[IGSMAIL-8634] Switch to IGc20 reference frame

[Paul Rebischung]

Dear colleagues,

As announced in IGSMAIL-8619<https://lists.igs.org/pipermail/igsmail/2025/008615.html>, the second annual update of ITRF2020, ITRF2020-u2024, was recently released. The IGS intends to adopt this update, in the form of a new IGS reference frame called IGc20, for the alignment of its products, starting with the products of GPS week 2401 (11 January 2026).

IGb20 is a simple extract from ITRF2020-u2024. It includes reference coordinates for 346 well-performing, operational and historical IGS stations. The same 343 stations as in the current IGS reference frame, IGS20, were retained. 3 new stations were added in areas currently sparsely covered by IGS reference frame stations:

  *
CASC (Cascais, Portugal): historical EUREF station with 28-year long position time series, which recently joined the IGS network.
  *
MANA (Managua, Nicaragua): historical IGS station with 25-year long position time series. Although MANA was displaced by many earthquakes over its observation history, it was nevertheless selected in IGc20 because of its strategic location in Central America, where the only two other IGS reference frame stations are either dormant (GUAT) or without up-to-date ITRF2020-u2024 coordinates due to a recent earthquake (SSIA).
  *
MAYG (Dzaoudzi, Mayotte): the 11-year long position time series of the station features a large transient deformation pattern due to the 2018–2022 volcano-tectonic episode in Mayotte. While this pattern was roughly approximated by a piecewise linear model in previous ITRF releases, it is now accurately represented by a physics-based "double-exponential" model in ITRF2020-u2024.

The IGc20 files are available at ftp://igs-rf.ign.fr/pub/IGc20:

  *
IGc20.snx.gz : IGc20 SINEX file (gzip compressed)
  *
IGc20.ssc : IGc20 SINEX file without covariance matrix
  *
psd_IGc20.snx : post-seismic deformation models to be used with IGc20
  *
soln_IGc20.snx : discontinuity list to be used with IGc20
  *
IGc20_core.txt : list of "core" stations recommended for the alignment of global solutions to IGc20

Since ITRF2020-u2024 is aligned in origin, scale and orientation to ITRF2020, just like the previous ITRF2020-u2023 update, IGc20 should not be considered as a complete new reference frame, but rather as an update to individual IGS20/IGb20 station coordinates. This alignment implies that the transformation parameters between ITRF2020 (IGS20), ITRF2020-u2023 (IGb20) and ITRF2020-u2024 (IGc20) are all zero. It also implies that the switch from IGb20 to IGc20 will not introduce any significant "datum" change in the IGS products. On the other hand, the precision and accuracy of the alignment of the IGS products to the reference frame will benefit from the adoption of updated reference station coordinates.

More details on the usage of the IGc20 reference frame may be found in the comment section of the SINEX file.

Best regards,
Paul Rebischung, on behalf of the IGS Reference Frame Committee

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.igs.org/pipermail/igsmail/attachments/20251117/affd0c37/attachment-0001.htm>