The present invention relates to a detector for attachment to a land vehicle, and in particular to a detector that may be suited to the detection of non-metallic objects buried beneath a ground plane.

It is known, for example from US 5712441 , to have a detector comprising a frame for attachment to a land vehicle. The frame extends beyond the front of the land vehicle and is provided, towards the foremost extremity, with a detection means that is intended to enable the detection of buried objects.

A frame as disclosed in US 5712441 may be adapted for attachment to a wide variety of vehicles. As such the frame allows a wide variety of land vehicles to be selectively given the additional functionality of being able to detect objects.

In the detector disclosed in US 5712441 , the detection means utilises an electromagnetic coil for detecting objects that conduct electricity and as such is capable of detecting buried objects which are metallic, or capable of conducting electricity in a comparable manner.

US 5712441 is intended to be used in detecting land mines and, provided that the land mines it seeks are metallic, can be capable of doing so.

However, as modern munitions become ever more sophisticated, it may no longer be safe to assume that all threats comprise an amount of metal sufficient to render the threat detectable to a detection means utilising an electromagnetic coil.

Accordingly, the present invention provides for a detector for attachment to a land vehicle, the detector comprising: an axle provided with at least one wheel for contacting the ground, a rotational axis defined by the axle, and a ground penetrating radar system, wherein the ground penetrating radar system comprises a first antenna unit mounted at the axle and; a second antenna unit mounted at the axle, wherein, as the at least one wheel rotates, the first antenna unit is periodically directed towards the ground plane so as to be able to transmit and receive signals into the ground, and wherein as the at least one wheel rotates, the second antenna unit is periodically directed towards the ground plane so as to be able to transmit signals into and receive signals from the ground.

The detector therefore allows the ground to be scanned as the vehicle moves forwards. When the detector is mounted at the front of the vehicle and the vehicle drives forwards, the detector can sense what is in the ground before the ground is driven over by the land vehicle. This is particularly useful where the land vehicle is driving over a minefield or driving in proximity to other concealed explosive devices. However, the present invention is also able to recognise underground tunnels and other forms of soil disruption.

There may be N antenna units at a first site along the axle, the N units thereby defining a first antenna unit set, the N antenna units being equally spaced about the rotational axis, where N is any positive integer.

As such, the sampling frequency, per rotation of the wheel, of the detector can be increased.

There may be M antenna units at a second site along the axle, the M antenna units thereby defining a second antenna unit set, the M antenna units being equally spaced about the rotational axis, where M is any positive integer.

In some embodiments N is equal to M, in which embodiments the N antenna units and the M antenna units may be aligned, thereby grouping inter- set antenna units into pairs according to angular orientation about the rotational axis.

A set of antenna units at more than one site enables a longer strip of ground to have signals transmitted into and received therefrom. Thus a larger area of ground can be scanned per rotation of the wheel and a given area can be swept in a lesser time.

Alternatively at least one antenna unit in the first antenna unit set is at an angular orientation about the rotational axis, which angular orientation is distinct from any of the angular orientations of the antenna units about the rotational axis in the second antenna unit set. Such an arrangement, and in particular an arrangement of this type where each antenna unit in any given set has a unique angular orientation compared to the antenna units in proximate sets, tends to simplify the implementation of isolation within the detector. Such simplification arises when the separation between commonly oriented (and therefore apt for simultaneously energisation) antenna units is sufficient to render interference effects between the units as negligible.

The first antenna unit may comprise a first transmitter element and a first receiver element, and the second antenna unit may comprise a second transmitter element and a second receiver element.

Using an antenna such as this, a bistatic radar antenna, the system does away with the requirement for a transmitter/receiver switch. As such the radar may have a high pulse repetition frequency and so can detect objects at a very close range.

It may be that each of the first and second transmitter elements is a frequency-independent antenna element and each of the first and second receiver elements is a frequency independent antenna element.

As such, the antenna elements are suitable for use in a radar system that transmits a wideband pulse. Such radar systems can alternatively be referred to as carrier-free radar systems. A wideband pulse gives good range resolution and further, will tend to identify a broader variety of targets as the various component frequencies in the wideband pulse set up resonances in variously sized discontinuities in the volume. A discontinuity in the volume may be a crack in a dry patch of earth or it may be a cavity in a land mine.

Each of the first and second antenna units may be arranged at the axle such that the transmitter element and receiver element therein lie on an element axis that is generally coplanar with and parallel to the rotational axis.

At each of the first and second antenna units, the transmitter element and the receiver element may be mounted at a distance from the axle that is approximately equal to the radius of the wheel and, as such, the transmitter element and the receiver element are arranged to generally contact the ground when the antenna unit is directed towards the ground plane.

Alternatively each of the antenna elements of the first and second antenna units may further comprise a resilient mount, the resilient mount being biased to urge the element to protrude beyond the circumference of the wheel and the resilient mount allowing the element to retract if the weight of the bogie is applied.

This should tend to reduce the amount of clutter that the radar system has to filter out. Specifically, by having the antennae in close proximity to the ground, the amount of surface clutter received at the antenna will tend to be reduced. This can free up the radar processing resources to enable faster detections.

Further, the proximity of the antenna to the ground will tend to set up reactive coupling and so focus the beam into the ground, helping to accurately determine the location of objects.

The ground penetrating radar system may further comprise: a signal processing unit; a conduit operably connecting the signal processing unit to the antenna units; a data processing unit, operably connected to the signal processing unit; and a display, operably connected to the data processing unit, such that the display can be substantially displaced from the transmitter and receiver units.

In particular, the display may be displaced from the axle by approximately 6 to 14 metres or, in where the bogie is mounted at a robotic or autonomous vehicle, may be remote from the vehicle but interfaced with the bogie by a wireless communication protocol.

According to a second aspect of the invention, there is provided a method for scanning a section of ground using a detector according to any one of the preceding claims, the method comprising: directing the first antenna unit towards the ground; illuminating a first portion of ground using the first antenna unit; processing the signals returned to the first antenna unit and displaying them; simultaneously advancing and rotating the axle, thereby directing the second antenna unit towards the ground; illuminating a second portion of the ground that is advanced from the first portion of ground, and processing the signals returned to the second antenna unit and displaying an image representing the section of ground.

According to a third aspect of the invention there is provided an antenna unit comprising: a first antenna element for transmitting a wideband radar pulse; a housing for the first antenna element; a second antenna element for receiving returns from the wideband radar pulse; a housing for the second antenna element, wherein each antenna element is disposed within its housing and slideable so as to occupy a protruding position and a sheathed position, and wherein each antenna element is resiliently mounted and biased to protrude from the housing associated with that element.

So that the invention may be clearly understood, at least one exemplary embodiment shall now be described with reference to the following figures of which:

Figure 1 shows a schematic representation of a detector according to the invention, the detector comprising a bogie;

Figure 2 shows the cross section along line A— A from figure 1 and shows the bogie in more detail;

Figure 3 shows a front-on view of the bogie resting on a ground plane; and

Figure 4 shows a ground-up view of the bogie.

A detector 100, for attachment to an armoured vehicle 400, and as shown in figure 1 , comprises a bogie 300 that is connected to the armoured vehicle 400 at a frame member 340.

The bogie 300 comprises a first disc 351 , a second disc 352, and four further discs 353, 354, 355 and 356. The first disc 351 is mounted on the bogie 300 in a sidemost position. The second disc 352 is adjacent the first disc 351 .

Each disc has generally the same diameter as the others and is mounted to an axle and thus rotates about a rotational axis 370 defined by the axle.

As shown in figure 2, axle sections 360, which define the axle axis, or rotational axis 370, extend between adjacent discs to connect and separate the discs. For example, an axle section 360 is interposed between the first disc 351 and the second disc 352.

Further interposed between the discs are antenna units. Each antenna unit is generally flat and generally U-shaped, that is to say the form can be thought of as comprising a tube that has been bent through 90° in the same plane at two locations so that it defines three sides of a rectangle.

For convenience, the two generally parallel aspects of a U-shaped antenna unit shall be referred to as the first and second vertical aspects respectively and the interconnecting aspect of the U-shape shall be referred to as the horizontal aspect. The reader will understand that a strictly literal interpretation of these terms is not appropriate. (Though, coincidentally, the horizontal aspect of the antenna will tend to be in a generally horizontal orientation when in use on a horizontal ground plane).

Each antenna unit is arranged at the bogie 300 so that the horizontal aspect of the particular U-shaped antenna unit is attached to the respective axle section 360 and as such lies generally along the rotational axis 370. Thus each of the vertical aspects of each antenna unit extends perpendicular to the common axle axis 370 and is attached to a disc.

For example, a first antenna unit 310 has a first vertical aspect 31 1 arranged to extend along the first disc 351 , a second vertical aspect 313 arranged to extend along the second disc 352, and a horizontal aspect 319 of unit 310 extending along axle section 360.

A first transmitter antenna element 312 is disposed at the end of the first vertical aspect 31 1 of the antenna unit 310. A first receiver antenna element 314 is disposed at the end of the second vertical aspect 313 of the antenna unit 310. Each of the vertical aspects, 31 1 and 313, has the form of a sleeve at its tip and thereby defines a housing which may accommodate the respective elements 312 and 314 in a protruding or sheathed position. The receiver antenna element 314 and the transmitter antenna element 312 are supported within the antenna unit 310 on resilient mounts 315, 317 respectively which are biased so that the antenna elements are urged to protrude out of their housing and beyond the disc radius. However, the resilience of each mount (e.g. 315) is such that as the associated antenna element (e.g. 312) contacts the ground, the antenna element (312) retracts into its housing (e.g. 31 1 ) due to the weight of the bogie 300. Thus the discs take the majority of the bogie 300 weight whilst the resilient mounts 315, 317 tend to ensure that the respective antenna 312, 314 are held in contact with the ground.

A first antenna unit set 380, comprises the first antenna unit 310 and four further antenna units, including a second antenna unit 320, disposed between discs 351 and 352. The horizontal aspect of each antenna unit is generally disposed along the rotational axis 370 in an equivalent manner to the horizontal aspect 319 of the antenna unit 310. This first set of antenna units 380 occupies a first site along the length of the axis 370.

At the first antenna unit set 380 the antenna units each have a unique angular orientation, that is to say unique within that set, and are equally spaced about the common axle axis 370. Thus, as there are five antenna units, the antenna units are angularly displaced from either neighbour by approximately 72° about the axis 370.

To maintain the simplicity, and therefore the clarity, of Figure 1 , a full set of interposed antenna units are shown only at the site between discs 351 and 352. A third antenna unit 330 of a further antenna unit set is also shown. However, in the present embodiment, three further antenna unit sets (each set having five units) are mounted on the bogie 300 in addition to the first set. A second set is mounted at a second site along the axis 370 between discs 352 and 353, a third set between discs 354 and 355, and a fourth set between discs 355 and 356. These further sets of antenna units also have their antenna units equally spaced around the axis 370.

Further, antenna units are grouped according to common angular orientation about the axis 370. For example a first group 390, as bounded by the bracket in Fig 2, composes units 310 and 330.

Referring back to figure 1 , it is clear from the relative positions of the first antenna unit 310 from the first set and the third antenna unit 330 from the third set, both of which antenna units point vertically upwards at the same time, each set of antenna units is arranged in phase with the other sets.

The frame member 340 is structurally connected at a first, frontmost end to the bogie 300. As can be seen from figures 3 and 4, struts 344 are rigidly fixed to the first end of the frame member 340 and extend upwards from the frame member 340. The struts 344 are rigidly connected at their uppermost ends to a support beam 342. Support beam 342 extends generally along the length of the axle (as comprised by the axle sections 360) and is generally parallel with the axle. A plurality of bearing struts 346 are rigidly connected to the support beam 342 at their uppermost ends and each is connected to the axle portion 360 at their lowermost ends by means of a bearing 343. In particular, a stator portion of a bearing 343 is fixedly attached to a respective bearing strut 346 and the rotor portion of that bearing 343 is attached to a respective axle portion 360. Each bearing 343 is arranged with the axis of rotation collinear with the axle in order to allow rotation about the axis 370.

The interconnected arrangement of the struts 344, the support beam 342 and the bearing struts 346, all of which are generally rigid, establishes a major load path between the bogie 300 and the frame member 340.

A second, backmost end of the frame member 340 is structurally connected to the vehicle 400. The fixture (not shown) between the frame member 340 and the vehicle 400 is flexible so that the worst likely rotation of the frame member 340 relative to the vehicle 400 in all three axes can be tolerated. The detector 100 further comprises a ground penetrating radar processor unit 200 arranged on board the vehicle 400.

The processor unit 200 comprises a display 250 that is operably connected to a data processor 240. The data processor 240 is in turn operably connected to a signal processor 230. The signal processor 230 is further operably connected to a receiver unit 220. The processor unit 200 further comprises a transmitter unit 210.

The transmitter unit 210 is connected to four electrical conduits (one conduit per antenna unit set) such as coaxial cables, which run the length of the frame member 340. Two of these conduits connect to a first rotary connector 345, and two of these conduits connect to a second rotary connector 357.

For example, and as shown in figures 1 , 3 and 4, a first electrical conduit 341 a connects the transmitter unit 210 to the rotary connector 345. Further conduits extending between the transmitter unit 210 and the rotary connector 345 are omitted from the figures for sake of clarity.

The first rotary connector 345 is further connected by electrical conduits to each one of the transmitter antenna elements in each of the third and fourth antenna unit sets. For example, the rotary connector 345 is connected to the transmitter antenna element 332 of antenna unit 330 by the electrical conduit 341 c.

The second rotary connector 347 is further connected to each one of the transmitter antenna elements in each of the first and second antenna unit sets by further electrical conduits (not shown).

Each rotary connector 345, 347 is configured such that only one transmitter antenna element (e.g. 332) per antenna unit set may be operably connected to the transmitter unit 210 at a time. However, the connection can commute from one transmitter antenna element to the following transmitter element in the same set. In particular, the connection is established between the transmitter antenna element that is directed perpendicularly into the ground and for approximately 5 to 10° either side (the energisation window). ln an equivalent manner, the receiver unit 220 is connected to four further electrical conduits, such as coaxial cables, which run the length of the frame member 340. Two of these conduits connect to the first rotary connector 345, and two of these conduits connect to the second rotary connector 357.

For example, and as shown in figures 1 , 3 and 4, a second electrical conduit 341 b connects the receiver unit 220 to the rotary connector 345. Further conduits extending between the receiver unit 220 and the rotary connector are omitted from the figures for sake of clarity.

The rotary connector 345 is further connected, by electrical conduits, to each one of the receiver antenna elements in each of the third and fourth antenna unit sets. For example, rotary connector 345 is connected to receiver antenna element 334 by electrical conduit 341 d.

Each rotary connector 345, 347 is configured such that only one receiver antenna element (e.g. 334) per antenna unit set may be operably connected to the receiver unit 220 at a time. However, the connection can commute from one receiver antenna element to another in the same set. In particular, the connection is established between the receiver antenna element that is directed perpendicularly into the ground and for approximately 10-35° either side, and connects to another antenna element if that antenna element becomes directed perpendicularly into the ground or is 10-30° either side.

Given the selectively applicable connections described above, it can be appreciated that the transmit antenna element and the receiver antenna element of a given antenna unit will become operably connected to the processor unit 200 at the same time, which is the time when the antenna elements are directed towards the ground.

In operation, the bogie 300 is arranged to rest on the ground plane P ahead of the vehicle 400 so that the discs 351 , 352, 353, 354, 355, and 356 contact the ground plane P.

As the vehicle 400 advances, the axle 360 rotates at the bearings 343 about the axis 370. Such axle rotation periodically positions a group of antenna units so that the group is directed towards the ground, generally perpendicular to the ground plane P.

In the position shown in figure 1 and with the vehicle advancing, the first group of antenna units to be directed towards the ground will be the group including antenna unit 320.

As a group of antenna units approaches a position where the group will be directed towards the ground, electrical connections between the processing unit 200 and that group of antenna units are established. Further, because each transmitter antenna and receiver antenna is spring loaded so as to encourage contact with the ground, as the group approaches the position, the antennae will tend to retract and the discs 351 , 352, 353, 354, 355, and 356 will tend to take the majority of the weight by the bogie 300.

The discs act as wheels insofar as they take the majority of the weight of the bogie and promote rotation about the axis 370 as the bogie advances.

With the electrical connections established and the group of antenna units momentarily directed towards the ground (as shown in Fig 344 for the group 390) the transmitter 210 generates an appropriate radar signal (such as a wideband pulse) that is emitted by the transmitter antennae. In the brief moment in which the antenna unit is directed towards the ground, the pulse thereby illuminates the ground Q beneath the bogie 300 and is returned to the receiver antennae.

Signals from the receiver antenna are relayed to the receiver unit 220 before that group of antenna units are disconnected. The signals are then processed at the processor unit 200, for example, to remove clutter and so as to present a map of the ground at the display. An operator can monitor the display 250 to identify buried objects and respond accordingly. This processing is done according to known practices.

As the vehicle 400 continues to advance, the next group of antenna units will become operably connected to the processor unit 200, brought into contact with the ground, energised so as to illuminate the ground Q directly beneath the bogie 300 and relay received signals back to the receiver unit 220, and lastly disconnected from the processor unit 200.

The frame member 340 should be of sufficient length so that when the vehicle 400 is advancing at a reasonable speed, the processor unit 200 has time to analyze the returns. In the described embodiment, the frame member 340 extends approximately 10m, or other safe distance, in front of the vehicle, taking into account the available protection.

As an alternative or in addition to the display 250, the processor unit 200 may comprise a detection processor for analysing the returns so that the processor can recognise certain objects. Further, this processor may be interfaced with an I/O device so that if a certain identified object (e.g. an explosive device) merits a certain response (e.g. stopping the vehicle), this response can be effected.

Each antenna should be isolated from other antennae to prevent interference. In the described embodiment the antenna units comprise vertical aspects of approximately 1 m length and horizontal aspects of approximately 1 m length. By such separation, intra-antenna-unit antennae are isolated from each other. To accommodate such antenna units arranged in sets of five, the discs 351 , 352, 353, 354, 355, and 356 should have a radius of approximately 1 m.

Inter-antenna-unit isolation is achieved in the described embodiment by providing that the discs 351 , 352, 353, 354, 355, and 356 and the axle sections 360 comprise a layer of suitable dielectric material as would be well known in the art.

In the embodiment presented in figures 1 to 4 and described above, the electrical connection scheme comprises firstly having a single conduit (e.g. 341 a) per antenna set extending between the transmitter 210 and the rotary connector (345 or 347) and secondly having a single conduit per transmit antenna extending between the antenna and the rotary connector.

As a first alternative to this electrical connection scheme, the detector 100 may be provided with a single conduit per transmitter antenna element, each conduit extending between the transmitter 210 and the transmitter antenna via a suitable rotary connector. Implementing such a scheme in the embodiment of Figure 1 would require a total of 20 electrical conduits connected to the transmitter 210. Further, since this scheme would permanently establish an operable connection between the transmitter 210 and each of the transmitter antennas, the transmitter 210 would need to be able to identify which of the antenna units were facing the ground prior to outputting a radar pulse to a limited group of conduits. Accordingly in such an embodiment a rotary encoder would be provided at the bearings 343, operably connected to the transmitter 210 to identify which antenna units were facing the ground. A potential benefit of this alternative electrical connection scheme is that a simpler and potentially more robust connector (for example a mercury-wetted slip ring) could be provided as the rotary connector 345 and 347.

In an equivalent manner to the first alternative electrical connection scheme for the transmitter antennae, the electrical connection scheme for the receiver antennae, which in the above embodiment is a single conduit per antenna set extending between the receiver 220 and the rotary connector (345 or 347), may alternatively be embodied with a scheme providing a single conduit per receive antenna.

In a further variation on the embodiment presented in figures 1 to 4 and described above, an alternative bogie is also provided for by the invention, which has only one antenna unit disposed between each pair of discs (i.e. there is one antenna unit per set). Each of these antenna units generally occupy a common plane (i.e. they are in phase) and thus define a single group of antenna units. The dimensions of the antenna units are the same as in the bogie 300 and so to ensure the same spatial interval between scans, the diameter of the discs of bogie 302 is approximately 320mm. Such an embodiment should tend to simplify the electrical connections required.

The inter-set phase scheme implemented in the alternative bogie and the bogie embodied in the figures provides for sets of antenna units arranged in groups so that antenna units from different antenna unit sets occupy a common plane and so may be directed towards the ground, and hence energised, at the same time. A first alternative to this inter-set grouping scheme is to offset successive antenna unit sets about the axis 370 by 5-30° (any offset is to be coordinated with the energisation window so as to prevent detrimental overlaps). Thus each antenna unit has a unique angular orientation about the axis 370 as compared to its antenna unit set and its neighbouring antenna unit sets.

By further providing a suitably adapted electrical connection scheme it would be possible to reduce the number of antenna units energised at any given time. Reducing the units energised at any one time should tend to mitigate any interference between antenna units, provided that those units energised at the same time are arranged to be sufficiently separated.

Further alternatives would be obvious to the skilled reader. Moreover, it is to be understood that, whilst a plurality of embodiments have been disclosed above, each having various features, the skilled reader would readily identify without requiring invention, which features from one embodiment may be inserted into another embodiment in addition to or instead of an equivalent feature.