There are four major modes that the waves transmitted from a transmitter may follow to reach the destination and they are

(a) Surface wave propagation

(b) Space wave propagation

(c) Troposphere propagation

(d) Ionosphere propagation.

The first two propagation modes are grouped into ground wave propagation, but the behave differently enough for separate consideration.

Meaning of different terms in Figure :

T_X = Transmitting antenna

R_X = Receiving antenna

Path A = Ground wave propagation

Path B = Sky or ionosphere propagation

Path C = Space wave propagation