7.6 Systems using transponders7.6.1 In rivers, where it is impractical to install cables across the river, transponders (which are analogous to reflectors) may be used.A transponder is a system of transducers and electronic circuits, installed as indicated in Figure 6. When B receives a signal from A, it causes a new signal to be transmitted from C to D. When C receives a signal from D, it causes a new signal to be transmitted from B to A.