1.Principal methods There are two principal methods of microwave inspection of fibre-reinforced plastic composites: Method 1: Reflection mode Method 2: Through-transmission mode This draft specifies an operational procedure based on ‘Method 1: Reflection mode’. It is anticipated that future versions of this draft will provide additional procedural guidance for Method 2: Through-transmission mode. Although the microwave frequency range generally lies between a few hundred megahertz (MHz) and a few hundred gigahertz (GHz), the method specified in this draft is based on work undertaken using, and applicable to, transceivers operating in the frequency range 10 GHz to 34 GHz. 2. Measurements For the method of operation detailed in this working draft, two key measurand outputs are considered based on the use of inspection systems using transceivers with two receiver diodes in quadrature: (i) Magnitude of the return signal as determined from the square root of the squares of the voltage signals measured at each receiver diode as function of (x, y) position. This output can be used to determine the geometric (e.g. minimum and maximum dimensions, area etc.) characteristics of certain defect types orientated normal to the direction of the microwave signal. (ii) Phase of the return signal as determined using the arctan2 (arctan function with two arguments) function performed on the voltage signals measured at each receiver diode as function of (x, y) position Consideration of the magnitude and phase outputs together can provide additional information for certain types of defect such as remaining wall thickness. Note: Inspection systems may utilise microwave transceivers comprising one or more receiver diodes. 3. Applicable materials and structures This draft is applicable to both plain laminates with and without curved surfaces. This draft may be applicable to the inspection of pipe structures provided that either the inner surface or substrate of the pipe comprises of a material that reflects microwaves (e.g. a metal substrate) or that the geometry of the pipe is such that there are no reflections from the far-side wall of the pipe. The method is suitable for use with the following range of materials: Filled and reinforced compounds, including laminates. Glass fibre-reinforced thermoset and thermoplastic matrix composites incorporating unidirectional or non-unidirectional reinforcements in either a continuous or discontinuous format. These composites include; mat, woven fabrics, woven rovings, chopped strands, combinations of reinforcement (glass or aramid fibre only) and hybrid rovings.