The ICRF, ITRF and VLBA Chopo Ma NASA’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 J 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 defining sources Position uncertainty ≥ 250 µas Accuracy of axes ~30 µas Orientation independent of equator, ecliptic and equinox

ICRF2 S/X data through March 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

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 requested sources per year re-observe weak sources per year provide snapshots of 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 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).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 scatter than 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. VLBA Non-VLBA HN-FD wrms =3.7 mmMK-SC wrms = 9.2 mmKP-PI mean length = 3,623, m mean length = 417, m mean length = 8,611, 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 – 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