Doppler Log
A Doppler log is a navigation instrument that uses the Doppler effect to measure a ship’s speed and distance travelled — either through water or over the ground. It is the most accurate speed-measuring instrument available on most modern commercial vessels.
Answer in Brief
The Doppler log transmits sound pulses (acoustic signals) from transducers in the ship’s hull. The signals reflect off either water particles or the seabed. The frequency shift of the returning signal (Doppler effect) is directly proportional to the ship’s speed. The instrument calculates and displays speed in knots and cumulative distance in nautical miles.
The Doppler Effect — How It Works
The Doppler effect is the change in frequency of a wave (sound or electromagnetic) when the source and observer are moving relative to each other.
Familiar example: An ambulance siren sounds higher pitched as it approaches and lower as it moves away. This frequency change is the Doppler shift.
In a Doppler log:
- Transducers in the hull transmit sound pulses downward at a known frequency
- Sound reflects off water (bottom-track) or seabed particles (water-track)
- The reflected signal returns at a slightly different frequency due to the ship’s movement
- The frequency difference (Doppler shift) is measured and converted to speed
Formula: Speed = (Frequency shift × Speed of sound in water) / (2 × Transmission frequency × cosine of beam angle)
Two Modes of Operation
Bottom Track Mode (Speed Over Ground)
Sound pulses reach the seabed and reflect back. Gives speed over ground (SOG) — actual movement over the earth’s surface.
- Accurate in shallow water (typically effective to 200–300 metres depth)
- Not affected by currents — gives true ground speed
- Used in port approaches, channels, canals, and shallow coastal waters
- Most accurate mode — directly measures movement over the fixed seabed
Water Track Mode (Speed Through Water)
Sound pulses reflect off water particles (sediment, micro-organisms, bubbles) in the water column. Gives speed through water (STW).
- Works in any depth
- Affected by current — shows ship’s movement through the water mass, not over ground
- Used in deep water where bottom-track is not possible
Doppler Log vs Electromagnetic (EM) Log
| Feature | Doppler Log | EM Log |
|---|---|---|
| Principle | Acoustic Doppler effect | Faraday’s law (electrical induction) |
| Accuracy | Higher | Moderate |
| Deep water | Water track only | Works in all depths |
| Cost | Higher | Lower |
| Output | Speed + distance | Speed + distance |
Most modern vessels have a Doppler log. Older vessels may use EM logs.
Display and Integration
The Doppler log output feeds into:
- ECDIS — for automatic speed input to position calculations
- ARPA/Radar — for relative motion calculations
- Autopilot — speed-based steering adjustments
- Main display — bridge console shows STW and SOG, trip distance, total distance
Log book entry: Speed entered in the deck log is typically speed through water (from log) and speed over ground (from GPS) — both recorded at watch handover.
Calibration and Errors
Common errors:
- Air bubbles under transducers: If the ship is pitching or in following seas, air can be entrained under the hull. This causes signal loss or erratic readings. Modern logs have bubble detection and automatic correction.
- Shallow banks near transducers: Reflections from underwater obstructions can give false readings
- Strong temperature layers (thermoclines): Can affect signal propagation in water-track mode
Calibration: Doppler logs are calibrated against a measured mile (timed run over a known distance) or against GPS/DGPS speed over measured course.
Frequently Asked Questions
Q: Can a Doppler log give speed over ground in deep ocean? No — only water-track speed in deep water. For speed over ground in deep water, GPS/GNSS is used.
Q: Why is Doppler log speed different from GPS speed? Doppler log in water-track mode measures speed through the water. GPS measures speed over ground. The difference is the current effect.
Q: Is a Doppler log mandatory on ships? SOLAS requires a speed measuring instrument. The type (EM log or Doppler) is not specified — but Doppler is the standard on modern vessels.
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