There is a growing need to detect newly developing defects on the surfaces of rails.

As train speeds increase and traffic becomes heavier the likelihood and consequences of catastrophic failure present a serious problem. Various types of cracks can occur in rails which, if undetected, can progress to track failure. One of the most critical defects are gauge corner cracks induced by rolling contact fatigue where the rails and wheels are in contact. This type of defect occurs especially on the high rail at bends.

The simplest form of detection is visual inspection. However this is a very time consuming process. Further it is desired to be able to detect forming defects before they are visible.

Current fault detectors are known which use ultrasonics. These systems pass an acoustic signal into the rail and monitor the signal to detect cracks. Portable ultrasonic detectors are known but these are time consuming to use and pedestrian based inspection of rails can involving interruptions to normal train running. Train mounted ultrasonic detectors are also known but do not work at normal train operating speeds. Therefore using this type of detector interferes with normal operation.

Another type of known detector is the eddy current detector. An eddy current is induced in the rail and a return signal detected. Any cracks will prevent formation of the eddy current and the drop in signal can be detected. Eddy current sensors are similar to acoustic detectors however in that they only work at the centre of the rail and are therefore not good for detecting gauge corner cracks. Again eddy current sensors are known which are pedestrian or train mounted but the train mounted sensor do not operate at normal train speeds which are typically in the region of 90 miles per hour.

Rails are not the only surfaces where defects can cause problems however. For instance, the internal surfaces of pipes must be inspected to ensure no leakage of the fluids being carried by the pipes, especially when the fluid is carried under pressure.

Therefore it is an object of this invention to provide a detector capable of detecting defects in a surface which mitigates at least some of the disadvantages of the above mentioned systems.

Thus according to the present invention there is provided a method of investigating a conducting surface for defects comprising the steps of illuminating the surface under investigation with millimetre wave electromagnetic radiation, and looking for variations in the reflection direction of radiation caused by surface variations associated with defects.

Millimetre wave radiation transmitted toward a continuous or smooth conducting surface at a particular angle will generally be reflected therefrom in a known way, i. e. the incident radiation will be reflected from the surface at a known angle. Therefore if the surface were defect free and a narrow beam of radiation were directed from a particular direction most of the radiation would be reflected in a relatively narrow reflected beam in a known direction. However if the surface has defects therein the defects will interact with the incident radiation and scatter it into different directions.

Whilst this scattered radiation could be used as an indicator of defects the amount of scattered radiation is relatively low. It has been found however that defects such as crack in rails are associated with undulations in the surface of the rail. This change of surface contour will be localised at the defect and relatively small but will mean that radiation transmitted to such a cracked surface may be reflected in directions other than expected reflection direction. Therefore by looking for variations in the reflection direction surface contour changes associated with defects can be identified.

As used herein the term millimetre wave radiation is taken to mean electromagnetic radiation having a frequency of between 30-300 GHz.

Using a radar based system like this allows very fast detection of cracks. Irradiation of the track and measurement thereof to determine whether any radiation has been scattered can happen extremely quickly. Therefore the method of the present invention can be used to investigate surfaces even when moving rapidly there over.

Indeed the method of the present invention could be applied to the investigation of rail surfaces at normal train operating speeds.

Preferably the step of looking for variations in the reflection direction of radiation comprises locating a receive antenna at an expected direction of reflection from a defect free surface and identifying any changes in measured intensity due to a change in reflection direction. If the surface is defect free a certain amount of the transmitted energy, which may be known prior to use through calibration, will be received by the receive antenna. Taking account of general losses involved in transmission, reflection and detection, which can be measured and accounted for, the power of the received signal would be expected to be constant. When the reflection direction changes due to the radiation impinging on an undulating surface the amount of received radiation will change. This change in received power can be used as an indication of the presence of defects. In a moving system the periodicity of the changes in received power will also give information about the defects providing certainty that a defect has been identified.

Preferably the receive antenna is located towards the edge of the expected reflected beam of radiation. As the skilled person will appreciate the radiation will be reflected from the surface in a beam having a power distribution. The middle of the beam will have the greatest intensity with the power falling off towards the beam edges. The undulations in the surface under test may cause relatively small changes of the reflection direction effectively changing the position of the receive antenna with respect to the received beam. A change of position of the receive antenna relative to the beam is more noticeable if the starting position was towards the side of the beam rather than the centre because the rate of change of intensity with position is greater towards the edges. Therefore locating the receive antenna towards the side of the reflected beam from a defect free surface will give a greater power change where

defects change the reflection direction. It should be noted that in such an arrangement the change in reflection direction could result in an increase in received radiation.

The method may further include the step of determining the power of the received radiation at different polarisations, such as orthogonal linear polarisations. The transmit antenna may be adapted to emit circularly polarised radiation or may be adapted to emit linearly polarised radiation and periodically alter the plane of polarisation of the transmitted radiation.

Measuring the polarisation properties of the scattered radiation can help in determining the type of defect and reduce false alarms. By irradiating with a particular polarisation at a particular time, as is achieved with circularly polarised radiation, and measuring the components of polarisation of the scattered radiation in two orthogonal directions the radiation pattern of the defect can be determined. As will be understood by one skilled in the art the radiation pattern can give information about a particular defect, for instance the derived polarisation angle of the scattered radiation can give an indication of orientation of a crack and the number of lobes in the radiation pattern can be indicative of its length. Further polarisation characteristics of non-defect echoes will generally differ from the expected defects.

The step of illuminating the conducting surface preferably comprises illuminating with short pulses of radiation. For instance pulses having a duration of less than 5ns or less than Ins may be suitable. Using very short pulses ensures that the system has a low depth of view. Having a low depth of view means that only the surface under investigation contributes to any signal detected by the receive antenna. This is especially useful when investigating rails and the like as it removes the chance of interference from the underlying rail structure such as the supporting tie plates, sleepers and track ballast etc.

The illuminating radiation preferable has a wide bandwidth, for instance a bandwidth of at least 2GHz.

The method preferably further comprises the step of recording the location of any identified defects. This is especially useful when the method involves moving the measuring equipment relative to the surface to be measured so as to investigate a length of area of the surface. When used in a rail inspection system the method may be implemented on a moving vehicle and it is obviously desirable to record the location of any defects detected, for more detailed inspection and/or repair. A variety of methods for recording the location of the defects will be apparent. For instance GPS positional information could be recorded to allow for identification of the location of defects, a velocity measurement system could be used to record how fast the apparatus has been travelling and how long for which would define the location for a linear surface such as a rail or the inside of a pipe.

Preferably the method involves identifying any movement of the receive antenna relative to the surface under investigation. Obviously the power received by the receive antenna will depend upon its orientation with respect to the transmitted radiation and the surface under investigation. A variation in distance will also affect the received power. Therefore a variation in movement of the receive antenna with respect to the surface under investigation would lead to a change in detected power even from a defect free surface. To address this issue and ensure that any change in power due to a change in reflection direction can be identified the extent of any movement of the receive antenna relative to the surface is monitored. Monitoring the relevant movement allows for compensation for any change in measured intensity due to any determining movement of the receive antenna relative to the surface under investigation. This could be implemented by a suitable signal processor.

Several methods for determining relative movement will be apparent to the skilled person. For instance the receive antenna could be provided with accelerometers to determine all motions and calculate the relative position. Alternatively the range to the surface could be monitored by a ranging device such as a laser ranger.

The received signal itself may even be used to determine any changes in the path length of the received radiation by monitoring the phase of the received radiation.

In another aspect of the present invention there is provided an apparatus for investigating surfaces comprising a transmit antenna for transmitting millimetre wave electromagnetic radiation towards a surface under investigation and a receive antenna positioned with respect to the transmit antenna so as to detect any variations in the reflection direction of radiation caused by surface variations associated with defects.

Conveniently the receive antenna is arranged so as to receive radiation reflected from a defect free surface.

As mentioned above in relation to the first aspect of the invention if a surface were defect free substantially all the transmitted radiation would reflect from the surface toward the receive antenna and be detected and, taking account of general losses involved in transmission, reflection and detection, which can be measured and accounted for, the power of the received signal would be expected to be constant.

However if the surface under investigation had defects therein some of the transmitted radiation would be reflected in directions other than the expected reflection direction because of the variations in surface contour associated with defects.

As described above the receive antenna is located towards the edge of the expected reflected beam of radiation.

Preferably the apparatus comprises a plurality of receive antennas, each receive antenna arranged to receive radiation reflected from a different part of the surface.

The general shape of the surface will influence the direction in which the incident radiation is reflected. For instance consider a rail which has a cross section having a largely flat or slightly curved top surface with two highly curved corners. Radiation incident on the rail will be reflected in different directions from the top surface and the two corners. Having multiple receive antennas the radiation reflected from each . corner and the top surface can be detected and analysed for any defects.

For investigation apparatus intended to be used in outdoor environments the frequency used may lie in the range 60-66 GHz. This lies within an atmospheric absorption band. Usually atmospheric absorption bands are avoided in radar systems as the signal attenuates too quickly. However atmospheric absorption will have little

effect on the apparatus of the present invention due to the short propagation distances involved-typically the transmit and receive antennas may be located within a few cm of the surface under test. Therefore there will be minimal attenuation due to atmospheric effects. However using an atmospheric absorption band reduces the likelihood of stray signals from other sources reaching the receive antenna as the stray signal would be attenuated before reaching the receive antenna. Further the signals transmitted by the apparatus are unlikely to interfere with other systems, which could be a factor in some applications.

Generally though the wavelength of radiation is chosen to be of the same order as the size of the expected defects. It has been found that cracks in rails can be up to 30mm long and occur at between 3 and 15 mm intervals along the length of the track. It is the interval between the cracks that can be important as this gives rise to the characteristic surface contour undulations. A wavelength of 3mm corresponds to a frequency of around 95GHz so another useful band of frequencies would lie between 90-100 GHz.

Conveniently the apparatus comprises processing means for processing the received signal so as to distinguish different types of defect.

Different forms of defect will have different properties. For instance corrugations in a rail will be generally periodic but are changes in the surface contour without being discontinuities. Gauge corner cracks do act as discontinuities and so will scatter differently, similar to a slot in a waveguide. Surface damage such as wheelburn can alter the conduction properties of the rail as can internal cracks. All these defects will have different patterns and these can be analysed. The information received from an investigation apparatus travelling over a surface can therefore be recorded and processed to determine what type of defect is present and also reduce false alarms due to spurious reflections from minor blemishes and diffraction effects. Suitable processing techniques will be apparent to the skilled person depending upon what type of surface is being investigated and what defects are to be detected. Suitable processing routines could involve the use of Fourier transforms or other time- frequency transforms.

Conveniently, when detecting scattered radiation, the apparatus may be adapted to measure the power of scattered radiation at different polarisations, such as orthogonal linear polarisations. When one receive antenna is used the apparatus may comprise a means of selectively altering the polarisation which the receive antenna will detect, such as a rotatable polarising grid disposed in front of the antenna. Preferably though, especially in applications where the investigation apparatus is to be moved over the ,., surface under investigation at speed, at least two receive antennas are adapted to simultaneously receive radiation at orthogonal polarisations. The transmit antenna may also be adapted to emit circularly polarised radiation or may be adapted to emit linearly polarised radiation and periodically alter the plane of polarisation of the transmitted radiation.

Measuring the polarisation properties of the scattered radiation can help in determining the type of defect and reduce false alarms. By irradiating with a particular polarisation at a particular time, as is achieved with circularly polarised radiation, and measuring the components of polarisation of the scattered radiation in two orthogonal directions the radiation pattern of the defect can be determined. As will be understood by one skilled in the art the radiation pattern can give information about a particular defect, for instance the derived polarisation angle of the scattered radiation can give an indication of orientation of a crack and the number of lobes in the radiation pattern can be indicative of its length. Further polarisation characteristics of non-defect echoes will generally differ from the expected defects.

In some applications the distance of the transmit and receive antennas from the surface under investigation may fluctuate. For instance if mounted on the bogie of a train the vibration of the bogie can cause'lift-off'of the apparatus from the rail which would affect the readings collected. In this event it is important to know the extent of fluctuation so the effect thereof can be compensated for. Preferably therefore the apparatus includes a means for compensating for any fluctuation in distance of the apparatus from the surface under investigation.

The apparatus may therefore comprise a means for determining the extent of any such fluctuation in distance. This could comprises a range finder, such as a laser range finder, to determine the actual distance from the surface, or could comprises an accelerometer device to determine the extent of movement of the apparatus.

Alternatively the apparatus could comprise means of determining the phase of the detected radiation. If there is any change in distance of the apparatus from the surface under investigation the path length of the radiation and hence the phase of the received radiation will change. This phase shift can be detected and used to determine the extent of any change in distance.

Instead of determining the extent of fluctuation in distance the effects thereof could be compensated for directly. For instance features in the detected signal at the frequency of vibration of the carrier of the apparatus could be filtered.

The invention may be used in a rail inspection apparatus comprising a carrier for travelling over a rail to be measured and an apparatus as described above mounted on the carrier such that, in use, radiation transmitted from the transmit antenna is directed toward the rail surface. The rail inspection system may further comprise a means of determining the position of the apparatus along the track.

As mentioned above the defects themselves will cause scattering of the incident radiation out of the expected reflection direction. This therefore may be used as an additional or indeed alternative method of determining the presence of defects in a conducting surface. In another aspect therefore there is provided a method of investigating a conducting surface to identify defects in the surface comprising the steps of illuminating the surface under investigation with millimetre wave electromagnetic radiation such that there is an expected reflection direction and determining whether any transmitted radiation has been scattered into a direction other than the expected reflection direction At least one receive antenna is located in a direction where no radiation would be expected to be reflected from a defect free surface. Detection of any received radiation could then be used as an indicator of the

presence of defects. The or each receive antenna could be arranged to detected back scattered radiation from any defects in the surface.

Because of the relatively low level of scattering from defects the apparatus would preferably employ a plurality of receive antennas and would employ multiple channel processing to determine the presence of any defects.

The invention will now be described by way of example only with reference to the following drawings, of which; Figure 1 shows a schematic of a first embodiment of the invention, Figure 2 illustrates the variation in reflected direction of a cracked surface, Figure 3 shows a cross sectional view of the system, Figure 4 shows the variation in power with angle within the reflected beam, Figure 5 shows a plot of amplitude of received signal over distance for a) a cracked rail and b) an intact rail, Figure 6 shows a plot of amplitude against frequency for a) a cracked rail and b) an intact rail.

Referring now to figure 1 an investigation module according to the present invention is generally indicated as 2. The module 2 consists of a transmit antenna 4 arranged to illuminate a surface. The module also has a receive antenna 38.

The apparatus shown is for use on a train mounted rail inspection system. Transmit antenna 4 and receive antenna 38 are therefore mounted on a carrier 10. This carrier could be the train chassis or bogie or could be a separate unit mounted to the chassis or bogie. Obviously each rail will require its own module.

The transmit antenna 4 and receive antenna 38 are located a few cm from the rail, generally indicated 12. The rail 12 has a rail head 14 having a surface 16. The rail also consists of a web 18 and a foot 20. Because of points and crossings the transmit

antenna 4 and receive antenna 38 both have to be located above the top of the rail surface 16. When only one module is used per rail the transmit antenna is generally mounted so as to illuminate the whole of the rail surface 16. However more than one module could be used on each rail and mounted at different parts of the train. In this instance the modules may be offset by different amounts to look at different parts of the rail surface. Where more than one module is used and two modules are positioned close to one another the frequency of operation of each module is chosen to be different so as to avoid any cross talk between modules. Alternatively modules with multiple receiver channels, i. e. more than one receive antenna could be used with, the receive antennas appropriately positioned.

In use the transmit antenna 4 emits a short pulse of millimetre wave radiation toward the rail 12. Millimetre wave frequency is used as the expected defects sizes are the order of millimetres. There are various types of defects that can occur in rails. Some defects, for example squats, will be fairly large and separate defects. Others, such as gauge corner cracks, consist of sequences of small defects which occur at intervals of a few millimetres. Modulation of the rail surface height, such as a corrugation, which may not be a problem of its itself but leads to increased noise and vibration which can promote formation of other more dangerous defects, will have a period of several millimetres. Millimetre wave radiation will interact with all these types of defects and give detectable signals. Also, use of millimetre wave radiation will cause induced currents in the rails surface to be concentrated in a region much less than 1 micrometre from the surface making the present invention sensitive to changes in surface characteristics of the rails such as conduction which could be caused by wheelburn or internal cracks. As used in this specification the term millimetre wave shall be taken to means a frequency range of 30-300 GHz.

Operating at a frequency range of 60-66 GHz can be advantageous as it lies in an atmospheric absorption band. Therefore radiation travelling through the atmosphere in this frequency is attenuated over distance. This has a minimal effect on the short distances involved in this invention but reduces the possibility of a stray signal - teaching receive antenna 38 from some external source. Also-although the module is low power working in this range reduces the likelihood of transmitted radiation

interfering with another train or trackside system. Another useful frequency range is 90-lOOGHz as the wavelengths in this range show good correspondence to the ripples caused by defects. For instance a frequency of 94GHz has been used and shown to give good identification of cracked rails.

Short pulses are transmitted, typically about lns, to ensure that the module has a small depth of view. Given that a short pulse is transmitted the receive antennas can be gated to receive signals over a short window of time. This is timed to ensure that only signals scattered from the rail surface 16 have time to reach the receive antenna 38 and any signals scattered from, say, the foot of the rail 20 would not have time to have reached the antenna. This minimises the possibility of false alarms.

The pulses typically have a bandwith of 2 GHz.

The beam emitted by the transmit antenna 4 is shown as reference 22. Were the rail surface 16 defect free the incident radiation would be reflected as reflected beam 24.

However the presence of defects will lead to surface contour variations in the rail surface which will tend to change the reflection direction of the radiation.

Figure 2a shows the situation for reflection from a smooth, defect free surface. The incident radiation beam 22 is reflected as beam 24 and is received by the receive antenna 38. Figures 2b and 2c illustrate the situation for a cracked surface. As mentioned cracks in the surface will cause an undulation in the contour of the rail surface. Therefore in some situations, as shown in figure 2b, the incident radiation may be incident on a smooth patch of rail and the reflected radiation will still be reflected in the same direction as for the defect free surface. However at another time the transmitted radiation will be incident on a different part of the surface and the reflection direction will be altered. As shown this could cause the reflected radiation to be reflected in a different direction to one which the receive antenna 38 is located leading to a drop in detected power.

As the inspection apparatus moves over the rail the ripple effect of the cracks on the rail surface contour will lead to a characteristic ripple on the detected power of received radiation which can be used to identify the presence of defects.

It should be noted in figure 2a that the receive antenna 38 is located towards the edge of the beam of expected reflected radiation and not in the middle. As shown in Figure 4 the intensity or power of radiation in the reflected beam varies within the beam with angle 0. Most of the power is concentrated in the centre of the beam. Thus where the receive antenna located in the centre of the expected beam, position 46, a small angular deflection either way would not have a large impact on the detected power whereas at position 48 the rate of change of intensity is greater and the same angular deflection has a much greater effect.

The power received by receive antenna 38 is monitored by processor 32. Processor 32 may perform various processing routines to help identify proper readings and discount false alarms and also categorise the type of defect. Various processing techniques are known to those skilled in the art. For instance time-frequency transforms such as Fourier Transforms can help in looking at the frequency characteristic of the received signals. As mentioned certain defects such as gauge corner cracks or corrugations comprise sequences of defects. Therefore the contribution to the received signal from such defects will be proportional to the train speed whereas independent defects such as wheel burns and squats will be independent of track speed. Other effects may influence the received signal however.

For instance non-uniformities of the train wheels could affect the signal and would also be proportional to train speed and carriage vibrations may also influence which would be related to train speed but also dependent on additional factors. Therefore some of the potential false alarms can be accounted for by knowing what to look for and defects can be categorised.

Further information can be determined by looking at the signals received at different polarisations. Transmit antenna 4 could be adapted therefore to transmit pulses of circularly polarised radiation as would be well understood by one skilled in the art.

Receive antenna 38 could then be arranged to be receptive to different polarisations at

different times, say first one linear polarisation and then the orthogonal polarisation.

This could be achieved by having a switchable polariser in front of the antenna.

Alternatively there may be an additional receive antenna (not shown) located adjacent receive antenna 38 and each receive antenna could be adapted to receive radiation at different orthogonal polarisations. As the transmit antenna 4 transmits circularly polarised radiation the defect is illuminated with radiation having a polarisation vector which changes with time and so the processor 32 can generate information regarding the radiation pattern of the defect. The polarisation angle of the received signal can be used to give an indication of the orientation of the crack and the number of lobes in the radiation pattern of a crack can indicate its length. The radiation pattern would of course be built up from a series of measurements as the train passes a crack.

The processor 32 may either process the information received in real time, may record it for subsequent analysis or may do both. Real time processing may be used to send warning signals to the driver or automatically slow the train if certain conditions are detected.

When the information is to be recorded the processor 32 is provided with a memory.

In this case it may be useful to store information regarding the position of train when the data was collected. The module is therefore be linked to a position sensor 34. The position sensor could measure the elapsed distance by counting wheel rotation or linking to systems for measuring the train's speed. An alternative would be to utilise a GPS (Global Positioning Sensor) type system with a GPS antenna situated elsewhere on the train. In any case, as mentioned, information about the speed of the train can be useful as speed obviously influences the frequency with which any periodic features on the track would contribute to the signal.

The processor 32 is also able to compensate for any fluctuations in the distances of the antenna from the rail, both vertical and transverse displacements. Mounting the module on a trains chassis is unlikely to give a consistent antenna-rail separation.

Even mounting the module on the bogie is unlikely to give a constant separation due to vibrations of the bogie. Therefore the module 2 has a module position monitor 40.

This acts to determine movement of the module antennas with respect to the rail.

Module position monitor 40 could comprise an array of accelerometers arranged to measure movement. Suitable accelerometers will be apparent to those skilled in the art. Alternatively the range to the rail could be measured by appropriately positioned laser range finders or other range finding systems.

In another embodiment module position monitor 40 comprises a phase detector linked to the receive antenna 38. Were the module to move upwards and increase the separation of the module 2 from the rail surface 16 the path length of radiation, transmitted by transmit antenna and received at the receive antenna, would increase.

This would therefore alter the phase of the radiation received at receive antenna 38.

Phase detector 40 detects the phase difference and passes this information to the processor 32 which can then compensate for the increased separation.

Using information about the speed of the train the processor 32 could also be adapted to filter signals at characteristic frequencies of vibration of the bogie. which could have been measured previously.

With reference to figure 3 more than one receive antenna may be used. Figure 3 shows the rail in cross section and illustrates that transmit antenna 4 is generally located in line with the rail to facilitate good illumination. Receive antenna 38 may also be located in line with the rail and has been omitted from figure 3 for the sake of clarity. Additional receive antennas 42 and 44 may be located adjacent receive antenna 38 but rotated slightly out of the plane so as to receive radiation reflected from the corners of the rail head 14. Thus the information from each of the three receive antennas gives information about a different part of the rail surface.

Referring back to figure 1 the apparatus may further comprise additional receive antennas 6 and 8 are arranged such that they would receive substantially no radiation reflected from a defect free surface. Antennas 6 and 8 are instead arranged to detect any back scattered radiation. Receive antennas 6 and 8 may be arranged adjacent one another or may be oriented to receive radiation scattered from different parts of the rail surface 16.

Where the surface has a defect incident radiation will be scattered into other directions. Apart from any direct reflection from the defect itself any induced current in the rail surface 16 will be disrupted by the defect and radiated into different directions.

The effect of defect 26 will be to scatter some radiation 28 in a direction where it will be detected by receive antenna 6. Similarly some radiation 30 to be scattered into a direction where it can be detected by receive antenna 8. Receive antennas 6 and 8 are connected to processor 32. In the simplest form merely detecting a sufficient signal at receive antennas 6 and 8 could be used as an indication that the surface may have defects therein. However further information about the type of defect can be gained by looking at the relative strength of radiation received at each receive antenna.

As mentioned previously different types of defects can be observed. Analysis of used rails Shows that lots of cracks can occur, at differing intervals of the order of millimetres, all generally running across the rail surface. The present invention is capable of detecting such defects using a non-contacting, non destructive system.

Further the invention can operate at normal train speeds and so does not necessitate any disruption to normal service and can be used to provide positional information about surface defects as well as information about the likely nature of the defect. An inspection module of any of the above described embodiments passing over a section of rail such as described would detect the radiation scattered from the cracks. From the radiation profile gathered as the train passed, together with information about the frequency of scattered radiation, the processor could determine the likely type of crack as a gauge corner track. Following such a determination a signal could be sent to impose a speed limit on the defective section of track until a visual inspection team or repair unit such as a rail grinder could be sent to the recorded location.

Referring now to figure 5 some experimental data is shown from measurements of actual rails. The apparatus used was a bistatic arrangement with the receive antenna located in the expected reflection direction, as antenna 38 is positioned in figure 1.

The frequency of operation was 94GHz. The antennas were oriented to look at the top of the rail and the gauge corner. The received power at the receive antenna was

measured and processed to give an indication of the drop in expected power as the rail was traversed.

Figure 5a shows a typical example of the signals received by the receive antenna when illuminating the cracked edge of the rail head. In this trace, ripples are observed in the amplitude of the received signal, with a period of around 0.5s corresponding to 3mm along the rail. The ripples are not observed in the trace shown in figure 5b corresponding to the smooth edge of the rail. These characteristic amplitude ripples were observed repeatably on the cracked rail edge, but not on the smooth edge.

The Fourier transforms of the traces of Figure 5 are presented in Figure 6. These show the amplitude of a received signal as a function of the period of any ripples within it. When the radar illuminated the smooth side of the rail, no significant ripples were observed-this can be seen in the Fourier transform as a very low amplitude for ripples with a period shorter than the length of the experiment. On the cracked edge of the rail, however, there is a large amplitude for ripples with a period of between 6 and 10mm. This simple analysis could form the basis of a threshold detection criterion in a practical system.

It can therefore be seen that the system according to the present invention can provide a fast, accurate and simple apparatus for checking the integrity of rails as the trains go about their normal running.

Although the invention has been described by way of reference with application to the detection of rails it should not be construed as being limited thereto. The invention could be used on any surface where it is wished to inspect the surface for defects.

Other applications could include investigation of conducting surfaces in machines, rollers and the like, where visual inspection is not possible, or inspection of the inside surfaces of pipes. Pipes carrying fluids such as oil or gas can cause severe problems if leakages occur. Traditional methods of inspecting pipes can be visual or can involve detecting leaks after they have occurred.

Often in such pipes inspection machines are forced through the pipes to check for blockages therein. Modules according to the present invention could be mounted on the inspection devices and could be used to give an indication of the integrity of the pipe's internal surface.

Other applications of the invention as well as other embodiments thereof will be apparent to the skilled person without departing from the concept of the invention.