From IDS Central Bureau                                                                                              November 2001, the 26th

Subject : Invitation to initiate IDS analysis

Dear colleagues of Analysis Centers,

The IDS steering committee is still looking for an analysis coordinator. The Central Bureau charged  CLS to initiate comparisons of station network coordinates data sets. CLS operates SSALTO, the DORIS multi-mission orbitography, altimetry and location ground segment. It is also responsible for the short delay (48h) station network positioning. The software we’ll use for analysis is CATREF, developed by Zuheir Altamimi and applied for ITRF realizations by IERS. Comparisons will be processed at CLS by myself and Laurent Soudarin.

That’s why we now invite every DORIS analysis centers to process data and send to us their solutions. To start with, we proposed June 2001 as a test period. Solar activity was at a low level and no maneuver was carried out on the SPOT2, SPOT4 and TOPEX-POSEIDON satellites. First comparisons would be based on :
- one solution of station positions averaged over June 2001,
- monthly (or weekly) position time series covering 1999-2000,
- global solution (positions and velocities).
Computations may be applied to mono or multi-satellite data sets. In practice, the expected  format will be the SINEX format (attached sinex-doris.snx). Details will be given very soon where to deliver the files, probably on the CDDIS Data Center.

As the strategy of data processing is important, we propose that each group fills the attached file form (blnckform.acn). The contains is very similar to one that can be found at the International GPS Service ( ftp://igscb.jpl.nasa.gov/igscb/center/analysis/ ). Items have been adapted to the DORIS context. Please send me your acn files as soon as possible ( Jean-Jacques.Valette@cls.fr ). They will be very soon accessible on an ftp server on the ids.cls.fr web site.

Via IDS web page related to IDS proposal ( http://ids.cls.fr/html/report/proposals.html ), we’d like to insist on the analysis contributors. So, we’d be very pleased if any group could give us a link to existing web pages presenting its activities or any useful information to insert.

Initiating such a preliminary comparison of future geodetic DORIS products before the end of the year would be useful as a first step for everybody in preparation of the IDS workshop on 13th of June 2002 in Biarritz, France (http://www.jason.oceanobs.com/html/portail/general/welcome_uk.php3 ).

The annex A gives recommendations on the solution computations in particular on the constraints used to define the Terrestrial Reference Frame datum.

Thanks in advance for your contribution,

Best regards,

Jean-Jacques Valette, Gilles Tavernier, Laurent Soudarin, Jean-Pierre Granier, Hervé Fagard

P.S :  This mail is send to all the persons responsible for an analysis center. They are :

Suriya Tatevian, INASAN-RAS, Moscow, Russia
Ramesh Govind, AUSLIG/Australia
John Dow, ESA/ESOC, Germany
Jan Kostelecky, Geodetic Observatory Pecny Czech Republic
George Krasinsky, IAA-RAS, St Petersbourg, Russia
John Ries,  University of Texas/CSR USA
Philip Moore, University of Newcastle, UK
Pascal Willis,  JPL USA - IGN/LAREG, France
René Warnant, Royal Observatory of Belgium, Brussels,  Belgium
Jean-Paul Berthias,  CNES/SOD France
Jean-Francois Cretaux, LEGOS-GRGS / CLS,  Toulouse France

We know that not all of them have expressed they intention of producing station coordinates.

ANNEX  A

Recommendations for DORIS set of station coordinates

Epoch of reference

The epoch of reference in the solution/estimates record of the Sinex file corresponds to the median epoch of the data set.

Solution constraints for Terrestrial Reference datum definition

Constraints upon all or sub-set of stations added are one among this classical three cases :

1.	Removable constraints : solutions for which the estimated station positions and/or velocities are constrained to external values within an uncertainty sigma around 10-5  m for positions and  m/y for velocities.
The constraint matrix should be coded in the sinex block  "SOLUTION/APRIORI3".

2.	Loose constraints : solutions where the uncertainty applied to the constrainst is sigma greater than 1 m for positions and > 10 cm/y for velocities. The constraint matrix should be coded in the sinex block  "SOLUTION/APRIORI3".

3.	Minimum constraints used solely to define the Terrestrial Reference Frame using a minimum amount of required information. For more details on the concepts and practical use of minimum constraints, see for instance Altamimi et al., 2001. The Analysis Center is invited to give details of how the method has been applied.

References

Altamimi Z., C. Boucher and P. Sillard, New Trends for the Realization of the International Terrestrial Reference System, Submitted to Adv. Space Res., 2001.