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Evaluation of Global Ionosphere TEC by comparison with VLBI data

Evaluation of Global Ionosphere TEC by comparison with VLBI data. Mamoru Sekido, Tetsuro Kondo Eiji Kawai, and Michito Imae. Motivation. External Ionospheric delay correction is necessary for single frequency VLBI astrometry (e.g. Pulsar 1.4-2GHz)

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Evaluation of Global Ionosphere TEC by comparison with VLBI data

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  1. Evaluation of Global Ionosphere TEC by comparison with VLBI data Mamoru Sekido, Tetsuro Kondo Eiji Kawai, and Michito Imae

  2. Motivation • External Ionospheric delay correction is necessary for single frequency VLBI astrometry (e.g. Pulsar 1.4-2GHz) • GPS network and technology is growing rapidly. • Global Ionosphere map(GIM) is available at IGS analysis centers. • Global Ionosphere Map (GIM) is useful for • VLBI Astrometry (Pulsar etc…) • Radiometric measurements(Spacecraft Navigation, SELENE Project(0.2mm/s), …) • ,if it has enough accuracy!!.

  3. Propagation delay

  4. Content of the presentation • Comparison of GIM/CODE and VLBI TEC • Statistical comparison about error of GIM • Conclusion: • Bias error ~3TECU. Need more precise GIM including fine structure for correction. • Comparison on TEC rate • Conclusion: GIM is not good for correction for delay rate in VLBI at present. • Trial of using Japanese dense GPS network for TEC. • VLBI receiver bias– by product • It was not known until TEC comparison.

  5. Global Ionosphere Map(GIM) produced by GPS observations • IGS is working for providing Ionosphere TEC as one of the products. IAAC(CODE, NRCan, ESOC, JPL, UPC) • Benefit of GIM/CODE • Daily 12(13) GIMs with 2 hours interval and related subroutines are available anytime by FTP. • It is expressed with 12(15)-deg.8(15)-ord. Spherical Harmonic expansion • No data interruptions since 1995

  6. Global Ionosphere Map

  7. IGS sites used for GIM/CODE

  8. z’ z GIM/CODE • 12(13) GIMs with 2 hours interval • Expression in 15-deg.15-ord. (12-deg./8-ord.) Spherical Harmonics expansion • Spherical single layer shell model • Single Layer Mapping Function • Modified SLM

  9. TEC Comparison GIM/CODE-VLBI True Ionosphere TEC VTEC GIM/CODE

  10. known known How these comparison can evaluate error of the TEC MAP?

  11. VLBI data used for Comparisons • Scans of • KSP(100km) • 6855 scans of • CORE & NEOS

  12. GIM/CODE-VLBIKashima-Koganei(100km) • Correlation • 0.92 • Prop.cff • 0.87 • Offset • -3.1TECU

  13. s2GIM – El relation (100km) sGIM,u =0.3 –0.6TECU

  14. GIM/CODE-VLBIAlgonquin-Wettzell (6000km) • Correlation • 0.99 • Prop.cff • 1.13 • Offset • 57.6TECU

  15. s2GIM – El relation (long baseline) sGIM =1-7TECU

  16. s2GIM – Baseline length relation GIM can be used to predict better than 10% of Ionospheric delay in VLBI observation

  17. El, Baseline lengthdependency • Baseline: • 0-500km • 500-4000km • 4000-8000km • 8000km - • El Cut off test • El >=20 deg. • El >=40 deg. • El >=60 deg.

  18. Baseline a s2GIM(baseline length) =Structure function of GIM error Assumption Error of GIM is isotropic

  19. Error Spectrum of GIM Assumption Error of GIM is isotropic

  20. Delay (TEC) rate comparison Algonquin - Wettzell • Correlation on TEC rate was low even on long baseline • Reasons will be • Low spatial resolution • 2500x1700km • Low time resolution • 2 hours interval.

  21. Traveling Ionospheric Disturbances detected by GEONET Provided by A.Saito in Kyoto Univ. (Saito te al., GRL Vol.25, 3079-3082, 1998) This sort of TIDs can contribute in TEC rate 5m TECU/sec > 5.e-4 TECU/sec

  22. Regional Ionosphere Map with GEONET and GIM/CODE • Dr. Ping in Mizusawa /NAOJ and Dr. Saito in Kyoto Univ. are trying to use GEONET (about 1000 GPSs in Japan) to make precise TECMAP. • High Time and Spatial resolution (60 deg. 10min.) (Ping et al., EPS. Vol. 54 e13-16, 2002)

  23. VLBI receiver bias • Bias comes from VLBI receiver delay difference between X and S. • It used to be absorbed in clock offset and be not aware so far ( It has been pointed out by T. Herrings).

  24. Since we have no any a priori knowledge on the bias, we put a condition VLBI (S/X) receiver biases Actually, we experienced these biases were constant regardless with experiment series or date. We have to aware these delay differences are relatively exist.

  25. Conclusions • GIM/CODE can predict VTEC better than 10 % of its magnitude at present. • GIM/CODE seems to have RMS error ~3 TECU at low spatial frequency. • About 100 degrees of SH model might be necessary to achieve the same accuracy with S/X VLBI. • High resolution GIMs in space and time is necessary for using it for delay rate correction (f<. • VLBI Receiver bias was detected.

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