Guidelines for a GPS EPN
by W. Gurtner, Astronomical Institute - University of Bern - May 1995
1. Introduction
At the EUREF Symposium in Warsaw, 8-11 June 1994, the Technical Working Group presented a proposal for the establishment of a network of permanent GPS stations in Europe. This network should in fact consist of a selection of those permanent stations that have been or will be installed
and the data of which are made available to EUREF by the responsible organizations.
Apart from the stations themselves such a permanent network needs the following components :
- Operational Centres (OC) :
-
They perform data validation, data conversion to RINEX, data compression, data upload to a data centre through Internet. For some sites the OC are identical with the institution responsible for the respective sites.
- Local Data Centres (LDC) :
-
They collect the data of all stations of the respective network and distribute them to the (local) users. For many of the local networks the Local Data Centre will be identical with their Operational Centre. If the local network takes part of the regional or global network (EUREF, IGS), the LDC will forward (a selection of) the data to the Regional Data Centre.
- Regional Data Centre (RDC) :
-
It collects all the data needed for the regional processing and makes it available to the users, especially to the European Analysis Centre(s)
- Regional Analysis Centres :
-
They process part or all of the European data to generate a continuous record of positions and other parameters (e.g. troposphere or ionosphere)
- A Network Coordinator
-
responsible for the coordination of all activities related to this EUREF Permanent Network
With the exception of the Network Coordinator all components more or less already exist, either within the organizations for the primary objectives of the local network, or as part of the International GPS Service for Geodynamics.
This paper describes the requirements the stations and Operational Centres have to meet to be included into the EUREF Permanent Network.
2. Requirements for Permanent GPS Stations
As the user of the network will be mainly interested in the data and the necessary auxiliary informations about the tracking sites, we do not distiguish too strictly between the activities and responsibilities of the Stations and the Operational Centres.
As EUREF would like to make use of existing networks it is of course not possible to set up very strict rules and guidelines. This paper is rather intended to be used to check if a station or part of a local network can actually be used for the EUREF Network.
Many of these guidelines are identical with those in use within the International GPS Service for Geodynamics.
2.0 Sites
It is recommended to collocate permanent GPS sites with observatories of other physical phenomenae such as
Although modern receivers can run unattendedly and remotey controlled, stations regularly manned with staff are nevertheless preferable.
Because geodetic links over several kilometers can be done by GPS with millimeter accuracy, it is not absolutely necessary that the permanent station is put to an existing geodetic marker. Other criteria like the colocation with other techniques or the connection to host computers are more important.
2.1 Instrument
The GPS receivers should
2.2 Antenna
2.3 Marker
The marker should fulfill standard requirements for a first order geodetic monument with respect to stability, durability, long-term maintenance, documentation, and access.
Obstruction should be minimal above 15 degrees elevation.
Signal reception quality has to be verified, especially with respect to interference of external signal sources like radars, and with respect to multipath.
The antenna height corresponds to the vertical distance of the agreed-upon physical reference point (see antenna diagrams) on the antenna above the marker.
Local ties to other markers on the sites have to be determined in a coordinate system parallel enough to ITRS to guarantee the millimeter in all three directions. They are given in delta-x, delta-y, delta-z, x,y,z being the geocentric cartesian coordinates (ITRS).
2.4 Documentation
A standard IGS site description file should be filled out and sent to the EUREF Network Coordinator :
e-mail: [email protected]
Blank forms are available through anonymous ftp at IGS Central Bureau Information System (CBIS) :
Internet: igscb.jpl.nasa.gov
File: igscb/station/log/BLNKFORM.LOG
or
IGN Institut Geographique Nationale (CBIS mirror site) :
Internet: schubert.ign.fr
File: /pub/igscb/station/log/BLNKFORM.LOG
Additional site descriptions (photos, maps, etc) should be sent to the Network Coordinator.
3. Operational Centres
3.1 Responsibilities
The Operational Centres are the institutions controlling the sites of a particular (local) network from the operational point of view.
They are the link between the sites and the Data Centre that makes available the data to the Analysis Centres, other Data Centres or individual users.
The Operational Centres are responsible for
Within the International GPS Service for Geodynamics there are many independent tracking sites (e.g. Zimmerwald, Herstmonceux) that are not part of a local / special network and as such not connected to an actual Operational Centre. In this case the organization operating the site of course also performs the tasks outlined above.
3.2 Data Download From the Stations
The downloading from the receiver to the Operational Centre's computer system can either be done directly or indirectly through a small on-site computer, e.g. a PC :
---------- ------------
| receiver |------| on-site PC |-----
---------- ------------ | --------------------
|-----| Operational Centre |
---------- | --------------------
| receiver |-------------------------
----------
The PC could even download the data from the receiver continually, using e.g. some manufacturer-provided download software.
The communication between the Operational Centre and the stations is done through dialup modem, Internet, special-purpose data links, Inmarsat, etc.
3.3 Data Archive
As the exchange data format does not conserve all information found in the raw data it should not be used for the primary data archiving. The Operational Centre is responsible for the long-term data archiving unless this task has been delegated to the sites. Usually the original raw data files or compressed (e.g. zipped) raw data files are archived.
The EUREF Regional Data Centre does archive the RINEX files for the general benefit of EUREF only.
3.4 Data Format
The data has to be prepared in daily (24 hours) RINEX files, both for observations and broadcast navigation messages.
The daily observation files contain the observations collected between 00:00:00 and 23:59:30 GPS time. The sampling rate (observation interval) has to be 30 seconds. In case of a higher original observation rate a decimation of the data to the currently adopted 30 seconds is mandatory.
The header information, especially the station name, receiver and antenna information, antenna height has to be up-to-date and has to strictly follow the agreed-upon conventions (see blank log forms for the stations).
The navigation message file contains all messages with TOC/TOE (time of clock, time of ephemeris) between 00:00 and 23:59 GPS time of the respective day.
It is recommended to generate a combined daily RINEX navigation file
containing non-redundantly all navigation messages collected by all
sites of the local network. The filename (part "ssss", see below)
should then contain a 4-character code of the Operational Centre.
The RINEX files are prepared in a compressed form using the standard UNIX
compress program. Compress and decompress programs for other platforms
(PC/DOS, VAX and Alpha VMS) are available at the IGS CBIS (directory
/igscb/software/compress).
The files are to be named according to the following table:
------------------------------------------------------------------------------
| File Type ASCII File Compressed File |
| Unix VMS DOS |
------------------------------------------------------------------------------
|Observation File ssssdddf.yyO ssssdddf.yyO.Z ssssdddf.yyO_Z ssssdddf.yyY |
|Navigation File ssssdddf.yyN ssssdddf.yyN.Z ssssdddf.yyN_Z ssssdddf.yyX |
------------------------------------------------------------------------------
ssss: 4-character station code
ddd : day of the year of the first record
yy : year
f : file sequence number within the day
(0: containing all data of the day)
Files sent to another host have to be named on the target system in accordance with the target operating system:
Example:
Put a file from a Unix to a VMS system: binary
put zimm1230.94O.Z zimm1230.94o_z
3.5 Data Validation
Data should be checked before sent to a Data Centre. A minimum verification should consist of a check of
The use of a true quality check program is highly recommended, e.g. UNAVCO's QC program (available through the IGS Central Bureau Information System or at its mirror site at IGN, directory /igscb/software/qc).
3.6 Data Handling
It is highly recommended that all steps in the data handling (download from the receiver, reformatting, data validations, generation of statistics, data archving and transmissions be fully automated. Data failing to pass the validation step should be kept back for manual treatment.
3.7 Data Upload
The data has to be sent daily to the Regional Data Centre (Institute for Applied Geodesy, Frankfurt), via a Local Data Centre if appropriate, by anonymous ftp using binary (image) "put" :
Internet: igs.ifag.de
System: Unix
Directory: /indata
For initial contacts and additional information write to :
Heinz Habrich, IfAG : [email protected]
3.8 Timeliness of the Data Transfer
Currently (March 1995) there is one EUREF Analysis Centre only (CODE in Berne) that routinely processes the data of the European Permanent Stations. As this data analysis is performed more or less simultaneously with the global IGS analysis the data has to be available within 48 hours after observation at the Analysis Centre, or 72 hours for Friday data.
As soon as other Analysis Centres are operating, this schedule might be changed.
Early 1995 CODE will change its procedure to allow for later arrival of the data. It is planned to process data that arrives at CODE within 7 days after observation.
Data that is not guaranteed to be processed should not be accumulated at the Regional Data Centre.
(Depending on the future number of stations and Analysis Centres it might well be that the processing of some stations is limited to certain time periods (e.g. two weeks every two months)).
3.9 Documentation
The IGS Central Bureau Information System makes available a blank form for an Operational Centre description file (blankform.ocn in /igscb/center/oper). This form should be filled out by the Operational Centre or by the agency operating an independent permanent GPS station and sent to the Network Coordinator.
4. Local Data Centres
4.1 General
Depending on the policy of the respective agencies the tracking data and the auxiliary station information of the local network can be openly made available through computer networks or bulletin boards etc. This task would be the responsibility of a Local Data Centre.
4.2 Data Access
It is recommended to allow access to (all or part of) the data through Internet's anonymous ftp, currently being the most effective and easy to use access method, especially for automated data download. Another access procedure on Internet is through World Wide Web (WWW) servers with easy to use browsers such as Mosaic or Lynx.
4.3 Available Information
It is recommended to make available at least the following information :
****** B B G H H J K K K L M M M M M N O O P T U V W W Z
IFAG O R R E F O I I O A A A A D E Y H N O R P I E T I
****** R U A R L Z R T S M D S T V T A I S T O A L T Z M
1 S Z S K E U 3 G A R 1 E O S L G A S M D L T R M
******* Last Update : 8-MAR-95 20:20 (Day 067) ********
95-066 . 1 1 1 1 1 1 1 1 1 1 1 . . 1 1 1 1 1 1 . 1 1 . 1
95-065 2 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 . 1 1 1 1
95-064 2 1 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 1 1 1 2 1 1 1 1
95-063 3 1 1 2 1 1 2 1 1 1 1 2 1 2 1 1 1 1 1 1 3 2 1 1 2
95-062 1 1 1 1 1 1 3 1 1 1 1 3 1 2 1 3 1 1 1 1 4 3 1 1 1
95-061 . 1 1 1 1 1 1 1 2 1 6 1 1 3 1 1 1 1 1 1 1 1 1 1 1
95-060 2 1 1 1 1 1 1 1 . 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1
95-059 2 1 1 1 1 2 1 1 . 2 2 1 1 2 1 1 2 1 1 1 . 1 1 1 1
95-058 2 1 1 2 1 1 1 1 . 1 8 1 1 . 1 1 1 1 1 1 1 1 1 1 1
95-057 2 1 1 1 1 1 1 1 1 1 3 1 1 . 1 1 1 1 1 1 1 1 1 1 1
95-056 3 1 1 1 1 1 1 2 1 2 4 1 1 . 1 1 1 1 1 1 1 1 1 1 1
Example of a data holding file for IfAG.
4.4 Data Organization
It is recommended to organize at least the tracking data in a hierarchical directory structure. From the user's point of view it is
usually preferable to combine all the data of one day (or one week) into one directory than to have station-dependent
directories. Examples of directory organizations can be found in the IGS Data Centre description files (CBIS, directory
/igscb/station/dcn).
All the other information of course can be made available either through ASCII files in a directory tree or through
more advanced means like data bases or hypertext documents or, preferably, through both.
The Local Data Centre description file and the daily updated data holding file should be made available to the IGS Central Bureau Information System.
4.5 Data Transfer to the Regional Data Centre
The tracking data of stations that are also part of an upperlevel network (EUREF, IGS) have to be forwarded through Internet to the Regional Data Centre (Institute for Applied Geodesy, Frankfurt), by anonymous ftp using binary (image) "put" :
Internet: igs.ifag.de
System: Unix
Directory: /indata
For initial contacts and additional information write to :
Heinz Habrich, IfAG : [email protected]
5. Analysis Centres
All 7 IGS Analysis Centers process routinely at least some of the European permanent tracking stations. The Centre for Orbit Determination in Europe (CODE) currently processes all European sites made available to the European Regional Data Centre. With the growing number of permanent stations that could be used for the benefit of the European Reference System it might be necessary to distribute the processing to several Analysis Centres.
Such distributed processing is also in discussion in the IGS. The procedures of how to combine partial solutions (including an exchange format for solution exchanges) are currently developed.
The EUREF Analysis Centre(s) should generate a (combined) time series of coordinates of all permanent sites included in the EUREF Network with a resolution of 3 months or better.
The solutions are primarily based on the reference frames used within IGS, namely ITRF-yy, yy being some yearly realization. The transformation of the solutions to the European Reference System have to be performed according to the EUREF guidelines issued and updated by IGN.
6. Network Coordinator
The Network Coordinater coordinates and supervises all activities within the EUREF permanent network.
He/she is in permanent and close contact with
He/she proposes to the EUREF TWG the topology of the EUREF permanent network, i.e. the stations to be included/excluded, their data flow and coordinates their data analysis.
He/she monitors the data flow through the EUREF network.
He/she coordinates the flow of information within the EUREF network and towards the International GPS Service (IGS) and other organizations.
He/she prepares, in collaboration with all participating agencies, a yearly report.
Dr. Carine BRUYNINX Tel. +32 2 373 0292
Observatoire Royal de Belgique Fax: +32 2 374 9822
Avenue Circulaire 3 E-Mail: [email protected]
B - 1180 Brussels
has been appointed as EUREF Network Coordinator.
7. Information Exchange
The distribution of information within the network (i.e. between Operational, Data, and Analysis Centres) is preferably assured through e-mail. Stations operated by an independent Operational Centre do not have to be directly on the distribution lists. The information from and to these stations will flow through their Operational Centre.
The e-mail addresses should point to mailboxes that are checked at least once per working day. Fax and phone communication should be available, too.
It is recommended to subscribe to the IGSMail service run by the IGS Central Bureau (send e-mail to [email protected]).
8. Check List For Becoming a Permanent EUREF Station
- Contact the EUREF Network Coordinator concerning the intent to install the station, the schedule for implementation, and a statement of desire for the station to be considered as part of the permanent EUREF network.
- Follow the EUREF and IGS standards in installing the station.
- Once the station is installed and operational, a communication should be addressed to the EUREF Network Coordinator indicating data availability. Describe the planned data flow (including schedules) in details. Send a copy of the mail to the Regional Data Centre and to the EUREF Analysis Centre(s).
- The EUREF Network Coordinator will assist in the designation of the 4-character station identifier to prevent duplication
- The new station or its responsible Operational Centre has to create an
IGS station log (blank formats are available at the IGS Central Bureau
Information System and at its mirror site at IGN) :
IGN mirror site :Internet: schubert.ign.fr File: /pub/igscb/station/log/BLNKFORM.LOGIGS Central Bureau Information System (CBIS) :Internet: igscb.jpl.nasa.gov File: igscb/station/log/BLNKFORM.LOG - Send the EUREF log file to the Institut Geographique Nationale (IGN) :
E-Mail: [email protected]
and ask for the assignment of a DOMES number, and, if not yet known, a EUREF number. If appropriate, update the IGS log file with the DOMES number and send it to the EUREF Network Coordinator :E-Mail: [email protected]
- Whenever modifications of the log files are necessary, first download the currently valid file from the Information System and send the updated file back through e-mail
- A sample file of each RINEX file type should be sent to the EUREF Regional Data Centre for verification.