SC 1.3b: South and Central America

Chair: William Martinez (Colombia)

Vice-Chair: Virginia Mackern (Argentina)


Terms of Reference

Sub-commission 1.3b (South and Central America) encompasses the activities developed by the “Geocentric Reference System for the Americas” (SIRGAS). As such, it is concerned with the definition, realization and maintenance of a modern geodetic reference infrastructure for South and Central America and the Caribbean. This includes a geometric reference frame consistent with ITRS/ITRF and a gravity field-related vertical reference system, defined and realized globally.



The main purposes of the Sub-commission 1.3b are:

  • To determine, maintain and make available a geocentric reference frame (a set of stations with high-precise geocentric positions and their variation with time, as a regional densification of the global ITRF);
  • To support the SIRGAS countries in the establishment and maintenance of national geodetic reference networks as local densifications of SIRGAS in order to guarantee accessibility to the global ITRF at national and local levels;
  • To establish a unified vertical reference system supporting the determination and precise combination of physical and geometric heights as well as their variations with time;
  • To contribute to the GGOS program by developing and implementing state-of-the-art products based on the SIRGAS observational infrastructure;
  • To promote, support, and coordinate the efforts of the Latin American and Caribbean countries to achieve these objectives.



The structure of the Sub-commission 1.3b is based on the functioning bodies of SIRGAS. There are currently three Working Groups:

  • SC1.3b-WG 1 : Reference System
    • Chair : Víctor José Cioce (Venezuela)
  • SC1.3b-WG 2: SIRGAS at National Level
    • Chair : Roberto Pérez Rodino (Uruguay)
  • SC1.3b-WG 3 : Vertical Datum
    • Chair : Silvio Rogério Correia de Freitas (Brazil)

The SIRGAS Executive Committee (as it is named in the SIRGAS statutes) is composed of:

  • SC1.3b Chair : W. Martínez (Colombia)
  • SC1.3b Vice-Chair : Virginia Mackern (Argentina)
  • SC1.3b WG1 Chair : Víctor José Cioce (Venezuela)
  • SC1.3b WG2 Chair : Roberto Pérez Rodino (Uruguay)
  • SC1.3b WG3 Chair : Silvio Rogério Correia de Freitas (Brazil)


Program of Activities

Since the SIRGAS countries are improving their national reference frames by installing an increasing number of continuously operating GNSS stations, it is necessary to outline the best strategy for the appropriate integration of those frames into the continental frame. This includes:

  • Promotion of the IGS and IERS standards within the SIRGAS countries to ensure the adequate installation, maintenance, and analysis of continuously operating GNSS stations;
  • Establishment of a SIRGAS National Processing Centre in all the member countries;
  • Refinement of the SIRGAS station hierarchy. At present, two classes are considered: core and densification stations (the establishment of other categories is under consideration);
  • Promotion of the adequate usage of SIRGAS as a reference frame by means of capacity building activities. This comprises SIRGAS schools on reference frames, scientific processing of GNSS data, atmospheric analysis based on the SIRGAS infrastructure, etc.;
  • Promotion and implementation of real-time services based on the SIRGAS infrastructure to make available the reference frame to more users;
  • The kinematics of the SIRGAS frame, up to now, have been represented by linear station movements (i.e. constant velocities). This representation is not sufficiently precise due to existing seasonal variations in the station position time series and due to discontinuities caused by the frequent occurrence of seismic events in the SIRGAS region.

According to this, it is necessary:

  • To model non-linear station movements within the reference frame computation;
  • To implement a methodology aiming at a precise transformation between different epochs and, in general, between pre-seismic and post-seismic reference frame realizations in particular;
  • To evaluate the feasibility of computing and using near-real time reference frames instead of those based on epoch station positions and constant velocities.

The establishment of a unified vertical reference system continues to be a big challenge of SIRGAS. The related activities concentrate on:

 Continental adjustment of the national vertical networks in terms of geo-potential numbers;

  • Combined analysis of tide gauge registrations, GNSS positioning and satellite altimetry observations to determine the dynamic ocean topography at the classical vertical datums;
  • Determination of potential differences between the reference tide gauges and the global reference surface;
  • Stronger cooperation with the Sub-Commission 2.4b (Gravity and Geoid in South and Central America - GGSCA) to promote national initiatives regarding the modernization of the gravity reference networks and the computation of geoid models of high resolution.

Hourly SIRGAS ionospheric models (vTEC) based on the GNSS SIRGAS stations have been generated since 2006 on a regular basis. The SIRGAS ionospheric model is being upgraded to include a better distribution of the electron density based on the assimilation of ground- and space-based GNSS observations. In addition, SIRGAS is developing a service for computing water vapour estimations.