Evaluation of Global Ionosphere TEC by comparison with VLBI data

Mamoru Sekido, Tetsuro KondoMamoru Sekido, Tetsuro Kondo

Eiji Kawai, and Michito ImaeEiji Kawai, and Michito Imae

Motivation External Ionospheric delay correction is necessarExternal Ionospheric delay correction is necessar

y for single frequency VLBI astrometry (e.g. Puly for single frequency VLBI astrometry (e.g. Pulsar 1.4-2GHz)sar 1.4-2GHz)

GPS network and technology is growing rapidly.GPS network and technology is growing rapidly. Global Ionosphere map(GIM) is available at IGlobal Ionosphere map(GIM) is available at I

GS analysis centers.GS analysis centers. Global Ionosphere Map (GIM) is useful forGlobal Ionosphere Map (GIM) is useful for

VLBI Astrometry (Pulsar etc…)VLBI Astrometry (Pulsar etc…) Radiometric measurements(Spacecraft NavigaRadiometric measurements(Spacecraft Naviga

tion, SELENE Projecttion, SELENE Project(0.2mm/s)(0.2mm/s), …), …) ,if it has enough accuracy!!.,if it has enough accuracy!!.

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Propagation delay 2 fTECAION

Content of the presentation1.1. Comparison of GIM/CODE and VLBI TECComparison of GIM/CODE and VLBI TEC

Statistical comparison about error of GIMStatistical comparison about error of GIM Conclusion: Conclusion:

Bias error ~3TECU. Need more precise GIM Bias error ~3TECU. Need more precise GIM including fine structure for correction.including fine structure for correction.

2.2. Comparison on TEC rateComparison on TEC rate Conclusion: GIM is not good for correction for Conclusion: GIM is not good for correction for

delay rate in VLBI at present.delay rate in VLBI at present.

3.3. Trial of using Japanese dense GPS network for TEC.Trial of using Japanese dense GPS network for TEC.

4.4. VLBI receiver bias– by productVLBI receiver bias– by product It was not known until TEC comparison.It was not known until TEC comparison.

Global Ionosphere Map(GIM) produced by GPS observations

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

Benefit of GIM/CODEBenefit of GIM/CODE Daily 12(13) GIMs with 2 hours interval and relateDaily 12(13) GIMs with 2 hours interval and relate

d subroutines are available anytime by FTP.d subroutines are available anytime by FTP. It is expressed with 12(15)-deg.8(15)-ord. Spherical It is expressed with 12(15)-deg.8(15)-ord. Spherical

Harmonic expansionHarmonic expansion No data interruptions since 1995No data interruptions since 1995

Global Ionosphere Map

IGS sites used for GIM/CODE

GIM/CODE 12(13) GIMs with 2 hours interval12(13) GIMs with 2 hours interval Expression in 15-deg.15-ord. (12-deg./8-orExpression in 15-deg.15-ord. (12-deg./8-or

d.) Spherical Harmonics expansiond.) Spherical Harmonics expansion Spherical single layer shell modelSpherical single layer shell model

Single Layer Mapping FunctionSingle Layer Mapping Function

Modified SLMModified SLM

)sin()'sin( ,)'cos(

1)( z

HR

Rz

zzFm

)sin()'sin( ,)'cos(

1)( z

HR

Rz

zzFm

zz’

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TEC Comparison GIM/CODE-VLBI True Ionosphere

TEC

GIM/CODE

VTEC

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How these comparison can evaluate error of the TEC MAP?

)()( ,, xxGIMxyyGIMyGIM

VLBIVLBI

ElFmeVTECElFmeVTECdTEC

edTECdTEC

) baseline Long( )()(

) baselineShort ( )(2

2222

222,

2

yxVLBIyxGIM

yxVLBIuGIM

GIMVLBI

zzElFmElFm

zzElFm

dTECdTEC

known

known

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VLBI data used for ComparisonsDateDate ExperimentExperiment

7-18 Apr.20007-18 Apr.2000 KSP-VLBIKSP-VLBI

5-6 Jul.20005-6 Jul.2000 NEOS-A375NEOS-A375

10-11 Jul 200010-11 Jul 2000 CORE-1001CORE-1001

11-12 Jul 200011-12 Jul 2000 NEOS-A376NEOS-A376

12-13 Jul. 200012-13 Jul. 2000 CORE-3001CORE-3001

18-19 Jul. 200018-19 Jul. 2000 NEOS-A377NEOS-A377

25-26 jul 200025-26 jul 2000 NEOS-A378NEOS-A378

2683 Scans of KSP(100km)6855 scans of CORE & NEOS

GIM/CODE-VLBIKashima-Koganei(100km)

CorrelationCorrelation 0.920.92

Prop.cffProp.cff 0.870.87

OffsetOffset -3.1-3.1TECUTECU

GIM – El relation (100km)

) baselineShort ( )(2 22

,

22

yxuGIM

VLBIGIMVLBI

zzzFm

dTECdTEC

GIM,u

=0.3 –0.6TECU

GIM/CODE-VLBIAlgonquin-Wettzell (6000km)

CorrelationCorrelation 0.990.99

Prop.cffProp.cff 1.131.13

OffsetOffset 57.657.6TECUTECU

GIM – El relation (long baseline)

) baseline Long( )()( 222

22

yxyxGIM

VLBIGIMVLBI

zzElFmElFm

dTECdTEC

GIM

=1-7TECU

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GIM – Baseline length relation

GIM can be used to predict better than 10% of Ionospheric delay in VLBI observation

El, Baseline lengthdependency

Baseline: • 0-500km• 500-4000km• 4000-8000km• 8000km -

El Cut off test• El >=20 deg.• El >=40 deg.• El >=60 deg.

2GIM(baseline length)

=Structure function of GIM error

)()()( where,C(l)-C(0)2

))()(()( 2

lxerxerlC

lxerxerlR

Baseline

AssumptionError of GIM is isotropic

Error Spectrum of GIM

AssumptionError of GIM is isotropic

28.4(GHz)

(TECU) 05.0ps1

f

21.4(GHz)

(TECU) 015.0ps 10

f

Delay (TEC) rate comparison

Correlation on TEC rate was Correlation on TEC rate was low even on long baselinelow even on long baseline

Reasons will beReasons will be Low spatial resolutionLow spatial resolution

2500x1700km2500x1700km Low time resolutionLow time resolution

2 hours interval.2 hours interval.

2

414- 8.4(GHz)(TECU) 105sec)(sec/01

f

Algonquin - Wettzell

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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

Regional Ionosphere Map with GEONET and GIM/CODE Dr. Ping in Mizusawa /NAOJ aDr. Ping in Mizusawa /NAOJ a

nd Dr. Saito in Kyoto Univ. are nd Dr. Saito in Kyoto Univ. are trying to use GEONET (about 1trying to use GEONET (about 1000 GPSs in Japan) to make pre000 GPSs in Japan) to make precise TECMAP.cise TECMAP.

High Time and Spatial resolutioHigh Time and Spatial resolution (60 deg. 10min.)n (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 Bias comes from VLBI receiver delay difference receiver delay difference between X and S. between X and S.

It used to be absorbed in It used to be absorbed in clock offset and be not clock offset and be not aware so far aware so far ( It has been ( It has been pointed out by T. Herrings).pointed out by T. Herrings).

)(

)(

,

,

xxGIMx

yyGIMyGIM

VLBIVLBI

ElFmeVTEC

ElFmeVTECdTEC

edTECdTEC

BiasTEC

VLBI (S/X) receiver biasesStationStation Bias Bias

(ns)(ns)Error Error (ns)(ns)

AlgonquinAlgonquin -0.43-0.43 0.40.4

FortlezaFortleza -5.3-5.3 0.40.4

GilcreekGilcreek 0.130.13 0.40.4

HobartHobart 1.21.2 0.60.6

HartraoHartrao -16.5-16.5 0.60.6

KokeeKokee -18.4-18.4 0.40.4

WettzellWettzell 13.613.6 0.40.4

WestfordWestford 0.50.5 11

TsukubaTsukuba 5.65.6 0.60.6

NyalesNyales -0.4-0.4 0.60.6

OnsalaOnsala 14.414.4 0.80.8

MateraMatera 5.55.5 0.50.5

0, i

ibias

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 % GIM/CODE can predict VTEC better than 10 %

of its magnitude at present.of its magnitude at present. GIM/CODE seems to have RMS error ~3 TECU GIM/CODE seems to have RMS error ~3 TECU

at low spatial frequency.at low spatial frequency. About 100 degrees of SH model might be necessAbout 100 degrees of SH model might be necess

ary to achieve the same accuracy with S/X VLBI.ary to achieve the same accuracy with S/X VLBI. High resolution GIMs in space and time is necessHigh resolution GIMs in space and time is necess

ary for using it for delay rate correction (f<.ary for using it for delay rate correction (f<. VLBI Receiver bias was detected.VLBI Receiver bias was detected.