1. How does satellite positioning work?

Satellites are the cornerstones of modern positioning and navigation. They are used by vehicle navigation systems, smartphones and aircraft alike. On the basis of satellite signals, the receiver such as a smartphone can determine its position anywhere in the world with an accuracy of a few metres in less than a minute. Using augmentation systems such as the mobile network, the position can be pinpointed with an accuracy of a few centimetres.

There are four global satellite positioning systems: the US GPS system, the Russian GLONASS system, the European Galileo system which is in the deployment phase, and the Chinese BeiDou system. For this reason, satellite positioning is currently referred to as the Global Navigation Satellite System (GNSS).

Satellite positioning is based on measuring distances between satellites and the receiver. Because the signals transmitted by satellites travel at the speed of light, even the smallest errors in measured distances can have a significant impact on the accuracy of positioning. Each of the four satellite positioning systems consists of 20 to 30 satellites, orbiting at an altitude of approximately 20,000 kilometres. All of these satellites are equipped with a precise atomic clock, on the basis of which they transmit a time signal down to Earth, as well as other data that, for example, indicates the satellite position. To determine the position, the receiver requires a connection to at least four satellites.

2. What causes interference in positioning?

Interference in satellite signals can be caused not only by natural reasons such as disturbance in the ionosphere or a malfunction in the receiver but also by intentional interference. Obstacles in the environment, including buildings and forests, also disrupt the signal. Due to long distances, satellite signals are already weak when reaching Earth’s surface. This means that disruptions are not unusual.

Intentional interference comes in two ways: jamming and spoofing.

In jamming, disruptive signals are transmitted in the radio frequency range used by a satellite positioning system to weaken or block the original positioning signals. Jamming is usually carried out using a simple device called a jammer. The use of jammers is illegal throughout the EU, and a political initiative has also been made in Finland to prohibit their possession. Jammers are used by professional drivers who seek to conceal their location. Unwanted consequences include large-scale interference in satellite signals.

Spoofing is another form of intentional interference, in which the information carried by signals is falsified. False signals can be used to divert receivers so that their calculated position or time is inaccurate. As spoofing requires specific skills and devices, only professionals are basically capable of it. A recent example of GNSS spoofing was the counterfeiting of cargo vessel locations to evade financial sanctions.
 

3. How can it be ensured that critical systems remain operational?

The reliability of satellite positioning is verified in various ways. Functions critical for security in society are always protected by backup systems that enable their effectiveness even if satellite positioning was inaccurate. For example, air traffic does not rely on GNSS signals alone, as navigation is secured by several augmentation systems. A situation where a plane cannot land due to interference in positioning signals is unusual.

Accuracy is also verified in many ways. While the positioning accuracy of five metres is sufficient for many geospatial data applications, reliable and accurate positioning data is required by air traffic and rescue services, among others.

The accuracy and reliability of satellite positioning is improved by various augmentation services. These are based on a group of reference stations, the locations of which are known in great detail. Reference stations monitor the quality of GNSS signals and produce real-time data needed by different users. For example, the Finnish FinnRef network maintained by the National Land Survey of Finland (NLS) consists of dozens of reference stations to produce corrections that improve the positioning accuracy to within a few centimetres.

To ensure the level of reliability required for air traffic in particular, various satellite-based augmentation systems (SBAS) are used. For example, they can give a warning in a few seconds if there is a malfunctioning GPS satellite.

4. What impact does interference in positioning have on regular people?

Interference in satellite positioning systems can be seen as errors in positioning or occasional interruptions that have an impact on smartwatches, car navigators or GPS dog collars. For example, the hit game Pokemon Go is susceptible to interference because it is largely based on the smartphone position.

However, many modern consumer devices use several positioning systems, including mobile networks, alongside satellite positioning. As a result, interference rarely disables devices completely. Various natural factors interfere with positioning signals, including tall buildings in urban environments.

The extent of inaccuracies depends partly on the intensity of interference and partly also on the properties of the positioning device. For example, the high-accuracy receivers used by the NLS stop producing positioning data much more quickly than devices that are only intended to make a rough estimate of the location. In the latter case, the reliability of positioning data provided by the device decreases, and the location may “wander” more than usual. This means that the shown location may move around the actual location. The location shown may also be completely wrong.

5. How can research improve the reliability of positioning?

The research conducted by the Finnish Geospatial Research Institute (FGI) helps develop solutions to monitor, detect, classify and prevent GNSS interference. The goal is that users of satellite positioning can use the service continuously without any interruptions.

GNSS-Finland, a quality monitoring service for satellite signals, is a significant product of this research, providing information about the quality of satellite signals in Finland. It is a national satellite signal quality monitoring service developed by FGI, which is always operational and openly accessible by all. The service is based on research assigned and financed by the Finnish Transport and Communications Agency (Traficom), which coordinates the use of radio signals in Finland. The goal is to use research results and the service provided broadly in society.

Further information

Sanna Kaasalainen, Professor, Director of Navigation and Positioning, +358 29 531 4760

Mika Saajasto, Senior Research Scientist, +358 29 531 4314, firstname.lastname@nls.fi