With supreme timing stabilities, millisecond pulsars (MSPs) have been used to test gravitational theories, and to collectively probe the stochastic gravitational-wave background (GWB) at the nano-Hz regime. Recent years have seen the major pulsar timing array (PTA) consortia closing in on high-significance detections of the GWB, which marks the dawn of a new GWB era. To deepen our understanding of the sources of the GWB requires steady enhancement of the PTA sensitivity. The best strategy to sustain the enhancement is to continuously add new qualified MSPs to the PTAs. VLBI (very long baseline interferometry) astrometry of MPSs would empower this strategy by refining the pulsar timing model with precise astrometric measurements obtained in a relatively short timeline. In addition, VLBI astrometry of MSPs contributes to MSP-related/based studies in many ways: it can strengthen the tests of gravitational theories, probe the formation channels of millisecond pulsars, refine the Galactic interstellar medium distribution, and even sharpen the constraints on the Galactic dark matter distribution. So far, 24 MSPs have been astrometrically determined with VLBI. As the only southern-hemisphere-based VLBI network, the LBA has only contributed to one astrometric determination of MSPs. We propose to carry out the first LBA astrometric survey of MSPs, which will benefit the aforementioned scientific aspects, and significantly reduce sky-position-related selection effects on statistical studies based on MSP astrometry. This proposal aims at piloting the astrometric survey by observing 28 MSP candidates at 13cm.
