(http://www.dgfi.badw.de/cstg/)
H. Drewes (President)
The objectives of the Commission on International Co-ordination of Space
Techniques for Geodesy and Geodynamics (CSTG) are to develop links between
various groups engaged in the field of space geodesy, to co-ordinate the work
of these groups, and to elaborate, propose and follow projects implying
international co-operation. CSTG is the Commission VIII of the International
Association of Geodesy (IAG) and simultaneously the Sub-commission B2.1 of the
Scientific Committee on Space Research (COSPAR) of the International Council
for Science (ICSU).
CSTG is structured into five sub-commissions (SC) and one Project:
While the sub-commissions were designed for long-term activities, the
project has an objective limited in time and should serve only for one elective
period.
CSTG operates through the Executive Committee (EC). It is composed by
the commission’s president and past-president, the chairpersons of the
sub-commissions and the project, as well as each one representative of the
relevant IAG Services and of COSPAR. These were during the period 1999 – 2003
CSTG disseminates information about ongoing international activities and
research results in the field of space geodesy by publishing the CSTG
Bulletins. In the period 1999 – 2003 the following volumes were issued:
CSTG organised two International Symposia during the last four years
together with the COSPAR Panel on Satellite Dynamics (PSD):
There were seven meetings of the CSTG Executive Committee during the
period:
The minutes of all the EC meetings are published in the CSTG Bulletins.
In fulfilment of its objectives, CSTG continued its role of
co-ordinating IAG activities in space geodesy. During the past elective period,
1995 – 1999, two new services had been installed upon the initiative of CSTG,
the International Laser Ranging Service (ILRS) and the VLBI Service for Geodesy
and Astrometry (IVS). Two other services, the International Earth Rotation
Service (IERS) and the International GPS Service (IGS) had developed from CSTG
sub-commissions before. The co-ordination of specific activities of these
services and the co-operation with CSTG sub-commissions were continued. In the
following we mention some of the major joint efforts.
The CSTG Executive Committee decided in April 2003 to support the
Project on DORIS in its application to become an official IAG Service. The
intention was immediately forwarded to the IAG EC. The final decision will be
made during the IUGG General Assembly in Sapporo, July 2003.
The activities of the CSTG sub-commissions are summarised in the
following in very short abstracts. For more details we refer to the reports in
the CSTG Bulletin No. 18.
Sub-commission on the
International Space Geodetic Network (ISGN)
The ISGN Sub-commission reviewed all the existing space geodetic
observation stations with respect to its fulfilment of the CSTG/IERS criteria
set up in 1998 (CSTG Bulletin No. 15, 1999). A list of candidate sites to
become official ISGN stations and invitation letters to the station managers
were prepared. The issue came to a standstill because the technique-specific
services also started the classification of their network stations. A common
tuning seemed to be indispensable. Later on, the proposal of the IAG Project on
an Integrated Global Geodetic Observing System (IGGOS) seemed to require a
quite similar selection of sites which should not be predetermined.
A major problem of co-located space geodetic sites is the precise
determination of relative positions of observation stations of different
techniques (eccentricities, local ties). The ISGN Sub-commission is in charge
of a list of local ties since several years. The ITRS Product Centre of the
IERS also uses an own eccentricity file. In a joint effort of the ISGN
Sub-commission and the ITRF Product Centre and several working meetings, a list
of discrepancies between local surveys and differences of technique-specific
station coordinates was prepared. The organisation of an international team of
geodetic surveyors to assist in outstanding site survey omissions was begun.
In a regular way, the ISGN Sub-commission publishes the “Active Global
Space Geodetic Site Information Summary” in the CTSG Bulletins. It includes
approximately 300 sites with coordinates, host or managing agencies and
available space techniques. The chart is continuously being updated and may be
accessed by internet: http://cddisa.gsfc.nasa.gov/ggs/sgp_chart.html.
The Sub-commission started its activities by combining mainly GPS
processing results of different analysis centres. In the following, a main
issue was the updating of the SINEX format to include information appropriate
for all space geodetic systems and to enable the easy integration of different
techniques. This was done in very close co-operation with the IGS and the IERS.
In 2001, the IERS started its new structure including an Analysis
Co-ordinator (AC) and Combination Research Centre (CRC). The chairman of the
CSTG Sub-commission, Tom Herring, became the interim AC and brought the CSTG
Sub-commission and the CRCs together. The analysis is thus done in an
integrated manner, co-ordinated by the IERS AC (since 2002 Markus Rothacher.
The principal objective is to investigate and develop strategies for
combining results of geodetic data processing in a consistent and strict way.
Approaches of combination at the level of observation equations, normal
equations or complete solutions are studied. A main issue is the elimination of
(direct or indirect) constraints, the estimation of relative weights, and the
definition of a common datum.
The role of this Sub-commission was foreseen in supporting new and
emerging techniques other than GPS than are not yet organised as IAG
Service-oriented structure, namely GLONASS, DORIS, PRARE and Galileo.
Concerning GLONASS, a decision has been made after the final workshop of
the IGEX-98 campaign to include GLONASS within the IGS organisation through an
IGS project, called IGGLOS (International GLONASS Service). In early 2003, 36
GLONASS tracking stations are currently observing 7 available GLONASS
satellites. All these stations are collocated, and most are equipped with
combined GPS/GLONASS receivers. Russia has launched few new GLONASS satellites
to replace part of its still incomplete constellation. Several analysis groups
process GLONASS data using the same strategy as for GPS data within the IGS.
Current performances are now closer to GPS results (10 cm radial for orbit and
sub-cm accuracy for point positioning). At the end of the 4 years, the GLONASS
data collection, analysis and information are joined with GPS.
DORIS has started activities as a Pilot Project. In 1999, a general call
for participation has been issued and several groups proposed to serve as
analysis centres (see Project on DORIS below). In 2002, 3 additional satellites
have been launched by NASA, CNES and ESA. The DORIS constellation jumped from 3
to 6 satellites, including satellites with a new generation of receiver
(on-board orbit determination, improved accuracy, autonomy and multi-channel).
DORIS data and products can presently be found at CDDIS (cddisa.gsfc.nasa.gov)
and at IGN (ftp://lareg.ensg.ign.fr/pub/doris/). DORIS results have improved in
the last 4 years, showing orbit accuracy at 1 to 2 cm for TOPEX and JASON
satellites, point positioning accuracy at 1 to 2 cm for weekly solutions, or 1
cm and 2 mm/year, respectively, for global solutions (over the 10 year period).
The PRARE system is only operating on the ERS-2 satellite. There are
presently no plans to extend its activity to new satellites. Several groups
process the PRARE/ERS-2 data. It is not foreseen to organise the PRARE
community in an IAG-Service oriented structure.
The future European satellite navigation system Galileo has been decided
and the first satellites should be launched before 2006, first a demonstration
mode. As new actual Galileo data are available, the work of the sub-commission
has been to start contact with Galileo (through the European Commission and
ESA) and to prepare a possible geodetic use of the future Galileo satellites
through the IGS structure. A few initiatives led to the future use of ITRF as
the Galileo reference coordinates frame. In parallel, discussion with ESA could
bring a co-operation between EUREF and the EGNOS system (first development
phase of Galileo) concerning the monitoring EGNOS tracking stations
coordinates. Within the IGS, a working group has been created in order to
prepare a future inclusion of the geodetic applications of Galileo in the frame
of the IGS (chairman: J. Dow). In the IGS strategic planning recommendation it
is clear that Galileo has some specific interest for the IGS community and
that, following the example of GLONASS, future steps should be taken in the
future to incorporate Galileo data collection and data analysis in the frame of
the IGS structure.
The main objectives of the Sub-commission are to promote free access to
all available altimeter data for scientific investigations, to study synergies
among different altimeter missions with different space-time sampling and with
other remote sensing techniques, to set up the requirements for unified
multi-mission long-term records of altimeter data (structure, standards,
formats), to study new techniques and application areas, and to investigate the
establishment of an international altimeter service.
A concept for a multi-mission altimeter data base was developed in terms
of the organisation, functionality, contents, and structure. A prototype of
such a system, an Open Altimeter Data Base (OpenADB) with a generic data format
was prepared and tested. It allows the quick actualisation of parameters, i.e.,
adoption to actual gravity field models, different physical correction models
and new algorithms. It is also possible to extract data in a user defined
format or to add results from external groups.
The amount of altimeter data will grow tremendously in the next future
due to new missions and increasing sampling rates. It will be beyond the normal
storage capacities. A scaling into integer units and binary codes was developed
for the compression of data. A decoder program allows the selection and
formatting of data. The goal is to provide data of different missions for
selected regions and time intervals for the users.
Primary goals of the Sub-commission are the studies of different LEO
orbit modelling approaches, the impact of global parameters on LEO orbits,
mission-dependent data structures and standards, and the analysis of LEO orbits
obtained from data of different observation techniques.
Various data sets of LEO observations were made available to interested
groups for this purpose. The Sub-commission collected and distributed also
information on ongoing LEO satellite missions and data sets. Exchange formats
are defined and described. The activities are closely co-ordinated with the IGS
Working Group on LEOS and the Special Study Group 2.162 of IAG. While the IGS
WG is focussing more on the data formats and standards, the CSTG Sub-commission
pays to the orbit analysis, in particular using data from different techniques.
Besides the TOPEX, GPS/MET and SAC-C orbits, emphasis is laid on the new
satellite missions, CHAMP, GRACE and JASON-1. The orbits determined from GPS and
SLR observations and the obvious offset of 5 cm were analysed.
DORIS was developed for
precise orbit determination and precise positioning of ground tracking stations
on Earth. In 1999 it was decided
at the IAG General Assembly to create a DORIS Pilot Experiment. Such a Pilot
Experiment would give time to all engaged groups to co-ordinate and improve
their operation in order to create on the long term an International DORIS
Service (IDS). A Steering Committee manages the DORIS Pilot Experiment with the
help of a Central Bureau responsible for the co-ordination, communication,
meetings, workshops etc.
Two Data Centres support DORIS data and products archiving and
distribution for the space geodesy and geodynamics community. The Central
Bureau started a Station Coordinates Analysis Campaign in November 2001 which
was extended to new products in March 2002. The DORIS Station Selection
Committee defines site criteria and analyses proposals for new sites. There are eleven Analysis Centres willing to process
DORIS measurements, four of them regularly contributing to an
Analysis Campaign and delivering products.
For three years, a network renovation action has been conducted: today
more than half of the stations meet the new stability requirement. In the same
period, third generation beacons were developed. Deliveries and deployment
started in December 2001. Presently 56 stations are installed at the end of
2002. Three new satellites recently joined the three satellites already flying
with a DORIS receiver. They carry an improved second generation DORIS receiver
including two channels and DIODE navigator.
During the last CSTG EC meeting in Nice, 9 April 2003, the future of
scientific activities performed at present by CSTG were discussed. There is a
general consensus that the new IAG Commission 1 “Reference Frames” should
integrate the main CSTG objectives, i.e., the co-ordination of space geodesy in
IAG and among the services. Commission 1 should also represent the COSPAR Sub-commission
B2.1. The CSTG sub-commissions should go to the new Commission I or to the
relevant IAG Services.
The sub-commissions on the IGSN and on the Combination of Space Geodetic
Analysis should be integrated into the new IERS structure. The sub-commissions
on Precise Satellite Microwave Systems and on Precise Orbit Determination for
Low Earth Orbiting Satellites should completely continue in IGS. The
sub-commission on Multi-Mission Satellite Altimetry should become a Project in
the new Commission 1 with the goal of establishing a service. The DORIS Project
should become an IAG Service in 2003.
The project on the geocentric
reference system for the Americas (SIRGAS) was continued by observation
campaigns, symposia and meetings during the elective period 1999-2003. Two
symposia were organised by IAG entities in co-operation with SIRGAS: “Symposium
on Vertical Reference Systems”, Cartagena, Colombia, February 2001, and “Symposium
on Crustal Deformations in South America and Surrounding Areas”, Santiago,
Chile, October 2002. The SIRGAS Project Committee had official meetings in
Cartagena, Colombia, February 21 and 22, 2001, in Budapest, Hungary, September
6, 2001, and in Santiago, Chile, October 21 and 22, 2002. Two SIRGAS Bulletins
were issued, No. 6, February 2002 and No. 7, December 2002.
Approximately 130
participants participated in the IAG Symposium on Vertical Reference Systems,
Cartagena, February 20-23, 2001. More than 70 papers were presented (oral and
posters) under five topics: (1)
Vertical control systems, (2) Height determination techniques, (3) Height
reference surfaces, (4) Sea level and height systems, (5) Unification of
vertical reference systems. After a strict review process, 64 papers were
published in the IAG Symposia series (Vol. 124). A report is published in J. of
Geodesy (75) 679-680, 2001. The IAG Symposium on Crustal Deformation in South
America and Surrounding Areas, Santiago, Chile, 21.-25.10.2002 was organised in
conjunction with the 4th Chilean Symposium on Earth Sciences. It
included the sessions (1) Earthquake studies, (2) South American reference
frame, 3) Andean evolution, (4) Vertical motion (5) South American plate
kinematics, and (6) Sea level change with a total of some 60 oral and poster
presentations.
The SIRGAS Project, installed
in 1993 by IAG, PAIGH, and the US National Imagery and Mapping Agency (NIMA)
for the definition and realisation of the South American Geocentric Reference
System, was extended to Central and North America and the Caribbean. During the
seventh United Nations Regional Cartographic Conference for the Americas, New
York, January 2001, a resolution was released recommending to all the American
countries the integration of their national reference systems into SIRGAS.
Consequently, the SIRGAS Project Committee decided during its meeting in
Cartagena, Colombia, February 2001, to change the synonym of the abbreviation
SIRGAS to "Sistema de Referencia Geocéntrico para las Américas"
(Geocentric Reference System for the Americas) and to invite the Central and
North American as well as Caribbean countries to participate in the project and
nominate their delegates to the Project Committee.
New Statutes have been
released in 2002 by the SIRGAS Committee. A complete organisation structure of
the project is defined, the major components being the Executive Committee, the
Directive Council, the Scientific Council, and the Working Groups. The
sponsoring agencies, among those the IAG, is represented by a voting member in
the Executive Committee.
An extended GPS observation
campaign over ten days of each 24 hours was performed in May 2000 including 184
stations in North, Central and South America, the Caribbean as well as Galapagos,
Easter Islands and O’Higgins, Antarctica. For South America this was the first
repetition of the May 1995 observation campaign. SIRGAS Working Group I
"Reference System" will include a comparison of the 1995 and 2000
results to derive station velocities for the realisation of a kinematic
reference system. For the use of SIRGAS Working Group II "Geocentric
Datum" a number of additional stations of national networks were included
in order to improve the transformation parameters from the classical networks
to the SIRGAS (and ITRF) reference frame.
The establishment of a
"vertical datum" (SIRGAS Working Group III) was a major objective of
the 2000 GPS campaign. All the reference tide gauges that define national
height systems (levelling networks) and additional levelling points, in
particular at the borders between neighbouring countries, were included for
this purpose. The goal is to derive the relation between classical (normal or
orthometric) and modern (ellipsoidal) height reference systems. A close co-operation
with the IAG Gravity and Geoid Commission (XIII), in particular its
sub-commission for South America, was established.
The SIRGAS project made an
essential progress in the realisation of the geocentric reference system and
the unification of the vertical systems. The latter activities are also
performed in a global scale, in particular in co-operation with the European
Vertical Reference System (EVRS) towards a unified world height system.