  VLBI is unique, among the space geodetic techniques, in its contribution to
defining and maintaining the International Celestial Reference Frame, providing
precise measurements of coordinates of extragalactic radiosources. The quest
for increasing accuracy of VLBI geodetic products has lead to a deeper revision
of all aspects that might introduce errors in the analysis. The departure of
the observed sources from perfect, stable, compact and achromatic celestial
targets falls within this category. This paper is concerned with the impact of
unaccounted frequency-dependent position shifts of source cores in the analysis
of dual-band S/X VLBI geodesy observations, and proposes a new method to
measure them. The multi-frequency phase transfer technique developed and
demonstrated by Middelberg et al. (2005) increases the high frequency coherence
times of VLBI observations, using the observations at a lower frequency. Our
proposed SOURCE/FREQUENCY PHASE REFERENCING method endows it with astrometric
applications by adding a strategy to estimate the ionospheric contributions.
Here we report on the first successful application to measure the core shift of
the quasar 1038+528 A at S and X-bands, and validate the results by comparison
with those from standard phase referencing techniques. In this particular case,
and in general in the cm-wavelength regime, both methods are equivalent.
Moreover the proposed method opens a new horizon with targets and fields
suitable for high precision astrometric studies with VLBI, especially at high
frequencies where severe limitations imposed by the rapid fluctuations in the
troposphere prevent the use of standard phase referencing techniques.
