# Feasibility of Using Low-Cost Dual-Frequency GNSS Receivers for Land Surveying
## 3 Results and Discussion  
### Carrier-to-Noise Ratio Analysis  
A strong dependence of the C/N\({}_{0}\) on satellite elevation was noticed (Figure 2). For the L1 frequency, the highest C/N\({}_{0}\) ratios (26 to 53 dB Hz) were recorded for elevation angles between 35 and 70\({}^{\circ}\). For higher elevation angles, C/N\({}_{0}\) were higher than 26 dB Hz but remained lower than 50 dB Hz. Below 20\({}^{\circ}\), a significant amount of measurements  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Relative Positioning** & **Absolute Positioning** \\ \hline Software/service & RTKLib & CSRS-PPP \\ GNSS & GPS + GLONASS + Galileo & GPS + GLONASS \\ Satellite orbits and clocks & MGEX Final (COM) & MGEX Final (COM) \\ Technique & double-differencing & PPP \\ Frequencies & L1 + L2/L5 & Ionosphere-free from L1 + L2 \\ Measurement frequency & 1 Hz & 1 Hz \\ Elevation mask & 10\({}^{\circ}\) & 7.5\({}^{\circ}\) \\ Satellite PCO/PCV & sig14.atx & igs14.atx \\ Antenna PCO/PCV & none & none \\ Troposphere delay & Saastamoinen & estimated \\ Ambiguities & fixed & float \\ \hline \hline \end{tabular}
\end{table}
Table 1: Processing strategies for the relative and absolute positioning.  
with C/N\({}_{0}\) lower than 30 dB Hz was noticed. For the second frequency (L2/E5b/B2I), a nearly linear increase of the average C/N\({}_{0}\) was recorded with the increasing elevation angle. Multiple C/N\({}_{0}\) ratios lower than 30 dB Hz were noticed for elevation angles lower than 20\({}^{\circ}\). By assuming that typical C/N\({}_{0}\) ratios vary from 35 to 55 dB Hz with a minimum of around 28 dB Hz [29], a u-blox receiver combined with a low-cost antenna performed well, except for the low-elevation satellites.  
Compared to a geodetic grade antenna and receiver (Figure 3), C/N\({}_{0}\) ratios for a u-blox receiver resulted on average lower by 7.2 and 7.0 dB Hz for the first and second frequency, respectively. For the L1 frequency, 98.9% of measurements showed a lower C/N\({}_{0}\) ratio by using the low-cost receiver, rather than the geodetic-grade receiver. For the second frequency, this percentage dropped to 93.7%. Although for most cases the C/N\({}_{0}\) ratios were lower for the u-blox receiver, exceptional cases of stronger acquisition as well as the acquisition of signals weaker than 28 dB Hz, reflecting the implementation of a modern detection approach, which enhanced the acquisition of weak signals.  
Figure 3: Carrier-to-noise ratio (C/N\({}_{0}\)) difference (u-blox minus a geodetic grade receiver at WROC station) for (**a**) 1 st and (**b**) 2 nd frequency as a function of satellite elevation angle: the dots indicate individual measurements, while lines represent elevation mean. 22 September 2020 (24 h).  
Figure 2: Carrier-to-noise ratio (C/N\({}_{0}\)) for (**a**) 1 st and (**b**) 2 nd frequency as a function of satellite elevation angle for the u-blox receiver. The dots indicate individual measurements, while the lines represent the elevation mean. 22 September 2020 (24 h). G, R, E, C stand for GPS, GLONASS, Galileo, and BeiDou, respectively.  
The C/N\({}_{0}\) differences between the two frequencies for the same satellite (Figure 4) corresponded to the difference in signal transmission energy, i.e., for GPS the L1 signal resulted from 3 to 7.5 dB higher than the L2 signal, for GLONASS the L1 signal was 6 dB higher than L2, for Galileo the E1 signal was 2 dB lower than E5b, for BeiDou the B1 signal was 4 dB higher than B2.