GB2398946A

GB2398946A - Microwave radar detection of surface discontinuities

Info

Publication number: GB2398946A
Application number: GB0304085A
Authority: GB
Inventors: Michael Robert Buchan Dunsmore
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2003-02-22
Filing date: 2003-02-22
Publication date: 2004-09-01
Also published as: WO2004074823A3; GB0304085D0; WO2004074823A2

Abstract

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.

Description

Radar Detection of Surface Discontinuities This invention relates to an

apparatus and method for the investigation of surfaces using radar to identify any discontinuities present therein, in particular it relates to a device for non-contact testing of surfaces, such as rails, to detect defects therein.

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 0 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 momtor 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 tram 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.

s 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 an apparatus for lo investigating surfaces composing a transmit antenna for transmitting millimetre wave electromagnetic radiation towards a surface under investigation and a means for determining whether any transmitted radiation has been scattered by any surface defects.

Millimetre wave radiation transmitted toward a continuous or smooth 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 single reflection direction.

However if the surface has defects therein, and so is not smooth, the defects will interact with the incident radiation and scatter it into different directions. Determining whether any radiation has been scattered by the surface under investigation will give an indication of whether there are any defects in the surface.

As used herein the term millimetre wave radiation is taken to mean electromagnetic radiation having a frequency of between 10 - 200 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 an inspection apparatus according to the present invention can be used to investigate surfaces even when moving rapidly there over. Indeed an apparatus according to the present invention could be applied to the investigation of rail surfaces at normal train operating speeds.

Conveniently the means for determining whether any transmitted radiation has been scattered comprises a receive antenna arranged relative to the transmit antenna such that, in use, substantially no radiation reflected from a defect free surface would reach s the receive antenna.

As mentioned, a defect free surface would be expected to reflect the transmitted radiation in general along an expected reflection direction. If a receive antenna is arranged at an angle different to the expected reflection direction the presence of lo defects can be determined. Il substantially no reflected radiation is detected by the receive antenna this could indicate that the surface is defect free. However the detection of radiation at the receive antenna would indicate that defects are present as some of the incident radiation has been scattered therefrom.

The transmit antenna could also operate as the receive antenna, i.e. the apparatus could be monostatic. However because short pulses are preferred (for reasons that will be explained below) a bistatic arrangement, having separate transmit and receive antennas is preferred. Bistatic arrangements can generally cope with shorter pulse trains than monostatic systems as there is no need to duplex. However monostatic systems could be useful for some embodiments of the invention and could be used more widely if speed of operation of monostatic arrangements could be increased.

Preferably the receive antenna is arranged relative to the transmit antenna such that, in use, it receives radiation back scattered from the surface. Back scattered radiation, i.e. radiation which is scattered back toward the general direction from which the radiation was incident is a good indicator of defects in the surface.

More than one receive antenna could be used, each being arranged relative to the transmit antenna so as to receive substantially no radiation reflected from a defect free surface. Using more than one receive antenna allows the rail to be inspected from different angles which can increase sensitivity and help in determining the type of defect.

Additionally or alternatively the means for determining whether any transmitted radiation has been scattered may comprise a receive antenna arranged relative to the transmit antenna such that, in use, radiation reflected from a defect free surface would reach the receive antenna and a means for comparing the received signal power at the receive antenna with an expected power.

If a surface were defect free substantially all the transmitted radiation would reflect from the surface toward the receive antenna and be detected. Taking account of general losses involved in transmission, reflection and detection, which can be lo 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 scattered in directions other than the expected reflection direction. Therefore the signal received along the expected reflection direction would have a lower power than expected. This drop in power can be used as an indication that the surface has defects. As mentioned this could be used as an alternative to direct detection of scattered radiation by a receive antenna located away from the expected reflection direction or could be used in combination therewith.

Preferably the transmit antenna Is adapted to transmit pulses of radiation having a duration of Sns or less, more preferably Ins or less. 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 transmit antenna is preferably adapted to transmit pulses having a wide bandwidth.

For investigation apparatus intended to be used in outdoor environments the frequency used is preferably 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 s 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.

0 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 profile 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 scattering 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 2s frequency transforms.

Conveniently, when detecting scattered radiation, the apparatus may be adapted to measure the power of scattered radiation at different polarizations, such as orthogonal linear polarizations. When one receive antenna is used the apparatus may comprise a means of selectively altering the polarization 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 polarizations. 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 polarization of the transmitted radiation.

Measuring the polarization properties of the scattered radiation can help in determining the type of defect and reduce false alarms. By irradiating with a particular polarization at a parlcu]ar lime, as is achieved with circularly polarised radiation, and measuring the components of polarization of the scattered radiation in lo 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 polarization 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 ndcativc of its length. Further polarization IS 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 Logic 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 he 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 a receive antenna located relative to the transmit antenna so as to receive substantially all the radiation reflected from a defect free surface and a 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.

In the embodiment where a receive antenna is also used to determine the received s power of reflected radiation a single receive antenna to detect reflected radiation could be used to perform both tasks.

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 lo 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 IS toward the rail surface. The rail inspection system may further comprise a means of determining the position of the apparatus along the track.

In another aspect of the invention there is provided method of investigating a surface comprising the steps of illuminating a 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.

The invention will now be described by way of example only with reference to the following drawings, of which; s Figure 1 shows a schematic of a first embodiment of the invention, Figure 2 shows a schematic of a second embodiment of the invention, Figure 3 shows a third embodiment of the present invention, Figure 4 shows a section of rail suffering from gauge corner cracks, some cracks have been highlighted for clarity, 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 amphtudc against frequency for a) a cracked rail and b) an intact rail.

Referring now to figure l an mvestgation 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 two receive antennas 6 and 8.

The apparatus shown is for use on a train mounted rail inspection system. Transmit 2s antenna 4 and receive antenna 6, 8 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 antennas 6, 8 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 antennas 6, 8 all 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 rai l 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.

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 0 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. See Figure 4. 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 Is 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 l micrometre from the surface making the present invention sensitive to changes in surface characteristics of the r ails 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 lo - 200 GHz.

Operating at a frequency range of 60-66 GHz is advantageous as it lies in an 2s 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 reaching receive antennas 6, 8 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 backside system.

Short pulses are transmitted, typically about ins, 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 surl;ace l 6 have time to reach the receive antennas 6, 8 and any signals scattered from, say, the foot of the rail 20 would not have time to have I reached the antenna. This minimises the possibility of false alarms. s

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.

lo Receive antennas 6 and 8 are arranged such that they would receive substantially no radiation reflected from a defect tree surface and 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. For instance receive antenna 6 could look towards the middle of the rail surface and receive antenna 8 could be offset to look towards the gauge corner I surface.

Where the surface has a defect however 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.

Processor 32 may perform various processing routines to help identify proper readings and discount false alarms and also catcgorise 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 s speed whereas independent detects such as wheel burns and squats will be independent of track speed. Other effects may influence the received signal however.

For instance non-uniformites 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 detcrmincd by looking at the signals received at different polarizations. Transmit antenna 4 could be adapted therefore to transmit pulses of is circularly polarised radiation as would be well understood by one skilled in the art.

Receive antenna 6 could then be arranged to receive radiation corresponding to a particular linear polarization and recei ve antenna 8 could be arranged to receive radiation from the orthogonal linear polarization. Either the receive antennas could be appropriately oriented polarization sensitive antennas or polarisers could be placed in the receive beam path of each antenna. As the transmit antenna 4 transmits circularly polarised radiation the defect is illuminated with radiation having a polarization vector which changes with time and so the processor 32 can generate information regarding the radiation pattern of the defect. The polarization 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 lo compensate for any fluctuations in the distances of the antennas 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 36.

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

Module position monitor 36 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 r ange finding systems.

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. 2s

An alternative embodiment is shown in figure 2 where items performing the same function are designated with the same reference numerals. In this embodiment there is still a transmit antenna 4 and two reccve antenna 6, 8. However there is also an additional receive antenna 38 which is located in the direction of the expected reflected beam of radiation 24. As can be seen from figure 2 this additional receive antenna will therefore always receive radiation when in use. This fact can be used to give information about the separation of the module from the rail. Were the module to move upwards and increase the separation of the module 2 from the rail surface 16 the path length of radiation, transmLtcd 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.

Receive antenna 38 can also be used to give an indication that defects are present. As can be seen the presence of a defect 26 in the rail surface 16 will result in scattering of some of the radiation which otherwise would have been reflected toward receive lo antenna 38. Therefore the received power at receive antenna 38 will drop and this drop could be used to give an indication of surface defects, either on its own or, as shown in figure 2, in conjunction with other receive antennas.

Figure 3 shows yet another embodiment of the present invention. Here a monstatic arrangement is used with a single antenna 42 being used as both a transmit and a receive antenna.

Figure 4 is a picture of a defective rail having gauge corner cracks therein. Some of the cracks 50 have been highlighted for clarity. It can be seen that lots of cracks 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-eontaeting, 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 i 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 shown in figure 4 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 deterrnnation 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 S some experimental data is shown from measurements of actual rails. The apparatus used was a bistaticarrangement with the receive antenna located in the expected reflection direction, as antenna 38 is positioned in figure 2.

The frequency of operation was 94GHz. The antennas were oriented to look at the top of the rail and the gauge coiner. The rcccivcd power at the receive antenna was measured and processed to give an indication of the drop in expected power as the rail was traversed over a metre distance.

Figure Sa shows the amplitude of the power drop against distance for a lm section of lo a cracked rail similar to that shown in figure 4. Figure Sb shows the measurements taken using a defect free rail. It can be seen that the plot for the cracked rail has a great deal more fluctuation and a greater amplitude of power loss. Measurement for the intact rail still showed some loss, most likely due to spurious reflections and diffraction effects. However the rate of change was much reduced and the overall amplitude was lower.

Figure 6 shows the same data after it has undergone a Fourier Transform to the frequency spectrum. Figure 6a shows the amphtude against frequency plot for the cracked rail for 0 - 10 Hz. Figure 6b shows the same for the intact rail. Here it can be seen that there are much more relatively high frequency components in the cracked rail plot which would be an indication that the surface profile is not uniform.

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 contacting 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 s 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 in apparent to the skilled person without departing from the concept of the invention.

Claims

Claims 1. An apparatus for investigating surfaces comprising a transmit

antenna for transmitting millimetre wave electromagnetic radiation towards a surface under investigation and a means for determining whether any transmitted radiation has been scattered by any surface defects.
2. An apparatus as claimed in claim 1 wherein the means for determining whether any transmitted radiation has been scattered comprises a receive antenna arranged relative to the transmit antenna such that, in use, substantially no radiation rel lected from a defect free surface would reach the receive antenna.
3. An apparatus as claimed m claim 2 wherein the transmit antenna is also the receive antenna.
4. An apparatus as claimed in claim 2 wherein the receive antenna is an additional antenna to the transmit antenna.
5. An apparatus as claimed in any of claims 2 to 4 wherein the receive antenna is arranged relative to the transmit antenna to detect radiation which is back scattered from the surface under investigation.
6. An apparatus as claimed in any ol claims 2 to 5 wherein the means for determining whether any transmitted radiation has been scattered comprises more than one receive antenna, each receive antenna being arranged relative to the transmit antenna such that, In use, substantially no radiation reflected from a defect free surface would reach any ol the receive antennas.
7. An apparatus as claimed In any of claim 2 to 6 wherein the receive antenna is capable of measuring the received power at different polarizations.
8. An apparatus as claimed in claim 6 wherein each receive antenna is adapted to receive electromagnetic radiation at a different polarization.
9. An apparatus as claimed in claim 8 wherein two receive antennas are adapted to receive radiation at orthogonal linear polarizations.
10. An apparatus as claimed in any preceding claim wherein the means for determining whether any transmitted radiation has been scattered comprises a receive antenna arranged relative to the transmit antenna such that, in use, radiation reflected from a defect free surface would reach the receive antenna and a means for comparing the received signal power at the receive antenna with an expected power.
An apparatus as claimed in any preceding claim wherein the transmit antenna is adapted to transmit pulses of radiation having a duration of less than ins.
12. An apparatus as claimed In any preceding claim wherein the transmit antenna is adapted to transmit radiation having a bandwidth of 2GHz.
13. An apparatus as claimed in any preceding claim wherein the transmit antenna is adapted to transmit radiation having a frequency in the range of 60 - 66 GHz.
14. An apparatus as claimed In any preceding claim wherein the apparatus comprises processing means for processing the received signal so as to distinguish different types of defect.
15. An apparatus as claimed in claim 14 wherein the processing means is adapted to perform a Fourier Transform on the received signal.
16. An apparatus as claimed in any preceding claim further comprising a means for determining the extent of any fluctuation in distance of the apparatus and the surface under invcstgation.
17. An apparatus as claimed in claim 16 wherein the means for determining the extent of any fluctuation in distance comprises one of a motion detector and a range finder.
18. An apparatus as claimed in claim 16 wherein the means for determining the extent of any fluctuation in distance comprises a receive antenna arranged relative to the transmit antenna such that in use radiation reflected from a defect free surface would reach the receive antenna and a means for determining the phase of the detected radiation.
19. A rail inspection apparatus comprising a carrier for travelling over a rail to be measured and an apparatus as claimed in any of claims 1 to 18 mounted on the carrier such that in use radiation transmitted from the transmit antenna is directed toward the rail surface.
20. A rail inspection system as claimed m claim 19 further comprising a means of determining the position of the apparatus along the track.
21. A method of investigating a surl:ace comprising the steps of illuminating a surface under investigation with mi]limetre 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.
22. A method as claimed m claim 21 wherein the step of determining whether any transmitted radiation has been scattered into a direction other than the expected reflection direction comprises the step of disposing a receive antenna out of the expected reflection direction and detecting any radiation received.
23. A method as claimed in claim 22 wherein the method comprises the step of disposing at least two receive antennas out of the expected reflection direction and detecting any radiation received.
24. A method as claimed m claim 23 wherein each antenna is adapted to receive radiation having a different polarsation.