Satellites are the cornerstones of modern positioning and navigation. They are used by vehicle navigation systems, smartphones and land surveyors alike. On the basis of satellite signals, the receiver can define its position anywhere in the world with an accuracy of a few metres in less than a minute. In addition, the time can be defined as a by-product with an accuracy of approximately one hundred nanoseconds.
Using assistance systems, the position can be pinpointed with an accuracy of a few centimetres. Previously, the US-based GPS was considered to be synonymous to satellite positioning. Currently, the Russian GLONASS, as well as the European Galileo and the Chinese BeiDou which are at the deployment stage, are openly accessible to all. For this reason, satellite positioning is currently referred to as the Global Navigation Satellite System (GNSS).
Satellite positioning is not possible without time
Satellite positioning is ultimately based on the accurate transfer of time. Each of the four satellite positioning systems consists of some 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.
The difference between the signal transmission time and the reception time indicated by the internal clock of the receiver can be converted into a distance by multiplying it with the radio signal propagation speed, i.e. the speed of light. All the clocks of satellites belonging to a single system have been mutually synchronised. However, receivers typically include more affordable quartz clocks, and the reception time they indicate is not directly comparable to the transmission time.
In addition to three-dimensional position coordinates, the fourth factor to be solved is the difference between the receiver clock and the satellite clocks. This can be done by using at least four satellites, the positions of which are known – the use of several satellites improves the accuracy and reliability of positioning.
Because positioning is linked to time, timekeepers also make up a significant group of GNSS users. Using GNSS signals, it is possible to synchronise devices and clocks that are located far from each other. For example, electric power plants need to produce alternating current synchronously, and mobile base stations must be synchronised to prevent calls from being disconnected when switching from one base station to the next.
GNSS receivers do not automatically reveal their location
To use GNSS positioning, signals only need to be received. This means that users do not automatically reveal their location to the system administrator or anyone else. This is a particularly important feature considering the original user of satellite positioning, i.e. the military.
However, many devices that use satellite positioning, such as smartphones, tracking devices and autonomous vehicles can transmit their location via other channels, for example, to a cloud service or the device manufacturer for their purpose of use, to use performance-enhancing services or for crowdsourcing.
Many factors cause inaccuracy
Satellite measurements used to calculate positions are based on a chain of various factors. First, the system administrator needs to predict satellite orbits and any differences between satellite clocks and to send this data to the satellites from Earth. On the basis of this data, the satellites generate and transmit a signal which passes to Earth through the atmosphere. In the worst case, the signal is attenuated or reflected by any trees or buildings located close to the receiver.
The position of the receiver needs to be calculated by using inaccurate measurements: the estimated satellite orbits and differences in time are not perfect, the ionosphere and troposphere distort the signal’s path, and any dampening and reflections make it more difficult to track the signal.
Due to these factors, the typical accuracy of a GNSS receiver designed for consumers is approximately five metres, but it can also be worse in certain conditions. In the worst case, the position calculated by the receiver can differ from the actual position by hundreds of metres or even more due to a satellite malfunction or an unusually high environmental or atmospheric interference.
Another significant challenge in GNSS positioning is the low power of satellite signals. Signals are transmitted from a distance of approximately 20,000 kilometres with a power of a few dozen watts, due to which they are weaker than background noise when received on the Earth’s surface. The structure of these signals has been designed so that they can be tracked below the background noise level in normal conditions. However, any external factors, such as atmospheric activity or any interfering radio transmissions, may change this situation.
Malicious jamming is common when satellite positioning is used for military purposes. Unfortunately, jamming devices are also sold online to regular consumers with the purpose of protecting privacy. However, the use of jamming devices is illegal.
A localisation error is a safety risk
An accuracy of roughly five metres is fine for various geospatial data applications, such as vehicle navigation systems, emergency call positioning, geotagging of photographs, and dog tracking collars. However, many professional applications require a higher accuracy or better integrity with respect to measurement errors.
For example, an accuracy of a few metres based on satellite positioning is sufficient for landing aircraft, but any factors leading to significant measurement errors must be excluded: a localisation error of a hundred metres would be catastrophic. In land surveying and construction, the accuracy of geospatial data needs to be on a scale of a few centimetres.
In order to respond to these strict accuracy and reliability requirements, various augmentation systems have been developed for satellite positioning systems. These are based on a group of reference stations, the locations of which are known in great detail. Reference stations continuously 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 consists of dozens of reference stations to produce corrections that improve the positioning accuracy to 50 centimetres.
In addition to FinnRef, a group of commercial services are available. These have been developed to improve the accuracy to a few centimetres. At best, GNSS measurements can even be used to track the movements of tectonic plates.
SBAS satellites monitor the operation of GPS satellites
To ensure the level of reliability required, in particular, for air traffic, various satellite-based augmentation systems (SBAS) are used. They transmit augmentation information by using similar signals as positioning satellites. SBAS satellites are in geostationary orbits, similarly to communications satellites. The data they transmit is updated in real time at earth stations, which is why SBAS satellites are able, for example, to give warnings of damaged GPS satellites in just a few seconds.
Instead, GNSS satellites are not in constant contact with earth stations, and their response time can be as much as hours in the case of a malfunction. Unlike GNSS systems, SBAS systems do not cover the whole planet. The European SBAS system is entitled EGNOS, and the North American system is called WAAS.
First used by the military
Satellite positioning systems can be regarded as critical parts of the infrastructure. This is why four different parties want to maintain their own parallel systems. GPS is primarily intended for the military and it is maintained by the US Department of Defense. Some of its functions are only available to the military.
In fact, until the early 2000s, GPS intentionally reduced the accuracy of its signals intended for civilian users. However, these reductions are no longer in use. For a long time, GPS only offered a single signal at a single frequency to civilian users, even though devices designed for professional use have been able to track military signals, regardless of their encryption.
The most recent GPS satellites offer open signals to all users at three frequencies. Still, it will take years before all older satellites have been replaced by new ones: the service life of a single satellite is more than ten years. As multi-frequency receivers can correct any measurement errors caused by the ionosphere, the deployment of new frequencies will improve the accuracy of consumer devices.
The GLONASS system was developed as the Soviet version of GPS. As a result of the dissolution of the Soviet Union, the GLONASS system was no longer maintained and became practically unusable. During the 2000s, Russia, however, restored it back to operating condition.
GLONASS differs technically from the other systems in that each GLONASS satellite has a dedicated transmission frequency. In the other systems, all satellites transmit signals at a single frequency, and different satellites are identified on the basis of a code embedded in the signal. However, the use of satellite-specific transmission frequencies presents challenges, above all, in professional receivers, due to which next GLONASS satellite generations will also use code-divided signals in addition to frequency-based satellite identification.
Galileo is the first GNSS maintained by civilians
Unlike the older systems developed for military purposes, the European Galileo system is completely maintained by civilians. It offers openly accessible signals at two frequencies, in addition to which a third frequency is available for the authorities and additional services.
The Galileo system was designed from the first to be compatible with GPS. This is why both of these systems use the same key signal frequencies. In addition to offering navigation services, Galileo supplements the international COSPAS–SARSAT system, which transmits emergency reports to marine rescue organisations.
The BeiDou system is designed to cover the Chinese territory, in particular, which is also indicated by its satellite orbits. While the other systems use satellites orbiting at an altitude of roughly 20,000 kilometres, BeiDou also includes geosynchronous and geostationary satellites (at an altitude of approximately 35,000 kilometres) that remain over China’s longitudes. BeiDou was formerly known as Compass.
These four global GNSS systems are supplemented by a group of local systems, such as the Japanese QZSS and the Indian Navic.
- Global Navigation Satellite System (GNSS). A global satellite positioning system.
- Global Positioning System (GPS). A GNSS system maintained by the US Department of Defense.
- Geosynchronous satellite. A satellite with an orbital period of 24 hours, i.e. a single satellite can be seen in the same place at the same time, every day, when viewed from the Earth’s surface. The orbit’s altitude is approximately 35,000 kilometres.
- Geostationary orbit. A geosynchronous orbit following the Equator. A geostationary satellite seems to be stationary when viewed from the Earth’s surface.
- Medium Earth orbit (MEO). An orbit with a lower altitude than a geosynchronous orbit. Most positioning satellites use an MEO at an altitude of approximately 20,000 kilometres with an orbital period of 12 hours.
- Satellite-based augmentation system (SBAS). An augmentation system which does not produce any navigation signal but provides information about the reliability of GNSS signals.
- European Space Agency (ESA). ESA is responsible for the technical development of the Galileo system, together with the European Commission.
- European GNSS Agency (GSA). GSA is responsible for the services offered by the Galileo and EGNOS systems.
- European Geostationary Navigation Overlay Service (EGNOS). Europe’s SBAS.
- COSPAS–SARSAT. A satellite system that receives emergency signals transmitted from the Earth, locates their sender and transmits data to rescue authorities. The 406 MHz frequency is only reserved for this system.
- Ionosphere. The ionised part of the atmosphere, containing free electrons. The ionosphere distorts radio signals transmitted from satellites. The amount of this distortion depends on the signal transmission frequency and the number of free electrons that affect the signal. The ionosphere is the most significant source of interference for GNSS measurements.
- Troposphere. The layer of the atmosphere where all weather conditions take place. The troposphere slows the progress of radio signals transmitted from satellites. The magnitude of the delay depends on the weather, but not on the signal frequency.
- FinnRef. A network of GNSS measurement stations owned by the Government of Finland. The network was established for maintaining the national coordinate system. It also offers an openly accessible correction service, the use of which improves the accuracy of GNSS positioning.