# Position Accuracy Improvement by Implementing the DGNSS-CP Algorithm in Smartphones
## 4 Experiments and Results  
### Preliminary Test  
To verify our assumption about the multi-GNSS positioning algorithm of the location chipset in the target smartphones, we conducted a preliminary test to shift the coordinates of the devices considering the conceptual position-domain DGNSS. In a zero baseline static test, the RS and a user device installed at the same location should observe the same satellites, and can theoretically cancel out all errors other than the thermal noise. Thus, this test is very useful for verifying the validity of the algorithm. Instead of a signal splitter, which is generally used for the zero-baseline static test, a GNSS repeater should be used, because smartphones cannot be connected to external antennas. We placed one Novatel FlexPak 6 receiver (as the RS) and one Samsung Galaxy S5 smartphone (as the DGPS user) under the GNSS signal repeater to construct the zero-baseline static test, as shown in Figure 5. The GNSS repeater was connected to the Trimble Zephyr Geodetic II antenna on the roof of the Chungmu building at Sejong University. Thus, both devices simultaneously received the same GNSS signal. This preliminary test was conducted from 16:00 to 18:00.  
Figure 6 shows the Position-domain DGNSS results given by the smartphone. The correction term was generated by differentiating the real-time stand-alone position of the RS from the precisely surveyed coordinates. The mask angle was assigned a value of 0\({}^{\circ}\), and no atmospheric model was applied to the positioning options of the Flexpak receiver.  
Figure 5: Zero-baseline static test setup.  
Even though the Position-domain DGNSS was applied to the smartphone, the RMS of the stand-alone error (horizontal 1.3 m, vertical 3.1 m) increased to 3.5 m and 24.7 m, and the maximum error reached 37 m. The RS real-time coordinates without an atmospheric model cannot improve the position accuracy of the smartphone, but rather degrades its performance, which gives reasonable proof of our assumption that the smartphone calculates its position after compensating for the pseudorange error using Equation (1).  
Based on this assumption, we generated a new type of Position-domain DGNSS correction. Using the SETIONOTYPE and SETTROPOMODEL commands [30], we applied the Klobuchar Model and a troposphere model to the SPP. The new position-domain correction was generated by subtracting the model-applied real-time position from the surveyed coordinates. This is different from the generally known methods.  
The results in Figure 7 indicate that the errors were smaller than those in Figure 6, which means that compensated correction by the atmospheric model is more effective than the traditional method. Despite this advantage, the disagreement between the RS and the user regarding the visible satellites prevents further improvement in the stand-alone position result. This emphasizes the need to select the PRC set of visible satellites at the user-side, and that DGNSS-CP is the only method for correctly projecting the measurement-domain corrections onto the position-domain.  
There were generally fewer satellites visible to the smartphone than to the RS, which confirmed our assumption that the smartphone uses a different satellite selection algorithm from that of the commercial receiver. For example, at the GPSTime of 458,095 s, GPS satellites G4 and Beidou satellite B2, B5, B6 were not used by the smartphone although they were observed at the RS. This disagreement  
Figure 6: Zero-baseline Position-domain DGNSS results ((a) Horizontal; (b) Vertical).  
Figure 7: Zero-baseline Position-domain DGNSS results with the atmospheric model ((a) Horizontal; (b) Vertical).  
increased the stand-alone error by 3.1 m, whereas the errors were bounded in the range 1-2 m when the two devices observed the measurements from the same set of satellites.