Doppler Speed Log works on the principle of Doppler shift in frequency measurement i.e. an apparent change in frequency received when the distance between source and observer is changing due to the motion of either source or observer or both.
Doppler Log is used to indicate speed and distance are intended for general navigational and ship manoeuvring use. The minimum requirement is to provide information on the distance run and the forward speed of the ship through the water or over the ground,
As per SOLAS Chapter V, regulation 19.2.3, all ships of 300 gross tonnage and upwards and passenger ships irrespective of size should be provided with speed and distance measuring device, or other means, to indicate speed and distance through the water. This speed and distance measuring device should be as per the IMO Performance standards as mentioned below.
IMO PERFORMANCE STANDARDS FOR MARINE DOPPLER SPEED LOGS
- Devices to indicate speed and distance should comply with the following minimum performance requirements:
- The equipment should comply fully with its performance standards at ahead speeds up to the maximum speed of the ship and in water of depth greater than 3 m beneath the keel.
- Speed information can be made available in either analogue or digital form. Where a digital display, its incremental steps should not exceed 0.1knots. Analogue displays should be graduated at least every 0.5 knots and be marked with figures at least every 5 knots. If the display can present the speed of the ship in other than the forward direction, the direction of movement should be indicated unambiguously.
- Distance run information should s provbe presented in digital form. The display should cover the range from 0 to not less than 9999.9 nautical miles iand the incremental steps should not exceed 0.1 nautical miles. Where practicable, means should be provided for resetting readout to zero.
- The display should be easily readable by day and by night.
- Means should be provided for feeding distance run information to other equipment fitted on board.
- If equipment is capable of being operated in either the “speed through the water” or “speed over the ground” mode, mode selection and mode indication should be provided.
- If the equipment has provision for indicating speeds other than on a single fore and aft axis, then the forward and athwart speed through the water must be provided, and the forward and athwart speed over the ground may be offered as an additional option. Resultant speed and course information can also be made available as a switchable option. All such information should clearly indicate the direction, mode and validity status of the displayed information.
- The performance of the equipment should be such that it will meet the requirements of these standards when the ship is rolling up to + 10° and pitching up to + 5°.
- Where any part of the system is designed to extend from and retract into the hull of the ship, the design should ensure that it can be extended, operated normally and retracted at all speeds up to the maximum speed of the ship. Its extended and retracted positions should be clearly indicated at the display position.
- Errors in the indicated speed, when the ship is operating free from shallow water effect and the effects of wind, current and tide, should not exceed 2% of the speed of the ship, or 0.2 knots, whichever is greater.
- Errors in the indicated distance run, when the ship is operating free from shallow water effect and the effects of wind, current and tide, should not exceed 2% of the distance run by the ship in 1 h or 0.2 nautical miles in each hour, whichever is greater.
- If the accuracy of devices to indicate speed and distance run can be affected by certain conditions (e.g. sea state and its effects, water temperature, salinity, sound velocity in water, depth of water under the keel, heel and trim of ship), details of possible effects should be included in the equipment handbook.
- The system should be so designed that neither the method of attachment of parts of the equipment to the ship nor damage occurring to any part of the equipment which penetrates the hull could result in the ingress of water to the ship.