The ICRF, ITRF and VLBA

Chopo MaNASA’s Goddard Spaceflight Center

The ICRS is the idealized barycentric coordinate system to which celestial positions are referred. It is kinematically nonrotating with respect to the ensemble of distant extragalactic objects. It has no intrinsic orientation but was aligned close to the mean equator and dynamical equinox of J2000.0 for continuity with previous fundamental reference systems. Its orientation is independent of epoch, ecliptic or equator and is realized by a list of adopted coordinates of extragalactic sources.

International Celestial Reference System (ICRS)

The ICRF is a set of extragalactic objects whose adopted positions and uncertainties realize the ICRS axes and give the uncertainties of the axes. Note that the orientation of the ICRF catalog was carried over from earlier IERS radio catalogs and was within the errors of the standard stellar and dynamical frames at the time of adoption. Successive revisions of the ICRF are intended to minimize rotation from its original orientation. Other realizations of the ICRS have specific names (e.g., the Hipparcos Celestial Reference Frame). The current realization is designated ICRF2.

International Celestial Reference Frame (ICRF)

ICRF

• S/X data and analysis through 1995• ICRF-Ext.1, ICRF-Ext.2• 212 defining sources• Position uncertainty ≥ 250 µas• Accuracy of axes ~30 µas• Orientation independent of equator, ecliptic

and equinox

ICRF2

• S/X data through March 2009• 4540 sessions, 6.5 million observations• 3414 total sources• 295 defining sources• Position uncertainty ≥ 40 µas• Accuracy of axes ~10 µas• Orientation independent of equator, ecliptic

and equinox

ICRF2

VLBA Data Very Important

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Contribution of RDVs to IVS Data

RDV T2 R1&R4 Other

Importance of VLBA Sessions

• Impact on ICRF2• 28% of data• improved average position errors by 62% (α) and 54% (δ)• structure mapping (707) for selecting defining sources• densification – VCS (VLBA Calibrator Survey)

• Other astrometric uses of RDV sessions• observe 20-40 requested sources per year• re-observe 30-40 weak sources per year• provide snapshots of 80-90 sources per session - useful for time history

studies

Another Slide

The International Terrestrial Reference System (ITRS)

• The ITRS definition fulfills the following conditions:• It is geocentric, the center of mass being defined for the whole earth,

including oceans and atmosphere.• The unit of length is the metre (SI). Its orientation was initially given by the

BIH orientation at 1984.0.• The time evolution of the orientation is ensured by using a no-net-rotation

condition with regards to horizontal tectonic motions over the whole earth.•

The ITRS is realized by estimates of the coordinates and velocities of a set of stations observed by VLBI, LLR, GPS, SLR, and DORIS. Its name is International Terrestrial Reference Frame (ITRF).

ITRF2008 Network

VLBI sets ITRF scale

ITRF2008: Site distribution by Technique

RDV Sessions for ITRF

Precision of VLBA Results

Baseline Length Precision:

VLBA baselines have the lower scatterthan non-VLBA baselines especially for long baselines.

Time evolution (relative to mean lengths) of various VLBA baselines. Curvature in last plot is due to antenna tilt at Pietown.

VLBANon-VLBA

HN-FD wrms =3.7 mm

MK-SC wrms = 9.2 mm

KP-PI

mean length = 3,623,021.2526 m

mean length = 417,009.1252 mmean length = 8,611,584.6972

m

Importance of VLBA Sessions

• Impact on VLBI contribution to ITRF• 28% of data• largest networks - more than twice as large as R1s and R4s• highest precision baselines• improved non-VLBA site position precision by 30%

• Other geodetic uses of RDV sessions• supports monitoring of VLBA positions at 1-2 mm, sub-mm/yr• highest precision EOP measurements – information for FCN

VLBI 2010

• Current geodetic network: dual frequency S/X band– S-band delays contribution down by factor ( 8.4/2.3 )2 – Only need to concentrate on X-band source structure

• For VLBI 2010 – 24/7 operations– Reduced stochastic errors (i.e. troposphere, thermal,

clock drifts)• Source structure delay becomes more important

– 2 - 12 GHz feeds with four ~1 GHz optimally spaced bands

VLBI 2010

• Larger networks, faster slewing times, higher data rates, better uv coverage

• Should enable imaging of geodetic data and rapid determination of source structure corrections rapid determination of source structure corrections