Communicating with waves

The Earth, the satellites and the cable are drawn in one cut along a line of longitude, with London and Cape Town treated as if they lay on the same one (Cape Town is really about 18° further east). Nothing is to scale: the low orbit, 550 km up, is drawn much higher, the geostationary orbit, about 36 000 km up, much lower, and the sea much deeper. The times are worked out from the model’s distances and speeds: they are the travel times only, and real calls add time in the equipment. The Earth’s turning is left out, so in the model the other low-orbit satellites follow satellite E round the same orbit, and one crosses dish 1’s beam for about a quarter of a minute every 9 minutes: far too briefly for a television picture.

The noise is the model’s: the same amount on each stretch of cable, from fixed numbers, so every run gives the same result. Real undersea cables carry digital pulses of infrared light, and modern repeaters amplify the light itself; the analogue run shows how a signal like the word itself would fare. Bluetooth and wi-fi both work at about 2.4 GHz; the syllabus counts Bluetooth as radio waves, and so does the key.

With reduced motion each run happens at once: the signals arrive, the hour passes and the dishes are aimed straight away, and the panel shows the result at once.