Broadcast Network RTK - Transmission Standards and Results

Abstract

Network Real-Time Kinematic (RTK) positioning is the latest innovation in high precision GPS positioning. Network RTK involves the use of three or more reference stations to collect GPS data and extract information about the atmospheric and ephemeris errors affecting signals within the network. A central processing facility uses the reference station data to generate corrections that are then relayed to RTK users operating within the coverage region of the network. Single-reference station RTK is widely used for many centimetre-level applications. However, users must operate within say a 10-20km radius of the reference station. Beyond this limit, atmospheric biases degrade results – network RTK helps to overcome this limitation. Network RTK can be implemented in either a Virtual Reference Station (VRS) or broadcast mode. The former approach requires RTK rovers to send their location to the central processing facility in order to receive a corrected data-stream from the network. The broadcast mode places the onus on the rover to interpolate atmospheric and ephemeris corrections that are transmitted by the network. Data transmission bandwidth comes at a premium for the broadcast mode and careful consideration must be given to a data transmission standard used for network RTK. A standard must be efficient in terms of size, but flexible enough to allow different manufacturers to implement their own innovative approaches for using correction information. In particular manufacturers should be able to use their own correction interpolation schemes and not be bound to a single strategy. The following paper presents a new Network RTK data transmission format that has been formulated within the RTCM-Version 3 framework. The new format encompasses both single-reference and network correction layers and is scaleable to city-, county- and nation-wide coverage. Furthermore, the standard is designed to support data archival which is important for legal traceability purposes. Performance results from Network RTK positioning are compared to single-reference station RTK using datasets collected from an operational network in Germany. The Network RTK approach was found to provide improvements in initialisation reliability and timeto- initialise metrics in cases where ionospheric biasing was significant.