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4.0 Improved Measurement Accuracy Support

During the last decade, the precision of GPS measurements of station positions has improved dramatically, especially on the global scale. There has been an orders-of-magnitude increase in the amount of GPS data collected. There has also been a considerable increase in the diversity of the GPS receivers and antennas used in field campaigns and an increase in the use of mixed-mode observation strategies, often with different types of receivers. Expectations of the accuracy of GPS measurements have increased and therefore the magnitudes of the signals to be studied with GPS measurements have become, in many cases, much smaller with time. Many experiments being conducted currently need to resolve velocities with uncertainties of less than 1 mm/yr.

Within the framework of this greater diversity of measurements and equipment has come the realization that there are a number of limitations to the accuracy of the GPS measurements which, if overcome, would yield more accurate results. Higher accuracy GPS measurements will shorten the duration (and cost) of measurements to resolve the problems being addressed by the UNAVCO Community. The UNAVCO Boulder Facility has been involved since its inception in a broad range of issues affecting high precision GPS measurements. The results of this work have been presented in papers, technical presentations to the community, and recently via the Internet on the UNAVCO Home Page. The major issues are covered in detail in two comprehensive papers, Meertens and others, 1996, and Rocken and others, 1995. The FY96 activities of the UNAVCO Facility Development and Testing Group (DTG) were done in cooperation and coordination with the UNAVCO and extended GPS community and generally fall under the category of modeling in-situ characteristics of GPS antennas and receivers. Specific guidance was given to the Facility by Working Group 3 (WG3) on Technology and Development. WG3 defined the areas of responsibility for DTG as follows:

  • testing and standardization of receiver and ancillary equipment specifications
  • development of techniques for mixed-mode operations
  • development of techniques for establishment and maintenance of continuous networks
  • development and evaluation of advanced data communication techniques
  • development and standardization of monumentation techniques
  • development and standardization of supporting software
  • approaches to down-sized equipment
  • evaluation of advanced power sources
  • evaluation of improved training techniques
  • communication of applicable policies and procedures to user community
  • expanded applications of GPS technology
  • other areas as deemed appropriate, in consultation with the Steering Committee

Based on these guidelines, the DTG focused in FY96 on: (1) antenna field and anechoic chamber tests; (2) receiver performance tests and comparisons; (3) mixed antenna/receiver system tests; (4) development of receiver independent tools for communication of real time GPS data streams, and; (5) continued support of quality check and RINEXing software. Figure 4-1 shows a summary of error sources related to the antenna, receiver and monument and their approximate maximum effect on measurement accuracy. The photos in the center of the figure illustrate the problems encountered in episodic and continuous GPS data collection.

Figure 4-1. Summary of Primary GPS Error Sources and Magnitude of Possible Errors.

The UNAVCO Boulder Facility has investigated the problems highlighted with an underline in FY96. Below are the summarized findings in each of these areas.

1996 Annual Report - 30 SEP 1997

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