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Instruments and Navigation Methods at Sea

Basic instruments: compass (direction), log (speed), echo sounder (depth), clock and chronometer (time). Position is fixed by coastal navigation (bearings of landmarks), radio navigation (GNSS satellites, radar) or celestial navigation (sextant sights of Sun and stars). Two or more position lines that cross give a fix.

🎬 Step-by-step story

  1. On the bridge there are four basic instruments: compass for direction, log for speed, echo sounder for depth, and a clock for time.
  2. The compass needle always points to north. Turn the ship with the slider and watch its heading change.
  3. Coastal navigation: take the bearing of two landmarks. Where the two lines cross is the ship. Move the ship and watch the bearings change.
  4. Radio navigation: three satellites give three distances. Where the three lines meet is the ship's position.
  5. Celestial navigation: a sextant measures the Sun's height. On an equinox day, latitude = 90° − height.
  6. Free play: press any one of the four methods and move the slider to try it yourself.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does the compass needle point north, not the ship's direction?

The needle follows the Earth's magnetic field, so it points north. The ship turns around it, and the heading is read where the ship's bow meets the card.

Why do I need two landmarks?

One bearing gives only a line the ship is on. A second bearing gives a second line. The ship is where they cross.

Why do we need three or four satellites?

Each satellite gives a distance, and one distance only says "somewhere on a sphere". More distances narrow it to one point, and the fourth fixes the clock error.

Why does the Sun being higher mean a lower latitude?

Near the equator the Sun comes almost overhead at noon. Further from the equator it stays lower. So latitude = 90° − altitude.

Which instrument measures speed?

The log. The echo sounder is for depth. The bridge scene in the first step shows all four basics.

Basic navigation instruments

True course = magnetic course + variation (east +, west −), after correcting deviation.

Coastal (pilotage) navigation

Near the coast the navigator uses visible landmarks such as lighthouses, towers and headlands. He takes the bearing of each landmark with a compass (or the angle to it) and draws the lines on the chart. A position line is a line on which the ship must lie. Two bearings that cross give a fix; a third bearing makes a triangle (the "cocked hat"): a small one means a good fix. Other tools: transit (two objects in a line show a safe path), radar range and soundings compared with the chart. Choose objects that are about 90° apart for the best fix.

Radio navigation

GNSS (Global Navigation Satellite Systems) such as GPS (USA), GLONASS (Russia), Galileo (Europe), BeiDou (China) and NavIC (India, regional). Each satellite sends its position and the exact time. The receiver measures how long the signal took (speed of light 3 × 10⁸ m/s), finds its distance to each satellite, and the crossing of those distances is the position. Four satellites give position and fix the receiver's clock error.

Radar sends a radio pulse and times the echo: distance = (3 × 10⁸ × time) ÷ 2. It shows other ships, land and buoys even in fog and gives range and bearing. AIS makes ships broadcast their name, position and course so others can see them.

Celestial navigation

Far from land the navigator can use the sky. A sextant measures the altitude (angle above the horizon) of the Sun, Moon, a planet or a star. At local noon the Sun is highest. On an equinox the Sun is above the equator, so latitude = 90° − noon altitude. On other days a small correction (declination, from the almanac) is added or subtracted.

Longitude comes from time. The Earth turns 360° in 24 h, which is 15° per hour (1° = 4 minutes). If local noon happens 3 hours after noon at Greenwich, the ship is 45° west. This is why an exact chronometer is vital. Celestial navigation is slower than GPS but needs no electricity from outside, so it is a good back-up.

Try it

At home: stand at a window, point a pencil at a far tree and write down the compass bearing (use a phone compass). Move to another window and take the bearing of the same tree and a second landmark. Where would the lines cross on a sketch? That crossing is your fix.

In 3D: in the last step choose "Celestial", set the Sun height to 62° and read the latitude. Then try 30°.

Key formulas and definitions

Worked examples

1. An echo sounder gets its echo after 0.04 s. Find the depth (sound speed 1,500 m/s).

Depth = 1,500 × 0.04 ÷ 2 = 30 m.

2. A radar echo returns after 123.5 microseconds (1.235 × 10⁻⁴ s). How far is the target?

Distance = 3 × 10⁸ × 1.235 × 10⁻⁴ ÷ 2 = 18,525 m ≈ 10 nautical miles.

3. Local noon happens when the clock at Greenwich shows 15:00. Find the longitude.

Difference = 3 h. 3 × 15° = 45°. Noon is later than at Greenwich, so the ship is 45° West.

4. On an equinox the noon Sun's altitude is 62° (Sun due south). Find the latitude.

Latitude = 90° − 62° = 28° (north).

5. The magnetic course is 075° and the variation is 3° West. Find the true course.

West variation is subtracted: 075° − 3° = 072° true.

6. A GNSS signal travels 20,200 km. How long does it take (speed 3 × 10⁵ km/s)?

Time = 20,200 ÷ 300,000 ≈ 0.067 s.

Common mistakes

Practice quiz

1. Which instrument measures the depth under the ship?
2. A sextant measures:
3. Two bearings that cross give a:
4. The Earth turns through how many degrees in one hour?
5. Radar distance is half of speed × time because:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is the difference between a magnetic compass and a gyro compass?

A magnetic compass points to magnetic north and has variation and deviation. A gyro compass uses a spinning wheel to find true north.

How does GPS find my position?

The receiver measures the time signals take from at least four satellites. Time × speed of light gives distances. Where the distances cross is your position.

Why learn celestial navigation when GPS exists?

Electronics can fail, so a navigator keeps a sextant and an almanac as a back-up. It is also part of many maritime exams.

Where this is taught

Japan高校（専門学科）1〜3年Navigation and Instruments

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