A new technique has been developed to remove the ionosphere ’ s distorting effects from low frequency VLBI data .
By fitting dispersive and non-dispersive components to the phases of multi-frequency data , the ionosphere can be effectively removed from the data without the use of a priori calibration information .
This technique , along with the new gating capability of the VLBA correlator , was used to perform accurate astrometry on pulsar B0950+08 , resulting in a much improved measurement of this pulsar ’ s proper motion ( \mu _ { \alpha } = -1.6 \pm 0.4 mas/yr , \mu _ { \delta } = 29.5 \pm 0.5 mas/yr ) and parallax ( \pi = 3.6 \pm 0.3 mas ) .
This puts the pulsar at a distance of 280 \pm 25 parsecs , about twice as far as previous estimates , but in good agreement with models of the electron density in the local bubble .