Network Real Time Kinematic (NRTK) Positioning - Description, Architectures and Performances

In this last years, the differential GNSS positioning has had an intensive expansion, especially due to the development and realization of GNSS CORSs (Continuous Operating Reference Stations) networks. One of the main goals of these networks is to give the possibility to the users to extend the differential positioning (whether in real time or post-processing) up to 25-50 km, allowing a positioning useful for applications such as surveying, monitoring and precise navigation. It is possible to find in bibliography some studies that shown the performances of this type of positioning, with their limitations and peculiarities, also in function of the network size. The Network Real Time Kinematic (NRTK) positioning is a very common practice not only in academia but also in the professional world. Since its appearance, over 10 years ago, a growing number of people use this type of positioning not only for topographic applications, but also for the control of vehicles fleets, precision agriculture, land monitoring, etc. This chapter wants try to focus the attention on the methods and characteristics of NRTK positioning and to summarize principles and peculiarities of this type of GNSS positioning: the goal is to show how networks for NRTK positioning work and also to show the differential corrections that are available today, with their performances in order to obtain acceptable results, always considering the accuracy required by the purposes described above. So, first of all the GNSS network positioning will be discussed, starting from the network biases estimation to the rover positioning with particular attention to the differential GNSS corrections such as the Master Auxiliary Concept (MAC), Virtual Reference Station (VRS) and Flachen Korrektur Parameter (FKP). Particular attention will be also devoted both to the transmission of the differential corrections and their problems, such as the GSM coverage, and to the analysis of the RTCM protocol. A theoretical description of the network calculation and biases interpolation will be made and brief results for each correction will be shown, in order to give a practical example