This part of ISO 17208 describes the general measurement systems, procedures and methodologies to be used to measure underwater sound pressure levels from ships at a prescribed operating condition. It presents a methodology for the reporting of one-third-octave band sound pressure levels. The resulting quantities are the sound pressure levels normalized to a distance of 1 m. Since the underwater sound pressure levels are affected by the presence of the free surface (and sometimes the bottom), such quantities are sometimes called “affected source levels” (see ANSI/ASA S12.64-2009). This part of ISO 17208 refers to the result of these measurements as “radiated noise levels”. The underwater sound pressure level measurements are performed in the geometric far field and then adjusted to the 1 m normalized distance for use in comparison with appropriate underwater noise criteria. However, this part of ISO 17208 does not specify or provide guidance on underwater noise criteria or address the potential effects of noise on marine organisms. This part of ISO 17208 is applicable to any and all underway surface vessels, either manned or unmanned. Its methods have no inherent limitation on minimum or maximum vessel size. It is not applicable to submerged vessels or to aircraft, and is limited to vessels transiting at speeds no greater than 50 knots (25,70 m/s). The measurement methods mitigate the variability caused by Lloyd's mirror surface image coherence effects, but do not exclude a possible influence of propagation effects such as bottom reflections, refraction and absorption. No specific computational adjustments for these effects are given. A specific ocean location is not required for the application of this part of ISO 17208, but requirements for an ocean test site are provided. Among the applications of this part of ISO 17208 are the showing of compliance with contract requirements, the enabling of periodic signature assessments and in research and development. Intended users include government agencies, research vessel operators and commercial vessel owners operating in acoustically sensitive waters. This part of ISO 17208 offers three grades of measurement — A, B and C — each with a stated applicability, test methodology, uncertainty, system repeatability and complexity. A summary of the attributes of each grade is given in Table 1. Application of the three grades of measurement to the same ship under the same conditions does not necessarily result in the same radiated noise level.