# Assessment of GNSS Galileo Contribution to the Modernization of CROPOS's Services
## 1 Introduction  
The single-base Real-Time Kinematic (RTK) GPS (GNSS) is a differential positioning method based on a double differenced carrier phase algorithm which relies on the On-The-Fly (OTF) ambiguity resolution method providing results with cm-level accuracy in real time [1]. The RTK method involves a reference receiver transmitting its raw measurements or observation corrections to a rover receiver via some sort of data communication link. The data processing at the rover site includes ambiguity resolution of the differenced carrier phase data and coordinates estimation of the rover position [2] with the accuracy of 1 cm + 1 ppm over a short range [3]. More specifically, [PERSON] and [PERSON] [4] have reported that the RTK horizontal and vertical accuracy can be specified versus the rover-base distances as 1 cm + 0.5 ppm and 2 cm + 1 ppm, respectively. The basic concept of a single-base RTK is to use real-time corrections from a reference station to reduce and remove errors common to a base and rover pair to achieve centimeter-level accuracy. The system concept was developed in the late 1980s and early 1990s by the academia, science, private, and government sectors to improve the efficiency, accuracy, and timeliness of dredging and hydrographic surveying. In 1993, the Trimble company released its first commercial RTK product. A year later, it released the first commercial RTK receiver with OTF capabilities. That opened the door to the utilization of RTK by surveyors and geospatial professionals. A comprehensive history of the RTK was portrayed by [PERSON] [5, 6, 7, 8]. Although RTK provided a substantial benefit to position determination, the weakness of the methodsoon became evident: the method could be applied up to distances of 10-20 km from the base receiver because of the spatial decorrelation of distance-dependent errors induced by the ionosphere, troposphere, and orbital errors [9; 10]. Distance-dependent errors can be separated into a dispersive component, consisting mainly of ionospheric refraction, and a non-dispersive component, consisting of tropospheric refraction and orbit errors [10]. The ionosphere is subject to rapid and localized disturbances, and as such, it represents the main restriction on the reference station density, whereas the tropospheric and orbit errors are known to change only slowly with time [2; 11]. To mitigate those weaknesses of the single-base RTK, the Network RTK (NRTK) was developed. It provides larger service area coverage, increased robustness, and higher positioning accuracy due to more precise accounting for spatially auto-correlated errors in GNSS measurements [12; 13]. Compared with the single-base RTK, the advantage of network RTK is that large portions of ionospheric and geometric errors are removed through network corrections, enabling a considerable improvement in RTK initialization and positioning accuracy [14]. Continuously Operating Reference Stations (CORS) network density is usually restricted to 70-100 km to allow for quick and reliable ambiguity resolution [3; 10]. Over the years, several concepts of NRTK have been developed [15], the Virtual Base Station (VRS) concept being among the most used ones [2; 10; 16]. The VRS concept was announced in 2000 by Spectra Precision Terrasat company [8; 17]. The VRS concept was outlined in, e.g., [PERSON] et al. [18], its concept and performance in [PERSON] [15], the VRS observation algorithm was given in, e.g., [PERSON] et al. [19], whereas the VRS data generation approach was given in, e.g., [PERSON] et al. [12]. An overview of the development of the VRS is portrayed in Schrock [16]. The VRS requires a two-way communication channel between rover and server, where the corrections are streamed via the NTRIP (Networked Transport of RTCM via Internet Protocol). The GNSS errors are usually reduced via the Observation Space Representation (OSR) modelling method, which provides a single compound of ranging corrections as observed in a nearby (real or virtual) reference station [3].