|
A
unique reference frame for the American
continent was established by two extremely
successful geodetic observation campaigns,
SIRGAS95 and SIRGAS2000. The general outlines
for the future tasks are presented in this
paper |
|
SIRGAS
(Sistema de Referencia Geocéntrico para
las Américas) is a joint project of South,
Central and North American countries in cooperation
with some international institutions for the establishment
and maintenance of a geocentric reference frame
for practical and scientific use. A unique reference
frame for the American continent was
established by two extremely successful geodetic
observation campaigns, SIRGAS95 and SIRGAS2000.
It provides the basis for the requirements of
modern geodesy, and is one of the most important
enterprises within geodetic science. At present,
its goals aim at the complete integration of all
American countries, the maintenance and processing
of a continuously observing stations network in
the continent, and the adoption of a unified vertical
system (height datum). The general outlines for
the future tasks are presented in this paper. |
| SIRGAS history, structure
and present situation |
SIRGAS
was established during an international meeting
in Asunción, Paraguay, in October 1993
by representatives of most of the South American
countries, the International Association of Geodesy
(IAG), the Pan American Institute of Geography
and History (PAIGH), and the National Imagery
and Mapping Agency (NIMA), now National Geospatial-
Intelligence Agency (NGA), as the “Sistema
de Referencia Geocéntrico para América
del Sur” (South American Geocentric Reference
System) Project. The main objectives were - to
define and establish a geocentric reference system
for the continent, - to define and establish ageocentric
datum, - to define and establish a unifi ed vertical
datum.
The project structure includes an Executive Committee,
made up by the representatives of all participating
countries and international institutions, a Directive
Board, a Scientific Council, and three Working
Groups: WGI “Reference Systems”, WGII
“Geocentric Datum”, WGIII “Vertical
Datum”. The objectives, responsibilities
and activities of each of these entities are established
by the Project Statutes, which have been approved
in 2003. In 2000 the project was extended to Central
and South America. As a consequence it was renamed
“Geocentric Reference System for the Americas”
keeping the acronym SIRGAS. In 2001 the United
Nations Regional Cartographic Conference for the
Americas (UNRCCA) recommended the countries in
the region to adopt SIRGAS2000 as their national
geodetic reference frames. |
| Working Group I: Reference
System |
The objectives
of WGI are clearly stated by the SIRGAS statute
and encompass the definition of a tridimensional
geocentric reference system for the Americas,
as well as its realization and maintenance through
a reference frame materialized by a set of stations
coordinates and velocities. The first SIRGAS observation
campaign was performed from May 26th to June 4th
1995. A total of 58 stations in the South American
continent and neighboring areas were measured
simultaneously by GPS observations (Figure 1)
establishing one of the most precise continental
geodetic networksall over the world. The observations
were processed by two analysis centers at DGFI,
Germany, and DMA, USA and combined for a unique
solution in the International Terrestrial Reference
Frame 1994 (ITRF 94), reference epoch 1995.4.
The precision of threedimensional coordinates
is about ±
5 mm for each component. The final results were
reported at the IAG Scientific Assembly in Rio
de Janeiro, August 1997 (SIRGAS 1997).
|
 |
The second
SIRGAS observation campaign was performed from
May 10th to 19th 2000. In South America 56 points
of the first campaign were measured again. The
network was densifi ed in some countries and extended
to Central and North American countries by 85
new stations. In addition 43 points at tide gauges
were included in order to connect the existing
height datums to the geocentric reference system.
Altogether the network consists thus of 184 stations
all over the American continent (Figure 2). The
data were processed by three processing centers
at IBGE, DGFI and the Bavarian Commission of Geodesy
(BEK), Germany. The final combined station coordinates
refer to ITRF2000, reference epoch 2000.4 (Drewes
et al. 2005). |
 |
In the
meantime, a network of continuously operating
GPS receivers has been installed by SIRGAS in
Central and South America. It is operational since
1996 and nowadays involves more than 80 stations.
The Regional Network Associate Analysis Center
(RNAAC) for SIRGAS – one of the regional
analysis centers belonging to the International
GNSS Service (IGS) – computes the network
coordinates and velocities (Seemüller 2004).
A set of station coordinates iscomputed every
week. Linear station velocities are derived by
accumulating several years of weekly solutions,
the latest including all data up to end of 2005
(Figure 3). The DGFI is in charge of processing
the IGS RNAAC-SIR.
|
 |
A station
velocity field was derived by finite element and
least square collocation approaches (Figure 4,
Drewes and Heidbach 2005) from the SIRGAS coordinate
changes 1995 to 2000, the continuously observing
GPS stations of the IGS RNAACSIR, and the geodynamics
projects Central and South America (CASA), South
America – Nazca Plate Motion Project (SNAPP),
Central Andes Project (CAP), and South America
Geodynamic Activities (SAGA). |
 |
As a result
of the SIRGAS WGI activities, the Americas have
gota geocentric reference system materialized
in accordance with the uppermost geodetic standards.
SIRGAS satisfies the highest demands of modern
geodesy and provides the fundamental layer for
America’s Spatial Database Infrastructure.
A WGI action plan for the coming years was discussed
during the SIRGAS meeting, held in Aguascalientes,
Mexico, December 2004. The general consensus was
to drive WGI efforts toward the consolidation
of the SIRGAS network of continuously operational
GPS stations as the cornerstone for the reference
frame maintenance. The following issues were appointed
as WGI priority: - to improve the performance
of presently operational stations, - to improve
data transfer to data centers,- to encourage and
to support the installation of new stations, and
- to install processing centers in American countries.
In addition, the SIRGAS executive committee encouraged
WGI for conducting a pilot project on ionospheric
mapping in South America, based on dual-frequency
GPS observations belonging to the continuously
operational SIRGAS network. A cooperative project
between Brazilian and Argentinean universities,
aimed to regional ionosphere mapping, is currently
being discussed.
The first assignment for accomplishing the action
plan was to identify the main problems that affect
the network performance. These problems may be
summarized as follows: - Difficulties for data
accessing on time (within three weeks after observation),
- Lack of information about changes in the observing
configuration (receiver, antenna, antenna height,
etc.), - Lack of information about the reason
for no data, - Lack of information about new stations
in the IGS RNAAC-SIR region, - Bad or very slow
internet connections, and - No data transfer during
the South American vacation period (December and
January).
Actions for mitigating these problems are currently
being discussed by WGI.
WGI is conducting a pilot project to establish
processing centers in the region, to carry out
a task analogous to that currently performed by
the IGS RNAAC-SIR. Preliminary, the UNLP, Argentina,
the IBGE, Brazil, and INEGI, Mexico, were identified
as potential candidates to carry out this task.
These institutions agreed to compute different
parts of the network and to deliver normal equations
from their weekly solutions to DGFI. During a
test period of two years DGFI will continue to
be in charge of the IGS RNAAC-SIR. |
| |
| Next...>>1
Of 3 |
| |
|
|
|
.gif)
