# Analysis of the Impact of Multipath on Galileo System Measurements
## 1 Introduction
A carefully selected signal correlation technique or distance between correlators significantly reduces the impact of signal distortions related to the multipath effect. An overview of such signal correlation methods, commonly used by measurement equipment manufacturers, was presented by [PERSON] [12]. The broadest group of methods for determining the multipath effect is methods related to data postprocessing. An important feature used in multipath analyses is the correlation of this phenomenon with the values of the SNR parameter. This correlation results from the fact that the presence of secondary waves in the registered signal affects the change in its amplitude. This correlation is used for modelling the SNR parameter in order to identify the main source of signal reflections. The resulting adjustments to the phase observations allow for a reduction in residual errors of the DD carrier-phase observations by 25% [13]. [PERSON] and [PERSON] [14] presented a method to analyze changes in the SNR parameter, allowing for a reduction of the value of this error by up to 47%. In both papers, the multipath impact on the DD carrier-phase observations was determined at the level of approximately 1 cm. In order to isolate the multipath impact on the accuracy of code observations from other measurement errors, the Code-Minus-Carrier (CMC) combination of code and phase observations was used [15]. On the other hand, an analysis of a feature related to the repetitiveness of the multipath characteristics with repetition of the satellite constellation, every sidereal day, was performed [16]. The differences in Global Positioning System (GPS) observations between two consecutive days allows for obtaining a series of code observations with an error due to the multipath effect at the level of 0.06-0.08 m, instead of approximately 0.5 m for undifferenced observations. Due to the close relationship between the multipath effect and the position of the satellites, the phenomenon is characterized by variability resulting from the movement of the satellites. The typical multipath period for GPS, under typical conditions, is approximately 15 min [4]. In turn, [PERSON] et al. [17], quoting [PERSON] and [PERSON] [18], pointed to the different multipath periods depending on the type of signal reflections. Long-period phenomena, with frequencies of 1-3 mHz, correspond to specular reflections of the signal, while short-period phenomena (5-20 mHz) are characterized by diffuse reflections. The authors used these features to determine the impact of both types of reflections on the accuracy of the code observations. A time-frequency analysis of the signal was conducted by the authors using the wavelet transform. In the examined case, the standard deviation (STD) of the multipath error for code observations due to specular reflections was 0.10 m, while for diffuse reflections, this value was 0.05 m. Wavelet analysis allows for the separation of components with frequencies typical of the multipath effect and measurement noise with a much higher frequency, from DD phase observations [6]. In turn, [PERSON] and [PERSON] [19] used Short Time Fourier Transform (STFT) in time-frequency analyses of time series of a CMC combination. Another method that allows for the elimination of the multipath is the analysis of the SNR parameter for signals at several different carrier wave frequencies. This method takes advantage of the fact that the relative phase of the secondary signal varies depending on the frequency of the measured signal [20]. The multipath effect can also be determined using a set of antennas located in close proximity (multi-antenna). Such an approach allows for modelling the reflector position and introducing corrections to the observations. The adjusted code observations have their error reduced by 65%, while the error of the determined coordinates is reduced by 55% [7].