  The use of the phase-delay improves substantially the accuracy obtained in
VLBI astrometry with respect to the group-delay observable. Recently, Ros et
al. (1999) have extended the related phase-connection technique to a triangle
of radio sources with relative separations up to 6.8deg (the S5 sources
BL1803+784/ QSO1928+738/ BL2007+777). This technique has also been extended for
separations up to 15deg in the studies of the pair of S5 radio sources
QSO1150+812/ BL1803+784 (Perez-Torres et al. 2000). We are carrying out a
long-term astrometric programme at 8.4, 15, and 43 GHz to determine the
absolute kinematics of radio source components in the 13 members of the
complete S5 polar cap sample. For each epoch, all the radio sources are
observed at the same frequency for 24 hours in total. Bootstrapping techniques
are used to phase-connect the data jointly for the 13 objects throughout the
observations. An accurate registration of the maps of the radio sources at
different frequencies will allow a study of jet components with unprecedented
precision and provide spectral information. This observing scheme could be
extended in the future to other regions of the sky and eventually lead to
global phase-delay astrometry.
