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Interaction of Atmosphere Modeling

and VLBI Analysis Strategy

Arthur Niell

MIT Haystack Observatory

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Objectives

Show dependence of height errors/uncertainties on minimum elevation and latitude

Compare atmosphere mapping function errors Illustrate result of combining these errors Point out choices that need to be made

Note: the results to be shown are a guide, not necessarily the final answer.

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Height vs Minimum Elevation

Estimates of height are sensitive to the minimum elevation Both the mean value and the RMS scatter are affected Usually a single value for minimum elevation is chosen for

all sites

Two effects are strongly elevation dependent and compete in determining what minimum elevation should be adopted Better geometric precision is obtained by using lower

elevation data. Uncertainty in the atmosphere delay model encourages

higher minimum elevation.

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Height Uncertainty

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Relative Height Uncertaintyfrom Mapping Function

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Atmosphere Mapping FunctionTrade-off

Data-independent mapping functions: NMF needs only site position and date; easy to use. GMF is more accurate and precise version of NMF.

Data-dependent mapping functions:IMF and VMF1 require external input from Numerical Weather Model. IMF can be calculated for arbitrary site location for

any date since 1980 (good for GPS). VMF1 must be calculated for specific site locations,

but is available for all VLBI sites.

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1997-98 El Niño

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Height Equivalent Mean Errorfor 5° Minimum Elevation

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Height Equivalent Scatterfor 5° Minimum Elevation

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Height Equivalent Scatterfor Combined Hydro and Wet MFs

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Combine Geometry and Mapping Function

Uncertainties

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Combine Geometry and Mapping Function

Uncertainties

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Conclusions

From comparison with radiosondes, VMF1 is the most precise mapping function for both hydrostatic and wet.

The best precision for azimuthally symmetric mapping functions is obtained with VMF1 and minimum elevation 5° - 7.5°.

For minimum elevation >7°, the difference in height uncertainty between IMF and VMF1 is less than 1 mm.

For accuracy over long time, e.g. years (TRF), all but NMF are comparable.

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Recommendations

Use either VMF1 or IMF in order to model geophysical variations in atmosphere.

Use VMF1 with a minimum elevation of 5°. If using NMF or GMF, use minimum

elevation of 7.5°. Do analysis for current schedules.

(Minimum elevation may vary with date of session, as well as antenna.)

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Site specific phase correction

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Site specific phase correction