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.