The International Maritime Organization (IMO) is the permanent international organization responsible for maritime safety. Currently 32 international conventions have been established by the efforts of the IMO. Eleven deal with marine safety, seven deal with marine pollution, seven deal with liability and compensation, and four deal with miscellaneous matters. Perhaps the most important conventions dealing with marine safety are the International Convention for the Safety of Life at Sea (SOLAS), 1974 and Convention on the International Regulations for Preventing Collisions at Sea (COLREGs), 1972.
The international COLREGs and parallel United States Coast Guard Inland Navigation Rules establish operational rules for avoiding collisions at sea. The relevant sections establish rules for usage of radar and encounters in restricted visibility or when the other vessel is detected by radar alone. The combined Navigation Rules; International - Inland are available for viewing or downloading from the USCG Navcenter.
The SOLAS Convention is relevant to us here because it details carriage requirements for radar, radar reflectors, and automatic identification systems (AIS). The relevant section of the SOLAS Convention is, SOLAS Chapter V: Safety of Navigation, in particular Regulation 19 "Carriage requirements for shipborne navigational systems and equipment". Relevant paragraphs from SOLAS Ch V Reg 19 are summarized below. My annotations are shown in square brackets.
2.1.7 [Radar reflector]: All ships irrespective of size shall have:...if less than 150 gross tonnage and if practicable, a radar reflector, or other means, to enable detection by ships navigating by radar at both 9 [X-band] and 3 GHz. [S-band]
2.3.2 [X-band radar]: All ships of 300 gross tonnage and upwards and passenger ships irrespective of size shall...be fitted with: a 9 GHz. radar , or other means to determine and display the range and bearing of radar transponders and of other surface craft, obstructions, buoys, shorelines and navigational marks to assist in navigation and in collision avoidance;
2.4 [AIS]: All ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with and automatic identification system (AIS),...
2.5.5 [ARPA]: All ships of 500 gross tonnage and upwards shall,...have: an automatic tracking aid, or other means, to plot automatically the range and bearing of other targets to determine collision risk.
2.7.1 [S-band radar]: All ships of 3000 gross tonnage and upwards shall,...have: a 3 GHz [S-band] radar or where considered appropriate by the Administration a second 9 GHz [X-band] radar, or other means to determine the range and bearing...which are functionally independent of those referred to in paragraph 2.3.2; and ...
Paragraph 2.1.7 recognizes that small vessels are hard to detect by radar, hence the requirement to carry an RTE. The term "radar reflector" clearly means a passive device. The term "or other means" has been interpreted to mean an active device.
The first bullet has been interpreted to mean that the device used must enable detection at both X-band and S-band. This would mean that the active RTE on the market, which do not operate at S-band, do not satisfy the requirement. However, the first bullet says that the ship is operating at both frequencies, hence it will always be operating the X-band radar. This matches the last bullet which indicates that S-band radar, when installed, is in addition to the X-band radar. One could argue that, under the stated requirements, no vessel would be operating only an S-band radar, at least absent failure of the X-band radar. It seems to me that SOLAS does not require RTE performance at S-band though prudence dictates that it should. It seems to me that there is a grammatical issue here: is the vessel navigating with both X-band and S-band or does the RTE have to be detected by both bands? In any case, the "...if practical..." modifier makes the question moot.
The SOLAS Convention requires that certain small vessels carry a radar reflector. It does not specify technical performance requirements. Technical specifications are left to the ITU for things that emit electromagnetic energy, i.e. active RTE and beacons and to the ISO for things that do not emit energy, i.e. passive radar reflectors. That said, the Maritime Safety Committee (MSC) of the IMO promulgates recommendations for technical specifications.
The current standard for passive radar reflectors established by International Organization for Standardization (ISO) is ISO 8729 "Ships and marine technology - Marine radar reflectors" (more specifically ISO 8729:1997(E)). This is being updated and the more relevant document is the recommendation of the Maritime Safety Committee (MSC) of the IMO. Major points from Resolution MSC.164(78) (adopted on 17 May 2004) "Revised Performance Standards for Radar Reflectors" are listed below:
[Frequency coverage]: Performance requirements should apply to either active or passive radar reflectors in both 9 GHz and 3 GHz bands.
[Stated Performance Level]: The radar reflector should have a "Stated Performance Level" (SPL), i.e. RCS, of at least 7.5 m2 in X-band and 0.5 m2 in S-band.
[Azimuth coverage]: The RCS should be at least the SPL over at least 280° deg azimuth; not remain below SPL over any single arc of more than 10° deg - a null; and not have distances between nulls of less than 20°.
[Elevation coverage]: This performance should be maintained through angles of heel 10° for power driven vessels and sailing vessels designed to operate with little heel [multihulls] and through 20° for other sailing vessels.
[Mounting height]: The recommended mounting height of 4 m ...should be...marked on the reflector.
[Active RTE]: Active reflectors should conform to Recommendation ITU-R M.1176.
The current standard for active RTE established by the International Telecommunication Union (ITU) is ITU-R M.1176. Major technical requirements are listed below:
[Frequency coverage]: 9 GHz and/or 3 GHz bands. [i.e. X-band and/or S-band]
[Polarization]: Horizontal polarization at 9 GHz [X-band] and both horizontal and vertical polarization at 3 GHz [S-band].
[Amplification]: Minimum 50 dB including antenna gain.
[Azimuth coverage]: 360° within ±3 dB.
[Elevation coverage]: ±15° to ±3dB.
[Power output]: EIRP not greater than 10 W.
It should be noted that the minimum gain of 50 dB corresponds to a minimum effective RCS of 8 m2 at X-band and 0.8 m2 at S-band, which are slightly larger than the values required by MSC.174(78).