************************************************************************
IERS Message No. 137                                   November 11, 2008
************************************************************************
ITRF2008 Call for Participation

Dear colleagues,

Please find below the call for participation for the ITRF2008.

Best regards
Zuheir Altamimi
------------------------------------------------------------------------

ITRF2008 Call for Participation

Background

For the first time in the ITRF history, the ITRF2005 used time series of station positions and EOPs: weekly from satellite techniques (SLR, GPS, DORIS) and session-wise from VLBI. Time series are now essential to account not only for station non-linear motions, but also to evaluate the stability over time of the frame parameters, mainly the physical ones, the origin and the scale. For the first time also, the ITRF2005 rigorous combination provides self-consistent series of EOPs, including Polar Motion from VLBI and satellite techniques and Universal Time and Length of Day from VLBI only.

While the ITRF2005 marked considerable progress compared to past ITRF versions, some weaknesses were underlined in the individual technique solutions that were used as well as their co-locations. An ITRF2008 becomes now necessary to benefit from the expected improvement of the reprocessing effort being undertaken by the Technique Centers (IVS,
ILRS, IGS, IDS).

ITRF2008

Since the release of the ITRF2005 we can note the following changes:

- Three years of additional data have become available
- New sites added to the ITRF network
- A few new local ties are now available
- The processing strategies of the individual techniques have
 improved and self-consistent reprocessed solutions are  expected to be available.

In order to take benefit of these new developments, the ITRS Center, together with the ITRS Combination Centers, is planning to generate a new ITRF solution (ITRF2008) and solicits inputs from the IERS Technique Centers (TC) and their related Analysis Centers. Official combined time series of the four TCs are preferred. In case that this requirement could not be reached by the TCs within the ITRF2008 schedule (see below), at least three individual AC solutions are requested. The TCs are requested to provide as long time series as possible and preferably covering the full history of observations of each technique, such as for instance VLBI and SLR data since the eighties.

It is requested NOT to correct for any geophysical fluid loading effects, except for tidal ocean loading.

Long Time Series Contributions

Contributed time series (weekly/daily) solutions to be included in the ITRF2008 should be provided in SINEX format and comply with one of the following constraint categories:

- Solutions with removable constraints
- Loosely constrained solutions (constraint level: Sigma > 1 m)
- Solutions with TRF minimum/inner constraints
- Free singular normal equations

The SINEX files have to follow the SINEX Version 2.0 format standard and should contain for one week (Sunday to Saturday) - a session in the case of VLBI - station positions, a set of EOPs for each day (offsets and rates fitted over 24-hr intervals for polar motion, UT1 and LOD, where only VLBI provides UT1). IVS is highly encouraged to provide nutation offsets and quasar coordinates for future studies by the ITRS Combination Centers. If the SINEX files contain a variance-covariance matrix (eventually except minimal constraints or loose constraints), all constraints applied to the solution should be given in the a priori variance-covariance matrix. All solutions should conform to the IERS Conventions, including updates posted at: http://tai.bipm.org/iers/convupdt/convupdt.html

Whenever departures from the recommendations of the IERS Conventions are preferred, it is requested that the effects of those deviations be documented. Based on experiences with ITRF2005 some specific model updates are requested:

- Thermal expansion of the VLBI antennas causes a significant annual  frame scale variation if it is not modeled. If feasible, IVS is  encouraged to consider modeling that effect, provided that no net  artifactual scale offset/drift is introduced.

- Appropriate modeling of SLR station range biases is needed to minimize  frame scale effects.

- Long-term GPS solutions compatible with modern operational products  are needed for full consistency.

- Appropriate DORIS analysis strategy to minimize possible scale  discrepancies between ACs.

Solicited solutions to be considered are of three types:

A) Official single-technique combined time series solutions from the   Technique Centers

B) If solutions of type A are not available, or for specific reasons   (e.g. longer time span, more stations, more homogeneous processing),   individual Analysis Center time series solutions are solicited under   the coordination of the corresponding Technique Center.

C) Solutions that result from a combination of various techniques at the  observation level may be submitted as well.

A summary file describing the strategy adopted to generate the time series should be submitted together with the SINEX files, as well as all information related to station position discontinuities.

ITRF2008 Analyses and Results

The general analysis strategy planned by the ITRF CCs will follow closely the procedure used for ITRF2005 and will consist of the following steps:

- Remove original constraints (if any)
- Use as they are the minimally constrained solutions
- Perform per-technique combinations (TRF + EOP) of each individual time series: stacking/accumulating the weekly time series. The outputs of this step will be full SINEX file per technique/AC, containing station positions, velocities and daily EOPs.
- Identify and reject outliers and properly handle discontinuities, using break-wise approach
- Combine if necessary all solutions of a given technique into a unique solution
- Combine the per-technique combinations adding local ties in co-location sites. This global combination will constitute the ITRF2008 solution.

The expected main results will be:

- Positions and velocities for a global network of tracking stations and  related markers of the four techniques (VLBI, SLR, GPS, DORIS), with  full variance/covariance information provided in SINEX format.
- Fully consistent EOPs: daily polar motion and their rates, UT1 and LOD

- Per-technique solutions of station positions and velocities and daily EOPs, with full variance/covariance information provided in SINEX format.

- Residual time series as result from the per-technique time series combinations

- Geocenter offset time series from satellite solutions (SLR, GPS, DORIS)

Instructions

The Technique Centers and individual Analysis Centers that intend to contribute their long time series of SINEX files should submit these series by February 10, 2009 at the latest. Earlier submissions are most welcome and very much encouraged.

To submit a time series of SINEX files please send an e-mail to:

ITRS Center: itrf@ensg.ign.fr and IERS Central Bureau: central_bureau@iers.org

The e-mail should contain the detailed instructions on where the solutions can be downloaded, their present naming convention, etc. The submitted SINEX files will be archived in the IERS Data and Information
System and the ITRS Center.

Schedule:

November 11, 2008 Dissemination of the Call for Participation
February 10, 2009 Deadline for solution submissions by Technique/Analysis Centers
April 19-24, 2009 First results and discussions at EGU General Assembly
Until May 10, 2009 Analysis by the ITRS CCs
May 10 - June 10, 2009 Preliminary ITRF2008 solution available for evaluation by the Technique/Analysis Centers
July 15, 2009 Final ITRF2008 solution ready.