Abstract.
Properly designed and structured ground-based
geodetic networks materialize the reference systems to support sub-mm global
change measurements over space, time and evolving technologies. Over this past
year, the Ground Networks and Communications Working Group (GN&C WG) has
been organized under the Global Geodetic Observing System (GGOS) to work with
the IAG measurement services (the IGS, ILRS, IVS, IDS and IGFS) to develop a
strategy for building, integrating, and maintaining the fundamental network of
instruments and supporting infrastructure in a sustainable way to satisfy the
long-term (10-20 year) requirements identified by the GGOS Science Council.
Activities
of this Working Group include the investigation of the status quo and the
development of a plan for full network integration to support improvements in
terrestrial reference frame establishment and maintenance, Earth orientation
and gravity field monitoring, precision orbit determination, and other geodetic
and gravimetric applications required for the long-term observation of global
change. This integration process includes the development of a network of
fundamental stations with as many co-located techniques as possible, with
precisely determined intersystem vectors. This network would exploit the
strengths of each technique and minimize the weaknesses where possible. This
paper discusses the organization of the working group, the work done to date,
and future tasks.
Keywords.
Global Geodetic Observing System, GGOS, GEOSS, GPS,
SLR, VLBI, DORIS, Gravity, Tides, Geoid
1 Introduction
The Ground Networks and Commmunications Working Group (GN&C WG) of the Global Geodetic Observing System (GGOS) is charged with developing a strategy to design, integrate, and maintain the fundamental space geodetic network. In this report, we review the significance of geodetic networks and the GGOS project, and summarize the present state of as well as future improvements to and requirements on space geodetic networks, services, and products. The approach of the WG and preliminary conclusions follow.
1.1 Significance of the Terrestrial Reference Frame
Space geodesy provides precise position,
velocity and gravity on Earth, with resolution from local to global scales. The
terrestrial reference system defines the terrestrial reference frame (TRF) in
which positions, velocities, and gravity are reported. The reference surface
for height reckoning, the geoid, is defined through the adopted gravity model,
which is referenced to the TRF. The TRF is therefore a space geodesy product
that links every observable quantity, product and geophysical parameter on Earth.
Its position, orientation and evolution in space and time are the basis through
which we connect and compare such measurements over space, time, and evolving
technologies. It is the means by which we verify that observed temporal changes
are geophysical signals rather than artifacts of the measurement system. It provides
the foundation for much of the space-based and ground-based observations in
Earth science and global change, including remote monitoring of sea level, sea
surface and ice surface topography, crustal deformation, temporal gravity
variations, atmospheric circulation, and direct measurement of solid Earth
dynamics. A precise TRF is also essential for interplanetary navigation,
astronomy and astrodynamics.
The
realization of the TRF for its most demanding applications requires a mix of
technologies, strategies and models. Different observational methods have
different sensitivities, strengths and sources of error. The task is
complicated by the dynamic character of Earth’s surface, which deforms on time
scales of seconds to millennia and on spatial scales from local to global.
1.2 The Role of GGOS
In early 2004 under its new organization, the International
Association of Geodesy (IAG) established the Global Geodetic Observing System
(GGOS) project to coordinate geodetic research in support of scientific
applications and disciplines (Drewes, 2004; Rummel, 2002). GGOS is intended to
integrate different geodetic techniques, models and approaches to provide
better consistency, long-term reliability, and understanding of geodetic, geodynamic,
and global change processes. Through the IAG’s measurement services (IGS[1],
ILRS[2],
IVS[3],
IDS[4],
and IGFS[5]),
GGOS will ensure the robustness of the three aspects of geodesy: geometry and
kinematics, Earth orientation, and static and time-varying gravity field. It
will identify geodetic products and establish requirements on accuracy, time
resolution, and consistency. The project will work to coordinate an integrated
global geodetic network and implement compatible standards, models, and parameters.
A fundamental aspect of
GGOS is the establishment of a global network of stations with co-located
techniques, to provide the strongest reference frames. GGOS will provide the
scientific and infrastructural basis for all global change research and provide
an interface for geodesy to the scientific community and to society in general.
GGOS will strive to ensure the stability and ready access to the geometric and
gravimetric reference frames by establishing uninterrupted time series of
state-of-the-art global observations.
As
shown in Figure 1, GGOS is organized into working groups headed by a Project
Board and guided by a Science Council that helps define the scientific
requirements to which GGOS will respond.
Fig. 1. GGOS Organization
1.3 Role of the Ground Networks and Communications Working
Group
The ground network of GGOS is fundamental
since all GGOS data and products emanate from this infrastructure.
The
Charter of the Ground Networks and Communications Working Group (GN&C)
within GGOS is to develop a strategy to design, integrate and maintain the
fundamental geodetic network of instruments and supporting infrastructure in a
sustainable way to satisfy the long-term (10-20 years) requirements identified
by the GGOS Science Council. At the base of GGOS are the sensors and
observatories situated around the world providing the timely, precise and fundamental
data essential for creating the GGOS products. Primary emphasis must be on sustaining
the infrastructure needed to maintain evolving global reference frames while at
the same time ensuring support to the scientific applications’ requirements.
Opportunities to better integrate or collocate with the infrastructure and
communications networks of the many other Earth Observation disciplines now
organizing under the Global Earth Observation System of Systems should be taken
into account (Group on Earth Observations, 2005).
Recognizing
that the infrastructure and operations collectively contributing to the
Services of the IAG are possible solely due to the voluntary contributions of
the globally distributed collaborating agencies and their interest in maximized
system performance and sustainable long term efficient operations, the Working
Group is made up of representatives of the measurement services plus other
entities that are critical to guiding the activities of the Working Group:
·
IGS: Angelyn Moore, Norman Beck
·
ILRS: Mike Pearlman, Werner Gurtner
·
IVS: Chopo Ma, Zinovy Malkin
·
IDS: Pascal Willis
·
IGFS: Rene Forsberg, Steve Kenyon
·
ITRF and Local Survey: Zuheir
Altamimi, Jinling Li
·
IERS Technique Combination Research Centers:
Marcus Rothacher
·
Data Centers: Carey Noll
· Data Analysis: Erricos Pavlis, Frank
Lemoine, Frank Webb, John Ries, Dirk Behrend
·
IAS (future International Altimetry
Service): Wolfgang Bosch
2 Global Geodetic
Network Infrastructure
All infrastructure, and resulting analysis
and products of GGOS and its constituent services are made possible through the
goodwill voluntary contributions of national agencies and institutions and are
coordinated by the IAG governance mechanisms.
The
ground network of GGOS includes all the sites that have instruments of the IAG
measurement services either permanently in place or regularly occupied by
portable instruments. Some sites have more than one space
geodesy technique co-located, and knowledge of the precise vectors between such
co-located instruments (known as “local ties”) is essential to full and
accurate use of these co-locations.
Analysis
centers use the ground networks’ data for various purposes including positioning,
Earth orientation parameters (EOP), the TRF, and the gravity field. The ground
stations of the satellite techniques provide data for precise orbit determination
(POD). The individual sites’ reference points of the contributing space geodesy
networks are the fiducial points of the TRF.
2.1 IAG Measurement Services
Each service coordinates its own network,
including field stations and supporting infrastructure. Here we will review the current status of
each measurement service.
2.1.1
IGS
The foundation of the International GNSS
Service (IGS, formerly the International GPS Service) is a global network of
more than 350 permanent, continuously-operating, geodetic-quality GPS and
GPS/GLONASS sites. The station data are
archived at three global data centers and six regional data centers. Ten analysis centers regularly process the
data and contribute products to the analysis center coordinator, who produces
the official IGS combined orbit and clock products. Timescale, ionospheric, tropospheric, and reference frame
products are analogously formed by specialized coordinators for each. More than
200 institutes and organizations in more than 80 countries contribute voluntarily
to the IGS, a service begun in 1990.
The IGS intends to integrate future GNSS signals (such as Galileo) into
its activities, as demonstrated by the successful integration of GLONASS.
(Kouba et al., 1998; Beutler et al., 1999; Dow, 2003).
2.1.2
ILRS
The International Laser Ranging Service
(ILRS) currently tracks 28 retroreflector-equipped satellites for geodynamics,
remote sensing (altimeter, SAR, etc.), gravity field determination, general
relativity, verification of GNSS orbits, and engineering tests (Pearlman et
al., 2002). Satellite altitudes range from a few hundreds of kilometers to GPS
altitude (20K kilometers) and the Moon. The network includes forty laser
ranging stations, two of which routinely range to four targets on the Moon.
Satellites are added and deleted from the ILRS tracking roster as new programs
are initiated and old programs are completed. The collected data are archived
and disseminated via two centers[AM1], and several analysis centers voluntarily and routinely deliver
products for TRF, EOP, POD, and gravity modeling and development.
2.1.3
IVS
The International VLBI
Service for Geodesy and Astrometry (IVS) was established in 1999 and currently
consists of 74 permanent components: coordinating center, operation centers,
network stations, correlators, analysis centers, and technology development
centers. The IVS observing network includes about 30 regularly-observing IVS
stations and 20-30 collaborating stations participating in selected IVS programs
on an irregular basis (Behrend and Baver, 2005). 24-hr sessions twice per week
as well as other less frequent sessions are used to determine the complete set
of EOP (polar motion, celestial pole coordinates, UT1-UTC), station coordinates
and velocities, and the positions of the radio sources. Daily 1-hr single
baseline sessions are used to monitor Universal Time (UT1) with low latency
(Schlueter et al., 2002).
2.1.4
IDS
The International DORIS Service (IDS) was
created in 2003 (Tavernier et al., 2005). The current ground tracking network
is composed of 55 stations allowing an almost continuous tracking of the
current five satellites (SPOT-2, -3 and -4 used for remote sensing
applications, Jason-1 and Envisat used for satellite altimetry). The main
applications of the DORIS system are precise orbit determination, geodesy and
geophysics (Willis et al., 2005). Using improved gravity Earth models derived
from the GRACE mission (Tapley et al., 2004), DORIS weekly station positions
can now be regularly obtained at the 10 mm level (Willis et al., 2004). DORIS
data are available at the two IDS Data Center since 1990 (SPOT-2). In 1999 a
DORIS Pilot Experiment was created by the IAG (Tavernier et al., 2002) leading
gradually to the IDS. The French space agency (CNES) has the leading role in
the IDS.
2.1.5
IGFS
The
International Gravity Field Service (IGFS) was created in 2003 to provide
coordination and standardization for gravity field modeling. It supports the
IAG scientific and outreach goals and therefore GGOS, through activities such
as collecting data for fundamental gravity field observation networks (e.g., a
global absolute reference network, co-located with satellite stations and other
geodetic observation techniques), data collection and release of marine,
surface and airborne gravity data for improved global model development (e.g.,
EGM96 (Lemoine et al., 1998)), and advocating consistent standards for gravity
field models across the IAG services. Establishing new methodology and science
applications, particularly in the integration and validation of data from a
variety of sources, is another focus of the service. The IGFS is composed of a
variety of primary service entities: Bureau Gravimétrique International (BGI),
International Geoid Service (IGeS), International Center for Earth Tides
(ICET), and International Center for Global Earth Models (ICGEM), with the
National Geospatial-Intelligence Agency (NGA) participating as an IGFS
Technical Center.
2.2 Communications
Transmission of data from the network
instruments to data centers and processing or analysis centers is a function
critical to all the techniques. For the satellite services, data transmission
is normally via primarily the Internet thorugh terrestrial or satellite
communications networks.. Due to the volume of data (terabytes per station per
24 hrs), VLBI data are currently shipped on recorded media, but transmission of
data via high speed fiber is a future goal.
Control and coordination information is also routinely and primarily
sent via Internet. Sites are often
situated fortuitously where suitable access to communications networks, and
ideally Internet, Communications costs are borne by thei operating agency which
in remote areas is often at considerable expense. The GN&C WG will investigate the possibility of improving
efficiency through coordinated implementation of modern methods and additional
sharing of communications facilities
and infrastructure.
3 Synergy of the
Observing Techniques
At the dawn of space age about half a century ago, the individual
national classical systems that were then dominating geodesy started slowly to
be replaced by initially crude global equivalents (e.g., the SAO Standard Earth
models), and later on, when the first satellite navigation constellations like
TRANSIT became available, by more sophisticated “World Geodetic Systems” (e.g.,
the US DoD-developed WGS60, 66, 72, and WGS84). As space techniques
proliferated throughout the world, it soon became apparent that the optimal
approach would be to make use of all available systems, and to share the burden
of the development through international coordination and cooperation. This section reviews the synergistic
contributions of space geodetic techniques to various products.
3.1 The Terrestrial Reference Frame
The dramatic improvement of space geodesy techniques in the
eighties, thanks to NASA’s Crustal Dynamics Project and Europe’s WEGENER Project,
has drastically increased the accuracy of TRF determination. However, none of
the space geodesy techniques alone is able to provide all the necessary
parameters for the TRF datum definition (origin, scale, and orientation). While
satellite techniques are sensitive to Earth’s center of mass, VLBI is not. The
scale is dependent on the modeling of some physical parameters, and the
absolute TRF orientation (unobservable by any technique) is arbitrary or conventionally
defined through specific constraints.
The utility of multi-technique combinations is therefore recognized
for the TRF implementation, and in particular for accurate datum definition.
Since
the creation of the International Earth Rotation and Reference Systems Service
(IERS), the current implementation of the International Terrestrial Reference
Frame (ITRF) has been based on suitably weighted multi-technique combination,
incorporating individual TRF solutions derived from space geodesy techniques as
well as local ties of co-location sites. The IERS has recently initiated a new
effort to improve the quality of ties at existing co-location sites, crucial
for ITRF development.
The
particular strengths of each observing method can compensate for weaknesses in
others. SLR defines the ITRF2000 geocentric origin, which is stable to a few
mm/decade, and SLR and VLBI define the
absolute scale to around 0.5 ppb/decade (equivalent to a shift of approximately
3 mm in station heights) (Altamimi et al., 2002). Measurement of geocenter
motion is under refinement by the analysis centers of all satellite techniques.
The density of the IGS network provides easy and rigorous TRF access
world-wide, using precise IGS products and facilitates the implementation of
the rotational time evolution of the TRF in order to satisfy the
No-Net-Rotation condition over tectonic motions of Earth’s crust. DORIS
contributes a geographically well-distributed network, the long-term permanency
of its stations, and its early decision to co-locate with other tracking
systems.
The
TRF is heavily dependent on the quality of each network and suffers with any
network degradation over time. The current distribution and quantity of
co-location sites as depicted on Figure 2 (in particular sites with three and
four techniques) is sub-optimal.
Fig.
2. Distribution of space geodesy co-location sites
since 1999.
3.2 Earth Orientation Parameters
Earth orientation parameters measure the
orientation of Earth with respect to inertial space (which is required for
satellite orbit determination and spacecraft navigation) and to the TRF, which
is a precondition for long-term monitoring. Polar motion and UT1 track changes
in angular momentum in the fluid and solid components of the Earth system
driven by phenomena like weather patterns, ocean tides and circulation,
post-glacial rebound and great earthquakes. The celestial pole position, on the
other hand, is dependent on the deep structure of Earth. Only VLBI measures
celestial pole position and UT1, and VLBI also defines the ICRF (International
Celestial Reference Frame) (Ma et al., 1998), whose fiducial objects (mostly
quasars) have no detectable physical motion across the sky because of their
great distance. The two-decade VLBI data set contributes a long time series of
polar motion, UT1 and celestial pole position.
Satellite techniques (GPS, SLR and DORIS) measure polar motion and
length of day relative to the orbital planes of the satellites tracked. In
practice, recent polar motion time series are derived from GPS with a high
degree of automation, and predictions of UT1 rely on GPS length of day and
atmospheric excitation functions.
3.3 Gravity, Geoid, and Vertical Datum
Gravity is important to many scientific and
engineering disciplines, as well as to society in general. It describes how the
“vertical” direction changes from one location to another, and similarly, it defines
at each point the datum for height reckoning; therefore, it describes how “water
flows”. Global scale models of terrestrial gravity and geoid (Lemoine et al.,
1998) are now routinely delivered on a monthly basis by missions like GRACE,
with a resolution of 200 km or so, and high accuracy (Tapley et al., 2004). The
addition of surface gravity observations can extend the resolution of these
models down to tens of kilometers in areas of dense networks. Worldwide
databases of absolute and relative gravity, airborne and marine gravity are
collected and maintained by IGFS. Astronomically-driven temporal variations of
gravity (Earth, ocean and atmospheric tides) are also a product of this and
other IAG services. The combination of all these information is crucial in precisely
determining instantaneous position on Earth or in orbit, the direction of the
vertical and the height of any point on or around Earth, and the computation of
precise orbits for near-Earth as well as interplanetary spacecraft. Similarly,
the vertical datum is the common reference for science, engineering, mapping
and navigation problems. Achieving a globally consistent vertical datum of very
high accuracy has been a prime geodetic problem for decades, and only recently
(thanks to missions like CHAMP and GRACE) is a successful result in reach.
Strengthening and maintaining a close link between the “geometric” and
“gravimetric” reference frames is of paramount importance to the goals of GGOS.
3.4 Precise Orbit Determination
Precise orbit determination is one of the
principal applications of the satellite techniques (GPS, SLR, DORIS), and has
direct application to many different scientific disciplines such as ocean
topography mapping, measurement of sea level change, determination of ice sheet
height change, precise geo-referencing of imaging and remote sensing data, and
measurement of site deformation using synthetic aperture radar (SAR) or GPS.
The techniques have evolved from meter-level orbit determination of satellites
such as LAGEOS in the early 1980’s to cm-level today. The computation of
precise orbits allows these satellite tracking data to be used for gravity
field determination (both static and time-variable) and the estimation of other
geophysical parameters such as post glacial rebound, ocean tidal parameters,
precise coordinates of tracking sites, or the measurement of geocenter motion.
Precise
orbit determination, which requires precise UT1 and gravity models, underpins
the analysis that in parallel has resulted in improved station coordinate
estimation, and thereby improved realizations of the TRF (e.g., ITRF2000);
There is close synergy between POD and TRF realization. The density of data
available from GPS (and in the future from other GNSS including Galileo) allows
the estimation of reduced-dynamic or kinematic orbits with radial accuracy of a
few cm even on low-altitude satellites such as CHAMP and GRACE. Only a few
satellites carry multiple tracking systems, but space-based co-location is
invaluable. The detailed intercomparison of orbits computed independently from
SLR, DORIS, and GPS data confirms that Jason-1 orbits have a one-cm radial accuracy
(Luthcke et al., 2003). These techniques are complementary; the precise but
intermittent SLR tracking of altimeter satellites, such as Envisat or
TOPEX/Poseidon, is complemented by the dense tracking available from the DORIS network.
SLR tracking of the GPS, GLONASS or future Galileo satellites is and will be
vital to calibrating GNSS satellite biases and assuring the realization of a
high quality TRF.
4 Future
Requirements
The measurement requirements for GGOS will
be set by the GGOS Project Board with guidance from the Science Council
(Rummel, 2002). Until these requirements are formally specified, we judge the
practical useful target for the TRF and space geodetic measurement accuracy to
be roughly a factor of 5 to 15 below today’s levels. Given that the TRF and
global geodesy are now accurate to the order of 1 cm (or 5-15 mm for different
quantities) and 2 mm/yr, we foresee near-term utility in global measurements
with absolute accuracies at or below 1 mm and 0.2 mm/yr. Corresponding levels
of improvement are required for Earth orientation and gravity.
5 Evolution of the Techniques
Each of the GGOS Services techniques
envisions technological and operational advances that will enhance measurement
capability. Some advances are currently being implemented while others are in
the process of design or development.
In addition, each technique related service
is seeking to improve on not only data quality and precision, but also on
reliability of data and product delivery, performance, continuity, station
stability, and data latency (which in the case of GNSS includes real-time).
While making these improvements, contributors seek operational efficiencies in
order to minimize costs.
5.1 GNSS
Geodetic GNSS has already evolved from GPS-only
operations to inclusion of GLONASS, and upgrades to next-generation receivers
will allow full benefit from modernized GPS signal structures, Galileo signals,
and GLONASS signals. Studies leading to
improved handling of calibration issues such as local signal effects (e.g.,
multipath) and antenna phase patterns are underway, as are initiatives to fill
remaining network gaps, particularly in the southern hemisphere. Elsewhere, station density is less
problematic and the focus has shifted to consolidation of supplementary
instrumentation such as strain meters and meteorological sensors.
5.2 Laser Ranging
Newly designed and implemented laser
ranging systems operate semi-autonomously and autonomously at kilohertz
frequencies, providing faster satellites acquisition, improved data yield, and
extended range capability, at substantially reduced cost. Improved control
systems permit much more efficient pass interleaving and new higher resolution
event-timers deliver picosecond timing. The higher resolution will make
two-wavelength operation for atmospheric refraction delay recovery more
practical and applicable for model validation. The current laser ranging
network suffers from weak geographic distribution, particularly in Africa and
the southern hemisphere. The comprehensive fundamental network should include
additional co-located sites to fill in this gap.
Improved
satellite retroreflector array designs will reduce uncertainties in
center-of-mass corrections, and optical transponders currently under development
offer opportunities for extraterrestrial measurements.
5.3 VLBI
The VLBI component of the future
fundamental network will be the next-generation system now undergoing
conceptual development. Critical elements include fast slewing; high efficiency
10-12 m diameter antennas; ultra wide bandwidth front ends with continuous RF
coverage; digitized back ends with selectable frequency segments covering a substantial
portion of the RF bandwidth; data rate improvements by a factor of 2–16; a
mixture of disk-based recording and high speed network data transfer, near real
time correlation among networks of processors, and rapid automated generation
of products. Better geographic distribution, especially in the southern
hemisphere, is required.
5.4 DORIS
The DORIS tracking network is being
modernized using third-generation antennae and improvements to beacon
monumentation (Tavernier et al., 2003; Fagard, in preparation). Efforts are
underway to expand the network to fill in gaps in existing coverage. DORIS
beacons are also being deployed to support altimeter calibration, co-location
with other geodetic techniques, or specific short-term experiments. A specific
IDS working group is selecting sites and occupations for such campaigns, using
additional DORIS beacons provided by CNES to the IDS.
5.5 Gravity
Gravity observations are most sensitive to
height changes; they therefore provide an obvious way to define and control the
vertical datum. A uniformly-distributed network of regularly cross-calibrated
absolute gravimeters supported by a well-designed relative measurement network
that will be repeatedly observed at regular intervals, and a sub-network of
continuously operating superconducting tidal gravimeters are expected in a
fundamental network of co-located techniques. These permanent networks should
be augmented with targeted airborne and ship campaigns to collect data over
large areas that are devoid of gravimetric observations. A well-distributed
global data set of surface data is necessary to calibrate and validate products
of the recent (CHAMP and GRACE) and upcoming (GOCE) high-accuracy and
-resolution missions. Eventually, gravimetry will need to devise a method
analogous to InSAR, to continuously “map” changes in the field with resolution
many orders of magnitude higher than currently achievable from any geopotential
mapping mission.
6 Approaches to Network Design
The final
design of the GGOS network must take into consideration all of the applications
including the geometric and gravimetric reference frames, EOP, POD, geophysics,
oceanography, etc. We will first consider the TRF, since its accuracy
influences all other GGOS products. Early
steps in the process are:
1.Define the critical
contributions that each technique provides to the TRF, POD, EOP, etc.
2.Characterize the improvements
that could be anticipated over the next ten years with each technique.
3.Examine the effect in the TRF
and Earth orientation resulting from the loss of a significant part of the
current network or observation program.
4.Using simulation techniques,
quantify the improvement in the TRF, Earth orientation and other key products
as stations are added and station capability (co-location, data quantity and
quality) is improved. We will also explore the benefit of adding new SLR
targets.
6.1 Impact of Network Degradation on the TRF
Preliminary results (Govind, 2005) indicate
the origin drift caused by removal of one station, Yarragadee (Australia), from
SLR analysis. The drift is about 0.6, 1 and 1mm/yr over the origin components
around the three axes X, Y Z, respectively. This drift is at least three times
larger than requirements for high -precision Earth science applications such as
sea level change and other geophysical processes.
6.2 Effect of System and Network Degradation on Other GGOS
Products
The TRF is a primary space geodesy product,
but it is also the basis on which every other product is referenced. As such,
degradation in its definition and maintenance influences the quality of these
other products and services, such as EOP, geocenter motion, temporal global
gravity variations, and POD.
The
degradation can originate in two ways: geometric changes (as those shown by the
example of sec. 6.1) and changes in the type, amount and spatiotemporal
distribution of the observations. In practice what happens is a combination of
both. To quantify the resultant errors is not an easy task because there are
infinite possible variations in the network of TRF stations, supporting
techniques, and selection of data. Examination of particular station deletions
that either happened in practice or had been proposed indicates (Pavlis, 2005)
that even moderate degradations impact results significantly more than their
quoted accuracies. This confirms the present ILRS network is not robust to any
contraction; the smallest perturbation of the system yields large uncontrolled
changes in the products.
The
closing of the Arequipa and Haleakala SLR sites for example, degraded origin,
orientation and scale of the by 3-4 times the standard deviation of the relevant
parameters. Impact on geocenter motion was almost two times worse. Temporal
gravity variations are less sensitive due to their nature as proxies of global
scale changes, but were still degraded by several standard deviations. On the
positive side, for a modest improvement from an old TRF (ca. 1995) to the
current one (ITRF2000), POD-based products (such as altimeter derived Mean Sea
Level) improved by 30%.
Much
more work is required to assess the effects of such changes in the tracking
networks of all space geodesy techniques, and their combined effect on the
final products. The sizes of these separate networks and the infinite possible
variations in their design, overlap and operation, and the quality of their
data and the targets used for collecting their observations complicate this
task, but a few well-thought-through scenarios will be tested with future
simulations.
6.3 Improvements in the TRF and Other Key Products
Expected advances in instrumentation, as
described in section 5, will cause improvements in the TRF and the various
products, but the accuracy needed for future science applications will require
optimization of the ground network. Simulation capabilities will be developed
that will allow for evaluation and optimization of the locations of potential
sites.
In
addition, the benefit of introducing a few new SLR targets needs to be
evaluated. Target interaction with the current large LAGEOS satellites is one
of the principal limitations in mm-level SLR, and smaller targets would support
the necessary accuracy. New lower-altitude targets would allow more observation
opportunities per day, increased probability of tracking from lower-power
systems (particularly during daylight) and a more accurate determination of the
Earth’s mass center, critical for both controlling the drift in the origin of
the TRF as well as observing the seasonal geocenter motions associated with
large-scale mass transport within the Earth system.
Simultaneously,
enhanced performance of each of the individual techniques should result as each
technique’s data and analysis outputs are further combined and compared and
eventually integrated.
7 Sustaining the Ground Network Over the Long Term
The
measurement techniques Services have each maintained their own networks and
supporting infrastructure, routinely producing data, but suffer from severe
budget constraints of the voluntarily contributing agencies that prevent
appropriate maintenance and development of physical and computational assets.
This degradation of the observing network capability coincides with high value
science investigations and missions, such as sea level studies from ocean and
ice-sheet altimetry missions, eroding their scientific return and limiting
their ability to meet the mission goals.
Many of the elements of the current networks are funded from year to year and depend upon specific activities. Stations are often financed for capital and maintenance and operations costs through research budgets, which may not constitute a long-term commitment. Sudden changes in funding as priorities and organizations change have resulted in devastating impacts on station and network performance. On the other hand, missions and long term projects have assumed that the networks will be in place at no cost to them, fully functioning when their requirements need fulfillment. GGOS will be proactive in helping to persuade funding sources that the networks are interdependent infrastructure that needs long term, stable support. The GGOS community must secure long-term commitments from sponsoring and contributing agencies for its evolution and operations in order to support its users with high-quality products. In view of the difficulties in securing long-lasting and stable financial support by the interested parties, new financial models for the networks must be developed. This Working Group will work with the Strategy and Funding Working Group to develop an approach.
Since the present networks must support current as well as future requirements, the GGOS network must evolve without interruption of data and data products. In particular, the TRF relies on a long continuous history of data for its stability and robustness. New and upgraded systems, changes in stations locations, and changes in the way products are formed must be planned and phased so that the impacts are well documented and well understood.
The analysis and simulation procedures being undertaken by the Working Group will identify network voids and shortcomings. The Ground Networks and Communications Working Group, in concert with the other GGOS entities, will work with agencies and international organizations toward filling in these gaps.
8 Summary
A permanent geodetic network of
complementary yet interdependent space geodetic techniques is critical for geodetic
and geophysical applications and underpins the Global Earth Observation System
of Systems. Thanks to the generous and voluntary contributions of many national
agencies and institutions around the world, the IAG has been able to coordinate
global collaborations for geodetic technique based services from which all
benefit. There is a strong need for coordination of the planning, funding and
operation of future integrated geodetic networks to maximize performance in
meeting evolving requirements while taking into account the need for
sustainable infrastructure and efficient operations. The GGOS Ground Networks
& Communications Working Group has initiated studies which will guide the
services in infrastructure planning for optimal benefit to Earth science and associated
engineering and societal concerns.
Acknowledgments
The authors would like to acknowledge the
support of IAG services (IGS, ILRS, IVS, IDS, IGFS, and IERS) and their
participating organizations. Part of this work was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under a contract
with the National Aeronautics and Space Administration.
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