1. IVS GM 20061 January 10, 2006 Interaction of Atmosphere Modeling and VLBI Analysis Strategy Arthur Niell MIT Haystack Observatory

2. IVS GM 20062 January 10, 2006 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.

3. IVS GM 20063 January 10, 2006 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.

4. IVS GM 20064 January 10, 2006 Height Uncertainty

5. IVS GM 20065 January 10, 2006 Relative Height Uncertainty from Mapping Function

6. IVS GM 20066 January 10, 2006 Atmosphere Mapping Function Trade-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.

7. IVS GM 20067 January 10, 2006 1997-98 El Niño

8. IVS GM 20068 January 10, 2006 Height Equivalent Mean Error for 5° Minimum Elevation

9. IVS GM 20069 January 10, 2006 Height Equivalent Scatter for 5° Minimum Elevation

10. IVS GM 200610 January 10, 2006 Height Equivalent Scatter for Combined Hydro and Wet MFs

11. IVS GM 200611 January 10, 2006 Combine Geometry and Mapping Function Uncertainties

12. IVS GM 200612 January 10, 2006 Combine Geometry and Mapping Function Uncertainties

13. IVS GM 200613 January 10, 2006

14. IVS GM 200614 January 10, 2006 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.

15. IVS GM 200615 January 10, 2006 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.)

16. IVS GM 200616 January 10, 2006

17. IVS GM 200617 January 10, 2006 Site specific phase correction

18. IVS GM 200618 January 10, 2006 Site specific phase correction