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Geodesy in the Year 2000 (1990) / Chapter Skim
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The Accuracy of Position Determinations by VLBI Expected by the Year 2000
Pages 114-123

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From page 114...
... of signals from distant quasars is measured by recording digitized samples on magnetic tape for crosscorrelation at a central processor. A hydrogen maser frequency standard provides a precise frequency reference for the receiver's local oscillators as well as a time standard for the digitization and data formatting as illustrated in Figure 2.
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From page 115...
... Other Instrumental Errors. Antenna flexure and changes in the axis intersection with tower expansion are significant error sources in some of the larger antennas used as base stations although these errors can be accurately modelled.
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From page 116...
... Both methods assume azimuthal symmetry for the dry delay and hence, are equally limited by the presence of horizontal gradients. Perhaps the real promise for future improvements will come with an understanding of the biases between WAR determined path delays and the wet path delay solutions using VLBI data which may well be errors in the model for the dry delay.
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From page 117...
... Instrumental errors and errors due to source structure are likely to be reduced to a negligible level in the next decade. It is likely that the fractional accuracy of baseline length measurements will improve to 1 x 10-9 or perhaps 5 x 10-~°.
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From page 118...
... Remote Sens., GE-23, Crustal Dynamics Project Users Davis, J L., et al., Geodesy by Radio Interferometry: Effects of Atmospheric Modeling Errors on Estimates of Baseline Length, Radio Sci.
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From page 119...
... _ | CLOCK Geodetic VLBI _ ~ REC~WR __ ~ r l _ TAPE _RECORDER .
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From page 120...
... 120 cn ~ o Z > 0 Z >a O J By O 1 L U]
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From page 121...
... SIX Frequencies Recorded for "Bandwidth Synthesis"
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From page 122...
... Present Levelof Accuracy 5mm plus 2 x 10-. times tl's length Table 1.
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From page 123...
... ~ 5,nm <5 mm ~ vertical - ~ i Lower system temperatures and Aver according rates frill improve | ~mpors~t for large as~tennas can be motelled can be modelled Antenna tower e~cp~ion Spurious sign^/cal errors Polarization impurity Cloci; instability Ionosphere Source structure _ _ ~ a mm _ .. _ ~ 5 trim _ _ e: 2 ~ _ I_ be _ ~ ~ =:~-~V 2 Am a: 1 mm (1 x 10_14 ~r 10 [L Ia ~ .
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Key Terms

  • atmospheric delay
  • geodetic vlbi
  • error sources
  • mapping function
  • dry delay

  • path delay
  • haystack observatory
  • baseline length
  • vapor radiometry
  • direct calibration

  • source structure
  • path delays
  • dynamics project
  • geodetic measurements
  • wet path

  • errors due
  • ground pressure
  • radio source
  • crustal dynamics
  • wet delay

  • local oscillators
  • water vapor
  • data recording
  • troposphere lomm
  • negligible level

  • war determined
  • effective aperture
  • astronomical study
  • standard provides
  • spacing provided


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