Communicating with waves — return questions key

1. CR1 · Return

Model answer. (a) A geostationary satellite stays above one point on the equator, so it covers only the part of the Earth that faces it. As the ship sails a long way east, the first satellite sinks lower and lower in its sky, until the phone switches to the next satellite, which stays above a point further east. (b) Bluetooth uses radio waves. They pass through walls, but the signal is weakened on doing so: after one wall it is still strong enough, but after four walls it is too weak, and the printer receives nothing.

A full answer includes

• (a) I wrote that each geostationary satellite stays above one point on the equator.
• (a) I wrote that one satellite covers only the part of the Earth that faces it.
• (a) I wrote that far enough east, the first satellite is too low in the ship’s sky, so the phone uses the next one.
• (b) I wrote that Bluetooth uses radio waves.
• (b) I wrote that each wall weakens the signal.
• (b) I wrote that after four walls the signal is too weak.
• (a) I did not write that the satellite moves away from the ship.

Also accept

• (a) each satellite stays above one point and covers only part of the Earth; the ship sails into the next one’s area (b) radio waves; weakened by each wall, too weak after four

Watch for

• “(a) The phone switches because the geostationary satellite moves away across the sky.” — A geostationary satellite stays above one point on the equator. It is the ship that moves, out of the area the first satellite covers.
• “(a) One geostationary satellite can cover the whole Earth.” — It covers only the part of the Earth that faces it. That is why a service uses several, spread round the equator.
• “(b) Radio waves cannot pass through walls.” — The printer worked through one wall, so they do. Each wall weakens the signal.

2. CR2 · Return

Options as numbered on the pupil sheet: 1 — Station D only; 2 — Both stations; 3 — Station F only.

Model answer. (a) Both signals began as the same varying voltage from a microphone at the concert, and both pick up noise on the way to the car. Station F sends out a wave that varies smoothly, by any amount, as the sound does. Station D sends pulses, and the car radio decides whether each pulse it receives is on or off. (b) On the analogue FM signal, the noise picked up on the way is heard directly, as hiss. Further from the transmitter the signal is weaker, so the noise is bigger compared with it, and the hiss grows. The DAB radio only has to tell each 1 from each 0, so while the noise is small it makes no difference and the sound is perfect: the same reason a repeater can regenerate digital pulses accurately. Far enough away, the noise becomes big enough for 1s and 0s to be read wrongly, and the sound suddenly breaks up.

A full answer includes

• (a) I matched the varying voltage from a microphone to both stations.
• (a) I matched deciding whether each pulse is on or off to station D only.
• (a) I matched the wave that varies smoothly, by any amount, to station F only.
• (a) I matched picking up noise to both stations.
• (b) I wrote that noise on the analogue FM signal is heard directly, as hiss.
• (b) I wrote that further from the transmitter the signal is weaker.
• (b) I wrote that the noise is then bigger compared with the signal, so the hiss grows.
• (b) I wrote that the DAB radio only has to tell each 1 from each 0, so small noise makes no difference.
• (b) I wrote that once the noise is big enough, 1s and 0s are read wrongly and the sound breaks up.
• (b) I did not write that the DAB signal picks up no noise.

Also accept

• (a) microphone voltage both; deciding on or off D only; varies smoothly F only; picks up noise both (b) FM: noise heard as hiss, growing with distance; DAB: 1s and 0s read right until the noise is too big

Watch for

• “Only station F’s signal began as a varying voltage from a microphone.” — Both did. For DAB, the microphone’s varying voltage was then turned into pulses.
• “The FM radio decides whether each pulse it receives is on or off.” — FM is analogue: its wave varies smoothly, with no pulses. Only the DAB radio receives pulses and decides each one.
• “Only station F’s signal picks up noise.” — Both pick up noise. The DAB radio can ignore small noise, because it only has to tell 1 from 0.
• “(b) The DAB programme stays clear because a digital signal picks up no noise.” — It does pick up noise. It stays clear while the noise is too small to change a 1 into a 0, or a 0 into a 1.