pub type FieldGain<'a> = &'a dyn Fn(f64, f64, &mut [f64]);
The gain of the field at every frequency bin (FFT order) between two positions of a span (km): gain(z_0, z_1, out) writes $\sqrt{G(z_{1}, f) / G(z_{0}, f)}$ into out.
gain(z_0, z_1, out)
out