View source for International Celestial Reference System and its realizations

{{short description|Current standard celestial reference system and frame}}
{{Use dmy dates|date=September 2019}}

The '''International Celestial Reference System''' ('''ICRS''') is the current standard [[celestial reference system]] adopted by the [[International Astronomical Union]] (IAU). Its origin is at the [[barycenter]] of the [[Solar System]], with axes that are intended to "show no global rotation with respect to a set of distant extragalactic objects".<ref name = "IAU91"/><ref name = "IAU97"/> This fixed reference system differs from previous reference systems, which had been based on [[Catalogues of Fundamental Stars]] that had published the positions of stars based on direct "observations of [their] [[Equatorial coordinate system|equatorial coordinates]], right ascension and declination"<ref>{{ Citation | last1 = Walter | first1 = Hans G. | last2 = Sovers | first2 = Oscar J. | date = 2000 | title = Astrometry of Fundamental Catalogues: The Evolution from Optical to Radio Reference Frames | publisher = Springer | place = Berlin | page = 1 | isbn = 9783540674368 }}</ref> and had adopted as "privileged axes ... the mean equator and the dynamical equinox" at a particular [[Epoch (astronomy)#Date-references for coordinate systems|date and time]].<ref>{{ Citation | last1 = Arias | first1 = E.F. | last2 = Charlot | first2 = P. | last3 = Feissel | first3 = M. | last4 = Lestrade | first4 = J.-F. | date = 1995 | title = The Extragalactic Reference System of the International Earth Rotation Service, ICRS | journal = Astronomy and Astrophysics | volume = 303 | pages = 604–608 | bibcode = 1995A&A...303..604A | url = https://adsabs.harvard.edu/pdf/1995A%26A...303..604A}}</ref>

The '''International Celestial Reference Frame''' ('''ICRF''') is a realization of the International Celestial Reference System using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a [[frame of reference|reference ''frame'']] (RF) is the physical realization of a reference ''system,'' i.e., the reference frame is the set of numerical coordinates of the reference sources, derived using the procedures spelled out by the ICRS.<ref name="USNO">{{cite web|title=International Celestial Reference System (ICRS)|url=https://aa.usno.navy.mil/faq/ICRS_doc |access-date= 23 June 2022 |website=aa.usno.navy.mil|publisher=US Naval Observatory |language=en}}</ref>

More specifically, the ICRF is an [[inertial frame of reference|inertial]] [[Barycentric coordinates (astronomy)|barycentric]] reference frame whose axes are defined by the measured positions of [[extragalactic astronomy|extragalactic sources]] (mainly [[quasar]]s) observed using [[very-long-baseline interferometry]] while the ''Gaia''-CRF is an inertial barycentric reference frame defined by optically measured positions of extragalactic sources by the [[Gaia (spacecraft)|''Gaia'' satellite]] and whose axes are rotated to conform to the ICRF. Although [[general relativity]] implies that there are no true inertial frames around [[gravitation|gravitating]] bodies, these reference frames are important because they do not exhibit any measurable angular [[rotation]] since the extragalactic sources used to define the ICRF and the ''Gaia''-CRF are so far away. The ICRF and the ''Gaia''-CRF are now the standard reference frames used to define the positions of [[astronomical object]]s.<ref name = "IAU21"/>

==Reference systems and frames==

It is useful to distinguish reference systems and reference frames. A reference frame has been defined as "a catalogue of the adopted coordinates of a set of reference objects that serves to define, or realize, a particular coordinate frame".<ref name = "Wilkins_90">{{Cite journal| last = Wilkins | first = G.A. | editor-last = Lieske | editor-first = J. H. | editor2-last = Abalakin | editor2-first = V. K | date = 1990 | title = The Past, Present and Future of Reference Systems for Astronomy and Geodesy | journal = Symposium - International Astronomical Union | volume = 141 | publisher = Cambridge University Press | pages = 39–46 | doi = 10.1017/S0074180900086149 }}</ref> A reference system is a broader concept, encompassing "the totality of procedures, models and constants that are required for the use of one or more reference frames".<ref name = "Wilkins_90"/><ref name = USNO_BKGD>{{cite web |title=International Celestial Reference System (ICRS) |url=https://aa.usno.navy.mil/faq/ICRS_doc#BKGD |access-date=24 June 2022|publisher=U.S. Naval Observatory, Astronomical Applications | quote = A reference system ... defines the origin and fundamental planes (or axes) of the coordinate system. It also specifies all of the constants, models, and algorithms used to transform between observable quantities and reference data that conform to the system. A reference frame consists of a set of identifiable fiducial points on the sky (specific astronomical objects), along with their coordinates, that serves as the practical realization of a reference system.}}</ref>

==Realizations==

The ICRF is based on hundreds of [[Astronomical radio source|extra-galactic radio sources]], mostly [[quasar]]s, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured very accurately by [[Very Long Baseline Interferometry]] (VLBI). The positions of most are known to 1 [[Minute and second of arc#Symbols and abbreviations|milliarcsecond (mas)]] or better.<ref>{{cite web |title=International Celestial Reference System (ICRS) |url=https://aa.usno.navy.mil/faq/ICRS_doc |access-date=23 June 2022|publisher=U.S. Naval Observatory, Astronomical Applications}}</ref>

In August 1997, the [[International Astronomical Union]] resolved in Resolution B2 of its XXIIIrd General Assembly "that the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths."<ref name = "IAU21" /> The ''Hipparcos'' Celestial Reference Frame (HCRF) is based on a subset of about 100,000 stars in the [[Hipparcos Catalogue|''Hipparcos'' Catalogue]].<ref>{{cite web|title=International Celestial Reference System (ICRS)|url=https://aa.usno.navy.mil/faq/ICRS_doc#FROPT|access-date=22 June 2022|publisher=U S Naval Observatory, Astronomical Applications Department}}</ref> In August 2021 the International Astronomical Union decided in Resolution B3 of its XXXIst General Assembly "that as from 1 January 2022, the fundamental realization of the International Celestial Reference System (ICRS) shall comprise the Third Realization of the International Celestial Reference Frame (ICRF3) for the radio domain and the Gaia-CRF3 for the optical domain."<ref name= "IAU21">{{cite web | url = http://www.iau.org/static/archives/announcements/pdf/ann21040c.pdf | title = Resolution B3, On the Gaia Celestial Reference Frame | author= The XXXIst General Assembly of the International Astronomical Union | date = 2021 | website = Resolutions adopted at the General Assemblies | publisher = International Astronomical Union | access-date = 9 June 2022 | quote = }}</ref>

===Radio wavelengths (ICRF)===

====ICRF1====

The ICRF, now called ICRF1, was adopted by the [[International Astronomical Union|International Astronomical Union (IAU)]] as of 1 January 1998.<ref name = "IAU97" /> ICRF1 was oriented to the axes of the ICRS, which reflected the prior astronomical reference frame [[Fifth Fundamental Catalog|The Fifth Fundamental Catalog (FK5)]]. It had an angular [[noise floor]] of approximately 250 [[microarcsecond]]s (μas) and a reference axis stability of approximately 20 μas; this was an order-of-magnitude improvement over the previous reference frame derived from (FK5).<ref>{{ Citation | last1 = Ma | first1 = C. | last2 = Arias | first2 = E. F. | last3 = Eubanks | first3 = T. M. | display-authors = etal  | date = July 1998 | title = The International Celestial Reference Frame as Realized by Very Long Baseline Interferometry | journal = Astronomical Journal | volume = 116 | issue = 1 | pages = 516–546 | doi = 10.1086/300408 | s2cid = 120005941 | doi-access = free | bibcode = 1998AJ....116..516M }}</ref><ref name=ICRF2_tn35>{{cite web|title=IERS Technical Note No. 35: The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry|url=https://www.iers.org/SharedDocs/Publikationen/EN/IERS/Publications/tn/TechnNote35/tn35.pdf?__blob=publicationFile&v=1|publisher=International Earth Rotation and Reference Systems Service (IERS)|access-date=5 April 2014|url-status=dead|archive-url=https://web.archive.org/web/20150725164148/http://ww2.iers.org/SharedDocs/Publikationen/EN/IERS/Publications/tn/TechnNote35/tn35.pdf?__blob=publicationFile&v=1%2CtemplateId%3Draw%2Cproperty%3DpublicationFile.pdf%2Ftn35.pdf|archive-date=25 July 2015}}</ref> The ICRF1 contains 212 defining sources and also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue. ICRF1 agrees with the orientation of the Fifth Fundamental Catalog (FK5) "[[J2000.0]]" frame to within the (lower) precision of the latter.<ref name = "IAU97" />

====ICRF2====

An updated reference frame ICRF2 was created in 2009.<ref name=ICRF2_tn35/><ref name="Steigerwald2009">{{cite web |last1=Steigerwald |first1=Bell |title=NASA - New Celestial Map Gives Directions for GPS |url=https://www.nasa.gov/centers/goddard/news/topstory/2009/icrf2.html |website=www.nasa.gov |publisher=NASA |access-date=5 June 2018 |language=en |archive-date=3 December 2020 |archive-url=https://web.archive.org/web/20201203205556/https://www.nasa.gov/centers/goddard/news/topstory/2009/icrf2.html |url-status=dead }}</ref> The update was a joint collaboration of the [[International Astronomical Union]], the [[International Earth Rotation and Reference Systems Service]], and the [[International VLBI Service for Geodesy and Astrometry]].<ref name="Fey2009">{{cite web |last1=Fey |first1=Alan L. |title=The International Celestial Reference Frame |url=https://aa.usno.navy.mil/faq/ICRS_doc | website=usno.navy.mil | publisher=US Naval Observatory (USNO) | access-date=23 June 2022}}</ref> ICRF2 is defined by the position of 295 compact radio sources (97 of which also define ICRF1). Alignment of ICRF2 with ICRF1-Ext2, the second extension of ICRF1, was made with 138 sources common to both reference frames. Including non-defining sources, it comprises 3414 sources measured using [[very-long-baseline interferometry]]. The ICRF2 has a noise floor of approximately 40&nbsp;μas and an axis stability of approximately 10&nbsp;μas. Maintenance of the ICRF2 will be accomplished by a set of 295 sources that have especially good positional stability and unambiguous spatial structure.<ref name="boboltz">{{cite journal |last1=Boboltz |first1=David A. |last2=Gaume |first2=R. A. |last3=Fey |first3=A. L. |last4=Ma |first4=C. |last5=Gordon |first5=D. |title=The Second Realization of the International Celestial Reference Frame (ICRF2) by Very Long Baseline Interferometry |journal=American Astronomical Society Meeting Abstracts #215 |url=https://ui.adsabs.harvard.edu/abs/2010AAS...21546906B/abstract |publisher=AAS |access-date=18 July 2022 |pages=469.06 |date=1 January 2010|volume=215 |bibcode=2010AAS...21546906B }}</ref>

The data used to derive the reference frame come from approximately 30 years of VLBI observations, from 1979 to 2009.<ref name=ICRF2_tn35/> Radio observations in both the S-band (2.3&nbsp;GHz) and X-band (8.4&nbsp;GHz) were recorded simultaneously to allow correction for [[ionospheric]] effects. The observations resulted in about 6.5 million group-delay measurements among pairs of telescopes. The group delays were processed with software that takes into account atmospheric and geophysical processes. The positions of the reference sources were treated as unknowns to be solved for by minimizing the [[mean squared error]] across group-delay measurements. The solution was constrained to be consistent with the [[International Terrestrial Reference Frame]] (ITRF2008) and [[earth orientation parameters]] (EOP) systems.<ref name="Fey2015">{{cite journal |last1=Fey |first1=A. L. |last2=Gordon |first2=D. |last3=Jacobs |first3=C. S. |last4=Ma |first4=C. |last5=Gaume |first5=R. A. |last6=Arias |first6=E. F. |last7=Bianco |first7=G. |last8=Boboltz |first8=D. A. |last9=Böckmann |first9=S. |last10=Bolotin |first10=S. |last11=Charlot |first11=P. |last12=Collioud |first12=A. |last13=Engelhardt |first13=G. |last14=Gipson |first14=J. |last15=Gontier |first15=A.-M. |last16=Heinkelmann |first16=R. |last17=Kurdubov |first17=S. |last18=Lambert |first18=S. |last19=Lytvyn |first19=S. |last20=MacMillan |first20=D. S. |last21=Malkin |first21=Z. |last22=Nothnagel |first22=A. |last23=Ojha |first23=R. |last24=Skurikhina |first24=E. |last25=Sokolova |first25=J. |last26=Souchay |first26=J. |last27=Sovers |first27=O. J. |last28=Tesmer |first28=V. |last29=Titov |first29=O. |last30=Wang |first30=G. |last31=Zharov |first31=V. |title=The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry |journal=The Astronomical Journal |date=24 July 2015 |volume=150 |issue=2 |pages=58 |doi=10.1088/0004-6256/150/2/58 |bibcode=2015AJ....150...58F |hdl=11603/17528 |s2cid=3281444 |url=https://iopscience.iop.org/article/10.1088/0004-6256/150/2/58 |issn=1538-3881|hdl-access=free }}</ref>

====ICRF3====
[[File:ICRF3_Figure_13_Defining_Sources_Only.png|thumb|Sky distribution of the 303 "defining sources" in the ICRF3]]
ICRF3 is the third major revision of the ICRF, and was adopted by the IAU in August 2018 and became effective 1 January 2019. The modeling incorporates the effect of the [[Aberration (astronomy)#Secular aberration|galactocentric acceleration]] of the solar system, a new feature over and above ICRF2. ICRF3 also includes measurements at three frequency bands, providing three independent, and slightly different, realizations of the ICRS: dual frequency measurements at 8.4&nbsp;GHz ([[X band|''X'' band]]) and 2.3&nbsp;GHz ([[S band|''S'' band]]) for 4536 sources; measurements of 824 sources at 24&nbsp;GHz ([[K band (IEEE)|''K'' band]]), and dual frequency measurements at 32&nbsp;GHz ([[Ka band|''Ka'' band]]) and 8.4&nbsp;GHz (''X'' band) for 678 sources. Of these, 303 sources, uniformly distributed on the sky, are identified as "defining sources" which fix the axes of the frame. ICRF3 also increases the number of defining sources in the southern sky.<ref>{{Cite journal | last1 = Charlot | first1 = P. | last2 = Jacobs | first2 = C.S. | last3 = Gordon | first3 = D. | display-authors = etal | date = 2020 | title = The third realization of the International Celestial Reference Frame by very long baseline interferometry | journal = Astronomy and Astrophysics | volume =  644 | issue = A159 | pages = A159 | doi = 10.1051/0004-6361/202038368 | s2cid = 225068756 | url = https://www.aanda.org/articles/aa/abs/2020/12/aa38368-20/aa38368-20.html | accessdate = 16 June 2022 | doi-access = free | arxiv = 2010.13625 | bibcode = 2020A&A...644A.159C }}</ref><ref>{{cite web |url=http://hpiers.obspm.fr/icrs-pc/newwww/icrf/index.php |title=The ICRF |website=IERS ICRS Center |publisher=Paris Observatory |access-date=25 December 2018 }}</ref><ref>{{cite web |url=https://www.iers.org/IERS/EN/Science/ICRS/ICRS.html |title=The International Celestial Reference System (ICRS) |website=International Earth Rotation and Reference Systems Service |access-date=11 February 2020 }}</ref>

===Optical wavelengths===

====''Hipparcos'' Celestial Reference Frame (HCRF)====

In 1991 the [[International Astronomical Union]] recommended "that observing programmes be undertaken or continued in order to ... determine the relationship between catalogues of extragalactic source positions and ... the [stars of the] [[FK5]] and [[Hipparcos catalogue]]s."<ref name = "IAU91" >{{cite web | url = https://www.iau.org/static/resolutions/IAU1991_French.pdf | title = Resolution No. A4; Recommendations from the Working Group on Reference System | author = The XXIst General Assembly of the International Astronomical Union | date = 1991 | website = Resolutions adopted at the General Assemblies | publisher = International Astronomical Union | access-date = 13 June 2022 | quote = }}</ref> Using a variety of linking techniques, the coordinate axes defined by the ''Hipparcos'' catalogue were aligned with the extragalactic radio frame.<ref>{{ Cite journal | last1 = Kovalevsky | first1 = J. | last2 = Lindegren | first2 = L. | last3 = Perryman | first3 = M.A.C. | display-authors = etal | year = 1997 | title = The Hipparcos Catalogue as a realisation of the extragalactic reference system | journal = Astronomy and Astrophysics | volume = 323 | pages = 620–633 | url = https://hal.archives-ouvertes.fr/hal-02053947/ | accessdate = 20 June 2022 }}</ref> In August 1997, the International Astronomical Union recognized in Resolution B2 of its XXIIIrd General Assembly "That the Hipparcos Catalogue was finalized in 1996 and that its coordinate frame is aligned to that of the frame of the extragalactic sources [ICRF1] with one sigma uncertainties of ±0.6 milliarcseconds (mas)" and resolved "that the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths."<ref name = "IAU97" >{{cite web | url = https://www.iau.org/static/resolutions/IAU1997_French.pdf | title = Resolution No B2; On the international celestial reference system | author = The XXlIIrd General Assembly of the International Astronomical Union | date = 1997 | website = Resolutions adopted at the General Assemblies | publisher = International Astronomical Union | access-date = 13 June 2022 | quote = }}</ref>

====Second ''Gaia'' celestial reference frame (''Gaia''–CRF2)====

The second ''Gaia'' celestial reference frame (''Gaia''–CRF2), based on 22 months of observations of over half a million extragalactic sources by the [[Gaia (spacecraft)#Data releases|''Gaia'' spacecraft]], appeared in 2018 and has been described as "the first full-fledged optical realisation of the ICRS, that is to say, an optical reference frame built only on extragalactic sources." The axes of ''Gaia''-CRF2 were aligned to a prototype version of the forthcoming ICRF3 using 2820 objects common to ''Gaia''-CRF2 and to the ICRF3 prototype.<ref>{{ Citation | last1 = Gaia Collaboration | last2 = Mignard | first2 = F.  | last3 = Klioner | first3 = S. | last4 = Lindegren | first4 = L. |display-authors=etal | date = 2018 | title = ''Gaia'' Data Release 2. The celestial reference frame (''Gaia''-CRF2) | journal = Astronomy & Astrophysics | volume = 616 | issue =  A14 | pages = A14 | doi = 10.1051/0004-6361/201832916 | arxiv = 1804.09377 | bibcode = 2018A&A...616A..14G | s2cid = 52838272 }}</ref><ref>{{ Citation | last1 = Lindegren | first1 = L. | last2 = Hernandez | first2 = J. | last3 = Bombrun | first3 = A. | last4 = Klioner | first4 = S. |display-authors=etal | date = 2018 | title = ''Gaia'' Data Release 2 – The astrometric solution | journal = Astronomy & Astrophysics | volume = 616 | issue = A2 | pages = A2 | doi = 10.1051/0004-6361/201832727 | arxiv = 1804.09366 | bibcode = 2018A&A...616A...2L | s2cid = 54497421 }}</ref>

====Third ''Gaia'' celestial reference frame (''Gaia''–CRF3)====

The third ''Gaia'' celestial reference frame (''Gaia''–CRF3) is based on 33 months of observations of 1,614,173 extragalactic sources. As with the earlier ''Hipparcos'' and ''Gaia'' reference frames, the axes of ''Gaia''-CRF3 were aligned to 3142 optical counterparts of ICRF-3 in the ''S/X'' frequency bands.<ref>{{ Citation | author = Gaia Collaboration | date = 2022 | title = Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3) | journal = Astronomy & Astrophysics | volume = 667 | doi = 10.1051/0004-6361/202243483 | s2cid = 248405779 | doi-access = free | arxiv = 2204.12574 | bibcode = 2022A&A...667A.148G }}</ref><ref name="EDR3">{{ Cite web |website = ESA | title = Gaia Early Data Release 3 (Gaia EDR3) | url = https://www.cosmos.esa.int/web/gaia/earlydr3 | access-date = 12 December 2020 }}</ref> In August 2021 the International Astronomical Union noted that the ''Gaia''-CRF3 had "largely superseded the Hipparcos Catalogue" and was "de facto the optical realization of the Celestial Reference Frame within the astronomical community." Consequently, the IAU decided that ''Gaia''-CRF3 shall be "the fundamental realization of the International Celestial Reference System (ICRS) ... for the optical domain."<ref name= "IAU21"/>

==See also==
* [[Astrometry]]
* [[Astronomy]]
* [[Barycentric and geocentric celestial reference systems]]
* [[International Terrestrial Reference System and Frame]]

==References==
{{reflist}}

==Further reading==
* Kovalevsky, Jean; Mueller, Ivan Istvan; Kołaczek, Barbara (1989) ''Reference Frames in Astronomy and Geophysics'', ''Astrophysics and Space Science Library'', Volume 154 Kluwer Academic Publishers {{ISBN|9780792301820}}
* {{cite tech report | title=IERS Technical Notes - The International Celestial Reference System and Frame | url=https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn34.html?nn=94912 | editor1 = Jean Souchay | editor2 = Martine Feissel-Vernier |id=IERS Technical Note 34 |location=Frankfurt am Main |publisher= International Earth Rotation and Reference Systems Service (IERS) and Verlag des Bundesamts für Kartographie und Geodäsie | date=2006 |ISBN=3-89888-802-9}}

== External links ==
* [https://aa.usno.navy.mil/faq/ICRS_doc  International Celestial Reference System (ICRS)] from [[United States Naval Observatory|USNO]]
* [http://www.usno.navy.mil/USNO/astronomical-applications/data-services/icrs-links?searchterm=ICRS+ICRF Overview of ICRS and ICRF] {{Webarchive|url=https://web.archive.org/web/20120901201251/http://www.usno.navy.mil/USNO/astronomical-applications/data-services/icrs-links?searchterm=ICRS+ICRF |date=1 September 2012 }}
* [http://www.iers.org/MainDisp.csl?pid=46-25776 IERS Conventions 2003] {{Webarchive|url=https://web.archive.org/web/20140419043412/http://www.iers.org/MainDisp.csl?pid=46-25776 |date=19 April 2014 }} (defines ICRS and other related standards)
* [http://www.iers.org/MainDisp.csl?pid=40-25728 ICRF page from the International Earth Rotation Service] {{Webarchive|url=https://web.archive.org/web/20120204050139/http://www.iers.org/MainDisp.csl?pid=40-25728 |date=4 February 2012 }}
* [http://www.iers.org/IERS/EN/DataProducts/ICRS/icrs.html General information] on the ICRS from [[IERS]]
* [http://hpiers.obspm.fr/icrs-pc/ ICRS Product Center]

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[[Category:Astronomical coordinate systems]]
[[Category:Astrometry]]
[[Category:Frames of reference]]