest of the equipment has been carefully selected. The receiver electronics are also protected against interference from other equipment.
Poor design or a device optimised for other user needs can easily render the GNSS location data ten times more inaccurate. For example, in a conventional mobile phone, the GNSS antenna is a very small component mounted directly on the circuit board. Therefore, it is susceptible to interference, and users can easily obstruct its view of the sky with their own bodies. In many cases, vehicle navigators are also integrated in between other vehicle electronics. GNSS receivers often need to be used in a built environment that is challenging due to the limited view of the sky and the reflection of satellite signals.
Accuracy of location data experienced by the user
If we return to the initial question and think about the accuracy of the location data provided by GNSSs, it is difficult to give an unambiguous one-digit response. Although the internal uncertainties of the GNSS in itself are reasonably easy to manage, many uncertainties beyond the system operator's control affect the accuracy of the location information experienced by the end user.
The reported performance capabilities of the GNSSs indicate the basic level of accuracy of the service provided that the user could achieve when the uncertainties external to the system are minimized.
The accuracy experienced by the end user consists of the basic accuracy of GNSSs, to which the distortions caused by external uncertainties are added layer by layer. Some uncertainties, such as the quality of the receiver used, the users can influence by their own choices. The opportunities for the end users to influence their operating environment or other external factors are often weak.
Therefore, the answer to the question about the accuracy of location data could be: “the GNSSs produce the necessary initial data for determining location data with an accuracy of 1 – 2 metres, but in city centres the location data computed by the mobile phone may vary between 5 to 30 metres”.
If the end users desire location data of uniform quality, they must also use careful consideration when selecting terminal equipment and planning their use. The GNSS administrator cannot correct the poor choices made by the user. If the operating environment of the end user causes too many uncontrolled uncertainties, other methods can be used to support GNSSs in the provision of location data.
More information: gnss(at)traficom.fi