We use VLBI imaging of the interstellar scattering speckle pattern associated with the pulsar PSR 0834+06 to measure the astrometric motion of its emission .
The \sim 5AU interstellar baselines , provided by interference between speckles spanning the scattering disk , enable us to detect motions with sub nanoarcsecond accuracy .
We measure a small pulse deflection of \sim 18 \pm 2 km ( not including geometric uncertainties ) , which is 100 times smaller than the native resolution of this interstellar interferometer .
This implies that the emission region is small , and at an altitude of a few hundred km , with the exact value depending on field geometry .
This is substantially closer to the star than to the light cylinder .
Future VLBI measurements can improve on this finding .
This new regime of ultra-precise astrometry may enable precision parallax distance determination of pulsar binary displacements .