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Evaluation of Global Ionosphere TEC by comparison with VLBI data

Analyzing Global Ionosphere Maps for VLBI error correction, GIM/CODE comparison, TEC rate assessment, and potential of Japanese dense GPS network. Evaluation of TEC precision and implications for VLBI astrometry and spacecraft navigation.

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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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