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Simultaneous observations at multiple frequency bands have the potential toovercome the fundamental limitation imposed by the atmospheric propagation inmm-VLBI observations. The propagation effects place a severe limit in thesensitivity achievable in mm-VLBI, reducing the time over which the signals canbe coherently combined, and preventing the use of phase referencing andastrometric measurements. We carried out simultaneous observations at 22, 43,87 and 130 GHz of a group of five AGNs, the weakest of which is ca. 200 mJy at130 GHz, with angular separations ranging from 3.6 to 11 degrees, using theKVN. We analysed this data using the Frequency Phase Transfer (FPT) and theSource Frequency Phase Referencing (SFPR) techniques, which use theobservations at a lower frequency to correct those at a higher frequency. Theresults of the analysis provide an empirical demonstration of the increase inthe coherence times at 130 GHz from a few tens of seconds to about twentyminutes, with FPT, and up to many hours with SFPR. Moreover the astrometricanalysis provides high precision relative position measurements between twofrequencies, including, for the first time, astrometry at 130 GHz. Finally wedemonstrate a method for the generalised decomposition of the relative positionmeasurements into absolute position shifts for bona fide astrometricregistration of the maps of the individual sources at multiple frequencies, upto 130 GHz.

The Power of Simultaneous Multi-Frequency Observations for mm-VLBI: Astrometry up to 130 GHz with the KVN