Report on the South American Geocentric Reference System (SIRGAS)
Hermann Drewes
Deutsches Geodaetisches Forschungsinstitut
Muenchen, Germany
IAG Representative in the SIRGAS project



The project for the establishment of a South American Geocentric Reference System (Sistema de Referencia Geocéntrico para América del Sur, SIRGAS) was initiated in October 1993 during an international conference held in Asunción, Paraguay, and organized by the International Association of Geodesy (IAG), the Panamerican Institute of Geodesy and History (PAIGH), and the U.S. Defence Mapping Agency (DMA). The objectives of the project defined at this meeting were
? to define a reference system for South America,

? to establish and maintain a reference frame,

? to define and establish the geocentric datum for all South American countries.

The first objective was achieved already during the constitutive meeting in Asunción. It was decided that
? the SIRGAS reference system shall coincide with the IERS standards and refer to the IERS Terrestrial Reference Frame (ITRF),

? the geocentric datum shall be based on the SIRGAS reference system and the ellipsoid parameters of the Geodetic Reference System 1980 (GRS 80).

To realize the other objectives, two Working Groups were initially installed:
? Working Group I: Reference System

? Working Group II: Geocentric Datum.

The Working Groups started their activities in 1993 and held some meetings to define the project plan in Bogotá, Colombia, April 1994 (WG II) and La Plata, Argentina, October 1994 (WG I and WG II). Later, in September 1997, a third Working Group was created:
? Working Group III: Vertical Datum.


Realization of the SIRGAS Reference Frame

A total of 58 sites on the South American mainland and the surrounding areas were selected to form the SIRGAS reference frame. All these sites were observed simultaneously during a continuous GPS campaign from May 26, 1995, 0:00 UT to June 4, 1995, 24:00 UT. The observation data files were collected and are available at two data centers at Deutsches Geodaetisches Forschungsinstitut (DGFI), Munich, Germany and Instituto Brasileiro de Geografia e Estatística (IBGE), Rio de Janeiro, Brazil.

The data processing was done by two processing centers at DGFI and the National Imagery and Mapping Agency (NIMA, formerly DMA), St. Louis, MO/USA using different softwares (Bernese at DGFI and GIPSY at NIMA). The two independent results were combined and transformed to the ITRF 94 by means of nine identical stations of the International GPS Service (IGS). The precision of station coordinates is in the sub-centimeter level.

All the processing procedure was discussed during project meetings at Santiago, Chile, August 1996 and Isla Margarita, Venezuela, April 1997. At the latter meeting the final station coordinate solution was adopted as the official SIRGAS reference frame and recommended to be used by all South American countries as the basis for the national reference frames.


Realization of the Geocentric Datum

The South American countries started to refer their national reference networks to the SIRGAS frame immediately after its realization. In most cases completely new GPS networks were installed including some identical stations with the existing classical triangulation networks. By this means, a transformation from the old datums (e.g. the Provisional South American Datum 1956, PSAD 56, or the South American Datum 1969, SAD 69) to the new SIRGAS datum is feasible.

At present, in nearly all South American countries a national GPS network within the SIRGAS frame has been installed. Thereby a dense station distribution covering the total continent with a unique datum for its coordinates is established. The typical spacing between stations is 100 km. A further densification (second order networks) has started in several countries.


Vertical Reference System

During the IAG Scientific Assembly in Rio de Janeiro 1997, the SIRGAS Working Group III "Vertical Datum" was created. The objective of this WG is to define and to establish a unique vertical datum for the existing and future height systems in all South American countries.

The WG III met for a Workshop in Santiago, Chile, in August 1998 and released the main resolutions
? to adopt a unique vertical reference system for all South America with two types of heights: ellipsoidal heights and another type based on geopotential numbers,

? to realize the vertical reference system by a set of stations which have levelled heights, gravimetric measurements, and coordinates in the SIRGAS system, including those tide gauges that define the classical vertical datum.

With respect to the first resolution it was recommended to introduce normal heights as the second type besides the ellipsoidal heights. A network of stations realizing the vertical reference frame and being observed by GPS is in preparation.

Future Acitivities

The activities of the SIRGAS Working Groups will continue during the next years. In order to monitor the coordinate changes of the SIRGAS reference frame (station velocities) it‘s striven for permanent GPS observations in most of the stations. Presently, about 20 sites are included in the IGS regional network weekly processed by the Regional Network Associate Analysis Center for SIRGAS (RNAAC-SIR) at Deutsches Geodaetisches Forschungsinstitut (DGFI), Munich. A repetitive GPS campaign shall be planned for those stations not included in the RNAAC network.



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