Editor's Points: Positioning with Confidence

October 28, 2010
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Earlier this year, I posted a note on, POB’s online professional community, asking what topics readers would like to see covered in upcoming issues.

One of the first responses was “RTK.” Although real-time kinematic GPS technology has been in use for nearly two decades, it is still widely misunderstood. As equipment has advanced and the number of satellites in the Global Navigational Satellite System has grown, so, too, has confusion about best practices for field procedures. With conventional single-base systems, errors and ambiguities abound. And although the emergence of networks has resolved some of these issues and has made the correction process much faster and easier, RTK positioning is still far from fool-proof.

In this issue, William Henning, PLS, senior geodesist for NGS, tackles the first of seven best practices for RTK positioning, which NGS has dubbed “The Seven C’s:” checking, communication, conditions, constraints, coordinates, collection and confidence. Although his column will focus primarily on single-base RTK, Henning is also hard at work leading a group that is developing guidelines for using real-time networks. Chief among the recommendations:
  • Know the networks. Users should be familiar with the different GNSS manufacturers’ RTN solutions, including non-physical (virtual) reference stations, master-auxiliary types and reverse processing concepts.

  • Record the metadata. These include details such as the RTN coordinate basis (datum, adjustment, epoch), any calibrations performed, hardware, firmware and software versions, data collection criteria (number of epochs of data, intervals used, DOP settings, minimum satellites present during collection), conditions such as local weather and space weather, possible multipath conditions, and obstructions.

  • Understand the concepts of precision (a measure of alignment to the RTN or to the constrained passive marks as a local projection) and accuracy (the alignment to the national datums) in differential GNSS positioning. NGS encourages all RTNs to align to the national datums at certain levels horizontally and vertically using the national CORS network as truth and to produce positions that are consistent at high accuracy from overlapping or abutting RTNs. NGS also encourages all RTNs to provide their data in formats that all users can use regardless of their GNSS hardware.

  • Follow the NGS best practices for GNSS field locations--i.e., “The Seven C’s”--which are the same for networks as they are for single-base RTK. 
According to Henning, the draft RTN guidelines document will comprise five chapters (Site Considerations, Planning, Aligning RTN to the NSRS, Administration and Users) and will be approximately 60 pages long. The document is currently in the review and editing process and is expected to be released for public comment in early 2011.

By following the guidelines, users should be able to achieve GNSS RTN positioning with high accuracy at a 95 percent confidence level. It still won’t be fool-proof--but it’s getting closer.

Share your thoughts on this column here or To contact the editor, send an e-mail to

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