## 5 Discussion  
It is becoming more widely acknowledged that the ionosphere-free PPP technique is the preferred solution for attaining cost-effective high-precision GNSS-based positioning. The advanced GNSS receivers that can track multiple frequencies are the bare minimum requirement for such an approach. Recently, advancements in GNSS technology, particularly the emergence of low-cost dual-frequency receivers such as smartphones and the widespread availability of freely accessible GNSS correction products, have significantly enhanced the feasibility and accessibility of the IF-PPP approach, thereby employing it in a wider range of land use and urban sustainability applications. To further analyze the attained positioning accuracy, the RMSE is computed. The RMSE can be estimated using the following equation:  
\[RMSE=\pm\sqrt{\frac{\sum Error^{2}}{n}} \tag{6}\]  
Figure 9 demonstrates the following:  
* For a GPS-only solution, less than 50 cm RMSE values in horizontal, vertical, and three-dimensional (3D) directions using CNES archived real-time products on DOY 70 and DOY 71. However, a notable degradation in positioning accuracy was observed, with RMSE values exceeding 50 cm in the horizontal and 3D component on DOY 72. The final WUM products demonstrated superior performance for the GPS-only solution, yielding RMSE values of approximately 22 cm, 11 cm, and 24 cm in the horizontal, vertical, and 3D directions, respectively. * The Galileo-only solution exhibits higher RMSE values compared to the real-time GPS-only PPP solutions counterpart for DOY 70 and DOY 71. When employing the CNES real-time products, the Galileo-only solution achieved RMSE values close to 2 m across horizontal and 3D components on DOY 71, as well as vertical and position 3D components on DOY 70. A significant improvement in RMSE values in all components was observed on DOY 72 with 24 cm, 16 cm, and 29 cm in horizontal, vertical, and position 3D components, respectively. Using the final post-processing products, the Galileo-only solution yielded RMSE values of approximately less than 30 cm for all products. A notable degradation in positioning accuracy was observed across all components on DOY 70 using the GRG product. Among the evaluated final products, the GRG solution delivered the best performance on DOY 72, attaining RMSE values of 21 cm, 13 cm, and 29 cm in the horizontal, vertical, and position 3D components, respectively. * The combined GPS/Galileo solution demonstrates more stable accuracy compared to single-constellation solutions. Using the real-time CNES products, it achieved RMSE values of approximately 50 cm for all directions. On DOY 72, the CNES solution achieved superior RMSE values of 13 cm, 15 cm, and 20 cm in horizontal, vertical, and 3D components, respectively. For the final products, the combined GPS/Galileo solution achieved RMSE values less than 50 cm in all directions, with the WUM products demonstrating superior performance, achieving 21 cm, 19 cm, and 28 cm in horizontal, vertical, and 3D components, respectively, on DOY 72. However, on DOY 70, the combined solution achieved RMSE values greater than 50 cm using GRG products in vertical and 3D components. * Generally, ionosphere-free PPP solutions with smartphones can achieve less than 50 cm positioning 3D accuracy. Even with a notable degradation in RMSE values reaching 1 m level in real-time and close to 1 m using post-processing products. The observed degradation in PPP solutions can be attributed to poor signal strength, which results in data losses. These losses subsequently reduce the number of observable satellites and increase the PDOP, thereby adversely affecting positioning accuracy. Although the positioning accuracy using low-cost Xiaomi 11T modules is encouraging, it is subject to several limitations, including the necessity for pre-mission planning because Xiaomi 11T tracks only the L5 signal, which is currently transmitted by only 17 out of 32 GPS satellites. This constraint is particularly impactful for dual-frequency positioning.