METHODS AND APPARATUS FOR OBTAINING POSITIONAL INFORMATION

Abstract: Apparatus for and methods of obtaining positional information about one or more objects in a detection field are disclosed. An array including a transmitting element and a plurality of receiving elements is provided. In one aspect a truncated cross-correlation function is applied to determine the interval between signals received by a plurality of the receiving elements, thereby to determine an angular position of an object. In another aspect a warning zone is defined and it is determined whether an object is within the warning zone. Also disclosed are techniques for stretching received signals, and techniques for obtaining positional information relating to an object using non-Doppler radar. Various implementations, modifications and applications of the techniques described are disclosed. Typical applications of the techniques described are with vehicles.

Description: METHODS AND APPARATUS FOR OBTAINING POSITIONAL INFORMATION The present invention relates to techniques for obtaining positional information about one or more objects More specifically, the invention relates to methods of and apparatus for determining distance and angular positional information of an object or a plurality of objects with respect to a datum Many possible applications of this invention are envisaged One specific field of application is withm the automotive industry where a system embodying the invention might be deployed on a vehicle with a field of detection withm or without the vehicle or both. In general, a reference to a vehicle in this specification may have particular application to a land vehicle, or, more particularly, a motor land vehicle such as a powered road vehicle The vehicle would usually be wheeled or tracked and/or be incapable of flight. For example, there exists a known problem with safety air bags that they can cause serious injuries if deployed while a person is sitting too close to the bag Moreover, if the vehicle were not fully occupied, it would be desirable to deploy only those bags needed to protect the occupants This would minimise the pressure pulse that is generated withm the vehicle upon deployment and also reduce the considerable cost of replacement of the air bags if the vehicle is subsequently repaired. A system embodying the invention might be employed to determine the number and position of the occupants in a vehicle and control the deployment of air bags accordingly; such a system could apply to any or all of the air bags, whether side or front air bags This information could further be used to send an automatic notification to the emergency services in the event of an accident. Use of the system withm a vehicle might be extended to act as an intruder detection system, to monitor the contents of a load bay or boot, or even to detect movements of the dnver characteπstic of an onset of drowsiness A system embodying the invention could also be employed to determine the relationship between a vehicle and surrounding obj ects. For example, a system could provide a dnver with a manoeuvring aid to avoid contact with objects or people duπng reversing or parking manoeuvres, and to measure a space to determine whether the vehicle can fit into it. An indication might be provided to warn a dnver of the presence of a vehicle m a mirror blind spot to assist the dnver when changing lane or merging into traffic. Furthermore, a warning could be issued or the brakes applied to prevent the vehicle from hitting the one in front m slow-moving or congested traffic. Of course, such applications are not limited to use on passenger vehicles. They might equally be applied to commercial vehicles or earth-moving equipment. Such a system can provide an operator of a vehicle with a complete assessment of the situation existing in the vicinity of the vehicle. A system embodying the invention might also have aeronautical applications, such as collision warning for aircraft, or as a landing aid, particularly for helicopters. Further applications of a system embodying the invention might include position monitoring for articles m a handling system such as a production line or articles in a post handling system; intruder detection in a building or an open space as part of a secunty system; liquid level detection; imaging the intenor of a room from outside its walls and gathenng information through walls generally, detecting the presence of articles withm structures such as walls; amongst many other possibilities. As a further example, there is often a need to determine the location of objects, such as cables, pipes or reinforcement rods withm walls It is often essential to locate such objects with accuracy pnor to carrying out operations on the wall, such as dnlhng or demolition. A system embodying this invention can be use to provide an accurate indication of the location of such objects A system embodying the invention may also provide data for further processing for classifying, tracking and measuπng moving objects US-A-5829782 discloses a system for monitonng occupancy of a vehicle The disclosed system operates by emitting signals within the vehicle and, by means of suitable detectors, monitonng reflections of these waves The reflected signals are analysed algonthmically using a vanety of pattern-recognition techniques to look for the presence of characteπstic qualitative and/or quantitative features. The results of the analysis are compared with results obtained under controlled conditions in order to draw an inference about the occupancy of the vehicle. Since the system operates by companng a signal with previously obtained patterns, it does not enable accurate location of arbitrary objects withm its detection field WO98/00729, proceeding in the name of the present patent applicant, discloses a radar system, particularly for use on a vehicle, to provide a dnver with a warning of potential obstructions in the vicinity of the vehicle. The system establishes a plurality of static, scanning and tracking range gates at which objects can be detected. The disclosure of WO98/00729 is hereby incorporated into this specification by reference. WO97/14058, proceeding m the name of the present patent applicant, discloses a system m which a signal is transmitted into a detection region, and signals reflected from an object m that region are detected at a plurality of locations By monitonng these reflections as the object passes through a plurality of range gates, the trajectory of the object can be established While this system is highly effective for its intended purpose, it has been found pursuant to the present invention that it is not ideally suited to detection of objects that are stationary withm the detection region. This can limit the effectiveness of such apparatus in applications of the type descnbed above. The disclosure of WO97/14058 is hereby incorporated into this specification by reference. Angular resolution by cross-correlation From a first aspect, the invention provides apparatus for obtaining positional information relating to an object, the apparatus compnsing: an (optionally fixed) array compnsing a transmitting element and a plurality of receiving elements; a signal generating stage for applying a senes of pulses to the transmitting element to cause it to transmit a signal, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; a detection stage for detecting signals reflected to the receiving elements and for generating output signals representative of the received signals; and a processmg stage operable by application of a truncated cross-correlation function to detect, measure or determine the interval between signals received by a plurality of the receiving elements, whereby to determine an angular position of an object from which the transmitted signal has been reflected. Application of a truncated cross-correlation function to signals received by the array may provide a coπelation result that is less prone to error than is the case for conventional correlation functions. Moreover, the correlation can be achieved more quickly than is the case for conventional correlation functions. By carrying out the cross- correlation of the received signals in the time domain, the value of the interval is obtained directly. Operation of a system according to this aspect of the invention can be contrasted with the operation of a phase companson monopulse radar system. In such a system, a relationship between phase and frequency of received signals is first obtained. In many cases, this is done after mixing with a local oscillator. A delay is then calculated from the relationship between phase and frequency. However, it is recognised that such systems suffer from intrusion of phase noise that is inherent m many systems. Moreover, to preserve the unambiguous phase relationship, it is necessary for two receiving antennas to be placed closer together than the wavelength of the signal with the result that good angular resolution is possible with such systems only by providing several such spaced-apart antennas Conventional cross-correlation between two signals includes the step of summing the products of the two signals over a penod (the interval of correlation) corresponding to the duration of the waveform contained m the signals This step is earned out for a senes of cases between which one of the signals is shifted in time with respect to the other over a range which, m conventional cross-correlation, is also comparable with the duration of the waveform. In the first aspect of the invention, a truncated cross-correlation function is applied to the signals received by two or more of the receiving elements By truncated cross-correlation it is preferably meant that the range over which one of the signals is shifted with respect to the other is less than that m conventional cross-correlation Thus the truncated cross-correlation function is preferably operable to shift one output signal with respect to another over a range which is less than the duration of the signals and preferably less than the duration of a pulse. The maximum time interval between receipt of the same signal at two receiving elements is equal to the time taken for the signal to propagate directly between them, which is equal to the distance between the elements divided by the speed of propagation of the transmitted signal. Therefore, it may be appropnate to limit the extent of cross-correlation to this time value, or to some related value Thus the truncated cross-correlation function is preferably operable to shift one output signal with respect to another over a range m which the maximum offset in either direction is less than 5 times the time that would be taken for the transmitted signal to travel directly from one receiving element to another, and preferably less than or equal to 3, 2 or 1 times this value In this way, the amount by which a signal received by one of the receiving elements is shifted with respect to a signal received by another receiving element can be limited to a value corresponding, for example, to the maximum anticipated delay between the two. This may keep to a minimum the likelihood of false correlation peaks bemg generated and reduce the amount of processmg required. In certain circumstances it may be appropnate to limit the maximum offset to less than the time taken for the transmitted signal to travel directly from one receivmg element to another. Such a situation may aπse, for example, if the array has a field of view of less than 180°, in which case the maximum offsets may be reduced accordingly. The maximum positive offset and the maximum negative offset need not be equal, and indeed they may be different, for example if the field of view is not symmetrical. In a typical embodiment, the signal has a charactenstic wavelength λ and the interval of correlation (that is the range of signals used m the cross-correlation) is a small multiple of λ. For example, the interval of correlation may be less than 10, 5, or 2 λ , or even lλ. As a minimum, the cross-correlation function is applied over two samples, although in many embodiments a greater number of samples is used The interval of correlation may also (or alternatively) be related to the receiving elements. Specifically, where the receiving elements are spaced apart by a distance D (that is, the distance between adj acent receiving elements is D; however, the distance may also vary such that the D from a first receiving element to a second may be different than the D between either the first or second receiving element and any other receivmg element), the truncated cross-correlation function may have an interval that is less than a small multiple of D. For example, the interval of correlation may be less than 5, 2, 1.5 or ID, or, where the detection field of at least one element is less than 180 degrees, it may be 0.9D, 0 8D or 0.7D. This is a relevant consideration because the maximum difference in the length of the flight between the same signal received by two receiving elements is the spacing between those elements Typically the processing stage is operative to identify a maximum value of the cross correlation function The maximum identifies the portions of greatest similanty between the two signals, and, from this, the interval between receipt of the two signals can be deduced Pulses preferably occupy a constant band of frequencies and are preferably relatively short The first aspect of the invention also provides apparatus for obtaining positional information relating to an object, optionally including any of the above mentioned features, the apparatus compnsing transmitting means for transmitting a signal for at least partial reflection by the object, a plurality of receiving means for receiving a signal reflected by the object; and processing means for applying a truncated cross-correlation function to signals received by a plurality of the receiving means, thereby to determine a position of the object In the first aspect there may also be provided, optionally in dependence of any apparatus descnbed above, apparatus for obtaining positional information relating to an object in which the or a processing stage is operable to detect the interval between a signal being received by a first set of any two or more of the receiving elements and to determine a first angular position of an object from which the transmitted signal has been reflected; and to determine the interval between a signal being received by a second set (which may include one or more of the elements of the first set) of any two or more of the receiving elements and to determine a second angular position of an object from which the transmitted signal has been reflected; the processing stage being preferably operable by application of a truncated cross-correlation function. In preferred embodiments, the first and second angular positions are measured m planes that are substantially not parallel to one another. These angular positions can be used to provide two angular values of 3 -dimensional polar co-ordinates of the object Most favourable resolution of both angles can be achieved if the said planes are approximately normal to one another. In apparatus embodying the invention, the processing stage may be operative to determine the distance from the array of an object from which a signal has been reflected. This can provide the third measurement required to obtain the co-ordinates of an object in three-dimensional space. Thus, the three-dimensional position of an object may be determined with precision by measunng its angular position within what might be a relatively broad antenna beam, rather than using a larger antenna or a higher frequency to provide a precisely-tailored, narrow beam. This may permit the use of devices that have a smaller frequency/aperture ratio than has hitherto been possible The processing stage may be operable to determine a coordinate in three-dimensional space of an object from which the transmitted signal has been reflected, for example by using the angles determined as descnbed in the last two preceding paragraphs In a favoured configuration, at least one of the first and second set of receiving elements includes three or more elements disposed such that that set includes at least two pairs of elements, the spacing of elements m the two pairs being unequal This allows the same angular information to be obtained from two different pairs of receiving elements, to permit compensation for artefacts of the array, for example, grating lobes. For example, the spacing (D) between the elements of one pair may be approximately equal to (for example between 50% and 200% or between 75 and 150% of) a charactenstic wavelength λ of the signal, the spacing between a second pair of elements may be approximately equal to 3λ/4, or 3 D/4, and the ratio of the spacing of the elements in one of the first and second pairs to the spacing of the elements in the other of the first and second pairs may be between 0.5 and 1 or 0.75 and 0.9 Preferably the relative spacing is arranged so that grating lobes which anse in the pairs line up differently, that is, a positive grating lobe for one pair substantially lines up with a negative or zero grating lobe for the other In embodiments according to the last-precedmg paragraph, the processing stage may be operative to perform a truncated cross-conelation between the signals received by each pair of elements and to calculate the product of, or otherwise compare, the results of the cross-correlations This can allow mutual calculation of errors or other artefacts in the received signals. Apparatus embodying the invention may further compnse an output stage operative to generate an output for presentation of positional information relating to the object to a user. For example, the output may include at least one of an audible and a visual signal. Thus apparatus embodying the invention can be used to inform a user directly of the presence of an object in its field of detection. Warning zone From a second aspect (which may optionally be provided in combination with the first aspect), the invention provides apparatus for obtaining positional information relating to an object, the apparatus compnsing: a warning zone definition stage for defining a warning zone (m two or three dimensions) withm a detection field of the apparatus; and a discnmination stage for determining whether a detected object is within the warning zone; in which the warning zone is preferably defined as a three-dimensional region withm the detection field. This aspect of the invention allows a warning zone to be defined that is largely independent of the shape of the beam generated by the transmitting element, that is, the antenna design can be decoupled from the zone definition. This can allow the system to operate at frequencies at which the beam formed by the antenna alone would be too wide, thereby allowing comparatively low frequency (and therefore low cost) apparatus to be used, and to use antennas which are smaller than would otherwise be applicable, and also less costly. Preferably the apparatus further compnses an object location stage for determining the position of a detected object within the detection field of the apparatus. The discnmination stage may then be operable to compare the coordinates of the detected object to the coordinates of the warning zone to determine whether the object is withm the warning zone. As will be seen, this arrangement acts to disconnect the function of generating a warning from detection of an object. Typically, the warning zone may be contained withm and may be smaller than the detection field of the apparatus. More specifically, the shape of the warning zone may be dissimilar from the shape of the detection field of the apparatus and/or may be non-circular or non-sphencal. The shape of the warning zone may therefore be tailored to the needs of a particular application, largely independently of the shape of the detection field. For example, the warning zone may include a region (such as a planar surface) defined in two dimensions withm the detection field. The warning zone definition stage may include an algonthm that defines a warning zone as a function of a coordinate withm the detection field Such a region may have an essentially arbitrary shape as defined by the algonthm Alternatively or additionally, the warning zone definition stage may define at least a limiting value of one or more ordinates of a coordinate within the detection field Each such limit effectively defines a cut-off of the warning zone m a particular direction For example, the warning zone definition stage may define at least a limiting value of one or more angles of a polar coordinate within the detection field Alternatively or additionally, the warning zone definition stage may define at least a limiting value of a range of a polar coordinate within the detection field Such is the essentially arbitrary shape of a warning zone, that it may include a plurality of discontinuous spatial regions Apparatus embodying this aspect of the invention may further compnse an object location stage for determining the position of a detected object within the detection field of the apparatus More specifically (or alternatively) the discnmination stage may include a coordinate generating stage that generates a coordmate of a detected ob j ect, which coordinate is then compared with the warning zone. In such embodiments, the discnmination stage is typically operable to determine the coordinates of the detected object and compare the determined coordinates with the coordinates of the warning zone to determine whether the obj ect is withm the warning zone In embodiments of this aspect of the invention, the discnmination stage may be operative to generate an output signal indicative that the object is within the warning zone Apparatus embodying this aspect of the invention may be further operative to issue a warning, for example at least one of an audible, a visual or a tactile warning to a user upon detection of an object in the warning zone, thereby providing an immediate warning to a user. As a development of this aspect of the invention, the warning zone definition stage defines a plurality of warning zones. The warning zones may be non-coextensive (overlapping, separated or spatially different) and/or alternatively defined, by which it is meant that different charactenstics are used for determining whether an object is in the relevant warning zones. Thus the discnmination stage may be operable to apply different logic to at least two of the zones For example, different zones may be provided for detecting different speeds or different sizes of objects. This can, for example, be used to provide warnings of multiple levels of seventy, depending upon the position or other charactenstics of a detected object Apparatus according to the last-preceding paragraph may be operative to generate an output signal indicative of which of the plurality of warning zones contains the object. For example, it may be further operative to issue at least one of an audible and a visual warning to a user upon detection of an object in a warning zone. The warning zone may be limited in range and/or may be approximately cuboid In another development of this aspect of the apparatus, the discnmination stage is operative to analyse charactenstics of objects outside of the warning zone Such charactenstics may be, for example, size of the object, distance of the object from the apparatus and/or the warning zone, direction of movement of the object relative to the apparatus and/or the warning zone, and relative speed of the object. As an example, the discnmination stage may be operative to track objects outside the warning zone and to predict their entry into the warning zone The apparatus may be operative to issue a pre-warnmg based on the analysis. For example, if the apparatus is mounted on a vehicle (as descnbed below) in order to provide the dnver with a parking aid, the apparatus may issue a pre-warnmg in the way descnbed above if a large object is convergmg with the vehicle, even though that object may be outside of the warning zone This may be particularly desirable, for example, if the object itself is heading for the vehicle m the same direction that the vehicle is heading for the object, which may give an increased nsk of collision Apparatus embodying this or any other aspect of the invention may be suitable for use on a vehicle and may optionally include the vehicle In such apparatus, at least one of the shape and a relevant dimension of a warning zone may at least in part be determined in dependence on a coπesponding shape and dimension of the vehicle For example, a warning zone may have a width that is equal to the width of the vehicle plus a predetermined amount (preferably which is less than the width of the vehicle) in order to assist an operator of the vehicle when navigating naπow openings A warning zone may have a height that is equal to the height of the vehicle (including any accessones, such as a radio antenna) plus a predetermined amount (preferably which is less than the height of the vehicle), for example to allow for error and for suspension displacements. A warning zone may have a lower surface that is substantially planar and spaced above a road surface upon which the vehicle is supported. Spacing the lower surface above the road can exclude false alarms generated by low kerbs, and small, insignificant objects on the road surface. The shape and a relevant dimension of the warning zone of apparatus for use on a vehicle may be changed in response to vehicle operating conditions (by the warning zone definition stage). Such vehicle operating conditions may include at least one of speed, direction of travel, vehicle controls (such as accelerator, brake and clutch position) and ambient environmental conditions (amongst other possibilities). For example, the warning zone may extend further forward from the vehicle at high speed, and/or the zone may be extended further to one side of the vehicle when cornenng. Frequency considerations Typical embodiments of the invention in any aspect are operative to transmit an electromagnetic signal, typically radio frequency, for example, microwave, radiation. Such radiation is readily controllable and well-suited for the purposes of the present invention. For example, the transmitted signal may have a frequency of between 0.5 or 1 and 77 GHz, of between 2 and 25 GHz, of approximately one of 0.5 GHz, 1 GHz, 6 GHz, 10 GHz or 2-2.5 GHz. Such signals have a useful ability to penetrate solid objects, including objects made of wood, plastic mateπal, concrete, bnck and other non- metal matenals. This might, for example, allow the apparatus to be located on the opposite side of a wall from a region that is to be monitored or withm a structure of a vehicle such as a bumper. Moreover, such signals may be used to detect solid metal objects embedded withm non-metal objects. These frequency ranges may be implemented using apparatus that is of advantageously low cost, and may have an advantageous ability to penetrate solid matenal. In a typical embodiment, the frequency may be approximately 2.45 GHz, possibly in order to meet applicable legislative requirements. In general, a particular frequency will be chosen on the basis of several cntena: cost (low frequency is better), size of the antenna (high frequency is better), receiver performance (low frequency is better), scattenng performance (this depends upon geometry, but a wide relative bandwidth (high bandwidth and low frequency) reduces glint), and applicable legislative requirements. The transmitted signal may have a relative bandwidth (say, to the centre frequency) of approximately 15%, say between 3 and 33%, 5 and 25% or 10 and 20%. By virtue of this invention, apparatus operating with a frequency and/or bandwidth set forth in the preceding paragraphs may be operative to resolve an angular position of an object with respect to a predetermined datum Pulse length in relation to target/array From a third aspect (which may optionally be provided in combination with either or both preceding aspects or any other aspects), the invention provides apparatus for obtaining positional information relating to an object which is operative to transmit a signal into a detection field and to detect a signal reflected from an object in the detection field, in which the spatial length of the transmitted signal dunng its propagation is approximately the same as (say between 50 and 200% or 75 and 150%) a dimension of the smallest objects (for example, posts, rails, human limbs, vehicle components and equipment, furniture, aircraft components, etc ) that the apparatus is intended to resolve. This may help to ensure that a reliable coπelation can be achieved between signals received at different receiving elements. In order that the position of objects can be resolved to an accuracy of several centimetres, the spatial length of the transmitted signal dunng its propagation is preferably, in order of magnitude, not greater than, say, 1.0 m, 0.3 m, 0.1 m, 0.03 m or 0.01 m. Two similar, closely spaced objects may give similar return signals. If these signals are one half wavelength out of phase with each other, or thereabouts, the two signals could interfere destructively, resulting in little or no signal. In order to reduce the nsk of destructive mterference, the wavelength of the transmitted signal is preferably not very much shorter than the spatial length, or, put another way, each pulse of the transmitted signal may compnse only a few wavelengths. For example, the spatial length of the transmitted signal dunng its propagation may be less than 10 wavelengths, or less than 6, 5, 3, 2 or even 1 wavelengths, although pulses of longer than any of these values may also be provided. By restπctmg the number of wavelengths per pulse, the likelihood of destructive interference occurnng due to two closely spaced objects is reduced. If the receiving elements are spaced too far apart then ambiguities may be introduced due to signals arnvmg at respective receiving elements with a delay shift of more than a half wavelength. On the other hand, if the receiving elements are too close together there may be insufficient angular resolution. Thus will be appreciated that the choice of distance between receiving elements is a trade off between achieving reasonable angular resolution by having a sufficiently large distance between the receiving elements, and avoiding ambiguity by having a sufficiently small distance between the receiving elements. In prefeπed embodiments of the invention, the signal is received at the apparatus at receiving elements spaced by a distance being of the same order of magnitude as a charactenstic wavelength λ of the signal. For example, the receiving elements may be spaced apart by a distance nλ where n is a real number and 0.5 < n ≤ 10, preferably 1 ≤ n ≤ 5. Time scale adjustment/stretching In prefeπed embodiments, the signal generating stage applies a senes of m pulses to the transmitting element to cause it to transmit a signal at times t„ where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; and the detection stage detects a signal reflected to the receiving elements at times r n and generates an output signal representative of the received signal; wherem the value of r n - 1„ vanes as some function of n By this arrangement, the flight time of a detected signal can be determined given only the knowledge of the value of n at which it was received and knowledge of the function of n. For example, it may be that the value of r n - t n changes linearly with n, or it may vary in some other manner, for example in a pseudo-random sequence. In a typical embodiment, the value of r n - t„ increases or decreases linearly with n, by which it will be understood that the delay between a transmit time t„ and a coπesponding receive time r„ mcreases or decreases hnearly with n Preferably, the delay vanes from one pulse to an adjacent pulse, this can be a convenient way of putting the invention into practice Although in one prefeπed embodiment the delay vanes with each successive pulse, this is not necessanly the case, a first senes of pulses at a first delay may be followed by a second senes of pulses at a second, different, delay, and more than two different delays may be used. It will be appreciated that usually the delay is considered to be the delay with respect to the time at which the relevant pulse is transmitted Successive outputs of the detection stage may be stored in a storage means, the storage means being operable to output a signal of substantially the same shape as the received signal, but with a duration that is increased in time Preferably, in apparatus embodying the invention, the duration of each transmitted signal is less than the interval between transmitted signals. Most typically, the ratio between the duration of each transmitted signal and the mean interval between each transmitted signal is less than 1/10, for example less than 1/20 or 1/50, and greater than 1/1000, for example greater than 1/500 or 1/200. Preferably, the detection stage is operable to detect the reflected signal dunng a detection aperture penod, which is shorter than (preferably very much shorter than) the time between successive pulses. In this way, one or more (if a plurality of detection aperture penods is provided) so-called "range gates" may be provided, as descnbed in WO97/14058; these might typically have widths coπesponding to distances of between 1 and 2 cm With the present invention, however, the range gates would move rather than, as in the pnor art, remain stationary. A timmg stage may be provided to supplying timing signals to the detection stage and or the signal generatmg stage. Preferably, the timing stage is adapted to operate the detection stage by means of a timing pulse. A timing pulse (or pulses) is an efficient way to generate the range gate (or gates). Preferably, a plurality of spaced receiving elements is provided. With a sufficient number of elements a precise location of the object in space can be determined Moreover, if as is preferred the transmitting elements have a wide beam and little individual angular resolution, sufficient elements can be provided so that the angular position of objects can be determined by tnlateration; that is, by precise path-length companson Preferably the apparatus further compnses a computation stage for processing signals detected by the detection stage, determining the time interval between a single reflected signal arnving at a plurality of the receiving elements, and thereby obtaining positional information relating to the object from which the signals were reflected The signal generating means preferably operates m an operation cycle to generate a sequence of spaced pulses simultaneously with or at a fixed time after each of a plurality of transmitting tngger instants The detection stage may be adapted to detect the signals from the receiving elements simultaneously with or at a fixed time after each of a plurality of receiving tngger instants, each of which occurs at a time in predetermined relation to the transmitting tngger instants. For example, for each transmitting tngger instant t n , a coπespondmg receiving tngger instant may occur at r n , the time interval r n - t„ being a predetermined function of n. In such a system, the interval r„ - 1„ may be a function of the general form of T„ + nT where T 0 and T are constants and n = 1, 2 ... m Preferably the value oft / T (that is, the ratio between the mean interval between transmit tngger instants and the increment in the delay between transmit and receive tngger instants) is of several orders of magnitude; for example, t / T may be between 10 3 and 10 7 , or between 10 4 and 10 6 , typically about 10 5 By aπangmg the value of r„ - t n to increase (or decrease) as a function of n the received signals may be stretched in the time domain by a factor of t / T, as will now be explained The output of the detection stage may be fed to a storage means, such as a capacitor or a sample and hold stage, which stores the value of the signal which is received at each receiving tngger instant r π The signal received at a receiving tngger instant r„ may thus be stored until the next receiving tngger instant r n+1 (that is, the signal is sampled and held at each receiving tngger instant) Since each receiving tngger instant has a slight increment T in the delay from the coπesponding transmit tngger instant, the value of the signal stored by the storage means changes with each new tngger instant It therefore takes t / T repetitions to complete the waveform. Assuming that the reflected signal is essentially unchanged between tngger instants (such as is the case if the ob j ect does not move significantly relative to the transmitting and receiving elements), then a strobe effect takes place which results m the detected signal being stretched by a factor of t / T. In this way the output of the storage means may be of duration greater than that of the received signals by a factor t / T and of frequency less than that of the received signals by a factor t / T. The last-descnbed aπangement can stretch the received signal in time without changing the signal shape. This is beneficial because it can reduce by a factor oft / T the frequency at which the computation stage can operate. The value of t / T may be considered as a constant divisor of frequency and multiple of time. This allows processing of signals output from the detection stage to take place at a frequency substantially less than the frequency of the received signal, with a consequent advantage m complexity and cost of processing apparatus. Thus the apparatus may further compnse a storage means for stonng values of the output signal coπesponding to signals received at times r„. The storage means may be operable to output a signal of substantially the same shape as the received signal, but with a duration which is increased in time (by a factor of t / T). Sampling stage/single control line Apparatus embodying the invention may include a sampling stage operative under the control of the timing stage selectively to pass or to interrupt the passage of signals from the receiving elements to the detection stage. The sampling stage may pass signals to the detection stage for an aperture time t a . Typically, such apparatus compnses a respective sampling stage for each receiving element. Advantageously, each sampling stage is connected to the timing stage by a respective signal delay line, withm which delay line a signal is delayed by a time not less than t a / 2. This ensures that signals cannot travel from one sampling stage to another through the timing stage dunng the aperture time, thereby minimising crosstalk between the sampling stages. To facilitate processing of the received signals, the detection stage typically includes an analogue to digital conversion stage such that the output of the detection stage is a digital signal. The output of the detection stage advantageously includes an indication of the amplitude of the received signals From a further aspect, the invention provides apparatus for obtaining positional information relating to an object according to any preceding claim, contained withm a single housmg. Such apparatus may be hand-holdable in use. For example, the apparatus may be intended to provide information about the location of objects within a wall Advantageously, apparatus embodying the invention may comprise an antenna aπay and processing means constructed as a single assembly. In such embodiments, the processing means operates to provide all functional electncal signals to and receive all functional electncal signals from the aπay Apparatus embodying the invention may be intended for use in a vehicle Angle measurement in vehicle radar Apparatus for obtaining positional information relating to an object, according to any aspect of the invention, is advantageously contained withm a single housing Such apparatus may be particularly for use in a land vehicle This may greatly simplify its installation, for example, m a vehicle Such apparatus may alternatively be hand-holdable for use. This is of particular application in cases where the apparatus is embodied in a handheld tool, such as a device for obtaining information about objects withm a wall. This feature of the invention may be provided independently. Accordingly there is apparatus for obtammg positional information relating to an object which is advantageously contained within a single housing. Such apparatus may be particularly for use in a land vehicle. This may greatly simplify its installation, for example, in a vehicle. Such apparatus may alternatively be hand-holdable for use This is of particular application in cases where the apparatus is embodied in a hand-held tool, such as a device for obtaining information about objects within a wall. From a fifth aspect (which may optionally be provided in any combination with any other aspect), the invention provides apparatus for obtaining positional information relating to an object, for use on a vehicle, for resolvmg the angular position of an object preferably using non-Doppler radar. There may also be provided with any aspect of the present invention, particularly for a land vehicle, for obtaining positional information relating to an object, apparatus for compnsing: means for transmitting a probe signal towards the object; means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object; and detecting means, coupled to the receiving means, for detecting the relative timing of the returned probe signals as received at the plurality of locations; whereby the positional information for the object can be determined from said relative timing. Apparatus, particularly for a land vehicle, for obtaining positional information relating to an object, embodymg any combination of aspects of the invention for use on a vehicle may be for obtaining positional information relating to an object external of or internal to the vehicle, the apparatus being operative to generate 3- dimensional positional data for the object. This can, for example, provide an operator of the vehicle with a warning of an obstruction nsk, or it may overnde controls of the vehicle Alternatively or additionally, apparatus embodying any combination of aspects of the invention for use on a vehicle may be for obtaining positional information relating to an object internal to the vehicle, the apparatus being operative to generate 3-dιmensιonal positional data for the object. Typically, such apparatus has a detection field within a passenger compartment of the vehicle In apparatus according to any of the last three preceding paragraphs, the positional data may include at least one of the range, azimuth and elevation of the object. Most typically, the antenna aπay of an embodiment of the invention for use on a vehicle is earned on a fixed location on the vehicle. Advantageously, the antenna aπay is located withm a component of the vehicle, preferably a non-metallic component. Thus, visual conflict with the styling of the vehicle can be avoided and the aπay can be protected For example, the antenna aπay may be located within a bumper (or other portable enclosure) of the vehicle, such as a (preferably a non-metallic) bumper, from which it can generate a detection zone to the front or to the rear of the vehicle. In (for example) a vehicle installation, apparatus embodying the invention may further compnse alerting apparatus for alerting a vehicle dnver to the presence of a detected object. Such apparatus may provide to the dnver (or another person, or a user of the apparatus, as the case may be) information that would otherwise be unavailable. In such embodiments, the alerting apparatus is operative to generate an audible warning, which, for example, may include a verbal warning that may convey information about detected object(s). Alternatively or additionally, the alerting apparatus may be operative to generate a visual warning For example, the visual warning may include a visible representation (an image) of the position of an object detected by the apparatus Apparatus embodying the invention may further compnse a display upon which is presented a visual representation e.g. an image of a detection field of the apparatus and an object withm the detection field Imaging/Pattern Matching From a sixth aspect (which may optionally be provided m any combination with any other aspect), the invention provides apparatus for obtaining positional information relating to an object, for use on a vehicle, preferably using non-Doppler radar and being operative to determine a radar cross-section of an object. From a seventh aspect (which may optionally be provided in any combination with any other aspect), the invention provides apparatus, particularly for use on a (for example land) vehicle, for obtaining positional information relating to an object, including a transmitting element for transmitting radiation mto a detection field, a receiving element for receiving radiation reflected from an object in the detection field, and a processing stage, which is operative to analyse the signals from the receiving element to denve qualitative information relatmg to the object. This permits the apparatus to provide extended data relating to objects that it detects. For example, the processing stage may be operative to compare information relating to an object at successive different angular positions against a look-up table. This allows vanation in the gain of the antenna aπay with angle to be accounted for. The processing stage may be operative to determine a radar cross-section of the object. In such embodiments, the processing stage may be operative to compare the radar cross section with a threshold value of radar cross- section and to generate a warning signal in dependence upon the result of the companson. Alternatively or additionally, the processing stage may be operative to determine an evolution of angular position of an object with time. In one example, the processing stage is operative to determine the rate of change of angular position of the object. In another example, the way in which the angular position evolves with time is determined. By determining not just the rate of change of angular position, but also how the angular position vanes with time, the accuracy with which the relative movement of the vehicle and the object can be predicted is mcreased For example, if the vehicle is moving towards a kerb which is below the level of the sensor, the vertical angular position of the kerb will change as a hyperbola as the vehicle moves towards and over the kerb By defining the charactenstic hyperbola, the movement of the vehicle relative to the kerb can be predicted In this way, it can be predicted whether the vehicle will hit the kerb or not, and by how much the vehicle will miss the kerb In another example, if the vehicle is moving towards an object, such as a post, which is not directly in the line of travel, then the honzontal angular position of the object will change as a hyperbola as the vehicle moves towards and past the object By defining the charactenstic hyperbola, it can be predicted whether or not the vehicle will hit the object, and by how much In a further example, if, as the vehicle moves, a target not on the centre line maintains a constant angular position as the vehicle moves, this may indicate a hazard such as a moped which is moving alongside the vehicle at the same speed. The above situations may be also be analysed by considenng how the distance away from the object changes as a function of distance moved by the vehicle. For example, if the distance away from an object changes as a straight line against distance moved by the vehicle then it may be predicted that the vehicle will hit the object, and the point of collision may be predicted by extrapolating the line. If the distance away from the object changes as a curve against distance moved, then it may be predicted that the vehicle will miss the object. The amount by which the vehicle will miss the object, and the point at which the object will be closest to the vehicle, may be predicted by estimating the evolution of the curve Thus the processing stage may be operative to predict a path of movement of the object, preferably relative to the apparatus. If the evolution of the angular position of the object differs from that which would be produced by a point object, then it may be mfeπed that the object has an lπegular outline, and the outline of the object may be predicted based on the difference. For example, if the vehicle is moving towards an object (such as another vehicle) with a curvilinear outline, the radar may see the nearest point of the object, but not the part of the object closest to the path of the vehicle. By predicting the outline of the object, it can be predicted where the vehicle will hit the object, even if that part of the object is not yet visible. The invention further provides apparatus for obtaining positional information relating to an object substantially as herein descnbed with reference to the accompanying drawings. Applications From an eighth aspect, the mvention provides a vehicle equipped with apparatus embodying any of the above- mentioned aspects of the invention. More specifically, the invention may provide a (preferably motor) road vehicle being equipped with a dnver warning system compnsing apparatus embodying any of the above-mentioned aspects of the invention for obtaining positional information relating to an object external of the vehicle, the apparatus being operative to generate 3-dιmensιonal positional data for the object In such a vehicle, the aπay of the apparatus may be contained within a non-metallic bumper of the vehicle. A vehicle embodying this aspect of the invention may compnse a display instrument operative to process information obtained by the apparatus and to generate a display therefrom for an operator of the vehicle From a ninth aspect, the invention provides a control system for air bags m a vehicle, the control system compnsing apparatus embodying the invention, in which the processing stage is operable to determine the occupancy of a seat equipped with a passenger air bag, and to suppress deployment of the bag in dependence on the occupancy of the seat. For example, the processing stage may be operable to determine whether or not the seat is occupied and to suppress deployment of the bag if the seat is unoccupied, and/or to determine if the occupant is too close to the air bag (such that release of the air bag would represent a hazard to the occupant) and to suppress deployment of the bag if the occupant is too close. This invention also provides embodiments being hand-held tools and devices for obtaining information about objects withm a wall compnsing a apparatus according to any of the preceding aspects of the invention. Antenna array From a tenth aspect, the invention provides an electromagnetic (for example, microwave) antenna array optionally for use in combination with any other aspect of the invention, the aπay including a transmitting element and a plurality of receiving elements, the transmitting and receiving elements being disposed on a common substrate. This feature of the invention may be provided independently. Accordingly the invention provides an electromagnetic (for example, microwave) antenna aπay, the aπay including a transmitting element and a plurality of receiving elements, the transmitting and receiving elements being disposed on a common substrate. An elecromagnetic antenna aπay embodying this aspect of the invention may include a single transmitting e elleemmeenntt,, o orr a a p olluurraahlittvy ooff ttrraannssmmiittttiinngε eelleemmeennttss. An elecromagnetic antenna aπay embodying this aspect of the invention may include three (or more) receiving elements aπanged non-colhnearly. For example, the receiving elements are aπanged substantially at the vertices of a nght-angled triangular locus (that is, in an L-shaped pattern). In one embodiment of the invention there is an elecromagnetic antenna aπay optionally for use in apparatus for obtaining positional information relatmg to an obj ect, the aπay including a transmitting element and at least three receiving elements aπanged non-colhnearly, the transmitting and receiving elements being disposed on a common substrate. The receiving elements may be spaced apart by a distance that is the same order of magnitude as the wavelength λ of the radiation that it is intended to transmit and receive. For example, the receiving elements may be spaced apart by a distance mλ where m is less than 10, and preferably less than 8,5,3 or 2, and m is greater than 0.1 , and preferably greater than 0.2, 0.3, or 0.5. The efficiency with which the elements can radiate or detect radiation decreases as the size of the elements decreases, since the radiation impedance of the elements may decrease with size, and thus the elements themselves should not be too small. On the other hand, the elements should not be too large, because they may become physically too big for the aπay, and because grating lobe effects may occur at larger sizes. In general, the size of the elements (whether receiving or transmitting) is preferably less than lOλ or 4λ and greater than about λ/4. In prefeπed embodiments, the size is in the region of λ/4 or λ/2, although other values may be used. In one particular example, the elements have a size of about 1.5 cm with a wavelength of about 5 cm. More advantageously, an elecromagnetic antenna aπay according to the present aspect may include four receivmg elements aπanged non-collinearly For example, the receiving elements may be aπanged substantially at the vertices of a quadnlateral locus, more specifically, a trapezoidal or rectangular locus, in which the quadnlateral has long and short parallel sides In accordance with this aspect of the invention the elecromagnetic antenna aπay may include at least three receiving elements aπanged non-colhnearly such that there is an axis about which the aπay is asymmetrical This feature may be provided independently. Accordingly there is an elecromagnetic antenna aπay including at least three receiving elements aπanged non-colhnearly such that there is an axis about which the aπay is asymmetrical. In embodiments where the locus is a trapezium (that is, a quadnlateral having only two sides parallel) , this aπangement can ensure that two unequally-spaced pairs of antennas in parallel planes can be selected, with dissimilar artefacts (for example, grating lobes) in their sensitivity patterns. Advantageously, the short side may be between 0.5 and 1 times (or approximately three-quarters of) the length of the long side. As a specific example, where the trapezial locus has long and short parallel sides, the length of the shorter side being approximately the wavelength λ of the radiation that the array is intended to transmit and receive, and the length of the longer side is approximately 3λ/2. By suitable processing of signals from such an array, the effect of grating lobes can be substantially reduced. Alternatively, the quadnlateral may have two opposing angles which are substantially nght angles, while the other two angles are not nght angles. This aπangement can ensure that the main grating lobe for each will point in the coπect 3-D direction, while the artefacts will be different, and thus will cancel out. More generally, to achieve the advantages discussed in the last-preceding paragraph, the invention may provide a microwave antenna aπay for use in apparatus for obtaining positional information relating to an object, which apparatus may be in accordance with any other aspect, the aπay including a transmitting element and a plurality of receiving elements, in which the spacing of two pairs of (the) receiving elements m a common direction is unequal This can be achieved in a wide range of embodiments, including that discussed above. Further aspects From an eleventh aspect, the invention provides an electromagnetic sensor for use on a vehicle, whether ground-, water- or air-borne, contained m a single enclosure, compnsing timing control means, transmitting means, receiving means, processing means and interface means, in which: said transmitting means includes a fixed antenna capable of emitting a sequence of short pulses of electromagnetic radiation in the ultra-high-frequency microwave or millimetre wave band in response to tngger signals from said timmg means into a field of view in excess of +/- 15 degrees in width and +/- 10 degrees m height from a pointing direction of the antenna, said receiving means is responsive to further tngger signals from said timing means and includes one or more fixed receive antennas responsive to signals corresponding to received fractions of such pulses after transmission, propagation to and scattenng from one or more obstacles within such field of view, adapted to convert such signals to signal information in which each frequency component of such signal is converted to a respective component of such signal information at a frequency related to such frequency component by a constant divisor for all such respective frequency components, said processing means is adapted to acquire said signal information thus responsive to said received fractions at each of said receive antennas and calculate information concerning said obstacles, said interface means are adapted to communicate either to a user of said vehicle or to other electronics systems on the vehicle, which is adapted to measure and provide via such interface means indications of the presence or absence of one or more obstacles within a volume which is not coextensive with the field of view of said antennas and can be defined independently of such field of view. In a sensor according to the last-preceding paragraph the processing means may be adapted to provide via such interface means information concerning the distances, azimuth angles and elevation angles from said sensor to said one or more obstacles. In a sensor embodying this aspect of the invention, the said volume may be contained within one or more surfaces, such as a plurality of effectively planar surfaces. The processing means may also be adapted to provide via such interface means indications of the reflecting strength of the obstacle, proportional to its radar cross section, and independent of its distance or position in the field of view of the sensor. In typical embodiments the antenna aπay is fixed with respect to its mounting; that is to say, it does not rotate A prefeπed embodiment of sensor according to this aspect of the invention is adapted to fit withm a bumper (or other portable enclosure) of a vehicle, or withm a handheld tool or enclosure. From a further aspect, the invention provides apparatus for obtaining positional information relating to one or more objects, the apparatus being operative in an operating cycle for each of m steps in which n = 1 , 2 .. m, the apparatus including: a signal generating stage operative, simultaneously with or at a fixed time after a transmitting tngger instant t n to generate a signal, and a transmitting element to transmit said signal mto a detection field; a plurality of spaced receiving elements operative simultaneously with or at a fixed time after a receiving tngger instant r n to receive a portion of the signal reflected from one or more objects in the detection field, the interval r n - t n varying as a function of n and having a magnitude in a range coπesponding to the times of travel of a signal reflected from an object withm the detection field; means for identifying the values of n at which signals reflected from one object are received at two or more receiving elements and thereby detecting the time taken, and therefore the distance travelled, by the signals from the transmitting element to the vanous receiving elements; and means for calculating the position of the object from the vanous path lengths thereby identified. In an electromagnetic imaging sensor according to this aspect of the invention, the transmitting and/or the receiving means are embodied in a single microchip, with the associated antenna elements pnnted on one or more adj acent pnnted circuit cards , and the timing signal generator means and control and processing means are embodied in the same single microchip. The transmitting means may compnse a semiconductor switching device or amplifier external to the said single microchip. Moreover, the receiving means may compnse one or more semiconductor switching devices or amplifiers external to the single microchip. More specifically, both transmitting and receiving means may compnse external semiconductor switching devices or amplifiers, and the timing signal generator and control and processing means are embodied in a single microchip. Yet more specifically, both transmitting and receiving means compnse semiconductor switching devices or amplifiers, and the timing signal generator and control and processing means may be embodied m two separate microchips Receivmg means of embodiments of this aspect of the invention may compnse switching samplers in which the switch may be closed for an aperture time less than or comparable with one half the penod of said dominant penod. In such embodiments, the switches may all be closed by a common signal without intervening pulse generating circuits Advantageously, the switching amplifiers may be electncally separated by lengths of transmission line whose electπcal length exceeds or is comparable to one half the duration of said aperture time The antennas m this aspect of the invention may be stacked microstnp patches. Suitably, the stacked microstnp patches may be fed by slots in the circuit card carrying the transmitting means The delay measurement process may, m prefeπed embodiments, include a cross-coπelation process Such a coπelation process may be a truncated coπelation process in which the range of the coπelation is calculated between the signals received by any two elements is related to the spatial separation of those two elements. For example, the delay measurement process compnses measurement of the timing difference between comparable features of the waveform such as zero-crossings, peaks, troughs, etc. The control and processing means of this aspect of the invention may also compnse classification means to identify classes of object near to a vehicle by pattern matching with said image. The said distance and said angular position of each said object may be used to identify the positions of a plurality of said objects in two or three dimensions. The positions of said plurality of said objects may be combined to form an image of the contents of the space near or in front of the sensor. In embodiments of this aspect of the invention, the control and processing means may compnse or may be connected to further processing means m which successive such images or the signals from which they were generated are used in a synthetic aperture or inverse synthetic aperture process to further detail the image of the contents of the space near or in front of the sensor. Such an imaging sensor may store digitally a descπption of a volume of space near the antenna. The volume of space may be other than a sphere or ellipsoid. Moreover, the measured position of each such object may be compared with such volume of space near the antenna to determine its location inside or outside such volume. The processing means may store a descnption of the gains of the antennas as a function of solid angle. The measured position of each object may be used with such descnption of the gains to determine the gam of each of the antennas in that direction. In such embodiments (and others), the measured signal strength aπsing for each ob j ect may be divided by the product of the antenna gains coπesponding to its position and multiplied by the fourth power of the measured range to provide a value proportional to its radar cross-section. For example, the denved value of radar cross-section may be compared with a cross-section threshold to determine the significance of the target. The cross-section threshold may be divided by the fourth power of the measured range and multiplied by the product of the antenna gams m the direction of the object before companson with the measured signal strength. Embodiments of the invention also provide apparatus according to any of the above-defined aspects for generating an image of objects within or through a solid object. Typically, the solid object may be a wall. Further embodiments of the invention provide apparatus according to any of the above-defined aspects for providing an image of an environment in conditions that human vision is compromised. For mstance, vision may be compromised by the physiological condition of a user (such as a physical handicap). Alternatively, vision may be compromised by environmental conditions, such as darkness, smoke or fog. Method Aspects Angular resolution by cross-correlation From a first method aspect the invention provides a method for obtaining positional information relating to an ob j ect, optionally in apparatus in accordance with any one of the preceding aspects of the invention, compnsing applying a senes of pulses to a transmitting element of an aπay to cause it to transmit a signal, such that at least a portion of the signal is reflected from the object to be received by the receiving elements; detecting signals reflected to receivmg elements of the aπay and generating output signals representative of the received signals; and applying a truncated cross-coπelation function to the output signals to detect the interval between signals received by a plurality of the receiving elements, whereby to determine an angular position of an object from which the transmitted signal has been reflected. Preferably the truncated cross-coπelation function compnses shifting one output signal with respect to another over a range which is less than the duration of the signals and preferably less than the duration of a pulse. For example, the truncated cross-coπelation function may compnse shifting one output signal with respect to another over a range in which the maximum offset in either direction is less than 5 times the time that would be taken for the transmitted signal to travel directly from one receiving element to another, and preferably less than or equal to 3, 2 or 1 times this value The signal may have a charactenstic wavelength λ and the truncated cross-coπelation function may have an interval of coπelation which is a small multiple of λ. For example, the interval of coπelation may be less than 10, 5, or 2 λ , or even λ. As a minimum, the cross-coπelation function is applied over two samples, although preferably a greater number of samples is used. In a method embodying this aspect of the invention in which the receiving elements are spaced apart by a distance D, the truncated cross-coπelation function may have an interval that is less than a small multiple of D. The interval of coπelation may be less than 5, 2, 1.5 or ID. Such a method may further include determining the distance from the array of an object from which a signal has been reflected. This can, for example, be achieved by multiplying the time taken for the signal to be received by the speed of propagation of the signal Typically, a maximum value of the cross coπelation function is identified. A method embodying the invention may further include a step of determining the distance from the aπay of an object from which a signal has been reflected. This may be achieved by multiplying the time taken for the signal to be received by the speed of propagation of the signal. The first method aspect of the invention also provides a method of obtaining positional information relating to an object, optionally including any of the above mentioned features, the method compnsmg: transmitting a signal for at least partial reflection by the object; receiving signals reflected by the object; and applying a truncated cross-coπelation function to received signals, thereby to determine a position of the object. In the simplest case the signals are received by two receiving elements and the interval between signals received by those two receivmg elements is determined. If more than two receiving elements are provided, a truncated cross-coπelation function may be applied to some or all of the vanous pairs of received signals, such that a plurality of angular positions are determined In this way the accuracy of the measurement may be improved and/or another dimension added to the measurement. Thus, a method embodying this aspect of the invention may include the steps of determining the interval between a signal being received by a first set of any two or more of the receiving elements; calculating a first angular position of an object from which the transmitted signal has been reflected, determining the interval between a signal being received by a second set of any two or more of the receiving elements (which may include one or more of the elements of the first set); and calculating a second angular position of an object from which the transmitted signal has been reflected Preferably, the first and second angular positions are measured in planes that are substantially not parallel to one another, and more optionally, the said planes are approximately normal to one another. The method may include determining a coordinate in three-dimensional space of an object from which the transmitted signal has been reflected In a method embodying the invention at least one of the first and second set of receiving elements includes three or more elements which may be disposed such that that set includes at least two pairs of elements, the spacing of elements in the two pairs being unequal. In an advantageous embodiment, the spacing (D) between the elements of one pair is approximately equal to (for example between 50% and 200% or between 75 and 150% of) a characteπstic wavelength λ of the signal, and preferably the spacing between a second pair of elements is approximately equal to 3λ/4, or 3 D/4, and preferably the ratio of the spacing of the elements m one of the first and second pairs to the spacing of the elements in the other of the first and second pairs is between 0.5 and 1 or 0.75 and 0.9. In such embodiments, a truncated cross-coπelation may be performed between the signals received by each pair of elements and the product of the result of the cross-coπelations may be determined. A method embodying the invention may further compnse generating an output for presentation of positional information relating to the object to a user. The output may include at least one of an audible and a visual signal. Warning zone From a second method aspect, the invention provides a method for obtaining positional information relating to an object, optionally in combination with the first aspect, compnsmg: defining a warning zone within a detection field; and determining whether a detected object is with the warning zone: wherein the warmng zone is preferably defined as a three-dimensional region within the detection field.. Typically, the warning zone is contained within and is smaller than the detection field. Moreover, the shape of the warning zone may be dissimilar from the shape of (and or may be smaller than) the detection field Preferably the method includes the step of determining the position of a detected object withm a detection field The warmng zone may include a region defined m two dimensions within the detection field. For example, the warning zone may be a planar surface withm the detection field. Alternatively, the warning zone may be defined as a three-dimensional region withm the detection field The warning zone may be defined by an algonthm as a function of a coordmate withm the detection field. Alternatively or additionally, the warning zone may be defined by at least a limiting value of one or more ordinates of a coordinate withm the detection field. For example, the warning zone may be defined by at least a limiting value of one or more angles of a polar coordinate within the detection field. Moreover, the warning zone may be defined by at least a limiting value of a range of a polar coordinate withm the detection field. The warnmg zone may include a plurality of discontinuous spatial regions A prefeπed method according to this aspect of the invention further includes generation of a co-ordinate of a detected object In such a method, the generated co-ordinates may be compared with co-ordinates of the warning zone to determine whether the object is within the warning zone. A method embodying the invention may further compnse the step of generating an output signal indicative that the object is within the warning zone. Typically, the method further compnses the step of issuing a warning to a user upon detection of an object in the warning zone. In a modification to a method embodying the invention, there is defined a plurality of non-coextensive warmng zones. In such embodiments, there may be included a step of generating an output signal indicative of which of the plurality of warmng zones contains the object. In another development of this aspect of the invention the method may further compnse the step of analysing a characteπstic of an object outside of the warning zone. The step of analysing a charactenstic may compnse tracking an object outside the warning zone and predicting its entry into the warning zone. Typical methods embodying the invention further comprise a step of issuing at least one of an audible and a visual warning to a user upon detection of an object in a warning zone. A method embodying this aspect of the invention may be earned out on a vehicle. In such methods, at least one of the shape and a relevant dimension of a warning zone may be at least m part determined by a coπespondmg shape and dimension of the vehicle. A method as set forth in the last-preceding paragraph typically includes monitonng operating conditions of the vehicle and changing at least one of the shape and a relevant dimension of the warning zone in response to vehicle operating conditions. Such operating conditions my include (amongst other possibilities) at least one of speed, direction of travel, and ambient environmental conditions. For example, the distance to which the warning zone extends in the direction of travel of the vehicle may be increased with the speed of the vehicle Alternatively or additionally, the extent to which the warmng zone extends to one side of a longitudinal axis of the vehicle may be increased m a direction in which the vehicle is turning. Frequency considerations In a method embodying the invention, the signal is most typically an electromagnetic signal. More specifically, in a method embodying the invention, the electromagnetic signal is most typically microwave radiation. For example, the electromagnetic signal may have a frequency of between 0.5 or 1 and 77 GHz, of between 2 and 25 GHz, or it may be approximately one of 0.5 GHz, 1 GHz, 6 GHz, 10 GHz or 2-2 5 GHz. For example, the frequency of the electromagnetic signal may be approximately 2.45 GHz. The transmitted signal may typically have a relative bandwidth (say, to centre frequency) of approximately 15%, preferably between 3 and 30%, 5 and 25%, or 10 and 20%. The transmitted signal may have a relative bandwidth to the centre frequency between 10 and 20%, between 5 and 25%, between 3 and 33% or approximately 15%. In a method embodying any of the last three preceding paragraphs, an angular position of an object may be resolved with respect to a predetermined datum. From a third method aspect, the invention provides a method of obtaining positional information relating to an object, optionally in combination with any of the other method aspects, compπsing transmitting a signal into a detection field and detecting a signal reflected from an object m the detection field, in which the spatial length of the transmitted signal dunng its propagation is approximately the same as a dimension of the smallest object that the apparatus is intended to resolve. In such a method, the spatial length of the transmitted signal dunng its propagation may be, in order of magnitude, not greater than say, 1.0 m, 0.3 m, 0.1 m, 0.03 m or 0.01 m For example, the spatial length of the transmitted signal dunng its propagation may be less than 10 wavelengths, or less than 6, 5, 3, 2 or even 1 wavelengths Typically, in a method embodying this aspect of the invention, the signal is received at receiving elements spaced by a distance being of the same order of magnitude as a charactenstic wavelength λ of the signal. More specifically, the receiving elements may be spaced apart by a distance nλ where 0.5 ≤ n ≤ 10, or 1 ≤ n ≤ 5 Time scale adjustment/stretching A method embodying this aspect of the invention may include a senes of pulses compπses m pulses to cause the transmitting element to transmit a signal, at times t n where n = 1, 2 ... m; and reflected signals are detected by the receiving elements at times r n ; compnsing the steps of generating an output signal representative of the received signal; wherein the value of r n - t n vanes as some function of n. In prefeπed embodiments of a method embodying this aspect of the invention, the value of r n - t n changes linearly with n. Alternatively, it may vary in some other manner, for example in a pseudo-random sequence. Preferably the method further compnses stonng values of the output signal coπesponding to signals received at times r π . The method may further compnse outputtmg a signal of substantially the same shape as the received signal, but with a duration which is increased in time. For example, the duration may be increased by several orders of magnitude, for example, by between 10 3 and 10 7 , or between 10 4 and 10 6 . Angle measurement in vehicle radar From a fourth method aspect, the invention provides a method for obtaining positional information relating to an object, performed on a vehicle optionally in combmation with any other method aspect of the invention, for resolving the angular position of an object using non-Doppler radar. A method embodying the invention may be performed on a vehicle for obtaining positional information relating to an object external of the vehicle, in which 3 -dimensional positional data for the object is generated. A method embodying the invention may be performed on a vehicle for obtaining positional information relating to an object internal to the vehicle, in which 3-dιmensιonal positional data for the object is generated. In a method embodymg the invention, a detection field may be withm a passenger compartment of the vehicle. Positional data obtained by a method embodying the invention may include at least one of the range, azimuth and elevation of the object. A method embodying the invention may be earned out by apparatus including an antenna aπay that is earned on a fixed location on the vehicle Such an antenna aπay may be located withm a (preferably non-metallic) component of the vehicle For example, the antenna aπay may be located within a (preferably non-metallic) bumper of the vehicle. A method embodying the invention may further compnse a step of alerting a vehicle dnver to the presence of a detected object In such embodiments, the alerting step may include generating an audible warning. The audible warning may, for example, include a descnptive verbal warning. Alternatively or additionally, the alerting step may include generating a visual warning. The visual warning may include a visual representation (an image) of the position of detected objects. Such a method may further compnse a step of presenting a visual representation of a detection field and objects within the detection field Imaging/Pattern Matching From a fifth method aspect, the invention provides a method for obtaining positional information relating to an object, performed on a vehicle optionally in accordance with any other method aspect, usmg non-Doppler radar, the method including determining a radar cross-section of a object. From a sixth method aspect, the invention provides a method for obtaining positional information relating to an object, optionally in accordance with any other method aspect, including transmitting radiation into a detection field, receiving radiation reflected from an object in the detection field, and m an analysis step analysing the signals from the receiving element to denve qualitative information relating to the object. In a method embodying this aspect of the invention, a radar cross-section of the object may be determined. The radar cross section may be compared with a threshold value of radar cross-section and a warning signal may be issued m dependence upon the result of the companson. Alternatively or additionally, in such a method, an evolution of angular position of an object may be determined, and/or a path of movement of the object may be predicted. In the analysis step of a method embodying this aspect of the invention, the signals may be modified to compensate for angular vaπation in sensitivity of the receiving element. Moreover, in the analysis step the signals may be modified to account for the range of the object from which the signals are reflected. In a method embodying this aspect of the invention, the analysis step may include making a companson between the received signal and a pattern coπesponding to signals received from a known class of objects. Such an analysis may include identification of charactenstic features of the received signals. The charactenstic features may include at least one of minima, maxima and zero-crossings. This invention also provides a method for obtaining positional information relating to an object substantially as herein descnbed with reference to the accompanying drawings. From another aspect, the invention provides a method of controlling deployment of air bags in a vehicle, in which a method according to any other method aspect of the invention is applied to determme the occupancy of a seat equipped with a passenger air bag, and deployment of the bag is suppressed m dependence on the occupancy of the seat, for example, if the seat is unoccupied or if the occupant is too close to the air bag. A portion of the signal emitted from the transmitting element will propagate directly to the receiving elements without being reflected off an object in the detection field. This will be detected at the receivmg elements a very short time after the transmitting tngger instant with the result that the ability of the apparatus to resolve objects at short range may be limited The receiving elements may be disposed in a symmetncal relationship with the transmitting element whereby such signals will be detected by the receiving elements substantially simultaneously and with a substantially similar signal shape This aπangement can simplify the processing required to compensate for the existence of these signals. However, in some circumstances the best results may be obtained if the receiving elements are not m a symmetncal relationship. From another aspect, the invention provides a method of obtaining positional information relating to an object, preferably in apparatus as aforesaid, the method compnsing an operating cycle having m steps in which n = 1 , .. m, each step compnsing (a) generating a signal at a given timing relationship with respect to a transmitting tngger instant t„ and transmitting it into a detection field; and (b) receiving at a given timing relationship with respect to a receiving tngger instant r n at least a portion of the signal reflected from the object, wherein the interval r n - t„ vanes as a function of n. Preferably the method further compnses. c) providing at least two (preferably spaced) receiving elements and identifying the values of n at which signals reflected from the object are received by the receiving elements In this way, the time taken, and therefore the distance travelled, by the signals from the transmitting element to the vanous receiving elements can be determined. In this method, step c) typically includes a step of cross-coπelation of the signals received by two of the receiving elements. Preferably the cross-coπelation function is a truncated cross-coπelation function and compnses shifting one output signal with respect to another over a range which is less than the duration of the signals. Typically one output signal is shifted with respect to another over a range in which the maximum offset in either direction is less than the time that would be taken for the transmitted signal to travel directly from one receiving element to another In order to resolve a three-dimensional position of the object, step c) preferably includes a plurality or even multiplicity of steps of cross-coπelation of the signals received by vanous of the receiving elements. More information about the position and/or the nature of the object from which the signals were reflected may be obtained by including in step c) a companson of the amplitude of signals received by vanous of the receivmg elements. Moreover, step c) may include companson of characteristic features of the received signals, such features including at least one of zero-crossmgs, maxima and mimma. In a prefeπed embodiment of this process, in which the coπelation coefficient is approximately a cosmusoidal function of each offset angle (θ-θ 0 ), where θ 0 is the angle offset of the object m the plane containing the relevant antenna parr and the antenna boresight direction), it may not be necessary in step c) to carry out a finely-stepped cross-coπelation, but to determine the coπelation coefficients at a small sample of angles, separated by less than half the sinusoidal wavelength, to estimate the direction of the maximum, followed by further samples close to that direction to refine the estimate. This further shortens the truncated cross-coπelation process. After carrying out the truncated cross-coπelation process, the method may include a step of selectmg reflections denoted by coπelation maxima which exceed a predetermined signal threshold (which may depend on the noise amplitude observed), and dividing each such correlation maximum by the gam of the transmitting and receiving antennas in the measured direction. Each such coπelation maximum may then be multiplied by the fourth power of the measured range, to obtain a value proportional to the radar cross-section of the object. This value may then be compared to a cross-section threshold such that objects whose cross-section exceeds such threshold are reported or may be subjected to further processing In a further processing step each three-dimensional position determined in step c) may be compared with a descnption, either m the form of a look-up table or of an algonthm, which descnbes a volume of space near the antenna. The space may be selected such that the presence of an object within it is of relevance to a particular situation in which the method is being employed This space will be refeπed to as "the warning zone". Objects that are found to be within the warning zone and which exceed either a signal threshold or a cross-section threshold may then be reported, may give nse to a warning, or may be subjected to further processing. In general, the range of values of r n - t π for 1 ≤ n ≤ m may cover a range of time within which it is expected that a signal reflected from the object will be received. This can ensure that a reflection can be received from an object located anywhere withm an intended detection field of the apparatus. Further aspects In a prefeπed apparatus embodying the invention, an aπay of electromagnetic antennas is used in conjunction with processing electronics in a wideband microwave or millimetre imaging sensor suitable for obstacle detection, proximity and approach sensing or inspection. Objects in the volume in front of the aπay are resolved in range so that substantially only one is found at any one value of range. The range of any such object is determined b the time of flight of a pulsed signal reflected from it, and its angular position is then resolved, as taught in published International patent application No. WO97/14058, by determining the relative times at which such signals received from that object arnve at different elements of the array. In embodiments of the invention an aπay is configured with two or more elements separated by a distance which is less than or comparable with the dominant wavelength in the radiated pulse; the signals are pre-processed by frequency scaling rather than or in addition to frequency shifting, and both angular resolution and high detection performance are obtained by processing a combination of the channel outputs according to the element separation to determine timing offsets and angular positions. Wide bandwidth combined with precise mter-element receiver timing allows many targets withm the range of the sensor to be resolved m range and uniquely positioned in angle. A method embodying the invention may provide an image of an environment in conditions that human vision is compromised. For example, vision may be compromised by the physiological condition of a user (e.g. due to a physical disability). Alternatively or additionally which vision may be compromised by environmental conditions such as darkness, smoke or fog. A method embodying the invention may also be applied to imaging withm or through a solid object, such as a wall. From yet another aspect, there is provided an electromagnetic imaging sensor compnsing wideband signal transmitting means and a plurality of wideband receiving means, with associated antenna means in fixed relative positions within an assembly suitable for attachment to road vehicles or as portable equipment, timing signal generator means, control and processing means and connection means for connection to alarms, indicators or other systems, in which: the transmitting means is operable to emit a tra of electromagnetic pulses each less than or comparable with 1 nanosecond in duration, and charactensed by a dominant wavelength or penod, but contammg substantially a small number of cycles of such wavelength or penod, the receivmg means and timing signal generator means are operable to control the transmitting means to emit electromagnetic pulses and the receiving means to receive reflections of such pulses from objects in the field of view of the sensor and convert them to signals in which each frequency component is reduced by a constant factor, and the time domain waveform is stretched by the inverse of the same factor, and the arrival time of each component at the receiving elements is measured, being proportional to the distance to the object, the pulses are of sufficiently short duration that reflections from such objects separated by a distance comparable with 0.1 metre or more in range are substantially resolved in time as they return to the receiving elements, the arrival times of the reflections from each such object at each receiving element are subjected to a delay measurement process to determine the relative delays in arrival and therefrom derive the direction of the arrival from that object, thus determining its angular position in one, two or three dimensions with respect to the aπay of antennas. In an electromagnetic imaging sensor according to the last-preceding paragraph, the transmitting and/or the receiving means may be embodied in a single microchip, with the associated antenna feed elements printed on one or more adjacent printed circuit cards. The antennas themselves may be either part of the cards, or separate from them. The timing signal generator means and control and processing means are preferably embodied in the same single microchip. In such embodiments, the transmitting means may comprise a semiconductor switching device or amplifier external to the said microchip. Moreover, the receiving means may comprise one or more semiconductor switching devices, amplifiers, filters (which may be adaptive/time-variable) or pulse shaping networks external to the said microchip. In one embodiment, both transmitting and receiving means comprise external semiconductor switching devices or amplifiers, and the timing signal generator and control and processing means are embodied in a single microchip. Alternatively, both transmitting and receiving means comprise semiconductor switching devices or amplifiers, and the timing signal generator and control and processing means are embodied in two separate microchips, or on a single chip. In embodiments of the invention, the receiving means may comprise switching samplers in which the switch may be closed for an aperture time less than or comparable with one half the period of said dominant period. Most preferably, the receiving means comprise switching samplers in which the switches are all closed by a common signal without intervening pulse generating circuits. In such embodiments, the switching samplers are advantageously electrically separated by lengths of transmission line whose electrical length exceeds or is comparable to one half the duration of said aperture time. In a prefeπed construction, the antennas are patch antennas such as microstrip patches. This aπangement has advantages for manufacture. The antennas may be fed by a slotline feed in the circuit card carrying the transmitting means. Advantageously, the patches may be stacked patches. This aπangement assists in achieving the bandwidth necessary for the antennas. The delay measurement process may include a cross-coπelation process. Advantageously, the cross-coπelation process is a truncated coπelation process in which the range of the coπelation determined between the signals received by any two elements is related to the spatial separation of those two elements. Additionally or alternatively, the delay measurement process comprises measurement of the timing difference between comparable features of the waveform such as zero-crossings, peaks, troughs, etc.. The control and processing means may also comprise classification means to identify classes of object near to the sensor by pattern matching with the reflected signals. Thus, the distance and angular position of each object from which signals are reflected may be used to identify the positions of a plurality of those objects m two or three dimensions Thereafter, the positions of said plurality of said obj ects may be combined to form an image of the contents of the space near or m front of the sensor. In embodiments of this aspect of the invention, the control and processing means may compnse or may be connected to further processing means in which successive such images or the signals from which they were generated are used in a synthetic aperture or inverse synthetic aperture process to further detail the image of the contents of the space near or m front of the sensor. When two objects occur at substantially the same range from the apparatus, a single angular position may be denved coπesponding substantially to the mean of the positions of the objects weighted by the amplitudes of said reflections of said pulses. In a related aspect, the invention provides apparatus for obtaining positional information relating to one (or more) object(s), the apparatus compnsing: an aπay including a transmitting element and a receiving element (preferably a plurality of spaced receiving elements); signal generating means for applying (operable to apply) a senes of pulses to the transmitting element to cause it to transmit a signal, such that at least a portion of the signal can be reflected from the object to be received by the receiving element; detection means for detecting (operable to detect) a signal reflected to the receiving element (dunng a detection aperture penod) and for generating (operable to generate) an output signal representative of the received signal; and timing means for operating the detection means at a varying delay. The timing means may be adapted or operable to initiate operation of the detection means after a vanable interval following each operation of the signal generating means, the said interval varying with successive pulses If a plurality of receiving elements is provided, the computation means may be adapted or operable to process signals detected by the detection means, assess the time interval between a single reflected signal arnving at two or more of the receiving elements, and thereby determine the position of the object from which the signals were reflected. In a further related aspect, the invention also provides apparatus for obtaining positional information relating to an object, the apparatus being operative m an operating cycle having m steps in which n = 1, ... m, the apparatus compnsing: a signal generating stage for generating a signal at a given timing relationship with respect to a transmitting tngger instant t n ; a transmitting element to transmit the said signal into a detection field; and a receiving element for receiving at least a portion of the signal reflected from the object at a given timing relationship with respect to a receiving tngger instant r„; wherein the interval r„ - t„ vanes as a function of n. The invention may also extend to apparatus for obtaining positional information relating to one or more objects, the apparatus compnsmg: an aπay including a transmitting element and a plurality of spaced receiving elements, stgnal generating means operative to apply a senes of pulses to the transmitting element to cause it to transmit a signal, such that a portion of the signal can be reflected from one or more objects to be received by the receiving elements; detection means operative to detect signals reflected to the receiving elements dunng a detection aperture penod and to generate an output signal representative of the received signals, timing means operative to initiate operation of the detection means after a vanable interval following each operation of the signal generating means, the said interval varying with successive pulses; computation means operative to process signals detected by the detection means, assess the time interval between a single reflected signal arπvmg at two or more of the receiving elements, and thereby determine the position of the object from which the signals were reflected The invention may further extend to apparatus for obtaining positional information relating to one or more ob j ects, the apparatus being operative in an operating cycle for each of m steps in which n = 1, 2 .. m, the apparatus including: a signal generating stage operative, simultaneously with or at a fixed time after a transmitting tngger instant t„ to generate a signal, and a transmitting element to transmit the said signal into a detection field; plurality of spaced receiving elements operative simultaneously with or at a fixed time after a receiving tngger instant r n to receive a portion of the signal reflected from one or more objects in the detection field, the interval r„ - 1„ varying as a function of n and having a magnitude in a range coπesponding to the times of travel of a signal reflected from an object within the detection field; means for identifying the values of n at which signals reflected from one obj ect received at two or more receiving elements and thereby assessing the time taken, and therefore the distance travelled, by the signals from the transmitting element to the vanous receiving elements; and means for calculating the position of the object from the vanous path lengths thereby identified. The invention additionally provides apparatus for generating an image of objects withm or through a solid object m accordance with any of the preceding aspects of the invention. In such cases, the solid object is a typically a wall. The invention further provides apparatus in accordance with any of the preceding aspects of the mvention for providmg an image of an environment in conditions that human vision is compromised. For example, in such embodiments, vision may be compromised by the physiological condition of a user. Alternatively or additionally, vision might be compromised by environmental conditions. In addition to all of the above, the invention provides apparatus for obtaimng positional information relating to an ob j ect, compπsing: transmitting means for transmitting a probe signal towards the object, the transmitting means compnsing: a signal generating stage; and at least one transmitting element; receiving means for receivmg, at a plurality of spaced apart locations, the probe signal as returned by the object, the receiving means compnsing; at least one receiving element at the plurality of spaced apart locations; and detecting means for detecting the relative timing of the returned probe signals as received at the plurality of locations, the detecting means compnsing; a detection stage, coupled to the receiving means; whereby positional information for the object can be determmed from the relative timing; and wherem: the signal generating stage applies a senes of m pulses to the transmitting element to cause it to transmit a signal, at times t n where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received by the receivmg elements; the detection stage detects a signal reflected to the receiving elements at times r„ and generates an output signal representative of the received signal; and wherem the value of r n - t n vanes as some function of n. The invention provides apparatus for obtaining positional information relating to an object, compnsing means for transmitting a probe signal towards the object, said transmitting means compnsing a transmitting element, means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object, said receiving means compπsing a plurality of receiving elements, and detecting means, coupled to the receiving means, for detecting the relative timing of the returned probe signals as received at the plurality of spaced apart locations, whereby the positional information for the object can be determined from said relative timing; and wherein the transmitting element and receiving elements are disposed on a common substrate And it provides an antenna aπay optionally for use m apparatus for obtaining positional information relating to an object embodying one or more aspects of the invention, the aπay including a transmitting element and a plurality of receiving elements, the transmitting and receivmg elements being disposed on a common substrate The invention further provides a method of obtaining positional information relating to an object, compnsing the steps of: transmitting a probe signal towards the object, receiving, at a plurality of spaced apart locations, the probe signal as returned by the object, detecting the relative timing of the returned probe signals as received at the plurality of locations; and determining positional information for the object from the relative timing, wherein the transmitting step compnses applying a senes of m pulses to a transmitting element to cause it to transmit a signal, at times t„ where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received at the plurality of spaced apart locations, the detecting step compπses detecting a signal reflected to the receiving elements at times r n and generating an output signal representative of the received signal; and wherein the value of r n - t n vanes as some function of n. The invention also provides a method of obtaining positional information relating to an object using an apparatus compnsing a transmitting element, a receiving means compnsing a plurality of receiving elements and a detecting means, the method compnsing: transmitting a probe signal from the transmitting element towards the object; receiving, at a plurality of spaced apart locations, the probe signal as returned by the object; and detecting, at the detecting means, the relative timing of the returned probe signals as received at the plurality of spaced apart locations; determining the positional information for the object from said relative timing; wherein the detecting means is coupled to the receiving means and the transmitting element and receiving elements are disposed on a common substrate. The invention further provides the use of an electromagnetic antenna aπay in a method descnbed above in which the receivmg elements are spaced apart by a distance that is the same order of magnitude as the wavelength λ of the radiation that it is intended to transmit and receive, the electromagnetic antenna array including at least three receiving elements arranged non-colhnearly such that there is an axis about which the aπay is asymmetrical The invention also provides the use of an electromagnetic antenna aπay in a method of obtaining positional information relating to an object, the aπay including a transmitting element and at least three receiving elements aπanged non-colhnearly, the transmitting and receivmg elements being disposed on a common substrate The invention further provides the use of an electromagnetic antenna aπay in a method of obtaining positional information relatmg to an object, the aπay including at least three receiving elements arranged non-colhnearly such that there is an axis about which the aπay is asymmetncal The invention provides for a vehicle substantially as herein descnbed and with reference to the accompanying drawings Additionally, the invention provides a control system for air bags in a motor road vehicle substantially as herein descnbed and with reference to the accompanying drawings. The invention also provides for apparatus for a land vehicle substantially as herein descnbed and with reference to the accompanying drawings. The invention provides for a device for obtaining information about objects with or through a wall substantially as herein descnbed and with reference to the accompanying drawings The invention also provides for a hand-held tool substantially as herein descnbed and with reference to the accompanying drawings. There is also provided an electromagnetic microwave antenna aπay substantially as herein descnbed and with reference to the accompanying drawings. There is also provided apparatus for generating an image of objects within or through a solid object substantially as herein descnbed and with reference to the accompanying drawings. There is also provided apparatus, optionally for use on a (for example, land) vehicle, for obtaining positional information relating to an object substantially as herein descnbed with reference to the accompanying drawings. There is also provided a method for controlling deployment of air bags in a vehicle substantially as herein descnbed Features of any aspect of the invention may be combined with or interchanged with features of any other aspect as desired. Method features may be applied to apparatus aspects and vice versa. Features which are provided independently may be provided dependently, and vice versa. Although the embodiments of the invention that will be descnbed below operate by radiatmg ultra-high-frequency, microwave or millimetre wave radiation, in pnnciple a range of other types of signals could be used in alternative embodiments. For example, embodiments could be constructed which operate in other parts of the RF spectrum, they could use light, or they could use sound waves such as ultrasound. Prefeπed features of an embodiment of the invention will now be descnbed m detail, purely by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a diagram showing the path of signals from a transmitting element to receiving elements of an embodiment of the invention; Figure 2 is a representation of an antenna array of transmitting and receiving elements being a component of an embodiment of the invention; Figure 3 is a representation of the mam circuit elements of an embodiment of the mvention; Figure 4 is a diagram illustrating the process of tnlateration; Figure 5 is a flow diagram of the processing steps earned out by apparatus embodying the invention; Figure 6 shows an antenna aπay for use m an alternative embodiment of the invention; Figure 7 is a graph illustrating the outputs of the aπay of Figure 6; Figure 8 shows an installation of apparatus embodying the invention m a motor road vehicle; Figures 9 and 10 are, respectively, side and plan views of the vehicle of Figure 8 illustrating a warning zone of the apparatus; Figure 11 shows an installation of apparatus embodying the invention for controlling deployment of vehicle air bags; and Figure 12 shows an implementation of apparatus embodying the invention implemented m an integrated unit. Principles of Operation With reference first to Figure 1, apparatus embodying the invention includes a transmitting element 10 located on an axis 20. In this simplified diagram, just two receiving elements 12,14 are shown, located equidistantly on opposite sides of the axis 20. The transmitting element 10 and the receiving elements 12, 14 are located on a common plane disposed normally to the axis 20. Radiation is emitted from the transmitting element 10 in a broad spread into a detection field of the apparatus. A portion of the radiation stnkes and is reflected or scattered from first and second objects 16, 18 m the detection field. The first object 16, in this example, is located on the axis 20. A portion of the radiation emitted from the transmitting element 10 travels along the axis 20 and stnkes the first object 18. Some of this radiation is reflected back to stnke the receiving elements 12,14. The total distance travelled by the radiation from the transmitting element 10 to each of the receiving elements 12,14 is equal. As a consequence, the reflected radiation is received by the two receiving elements 12,14 simultaneously. Provided that the speed of propagation of the radiation is known, and the round-tnp time can be measured sufficiently accurately, the distance from the elements 10,12,14 to the first object 14 can be determined. The second object 16 is located away from the axis 20. As before, a portion of the radiation emitted by the transmitting element 10 stnkes the object 16, and some is reflected back to each of the receiving elements 12,14. However, the length of the path followed by the radiation is less in the case of radiation that stnkes the first receivmg element 12 than it is for radiation that stnkes the second receiving element 14. This means that there is a delay between detection events m the first and second receiving elements 12,14. The length of each of the two reflected radiation paths can be determined directly from the total round trip time for the radiation. Once the difference between the lengths of the paths is known, it is a straightforward problem in tngonometry to calculate the angular position of the second object 16. Figure 1 shows only two receiving elements 12,14 aπanged m a line with the transmitting element 10. This is sufficient to determine the position of an object in two dimensions. The pnnciple can be extended to three dimensions through use of one or more additional receiving elements . Figure 2 shows one possible arrangement of a transmitting element and receivmg elements, as is used in the embodiment that will now be descnbed. Antenna Array With reference to Figure 2, there is shown a diagrammatic representation of an antenna array 30 suitable for use m an embodiment of this invention. In this embodiment, the radiation generated by the apparatus is constituted by RF signals m the microwave band, and the antenna aπay 30 is constructed accordingly. The antenna aπay 30 is constructed on a substrate 32. In this case, the substrate is a block of plastic or glass- fibre composite matenal having a flat supporting surface. In order that embodiments of the invention are available for use where space is restncted, the antenna array is compact, having a penpheral size of approximately 10 x 12cm. Antenna elements are formed on the supporting surface of the substrate as conductors pnnted onto the surface The antenna elements may be dipoles (for example, bow-tie dipoles), TEM horns, microstnp patches, stacked patches, or any other compact element or conductive structure suitable for operating at the required signal frequency. In this embodiment, the aπay 30 has five antenna elements in total. Four of these elements are first, second, third and fourth receiving elements 34,36,40,38 although other numbers of receiving elements, such as two, three, four or more, may be provided The fifth element is a transmitting element 42. The receiving elements 34,36,38,40 are disposed at the vertices of a trapezium-shaped (which may, in a special case be rectangular) locus, and with more elements these could be disposed at the vertices say of a trapezoid or cuboid. The transmitting element 42 is disposed at the centre of the same locus. For many applications, the size of the antenna aπay must be kept to a minimum For example, the spacing between the elements may be m the order of no more than a few centimetres, say between 1 and 10 cm, preferably between 3 and 8 cm A hypothetical axis coπesponding to the axis 20 discussed with reference to Figure 1 can be considered to extend normal from the supporting surface through the centre of the transmitting element 42 For reference below, the spacing between the first and second receiving element will be denoted D 12 , the spacing between the second and third receiving element as D 23 , and so forth. As a specific example, if the apparatus is designed for operation with signals of frequency m the region of 6.5 GHz, the antenna elements may be dipoles of approximately 18mm in length, and may be fed with a balanced line feed. In an alternative form of construction, the antenna elements may be located withm a dielectnc radome. Associated signal processing circuitry may also be located within the radome m order to provide the apparatus as a self-contained package. Turning now to Figure 3, the circuit elements of the apparatus embodying the invention will now be descnbed. The apparatus includes a control and processing stage 66 that controls the operation of other components of the apparatus. The control and processing stage 66 has a data output that sends data relating to the position of one or more objects located within the detection field of the apparatus. Such data may be received by a terminal unit 90, possibly including an alarm, for further processing, for display to a user, and/or for transmission to a remote system, " as required in any particular application. A pulse generator and filter stage 46 is connected to the transmitting element 42 of the array 30. The pulse generator may, for example, be implemented using step-recovery diodes ("SRDs"), GaAs FETs, or SiGe transistors, the aim bemg to produce a sharp pulse waveform, which is then filtered on transmission to generate the transmitted signal. Preferably, the nse and fall time of the waveform is in the order of less than 0.5ns. Each of the receiving elements 34,36,38,40 of the aπay 30 is connected to a respective filtenng and amplifying stage 48,50,52,54. The received signal is filtered to generate the output signal. Each of the filtenng stages 46; 48,50,52,54 includes a bandpass filter m the signal path from the transmitter to the transmittmg element 42 and from the receiving elements 34,36,38,40 to the receiver circuitry. Filtenng is a standard technique used to ensure that the generated signal is suitable for the antennas, and for compliance with regulatory requirements. Sampling Circuitry and Delay Lines Signals from each of the filtenng and amplifying stages 48,50,52,54 are fed to a signal input of a respective switched sampling stage 58,60,62,64. The output of each switched sampling stage 58,60,62,64 is connected to a respective input of the control and processing stage 66. Each switched sampling stage 58,60,62,64 has a gate input, which, when activated by a suitable signal, passes signals on the input line onto the output line. Each of the gate inputs is connected through a respective delay line 68,70,72,74 to a common strobe line 76. The strobe line 76 is fed with signals from a sampling strobe signal generation stage 78. Each of the delay lines 68,70,72,74 imposes a delay as near as possible identical to each other on signals. The delay lines 68,70,72,74 may be constructed as lumped capacitors and inductors, but more preferably are equal lengths of pnnted transmission line of length L d . This delay will be refeπed to as t d , and will be discussed further below. A timing signal generator 80 of the apparatus has two output lines 82,84. A first of the output lines 82 is connected to a control input of the pulse generator and filter stage 46. A second of the output lines 84 is connected to a control input of the sampling strobe signal generation stage 78. A control input of the timing signal generator 80 receives signals from the control and processing stage 66. The timing signal generator 80 operates to generate pulses at both the transmitter tngger instants and the receiver tngger instants. In this embodiment, the timing signal generator 80 includes an oscillator and logic components. The oscillator includes a crystal-controlled clock, an output of which is fed to the logic components. Upon initiation of a timing cycle, the logic components use the signals received from the clock to generate a linear ramp signal. The linear ramp signal is fed to one input of a fast comparator, the other input of which is fed with an external voltage source. The comparator has an output upon which a signal is generated that is indicative of the relative magnitudes of the signals on its two inputs. Thus, the signal on the output changes its state after the initiation of a ramp cycle at a time interval which is dependent upon the external voltage. The timing signal generator 80 stage is configured to generate a signal on its first output line upon initiation of the timing cycle, thereby creating a transmitter tngger instant, and on its second output line upon the change of state of the comparator output thereby creating a receiver tngger instant. Alternatively, two comparators may be provided, each of which has one input connected to a different external voltage source, and the other input connected to the ramp signal. The first comparator generates an output signal when the ramp exceeds a first voltage, thereby creating a transmitter tngger instant, and the second comparator generates an output signal when the ramp exceeds a second voltage, thereby creating a receiver tngger instant. In this way, even if the ramp signal dnfts up or down, the interval between the transmitter tngger instant and the receiver tngger instant will remain constant. In an alternative embodiment, the timing signal generator includes first and second crystal-controlled clocks, the second operating at a frequency slightly below that of the first. Thus, there is a slow vanation in phase between the two clocks, whereby a slowly varying time delay can be generated. Either of the two above-descnbed embodiments (more straightforwardly in the case of the former) permit and allow random or quasi-random vanation in the timing of the transmitter and receiver tngger instants. This may be achieved, for example, by phase modulation of the timing signals, level shifting of the ramp, time modulation of initiation of the timing cycle or random signal inversion. In alternative embodiments, the switch circuits may operate independently or be tnggered in common. Dunng post-processing of the signals, symmetncal leakage signals between the transmitting element 42 and the receiving elements 34,36,38,40 can be used to correct for any differences between the timing of the vanous switched sampling stages 58,60,62,64. A most important consideration in the design of the timing and sampling stages is that inter-channel timing eπors are minimised so that an accurate companson can be made of the times at which signals are received by the vanous channels The filter stage 46 is designed to ensure that the signal fed to the transmitting element 42 causes signals to be radiated that meet appropnate regulatory requirements, for example, in respect of their power and/or frequency, and to ensure that the signals are unlikely to interfere with nearby equipment such as communication or sensing devices The filter stage 46 may be implemented using a known broadband amplifier, associated with microstnp or lumped-element filters, selected to pass signals of frequency in the operating range of the device The switched sampling stages may suitably be implemented using switching diodes such as Schottky diodes. These may be configured in, for example, a bndge aπangement well-known to those skilled m the technical field, and are configured to be tnggered by pulses generated by the timing signal generator 80. The control and processing stage 66 is constructed in accordance with the specific requirements of the particular application in which an embodiment of the invention is to be used. Sequence of Operation A flow diagram of the processing steps earned out by apparatus embodying the invention is shown m Figure 5. This illustrates the manner in which signals from four receiving elements are filtered, digitised, and amplified. Then, pairs of signals are combined in a truncated cross-coπelation process, to provide a single output signal. This signal is then processed as required by any particular application. Such processing might mclude target detection, measurement and tracking, and selection, before finally generating a report. The sequence of operation of a prefeπed embodiment will now be descnbed. When a scanning sequence is to be initiated, the control and processing stage 66 applies an enablmg signal to the control input of the timing signal generator 80. The timing signal generator 80 then generates a train of pulses on both of its output lines 82,84. On the first output line, the pulses occur at times which will be designated as t„ t 2 , t 3 , and so on. The pulses on the first output line 82 tngger the pulse generator and filter stage 46, and consequently cause a signal to be emitted by the transmitting element 42. These times t„ t 2 , t 3 , ... will therefore be refeπed to as "transmitting tngger instants". The transmitting tngger instants can be generated at regular intervals, but this is not a requirement. Some other pre-determined pattern of intervals may be used, or they may even be generated randomly. However, m this example, it should be assumed that the separation between the transmitting tngger instants is an approximately constant time t. The time at which each of the transmitter tngger instants is generated is determined by programming of the control and processmg stage 66. On the second output line 84, the pulses trigger the sampling strobe signal generation stage 78. The time of occuπence of pulses on the second output line 84 will therefore be referred to as "receiving tngger instants". In the prefeπed embodiment, there is a coπesponding receiving trigger instant for each transmittmg tngger instant, although m other embodiments there may be a plurality of receiving tngger instants for each transmittmg tngger instant, typically each at a different delay. The receiving tngger instants occur m a sequence which will be represented as r„ r 2 , r 3 , where r,=t,+T 0 +T, r 2 =t 2 +T 0 +2T, r 3 =t 3 +T 0 +3T and so forth. In the above, m this embodiment, T 0 is a constant which may be greater than or less than or equal to 0 and T is a non-zero constant In this embodiment, T is a constant greater than 0. However, T may be chosen to be less than or equal to 0, whereby at least one receiving tngger instant occurs before or simultaneously with a first transmitting tngger instant Signals received from the receiving elements 34,36,38,40 dunng this receiving tngger instant can be analysed to obtain a sample of noise pnor to transmission. Also, signals denved from a simultaneous receiving and transmitting trigger instant can be analysed to obtain a sample of transmission leakage between the transmitting element 42 and the receiving elements 34,36,38,40. The results of these analyses can be used to facilitate analysis of signals received dunng subsequent operation of the apparatus. In alternative embodiments, the values of T may be chosen to follow a predetermined but discontinuous or partially continuous sequence. In cases where T > 0, as the sequence continues, each receiving tngger instant occurs at an increasing time after the coπesponding transmitting tngger instant. That is to say, as n increases the value of r n - t n also increases. The sequence continues until the value of r n - t„ reaches a predetermined maximum (typically in the order of several or several tens of nanoseconds). The maximum time coπesponds to the longest expected round-tnp time for a signal emitted by the transmitting element 42 to be reflected from an object in the field of detection of the apparatus, and received by the receiving elements 34,36,38,40, a time governed by the maximum range of operation of the apparatus. In an alternative embodiment, the value of T 0 is comparatively large and positive, and the value of T is negative. In such embodiments, the value of r„ - t n starts at a maximum when n = 0, and decreases as n increases. At a fixed time interval after each transmitting tngger instant, the pulse generator and filter stage 46 generates a transmitting signal. The signal is filtered to meet the appropnate regulatory standards, and is then passed to the transmitting element 42 from which it is radiated into the field of detection of the apparatus. In this embodiment, the emitted signal may have a frequency of 2.45 GHz or 6.5 GHz, which is made available by European and US regulatory authonties for applications such as this. At a fixed time after each receiving tngger instant, the sampling strobe signal generation stage 78 generates a pulse. This pulse is passed to the gate input of each of the switched sampling stages 58,60,62,64 through the respective delay line 68,70,72,74. The effect of this pulse arnving at the gate inputs is to close simultaneously each of the switch sampling stages 58,60,62,64 for an aperture time ta, dunng which the signals on the input of each is passed to its output. Dunng the aperture time ta, the signals received from receiving elements 34,36,38,40 and processed by the filtenng and amplifying stages 48,50,52,54 are conveyed to the control and processing stage 66. Avoidance of Crosstalk Between Samplers It is likely that there will be some signal leakage from the input of the switched sampling stages 58,60,62,64 to their gate inputs. The purpose of the delay lines 68,70,72,74 is to avoid this resulting in crosstalk between signals from the vanous receiving elements. It will be seen that the minimum time taken for a signal to pass from one switched sampling stage 58,60,62,64 to another is not less than 2td (smce the signal must pass through two of the delay lines). The value of td is therefore chosen such that ta < 2td (and more preferably ta < td) whereby no signal can propagate from one switched sampling stage to another within the aperture time ta. This aπangement permits the switched sampling stages 58,60,62,64 to be tnggered from a common signal. This is m contrast to a more conventional aπangement m which a separate strobe pulse generator is used to generate a signal for each individual switched sampling stage; a more complex and costly aπangement that can lead to uncertainty in the relative timing between tnggenng of the vanous switched sampling stages 58,60,62,64. In embodiments m which the delay lines are constituted as pnnted transmission lines, the value of Ld (as defined above) is given by the formula Ld > VI . ta / 2 where VI is the speed of propagation of signals in the transmission line. For example, if the value of ta is in the order of 50ps, the value of Ld may be m the region of 10mm Calculation of Object Ranges As will be understood, when a signal is conveyed to the control and processing stage at a receiving tngger instant n, it is known that the signal had a round-tnp time from the transmitting element 42 to the receiving element m a time of T 0 + nT + C. In this expression, C is a constant that represents relative total delay between the transmitting tngger instants and the start of the transmission, and the receiving tngger instants and the start of the receiving aperture time. The distance of the object from which the signal is reflected is therefore given by c(T 0 + nT + C)/2. As will become apparent in due course, it is important to note that the distance of the object can be calculated by a formula that does not depend on t, nor does it depend upon the absolute value of t n . As a particular example, for a device operating at or around 6 GHz, T may be approximately 160 ps and ta may be approximately 50-80 ps. Calculation of Angle to Object - Azimuth and Elevation As was discussed with reference to Figure 1 , a proportion of the energy emitted by the transmitting element 42 at each transmitting tngger instant is reflected back towards the apparatus and is received by the receiving elements 34,36,38,40. In this embodiment, the receiving elements 34,36,38,40 are aπanged in a two-dimensional aπay. Consequently, the reflected energy arnves at the receiving elements 34,36,38,40 at different times dependent upon the object's three-dimensional location with respect to the apparatus. Assume now that there is an object located at a distance R5 from the transmittmg element and at distances RI, R2, R3 and R4 from the receiving elements 34,36,40 and 38 respectively. If an object is located on the axis of the aπay 30 (sometimes refeπed to as "the boresight"), it is equidistant from all four receivmg elements 34,36,38,40, with the result that it will arnve at all four receiving elements 34,36,38,40 simultaneously. On the other hand, if the object is located off-axis, for example at a location with elevation azimuth φ and elevation θ, there will be a time difference of approximately Dv sin θ / c between the signals arnvmg at vertically spaced receiving elements of the aπay 30 and a time difference of approximately Dh sin φ / c between the signals arnvmg at honzontally spaced receiving elements of the array 30 where Dv and Dh are, respectively, the vertical and honzontal distance between receiving elements in the array 30. In the above, c is the speed of light. More specifically, the times at which the reflected signals will be received by the receiving elements are times Trl, Tr2, Tr3, Tr4, respectively: Trl = (2 R5 + (-D14 sin(θ ) + D12 sin(φ)) / 2) / c Tr2 = (2 R5 + (-D23 sin(θ ) - D12 sin(φ)) / 2) / c Tr3 = (2 R5 + ( D23 sin(θ ) - D34 sin(φ)) / 2) / c Tr4 = (2 R5 + ( D14 sin(θ ) + D34 sin(φ)) / 2) / c where R5 is the distance between the transmittmg element 42 of the aπay 30 and the object from which the received signals have been reflected, these equations being approximations for small angles The above equations can be solved to obtain the angles θ and φ It will be observed that there is some redundancy m the information received from an array 30 of four or more elements This may be dealt with by selection or by an averaging or a weighting process A Cartesian coordinate (X, Y, Z) can then be calculated If R is the range to the object (calculated by multiplying the total round tnp time for a reflected signal by the speed of light) then. X = R cos θ sin φ Y = R cos θ cos φ Z = R sin θ In order to determine the arnval time of the reflected signals, the control and processing stage analyses signals appeanng from the switched sampling stages 58,60,62,64, as will now be discussed. In general, the distance D is small in companson with the distance between the aπay 30 and the object from which the signals have been reflected Thus, the assumption is made that the reflected signal compnses pnncipally plane waves, and that the signal does not change significantly between one receiving element and another. It is also assumed that each of the receiving elements 34,36,38,40 will react substantially identically to the signal. Moreover, each receiving element has a small angular resolution with respect to the object Time Scale Stretching Refernng again to Figure 3, each of the switched sampling stages 58, 60, 62, 64 is activated at a time r„ = t n + T 0 + nT, for n = 1, 2, ... m. When a sampling stage is activated, it samples the input waveform and holds that value until the next receivmg tngger instant r n+1 . As descπbed above, the value of r n - t π is aπanged to increase (or decrease) as a function of n. It therefore takes t / T repetitions to complete the waveform, where t is the mean separation between the transmitter tngger instants. Assuming that the reflected signal is essentially unchanged between tngger instants (such as is the case if the object does not move significantly relative to the transmitting and receiving elements), then a strobe effect takes place which results in the detected signal being stretched by a factor of t / T. In this way the output of the sampling stages is of duration greater than that of the received signals by a factor t / T and of frequency less than that of the received signals by a factor t / T. The last-descnbed aπangement stretches the received signal in time without changing the signal shape. This is beneficial because it reduces by a factor oft / T the frequency at which the processing stage 66 operates. The value of t / T may be considered as a constant divisor of frequency and multiple of time. In the particular case where t = T, the output of the switched sampling stages 58, 60, 62, 64 would be at the same frequency as the mput signal. However, if t≠ T, then the output of the switched sampling stages 58, 60, 62, 64 would be at a frequency that is reduced by a factor of t / T. For example, if t = 100 ns and T = 1 ps then the factor t / T will be 10 5 . For an incoming signal frequency of 5 GHz at the receiving elements, the output of the switched sampling stages 58, 60, 62, 64 will be a signal of 50 kHz. Under the transformation descnbed above, the shape of the received signals is preserved in the time domain, but their frequency is reduced by a common divisor. The resulting signals may be of a frequency, duration and data rate that can readily be processed by comparatively low-cost hardware. Frequency stretched signals from the switched sampling stages 58, 60, 62, 64 are converted to digital form before being passed to the control and processing stage 66 The particular processing then earned out on the signals, and the output data generated by the control and processing stage 66, is highly dependent upon the particular application and intended function of any given embodiment of the invention Determining a Time Interval Using Truncated Cross-Correlation In this embodiment, the analysis is performed by a modification of a known method of cross-coπelation. The analysis is earned out upon signals which are assumed to contain a plurality of cycles with similar, but non- ldentical, envelope shapes. Conventional cross-coπelation methods can give ambiguous results when earned out on signals of this type. Following on from the assumption descnbed in the last-preceding paragraph, it is possible to determine the time separation of the receiving instants m a process that compares the signals denved from two of the receiving elements 34,36,38,40, and determines the time difference at which these signals appear most similar. Conventional cross-coπelation between two signals includes the step of summing the products of the two signals over a penod (the interval of coπelation) coπesponding to the duration of the waveform contamed in the signals. This step is earned out for a senes of cases between which one of the signals is shifted in tune with respect to the other over a range which, in conventional cross-coπelation, is also comparable with the duration of the signal. For signals that include a plurality of cycles of similar shape, the results produced by conventional cross- coπelation can be ambiguous. The method used in the present embodiment (which method will be refeπed to herein as "truncated cross-coπelation") takes advantage of the fact that the maximum time difference between the same reflected signal being received by any two of the receiving elements is the time taken for the signal to propagate the distance between adjacent receiving elements. This can be expressed as a value D/c. In the present embodiment, the range over which one signal is shifted with respect to the other is limited to this value. This results in a signal containing only one or a few peaks, the actual number bemg determmed by the ratio of the element separation (D) to the wavelength (λ) of the transmitted signal. Thus, the method of truncated cross- coπelation is optimised by taking into account the geometry of the apparatus, rather than by reference to the signal wavelength. More specifically, the truncated coπelation process is as follows: Assume that the received signal from element 1 is an amplitude modulated wave Vj = V 0 .P(t), and that the received signal from an element 2 is an amplitude modulated wave V 2 = V 0 .P(t) 2 . P(t) is substantially non-zero for a time penod -t p to t p for both waveforms. Internally withm a data processing system, each of the waveforms is represented by a senes of numbers X(n) and Y(n) that represent, respectively, the values of Vj and V 2 at time t(n) where t(n) = t 0 + ndt and dt = Aλ / kc, and where A is a constant which depends on the amount by which the signal has been stretched (typically A = t / T) c is the speed of light, and k is an integer between 2 and (for example) 20 representing the number of digital samples per wavelength The truncated coπelation function is nmax C(m) = Σ (X(n).Y(n+ m)) for values of m from -kD/λ to kD/λ, where: m is the shifting index, which is an integer between -kD/λ and kD/λ, representing the index of truncated coπelation, and nmax and nmin define the interval of the coπelation, typically a few cycles (say, 2 cycles) of the waveform Thus it w ill be appreciated that in the truncated cross-coπelation according to the present embodiment, the range over which one signal is shifted with respect to another is truncated, m comparison to conventional cross- coπelation The range of the shift is related to the separation of the receiving elements For example, a transmitted pulse may have a length of 2λ whereas the separation between receiving elements may be λ/2. Since it is known that the maximum delay between the two signals is equal to the time it would take the transmitted signal to travel between the two receiving elements, the index of coπelation can be limited to a value coπesponding to that maximum delay. Thus, the range of shift is not related to the duration of the signals themselves, but rather to the distance between receivmg elements. It will be noted that the interval of coπelation is also truncated, m this case being bound by the values nmin and nmax, these values being selected to limit the cross-coπelation to the useful part of the waveform. As will be understood, this coπelation takes place in a short space of time in order that the system as a whole can generate images and other data sufficiently quickly. To achieve this, a coπelating function such as this can be earned out directly in hardware, for example, by digital signal processors connected to the switched sampling stages through analogue to digital converters. Alternatively, a software program running on suitable hardware, such as a microprocessor, may carry out the coπelatmg function. This truncated cross-correlation of the signals denved from any two of the receiving elements 34,36,38,40 produces a maximum at a value coπesponding to the sine of the angle between the axis 20 and a line to the location of the object projected onto the plane in which the receiving elements he By performing such coπelation with vanous of the receiving elements in combination, the angular position of the object in three dimensions can be obtained. In the prefeπed embodiment, the receiving elements have wide beams and little angular resolution, and angular positions are determined by tnlateration, that is, by precise path length companson, as illustrated in Figure 4, in which path distance D sin θ is shown as a function of offset angle θ and the distance D between the receivmg elements RI, R2. In the light of the above, it will be understood that prefeπed embodiments may be considered as a "delay monopulse" radar (with delay being measured directly in the coπelation procedure) Enhancing Resolution of the Arrav The angular resolution of the aπay 30 is relatively poor with respect to objects lying close to the plane of the aπay Further processing steps may be earned out to improve resolution in this respect. For example, the relative amplitudes of signals received by each of the receiving elements 34,36,38,40 can be compared In cases where the aπay is moving with respect to objects in its field of detection, the time at which reflected pulses are received by the receiving elements will vary from one scan sequence to the next. The changes can be analysed to gain information about the position and motion of the objects. One effect of this is to enhance the ability of the array to resolve closely spaced objects. As will be understood, the coπelation sequence described above will produce a maximum at a delay that coπesponds to the interval between the reflected signal being received at the two receiving elements concerned. This delay can be used to determine an angular direction of the object from which the signal was reflected. A combination of the results of the coπelation being applied to two or more non-colhnear pairs of receiving elements can be used to deπve the position of the object in three dimensions using the formulae set forth above. It is possible to carry out a first, relatively coarse cross-coπelation of the signals in order to obtain an approximate value for the angle at which the object is located, and then to carry out a further, relatively finer coπelation, using a larger number of samples, around the angle first estimated, and thereby refine the estimate Ameliorating the effect of grating lobes If the separation between receiving elements D is greater than λ/2 then grating lobes may occur, which may in turn give nse to false maxima. These false maxima can be distinguished by generating a significantly reduced coπelation between the channel pairs, thereby reducing the difficulties normally associated with narrow-band aπays. Grating lobes may also be reduced or eliminated by reducing the spacing between the elements to less than one half of the dominant wavelength of the signal (that is, by setting D < λ/2); however, the cost of this is reduced angular resolution and increased mter-element coupling. In prefeπed embodiments, the occuπence of grating lobes is controlled by using different hoπzontal separations for two pairs of receiving elements 634 ... 640 (that is to say, D 14 ≠ D 34 ). An array 630 embodying this modification is shown in Figure 6. In such an array, the position of the pnncipal correlation maximum will be the same for each pair, but the position of the false maxima caused by the grating lobes will differ to an extent dependent upon the difference in spacmg between the element pairs. As a specific example, if D 34 = λ and D 12 = 3λ 14, the coπelation peaks ansing from the grating lobes of elements 1 and 2 coincide in angle with the coπelation zeros ansing from the grating lobes of elements 3 and 4. If the truncated coπelation functions of the two pairs of elements are multiplied together, the "true" peaks that anse from the coπelation are enhanced, while the peaks that anse from the grating lobes will be greatly reduced m amplitude. A graph illustrating the output of both pairs of elements individually and as combmed as descnbed above is shown in Figure 7. In Figure 7, trace A shows the angular responses of a pair of receiving elements, while trace B shows the combined response illustrating the result of carrying out the multiplication descnbed above. As has been discussed above, the value of D may be m the order of 10 "2 m, whereby the value of D/c is in the order of 10 l0 s, mandating a signal frequency in the range of several GHz to achieve satisfactory resolution While it is possible to carry out signal processing upon signals having a frequency and duration of this order, the apparatus required is considerably more costly than may be the case for apparatus havmg lower speed capabihties In many embodiments, cost is of considerable importance For this reason, a processing step of time scale stretching is used to reduce the frequency requirements of the processing circuitry The transmitted signal is of duration such that its length in space is of magnitude similar to that of the objects that it is pnmanly intended to detect, and also of the same magnitude as the spacing between the receiving elements 34,36,38,40. A consequence of this is a small likelihood of combined reflections from several objects adding to produce a combined reflected signal This allows several objects to be resolved within the detection region of the apparatus, provided that these are not located very close together For objects that are close to the plane in which the elements are located, the sensitivity of the relative timing of the received signals to small changes in angular position of the object will be relatively poor. In this situation the sensitivities of each receiving element can be made different, allowing further angular discnmination on the basis of the received signal amplitude Thresholding and Pattern Matching In a first example of an enhancement to a system as descnbed above, the control and processing stage is operable to determine the amplitude of the reflection from each object in addition to its position It may then be operable to determine whether the object is m a position and of a cross-section sufficient (that is in excess of a threshold) to waπant say activating an alarm In the example of a warning system for a vehicle dnver, an alarm may be activated in such circumstances to warn a dnver of the proximity of a hazard Since the angular position of an object determines the sensitivity of the receiving elements in the direction of that object, determination of the amplitude of reflection typically is made after the angular position of the object has been determined m the cross-coπelation procedure descnbed above. Suitable further processing of the received signals may proceed in a process as will now be descnbed. First, the amplitude of the maximum value obtained from the coπelation (that is, the value used to calculate the angular position of a detected object) is obtained The gain of the aπay, as applicable to signals received from that angular position is then determined, typically from a look-up table stored in memory of the control and processing stage 66, and the amplitude value is divided by the gain to produce an angle-independent amplitude value. The angle-independent amplitude value is then multiplied by a value proportional to the fourth power of the calculated distance of the object (i.e. by a value proportional to D H ). This resulting value is proportional to the radar cross-section of the object, and will be refeπed to as the "object amplitude". In order to decide whether the object detected in the cross-coπelation process is sufficiently significant to waπant (say) activating an alarm, in typical embodiments the object amplitude is compared with a threshold, and the activation (or other action) is initiated m the event that the object amplitude exceeds the threshold. In such embodiments, the threshold may be determined by vanous techniques. In the simplest case, it may be a fixed value. Alternatively, it may be vaned in accordance with a vanety of rules. As a first example of such a rule, the threshold may be vaned as a function of the round-tnp time of the signals (effectively, as a function of the range R 5 ). Alternative or additional vanation to the threshold may be made as a function of, amongst other possibilities, levels of noise m the received signals, (in appropnate embodiments) the parameters of operation of a vehicle, or an externally applied signal, for example a signal applied by a user control. Alternatively or additionally, the signals received may be subjected to pattern matching analysis. For example, they may be compared qualitatively to pre-determined signal patterns denved from signals reflected from known classes of object. Such analysis may include companson of the shape of the received signals with a plurality of prototype signals previously measured and stored in memory accessible by the processing stage, or may include identifying charactenstic features of the received signals such as their duration, their amplitude nse and fall times and their frequency spread. In each case, the match found indicates that the signal has been reflected from a particular class target objects. Then a match is found, or charactenstic features are identified m the received signals, the processing stage may alter the seventy of a warning signal, or otherwise modify its output, in response to the class of objects identified. For example, in applications for use on a vehicle, particular pnonty may be given to signals that are identified as having been reflected from a person or animal in the path of the vehicle Warning Zone The processing steps descnbed above can determine the 3-dιmensιonal position of an object within a detection field of the system However, further, the detection region can be sub-divided into a first zone in which detection events are considered to be significant, and a second zone m which they are not significant. Effectively, the first zone defines a warning zone of the system. The sub-division can be earned out by the control and processing stage 66, typically by a software program. The control and processing stage 66 may operate to execute an algonthm that defines a 3-dιmensιonal volume of space withm the detection field near to the aπay 30 as a warning zone. For example, the warning zone may be defined to lie between spaced planes by specifying that it is bounded by minimum and maximum values of X, Y, and Z ordinates in a Cartesian coordinate system within the detection field of the array. Alternatively, the warning zone may have an arbitrary shape, defined by a look-up table or a mathematical formula. Thus, the warning zone can have substantially any shape that can be defined algonthmically, and can have any volume, provided that it is entirely contained within the detection field. The control and processing stage 66 is operative to issue a warning, for example at least one of an audible, a visual or a tactile warning to a user upon detection of an object in the warning zone. As a development of this embodiment, the control and processing stage 66 defines a plurality of wammg zones. The warmng zones may be non-coextensive (overlapping, separated or spatially different) and/or alternatively defined, by which it is meant that different charactenstics are used for determining whether an object is m the relevant warning zones. For example, different zones may be provided for detecting different speeds or different sizes of objects. This can, for example, be used to provide warnings of multiple levels of seventy, dependmg upon the position or other charactenstics of a detected object. In another development of this embodiment, the control and processing stage 66 is operative to analyse charactenstics of objects outside of the warning zone. Such charactenstics may be, for example, size of the object, distance of the object from the apparatus and or the warning zone, direction of movement of the object relative to the apparatus and or the warning zone, and relative speed of the object. As an example, the control and processing stage 66 may be operative to track objects outside the warning zone and to predict their entry into the warning zone. The apparatus may be operative to issue a pre-warnmg based on the analysis. For example, if the apparatus is mounted on a vehicle (as descnbed below) in order to provide the dnver with a parking aid, the apparatus may issue a pre-warnmg in the way descπbed above if a large object is convergmg with the vehicle, even though that object may be outside of the warning zone. This may be particularly desirable, for example, if the object itself is heading for the vehicle in the same direction that the vehicle is heading for the object, which may give an increased nsk of collision. An Application to Land Vehicles Apparatus embodying the invention can be installed in a motor road vehicle 200, as shown diagrammatically in Figure 8. In this example, the apparatus is intended to warn a dnver of objects or obstructions external of the vehicle The apparatus includes an aπay and processing circuitry mounted, as shown at 202 in a single enclosure withm a non-metallic front bumper 204 of the vehicle 200 The aπay and processing circuitry may be as descnbed below with reference to Figure 12 The apparatus implements a shaped warning zone, as illustrated m Figures 9 and 10. In this example, the warning zone might be shaped as a cuboid, as shown at 220, contained within a detection region 222 of the apparatus A lower surface of the cuboid is spaced a short distance from the ground 228 on which the vehicle 200 is standing so that a warning is not generated by the presence of very low kerbs or bumps in the road. An upper surface of the cuboid is disposed at the height of the highest part of the vehicle (for example, a radio antenna 208), plus some additional height as margin for eπor Similarly, the width of the warning zone is the width of the vehicle plus a margin for eπor A Cartesian coordinate of an object withm the detection field can be expressed with, for example, the X-axis across the vehicle, the Y-axis as fore and aft distance, and the Z-axis as height Thus a cuboidal warning zone can be defined as being all points that meet the requirement that. (Xmin < X < Xmax; Ymin < Y < Ymax; Zmin < Z < Zmax) where Xmm, Xmax, Ymin, Ymax, Zmin, and Zmax are either constants, or are vaned in response to changing operating conditions. The apparatus may change the shape or size of the warning zone in response to changing vehicle operating conditions. The apparatus receives and processes signals from sensors mounted on the vehicle that detect, for example, the vehicle's speed, steenng and throttle input, ambient conditions, and so forth. For example, at low speed, distant objects are of less concern, so the warning zone might be shortened, as shown at 224. Objects detected withm the volume thus removed from the warmng zone might be ignored. Alternatively, that space might be treated as a second warning zone. The apparatus may generate a low-level warning when an object is detected within the second warning zone and a higher-level warning for a closer object. The warmng zone could be defined to taper or to curve such that it is wider further from the vehicle to take into account possible vanations in the vehicle's course. For example, as shown in Figure 10, the warning zone might be extended in a direction m which the vehicle is steenng As illustrated, an additional region 226 has been introduced into the warning zone of a nght-turning vehicle. As another example of apparatus, with reference to Figure 11 apparatus 1110 may be provided to monitor the internal volume 1100 of a vehicle for the purpose (for example) of intruder monitonng or control of air bags When such an embodiment is used as an intruder monitor, the warning zone will typically be limited to a volume that is approximately co-extensive with the mtenor space 1100 of a vehicle. However, (with reference to Figure 11) when used to control air bags, the wammg zone will typically include a plurality of regions 1112, each coπesponding to a region that might be occupied by a passenger in a seat 1114 that is protected by an air bag 1116. Upon detection of an accident, and the requirement to deploy air bags to protect the occupants of the vehicle, the apparatus determines which regions 1112 of its warning zone are occupied, and deploy only the coπesponding air bags. Additional regions 1120 of the warning zone may also be defined close to the air bags 1116. In the event that the system detects that any such additional region 1120 is occupied, deployment of the respective air bag will be suppressed to reduce the risk of injury to a person occupying that space. Figure 12 shows one aπangement in which apparatus including an antenna aπay and associated processing circuitry can be aπanged. The apparatus is constructed around a metal substrate 1210, formed as a rectangular plate. On one side of the plate is mounted an antenna 1212 aπay comprising several polymer plates, between which is disposed a plurality of transmitting and/or receiving elements 1216. On the other side of the substrate 1210 is carried a printed circuit board 1214 upon which is constructed electronic circuits that exchange signals with the elements 1216. In this embodiment, the dimension a is approximately 12cm and the dimension b is approximately 10cm. The apparatus is therefore self-contained and compact. In summary, prefeπed embodiments measure the position of objects and determine directly whether an intruder is present, or whether a hazard exists due to the position of the object with respect to, for example, an airbag. Prefeπed embodiments consist in a fixed-aperture radar sensor for vehicles, with a wide field of view which transmits short pulses, and use a multi-element antenna, a multi-channel swept-delay sampling or mixing receiver, a cross-channel delay processor, a digitally-stored or constructed definition of an arbitrary 3- dimensional warning zone, and a digitally-stored or constructed description of the beam pattern of the antenna. Such a sensor can be used to measure the 3-dimensional position of one or more objects within its field of view, determine whether each lies within the warning zone, and determine the radar cross-section of each object. The microwave position measuring sensor uses a small array of antennas and a wideband signal to measure the positions or shapes of objects within its field of view by a radar-like technique. The signal is emitted by one element of the aπay, is scattered from objects in the field of view, and a scattered portion is received a time interval later at each of the receiving elements, the interval being proportional to the distance to the object. The wide band signal (in one embodiment a short pulse) allows multiple objects to be resolved in terms of distance from the sensor. Each object gives rise to one such signal received at each element of the array, and the direction to that object is calculated by measuring and comparing the time at which the relevant signal is received at each antenna. The advantage of this approach is that the need for precision in determining the presence of an obstacle in the warning zone is met by measuring its angular position within a relatively broad antenna beam, rather than by using a larger antenna or higher frequency to provide a precisely-tailored or narrow beam. This permits the use of a smaller frequency/aperture ratio than hitherto. The frequency can be chosen on the basis of cost of devices (low frequency is good), size of antenna (high frequency is good), receiver performance (low frequency is good), scattering cross-section (depends on geometry, but wide relative bandwidth (high B, low F) reduces glint), and regulations. The beam pattern of each aπay element is known and its description is stored within the sensor's processing electronics. A description of a 3-dimensional warning zone is also stored to determine whether an object is in a position where a warning is appropriate. The field of view of the sensor, as determmed by the antenna beam patterns, the transmitter power, the desired minimum detectable object cross-section, and the noise level, contains the warning zone and exceeds it in maximum range in most directions. Targets may be isolated by range resolution of centimetres, and their directions found by comparing reception delays between elements of a small or minimum array. This yields a small device, adequate for any short- range applications , capable of 3D location of many range-separated static targets and of extension to synthetic or inverse synthetic aperture processing for moving targets or a moving platform. A single fixed wideband transmitter and three, four or more fixed receiving elements form such an array. As an illustrative embodiment, the receiving array elements may be arranged at the corners of a rectangle or cuboid, with the sides of the array similar to or less in length than the wavelength, and the transmitter may be positioned near the centre of the array. The elements may be dipoles or microstrip patches, or other small radiating conductive structures, For ease of manufacture and wide bandwidth, stacked patches may be used with advantage. The array may be constructed as a single assembly of conductors printed on a glass-fibre or plastic substrate, or assembled within a dielectric radome with its associated electronics and processing computer. In operation, these circuits are used to process the received signals, and differ from conventional antenna array processing circuits by replacing frequency shifting, or phase shifting at radio frequency, by frequency scaling. For each receiving element the output signal contains frequency components each of which is derived from its original radio frequency value by a constant divisor. The value of the divisor depends either on the rate of change of delay at which samples are obtained, or on the ratio of the target's approach speed to that of light, or both. An array of fixed wideband, wide beam antenna elements is provided, suitable for fitting within assemblies such as car bumpers or portable enclosures, without radio frequency phase shifting or combining circuits, nor with frequency-shifting circuits or a carrier frequency local oscillator, but with a short pulse generator associated with a transmitting element and wideband, switched sampling circuits for frequency scaling associated with each receiving element. The switched sampling receiver circuits are designed with low inter-channel timing errors to allow accurate relative timing of the received signals to be measured. The imaging sensor also includes processing means which are operable to (a) calculate timing of, and intervals between similar signals received at each channel from objects, (b) calculate the 3D position of each object observed from such timing and offsets, (c) compare such 3D position with a stored or constructed 3D warning zone, (d) activate a warning or indication if such 3D position is inside such warning zone, (e) calculate the antenna gains in the direction of such 3D position, and (f) calculate the radar cross-section of such object from the amplitude of the signal received from it. In addition to allowing remote detection and position measurement of obstacles, the operation of such a sensor at microwave frequencies or below allows a useful degree of penetration of solid materials. Thus the sensor may be placed behind the bumper of a car without requiring holes or special materials or treatments. It may also be positioned to detect and measure objects and obstacles behind other materials such as wood, plastic, concrete, brick and other nonmetallic materials. Metal or other objects may be detected within such nonmetallic materials. Measured objects may be stationary. Additional processing including target classification, tracking and imaging may be applied to improve the detection and measurement of moving objects. The application of such a sensor may include automotive obstacle detection outside a vehicle; providing a dnver with a complete situation assessment near the car; occupant position sensing within a vehicle; occupant identification and behaviour monitonng; collision warning for aircraft; landing aids for helicopters; fluid level measurement; secunty sensors for buildings; surveillance sensors for secure rooms and areas; manoeuvring aids for vehicles; earth moving equipment; traffic presence and movement, vehicle identification, etc. The present invention has further application to embodiments that have diverse applications. Several of these will now be descnbed Since RF electromagnetic signals can penetrate building matenals, embodiments of the invention that use such signals can be used to form positional images of objects behind walls and obstructions of bnck, stone, concrete, cinder block, wood, plasterboard etc Such objects may be located in a space behmd such a wall or obstruction, allowing the user to detect and measure the position or height of objects, people, animals, vehicles or surfaces in the space. This may have applications in secunty activities, search and rescue activities (e g. for earthquake, landslide or avalanche victims) and many other activities m a built environment Such objects may also be located within the wall or obstruction itself, including remforcing bar, studding, pipes, drains, beams and girders, and voids, cracks, gaps, ducts etc This is useful for inspection purposes, to detect the presence and map the positions of such objects or voids etc., to facilitate works or repairs, to determme the state of the wall or obstruction, to identify the nature of such objects or voids and so forth. Embodiments may be used to discnmmate between objects exhibiting different radar cross sections, for example, pipes of different diameters or different matenals, flat surfaces of different matenals, etc. Further embodiments may be used as a prosthetic for those with unpaired eyesight, providing an obstacle sensor and directional warmng device to warn of the approach of people, animals, obstacles, kerbstones (especially given the 3-dιmensιonal nature of positional information provided), trees, walls, and so forth. In a similar manner, embodiments of the invention may be used as a mght vision aid, allowing the detection and position measurement of objects in the absence of light, in fog or smoke. Likewise, further embodiments may be used in addition to or m place of optical or infra-red sensors in conjunction with unmanned vehicles such as robots in secunty applications including bomb disposal, m manufactuπng, mining, warehousing and storage, tunnelling, decommissioning or construction of nuclear and other hazardous plant, demolition, building construction work, among other possibilities. Additionally, the sensor may be used withm a road vehicle to determine the position of objects which may be the head or chest of an occupant of the vehicle in relation to hazards associated with crashes and impacts, and with counter-measures such as airbags or belts. In this connection it may not be necessary to use the sensor itself with pattern matching techniques to classify the object, but merely to measure its position and motion. In this case the sensor is distinguished from pnor art in that, by companson with existmg radar sensors, which measure the distance between occupant and the airbag (or other item, as the case may be) directly, and which are vulnerable to interference with the beam from objects earned, worn or inadvertently placed by the occupant, such as arms, legs, feet, hands, books, newspapers, boxes, etc. Such an embodiment can measure the angular position of the object as well as the distance and can be located m a position from which its beam is less likely to be interrupted. It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. The Applicant asserts design right and/or copyright in the accompanying drawings. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims: CLAIMS 1 Apparatus for obtaining positional information relating to an object, the apparatus compnsing: an aπay compnsing a transmitting element and a plurality of receiving elements; a signal generating stage for applying a senes of pulses to the transmitting element to cause it to transmit a signal, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; a detection stage for detecting signals reflected to the receiving elements and for generating output signals representative of the received signals; and a processing stage operable by application of a truncated cross-correlation function to detect the interval between signals received by a plurality of the receiving elements, whereby to determine an angular position of an object from which the transmitted signal has been reflected. 2. Apparatus according to claim 1 in which the truncated cross-coπelation function is operable to shift one output signal with respect to another over a range which is less than the duration of the signals, and preferably less than the duration of a pulse. 3. Apparatus according to claim 1 or claim 2 m which the truncated cross-coπelation function is operable to shift one output signal with respect to another over a range in which the maximum offset in either direction is less than 5 times the time that would be taken for the transmitted signal to travel directly from one receiving element to another, and preferably less than or equal to 3, 2 or 1 times this value. 4. Apparatus according to any of the precedmg claims m which the signal has a charactenstic wavelength λ and the truncated cross-coπelation function has an interval of coπelation which is a small multiple of . 5. Apparatus according to claim 5 in which the interval of coπelation is less than 10, 5, or 2 λ , or lλ. 6. Apparatus according to any precedmg claim in which the receivmg elements are spaced apart by a distance D and the truncated cross-coπelation function has an interval that is less than a small multiple ofD. Apparatus according to claim 6 in which the interval of coπelation is less than 5, 2, 1.5 or ID, or, where the detection field of at least one element is less than 180 degrees, it may be 0.9D, 0.8D or 0.7D. Apparatus according to any preceding claim in which the processing stage is operative to determine the distance from the aπay of an object from which a signal has been reflected. 9. Apparatus according to any preceding claim m which the processing stage is operative to identify a maximum value of the cross coπelation function. 10. Apparatus for obtaining positional information relatmg to an object, preferably accordmg to any precedmg claim, in which the processing stage is operable to detect the interval between a signal bemg received by a first set of any two or more of the receiving elements and to determme a first angular position of an object from which the transmitted signal has been reflected; and to determme the interval between a signal being received by a second set of any two or more of the receivmg elements and to determine a second angular position of an object from which the transmitted signal has been reflected; the processing stage being operable by application of a truncated cross-coπelation function. 11. Apparatus according to claim 10 in which the first and second angular positions are measured in planes that are substantially not parallel to one another. 12. Apparatus according to claim 11 in which the said planes are approximately normal to one another. 13. Apparatus according to any preceding claim in which the processing stage is operative to determine the distance from the aπay of an object from which a signal has been reflected. 14. Apparatus according to claim 13 in which the processing stage is operable to determine a coordinate in three-dimensional space of an object from which the transmitted signal has been reflected. 15. Apparatus according to any one of claims 10 to 14 in which at least one of the first and second set of receiving elements includes three or more elements which are disposed such that the spacings between elements of at least two pairs of elements included in that set are unequal. 16. Apparatus according to claim 12 in which the spacing (D) between the elements of one pair is approximately equal to (for example between 50% and 200% or between 75 and 150% of) a characteristic wavelength λ of the signal, and preferably the spacing between a second pair of elements is approximately equal to 3λ/4, or 3 D/4, and preferably the ratio of the spacing of the elements in one of the first and second pairs to the spacing of the elements in the other of the first and second pairs is between 0.5 and 1 or 0.75 and 0.9. 17. Apparatus according to claim 10 or 16 in which the processing stage is operative to perform a truncated cross-coπelation between the signals received by each pair of elements and to determine the product or otherwise compare of the result of the cross-coπelations. 18. Apparatus according to any preceding claim further comprising an output stage operative to generate an output for presentation of positional information relating to the object to a user. 19. Apparatus according to claim 18 in which the output includes at least one of an audible and a visual signal. 20. Apparatus for obtaining positional information relating to an object, optionally in accordance with any preceding claim, the apparatus comprising: a warning zone definition stage for defining a warning zone (in two or three dimensions) within a detection field of the apparatus; and a discrimination stage for determining whether a detected object is within the warning zone; in which the warning zone is defined as a three-dimensional region within the detection field. 21. Apparatus according to claim 20 in which the warning zone is contained within and is smaller than the detection field of the apparatus. 22. Apparatus according to claim 20 or claim 21 in which the shape of the warning zone is dissimilar from the shape of the detection field of the apparatus. 23. Apparatus according to any one of claims 20 to 22 in which the shape of the warning zone is non- circular or non-spherical. 24. Apparatus according to claim any one of claims 20 to 23 in which the warning zone definition stage includes an algorithm that defines a warning zone as a function of a coordinate within the detection field. 25. Apparatus according to any one of claims 20 to 24 further comprising an object location stage for determining the position of a detected object within the detection field of the apparatus. 26. Apparatus according to any one of claims 20 to 25 in which the discrimination stage includes a coordinate generating stage for generating a coordinate of a detected object, which coordinate is then compared with the warning zone. 27. Apparatus according to claims 20 to 24 in which the discrimination stage is operable to determine the coordinates of the detected object and compare the determined coordinates with the coordinates of the warning zone to determine whether the object is within the warning zone. 28. Apparatus according to any one of claims 20 to 27 in which the warning zone definition stage defines at least a limiting value of one or more ordinates of a coordinate within the detection field. 29. Apparatus according to claim 28 in which the warning zone definition stage defines at least a limiting value of one or more angles of a polar coordinate within the detection field. 30. Apparatus according to claim 29 in which the warning zone definition stage defines at least a limiting value of a range of a polar coordinate within the detection field. 31. Apparatus according to any one of claims 20 to 30 in which the warning zone includes a plurality of discontinuous spatial regions. 32. Apparatus according to any one of claims 20 to 31 in which the discrimination stage is operative to generate an output signal indicative that the object is within the warning zone. 33. Apparatus according to any one of claims 20 to 32 further operative to issue a warning, for example at least one of an audible, a visual and a tactile warning to a user upon detection of an object in the warning zone. 34. Apparatus according to any one of claims 20 to 33 in which the warning zone definition stage defines a plurality of non-coextensive warning zones, and preferably in which the discrimination stage is operative to generate an output signal indicative of which of the plurality of warning zones contains the object. 35. Apparatus according to claim 34 in which the discrimination stage is operable to apply different logic to at least two of the zones. 36. Apparatus according to any one of claims 20 to 35 in which the warning zone is limited in range and/or is approximately cuboid. 37. Apparatus according to any of claims 20 to 36 in which the discrimination stage is operative to analyse a characteristic of an object outside of the warning zone. 38. Apparatus according to claim 37 in which the discrimination stage is operative to track an object outside the warning zone and to predict its entry into the warning zone. 39. Apparatus according to any one of claims 20 to 38 for use on a vehicle, and optionally including the vehicle. 40. Apparatus according to claim 39 in which at least one of the shape and a relevant dimension of a warning zone is at least in part determined by a coπesponding shape and dimension of the vehicle. 41. Apparatus according to claim 40 in which a warning zone has a width that is equal to the width of the vehicle plus a predetermined amount. 42. Apparatus according to claim 41 in which the predetermined amount is less than the width of the vehicle. 43. Apparatus according to any one of claims 39 to 42 in which the warning zone has a height that is equal to the height of the vehicle plus a predetermined amount. 44. Apparatus according to claim 43 in which the predetermined amount is less than the height of the vehicle. 45. Apparatus according to any one of claims 39 to 44 in which the warning zone has a lower surface that is substantially planar and spaced above a road surface upon which the vehicle is supported. 46. Apparatus according to any one of claims 39 to 45 in which at least one of the shape and a relevant dimension of the warning zone is changed in response to vehicle operating conditions. 47. . Apparatus according to claim 46 in which the vehicle operating conditions include at least one of speed, direction of travel, and ambient environmental conditions. 48. Apparatus according to any preceding claim operative to transmit an electromagnetic signal. 49. Apparatus according to claim 48 in which the electromagnetic signal is microwave radiation. 50. Apparatus according to claim 49 in which the transmitted signal has a frequency of between 0.5 and 77 GHz. 51. Apparatus according to claim 50 in which the transmitted signal has a frequency of between 2 and 25 GHz. 52. Apparatus according to claim 51 in which the transmitted signal has a frequency of approximately one of 0.5GHz, 1GHz, 6 GHz, 10 GHz or 2-2.5 GHz. 53. Apparatus according to claim 52 in which the signal is approximately 2.45 GHz. 54. Apparatus according to any one of claims 48 to 53 in which the transmitted signal has a relative bandwidth with respect to the centre frequency of between 3 and 33%. 55. Apparatus according to claim 54 in which the transmitted signal has a relative bandwidth with respect to the centre frequency of between 5 and 25%. 56. Apparatus according to claim 55 in which the transmitted signal has a relative bandwidth with respect to the centre frequency of between 10 and 20%. 57. Apparatus according to claim 56 in which the transmitted signal has a relative bandwidth with respect to the centre frequency of approximately 15%. 58. Apparatus for obtaining positional information relating to an object according to any preceding claim, which is operative to transmit a signal into a detection field and to detect a signal reflected from an object in the detection field, in which the spatial length of the transmitted signal during its propagation is approximately the same as a dimension of the smallest objects that the apparatus is intended to resolve. 59. Apparatus for obtaining positional information relating to an object according to claim 58 in which the spatial length of the transmitted signal during its propagation is between 50 and 200% of the smallest objects that the apparatus is intended to resolve. 60. Apparatus for obtaining positional information relating to an object according to claim 58 in which the spatial length of the transmitted signal during its propagation is between 75 and 150% of the smallest objects that the apparatus is intended to resolve. 61. Apparatus according to any preceding claim in which the spatial length of the transmitted signal during its propagation is, in order of magnitude, not greater than 1.0 m, or preferably not greater than 0.3 m, 0.1 m, 0.03 m, or even 0.01 m. 62. Apparatus according to claim 48 or claim 49 in which the spatial length of the transmitted signal during its propagation is less than 10 wavelengths, or preferably less than 6, 5, 3, 2 or even 1 wavelengths. 63. Apparatus according to any preceding claim in which the signal is received at the apparatus at receiving elements spaced by a distance being of the same order of magnitude as a characteristic wavelength λ of the signal. 64. Apparatus according to claim 63 in which the receiving elements are spaced apart by a distance nλ where 0.5 ≤ n < 10, preferably 1 ≤ n ≤ 5. 65. Apparatus according to any preceding claim, in which: the signal generating stage applies a series of m pulses to the transmitting element to cause it to transmit a signal, at times t n where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; the detection stage detects a signal reflected to the receiving elements at times r n and generates an output signal representative of the received signal; wherein the value of r n - 1„ varies as some function of n. 66. Apparatus according to claim 65 in which the value of r„ - t„ changes linearly with n. 67. Apparatus according to claim 64 in which successive outputs of the detection stage are stored in a storage means, the storage means being operable to output a signal of substantially the same shape as the received signal, but with a duration that is increased in time. 68. Apparatus according to any preceding claim including a sampling stage operative under the control of a timing stage selectively to pass or to interrupt the passage of signals from the receiving elements to the detection stage. 69. Apparatus according to claim 68 in which, upon activation by the timing stage, the sampling stage passes signals to the detection stage for an aperture time ta. 70. Apparatus according to claim 69 comprising a respective sampling stage for each receiving element. 71. Apparatus according to claim 70 in which each sampling stage is connected to the timing stage by a respective signal delay line, within which delay line a signal is delayed by a time not less than t a / 2. 72. Apparatus for obtaining positional information relating to an object according to any preceding claim, contained within a single housing. 73. Apparatus according to claim 72 that is hand-holdable in use. 74. Apparatus according to claim 73 intended to provide information about the location of objects within or behind a wall. 75. Apparatus according to any preceding claim comprising an antenna aπay and processing means constructed as a single assembly. 76. Apparatus according to claim 75 in which the processing means operates to provide all functional electrical signals to and receive all functional electrical signals from the array. 77. Apparatus according to any one of claims 1 to 76 for use in a vehicle. 78. Apparatus for a land vehicle for obtaining positional information relating to an object, according to any preceding claim, comprising: means for transmitting a probe signal towards the object; means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object; and detecting means, coupled to the receiving means, for detecting the relative timing of the returned probe signals as received at the plurality of locations; whereby the positional information for the object can be determined from said relative timing. 79. Apparatus for obtaining positional information relating to an object, comprising: a transmitting stage for transmitting a probe signal towards the object, the transmitting stage comprising: a signal generating stage; and at least one transmitting element; a receiving stage for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object, the receiving stage comprising; at least one receiving element at the plurality of spaced apart locations; and a detecting stage for detecting the relative timing of the returned probe signals as received at the ; plurality of locations, the detecting stage comprising; a detection stage, coupled to the receiving stag iee;; whereby positional information for the object can be determined from the relative timing; and wherein: the signal generating stage applies a series of m pulses to the transmitting element to cause it to transmit a signal, at times t„ where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; the detection stage detects a signal reflected to the receiving elements at times r„ and generates an output signal representative of the received signal; and wherein the value of r„ - t„ varies as some function of n. 80. Apparatus for obtaining positional information relating to an object, for use on a vehicle, optionally in accordance with any preceding claim, for resolving the angular position of an object using non-Doppler radar. 81. Apparatus optionally according to any preceding claim, for use on a vehicle, for obtaining positional information relating to an object external of or internal to the vehicle, the apparatus being operative to generate 3-dimensional positional data for the object. 82. Apparatus according to any one of claims 77 to 81 in which the apparatus has a detection field within a passenger compartment of the vehicle. 83. Apparatus according to any one of claims 77 to 82 in which the positional data includes at least one of the range, azimuth and elevation of the object. 84. Apparatus according to any one of claims 77 to 83 in which the antenna array is carried on a fixed location on the vehicle. 85. Apparatus according to any one of claims 77 to 84 in which the antenna aπay is located within a component of the vehicle, preferably a non-metallic component. 86. Apparatus according to any one of claims 77 to 85 in which the antenna array is located within a bumper of the vehicle. 87. Apparatus according to any one of claims 77 to 86 in which the antenna aπay is located within a non- metallic bumper. 88. Apparatus according to any one of claims 77 to 87 further comprising alerting apparatus for alerting a vehicle driver to the presence of a detected object. 89. Apparatus according to claim 88 in which the alerting apparatus is operative to generate an audible warning. 90. Apparatus according to claim 89 in which the audible warning includes a verbal warning. 91. Apparatus according to any one of claims 88 to 90 in which the alerting apparatus is operative to generate a visual warning. 92. Apparatus according to claim 91 in which the visual warning includes a visible representation of the position of an object detected by the apparatus. 93. Apparatus according to claim 92 further comprising a display upon which is presented a visual representation of a detection field of the apparatus and an object within the detection field. 94. Apparatus for obtaining positional information relating to an object, for use on a vehicle, optionally in accordance with any preceding claim, using preferably non-Doppler radar and being operative to determine a radar cross-section of an object. 95. Apparatus for obtaining positional information relating to an object, for use on a land vehicle optionally in accordance with any preceding claim, including a transmitting element for transmitting radiation into a detection field, a receiving element for receiving radiation reflected from an object in the detection field, and a processing stage, which is operative to analyse the signals from the receiving element to derive qualitative information relating to the object. 96. Apparatus according to claim 95 in which the processing stage is operative to compare information relating to an object at successive different angular positions against a look-up table. 97. Apparatus according to claim 95 or 96 in which the processing stage is operative to determine a radar cross-section of the object. 98. Apparatus according to claim 97 in which the processing stage is operative to compare the radar cross section with a threshold value of radar cross-section and to generate a warning signal in dependence upon the result of the comparison. 99. Apparatus according to any one of claims 95 to 98 in which the processing stage is operative to determine an evolution of angular position of the object. 100. Apparatus according to any of one claims 95 to 99 in which the processing stage is operative to predict a path of movement of the object. 101. Apparatus for obtaining positional information relating to an object substantially as herein described with reference to the accompanying drawings. 102. A vehicle equipped with apparatus according to any preceding claim. 103. A motor road vehicle being equipped with a driver warning system, which system comprises apparatus according to any one of claims 1 to 100 for obtaining positional information relating to an object external of the vehicle, the apparatus being operative to generate 3-dimensional positional data for the object. 104 A road vehicle according to claim 103 in which the aπay of the apparatus is contained within a non- metallic bumper of the vehicle. 105 A road vehicle according to claim 104 compnsing a display instrument operative to process information obtained by the apparatus and to generate a display therefrom for an operator of the vehicle. 106. A control system for air bags in a motor road vehicle, the control system compnsmg apparatus according to any one of claims 1 to 100 m which the apparatus has a detection field withm a passenger compartment of the vehicle, and m which the processing stage is operational to determme the occupancy of a seat equipped with a passenger air bag, and to suppress deployment of the bag in dependence on the occupancy of the seat, for example, if the seat is unoccupied, the occupant is too close to the air bag, or if the occupant is determined to have violated a substantially planar surface or a substantially cuboid volume. 107. A device for obtaining information about objects within or behind a wall compnsmg a apparatus according to any one of claims 1 to 100. 108 A hand-held tool incorporating apparatus according to any one of claims 1 to 100. 109 Apparatus for obtaining positional information relating to an object, compnsing: means for transmittmg a probe signal towards the object, said transmittmg means compnsmg a transmitting element; means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object, said receiving means compnsing a plurality of receiving elements; and detectmg means, coupled to the receiving means, for detecting the relative timing of the returned probe signals as received at the plurality of spaced apart locations; whereby the positional information for the object can be determined from said relative timing; and wherem the transmittmg element and receiving elements are disposed on a common substrate. 110 An electromagnetic (optionally microwave) antenna array optionally for use in apparatus for obtaining positional information relating to an object in accordance with any one of claims 1 to 100, the aπay including a transmitting element and a plurality of receiving elements, the transmitting and receivmg elements bemg disposed on a common substrate. 111. An electromagnetic antenna aπay according to claim 109 or 110 including a single transmittmg element. 112. An electromagnetic antenna aπay according to any one of clauns 109 or 111 includmg three receivmg elements arranged non-collinearly. 113. An electromagnetic antenna array optionally for use m apparatus for obtainmg positional information relatmg to an object, the array including a transmitting element and at least three receivmg elements aπanged non-colhnearly, the transmitting and receiving elements being disposed on a common substrate. 114 An electromagnetic antenna aπay according to claims 112 or 113 in which the receivmg elements are aπanged substantially at the vertices of a right-angled triangular locus. 115. An electromagnetic antenna aπay according to any one of claims 109 to 114 in which the receiving elements are spaced apart by a distance that is the same order of magnitude as the wavelength λ of the radiation that it is intended to transmit and receive. 116. An electromagnetic antenna aπay according to claim 115 in which the receiving elements are spaced apart by a distance mλ where m is less than 10, and preferably less than 8, 5, 3, or 2, and m is greater than 0.1 and preferably greater than 0.2, 0.3 or 0.5. 117. An electromagnetic antenna aπay according to claim 115 or 116 including four receiving elements arranged non-collinearly. 118. An electromagnetic antenna aπay according to any one of claims 115 to 117 including at least three receiving elements aπanged non-collinearly such that there is an axis about which the aπay is asymmetrical. 119. An electromagnetic antenna aπay including at least three receiving elements aπanged non-collinearly such that there is an axis about which the aπay is asymmetrical. 120. An electromagnetic antenna aπay according to any of claims 109 to 119 in which the receiving elements are aπanged substantially at the vertices of a quadrilateral. 121. An electromagnetic antenna aπay according to any of claims 109 to 120 in which the receiving elements are aπanged substantially at the vertices of a trapezial locus. 122. An electromagnetic antenna aπay according to any of claims 109 to 121 in which the trapezial locus is rectangular. 123. An electromagnetic a ntenna array according to any of claims 109 to 121 in which the trapezial locus has long and short parallel sides and preferably the short side is between 0.5 and 1 times the length of the long side, or in which the quadrilateral has two opposing angles which are substantially right angles, the other two angles not being right angles. 124. An electromagnetic antenna aπay according to any of claims 109 to 122 in which the trapezial locus has long and short parallel sides, the length of the shorter side being approximately the wavelength λ of the radiation that the aπay is intended to transmit and receive, and the length of the longer side is approximately 3λ/2. 125. An electromagnetic antenna aπay according to any one of claims 109 to 124 for use in apparatus according to any one of claims 1 to 100. 126. An electromagnetic antenna aπay optionally in accordance with any one of claims 109 to 125, for use in apparatus for obtaining positional information relating to an object, which apparatus may be in accordance with any preceding claim, the aπay including a transmitting element and a plurality of receiving elements, in which the spacing of two pairs of the receiving elements in a common direction is unequal. 127 Apparatus for obtaining positional information relating to one or more objects, the apparatus being operative in an operating cycle for each of m steps in which n = 1 , 2 . m, the apparatus including: a signal generating stage operative, simultaneously with or at a fixed time after a transmitting tngger instant t π to generate a signal, and a transmitting element to transmit said signal into a detection field; a plurality of spaced receiving elements operative simultaneously with or at a fixed time after a receiving tngger instant r n to receive a portion of the signal reflected from one or more objects m the detection field, the interval r„ - t n varying as a function of n and having a magnitude in a range coπesponding to the times of travel of a signal reflected from an object within the detection field; means for identifying the values of n at which signals reflected from one object are received at two or more receiving elements and thereby detecting the time taken, and therefore the distance travelled, by the signals from the transmitting element to the vanous receiving elements; and means for calculating the position of the obj ect from the vanous path lengths thereby identified 128 Apparatus for generating an image of objects within or through a solid object according to any one of clauns 1 to 101. 129 Apparatus according to claim 128 in which the solid object is a wall. 130 Apparatus according to any one of claims 1 to 101 for providing an image of an environment in conditions that human vision is compromised. 131 Apparatus according to claim 130 in which vision is compromised by the physiological condition of a user. 132 Apparatus according to claim 130 or claim 131 m which vision is compromised by environmental conditions. 133 A method for obtaining positional information relating to an object, optionally in apparatus in accordance with any one of claims 1 to 100, compnsmg: applymg a senes of pulses to a transmitting element of an (optionally fixed) array to cause it to transmit a signal, such that at least a portion of the signal is reflected from the object to be received by the receiving elements; detecting signals reflected to receiving elements of the array and generating output signals representative of the received signals; and applying a truncated cross-coπelation function to the output signals to detect the interval between signals received by a plurality of the receiving elements, whereby to determme an angular position of an object from which the transmitted signal has been reflected 134 A method according to claun 133 m which the truncated cross-correlation function compnses shiftmg one output signal with respect to another over a range which is less than the duration of the signals, and preferably less than the duration of a pulse 135 A method according to claim 133 or 134 m which the truncated cross-coπelation function compnses shiftmg one output signal with respect to another over a range in which the maximum offset in either direction is less than 5 times the tune that would be taken for the transmitted signal to travel directly from one receivmg element to another, and preferably less than or equal to 3, 2 or 1 tunes this value. 136 A method according to any of the preceding method claims in which the signal has a charactenstic wavelength λ and the truncated cross-coπelation function has an interval of correlation which is a small multiple of λ 137 A method according to claim 136 m which the interval of coπelation is less than 10, 5 , or 2 λ , or even λ 138 A method according to any of the preceding method claims in which the receiving elements are spaced apart by a distance D and the truncated cross-coπelation function has an interval that is less than a small multiple of D 139 A method according to claim 138 in which the interval of coπelation is less than 5, 2, 1 5 or ID, or, where the detection field of at least one element is less than 180 degrees, it may be 0.9D, 0 8D or 0.7D 140 A method according to any of the preceding method claims further including determining the distance from the aπay of an object from which a signal has been reflected 141 A method according to claim 140 in which determming the distance from the array includes a step of multiplying the time taken for the signal to be received by the speed of propagation of the signal. 142 A method according to any of the preceding method claims m which a maximum value of the cross coπelation function is identified. 43 A method according to any one of claims 133 to 142 including the steps of. determining the interval between a signal being received by a first set of any two or more of the receiving elements; calculating a first angular position of an object from which the transmitted signal has been reflected; determining the interval between a signal being received by a second set of any two or more of the receivmg elements; and calculating a second angular position of an object from which the transmitted signal has been reflected. 44 A method according to claun 143 in which the first and second angular positions are measured m planes that are substantially not parallel to one another. 45 A method accordmg to claim 144 m which the said planes are approximately normal to one another. 46 A method according to claim 145 includmg determming a coordinate in three-dimensional space of an object from which the transmitted signal has been reflected. 47 A method according to any one of claims 143 to 146 in which at least one of the first and second set of receiving elements includes three or more elements which are disposed such that that set includes at least two parrs of elements, the spacing of elements in the two pairs being unequal. 8 A method according to claim 147 m which the spacing (D) between the elements of one parr is approximately equal to (for example between 50% and 200% or between 75 and 150% of) a charactenstic wavelength λ of the signal, and preferably the spacing between a second pair of elements is approximately equal to 3λ/4, or 3 D/4, and preferably the ratio of the spacing of the elements m one of the first and second pairs to the spacing of the elements in the other of the first and second pairs is between 0.5 and 1 or 0.75 and 0.9 149 A method according to claim 147 or 148 in which the truncated cross-coπelation is performed between the signals received by each pair of elements and the product or another companson of the result of the cross-coπelations is determined. 150 A method according to any preceding method claim further compπsing generating an output for presentation of positional information relating to the object to a user. 151 A method according to claim 150 in which the output includes at least one of an audible and a visual signal. 152 A method for obtaining positional information relatmg to an object, optionally in accordance with any preceding method claim, compnsmg- defining a warning zone (in two or three dimenstions) within a detection field; and determining whether a detected object is within the warning zone: wherein the wammg zone is defined as a three-dimensional region within the detection field. 153 A method according to claim 152 in which the warning zone is contained within and is smaller than the detection field. 154 A method according to claim 152 or 153 m which the shape of the warning zone is dissimilar from the shape of the detection field. 155 A method accordmg to any one of claims 152 to 154 in which the warning zone mcludes a region defined m two dimensions within the detection field, preferably in which the warning zone includes a planar surface withm the detection field. 156. A method according to any one of claims 152 to 155 m which the warning zone is defined as a three- dimensional region withm the detection field. 157. A method accordmg to any one of clauns 152 to 156 in which the warning zone is defined by an algonthm as a function of a coordinate within the detection field. 158. A method according to any one of claims 152 to 157 further includmg generation of a co-ordmate of a detected object. 159. A method accordmg to claim 158 in which the generated co-ordinates are compared with co-ordmates of the warmng zone to determine whether the object is withm the warning zone. 160. A method accordmg to any one of claims 152 to 159 in which the warning zone is defined by at least a limiting value of one or more ordinates of a coordmate within the detection field. 161 A method according to claim 160 in which the wammg zone is defined by at least a limiting value of one or more angles of a polar coordinate withm the detection field. 162 A method according to any one of claims 152 to 161 in which the warning zone is defined by at least a limiting value of a range of a polar coordinate within the detection field 163 A method according to any one of claims 152 to 162 in which the warning zone includes a plurality of discontinuous spatial regions. 164 A method according to any one of claims 152 to 163 further compnsing the step of generating an output signal indicative that the object is within the warning zone. 165 A method according to any one of claims 152 to 164 further compnsing the step of issumg a warning to a user upon detection of an object in the warning zone. 166 A method according to any one of claims 152 to 165 in which there is defined a plurality of non- coextensive warning zones 167 A method according to claim 166 which includes a step of generating an output signal indicative of which of the plurality of warning zones contains the object. 168 A method according to claim 166 or claim 167 including a step of applying different logic to at least two of the zones. 169 A method according to any of claims 152 to 168 further compnsing the step of analysing a characteπstic of an object outside of the warning zone. 170 Apparatus accordmg to claim 169 in which the step of analysing a charactenstic compnses trackmg an object outside the warning zone and predicting its entry into the warning zone. 171 A method according to any one of claims 152 to 170 further compnsing the step of issumg at least one of an audible and a visual warning to a user upon detection of an object in a warning zone. 172 A method accordmg to any one of claims 152 to 171 earned out on a vehicle, m which at least one of the shape and a relevant dimension of a warning zone is at least in part determined by a coπespondmg shape and dimension of the vehicle. 173 A method according to claim 172 which includes monitonng operating conditions of the vehicle and changing at least one of the shape and a relevant dimension of a warning zone in response to vehicle operating conditions. 74 A method accordmg to claim 173 in which the vehicle operating conditions include at least one of speed, direction of travel, and ambient environmental conditions. 75 A method accordmg to 173 or claim 174 in which the distance to which the warning zone extends in the direction of travel of the vehicle is increased with the speed of the vehicle. 76 A method according to any one of claims 173 to 175 in which the extent to which the warning zone extends to one side of a longitudinal axis of the vehicle is increased in a direction m which the vehicle is turning. 177 A method according to any one of the preceding method claims in which the signal is an electromagnetic signal 178 A method according to claim 177 in which the electromagnetic signal is microwave radiation. 179 A method according to claim 178 in which the electromagnetic signal has a frequency of between 0.5 and 77 GHz. 180 A method according to claim 179 in which the electromagnetic signal has a frequency of between 2 and 25 GHz. 181 A method according to claim 179 in which the electromagnetic signal has a frequency of approximately one of 0.5GHz, 1GHz, 6 GHz, 10 GHz or 2-2.5 GHz. 182 A method according to claim 181 m which the electromagnetic signal has a frequency of approximately 2.45 GHz 183 A method according to any one of claims 177 to 182 in which the transmitted signal has a relative bandwidth to the centre frequency between 10 and 20%. 84 A method according to claim 183 m which the transmitted signal has a relative bandwidth to the centre frequency between 5 and 25%. 85 A method according to claim 184 m which the transmitted signal has a relative bandwidth to the centre frequency between 3 and 33% 86 A method according to claim 185 m which the transmitted signal has a relative bandwidth to the centre frequency of approximately 15%. 87 A method according to any one of claims 180 to 186 m which an angular position of an object is resolved with respect to a predetermined datum. 88 A method for obtaimng positional information relating to an object, optionally accordmg to any one of clauns 142 to 187 compnsing transmitting a signal into a detection field and detecting a signal reflected from an object in the detection field, m which the spatial length of the transmitted signal dunng its propagation is approximately the same as (say between 50 and 200% or 75 and 150% of) a dimension of the smallest objects that it is mtended to resolve. 89 A method accordmg to any precedmg method claim in which the spatial length of the transmitted signal dunng its propagation is, in order of magnitude, not greater than 1.0 m, and preferably not greater than 0.3 m, 0.1 m, 0.03 m, or even 0.01 m. 90 A method according to claim 188 or 189 in which the spatial length of the transmitted signal dunng its propagation is less than 10 wavelengths, or less than 6, 5, 3, 2 or even 1 wavelengths. 191 A method according to any preceding method claim in which the signal is received at receiving elements spaced by a distance being of the same order of magnitude as a charactenstic wavelength λ of the signal. 192 A method according to claim 191 m which the receiving elements are spaced apart by a distance nλ where 0.5 < n < 10, and preferably 1 ≤ n ≤ 5. 193 A method according to any preceding method claim in which a senes of pulses compnses m pulses to cause the transmitting element to transmit a signal, at times t n where n = 1, 2 ... m; and reflected signals are detected by the receiving elements at times r„, compnsing the steps of generating an output signal representative of the received signal; wherein the value of r n - t n vanes as some function of n. 194 A method according to claim 193 m which the value of r„ - t„ changes linearly with n. 195. A method according to claim 193 or 194 further compnsing stonng values of the output signal coπespondmg to signals received at times r n . 196 A method according to claim 195 further compnsing outputtmg a signal of substantially the same shape as the received signal, but with a duration which is increased in time. 197. A method according to any preceding method claim for obtaining positional information relating to an object, performed on a vehicle. 198. A method according to claim 197 for resolving the angular position of an object using non-Doppler radar. 199. A method according to claim 197 or 198 for obtaining positional information relatmg to an object external of the vehicle, in which 3 -dimensional positional data for the object is generated. 200. A method according to any one of claims 197 to 199 for obtaining positional information relatmg to an object internal to the vehicle, in which 3 -dimensional positional data for the object is generated. 201. A method according to any one of claims 197 to 200 in which a detection field is withm a passenger compartment of the vehicle. 202. A method according to any one of claims 197 to 201 in which the positional data includes at least one of the range, azimuth and elevation of the object. 203 A method according to any one of claims 197 to 202 which is earned out by apparatus including an antenna array that is earned on a fixed location on the vehicle. 204 A method according to any one of claims 197 to 203 in which the antenna aπay is located withm a non- metallic component of the vehicle. 205. A method according to any one of claims 197 to 204 in which the antenna aπay is located within a non- metallic bumper of the vehicle. 206. A method according to any one of claims 197 to 205 further comprising a step of alerting a vehicle 5 driver to the presence of a detected object. 207. A method according to claim 206 in which the alerting step includes generating an audible warning. 208. A method according to claim 207 in which the audible warning includes a descriptive verbal warning. 0 209. A method according to any one of claims 206 to 208 in which the alerting step includes generating a visual warning. 210. A method according to claim 209 in which the visual warning includes an image of the position of a 5 detected object. 211. A method according to claim 209 further comprising a step of presenting a visual representation of a detection field and an object within the detection field. 0 212. A method for obtaining positional information relating to an obj ect, performed on a vehicle optionally in accordance with any preceding method claim using non-Doppler radar, the method including determining a radar cross-section of an object. 213. A method for obtaining positional information relating to an obj ect, in accordance with any preceding method claim, including transmitting radiation into a detection field, receiving radiation reflected from an object in the detection field, and analysing received signals to derive qualitative information relating to the object. 214. A method according to claim 213 in which a radar cross-section of the object is determined. 215. A method according to claim 213 or 214 in which the radar cross section is compared with a threshold value of radar cross-section and a warning signal is issued in dependence upon the result of the comparison. 216. A method according to any one of claims 213 to 215 in which an evolution of angular position of the object is determined. 217. A method according to any one of claims 213 to 216 in which a path of movement of the object is predicted. 218. A method according to any one of claims 213 to 217 in which in the analysis step the signals are modified to compensate for angular variation in sensitivity of the receiving element. 219. A method according to any one of claims 213 to 218 in which in the analysis step the signals are modified to compensate for the range of the object from which the signals are reflected. 220. A method according to any one of claims 213 to 219 in which the analysis step includes making a companson between a received signal and a pattern coπespondmg to a signal from a known class of objects 221 A method accordmg to claim 220 in which the companson includes identification of charactenstic features of the received signal 222 A method according to claim 221 in which the characteπstic features include at least one of minima, maxima, and zero-crossmgs 223 A method for obtaining positional information relating to an object substantially as herein descnbed with reference to the accompanying drawings 224 A method of controlling deployment of air bags in a vehicle, in which a method accordmg to any preceding method claim is applied to determine the occupancy of a seat equipped with a passenger air bag, and deployment of the bag is suppressed m dependence on the occupancy of the seat, for example, if the seat is unoccupied or the occupant is too close to the air bag. 225 A method of obtaining positional information relating to an object in apparatus m accordance with any one of claims 1 to 100, the method compnsing an operating cycle having m steps in which n = 1, .. m, each step compnsing: (a) generatmg a signal at a given timing relationship with respect to a transmitting tngger instant t π and transmitting it into a detection field; and (b) receiving at a given timing relationship with respect to a receiving tngger instant r„ at least a portion of the signal reflected from the object; wherein the interval r„ - t n vanes as a function of n. 226 A method accordmg to claim 225 in which the method further compnses- c) providing at least two spaced receiving elements and identifying the values of n at which signals reflected from the object are received by the receiving elements. 227 A method according to claim 226 including m step c) a companson of the amplitude of signals received by vanous of the receiving elements. 228 A method accordmg to any one of claims 225 to 227 in which step c) includes companson of charactenstic features of the received signals, such features including at least one of zero-crossmgs, maxima and minima. 229 A method accordmg to any one of claims 225 to 228 m which a truncated correlation is earned out m step c), and in which the coπelation coefficient is approximately a cosinusoidal function of each offset angle (θ-θ 0 ), where θ 0 is the angle offset of the object in the plane contammg the relevant antenna pair in which the coπelation coefficients are determmed at a small sample of angles, separated by less than half the smusoidal wavelength, to estimate the direction of the maximum, followed by further samples close to that direction to refine the estimate. 230 A method according to any one of claims 225 to 229 including a step of selecting reflections denoted by coπelation maxima which exceed a predetermined signal threshold, and dividing each such coπelation maximum by the gam of the transmitting and receiving antennas in the measured direction 231. A method according to claim 230 in which each such coπelation maximum is multiplied by the fourth power of the measured range, to obtain a value proportional to the radar cross-section of the object. 232. A method according to claim 231 in which the said value is compared to a cross-section threshold such that objects whose cross-section exceeds such threshold are subjected to further processing. 233. A method according to any one of claims 225 to 232 in which the range of values of r n - t n for 1 ≤ n ≤ m cover a range of time within which it is expected that a signal reflected from the object will be received. 234. A method according to any one of the preceding method claims for providing an image of an environment in conditions that human vision is compromised. 235. A method according to claim 234 in which vision is compromised by the physiological condition of a user. 236. A method according to claim 234 or claim 235 in which vision is compromised by environmental conditions. 237. A method according to any preceding method claim for imaging within or through a solid object. 238. A method according to claim 237 in which the solid object is a wall. 239. Apparatus for obtaining positional information relating to an object, comprising: transmitting means for transmitting a probe signal towards the object, the transmitting means comprising: a signal generating stage; and at least one transmitting element; receiving means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the object, the receiving means comprising; at least one receiving element at the plurality of spaced apart locations; and detecting means for detecting the relative timing of the returned probe signals as received at the plurality of locations, the detecting means comprising; a detection stage, coupled to the receiving means,; whereby positional information for the object can be determined from the relative timing; and wherein: the signal generating stage applies a series of m pulses to the transmitting element to cause it to transmit a signal, at times t n where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received by the receiving elements; the detection stage detects a signal reflected to the receiving elements at times r„ and generates an output signal representative of the received signal; and wherein the value of r n - 1„ varies as some function of n. 240. Apparatus for obtaining positional information relating to an object, comprising: means for transmitting a probe signal towards the object, said transmitting means comprising a transmitting element; means for receiving, at a plurality of spaced apart locations, the probe signal as returned by the ob j ect, said receiving means compnsing a plurality of receiving elements; and detecting means, coupled to the receivmg means, for detecting the relative timing of the returned probe signals as received at the plurality of spaced apart locations; whereby the positional information for the object can be determined from said relative timmg; and wherein the transmitting element and receiving elements are disposed on a common substrate. 241. An antenna aπay optionally for use in apparatus for obtaining positional information relating to an ob j ect in accordance with any one of claims 1 to 100, the aπay including a transmitting element and a plurality of receiving elements, the transmitting and receiving elements being disposed on a common substrate. 242. A vehicle substantially as herem descnbed and with reference to the accompanying drawings 243. A control system for air bags m a motor road vehicle substantially as herein descnbed and with reference to the accompanying drawings 244. Apparatus for a land vehicle substantially as herein descnbed and with reference to the accompanying drawings. 245. A device for obtaining information about objects within or through a wall substantially as herein descnbed and with reference to the accompanying drawings. 246. A hand-held tool substantially as herein descnbed and with reference to the accompanying drawings. 247. An electromagnetic microwave antenna aπay substantially as herein descπbed and with reference to the accompanying drawings. 248 Apparatus for generating an image of objects withm or through a solid object substantially as herein descnbed and with reference to the accompanying drawings. 249 Apparatus, optionally for use on a (for example, land) vehicle, for obtaining positional information relatmg to an object substantially as herein descnbed with reference to the accompanying drawings. 250. A method for controlling deployment of air bags m a vehicle substantially as herein descnbed. 251. Method of obtainmg positional information relating to an object, compnsing the steps of: transmitting a probe signal towards the object; receivmg, at a plurality of spaced apart locations, the probe signal as returned by the object; detecting the relative timing of the returned probe signals as received at the plurality of locations; and determining positional information for the object from the relative timmg; wherem: the transmittmg step compnses applying a senes of m pulses to a transmitting element to cause it to transmit a signal, at times t n where n = 1, 2 ... m, such that at least a portion of the signal can be reflected from the object to be received at the plurality of spaced apart locations; the detecting step compnses detecting a signal reflected to the receiving elements at tunes r n and generating an output signal representative of the received signal; and wherein the value of r n - t π varies as some function of n. 252. Method of obtaining positional information relating to an object using an apparatus comprising a transmitting element, a receiving means comprising a plurality of receiving elements and a detecting means, the method comprising: transmitting a probe signal from the transmitting element towards the object; receiving, at a plurality of spaced apart locations, the probe signal as returned by the object; and detecting, at the detecting means, the relative timing of the returned probe signals as received at the plurality of spaced apart locations; determining the positional information for the object from said relative timing; wherein the detecting means is coupled to the receiving means and the transmitting element and receiving elements are disposed on a common substrate. 253. Use of an electromagnetic antenna aπay in a method according to claim 252 in which the receiving elements are spaced apart by a distance that is the same order of magnitude as the wavelength λ of the radiation that it is intended to transmit and receive, the electromagnetic antenna aπay including at least three receiving elements arranged non-collinearly such that there is an axis about which the aπay is asymmetrical. 254. Use of an electromagnetic antenna aπay in a method of obtaining positional information relating to an object, the aπay including a transmitting element and at least three receiving elements aπanged non- collinearly, the transmitting and receiving elements being disposed on a common substrate. 255. Use of an electromagnetic antenna aπay in a method of obtaining positional information relating to an object, the aπay including at least three receiving elements aπanged non-collinearly such that there is an axis about which the aπay is asymmetrical.

Inventors: BURCHETT MICHAEL HUGH, CLOUSTON ERIC NICOL, OSWALD GORDON KENNETH ANDREW, RICHARDSON ALAN TREVOR, TOUTOUNGI DANIELLE EMMA, BURCHETT, MICHAEL, HUGH, CLOUSTON, ERIC, NICOL, OSWALD, GORDON, KENNETH, ANDREW, RICHARDSON, ALAN, TREVOR, TOUTOUNGI, DANIELLE, EMMA
Applicants: BURCHETT MICHAEL HUGH, CAMBRIDGE CONSULTANTS, CLOUSTON ERIC NICOL, OSWALD GORDON KENNETH ANDREW, RICHARDSON ALAN TREVOR, TOUTOUNGI DANIELLE EMMA, BURCHETT, MICHAEL, HUGH, CAMBRIDGE CONSULTANTS LIMITED, CLOUSTON, ERIC, NICOL, OSWALD, GORDON, KENNETH, ANDREW, RICHARDSON, ALAN, TREVOR, TOUTOUNGI, DANIELLE, EMMA
Application year: 2001
Main classification: G01S13
IPC codes: B60R21, G01S7, G01S13, H01Q1, H01Q9, H01Q21