# Position Accuracy Improvement by Implementing the DGNSS-CP Algorithm in Smartphones
## 2 Previous Work and Contributions
### Motivations and Contributions  
DGNSS is a relative positioning technique that involves two receivers, namely a reference station (RS) and a user receiver. For real-time operations, the correction, which is generally broadcast via radio transmission or mobile communication [13], can be applied in two ways: \"Position-domain DGNSS\" or \"Range-domain DGNSS\", as shown in Figure 1.  
Range-domain DGNSS is more general and effective than Position-domain DGNSS, and corrections for the Range-domain DGNSS are provided free to all DGNSS users in Korea. One of the Korean DGNSS service providers, the National Maritime PNT Office, developed and distributed a service program running on smartphones to feed corrections to general GNSS receivers [14]. This cannot be used for the GPS module of smartphones, because Android applications can only provide the final position coordinates through LocationManager or GPS_Provider, and do not have the authority to access the GPS device [15]. Therefore, DGNSS techniques or the post-processing of SPP cannot be performed to correct or mitigate the error in the observable values, which translates to a positioning error. The only way to enhance the position accuracy of the smartphone is to shift the coordinates in the position-domain. Conceptually, Position-domain DGNSS is simple enough to mitigate the error in the smartphone position, because the correction is generated by differentiating the real-time GPS-derived position and the surveyed location of the RS. Although it seems possible to apply the block-shift method [16], this application can only be executed when the RS and the rover have exactly the same sets of visible satellites. When the two sets are different, the error is actually larger than for standalone GPS, and thus the block-shift method is impractical [17]. To overcome this problem, [PERSON] considered both the RS and the user to be operating a smartphone and showed an overall improvement in accuracy over [PERSON]'s work; however, the subsequent errors were occasionally greater than 25 m [12]. The inverted DGPS method [18], in which the position of the user and satellite combination are sent to the server and the correction information is sent to the user, can be considered. However, this cannot handle multiple users, as the server would become overloaded.  
The DGPS coordinate projection (DGPS-CP) method has recently been developed [19]. Under DGPS-CP, the rover selects the pseudorange correction (PRC) corresponding to the satellite combination used for rover positioning and projects it to the position area to correct its own position. This method is practical in smartphones, because the coordinate correction for visible satellites is generated at the user-side and the range-domain DGPS infrastructure (such as national DGPS (NDGPS) and the Satellite-Based Augmentation System (SBAS)) is used without modification. This technique is also convenient, as it uses the position standard format provided by smartphones, allowing users to apply the method within applications. In this paper, the DGPS-CP method is modified for smartphones, and the subsequent improvement in accuracy is verified through static and dynamic testing.