Ships and submarines use different sensing systems because air and water carry signals in very different ways. Radar is most useful above the ocean surface, where radio waves can travel through air and reflect from objects such as ships, aircraft, coastlines, and storms. Sonar is most useful below the surface, where sound waves travel well through water and can reflect from submarines, seafloor features, and marine life.
Understanding this difference helps explain how navigation, search, and safety work at sea.
Key Facts
- Radar stands for Radio Detection and Ranging and uses electromagnetic radio waves.
- Sonar stands for Sound Navigation and Ranging and uses sound waves.
- Distance by echo timing: d = vt/2, where v is wave speed and t is round trip time.
- Radio waves in air travel at about c = 3.0 x 10^8 m/s.
- Sound in seawater travels at about v = 1500 m/s, depending on temperature, salinity, and pressure.
- Radar works poorly underwater because water absorbs electromagnetic waves strongly, while sonar works well underwater because sound travels long distances in water.
Vocabulary
- Radar
- Radar is a system that sends radio waves and detects their reflections to locate objects and measure distance.
- Sonar
- Sonar is a system that sends or listens for sound waves in water to detect objects and measure distance.
- Echo
- An echo is a reflected wave that returns to a receiver after bouncing off an object or surface.
- Electromagnetic wave
- An electromagnetic wave is a wave made of changing electric and magnetic fields that can travel through a vacuum or air.
- Acoustic wave
- An acoustic wave is a pressure wave caused by vibrating particles in a material such as air or water.
Common Mistakes to Avoid
- Using the one-way distance formula d = vt for echo ranging is wrong because the signal travels to the object and back. Use d = vt/2 for radar and sonar echoes.
- Assuming radar works well underwater is wrong because seawater absorbs radio waves strongly. Submarines rely mainly on sonar because sound travels much farther in water.
- Assuming sonar and radar use the same wave type is wrong because sonar uses sound waves and radar uses electromagnetic radio waves. The correct system depends on the medium.
- Forgetting that wave speed changes with the medium is wrong because sound is much faster in water than in air, while radio waves are extremely fast in air. Use the speed that matches the situation.
Practice Questions
- 1 A ship radar sends a pulse that reflects from another ship and returns in 20 microseconds. Using c = 3.0 x 10^8 m/s, how far away is the other ship?
- 2 A submarine sonar ping returns from an object after 4.0 seconds. Using a sound speed of 1500 m/s in seawater, what is the distance to the object?
- 3 Explain why a surface ship uses radar to detect aircraft above the ocean but a submarine uses sonar to detect objects underwater.