## 1 Introduction
In addition, because the WADGPS has been developed for aviation, the constraint regarding the 40-50\({}^{\circ}\) elevation angle is not a major problem; however, it may be a major obstacle for users in urban and mountainous areas while receiving the signals from the GEO satellites. In other words, if a signal is interrupted while receiving corrections, the user needs to wait when the correction data are not being received. Hence, an effective and reasonable augmentation system is required to compensate for the error due to the spatial decorrelation even in the cases of long baselines while receiving the correction via separated communication from the LADGPS. A network real-time kinematic (RTK) solution is used to reduce the decorrelation error in a Carrier-phase DGPS system. Among the various network RTK methods, the Flachen Korrektur parameter (FKP), which is a suitable methodology to complement the current DGPS using an one-way communication environment, is also useful to complement the current LADGPS. KODGIS and NAWGIS services operated by the ASG-EUPOS in Poland are examples of systems to improve the low-cost receiver using the network RTK solution, which is very rare in other countries or infrastructures. While the KODGIS generates DGPS correction directly for the user's location based on the virtual reference station (VRS) concept, the NAWGIS generates DGPS correction for strictly defined points (the geometric centers of the station network are for northern and southern Poland) based on the VRS concept to compensate for the spatial decorrelation error of the DGPS [9; 10]. To provide the service, it uses a proprietary message from the radio technical commission for maritime (RTCM) for the solution. Thus, exclusive users who made a contract with the operator support the system. The proprietary message of the NAWGIS (messagetype 59) has been recently moved to the reserved message for private use in the RTCM version 2.3, and mass market low-cost receivers such as u-blox series do not support the private message. In this study, we propose a user-oriented FKP-DGPS algorithm as a new augmentation system by integrating the conventional LADGPS correction with the modified FKP correction. This system combines the existing LADGPS and FKP infrastructures, and subsequently, generates a new correction to be provided to the low-cost GNSS receivers based on the RTCM standard. It does not require a special infrastructure for the FKP-DGPS and follows the current standards. Thus, any LADGPS user can compensate for the spatial decorrelation error by itself. In addition to solving the spatial-decorrelation problem in the LADGPS, it can be a solution for the signal corruption of the WADGPS correction from the satellite based augmentation system (SBAS) GEO satellite. Because it is difficult to ensure a stable service for land users based on the SBAS corrections, this study focuses on the ground infrastructure that provides the LADGPS service; herein, the DGPS referred is the LADGPS. The remainder of this paper is organized as follows. Section 2 describes the spatial decorrelation of the LADGPS and introduces a method of reducing its influence using a modified FKP correction with the help of mathematical analysis. Section 3 focuses on the method of developing a real-time FKP-DGPS algorithm using the modified FKP correction; moreover, the method of applying the calculated FKP-DGPS correction to a low-cost L1 single frequency GPS receiver is presented. Section 4 shows the results of the static and dynamic user tests conducted to confirm the improvement in the accuracy obtained through the developed algorithm. Finally, the performance of the developed algorithm in determining the reduced positioning errors with sufficient coverage area is verified through the test results.