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Modul-8 : Sistem VLBI(Very Long Baseline Interferometry)

Lecture Slides of GD. 2213 Satellite GeodesyGeodesy & Geomatics Engineering

Institute of Technology Bandung (ITB)

Hasanuddin Z. AbidinGeodesy Research DivisionInstitute of Technology BandungJl. Ganesha 10, Bandung, IndonesiaE-mail : [email protected]

Version : April 2007

Very Long Baseline Interferometry (VLBI)

Teknik VLBI pertama kali dikembangkan dalam bidang radioastronomy dengan obyektif untuk mempelajari secara detil struktursumber-sumber gelombang radio di luar angkasa dengan resolusiketelitian angular yang tinggi.

Dalam bidang satelit geodesi, teknikVLBI dapat dipandang sebagai teknikpenentuan posisi relatif denganmenggunakan data fase dari gelombangradio yang dipancarkan oleh kuasar,yaitu benda langit pemancargelombang radio alamiah.

Dalam geodesi satelit, VLBI adalahteknik penentuan posisi relatifyang paling teliti.

Hasanuddin Z. Abidin, 1997

Contoh Stasion VLBI


Ref. : http://lupus.gsfc.nasa.gov/

Distribusi Stasion VLBI (2007)

http://ivscc.gsfc.nasa.gov/stations/ns-map.html

e-VLBI Network (2007)

www.haystack.mit.edu/.../evlbi/igrid2005.html

The radio astronomy satellite HALCA,launched by Japan's Institute for Spaceand Astronautical Science (ISAS) inFebruary 1997 into an orbit with inclinationof 31o , an apogee height of 21,400 kmabove Earth surface, a perigee height of560 km and an orbital period of 6.3 hours,is the main element of the VLBI SpaceObservatory Programme (VSOP),a complex international endeavor involvingover 25 ground radio telescopes, 5 trackingstations and 3 correlators.

The first dedicated space VLBI mission-VSOP (VLBI Space Observatory Programme)

Most VSOP observations are made of extragalactic radio sources to investigate thecompact cores and the milliarcsecond scale jets. These images can be complementary tothe ground VLBI observations in the reconstruction of an overall structure asdemonstrated in the study of a bright quasar PKS 1921-293, in which the jet emissionrevealed by the VSOP was not seen from the ground due either to its eakness at highfrequency (millimeter-wavelength), or to the poor resolution of the ground array at low-frequency http://www.asiaa.sinica.edu.tw/research/radio/extragalactic/vlbi.htm

Group delay (data utama )

Phase delay

Delay rate

DATAPENGAMATAN

B

S

S

kuasar

c.t

jamatom

regis -trasi

regis -trasi

jamatom

korelator

VLBI VLBI

Group delay (data utama )

Phase delay

Delay rate

PrinsipDasarVLBI


c.t = B . S

Ref. Seeber (1993)

Persamaan Dasar VLBI (1)


Ref. Cannon (1999)

c.tg(t) = B . S(t)

c.t

B

Squasar

tobs = tg + tclock + tinst + ttrop +tionos + trel + ….

tobs = delay hasil pengamatantg = delay geometriktclock = bias delay karena tidak sinkronnya jamtinst = bias delay dalam instrumenttrop = bias delay karena refraksi troposfirtionos = bias delay karena refraksi ionosfirtrel = bias delay efek relativitas

dimana :

Persamaan Dasar VLBI (2)

Hasanuddin Z. Abidin, 1999Ref. Seeber (1993)

c.t

B

Squasar

B . S(t) = bx.cos s.cos hs +by.cos s.sin hs +bz.sin s

(bx,by,bz) = komponen vektor baseline B(s,s) = spherical equatorial coordinates dari sumber gelombang radio

hs = GST - s = sudut waktu (terhadap Greenwich)dari sumber gelombang radio (quasar)

dimana :

c.tg(t) = B . S(t)

Bentuk kuasartidaklah seperti

objek titik

Contoh Bentuk Kuasar


! A Catalog of 2118 Compact Radio Sources in the Northern Sky!! SUMMARY: A catalog of 2118 compact radio sources was derived from! the Jodrell Bank - VLA Astrometric Survey (JVAS) of flat-spectrum! sources. Each compact VLA source (a) has a peak flux density at! 8.4 GHz >= 50 mJy at a resolution of 200 milliarcsec; (b) contains! 80% or more of the total source flux density; and (c) has a position! known to an rms accuracy of 12-55 milliarcsec. The 2118 sources are! uniformly distributed in the northern sky at Galactic latitudes! |b| >= 2.5 degrees. The catalog can be read by the NRAO VLBI! scheduling program SCHED.!! DEFINITIONS:! epoch : J2000.! calcode: M for the Jodrell Bank MERLIN.! source : Name derived from the J2000 position.! ra : Right ascension of the source at epoch J2000.! dec : Declination of the source at epoch J2000.! rem : Remark giving a code for the published reference plus the! total source flux density at 8.4 GHz quoted in that reference.! Code JVAS1:! Reference : Patnaik et al. 1992, mnras, 254, 655! Observing date: 1990 February! Position error: 12 milliarcsec rms! Code JVAS2:! Reference : Browne et al. 1998, mnras, 293, 257! Observing date: 1992 October! Position error: 14 milliarcsec rms! Code JVAS3:! Reference : Wilkinson et al. 1998, mnras, 300, 790! Observing date: 1991 June! Position error: 55 milliarcsec rms (See footnotes in JVAS3 paper! regarding smaller [*] and larger [+] errors.)!source='J0000+4054' ra=00:00:53.0815 dec= 40:54:01.806 rem='JVAS1 379 mJy' /source='J0001+1914' ra=00:01:08.6226 dec= 19:14:33.816 rem='JVAS2 387 mJy' /source='J0003+1837' ra=00:03:01.1613 dec= 18:37:59.039 rem='JVAS2 68 mJy' /source='J0003+2129' ra=00:03:19.3497 dec= 21:29:44.436 rem='JVAS3 259 mJy' /source='J0003+4807' ra=00:03:46.0413 dec= 48:07:04.134 rem='JVAS1 75 mJy' /source='J0004+4615' ra=00:04:16.1293 dec= 46:15:17.968 rem='JVAS1 220 mJy' /

KATALOG RADIO SOURCES

Aplikasi-Aplikasi VLBI

Realisasi Kerangka ReferensiKoordinat.

Penentuan ERP(Earth Rotation Parameters).

Studi Geodinamika.


Dalam bidang geodesi,aplikasi teknik VLBIterutama adalah untukhal-hal berikut :

Parameter VLBI GPS SLR

Pergerakan kutub 0.2 mas 0.1 mas 0.4 mas

Laju pergerakankutub

? 0.15 mas/hari ?

UT1 0.01-0.03 ms ? ?

LOD 0.02 ms 0.02 ms 0.06 ms

Nutasi 0.15 mas 0.2 mas,periode 10 hari

-

Tingkat Ketelitian VLBI[Herring, 2000]



KetelitianBaseline VLBI

Ref.: Seeber (1993)

Ref : ftp://gemini.gsfc.nasa.gov/ Ref.: Lambeck (1988)

Baseline Westford - Fort Davis

Teknik Posisi - 3D(mm)

Kecepatan(mm/tahun)

VLBI 10 2

SLR 14 3

GPS 10 3

DORIS 34 9

WRMS = weighted rms (root mean squares)

Ketelitian Tipikal[Altamimi, 2000]


Aplikasi VLBI : Penentuan ERP


Standar deviasi yang diperoleh (Campbell, 1992)

0.001” untuk xp,yp 0.1 ms untuk UT1

600 500 400 300 200 100

-300

-200

-100

0

100

200

300

Y(0.001”)

X(0

.001”)

600 500 400 300 200 100

-300

-200

-100

0

100

200

300

Y(0.001”)

X(0

.001”)

Aplikasi VLBI : Geodinamika

79 81 83 85 87 89 91 93 95

Tahun

40

30

20

10

0

Milia

rcseco

nd

s

79 81 83 85 87 89 91 93 95

Tahun

40

30

20

10

0

Milia

rcseco

nd

sNutasi dari VLBI

[NASA-GSFC,2000]


Results ofSpace VLBI

http://www.isas.jaxa.jp/e/enterp/missions/halca/achiev.shtml

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