# Study of Global Navigation Satellite System Receivers' Accuracy for Unmanned Vehicles
## 1 Introduction  
The contemporary world needs more and more precise navigation. The main functions of Global Navigation Satellite Systems (GNSS) are to provide the moving object geographic coordinates, the parameters of the speed vector and accurate time. The accuracy of these parameters depends on lots of other parameters, such as the number of satellite signals available, system integrity, satellite positions, etc. One of the possible ways to improve position accuracy is multi-GNSS systems, which have become more common owing to an increase in the number of satellite systems [1, 2, 3, 4] as the spatial geometric distribution of GPS satellites has been significantly improved after the addition of BDS satellites [5]. Two GNSS services are offered with different accuracy and levels of access: standard accuracy service and precise service. These services are described in the Standard Positioning Service Performance Standard (SPS) [6], and the Precise Point Positioning Standard (PPP) [7]. In addition, the position accuracy of both services, considering a given probability and time accuracy, is provided with specific values on the European Space Agency website. The standard accuracy position and time service are provided by the C/A (coarse/ acquisition) ranging and acquisition code and the navigation message, which modulate the L1 carrier frequency and are intended for civilian users. The standard accuracy for position and time is determined based on a probability of 95% and only up to the signal-in-space (SIS) level. The accuracy improvement requires methods based on additional technical means [8; 9]. In some applications, such as unmanned aerial vehicles [10], transport infrastructure measurements [11], unmanned ground vehicles [12], navigation systems for measurements [13], etc., the accuracy requirements are higher since sub-meter or centimeter-level precision is critical. Through the C/A code, a standard service (SPS--Standard Positioning Service) is offered with an accuracy of 100 m horizontally, 156 m vertically and with a maximum inaccuracy of the receiver clock of 340 ns. The speed measurement error is 0.3 m/s. The data are at the 95% probability level at the navigation receiver input. The precision service is delivered by the P (Precise) code and the navigation message, which modulate the L1 and L2 carrier frequencies. Due to the fact that the code is spread over two frequencies, this makes it more robust than the C/A code. Also, ionosphere errors are overcome with both frequencies. The code itself is longer and this contributes to higher accuracy. The P code is a very long (267 days) 10.23 MHz PRN (Pseudo-Random Noise) code, with each satellite transmitting a weekly portion of the 267-day PRN sequence, restarting the code sequence at the end of each week. This code was later replaced by the cryptographically more secure Y code, which is a sum modulo two between the P code and an encryption W code, which is clocked at 0.511 MHz. The P code, as well as its Y variant, are intended for military purposes and provide the PPS (Precise Positioning Service), namely, 22 m horizontally and 27.7 m vertically. The receiver clock does not deviate more than 20 ns from the system time. Again, data are given at the 95% probability level at the navigation receiver input. Since 2018, the new satellites also broadcast a new civil code, L1C, which should ensure the interoperability of the system with other global navigation systems. To avoid interference with other signals located on the same channel, Multiplexed Binary Offset Carrier (MBOC) modulation is applied to the L1C. L5, which is another civilian signal whose function is to provide safe transportation and other high-performance applications. It is broadcast in a radio frequency band (1176 MHz) which is intended for services related to the safety of air transport. In combination with the L1C/A signal, it enables the elimination of ionospheric errors and higher reliability in determining the position.