  In this document we layout a new method to achieve "bona fide" high precision
Very-Long-Baseline-Interferometry (VLBI) astrometric measurements of
frequency-dependent positions of celestial sources (even) in the high
(mm-wavelength) frequency range, where conventional phase referencing
techniques fail. Our method, dubbed "Source/Frequency Phase Referencing" (SFPR)
combines fast frequency-switching (or dual-frequency observations) with the
source switching of conventional phase referencing techniques. The former is
used to calibrate the dominant highly unpredictable rapid atmospheric
fluctuations, which arise from variations of the water vapor content in the
troposphere, and ultimately limit the application of conventional phase
referencing techniques; the latter compensates the slower time scale remaining
ionospheric/instrumental, non-negligible, phase variations.
  For cm-VLBI, the SFPR method is equivalent to conventional phase referencing
applied to the measurement of frequency-dependent source positions changes
("core-shifts"). For mm-VLBI, the SFPR method stands as the only approach which
will provide astrometry. In this memo we layout the scope and basis of our new
method, along with a description of the strategy and a successful demonstration
of the application of this new astrometric analysis technique to the highest
frequency VLBA observations, at 86 GHz. Our previous comparative astrometric
analysis of cm-VLBI observations, presented elsewhere, produced equivalent
results using both methods.
