But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to
model it as
a reflection from a slightly higher level. that makes computation
of angles
of incidence and height a bit easier.

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The trigonometry is quite simple. Things become complicated when the
reflecting layer is not horizontal, ie., the layer is tilted.

The angle and direction of tilt are very difficult to predict.
Consequently, where on the surface of the Earth a ray returns is
anybody's guess.

This makes the vertical take-off angle, reported by antenna modelling
programs, even less useful.
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Reg.