Sonar: using sound to see underwater
Light does not travel far in sea water, but sound does. Sonar (or an echo sounder) sends a short sound pulse down from the boat. The pulse hits something, such as the sea floor or a fish school, and an echo comes back. The machine measures the time and works out the distance.
Sound in sea water goes about 1500 metres every second. The sound goes down and back, so the one-way depth is half of the total trip:
depth = 1500 × time ÷ 2 (depth in metres, time in seconds).
A strong echo from the middle of the water is often a fish school. A fish finder draws these echoes as colours on a screen.
Other information technology on boats
- GPS / satellite navigation: satellites send time signals; the receiver works out the position. It marks fishing spots and helps return to port.
- Radio and satellite phones: talk to port and send position and catch.
- AIS and vessel tracking: boats broadcast their position so ships do not collide.
- Satellite images: show sea surface temperature and plankton colour, where fish may gather.
- Sensors and buoys: record temperature, waves and salt and send the numbers by radio.
- Apps, spreadsheets and databases: keep the catch log, weights and prices.
Information security: keeping data safe
Positions of good fishing spots, boat records and passwords are valuable. Information security means keeping information safe from people who should not see or change it. Simple habits: a strong password (long, with words you can remember, not your name), a different password for each account, a screen lock on the phone, no sharing of passwords, keep software updated, and make a backup of the catch log. Be careful with unknown links and messages: they may be tricks to steal data.
Information ethics: using data fairly
Information ethics is doing the right thing with information. Do not copy or share another fisher's secret spots or private details without permission. Report catch and position honestly, because false data harms fish stocks and other people. Respect privacy of photos and data of others. Give credit when you use another person's data or pictures.
Try it: time the echo
Stand 20 metres from a big wall (use a big, empty space) and clap once. Listen for the echo and count with a stopwatch. Sound in air travels about 340 m per second, so the echo of 40 m should come back after about 0.12 seconds, which is hard to time exactly. In the 3D free play, set a depth of 60 m, then 180 m, and compare the time shown. Which gives a longer wait?
Key formulas and definitions
- depth = speed of sound × time ÷ 2 (sea water: about 1500 m/s)
- time = 2 × depth ÷ 1500
- 1 ms = 0.001 s (1000 ms = 1 s)
- Round trip distance = 2 × depth
Worked examples
1. An echo comes back after 0.2 s. How deep is the sea floor?
depth = 1500 × 0.2 ÷ 2 = 150 m.
2. The depth is 300 m. How long does the echo take?
time = 2 × 300 ÷ 1500 = 0.4 s = 400 ms.
3. A fish echo returns after 80 ms. How deep is the school?
80 ms = 0.08 s. depth = 1500 × 0.08 ÷ 2 = 60 m.
4. The floor echo comes after 0.2 s and a fish school echo after 0.12 s. How far above the floor is the school?
Floor: 150 m. School: 1500 × 0.12 ÷ 2 = 90 m. The school is 150 − 90 = 60 m above the floor.
Common mistakes
- Forgetting to divide by 2. The sound goes down and back.
- Mixing milliseconds and seconds. 80 ms = 0.08 s.
- Using the speed of sound in air (about 340 m/s) for sea water. In sea water it is about 1500 m/s.
- Using the same password everywhere, or sharing it by message.