Microwave radar detection of surface discontinuities

Disclosed is a means for high speed investigation of the surface of a railway track 16. A millimetre wave radar system 2 is provided to transmit short pulses of millimetre wave radiation 22 towards the surface under test 16. If the surface has no flaws or defects, most of the transmitted radiation is reflected in an expected direction 24. At least one receive antenna 6,8 is provided to detect whether any transmitted radiation 22 has been scattered 28,30 from a defect 26 in the surface in a direction other than that of the expected reflection direction 24. The transmit antenna 4 may also operate as the receive antenna i.e. monostatic, however a bistatic arrangement having separate transmit and receive antennas is preferred. More than one receive antenna may be used to inspect the rail from different angles to provide increased sensitivity and to help in determining the type of defect. The polarisation properties of the scattered radiation may be used in providing information about the type of defect. A further receive antenna (38,Fig 2) may be additionally or alternatively provided in the expected reflection direction 24 to detect a drop in power of the received signal when a defect 26 occurs.