# How to determine arrival time delay given dispersion measure?

In an online textbook, the following formula is given to calculate the arrival time delay between two frequencies, $$v_1, v_2$$ in a dispersed radio pulse:

$$t_1 - t_2 = 4.15 \cdot DM [(v_1/\text{GHz})^{-2} - (v_2/\text{GHz})^{-2}]$$

Where does the $$4.15$$ come from, and what exactly is “GHz”? Do I divide by 1 GHz or?

• Can you provide a link to where you found the equation online? That would help provide more context for your question. – ELNJ Jul 24 at 1:12