# Analysis of the Impact of Multipath on Galileo System Measurements
## 1 Introduction  
The multipath effect is one of the major source of errors in precise Global Navigation Satellite System (GNSS) positioning. This effect occurs when the receiver registers not only direct signals from the satellite but also indirect signals, reflected from the ground or other terrain obstacles. The properties of such signals are significantly different compared to direct signals. Indirect signals always cover a longer distance, reaching the receiver with a certain delay, and are in a different phase in relation to the direct signal. As a result of reflection, the amplitude of the wave (and possibly its polarization and frequency as well) also changes [1]. Since the receiver cannot distinguish between direct and reflected signals, the two waves interfere and the form of the direct signal is distorted. This results in code and phase observation errors as well as distortions of the Signal-to-Noise Ratio (SNR) parameter [2, 3]. The maximum multipath impact on the carrier wave phase accuracy is defined as a quarter of the wavelength [4]. This corresponds to approximately 5 cm for the frequencies used in GNSS systems. On the other hand, the impact of this effect on the measurement of the code pseudorange can be much greater. Under exceptionally unfavorable conditions, it can be as high as 100 m [5]. In practice, however, the use of precise measurement equipment minimizes such errors. The multipath impact on double-differenced (DD) carrier-phase observations has a value of up to 0.1 of the phase cycle, i.e., approximately 2 cm [6]. The value of the code multipath error is approximately 2 m [7]. Importantly, the accuracyof precise positioning does not depend solely on the accuracy of the phase measurements. It is the code observations that have a significant impact on the results of the float solution. The value of the multipath error depends on the nearest antenna environment. Due to the extremely local nature of this phenomenon, its impact cannot be eliminated by differential observations or by universal models describing the magnitude of this error. For this reason, the multipath effect is one of the most crucial factors among all errors in GNSS measurements, and the determination of its impact is a difficult and challenging task. The methods of elimination and identification of the multipath impact can be divided into three groups [5]: (a) elimination of the multipath effect as part of data postprocessing, (b) hardware methods at the stage of signal measurement, and (c) the appropriate selection of the place of measurement. An obvious way to reduce the multipath is to select a suitable location to take the measurements, away from terrain obstacles. In addition, placing the antenna directly on the ground can limit the possibility of the reception of waves reflected from the surface. This solution obviously has its limitations and can rarely be used in practice. Another option may be to use a horizontal elevation mask, adjusted to the actual environment of the antenna [8]. This allows for a partial reduction in the reception of signals that is highly likely to be affected by the multipath effect. [PERSON] et al. [9] proposed an elaboration of this method, using assumptions related to the radio wave propagation area and the Fresnel zone. This approach results in better determination of the wave ambiguity. In the examined case, the ambiguity success rate increased from 95.5% to 99.9%. Hardware methods include those related to the construction of the antenna and the receiver. GNSS antenna designs are based on signal polarization, allowing for wave attenuation with left-handed polarization (which is the typical polarization of reflected waves). In turn, the use of choke-ring antennas or antennas with increasing electrical resistance allows for additional protection against receiving surface waves of the signal reflected from the ground [10; 11]. The multipath effect can be significantly reduced at the stage of signal registration by the receiver.