The astrometric feasibility
and accuracy of VERA
Oyama, Tomoaki; Nakagawa, Akiharu; Kobayashi Hideyuki;
VERA team（National Astronomical Observatory of Japan）

1

The requirements of the VERA correlator

The correlation process spends within twice observation time at most.
( 8 hours observations, processing time within 16 hours)

VERA correlator process one or two narrow band width (16MHz or32MHz) with high frequency resolution (16.625kHz) and one wideband width with low resolution (250kHz) simultaneously.

The precise apriori model is needed to achieve a highly accurate
measurement with phase-referencing VLBI.

2Test observations (QSO pair)TargetReferenceSAPA

１J2218-0035

３C446

2.20

125

２

J1808+4542

OU+401

1.47

90

３J0831+0429

OJ038

2.050

４

NRAO512

３C345

0.5

80

Observations••••

Date 2004/11/～ 2009/2
Band K band
DIR2000(1Gbps, 16MHz*16) (all epoch) – VERA terminal
DIR1000(128Mbps, 16MHz*2）（2005/5/24）– VSOP terminal
3

The Comparison of apriori models
J2218-0335 phase referenced image(3C446 is reference source)
To achieve high accurateastrometry, apriori model is very
important. Therefore we havechecked and improved apriorimodels.

・ Two apriori models (fxcalc and DAP) developedby NAOJ have been used for VERA
（Jike et al.2005）

・ In a result of improvement, the difference of
fxcalc, DAP and CALC9 is only 10 psec(3mm),
mainly due to difference of tidal model.

The phase referenced image withuncorrected apriori model.

The corrected apriori model is used.
Self calibration is performed.
The scatter of the image is improved.However alittle scatter exist due to zenith atm delay offset

4Comparison between two different apriori
Dap (by honma,hirota) Vs Fxcalc (by jike,kurayama)

dap
Fxcalcdifference

ΔR.A (mas)0.12980.12620.0036

ΔDec (mas)0.21580.2205-0.0047

修正dap
The difference of the total delay
between dap and fxcalc is verysmalll. The value is within error
bar. Therefore, whichever apriori
we used, it is actually not soserious .

DapFxcalc5

The Comparison between K4 correlator andVERA 1Gbps FX correlator
To achieve high accurateastrometry and geodesy, total
delay and phase is very
important. Therefore thecomparison between
independent two correlators isneeded.
50psec

K4 correlator developed by NICT havebeen used for JADE* and IVS atGSI(Geographical survey institute).

12 hours


VERA terminal (DIR2000,1Gbps recorder)
have been used for VERA.

The difference of total delay between K4correlator and VERA 1Gbps FX correlator

The difference of the total delay between K4 correlator and VERA 1Gbps
FX correlator is only 50 psec(peak to peak) . This is in good agreement.
* ”JApanese Dynamic Earth observation by VLBI”

6

Test Observations(2 beam VS Single beam switching)

Phase referenced visibilities
J1753+4409(Target）

Phase referenced visibilities
J2218-03（Target）
Yellow: Single beam ,Blue : 2beam

7

Test Observations(2 beam VS Single beam switching)J1753+4409（Switching）
Peak Flux: 82.5 mJy

J1753+4409（2beam）
Peak flux：267.9 mJy

Position difference between 2beam and switching
RA. 118±50μas、Dec. -4.45±50μas（2beam-switching）

8

Test Observations (Astrometry)・By way of checking apriori models and correlator system, astrometric observations are considerabl

100μasOU+401&J1753+4409

OJ038&J0831+0429

3C446&J2218-0335

The accuracy of the direction of R.A is very high(about 10μas）.But thedirection of Dec is a little bad. Mainly due to apriori model error and zenith
atm delay offset?. The directions of offsets may be dependent on PA.
1.

To more improve the astrometric accuracy , improvement and verification ofgeophysical models ( ex., plate motion, ocean tide loading, non-tidal ocean loading,etc. ) are considerable.

2.

Moreover, we have to estimate the atmospheric zenith delay offset and apply the mostaccurate geophysical models.

9Several question
1) What? short period phase fluctuations（10 min～1 hour） to distort image
2) Why? the astrometric accuracy toward Dec ismore worse than toward R.A.
3) Are directions of offsets dependent on PA or RADEC?

10

Short period phase fluctuations（10m～1h） to distort image(Target Images J2218-0035、R05311A)

5 mas
1.3 JyNot phase reference
Fring searchSelfcal image0.47 Jy

Phase referenced image
Zenith atm delay offset
is not applied

0.96 Jy

Phase referenced and Zenith atm
Delay offset compensated image(MIZ 0.1nsec、IRK 0.06 nsec offset)

To compensate zenith atm delay, peak flux has improved by the twice.
however peak flux is 70% compared with self cal image⇒Need to check phases to be phase referenced.
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Short period phase fluctuations（10m～1h） to distort image( Target J2218-0335 (r05311a) phase to be compensated )Short phasefluctuations
10:00-12:00
expandMIZ 0.1nsec+IRK 0.06 nsec
Zenith atm dellay offset applied

Model = all components
1) Short period phase fluctuations distort image
2) Closure phase is stable as point source.
3) 2beam calibration table is stable.

Model = only center

4) Is it impossible to compensate short period
phase fluctuations using zenith atm delayof GPS data?
⇒Need to WVR, QSO pair obs hourly
and improve accuracy of GPS measurement
5) What are causes?
1. Time interval for fringe search ?
2, Accuracy of measurement of short atm fluctuations
using GPS ⇒next page3, Method of analysis?

Only fringe search (1beam)

2beam cal table

Method of zenith atm delay compensation
MSOLVE( geodetic VLBI by NAOJ ) VS GPS
The sequence of atm analysis for VERA･ GPS data + zenith atm delay offset which are applied to reference to phasevariation and image qualities.
IR-MZ
IR-OG
IR-IS

0.5

MSOLVE

0.12

0.3
0.2
0.1
0.0
-0.1
19.8

20.0

20.2

GPS0.15

Wet delay (m)dZatm (nsec)

0.4

IR-MZ
IR-OG
IR-IS

20.4

20.6

20.8

2006/Jan day

By Jike

0.09
0.06
0.03
0.00

0.1 nsec=3 cm-0.03-0.06-0.09
-0.12
19.6

19.8

20.0

20.2

20.4

20.6

20.8

2006/Jan day

The aspects of short and long phase variations are different between MSOLVE and GPS
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Zenith atm delay（Phase Fitting)

Fringe fitting (r05354c)

Clock (r05354c)
4 cm

5 cm

3 cm

4 cm

3 cm

ATM (r05354c) 60m

ATM (r06053b) By imai2005/11/25 鹿島34-つくば32基線
基準電波源へのモデルフィット

HYBSへのモデル適用

2時間

τ = τ 0 + τ(t − t0 ) − τ atm , x fm( El x ) + τ atm , y fm( El y )Why? the astrometric accuracy toward Dec ismore worse than toward R.A.
Problems and situations
 An aspect of long phase variation is different between SOLVE and GPS Accuracy is limited to 100μas（QSO pair、maser at several pc） due to lack of theaccuracy of estimation of zenith atm delay ?
 In case of near maser sources > also source structure ?（Orion、S-Crt、ρOｐｈ byhirota, nakagawa, imai et al. ）Other Status
 Accuracy of apriori have attained to several mm ( comparison to calc 9 ) VLBA astrometry have been attained to 10 micron accuracy toward (Xu et al .
2006 , Hachisuka et al. 2006）
Studying
 Comparison between Geometric observation and image optimization method.
(Honma et al in preparation)
 Studying the new method of geometric observations. ( Jike in this conf )16

Summary
 1-Gbit recording system and correlator are used

regularly. The good results of astrometry havebeen obtained.
 The accuracy of the direction of R.A is veryhigh(about 10μas）.

 The two questions have been discussing andstudying.
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