Integrity Monitoring for Network RTK Systems

From the advent of the satellite positioning techniques, civil users have always been trying to find a way to have more accurate and precise coordinates of their position. Differential concepts, from early days of GPS, have been considered. Applying the RTCM format, made the transmission of corrections possible from reference stations to the users. At first stage the corrections were casted to the users from one single station, which is called single RTK (Real Time Kinematic). This method is limited in some ways; degrading by increasing distance from CORS (Continuously Operating Reference Station), needed same signals at reference and rover and remaining the reference station errors. For compensating these shortages, the Network RTK concept appeared. In NRTK the corrections are produced using a network (at least three) of reference stations. The concept of Precise Point Positioning (PPP) is currently associated with global networks. Precise orbit and clock solutions are used to enable absolute positioning of a single receiver. However, it is restricted in ambiguity resolution, in convergence time and in accuracy. Precise point positioning based on RTK networks (PPP-RTK) overcomes these limitations and gives centimeter-accuracy in a few seconds. Nowadays, the use of automatic applications of navigation is increasing rapidly. Autonomous moving objects e.g. cars, UAV (Unmanned Aerial Vehicle), UMV (Unmanned Marine Vehicle) are examples of these kinds of applications. The main navigation engine in these examples is GNSS positioning and the best possible accuracy in kinematic mode is based on Network RTK solution. Because of the complicated moving environment for these autonomous objects and the importance of safety issues especially in urban areas, integrity monitoring is of great importance in navigation systems. Up to now, the integrity definition and standards have been well established for aviation applications especially around airports which are most based on SBAS and GBAS systems. But there is a lack of such standards for integrity in realm of RTK (PPP-RTK) positioning which can be useful in autonomous applications where high precision GNSS positioning (carrier phase measurement) must be used.