4The notation d0 is used elsewhere for a reference distance as in log(d∕d0) in one-slope models in earlier sections (e.g. in 3.1, 3.2 equation (3.3)), and later in 3.4; in this section however, the notation d0 refers to a distance for ray 0 – the most direct ray. Hopefully the two are different enough not to cause any confusion.