**Regional Densification of the IGS in Europe**

**Using the EUREF Permanent GPS Network (EPN)**

## Carine Bruyninx

Royal Observatory of Belgium

Avenue Circulaire 3

B-1180 Brussels, Belgium

## Matthias Becker

Federal Agency of Cartography and Geodesy

Richard Strauss Allee 11

D-60598 Frankfurt am Main, Germany

*Günter Stangl*

Space Research Institute of the Austrian Academy of Sciences

Elisabethstrasse 20

A-8010 Graz, Austria

***Summary***

> *In 1995 EUREF took the initiative to start coordinating the
> activities related to existing permanent GPS stations in Europe and
> created the EUREF Permanent Network (EPN). Part of this coordination
> task includes the creation of the structure to make tracking data and
> auxiliary data, as well as products of various kinds, publicly
> available.*
>
> *The establishment of the EPN Coordination Group (June 2000)
> guarantees the continued provision high quality data and services. It
> will allow the continuation of an efficient management of the
> expanding GPS tracking network and a growing array of related
> multi-disciplinary projects.*
>
> *Half of the EUREF stations are also belonging to the IGS network and
> consequently have their auxiliary information also available at the
> global level. Since EUREF is delivering, as RNAAC, regional network
> solutions to the IGS, a full compliance of standards between the IGS
> and EUREF network is primordial.*

### Introduction

EUREF is the subcommisison for Europe within Commission X \"Regional and
Continental Networks\" of the International Association of Geodesy.
EUREF is responsible for the maintenance of the European Reference
System (ETRS89). In 1995, EUREF took the initiative to create the EUREF
Permanent GPS Network (EPN), based on existing permanent GPS tracking
stations, data centres and analysis centres in Europe. At the same time,
EUREF proposed to the IGS to accept the EUREF network as the regional
densification for Europe of the global IGS network (Resolution no 2 of
the EUREF symposium in Helsinki, 1995). In May 1996, EUREF became one of
the IGS Regional Network Associate Analysis Centres (RNAAC).

### Network Components

***2.1 Stations***

Mid 2000, nearly 100 stations (Figure 1) were listed as part of the
EUREF Permanent Network, half of them belonging also to the IGS network.
Responding to a general request, since June 1998 EUREF accepts stations
from outside Europe (North Africa, Middle-East) for inclusion into its
network. These stations are known as \"*Associated EUREF stations\"* and
their inclusion in the EUREF network should allow to better assess the
motion of the European plate with respect to the neighbouring
continental plates.

Since mid 1999, tracking stations not fulfilling the IGS/EUREF standards
for more than three months receive the label *\"inactive\"* with as
direct consequence, that EUREF stops its engagement to monitor and
process the station data and auxiliary information.

![](media/image1.wmf)

Figure 1. Network of EUREF tracking stations, as of July 1, 2000

2.  *Data Flow*

The EPN is thoroughly diagnosing the performance of the data flow within
its network. Presently, about 95% of the daily RINEX files arrive at the
data centres within one day, most of them within a few hours after
midnight. However, missing files are not always announced as lost or
delayed. This part of the communication will be improved in the future,
the EPN Central Bureau (CB) plans to monitor data outages and distribute
information about station status in the case of data outages exceeding
one week.

Hourly data uploads have been initiated in October 1998; more than one
third of the EUREF stations (37) presently provide hourly data. Although
hourly data files are smaller, more communications have to be set up
increasing the danger of missing files. Nowadays the information about
missing or delayed hourly EPN data is close to zero. It is clear that
the availability of the hourly files has to be improved before replacing
the daily files by a concatenation of hourly files.

The BKG data centre, which is the IGS regional data centre for Europe,
runs a quality check program on all incoming RINEX data and makes the
resulting summary file available together with the original RINEX data
file. All EUREF data centres should apply this principle.

EUREF stations can make available their data at an EUREF local data
centre and/or at the regional data centre at BKG. 29 % of the EUREF
stations have presently no data redundancy. In order to avoid outages
due to the inaccessibility of one data centre, it is recommended to have
the data from the EUREF stations available at both a local data centres
and the regional data centre. Since 18 % of the EUREF stations make
their data only available to BKG, a fall back strategy should be
developed. Either a second server should act as a BKG mirror or the data
flow should switchover to agreed-upon local data centres (storage
network). This means that regulations for redirecting the data flow in
case of outage need to be defined.

# *2.3 Data Analysis*

# The EUREF data analysis is based on the distributed processing approach. Twelve Local Analysis Centres (AC) process each a well-defined subnetwork of the EUREF network. The quality of the combined solution is shown by the combination of the loosely constrained weekly solutions into a multi year combined solution. The average of the residual's rms is 1.6, 1.7 and 4.9 mm for the North, East and up components respectively.

All AC\'s follow specific analysis guidelines, adopted in April 1997 by
the AC\'s to guarantee the homogeneity of the EUREF solution (Bruyninx
et al., 1997). These analysis guidelines have aged and new analysis
guidelines will be investigated and implemented in 2000. Different
processing strategies will be simultaneously tested at the EUREF AC\'s
and the effect on the individual station coordinates as well as on the
overall network will be investigated before introducing any new
strategies that will influence the official combined EUREF solution.
This may help in reducing the systematic differences between AC's, e.g.
by consistently modelling ocean tidal loading, improving the
tropospheric zenith delay estimation (mapping function and elevation
cut-off) and more.

The EUREF Analysis Centres Workshop, \"Towards Multi-disciplinary EUREF
products\", (Marne la Vallée, France, September 1999) addressed issues
such as the creation of a future troposphere product. One of the
outcomes of the workshop was an endorsement of the strength of the
multi-centres data analysis (at least three analysis centres for each
EUREF station) and a formal engagement of the EUREF AC\'s to extend
their sub-networks in order to guarantee this principle. As a result, at
the end of 1999, 90 % of the EUREF stations were processed by 3 AC\'s, 8
% by 4 AC\'s and 2 % of the stations is only processed by two AC\'s.

### EUREF Contribution to the ITRF2000

Thanks to the IGS Densification Project, the EUREF stations are included
in the GNAAC P-SINEX solution submitted to the IERS. However, there is
no official engagement of the GNAACs to submit solutions for the EUREF
stations to the IERS. In addition to this, the auxiliary information
(such as station log files) of the EUREF stations is not maintained at
the IGS level, but at the regional (EUREF) level. Therefore, since 1997
EUREF started to submit its own regional solution to the IERS.

The EUREF submission for the ITRF2000 is a multi-year combination based
on the SINEX files of the weekly EUREF solutions. It comprises the time
period June 30, 1996 -- January 1, 2000 and includes solutions for 95
stations. At some sites discontinuities in one of the coordinate
components occurred which could not be associated to logged changes in
eccentricities or antennas. For them one velocity but two sets of
coordinates for different time periods were estimated. Sites with a
recording history shorter than 6 months were constrained to their NUVEL
1A NNR velocities. The a posteriori rms of this solution was 3.6 mm.

Two versions of the solution have been generated, a loosely constrained
solution and a solution constrained to the ITRF97 coordinates and
velocities of the following core sites: MATE, ONSA, POTS, WTZR, ZWEN.

For a first assessment of the accuracy of the EUREF contribution the
solution was compared to the ITRF2000 contribution of CODE (*Springer,
pers. comm.*). The rms of the differences of the 38 sites in common are
.9, 2.7 and 4.9 mm in North, East and Up components at a central epoch.
The agreement is excellent and is rather consistent with the internal
precision of the weekly solutions.

### 4 Management and Coordination {#management-and-coordination .unnumbered}

The EUREF Technical Working Group (EUREF TWG) is responsible for the
general management of the EUREF Permanent GPS network. It is made up
from representatives of national survey agencies, universities and
liaison person to the IAG and IERS.

In 1995, when EUREF started to coordinate the activities related to the
permanent GPS stations in Europe, the coordination task was attributed
to one network coordinator. The main usage of the network was at that
time purely reference frame related: the realization and maintenance of
the ETRS89.

Since that time, the EUREF network has evolved from about 30 permanent
tracking sites processed by four analysis centres to close to 100
stations and twelve analysis centres. In addition to this the data,
structure and results of the EPN have drawn the interest of a wide
variety of scientific users.

In order to continue to provide a high quality data and products, EUREF
decided, at its tenth Symposium in Tromsø, June 22-24, 2000 to formally
establish an EPN Coordination Group, an EPN Central Bureau and EPN
Special Projects.

The Coordination Group (CG) coordinates all activities related to the
permanent network and special projects and proposes policy to the EUREF
TWG. The CG consists of the network coordinator, data flow coordinator,
analysis coordinator, special project liaisons and an EUREF TWG
representative.

Special Projects are set up by the CG in order to introduce new
applications into the EPN, e.g. presently the generation of an EUREF
troposphere product, or study special aspects of the permanent network.

The EPN Central Bureau, managed by the network coordinator, is
responsible for the day-to-day general management of the EUREF permanent
network consistently with the directives, policies and priorities set up
by the EUREF TWG.

A completely re-styled EUREF web-site (http://homepage.oma.be/euref/)
including clickable maps, reviewed guidelines for the network
components, etc... was presented to the EUREF community in 2000. As part
of this effort, individual site information pages have been created. In
addition to the site description logs, these pages also contain
additional information (e.g. meteorological data, site pictures,
collocation with tide gauges) responding to a request for more detailed
documentation as needed by some other scientific applications.

### Relation to the IGS

Because of the large number of stations involved in the EPN, covering
more than 30 European countries and at least as much different agencies,
EUREF had to organise the coordination of its network efficiently. Since
half of the EUREF stations are also belonging to the IGS network and
consequently have their auxiliary information available at the global
level, EUREF has based its structure on the IGS structure and has
distributed and promoted the IGS standards within its network. Identical
site log and exchange formats are used. This means that presently IGS
stations also belonging to the EUREF network make identical auxiliary
information available to both the IGS CB and the EPN CB, which could
lead to some confusion.

Since the EPN delivers, as IGS RNAAC, weekly free network solutions to
the IGS it is clear that antenna and receiver name conventions (and
calibration values) have to be set up at global level guaranteeing that
all different types of analysis centres within IGS comply to the same
standards. EUREF distributes these conventions and requests from all its
network components to fully comply with them. In order to perform this
task efficiently, a close link between the IGS Central Bureau and the
EPN Central Bureau is necessary. Due to the extensive number of people
involved in the EPN, decisions made at the IGS level (such as a site log
change) can be the cause of a considerable additional workload on the
EPN CG.

Presently the communication between the EUREF RNAAC and the GNAAC\'s is
limited to the exchange of the GNAAC P-network reports. The EUREF RNAAC
is now setting up a routine which will fetch the global P-SINEX from
CDDIS and do some backward comparison with EUREF solutions and
cross-analysis.

The representation of the EUREF AC-coordinator in the IGS AC-group, is a
good step to foster and advance the integration and possibly the
improvement in the EUREF products. This will open a direct channel for
communication.

### Conclusion

The continuous extension of the EUREF permanent network, both through
its components and its applications, has lead to the establishment of
the EPN Coordination Group. It will focus on a continuing provision of
high quality data and products supporting a wide range of applications
and it will allow the network to flexibly adapt to future demands and
applications. Part of this effort will be the strengthening of the link
between the IGS and EUREF, both through the use of common standards, but
also by improving the quality and latency of the EUREF data and
products.

**7 References**

Bruyninx, C., D. Ineichen and T. Springer (1997), \"The EUREF RNAAC:
1997 Annual Report \", in International GPS Service for Geodynamics,
1997 Annual Report, Jet Propulsion Laboratory, Pasadena, California