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

VLBA Scientific Memorandum n. 31: Astrometric calibration of mm-VLBI using "Source/Frequency Phase Referenced" observations