Methods for forming 3D image data and associated apparatuses

Abstract

A method for forming 3D image data representative of the subsurface of infrastructure located in the vicinity of a moving vehicle. The method includes: rotating a directional antenna, mounted to the moving vehicle, about an antenna rotation axis; performing, using the directional antenna whilst it is rotated about the antenna rotation axis, a plurality of collection cycles in which the directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of collection cycles performed by the directional antenna.

Claims

1. A method for forming 3D image data representative of the subsurface of infrastructure located in the vicinity of a moving vehicle, the method including: rotating a directional antenna, mounted to the moving vehicle, about an antenna rotation axis with a plane perpendicular to the antenna rotation axis at least partially facing in a direction of movement of the vehicle, wherein the directional antenna is rotated through a full three-hundred and sixty-degree arc about the antenna rotation axis such that a primary radiation axis of the directional antenna traces a space curve helix relative to a stationary coordinate system; performing, using the directional antenna whilst it is rotated about the antenna rotation axis, a plurality of collection cycles in which the directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of collection cycles performed by the directional antenna: (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle; processing the radar data, the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle.

2. A method according to claim 1, wherein the method includes varying an angle θ between the antenna rotation axis and the direction of movement of the vehicle.

3. A method according to claim 1, wherein the method includes varying an angle θ between the antenna rotation axis and the direction of movement of the vehicle to a non-zero value such that the primary radiation axis of the directional antenna has a component of movement that is opposite to the direction of movement of the vehicle whilst the primary radiation axis of the directional antenna points beneath the moving vehicle.

4. A method according to claim 1, wherein the method includes varying an angle ϕ between the primary radiation axis of the directional antenna and a plane perpendicular to the antenna rotation axis.

5. A method according to claim 1, wherein the RF energy emitted by the directional antenna during each collection cycle includes an ultra-wideband pulse.

6. A method according to claim 1, wherein the directional antenna includes a transmit horn antenna configured to emit the RF energy and a receive horn antenna configured to receive reflected RF energy.

7. A method according to claim 1, wherein the method includes using position data collected by a GPS device to map the 3D image data to a global coordinate system.

8. A method according to claim 1, wherein the method additionally includes: rotating a further directional antenna, mounted to the moving vehicle, about a further antenna rotation axis; performing, using the further directional antenna whilst it is rotated about the further antenna rotation axis, a plurality of further collection cycles in which the further directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of further collection cycles performed by the further directional antenna: (i) further radar data representative of reflected RF energy received by the further directional antenna during the further collection cycle; (ii) further angular position data representative of an angular position of the further directional antenna about the further antenna rotation axis during the further collection cycle; and (iii) further translational position data regarding a translational position of the further directional antenna during the further collection cycle; processing the further radar data, the further angular position data and the further translational position data collected during each of the further plurality of collection cycles to form further 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle.

9. A method according to claim 8, wherein the directional antenna and further directional antenna may be configured to emit RF energies having different frequencies.

10. A method according to claim 9, wherein the directional antenna is configured to emit UWB pulses having a centre frequency in the range 250-750 MHz whereas the further directional antenna is configured to emit UWB pulses having a centre frequency in the range 1.5 GHz 2.5 GHz.

11. A method according to claim 1, wherein the vehicle is a rail vehicle.

12. An apparatus for collecting data for use in forming a 3D image representative of the subsurface of infrastructure located in the vicinity of a moving vehicle, the apparatus including: a base unit configured to be mounted to a vehicle; a directional antenna rotatably mounted to the base unit to allow the directional antenna to be rotated about an antenna rotation axis; a drive unit configured to rotate the directional antenna about the antenna rotation axis; wherein the apparatus is configured to perform, using the directional antenna whilst it is rotated about the antenna rotation axis, with a plane perpendicular to the antenna rotation axis at least partially facing in a direction of movement of the vehicle, through a full three-hundred and sixty-degree arc about the antenna rotation axis such that a primary radiation axis of the directional antenna traces a space curve helix relative to a stationary coordinate system, and whilst a vehicle to which the base unit is mounted is moving, a plurality of collection cycles in which the directional antenna emits RF energy and collects reflected RF energy; wherein the apparatus is configured to collect, during each of the plurality of collection cycles performed by the directional antenna: (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle.

13. An apparatus according to claim 12, wherein the apparatus includes a limb which is rotatable about a pivot/hinge, wherein the antenna is mounted to the limb such that an angle θ can be varied by rotating the limb about the pivot/hinge.

14. A method according to claim 1, wherein the method does not include varying an angle θ between the antenna rotation axis and the direction of movement of the vehicle to a non-zero value such that the primary radiation axis of the directional antenna has a component of movement that is opposite to a direction of movement of the vehicle whilst the primary radiation axis of the directional antenna points beneath the moving vehicle.

15. An apparatus according to claim 12, wherein the apparatus includes a processing unit configured to process the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle.

16. A computer-readable medium having computer-executable instructions configured to cause a computer to process radar data, angular position data and translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle, wherein the radar data, angular position data and translational position data has been collected according to a method for forming 3D image data representative of the subsurface of infrastructure located in the vicinity of a moving vehicle, the method including: rotating a directional antenna, mounted to the moving vehicle, about an antenna rotation axis, with a plane perpendicular to the antenna rotation axis at least partially facing in a direction of movement of the vehicle, wherein the directional antenna is rotated through a full three-hundred and sixty-degree arc about the antenna rotation axis such that a primary radiation axis of the directional antenna traces a space curve helix relative to a stationary coordinate system; performing, using the directional antenna whilst it is rotated about the antenna rotation axis, a plurality of collection cycles in which the directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of collection cycles performed by the directional antenna: (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle; processing the radar data, the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle.

17. A method according to claim 1, wherein a non-zero angle θ is made between the antenna rotation axis and the direction of movement of the vehicle during at least some of the plurality of collection cycles performed by the directional antenna.

18. A method according to claim 1, wherein an angle θ is made between the antenna rotation axis and the direction of movement of the vehicle; and the performing the plurality of collection cycles comprises: performing one or more first collection cycles at a first vehicle speed and at a first value of the angle θ, performing one or more second collection cycles at a second vehicle speed and at a second value of the angle θ, and wherein the first value of the angle θ is greater than the second value of the angle θ if the first vehicle speed is greater than the second vehicle speed, and the second value of the angle θ is greater than the first value of the angle θ if the second vehicle speed is greater than the first vehicle.

19. A method according to 1, wherein an angle θ is made between the antenna rotation axis and the direction of movement of the vehicle; and performing the plurality of collection cycles comprises: processing data about vehicle speed by a processing unit; using data about the vehicle speed to vary the angle θ from a first value at a first vehicle speed to a second value at a second vehicle speed; wherein the first value of the angle θ is greater than the second value of the angle θ if the first vehicle speed is greater than the second vehicle speed, and the second value of the angle θ is greater than the first value of the angle θ if the second vehicle speed is greater than the first vehicle.

20. An apparatus according to claim 12, wherein a non-zero angle θ is made between the antenna rotation axis and the direction of movement of the vehicle during at least some of the plurality of collection cycles performed by the directional antenna.

21. The method of claim 1, wherein the space curve helix traced by the primary radiation axis of the directional antenna is formed by the tip of a boresight vector representing the primary radiation axis maintaining contact with a cylinder whose center axis is coincident with the direction of the movement of the vehicle.

22. The method of claim 1, wherein the space curve helix traced by the primary radiation axis of the directional antenna corresponds to r(t), where: $r\left(t\right)=\frac{h}{\sqrt{{\left(\sin \left(\varphi \right)\sin \left(\theta \right)-\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)\right)}^2+{\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)}}\left(\left(\sin \left(\varphi \right)\sin \left(\theta \right)-\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)\right){\hat{x}}_1+\left(\sin \left(\varphi \right)\cos \left(\theta \right)+\cos \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)\right){\hat{x}}_2+\cos \left(\varphi \right)\cos \left(\Omega t\right)\right){\hat{x}}_3+vt{\hat{x}}_2$ wherein v is speed of the moving vehicle, wherein t is time, wherein the length of a boresight vector d extending in the direction of the primary radiation axis maintains contact with the cylinder of radius h whose centre axis is coincident with the direction of movement of the vehicle x.sub.2, wherein θ is the angle between the antenna rotation axis Ω and the direction of movement of the vehicle x.sub.2, wherein ϕ is the angle between the primary radiation axis Ω and a direction perpendicular to the antenna rotation axis xi, and wherein x.sub.3 is a vertical axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of these proposals are discussed below, with reference to the accompanying drawings in which:

(2) FIG. 1 shows an example radar apparatus for inspecting rail track ballast and subgrade made in accordance with conventional techniques and off-the-shelf components.

(3) FIG. 2 illustrates the kinematics of an example apparatus for collecting data for use in forming a 3D image representative of the subsurface of infrastructure located in the vicinity of a moving vehicle.

(4) FIG. 3A-D illustrates a space curve describing boresight motion relative to a stationary coordinate system 0 for the example apparatus of FIG. 2 for θ having the values of (a) 0 degrees, (b) 45 degrees, (c) 60 degrees and (d) 75 degrees.

(5) FIG. 4A and FIG. 4B shows the example apparatus of FIG. 2 in further detail.

(6) FIG. 5 shows the antenna assembly from the example apparatus of FIG. 2 in further detail.

DETAILED DESCRIPTION

(7) In general, the following discussion describes examples of our proposals that can be viewed as providing a time-varying incident angle subsurface investigation radar apparatus capable of generating three-dimensional volumetric images. In these examples, an antenna assembly is mechanically rotated allowing data collection from a full three-hundred and sixty-degree arc about the rotation axis. The incidence angle of the incoming radiation therefore varies over the rotation period. The angle θ between the antenna rotation axis and a direction of movement of a vehicle to which a directional antenna is mounted can be varied to achieve higher sampling densities within laterally localized areas, enabling surveys to be conducted more quickly in areas where full angular coverage is not required.

(8) A significant advantage of the examples described below is the ability to produce three-dimensional volumetric images using only a single transect, whilst collecting radar measurements obtained over a diffuse range of positions lateral to a direction of movement of the vehicle to which a directional antenna is mounted.

(9) To create such images with existing systems requires either the use of staggered antenna arrays (two or more rows of antennas, with antennas in each row being offset from antennas in other rows), or using a single antenna to obtain multiple adjacent transects (by obtaining measurements with the single antenna in a first fixed position and then moving the single antenna to a new fixed position to obtain further measurements and so on, which could be seen as staggering the multiple transects in time). However, using staggered antenna arrays is expensive and still suffers from scanning extent restrictions due to size limitations, and using a single antenna to obtain multiple adjacent transects is not feasible for transport applications involving road and rail networks (where measurements may need to be obtained quickly).

(10) Advantageously, the examples described below do not constrain sampling points to lie within discrete vertical planes, so the ratio of lateral to longitudinal sampling density can be comparatively high and the lateral sample distribution is more diffuse. Rotation of the directional antenna allows it to detect and image subsurface features at relatively large distances from the transect, which is advantageous for the evaluation of transport infrastructure; problems occurring outside the scanning region of current systems (e.g. the shoulder ballast 8, the damaged pipe 9 and the void 10 illustrated in FIG. 1) can be identified and corrected with targeted maintenance. For rail specific applications, an oblique incident angle of emitted RF energy (relative to a vertical axis) enables greater ballast evaluation capability under rails, since the wave vector of the emitted RF energy has a horizontal component, allowing the wave to move laterally under the rail (whilst there still is a radar shadow caused by the rails, this shadow is offset from the tracks). Evaluating rail ballast directly under the tracks is important because this is where defects often occur.

(11) Finally, the examples discussed below have a unique capability to inspect retaining walls and tunnels above and to the side of the moving vehicle. A report by the Transportation Research Board of the National Academies in the United States (“Mapping Voids, Debanding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings” published by the Transportation Research Board, Mar. 19, 2015) illustrates, using a fixed antenna, the effectiveness of air-coupled ground penetrating radar (“GPR”) to detect large air and water filled voids to a depth of 20 cm within tunnel walls. Using known techniques, smaller defects which could cause concrete delamination may be localized using the exemplified apparatus by determining the surface dielectric permittivity; normal concrete has a bulk relative permittivity of between 8-12. However, voids near the surface will produce backscatter that can interfere with the surface reflection. This will cause a fluctuation in the apparent permittivity calculated from measurements of reflected signal amplitude. The examples discussed below can scan the entire interior surface of a tunnel and locate variations in surface dielectric permittivity that can be further investigated with other diagnostic techniques.

(12) The kinematics of an example apparatus for collecting data for use in forming a 3D image representative of the subsurface of infrastructure located in the vicinity of a moving vehicle are illustrated in FIG. 2.

(13) For clarity, FIG. 2 only illustrates a directional antenna of the example apparatus that includes two antenna components: a transmit horn antenna 14 and a receive horn antenna 15. The transmit horn antenna 14 and the receive horn antenna 15 may be transverse electromagnetic (“TEM”) horn antennas.

(14) As shown in FIG. 2, the transmit and receive TEM horn antennas 14, 15 rotate about an antenna rotation axis defined by an angular velocity vector Ω. The transmit and receive TEM horn antennas 14, 15 are positioned directly over a centre line 16 of rail tracks at height h. The angular velocity vector Ω lies in the horizontal plane spanned by x.sub.1 and x.sub.2, where x.sub.2 is a direction of movement of the vehicle, which may be referred to herein as a longitudinal direction and xi is perpendicular to the direction of movement of the vehicle, which may be referred to herein as a lateral direction.

(15) An angle θ between the antenna rotation axis Ω and the direction of movement of the vehicle x.sub.2 is varied to change the angular position of Ω and its associated plane of rotation 13 (a plane perpendicular to the antenna rotation axis Ω). The angular position of the antenna assembly 14 in the plane of rotation 13 is Ωt with respect to x.sub.3 (the vertical axis), and the antenna rotation axis Ω can be rotated by angle ϕ about the vector (cos(θ)cos(Ωt), — sin(θ)cos(Ωt), sin(Ωt)), thereby tilting the boresight vector d out of the plane of rotation 13 such that there is an angle ϕ between the boresight vector d and the plane of rotation 13.

(16) For the purposes of illustrating the motion of the boresight vector d as shown in FIG. 3A-D, the length of d may be constrained such that:

(17) $.Math.d.Math.=\frac{h}{\sqrt{\begin{array}{c}{\left(\sin \left(\varphi \right)\sin \left(\theta \right)-\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)\right)}^2+\\ {\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)\end{array}}}$
which ensures that the tip of the boresight vector d maintains contact with a cylinder of radius h whose centre axis is coincident with the longitudinal axis x.sub.2. The parametric space curve describing boresight motion relative to the stationary coordinate system O is then:

(18) $r\left(t\right)=\frac{h}{\sqrt{\begin{array}{c}{\left(\sin \left(\varphi \right)\sin \left(\theta \right)-\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)\right)}^2+\\ {\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)\end{array}}}\left(\left(\sin \left(\varphi \right)\sin \left(\theta \right)-\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)\right){\hat{x}}_1+\left(\sin \left(\varphi \right)\cos \left(\theta \right)+\cos \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)\right){\hat{x}}_2+\cos \left(\varphi \right)\cos \left(\Omega t\right)\right){\hat{x}}_3+\mathrm{vt}{\hat{x}}_2$

(19) FIG. 3A-D illustrate this space curve for increasing values of θ assuming that an antenna assembly including the transmit and receive TEM horn antennas 14, 15 is moving translationally at 45 m/s (162 km/h) at height h=1 with ϕ=0, and rotating at 210 rad/sec (≈2000 rpm).

(20) FIG. 3A-D correspond to θ having the values 0, 45, 60 and 75 degrees, respectively.

(21) In the examples shown in FIG. 3B-D, the angle θ is changed to a non-zero value such that the primary radiation axis (boresight vector d) of the directional antenna has a component of movement that is opposite to a direction of movement of the rail vehicle whilst the primary radiation axis (boresight vector d) of the directional antenna points beneath the moving vehicle. The minimum distance between adjacent “windings” of the parametric space curve for a particular value of Ωt is l (illustrated in FIG. 3A-D) which is given approximately by

(22) $\ell \approx \frac{2\pi v}{\Omega }\sqrt{1-{\left({\hat{x}}_2.Math.\frac{\frac{\partial r}{\partial t}}{.Math.\frac{\partial r}{\partial t}.Math.}\right)}^2},\frac{\partial r}{\partial t}=\frac{h\Omega \cos \left(\varphi \right)}{{\left(\begin{array}{c}{\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)+\\ {\left(\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)-\sin \left(\varphi \right)\sin \left(\theta \right)\right)}^2\end{array}\right)}^{\frac{3}{2}}}\left(\cos \left(\varphi \right)\cos \left(\Omega t\right)\left(\sin \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)-\cos \left(\varphi \right)\cos \left(\theta \right)\right){\hat{x}}_1+\sin \left(\theta \right)\cos \left(\Omega t\right){\hat{x}}_2-\left(\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)-\sin \left(\varphi \right)\sin \left(\theta \right)\right)\left(\cos \left(\varphi \right)\cos \left(\theta \right)-\sin \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)\right){\hat{x}}_3\right)+v{\hat{x}}_2,.Math.\frac{\partial r\left(t\right)}{\partial t}.Math.=\frac{h\Omega \cos \left(\varphi \right)}{{\left(\begin{array}{c}{\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)+\\ {\left(\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)-\sin \left(\varphi \right)\sin \left(\theta \right)\right)}^2\end{array}\right)}^{\frac{3}{2}}}\begin{array}{c}({\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right){\left(\sin \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)-\cos \left(\varphi \right)\cos \left(\theta \right)\right)}^2+\\ \begin{array}{c}{\left(\sin \left(\theta \right)\cos \left(\Omega t\right)+\left(\frac{{\left(\begin{array}{c}{\cos }^2\left(\varphi \right){\cos }^2\left(\Omega t\right)+\\ {\left(\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)-\sin \left(\varphi \right)\sin \left(\theta \right)\right)}^2\end{array}\right)}^{\frac{3}{2}}}{h\Omega \cos \left(\varphi \right)}\right)v\right)}^2+\\ {{\left(\cos \left(\varphi \right)\cos \left(\theta \right)\sin \left(\Omega t\right)-\sin \left(\varphi \right)\sin \left(\theta \right)\right)}^2{\left(\cos \left(\varphi \right)\cos \left(\theta \right)-\sin \left(\varphi \right)\sin \left(\theta \right)\sin \left(\Omega t\right)\right)}^2)}^{1/2}\end{array}\end{array}$

(23) Thus, the minimum distance l separating adjacent segments of the curve in the region where the cylinder of unit radius contacts the ground plane (directly underneath the vehicle) decreases from approximately 1.25 m in (a) to 0.375 m in (d). The reciprocal of l yields the linear sampling density along a line segment that is in the ground plane and is approximately perpendicular to ∂r(t)/∂t at the point where the space curve intercepts the ground plane. The effect of increasing θ is to improve the sampling density from 0.8 samples/m to 2.67 samples/m along this line segment. Another measure of linear sampling density is along the space curve itself, which can be calculated by using

(24) Linear Sampling Density along Space Curve.=(scans/sec)/|∂r(t)/∂t|, where (scans/sec) is the number of scans per second given by the pulse repetition frequency (PRF) of the radar divided by the number of pulses per stack (a scan is functionally equivalent to a trace). The two dimensional sampling density, abbreviated as S.D., is measured in samples/m.sup.2 and is the product of the two aforementioned linear sampling densities:

(25) $S.D.\approx \frac{\Omega \left(\frac{\mathrm{scans}}{\sec }\right)}{2\pi v}\sqrt{\frac{1}{{.Math.\frac{\partial f}{\partial t}.Math.}^2-{\left({\hat{x}}_2\frac{\partial r}{\partial t}\right)}^2}}$

(26) From the above equation, it can be seen that the greater |∂r(t)/∂t|, the lower the S.D., as the antenna spends less time over a particular point. Therefore the sample density is inversely proportional to velocity.

(27) Of course, any increase of scan density below the vehicle results in a lessening of scan density above the vehicle. However the distance between adjacent “windings” of the space curve helix will increase as shown in FIG. 3A-D. Thus, it may be desirable to find the best balance of along curve and perpendicular to curve resolution as a function of θ for a particular antenna rotation rate and vehicle speed, accounting for the region to be scanned.

(28) Additionally, |∂r(t)/∂t| changes with θ, so an optimal value of θ can be found which minimizes |∂r(t)/∂t| and hence maximizes the linear sampling density along the space curve in a region of interest (which could e.g. be the ground plane, or a tunnel or retaining wall). This demonstrates the effectiveness of varying θ for maximizing the sampling densities both along and perpendicular to the space curve at the ground plane. When uniform sampling is required over three-hundred and sixty-degrees (such as when finding the surface impedance function of a tunnel interior) θ can be set to zero (shown in FIG. 3A), which will necessitate a reduced vehicle speed. For most rail applications, only the track and adjacent regions need investigation, and so θ can be selected depending on Ω, the lateral size of the region of interest, the sampling density needed, and the vehicle speed.

(29) The example apparatus for collecting data for use in forming a 3D image representative of the subsurface of infrastructure located in the vicinity of a moving vehicle is shown in more detail in FIG. 4A and FIG. 4B.

(30) The apparatus has a mounting plate 17 configured to be mounted to the vehicle, which carries brackets 18 that secure the longitudinal beam 19 and the stabilizing beams 20 and 21 to the plate. Preferably, the beam 19 is long enough to prevent the vehicle itself from blocking the radar boresight when θ is large (although as noted previously, some negative effects of a large θ can be mitigated by varying ϕ to be non-zero). The beams 20 and 21 act to stabilize the longitudinal beam against vertical and lateral motion, respectively. A linear actuator 22 connected to a moment arm 23 controls the value of θ through the pivot point 24. A platform 26 connected to the pivot carries an electric drive motor 28 in a mount 27, as well as associated control electronics 29. A drive axle 32 is attached to a non-rotating axle 30 through a gear assembly 31 which contains a high speed ball bearing for this purpose. This assembly also contains helical gears enabling the electric motor to rotate antenna assembly 33. Optical encoder 25 on the pivot and another optical encoder on the drive axle are configured to transmit data to the control unit which records angular position data corresponding to each trace (a trace=radar data representative of reflected RF energy as a function of depth received by the directional antenna during a collection cycle).

(31) Translational position data is attributed to each trace using tachometer and optionally

(32) GPS inputs. Normally, a tachometer would only give linear information, so position data collected by a GPS device may be used to take curves in a track into account and/or to map subsequently formed 3D image data (see below) to a global coordinate system.

(33) Data about vehicle speed may be used to vary both θ and Ω in real time.

(34) Components 17-33 of the apparatus can be viewed as a base unit configured to be mounted to a vehicle.

(35) An antenna assembly 33 contains the directional antenna, a radar transceiver and an analogue to digital converter. The antenna assembly 33 is mounted to the rail vehicle using with connection point 34 via the drive axle 32, thereby allowing the drive motor 28 to rotate the antenna assembly 33 and therefore the directional antenna contained in the antenna assembly 33. The antenna assembly 33 is connected to a processing unit (not shown) located in the base unit, via single mode fibre optic cable (a fibre optic rotary joint is used).

(36) The antenna assembly 33 is shown in more detail FIG. 5. The transmit and receive TEM horn antennas 14, 15 of the directional antenna are mounted on an inner gimbal 37 which allows angle ϕ to be varied; this gimbal rotates about the axis specified by 36. The truncated spherical shape of the cover of the antenna assembly 33 allows ϕ to vary up to 45 degrees. An outer gimbal 35 is rotated by the drive axle with angular velocity Ω thereby rotating the entire antenna assembly 33 about Ω. The TEM horns are connected to the transceiver 38 using a 50 Ohm coaxial cable 39. A bottom of the assembly cover 40 consists of radio frequency transparent material.

(37) The apparatus thus performs, using the directional antenna whilst it is rotated about the antenna rotation axis Ω, a plurality of collection cycles in which the transmit horn antenna 14 of the directional antenna emits RF energy and the receive horn antenna 15 of the directional antenna receives reflected RF energy.

(38) During each of the plurality of collection cycles performed by the directional antenna, the processing unit located in the base unit collects: (i) radar data representative of reflected RF energy received by the receive horn antenna 15 of the directional antenna during the collection cycle; (ii) angular position data (from the optical encoders) representative of an angular position of the directional antenna about the antenna rotation axis Ω during the collection cycle; and (iii) translational position data (from the tachometer and optionally the GPS device) regarding a translational position of the directional antenna during the collection cycle.

(39) All collected data is stored in a local hard disk included in the base unit. At a later time, a computer is used to processing the radar data, the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle.

(40) Associated with each sampling point on the ground, a reflection profile (known as a trace) exists. This is essentially a one dimensional set of reflection amplitude data (along a line) which the subsurface dielectric permittivity as a function of distance/depth. By knowing the precise linear and angular position of the antenna when a trace was taken, it is possible to determine the equation of that line in space and assign reflection amplitudes along it. By interpolating this data between all such lines in 3D space a 3D map of the subsurface can be obtained. Of course, this is only a very simple outline. Preferably, variations in surface impedance and topography would be compensated for to obtain the best possible imagery, which can be achieved using techniques known in the art.

(41) Accordingly, it would be straightforward for one skilled in the art to convert the data into basic 3D image data. More sophisticated algorithms could be envisaged to obtain a high quality 3D image, e.g. by compensating for variations in surface impedance and topography. Even when obtaining a basic 3D image, the calculations involved are computationally intensive, which is why offline processing is used in this example, though there is no fundamental reason why this processing could not be performed “on the fly” by a computer on board the apparatus.

(42) Although the antenna assembly 33 shown in FIG. 5 only includes a single directional antenna that includes transmit and receive TEM horn antennas 14, 15, the antenna assembly 33 may in some embodiments (not shown) include a further directional antenna that includes further transmit and receive TEM horn antennas. The further directional antenna may be used whilst it is rotated about the antenna rotation axis Ω to perform a plurality of further collection cycles in which the further directional antenna emits RF energy and receives reflected RF energy. As has already been described in detail above, the directional antenna and further directional antenna may be configured to emit RF energies having different frequencies.

(43) Although the example apparatus is described for a rail vehicle, the example mechanism could be used with another ground-based vehicle, e.g. a road vehicle.

(44) It would also be possible to use the mechanism with a water-based vehicle, preferably with compensation for yaw/pitch of the water-base vehicle being provided (either in the processing of the data or to prevent the antenna assembly from being affected by the yaw/pitch).

(45) It would also be possible to use the mechanism with an aerial vehicle, preferably with an inertial measurement unit being provided to compensate for yaw, pitch and roll, and preferably with a GPS device being used to obtain positional data. In the case of an aerial vehicle, there would most likely be altitude constraints beyond which meaningful subsurface data would not be obtained.

(46) In the example apparatus, a pulsed UWB radar technique is employed, meaning that non-dispersive antennas should be used. Although TEM horn antennas are used as the non-dispersive antennas in this example, these TEM horn antennas could be replaced by another type of non-dispersive antenna, e.g. a loaded dipole antenna. If another (non-pulsed) radar technique were used, other types of antenna could be used.

(47) The performance of any subsurface radar is dependent upon the electrical conductivity of the ground. Soils with high conductivity absorb radio frequency energy resulting in the rapid attenuation of electromagnetic radiation. Soils that are capable of holding high amounts of water and have a high cation-exchange capacity (the CEC is the total number of positive ions, or cations, a particular soil can hold) are not suitable for radar investigations. Clay, which is typically considered to be soil comprised of particles of 2 μm or less, is an example of this; the small particle size results in high particle density and the correspondingly large total surface area holds high water amounts compared to other soil types. This above described apparatus may have difficulty imaging subsurface features if clay with high moisture content is predominant, and might not function if the infrastructure under investigation is flooded.

(48) The example apparatus described above could be used for all manner of radar subsurface investigations. Most ground penetrating radar units used for conventional scanning use ground-coupled antennas, whereas the example apparatus described above is an air-coupled system. Ground-coupled systems transfer more energy into the ground because the antenna impedance is matched to that of the ground surface, hence the transmitted impulse propagates through the interface with low reflection. They therefore have better depth penetration than their air-coupled equivalents, however surface impedance can fluctuate rapidly over short distances so in practice it is impossible to always match the antenna with the ground. This mismatch will cause the antenna to ring, which results in obscured features and poor quality images. This problem is particularly difficult to remove through background removal processing, especially for large impedance variations. Air-coupled radar does not suffer from this problem as severely, and so can produce higher quality images at the expense of reduced penetration depth. Air coupled radar also has much higher data collection speeds and can be used more effectively over difficult or dangerous terrain (for example land mine clearance).

(49) The example apparatus described above could reduce the time required to complete a scan because its effective transect width is much larger than current systems.

(50) Fewer transects would be needed to survey a site, yielding both time and cost savings. The areas where this technology could be applied include utility location, forensics, archaeology, construction surveys, sinkhole/void detection, and unexploded ordnance/landmines.

(51) When used in this specification and claims, the terms “comprises” and “comprising”, “including” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the possibility of other features, steps or integers being present.

(52) The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

(53) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(54) For example, the antenna assembly 33 could be changed depending upon investigative requirements, or a different drive system could be used to rotate the antenna assembly (for example, a belt drive system).

(55) For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

(56) All references referred to above are hereby incorporated by reference.