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) External Ionospheric delay correction is necessary for single frequency VLBI astrometry (e.g. Pulsar 1.4-2GHz) GPS network and technology is growing rapidly. GPS network and technology is growing rapidly.  Global Ionosphere map(GIM) is available at IGS analysis centers. Global Ionosphere Map (GIM) is useful for 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!!.,if it has enough accuracy!!.

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology Propagation delay

Content of the presentation 1. 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. 2. Comparison on TEC rate  Conclusion: GIM is not good for correction for delay rate in VLBI at present. 3. Trial of using Japanese dense GPS network for TEC. 4. VLBI receiver bias– by product  It was not known until TEC comparison.

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) IGS is working for providing Ionosphere TEC as one of the products. IAAC(CODE, NRCan, ESOC, JPL, UPC) Benefit of GIM/CODE 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

Global Ionosphere Map

IGS sites used for GIM/CODE

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

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology TEC Comparison GIM/CODE-VLBI True Ionosphere TEC GIM/CODE VTEC

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology How these comparison can evaluate error of the TEC MAP? known

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology VLBI data used for Comparisons DateExperiment 7-18 Apr.2000 KSP-VLBI 5-6 Jul.2000 NEOS-A Jul 2000 CORE Jul 2000 NEOS-A Jul CORE Jul NEOS-A jul 2000 NEOS-A Scans of KSP(100km) 6855 scans of CORE & NEOS

GIM/CODE-VLBI Kashima-Koganei(100km) Correlation Correlation  0.92 Prop.cff Prop.cff  0.87 Offset Offset  -3.1 TECU

  GIM – El relation (100km)  GIM,u =0.3 –0.6TECU

GIM/CODE-VLBI Algonquin-Wettzell (6000km) Correlation Correlation  0.99 Prop.cff Prop.cff  1.13 Offset Offset  57.6 TECU

  GIM – El relation (long baseline)  GIM =1-7TECU

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology   GIM – Baseline length relation GIM can be used to predict better than 10% of Ionospheric delay in VLBI observation

El, Baseline length dependency Baseline: 0-500km km km 8000km - El Cut off test El >=20 deg. El >=40 deg. El >=60 deg.

 2 GIM (baseline length) =Structure function of GIM error  Baseline Assumption Error of GIM is isotropic

Error Spectrum of GIM Assumption Error of GIM is isotropic

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

On April 1, 2004 CRL and TAO will be reorganized as the National Institute of Information and Communications Technology Traveling Ionospheric Disturbances detected by GEONET Provided by A.Saito in Kyoto Univ. (Saito te al., GRL Vol.25, , 1998) This sort of TIDs can contribute in TEC rate 5m TECU/sec > 5.e-4 TECU/sec

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. 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.) High Time and Spatial resolution (60 deg. 10min.) (Ping et al., EPS. Vol. 54 e13-16, 2002) (Ping et al., EPS. Vol. 54 e13-16, 2002)

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

VLBI (S/X) receiver biases Station Bias (ns) Error (ns) Algonquin Fortleza Gilcreek Hobart Hartrao Kokee Wettzell Westford0.51 Tsukuba Nyales Onsala Matera Since we have no any a priori knowledge on the bias, we put a condition Actually, we experienced these biases were constant regardless with experiment series or date. We have to aware these delay differences are relatively exist.

Conclusions GIM/CODE can predict VTEC better than 10 % of its magnitude at present. 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. 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. 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<. High resolution GIMs in space and time is necessary for using it for delay rate correction (f<. VLBI Receiver bias was detected. VLBI Receiver bias was detected.