# A Systematic Review of Perception System and Simulators for Autonomous Vehicles Research
## 3 Position Estimation Systems  
### Global Positioning Systems  
Global Navigation Satellite System (GNSS) is the most widely used technology for vehicle positioning on land, sea, and air. The GNSSs provides the absolute position of a receiver with respect to a fixed reference and consists of a set of satellites orbiting approximately 20,000 km from the earth's surface. These emit signals with information about the satellite, its position, orbital parameters, etc. This system is complete with reception systems, which receive those signals and extract information about position, speed, and exact time. The best-known GNSS system is the Global Positioning System or GPS, developed by the USA in the 1970s, which consists of 24 satellites located in six planes separated by 55\({}^{\circ}\) and with a rotation period of 11 h and 58 min. Its configuration allows any receiver located on the earth's surface to receive signals from between 6 and 12 satellites.  
The operating principal of the GNSS is based on measuring the time of flight of the signal emitted by the satellite and that received by the receptor. The system is able to reliably obtain details of position and time (x, y, z, t) with a minimum of four visible satellites. Table 2 shows the characteristics of the most widely used GNSS systems for global positioning, their constellations, precision, coverage, period of rotation, height, and owner [22; 23].  
Figure 7: Comparison of the features of the different sensors used in environment perception systems.  
Satellite signals are influenced by numerous errors: (1) Synchronization of the atomic clocks of the orbiting satellites, (2) signal transmission by the ionosphere and troposphere, (3) noise in the receivers, (4) multipath effect due to signal reflections, and (5) geometric uncertainties.  
Differential GNSS (DGNSS) was developed to alleviate errors that affect signal measurement from satellite constellations. The DGNSS consists of two GNSS receivers, a ground or base station and a mobile station, known as a Rover. The base station knows its exact position and continuously communicates the signal corrections to the moving station. The signal corrections provide precisions of 0.7 m to 3 m in civil applications and improve the integrity of the measurement (ability of the system to provide timely warnings to users when the system should not be used for navigation). DGNSS needs a system of precisely georeferenced land stations and a communication system, usually UHF radio, with the mobile stations.  
There are other systems based on signal correction that come from satellites, such as the Satellite Based Augmentation System (SBAS) and Real-Time Kinematic (RTK).  
* SBAS was designed to improve air navigation by increasing the horizontal and vertical accuracy of the receptors and providing information about the quality of the signals. For this purpose, it has a set of stations distributed over large geographical areas that monitor the status of the satellite constellations, informing of any anomaly. These systems are operated by different institutions, governments, and even private companies [24].
* RTK systems use the satellite signal carrier to improve the position accuracy of the mobile stations; the base station retransmits the carrier to one or more rover stations and these compare the carriers with the signals received from the satellites and calculate the position accurately. These systems obtain precisions of up to 2 cm and usually use a radio modem to communicate between base stations and rovers within a range of 20 km. RTK has been successfully applied in autonomous driving [25] and precision agriculture [26].