The background of the THD
The amendment of a SOLAS chapter V has been discussed through the IMO Navigation sub-committee (NAV).
At the NAV.44, the TMHD (Transmitting Magnetic Heading Device) for heading information sensor equipment was adopted as a performance resolution of simple heading sensing device.
However this amendment now requires that RADAR, EPA and AIS should be installed on all passenger ships and ships of 300g/t and above. These three equipments require a common input of true heading information.
Carriage of Gyrocompass which satisfies the IMO performance resolution A.424 (XI), and ISO 8728 (2nd Ed.) standard for the vessel over 500 g/t is still a requirements of the regulation 20 of the SOLAS chapter V as amendment.
The standard for the test method and required test results for the TMHD is being considered in the compass working group in ISO/TC8/SC6, however, it should be recognised that the TMHD derives heading information using the principle of terrestrial magnetism. Thus it can not directly provide the true heading input required by RADAR, EPA and AIS without some additional correction device.
Furthermore, the NAV. 44 noted that there are many possibilities for the application of technology to providing a ship's heading sensor.
For example:
1. Magnetic
2. All independent of their surroundings
3. Radio compass
Nevertheless if the carriage requirement is restricted to such magnetic heading devices it is likely to obstruct the future technological developments and the application of other existing technology.
The input requirements of the equipments requiring an input of true heading have been studied in Japan by a group of specialists. As a result there is concern that the provision of heading input to the equipments installed on the size of vessels (300g/t) should not be limited to magnetic devices and that a general performance standard should be developed for application to installed equipment requiring the input of true heading information.
Japan would like to try to propose new work item of development of an international standard as THD (Transmitting Heading Devise) which is no restricted magnetic principle.
