Inverse Multiview I: Multi-Calibrator inverse phase referencing for
  Microarcsecond VLBI Astrometry
  Very Long Baseline Interferometry (VLBI) astrometry is a well established
technique for achieving $\pm10~\mu$as parallax accuracies at frequencies well
above 10~GHz. At lower frequencies, uncompensated interferometer delays
associated with the ionosphere play the dominant role in limiting the
astrometric accuracy. Multiview is a novel VLBI calibration method, which uses
observations of multiple quasars to accurately model and remove time-variable,
directional-dependent changes to the interferometer delay. Here we extend the
Multiview technique by phase referencing data to the target source ("inverse
Multiview") and test its performance. Multiple observations with a four-antenna
VLBI array operating at 8.3~GHz show single-epoch astrometric accuracies near
$20~\mu$as for target-reference quasar separations up to about 7 degrees. This
represents an improvement in astrometric accuracy by up to an order of
magnitude compared to standard phase referencing.
