HI line measurements of pulsars towards the Galactic Centre and the   electron density in the inner Galaxy
  We have measured 21-cm absorption and emission spectra in the direction of a further 7 southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained HI absorption measurements for PSRs J1602-5100, J1740-3015 and J1745-3040. We have also significantly improved the sensitivity and resolution on PSRs J1600-5044, J1752-2806, and J1825-0935, whose spectra have previously been measured, and have corrected an error in the published distance to PSR J1824-1945.   Since the Frail & Weisberg summary of pulsar distances in 1990, a further 23 pulsars now have measured HI distances, mainly through the efforts of the current group. We discuss the Taylor & Cordes electron density model in light of these new measurements and find that, although the model towards the Galactic Centre appears good, the line of sight through the Carina spiral arm is poorly fit by the model. 
Simon Johnston, B. Koribalski, J. M. Weisberg, W. Wilson