The Global Positioning System for the Geosciences Summary and Proceedings of a Workshop on Improving the GPS Reference Station Infrastructure for Earth, Oceanic, and Atmospheric Science Applications (1997) / Chapter Skim
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10 Static Positioning
10 Static Positioning
Pages 201-226
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10 Static Positioning
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From page 202... ...
Hence it is considered desirable to suppress surface noise to sub mm levels at all frequencies of interest. A second form of noise arises from the distortion of the tectonic strain-fields by elastic heterogeneity of the Earth's surface.
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Ice wedging is a well known source of noise in temperate and arctic regions, and the growth of halite in marine cliffs can result in the movement of loose blocks. Tree roots can also shift surface blocks, but by far the most common source of surface noise is the hydration of clay minerals caused by changes in soil moisture.
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From page 204... ...
Frequently no measure of local noise level exists before or after the installation of an engineered monument, and typically, the only test that can be applied to a control point is its identification as an outlier if its measured signal fails to fit a deformation field constrained by nearby monument motions. Those experiments that have been undertaken, illustrate that the decay of surface noise with depth is site dependent.
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From page 205... ...
However, the inclinometer method potentially provides a way to measure the lateral stability of geodetic control points. In principle, an inclinometer borehole can be incorporated into engineered monuments at little extra cost, and can be periodically measured to monitor long-term fluctuations relative to its base.
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From page 206... ...
MONUMENT DESIGN It has been known for many years that increased surface noise suppression is obtained by increasing the measurement length of a strainmeter or tiltmeter. Instruments with dimensions of the order of 1 km outperform instruments < lm long largely because the signal is increased by a factor of 1000 relative to the randomwalk noise-level of an attachment point.
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From page 207... ...
An inclinometer tube was cemented within this hollow central pier, and two inclined solid helical piers provided short term lateral stability of the attachment point as shown in Figure 6. Despite operating in a thick clay deposit of variable moisture content, the biaxial inclinometer data reveal that the tops of the piers have moved less than 0.5 mm during their first 18 months of operation.
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From page 208... ...
VERTICAL CONTROL Thus far the discussion of monument stability has been confined to monitoring lateral motions of control points relative to points at depth, partly because GPS vertical accuracies are currently much worse than horizontal accuracies, and partly because horizontal displacements are significantly more of a challenge to suppress or monitor than vertical measurements. Theoretically, the engineered monuments shown in Figure 6 all provide sub-mm vertical stability.
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From page 209... ...
The benefit to the geodesist is that these measurements provide a measure of long term stability of the control point, enhancing the integrity of the experiment. A future development that is clearly overdue given the limited precision of GPS geodesy, and the noisy surface environment to where these measurements are confined, is to consider future geodetic monuments that include not just measurements of surface noise, but measurements of crystal tilt and strain signals.
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and M Bryant, An overview of existing permanent GPS monuments in the US and Canada, Space Geodetic Site Subcommission, IUGG Int.
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The results are available in a series of UNAVCO technical reports.3 In the second field test, calibration corrections determined by the chamber tests were used in surveys between mixed antenna types on known baselines. OFFSET CALIBRATION Chamber measurements showed horizontal phase center onsets are as large as 3 mm (L1)
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C,, Oq, ~ -5 A ~ _1n. -154 -20 1 o AOA Rears rimble S~ , FIGURE 1 L1 phase center patterns are shown for several antennas (10 degrees of L1 phase is approximately 5 mm)
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4 6 213 - 6 4 2 E O _ ct L1J -2 -4 -6 -8 -10 8 10 FIGURE 2 Horizontal phase center offsets (labeled L1C, L2C, and L3C) determined from chamber tests.
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The least successful application of the offset and phase center pattern corrections has been with the Trimble SST antenna where the residual error is as large as SOmm.4 In-general, mixed antenna baseline solutions show vertical errors of 12 mm or less without tropospheric estimation, and up to 50 mm with tropospheric estimation. This compares with 1 mm errors achieved with unmixed antennas on short baselines with no tropospheric estimations.
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antenna tripod mounts with various GPS receivers and antennas. Baseline results using Trimble SSE GPS receivers and Trimble SST antennas with high and low antenna heights had vertical errors as large as 17 mm when tropospheric parameters were estimated.
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SNOW AND TROPOSPHERIC ESTIMATION EFFECTS Multipath effects have been demonstrated to affect vertical accuracy in the high-low antenna setups described above. In addition, changes in multipath conditions, such as snow at the site, can affect vertical baseline solutions when tropospheric delays are estimated.
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UNAVCO is conducting farther tests with a 1/4 inch acrylic cover used with AOA choke ring antennas at many IGS sites. Antenna Phase Center Variations | PROBLEM i STATIC POSITION ERROR LEVEL I : Mixed Antenna Types up to 8 mm horizontal | up to 90 mm vertical Like Antenna types with long baseline separation ("see" same satellite at different elevation)
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, by moving toward standard antennas,9 and by avoiding tropospheric estimation errors associated with low antennas. Using antenna phase pattern and offset corrections derived from anechoic chamber tests, the accuracy for mixed Tnmble SSI (patch antenna with removable ground-plane)
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In this paper the extent of the elevation dependent height errors for both similar and dissimilar antennas will be illustrated; measurements of the effect of the Ashtech radome (itself a scatterers on height estimates for a choke-ring antenna will be reported; and two potential solutions to the scattering problem for the specific geometry of the FlINN-lype monument and antenna mount will be summarized. ELEVATION DEPENDENT HEIGEIT ERRORS The results reported in this section will illustrate the type of errors that may affect all geodetic results and measurements of precipitable water vapor, regardless of antenna separation.
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From page 220... ...
(If the anechoic chamber measurements are correct, this implies that the roof/tripod mount simply "undoes" the intrinsic elevation dependence of the phase.) Similarly, the Ashtech chokering/radome could not use the same corrections as either The Global Positioning System for the Geosciences of the other choke-ring antennas.
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From page 221... ...
(Although both the leveling and the analysis are also candidates for the 10 mm discrepancy, the leveling was repeated to 1 mm, and the files associated with the analysis have been carefully reviewed.) The purpose of this illustration is to caution users that, although an antenna may be characterized by either or both anechoic chamber measurements and field measurements of their phase pattern (or phase pattern difference to another Me of antenna)
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Schaer, MeIvart, and G Beutler, Determination of Antenna Phase Center Vanations Using GPS Data, Paper presented at the 1995 IGS Workshop, Potsdam, Germany, May 15-17, 1995.
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A water vapor radi ometer was programmed to perform azimuth scans at a 30 degree elevation angle at Boulder Colorado. Skies were cloudless.
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The zenith vapor measurement represents some average of water vapor over the cone defined by the GPS elevation cutoff angle. In a 1992 experiment to determine if commercial GPS receivers were sufficiently sensitive to perform occultation measurements from a low earth orbiter, a TurboRogue and Dorne Margolin antenna were placed at the 6000 level on Flagstaff Mountain above Boulder and were allowed to track descending satellites to signal loss.
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From page 225... ...
The ability to give some definition to the spatial structure of water vapor, although integrated path values, is a strong constraint to forecast models. And if good measurements can be obtained at the horizon, sensing of tropospheric features to hundreds of miles beyond the horizon is possible.The magnitude of the value to meteorological forecasts is speculative, and simulations are needed.
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