Ultrasound travels at the speed of sound which is 330 m/s in air.

This is not fast enough to track fast moving aircraft or more distant objects.

So, instead of using ultrasound, radio waves are used as they travel at 300,000,000 m/s.

It works in the same way as sonar but with radio waves in place of ultrasound.

A short pulse of radio waves is transmitted and the time it takes for the reflection (the echo) to return is measured.

The distance is calculated using distance = speed x time.

As with sonar, it is important to remember to only use half of the time there and back when calculating the distance.

## Example

Meteorologists use radar to track a storm.

A pulse of radar is reflected by the storm and returns to the receiver after 3 ms.

1. How far away is the storm?
2. Why do the meteorologists use radar to track the storm rather than sonar?

1. How far away is the storm?

Radar uses radio waves which are electromagnetic waves and travel at a speed of 300,000,000 m/s.

Distance = speed x time

Speed = 300,000,000 m/s

Time for the radio waves to travel to the storm and back = 3 ms = $$\frac{\text{3}}{\text{1000}}$$ = 0.003 s

Time for the radio waves to travel to the storm = $$\frac{\text{0.003 s}}{\text{2}}$$ = 0.0015 s

Distance = 300,000,000 m/s x 0.0015 s = 450,000 m = 450 km.

The storm is 450 km away.

2. Why do the meteorologists use radar to track the storm rather than sonar?

The meteorologists uses radar because the distance involved is so large and ultrasound travels relatively slowly at 330 m/s.

It would take sonar too long to travel to the storm and back, during which time the storm would have moved.

This would make it impossible to determine its position accurately.

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