Quantitatively, the delay in arrival times between a high frequency and a low frequency pulse can be shown  to bed. This equation may be solved for d given a measurement of DM and a model of the free electron distribution calibrated from the 100 or so pulsars with independent distance estimates and measurements of scattering for lines of sight towards various Galactic and extragalactic sources [357, 391]. A recent model of this kind, known as NE2001 [88, 89], provides distance estimates with an average uncertainty of 30%.
Because the electron density models are only as good as the scope of their input data allow, one should be mindful of systematic uncertainties. For example, studies of the Parkes multibeam pulsar distribution [201, 227] suggest that the NE2001 model underestimates the distances of pulsars close to the Galactic plane. This suspicion has been dramatically confirmed recently by an extensive analysis  of pulsar DMs and measurements of Galactic H emission. This work shows that the distribution of Galactic free electrons to be exponential in form with a scale height of pc. This value is a factor of two higher than previously thought. A revised version of the NE2001 model which takes into account these and other developments is currently in preparation .
This work is licensed under a Creative Commons License.