Realizations of the Geocentric Reference System for the Americas

The first realization of SIRGAS (SIRGAS95) refers to ITRF94, epoch 1995.4. It is given by a high-precision GPS network of 58 points distributed over South America. In 2000, this network was re-measured and extended to the Caribbean, Central and North American countries. To account for this extension, the meaning of the acronym changed from the original “Sistema de Referencia Geocéntrico para América del Sur”, to the current “Sistema de Referencia Geocéntrico para las Américas”. The second realization (SIRGAS2000) includes 184 GPS stations and refers to ITRF2000, epoch 2000.4. The coordinate accuracy of these two realizations is about ±3 … ±6 mm.

The third and present realization of SIRGAS is given by a network of continuously operating GNSS stations distributed over Latin America called SIRGAS-CON (SIRGAS Continuously Operating Network). SIRGAS-CON is processed on a weekly basis to generate instantaneous weekly station positions aligned to the ITRF and multi-year (cumulative) reference frame solutions. The instantaneous weekly positions are especially useful when strong earthquakes cause co-seismic displacements or strong relaxation motions at the SIRGAS stations disabling the use of previous coordinates. The multi-year solutions provide the most accurate and up-to-date SIRGAS station positions and velocities. They are used for the realization and maintenance of the SIRGAS reference frame between two releases of the ITRF. While a new ITRF release is published more or less every five years, the SIRGAS reference frame multi-year solutions are updated every one or two years. SIRGAS station positions and velocities are available at ftp.sirgas.org. This server is maintained and managed by the IGS Regional Network Associate Analysis Centre for SIRGAS (IGS RNAAC SIRGAS).

The relationship between the different SIRGAS realizations (or its densifications) is given by the transformation parameters between the corresponding ITRF solutions and the reduction of the coordinates to the same epoch. This reduction is carried out in two ways, alternatively: i) those continuously observing stations (SIRGAS-CON) which are operating for more than two years apply the velocities computed in the latest multi-year solution of the IGS RNAAC SIRGAS, and ii) those stations with velocities not known from the IGS RNAAC SIRGAS solutions are reduced by means of the model VEMOS (Velocity Model for SIRGAS). The agreement between the different SIRGAS realizations at the same epoch is at the mm-level.

The realization of the new SIRGAS vertical reference system corresponds to the SIRGAS2000 frame, it contains the stations of the SIRGAS95 reference frame, the principal tide gauges of each country, and some stations at the borders to connect the first order levelling networks between neighbouring countries. These stations refer to the SIRGAS system, shall be connected by spirit levelling with the reference tide gauges, and their geopotential numbers shall be known. Since the SIRGAS national densifications include an increasing number of permanent stations of the SIRGAS-CON network, some of them are also integrated into the vertical reference frame.