Nexteq PP-RTK Technology and System

L. Urquhart, Y. Zhang, J. Chen, S. Lee, Y. Gao

Abstract:	Carrier phase integer ambiguity resolution is critical for high accuracy GPS positioning. Until recently the lack of integer ambiguity resolution for precise point positioning (PPP) users has meant long convergence times and relatively unstable solutions often with only decimeter accuracy. On the other hand, network real-time kinematic (RTK) positioning has provided users with fast ambiguity resolution and centimeter level accuracy but with large bandwidth requirements and baseline lengths restrictions due to the de-correlation of the error sources over long baseline distances. To overcome these limitations, Nexteq has introduced its Precise-Point Real-Time Kinematic (PP-RTK) positioning system. Nexteq’s PP-RTK system provides a fusion of PPP and network RTK techniques to provide both fast and accurate positioning capabilities for its users. The Nexteq PP-RTK system consists of a server and client architecture. On the server side, fractional carrier phase biases are estimated in an un-differenced manner to allow for ambiguity resolution at all network stations. Once the ambiguities are fixed for the network stations, the measurements are said to be on the same ambiguity “level” and can be used to provide orbit and integer clock corrections that enable PPP with ambiguity resolution to the end user. These corrections are valid over very wide areas and allow the user to operate without requiring any nearby base stations. In addition to the orbit-clock corrections, the network also provides high quality measurement-based corrections to the end user. These corrections allow for nearly instantaneous ambiguity resolution for the end user as is expected of a network RTK service. On the client side, corrections are received via a cellular modem using the NTRIP standards. Unlike traditional RTK, Nexteq’s PP-RTK system uses un-differenced measurements for rover processing. This allows users to seamlessly switch between PPP and network RTK modes without a decrease in performance. This feature also improves reliability by minimizing the reliance on individual base stations. In addition to the innovative network algorithms which allow for PPP with ambiguity resolution, user receiver algorithms have been designed to reduce ambiguity initialization time, ensure high confidence in ambiguity fixing and reduce user down time in the event of signal outages or data gaps. In the past these features have been a major limiting factor in the adoption of PPP. In this paper we describe the latest developments in Nexteq’s PP-RTK system for both the server and client. The system performance is tested under real world scenarios including both kinematic and static environments. Metrics for performance include ambiguity initialization time, positioning accuracy and ambiguity fixing reliability.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2243 - 2250
Cite this article:	Urquhart, L., Zhang, Y., Chen, J., Lee, S., Gao, Y., "Nexteq PP-RTK Technology and System," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2243-2250.
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