The current strategy adopted for the ITRF96 solution is twofold :

• - a simultaneous combination of positions and velocities using full variance/covariance matrices
• - a rigorous weighting scheme based on the analysis and estimation of the variance components using Helmert method.

The ItrF96 solution is based on a combination of :

• - a selected sets of individual solutions submitted to the IERS Central Bureau in 1997
• - some past data provided in SINEX format
• - recent submissions from GPS and DORIS specifically asked to improve ITRF by supplying new stations or recent positions to upgrade velocities.

The individual solutions selected for the ItrF96 analysis are 4 VLBI , 2 SLR , 8 GPS and 3 DORIS solutions, summarized in Table 1.

On the other hand, as a first step in the improvement of the use of the local ties, all the available vector eccentricities of colocated sites were converted into a complete set of positions for each site expressed in SINEX format.

Table 1 : Selected Solutions for the ITRF96 analysis