27. A radar installation required to be provided which is or was installed onboard a ship on or after 1st September 1984 shall provide a relative plan display having an effective diameter, without external magnification, of not less than:
(a)180 millimetres(1) on ships of 500 tons or over but less than 1600 tons;
(b)250 millimetres(1) on ships of 1600 tons or over but less than 10,000 tons;
(c)340 millimetres(1) in the case of one radar installation and 250 millimetres in the case of the other on ships of 10,000 tons or over.
Display diameters of 180, 250 and 340 millimetres correspond respectively to 9, 12 and 16 inch cathode ray tubes.