EP3213111A1 - Scanning bistatic perimeter intrusion detection system - Google Patents

Abstract

An electronically scanned bistatic radar system comprising a phased antenna array for transmission or reception at each node of a link being protected. Each node is electronically switched between a plurality of scan angle configurations. A target's baseline crossing point or direction of movement is determined more precisely by switching between a plurality of scan angle configurations focussed along a locus of points running substantially parallel to, or perpendicular to, the link. The system reduces the incidence of false alarms and allows greater target information to be gathered.

Description

SCANN ING BISTATIC RADAR PERIMETER I NTRUSION DETECTION SYSTEM Field of the Invention This invention relates to perimeter intrusion detection systems and, more particularly, to those that utilise a bistatic radar topology.

Background to the Invention Microwave perimeter intrusion detection systems generally have one of two basic configurations, comprising either a bistatic or monostatic radar system. The IEEE defines bistatic radar as 'a radar system that uses antennas at different locations for transmission and reception'. In the case of the angle between transmitter and receiver being equal to 180° the system may be described as a forward scatter (FS) radar. Typically in such systems a fixed beam antenna element or antenna array is used to transmit or receive at each node, for example as detailed in US Patent 3,877,002.

In a radar system the strength of the reflected signal from an object is dependent on the scattering properties of the object at the radar operating frequency, i.e. its radar cross section (RCS). The FS RCS of objects that are electrically large at a given frequency is significantly enhanced (relative to the backscattered or BS RCS) due to the effect of Babinet's principle. This effect may be observed for transmitter-target or receiver-target angles within ±10° of the baseline between transmitter and receiver.

A common problem in microwave bistatic radar perimeter intrusion detection systems is the false alarms that may result if processing of the received signal is not able to adequately distinguish an intruder from other fading effects. Also multipath signals may be difficult to observe in the presence of a large direct signal.

Summary of the Invention A first aspect of the invention provides an intruder detection system as claimed in claim 1.

A second aspect of the invention provides a method as claimed in claim 40.

In preferred embodiments, false alarm reduction is achieved, and greater target information gathered, through the implementation of an electronically scanned bistatic radar system, which preferably uses a phased antenna array for transmission or reception at each side of the link being protected.

Advantageously each detection node is electronically switched between a plurality of scan angle configurations, preferably with a frequency greater than 2n times that of the maximum frequency content of the received multipath signal, where n is the number of scan angle configurations implemented.

Advantageously, using an electronically scanned bistatic radar enables simultaneous, or like-for-like, comparisons to be made with stored intrusion signatures for each scan angle configuration, thereby ensuring that the probability of false alarms being triggered is significantly reduced, and detection probability increased, compared with a typical bistatic radar link.

Advantageously, correlation of time domain intruder signatures from a plurality of scan angle configurations enables a longer target visibility time and hence improves the received signal-to-noise ratio of multipath target signatures, thereby enabling more accurate application of pattern recognition algorithms.

In one mode of use, the preferred electronically scanned bistatic radar system helps to resolve a target's baseline crossing point or direction of movement more precisely through switching between a plurality of scan angle configurations focussed along a locus of points running substantially parallel to, or perpendicular to, the link, respectively. Knowledge of the exact baseline crossing point is useful with regard to interception of the target by on-site security as it pinpoints the exact location/direction of movement of the intruder, which may be especially useful for links that have a long baseline length. Determining the target's direction of movement reduces the likelihood of potential false alarm events due to movement or people or vehicles within the perimeter being protected.

In another mode of use, the preferred electronically scanned bistatic radar system helps to discriminate between movement parallel to, and movement perpendicular to, the link through switching between a plurality of scan angle configurations focussed along a locus of points running substantially perpendicular to the link. This reduces the likelihood of false alarms due to large reflectors, with high RCS, such as cars, moving parallel nearby to the link.

A threshold process is advantageously used to determine if objects with RCS above a certain level have been detected within any of the detection zones, and in particular focus regions, formed by the plurality of scan angle configurations of the link. If the variation in the RSSI (Received Signal Strength Indication) signal level is above this threshold then the target parameter/classification process may be triggered; also if a video camera, or other equipment, is linked to the system then recording of video footage of the link will commence. This means that the computational resources used by the system are minimised and current consumption thereby reduced, which is especially important for remotely positioned/battery powered nodes.

Preferred features of the invention are recited in the dependent claims. Further advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of a preferred embodiment and with reference to the accompanying drawings. Brief Description of the Drawings

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: Figures 1 (a)-(c) are alternative schematic views of an electronically scanned bistatic radar system scanning along a locus of points running perpendicular to the link, the system embodying one aspect of the invention and being suitable for use as an intruder detection system embodying another aspect of the invention; Figures 2(a)-(c) are alternative schematic views of the bistatic radar system scanning along a locus of points running parallel to the link;

Figure 3 is a schematic view of an electronically scanned bistatic radar system where both transmit and receive detection nodes are directed along the baseline of the link to be protected;

Figure 4(a)-(c) are block diagrams of a transmitter detection node, a receiver detection node and a phased antenna array respectively, being suitable for use in the electronically scanned bistatic radar system or intruder detection system of Figures 1 (a)-(c) and Figures 2(a)-(c); Figure 5(a)-(b) shows typical time-domain and frequency domain plots of RSSI variation as an intruder passes through an electronically scanned bistatic radar link which is scanned through a locus of points running perpendicular to the link baseline, for example of the intruder detection system of Figures 1 (a)-(c); Figure 6(a)-(b) shows typical time-domain and frequency domain plots of RSSI variation as an intruder passes through an electronically scanned bistatic radar link which is scanned through a locus of points running parallel to the link baseline, for example of the intruder detection system of Figures 2(a)-(c); Figure 7 illustrates a preferred target detection method suitable for use in the intruder detection system of Figures 1 (a)-(c) and Figures 2(a)-(c) or other system supporting an electronically scanned bistatic radar link;

Figure 8 illustrates a preferred method of target crossing point evaluation in the intruder detection system of Figures 1 (a)-(c) and Figures 2(a)-(c) or other system supporting an electronically scanned bistatic radar link; Figure 9 illustrates a preferred method of target direction of movement evaluation in the intruder detection system of Figures 1 (a)-(c) and Figures 2(a)-(c) or other system supporting an electronically scanned bistatic radar link;

Figure 10 (a)-(b) shows typical time-domain and frequency domain plots of RSSI variation as a relatively large reflector moves parallel to an electronically scanned bistatic radar link which is scanned through a locus of points running perpendicular to the link baseline, for example of the intruder detection system of Figures 1 (a)-(c); and

Figure 1 1 illustrates a preferred method of discriminating between movement parallel with, or perpendicular to, the intruder detection system of Figure 1 (a)-(c) or other system supporting an electronically scanned bistatic radar link. Detailed Description of the Drawings

Referring now to Figures 1 (a)-(c) and Figures 2(a)-(c) of the drawings there is shown an intruder detection system 10. The system 10 comprises first and second spaced apart detection nodes 12, 14, each node comprising a respective transmitter and/or receiver (or transceiver) for sending or receiving wireless signals 13 to or from the other node. As such, the system may be described as bistatic. In the illustrated system 10, the bistatic angle between the nodes 12, 14 (when their respective transmitter/receiver are scanned to a 0° angle) is 180°. In alternative embodiments of the invention, systems having other bistatic angles may be implemented. Each node 12, 14 is configured to create a scanned wireless link between the nodes 12, 14, i.e. a link that is established across multiple different paths in succession. To this end, the transmitting node is configurable to direct wireless signals selectably in any one of a plurality of transmit directions, and receiving node is configurable to receive wireless signals selectably from any one of a plurality of receive directions. Means for controlling the nodes 12, 14 is provided such that wireless signals 13 are directed between the nodes along a plurality of alternative main signal paths in succession, each signal path corresponding to a respective one of the transmit directions and a corresponding respective one of the receive directions. This action is referred to herein as scanning and the corresponding link between the nodes 12, 14 is referred to as a scanned link. The scanning is preferably performed repeatedly and continuously, at least while the system 10 remains in one or other of its operational modes,

In the preferred embodiment, a respective suitably controlled phased antenna array (also known as a phased array antenna) is provided in each node 12, 14 to create the scanned wireless link between the nodes. Advantageously, the angle of maximal transmitter or receiver radiation is electronically controlled, and hence the system 10 may be described as an electronically scanned bistatic radar system. In alternative embodiments, any other conventional antenna that is capable of adjusting its transmit/receive direction may be used instead of a phased antenna array, e.g. an antenna mounted on a mechanically movable structure. However, phased antenna arrays are preferred not least because of the speed at which transmit/receive direction can be changed. The nodes 12, 14 operate as a pair and, although Figures 1 and 2 illustrate a single pair, other embodiments of the system may comprise more than one pair of detection nodes. Hence, alternative embodiments of the system 10 may support more than one electronically scanned wireless link. Signals that travel directly between the nodes 12, 14, i.e. without deflection, may be referred to as direct signals and may be said to travel along a baseline between the nodes 12, 14. Signals that reach the receiving node after deflection from an object in a detection zone 16 defined between the nodes 12, 14 may be referred to as multipath signals. In preferred embodiments, the signals comprise electromagnetic signals, typically in the radio frequency or microwave frequency range, and so the link may be described as a radar link. The preferred intruder detection system 10 may therefore be said to comprise an electronically scanned bistatic radar system. In preferred embodiments, a continuous wave (CW) wireless signal is transmitted between the nodes 12, 14, meaning that in a FS configuration (e.g. where the link between nodes 12, 14 is scanned to angles within ±10° of the baseline between the transmitter and receiver) a received signal will typically comprise a relatively strong direct signal and a weaker multipath signal, which modulates the amplitude and phase of the direct signal. For configurations where the link is scanned to angles greater than ±10° away from the baseline between the transmitter and receiver, a received signal will typically comprise a relatively weak direct signal and a weaker multipath signal, which modulates the amplitude and phase of the direct signal. By scanning the nodes 12, 14 to different angles, relative to the transmitter-receiver baseline, the detection zone 16 of the system will change accordingly, as shown by way of example in Figures 1 (a)-(c) and Figures 2(a)-(c).

In the illustrated embodiment, it is assumed by way of example only that node 12 acts as the transmitting node and so comprises transmitter circuitry (for example as shown in Figure 4(b)), while node 14 acts as the receiving node and so comprises receiver circuitry (for example as shown in Figure 4(a)). It will be understood that in alternative embodiments (not illustrated), these roles may be reversed, or each node 12, 14 may comprise both transmitter and receiver circuitry, e.g. by means of a transceiver. In preferred embodiments, the respective transmitter/receiver circuitry of each node 12, 14 comprises a respective phased antenna array 18 (Figure 4(c)), the respective phased antenna arrays 18 of each node 12, 14 being aligned with each other to define the detection zone 16.

The antennas of bistatic radar systems typically have a relatively narrow beam width, for example having a 3 dB beamwidth of less than or equal to approximately 12°. Within the detection zone 16, the receiver node 14 is sensitive to, i.e. is capable of detecting, multipath signals scattered from a target 20 (Figure 3) in the zone 16. To achieve a suitable receiver sensitivity the ratio of the strength of a direct signal between the nodes 12, 14 to the strength of multipath signals scattered from the target 20 is typically less than a given threshold, for example approximately 30-40 dB. The shape and dimensions of the detection zone 16 are a function of any one or more of: (i) target RCS, as an electrically larger target will have a higher value/narrower beamwidth FS RCS lobe, (ii) target height, as propagation loss for target scattering is inversely related to (target height) for ground links, (iii) link length, as the propagation loss for target scattering is proportional to (link length)^A8 in a ground link, and (iv) phased antenna array gain, as a narrow beam width/low side lobe level transmit or receive antenna will focus transmitted signals or be sensitive to multipath scattered signals, respectively, within a narrower volume of space, (v) phased antenna array scan angle, as this will change the angle at which the antenna's main lobe is oriented and hence the orientation, relative to the transmitter or receiver node, of the volume of space in which the antenna focuses transmitted signals or is sensitive to multipath scattered signals. In use, an object (e.g. target 20) passing through the detection zone 16 interferes with the electromagnetic field associated with the bistatic radar link and this results in detectable changes (which may be referred to as a signature) in the output signal from the phased antenna array 18 at the receiving node. Hence, as the target 20 moves through the detection zone 16 a unique signature is detectable in the receiving phased antenna array 18 output, typically as a result of amplitude and phase modulation caused in the received signal by the object's movement. The signature may be evaluated using analogue and/or digital signal processing techniques to determine if an intrusion has occurred.

With reference to Figure 3, the instantaneous Doppler frequency f_d(t) of the scattered multipath signal created as the target 20 moves through the detection zone 16 of the bistatic radar system 10 is determined by the target's speed, v, the wavelength, A, of the continuous wave signal used in the system, the angles from transmitter to target, a_h(t), and from receiver to target, β^), and the baseline crossing angle, φ.

Therefore the maximum frequency content B of the multipath signal is:

2v

B =— [2]

A

It is evident from equation [2] that for objects moving at higher speeds, and/or systems that operate using higher frequency continuous wave signals, the bandwidth of the multipath signals created by the target 20 (which is assumed to be an intruder in the case where the system 10 is an intruder detection system) is relatively high and pose more constraints on the architecture proposed above, since the phased antenna arrays 18 in the transmitter node 12 and receiver node 14 have to switch more quickly between the various scan angle configurations supported by the system 10 to satisfactorily receive the full bandwidth of the multipath signal for each configuration of scan angles.

In the case of detecting human intruders it may be assumed that their speed will be less than 10 m/s and therefore, for system operation at 5.8 GHz for instance, the multipath signal bandwidth will be less than 387 Hz. Sampling at a rate of more than 774 Hz is therefore required for each channel (i.e. in each of the alternate operating states of the system 10), meaning that each switching of the scan angle configuration of the transmit/receive phased antenna arrays 18 has to occur within a (1.29/n) milliseconds time window, where n is the number of scan angle configurations implemented in the system, to enable sampling at this rate for each scan angle configuration. The primary constraint on phased antenna array switching time is the speed of the digital control interface used to change scan angles in digital phase shifters.

Figure 4(a) shows receiver circuitry suitable for use in the detection nodes 12, 14. In the present example it is assumed that detection node 14 comprises the receiver circuitry. The receiver circuitry comprises the phased antenna array 18, which is capable of sending and receiving signals via the wireless link between the nodes 12, 14, a receiver 22, filter 24 and a processor 26. The receiver 22 is coupled to the phased antenna array 18 such that it may receive signals. Typically, the receiver 22 comprises a superheterodyne receiver and, as appropriate, is operable to downconvert radio frequency (RF) signals to intermediate frequency (IF) signals. The receiver 22 is preferably configured to produce an output signal comprising an RSSI (Received Signal Strength Indicator) signal.

The output of the receiver 22 is provided to the filter 24 for removing unwanted components of the receiver output. The filter 24 typically comprises a high-pass filter (to eliminate low-frequency clutter signals from, for example, vegetation or rainfall) and a low pass filter (usually with a cut off frequency more than twice that of the maximum frequency content of the scattered multipath signal, thereby preventing aliasing from occurring during sampling of the signal). An analogue-to-digital converter (ADC) is usually provided for sampling the (filtered) output signal. The resulting digitised received output signal is provided to analysing means, typically comprising a suitably programmed processor 26, for analysis. The processor 26 may comprise a suitably programmed microprocessor, microcontroller or other digital signal processing (DSP) device. As is described in more detail hereinafter, the processor 26 is configured to detect the presence of an intruder in the detection zone 16.

Figure 4(b) shows transmitter circuitry suitable for use in the detection nodes 12, 14. In the present example it is assumed that detection node 12 comprises the receiver circuitry. The transmitter circuitry comprises a transmitter 28, which may take any convenient conventional form, a processor 30 and the phased antenna array 18. The processor 30 may comprise a suitably programmed microprocessor, microcontroller or other digital signal processing (DSP) device.

Figure 4(c) shows a preferred embodiment of the phased antenna array 18, which in this example comprises an architecture well known to one skilled in the art. When used for transmitting, transmit RF signals are input to a power divider network 32. This divides the RF input signals amongst N outputs and may take any convenient form, for instance Wilkinson power dividers may be used. The N outputs from the power divider network 32 are input to a respective digital phase shifter 34, which may be conventional. The magnitude of the phase shift, Αφ, applied to the RF signal at each digital phase shifter 34 may be incremented digitally using a typical digital control interface, such as the l²C or SPI protocols. The RF phase shift may be applied in the order of nanoseconds, however the speed at which switching between phase states may occur is limited by the speed of the digital control protocol, which is typically > 10 MHz in the case of the SPI protocol. Therefore switching may typically occur in the order of < 100 microseconds. An alternative embodiment of the system (not shown) may use analogue phase shifters, which have a voltage control line to allow switching between phase shift angles over a continuous range. The voltage control line of the phase shifter may be driven from a digital-to-analogue converter on a processor to enable convenient switching of phase shift angles. Each digital phase shifter 34 is connected to a respective antenna element or antenna sub-array 36. When used for receiving, the operation of the phased antenna array 18 is reciprocal to that described for transmitting, as would be apparent to a skilled person.

The steering angle, θ₀, of the main beam of the phased antenna array 18 is dependent on the RF phase shift applied at each phase shifter 34, Αφ, and the electrical spacing, d/A₀, between each antenna element, or antenna sub-array 36, measured in wavelengths. 9_n = sin [2]

v 2nd

The spacing between each antenna element, or antenna sub-array 36, determines the angle to which the transmitter and receiver phased antenna arrays may be scanned without grating lobes appearing, i.e. spacing of d/A₀≤ 0.5 allows scanning to ±90° while spacing of d/A₀ = 0.9 allows scanning to ±6.4°. Also, to achieve scanning action without distortion of the main lobe of the phased antenna array it is desirable to use, for instance, at least 8 antenna elements or antenna sub-arrays 36 in the phased antenna array 18.

The receiver detection node 14 may distinguish a received multipath signal from a received direct signal by using variations in RSSI levels to evaluate intrusions. For example, for a direct signal the RSSI remains at a relatively high and constant level, whereas for multipath signals the RSSI level varies. The processor 26 may be programmed or otherwise configured to perform such analysis of the received signals. The filter 24 may be configured to remove the dc content in the RSSI signal in order that only variations in RSSI level are analysed. Optionally, each node 12, 14 is configured to implement low bitrate amplitude shift keying (ASK) or frequency shift keying (FSK) modulation to uniquely pair the, or each, pair of transmitter and receiver nodes 12, 14 present in the system 10 via transmission of a unique identifier code between paired nodes.

In the preferred embodiment, in order to implement the scanned link between paired nodes 12, 14, the phased antenna array 18 of the respective transmitter and receiver nodes 12, 14 of a pair switch at intervals between operating at any one of a plurality of scan angle configurations and operating at the next of the scan angle configurations. Typically, the switching is performed until all supported scan angle configurations have been implemented and is then repeated. In preferred embodiments, switching between scan angle configurations occurs continuously, irrespective of whether a target 20 is present in the detection zone 16. In the illustrated embodiment, the respective processors 26, 30 control the switching between scan angle configurations. In the embodiment shown in Figure 4(c) the respective processor 26, 30 provides the digital control signals to the digital phase shifters 34 for this purpose.

Referring now to Figure 1 , the operation of the system 10 in a first mode is described. The system 10 may have a first operating state (illustrated in Figure 1 (a)) in which transmit and receive phased antenna arrays 18 are adopt a first scan angle configuration (T R-i) to focus on a region 40 on one side of the baseline, a second operating state (illustrated in Figure 1 (b)) in which transmit and receive phased antenna arrays 18 are directed along the baseline, i.e. a second scan angle configuration (T₂, R₂) where T₂, R₂ are equal to 0°, to focus on a region 42, and a third operating state (illustrated in Figure 1 (c)) in which transmit and receive phased antenna arrays 18 adopt a third scan angle configuration, (T_3, R₃), to focus on a region 44 on the opposite side of the baseline to the first scan angle configuration. In this mode of operation, the scan angle configurations are selected such that, as the system 10 switches operating state to the next, the focus region 40, 42, 44 where the transmit and receive directions intersect moves along a locus oriented substantially perpendicularly to the baseline. In the illustrated embodiment, the focus regions 40, 42, 44 are substantially equidistant from the nodes 12, 14 although this need not necessarily be the case in alternative implementations. In the example of Figure 1 , three operating states are shown although more generally two or more operating states may be supported.

Referring to Figure 2, operation of the system 10 in a second mode is described. The system 10 may have a first operating state (illustrated in Figure 2(a)) in which transmit and receive phased antenna arrays 18 adopt a first scan angle configuration, (T_a, R_a), to focus on a region 46 that is closer to the transmit node 12 than the receive node 14, a second operating state (illustrated in Figure 2(b)) in which transmit and receive phased antenna arrays 18 adopt a second scan angle configuration, (T_b, R_b), to focus on a region 48 midway between transmitter 12 and receiver 14, and a third operating state (illustrated in Figure 2(c)) in which the transmit and receive phased antenna arrays 18 adopt a third scan angle configuration, (T_c, R_c), to focus on a region 50 that is closer to the receiver 14 than the transmitter 12. In this mode of operation, the scan angle configurations are selected such that, as the system 10 switches operating state to the next, the focus region 46, 48, 50 where the transmit and receive directions intersect moves along a locus oriented substantially parallel with the baseline. In the illustrated embodiment, the focus regions 46, 48, 50 are substantially equidistant from the baseline although this need not necessarily be the case in alternative implementations. In the example of Figure 2, three operating states are shown although more generally two or more operating states may be supported.

The switching between scan angle configurations is conveniently performed periodically, preferably at a frequency greater than 2n times that of the maximum frequency content of the received multipath signal, where n is the number of scan angle configurations implemented in the system 10. This enables each channel to comply with the Nyquist sampling theorem, i.e. sampling of the RSSI variation at twice the rate of its maximum frequency content. The transmitter and receiver nodes 12, 14 may communicate with one another by any convenient means (not shown) to synchronise switching between different scan angle configurations. For example, the nodes may be linked by Ethernet in which case the Precision Time Protocol, defined in the IEEE 1588 standard, may be used for synchronisation of switching. This allows sub-microsecond synchronisation. Alternatively, a GPS module (not shown) may be provided in each transmitter or receiver node 12, 14 to allow synchronisation with atomic clocks on GPS satellites, which means synchronisation accuracy at the GPS module of typically 100 nanoseconds or less. Either of these techniques, or any other convenient technique, may be implemented in the system 10 using readily available off-the-shelf components.

The processor 26 may be programmed to perform pattern recognition (and/or other analysis) on the receiver output signals, and in particular in respect of the target signatures included in the output signals. The target signatures may be represented as plots (or other representation) of signal power, conveniently normalised received signal power, versus target frequency, conveniently Doppler frequency (where Doppler frequency describes the variation in the frequency of the received multipath signals over time), as shown in Figure 5(b), for the system configurations shown in Figures 1 (a)-(c). The signal power values may be normalised to the maximum power level in the signal. The instantaneous Doppler frequency defined in equation [1] above corresponds to a single point on the x-axis of this plot which, for a target 20 moving at a certain speed and for a known radar operating wavelength, will correspond to the position of a target relative to the transmitter 12 and receiver 14. The normalised signal power amplitude (on the y-axis) depends on the amplitude of the received multipath signal for this target position, which is dependent on target RCS/target height/link length/phased array antenna gain/phased array antenna scan angle.

The characteristics of the frequency domain, or Doppler frequency, plots used for pattern recognition are dependent on the feature extraction technique used. The frequency characteristics may be assigned automatically using for example Principle Component Analysis, which reduces the dimensionality of the frequency domain signatures down to Principle Components, the number of which may be chosen by the user. Alternatively the characteristics may be manually extracted, e.g. first main lobe width, second main lobe width, and/or number of lobes below a set threshold frequency. In the manual extraction technique the lobe widths and number of lobes are primarily affected by variations in the received signal level due to how the radar cross section of the target varies for particular target-receiver angles, /3_hffJ - radar cross section nulls occur at particular angles, corresponding to nulls at particular instantaneous Doppler frequencies as each

instantaneous Doppler frequency corresponds to a target position relative to transmitter/receiver.

In bistatic radar intrusion detection systems, target detection may be performed using threshold analysis, e.g. determining if the movement of the target through the direct path of the link has led to a drop greater than a threshold value in the amplitude of the received signal in the time domain, conveniently the RSSI amplitude, meaning that a target with an RCS greater than a threshold value has passed through the link. It is noted that the target does not necessarily have to cross the baseline of the link for it to cause a drop in the RSSI greater than the threshold value set at the receiver 14 for target detection. For example, for relatively large metal objects such as cars (with a large RCS) moving adjacent to the link, but not through the baseline, the RSSI may drop by an amount greater than the threshold value (which is typically set for smaller targets, such as people, with smaller RCS moving through the baseline of the link).

Hence, a simple threshold detection method is vulnerable to false alarms since multipath signals received from outside of the direct path may also cause signal amplitude drops of greater than the threshold value. Also, relatively subtle changes in the received signal caused by movement of a low RCS target, such as a crawling person, through the link may not be detected. In preferred embodiments of the invention therefore, (RSSI) threshold analysis is used to trigger subsequent target detection, for example application of signal processing algorithm(s), in order to classify the target that caused the RSSI change with greater accuracy and fewer false alarms.

To reduce false alarm probability, and increase detection probability for low RCS targets, preferred embodiments employ one or more pattern recognition algorithm to analyse the received signal, in particular the received multipath signal(s). Conveniently, the processor 26 is programmed to implement one or more pattern recognition algorithms. This may involve comparing one or more characteristics of the received multipath signal(s) (which may be said to be represented by a signature of the respective signal) with one or more of a plurality of stored comparable signatures (i.e. data representing one or more corresponding characteristics of a plurality of reference signals) that represent respective identifiable events, such as intrusion events or false alarm events. The stored signatures may be stored in local memory (not shown) in the node 14.

Multipath signals caused by movement of a target between the transmitter and receiver nodes 12, 14 are received in the time domain with a "signature" amplitude and phase variation. For the purposes of analysis, it is convenient to convert time domain multipath signals to the frequency domain, e.g. using FFTs, thereby creating a corresponding frequency signature for the target. The stored signatures for comparison with the frequency signatures obtained from the received multipath signal conveniently also comprise corresponding frequency domain signatures that facilitate comparison by signal processing. The frequency signatures preferably comprise Doppler frequency signatures.

In preferred embodiments, prior to comparison of the received signatures with the stored signatures, pre-processing of the received frequency signatures is advantageously performed to normalise them to a reference target speed and also to the maximum power level in the received signal. Also the baseline crossing point/angle is preferably evaluated to reduce the number of stored frequency domain signatures with which comparison is to be made, i.e. for particular intervals of baseline crossing point/crossing angle, frequency domain signatures of target types are stored. The pre- processing may involve an autocorrelation process, which compares the phase variation in the received time domain signal with that expected for a particular target speed, baseline crossing point and baseline crossing angle.

Pattern recognition algorithms well known to one skilled in the art, for example involving a neural network approach or a principle component analysis/K-nearest neighbour approach, may be used. In any event, the use of pattern recognition algorithms enables determination of intrusion with a high level of accuracy. Using an electronically scanned bistatic radar link enables simultaneous comparisons to be made with the stored database of intrusion signatures for each scan angle configuration, thereby further reducing the probability of false alarms, and increasing detection probability, compared with a typical bistatic radar link using a fixed beam antenna array.

From equation [1], the target's frequency signature, in particular its Doppler signature, is dependent on its speed, baseline crossing point and baseline crossing angle. In preferred embodiments, prior to any comparisons with reference intruder signatures stored in the database, the processor 26 performs a pre-processing process to normalise the received Doppler signatures to a selected (or reference) target speed. The processor 26 also determines a baseline crossing point and baseline crossing angle for the target. This allows the number of database signatures that the detected signature should be compared with to be reduced. In preferred embodiments, the stored reference signatures comprise respective Doppler signatures for a plurality of target types (e.g. a person running, walking, jumping, commando rolling, crawling on hands and knees or belly crawling), and optionally one or more anticipated false alarm Doppler signatures (e.g. representative of a small animal walking or a bird/flock of birds flying through the link, especially when close to either node, or a car moving parallel to the link), respective such reference signatures preferably being stored for respective intervals of baseline crossing point/crossing angle.

Figure 7 shows a flow chart of the preferred detection process used by the, or each, receiver node 14 in the evaluation of received signals for each scan angle configuration, the process conveniently being performed by the respective processor 26. At 701 the received signal, which in this example is assumed to have been filtered and digitised, is provided to the processor 26. It is also assumed in this example that the received signal is provided as, or at least comprises, an RSSI signal. At 702 the received signal is subjected to a threshold analysis to determine if a target 20 with an RCS above a certain level has been detected within the detection zone 16 of the link supported by the nodes 12, 14. The threshold analysis involves comparing a characteristic, typically the amplitude, of the received signal (in this case the RSSI signal, in particular the filtered RSSI level since it is the RMS amplitude of the ac content in the RSSI signal that is assessed in the preferred embodiment) against a threshold value. It is preferred to analyse variations in the multipath signal strength (RSSI level in this example) using RMS values. If the RMS amplitude exceeds the threshold value for a given measurement period of, for instance, 0.1 seconds then it is assumed that an object has been detected in the detection zone 16.

If the RSSI RMS level is determined to be above the threshold this indicates that an object has been detected in the detection zone 16 and, in preferred embodiments, the target analysis process is initiated (703 to 705), otherwise it is determined that no object is detected (706). In cases where the system 10 includes one or more activatable detection or monitoring devices, for example one or more video cameras for monitoring the detection zone (or elsewhere), such devices may be activated in response to the detection of an object. As a result, the computational resources used by the system 10 are minimised and current consumption thereby reduced, which is especially important for remotely positioned and/or battery powered nodes 12, 14.

The preferred target analysis process involves a transform, conveniently a Fast Fourier Transform

(703) , of the received time domain signal into the frequency domain. A pre-processing procedure

(704) is then employed to normalise the speed and baseline crossing point/angle to reference values, to facilitate comparison of the received signal with the stored signatures. This may involve the use of conventional pattern recognition algorithms such as neural network analysis or principle component/K-nearest neighbour analysis. A decision is made, based on the result of this process, to determine whether to cause an alarm signal to be rendered to an end user via any suitable interface (707) (e.g. comprising one or more visual and/or audio output device). Advantageously,

simultaneous processing of the received signal at the receiver node 14 for each of n scan angle configurations increases the probability of valid target detection and the probability of false alarms is reduced.

Figures 5(a)-(b) and Figures 10(a)-(b) show respective typical time-domain and frequency domain plots of the received signal for each scan angle configuration shown in Figures 1 (a)-(c), i.e. (T_1: R-,), (T₂, R2) and (T₃, R₃), respectively. Figures 6(a)-(b) show typical time-domain and frequency domain plots of the received signal for each scan angle configuration shown in Figures 2(a)-(c), i.e. (T_a, R_a), (T_b, R_b) and (T_c, R_c), respectively. In the case of Figures 5(a)-(b) and 6(a)-(b), the plots in particular show variations in the RSSI signal as an intruder (target 20) walks through the electronically scanned link between the nodes 12, 14. In Figures 5(a) and 6(a) the time at which the intruder crosses the baseline of the link equates to the time at the origin of the plot. Also, while in Figures 5(a)-(b) the intruder crosses the baseline at the midpoint of the link, in Figures 6(a)-(b) the intruder crosses the baseline at a point perpendicular to the focus region 46 of scan angle configuration (T_a, R_a). In the case of Figures 10(a)-(b) the plots show variations in RSSI signal as an object, for example a relatively large reflector such as a car (not shown), moves parallel to the electronically scanned link between the nodes 12, 14, passing through focus region 40 of Figure 1 (a). Such objects are not considered to be valid targets and it is desirable that the system 10 can distinguish between them and valid targets. A valid target 20 may for example be an object that crosses or at least moves in a direction towards the baseline between the nodes 12, 14. In Figure 10(a) the time at which the large reflector passes through focus region 40 of Figure 1 (a) equates to the time at the origin of the plot. The amplitude envelope of the time-domain signal varies according to any one or more of: the propagation loss (which is greater for lower target height, a longer link length and, in the case where the target crosses the baseline, baseline crossing points closer to the link centre), target RCS, phased antenna array beamwidth and phased antenna array scan angle. The phase shift of the time-domain signal is due to the varying propagation path (from transmitter 12 to target 20 to receiver 14) length, as detailed in equation [1 ], as the target 20 moves through the detection zone 16. With regard to the frequency-domain variations, the frequency resolution is equal to the sampling or observation time divided by the sampling rate.

In Figure 1 (a)-(c) the scanned link is scanned through scan angle configurations corresponding to focus regions 40, 42, 44 located along a locus running perpendicular to the baseline midpoint. For the scan angle configuration that is focussed along the link baseline, (T₂, R₂), shown in Figure 1 (b), as the intruder moves closer to the baseline the amplitude envelope of the time signature generated by their movement, shown in Figure 5(a), increases significantly in magnitude. This is due to the intruder's FS RCS increasing considerably due to Babinet's principle (assuming their cross section is electrically large at the radar operating frequency) and the transmit/receive antenna gain that is illuminating/viewing the target also increasing. For the scan angle configurations focussed on either side of the link baseline, (T_1: R-,) and (T₃, R₃), shown in Figure 1 (a) and 1 (c) respectively, the intruder's movement at locations further from the baseline results in a greater amplitude envelope magnitude in the plots of Figure 5(a), relative to the scan angle configuration focussed along the baseline, (T₂, R₂). This is due to the wider scan angle of the phased antenna array 18 relative to the baseline. Correspondingly, intruder movement at locations on the opposite side of the baseline to that at which the scan angle configuration is focussed result in a lower amplitude envelope magnitude. The significant increase in intruder RCS that occurs as they cross through the link baseline means that the amplitude envelope is still of a significant magnitude, despite the lower phased antenna array gain along the baseline, as while the absolute direct and multipath signal levels are at a reduced level, the ratio of received multipath and direct signals will be of a similar magnitude to the case where the phased antenna array scan angle configuration is focussed along the baseline. As is well known to one skilled in the art, the visibility time of the target 20 corresponds to the signal- to-noise ratio of the target's time domain signature. Hence by increasing the target visibility time it is possible to detect or classify targets with greater accuracy. By correlating time domain signatures obtained through scanning the phased antenna array 18 through multiple scan angle configurations with focus regions 40, 42, 44 along a locus substantially perpendicular to the baseline it is possible to obtain a greater target visibility time and hence use the resultant signature to classify targets, using the pre-processing/pattern recognition approach described in Figure 7, with a greater degree of accuracy. This may reduce the computational load of the processor 26, relative to that required to process different signatures for each scan angle configuration and separately compare each with a signature database, as a single resultant signature needs to be processed. The movement of a relatively large reflector parallel to the inter-node link - such as a car driving on a road running parallel to the link - is a common source of false alarms in conventional bistatic radar systems. This is due to the RCS of a reflector moving parallel to, but not through the baseline, having a similar magnitude to the RCS of a target moving through the baseline of the link, particularly in the case of targets moving close to the ground, such as crawling intruders. Referring to Figure 1 (a), for the scan angle configuration focussed on focus region 40, (T_?, R-,), the movement of the large reflector parallel with the link, through focus region 40, is detected by the system 10, a significant amplitude envelope being generated due to the relatively large RCS of the reflector. For the scan angle configuration focussed on focus region 42 (T₂, R2), on the baseline of the link, or on focus region 44 (T₃, R₃) on the other side of the baseline of the link to the focus region 40 through which the large reflector is moving, the amplitude envelope is significantly lower. This is due to the scan angle of the phased array antenna being focussed away from the movement of the large reflector, such that it illuminates said reflector less strongly and is less sensitive to multipath signals generated by said reflector. Hence, by analysing the relative amplitudes of the respective detected signals for respective scan angle configurations focussing along a locus of points running substantially perpendicular to the link (e.g. as shown in Figures 1 (a)-(c)), a determination can be made as to whether or not a detected object is moving parallel with the link. In particular, if the amplitude is significantly higher (e.g. higher by more than a threshold amount, where the threshold may be an absolute value but more typically a percentage value) for one or more scan angle configuration corresponding to a respective focus region on one side of the link than it is for one or more scan angle configuration corresponding to a respective focus region on the other side of the link (or closer to the link but on the same side), then it may be concluded that the detected object is moving parallel with the link on the side of the link corresponding to the higher amplitude. This analysis, which may conveniently be performed by the processor 26, is illustrated by way of example in Figure 1 1. Referring now to Figure 1 1 , there is shown a flow chart of a process that may be used, conveniently by processor 26, to evaluate whether a detected object is moving parallel with the baseline of the link, or through the baseline of the link. At 1 1 1 , the respective time domain signatures, and in particular the respective amplitude, captured as the system 10 scans along a locus of points running substantially perpendicular to the baseline, for example as shown in Figures 1 (a)-(c), are compared with one another. This may be achieved by use of a threshold process (1 12), which may be similar to that described above, to evaluate if the RSSI variation in the received signal increases beyond a threshold value, for example a preset RMS value, for respective scan angle configuration(s) on one side of the baseline compared to respective scan angle configuration(s) on the other side. If the difference in amplitude is sufficiently great, then it may be concluded that the detected object is a parallel-moving non-target (1 13) in which case no further action may be taken, otherwise it may be determined that the object is a potentially baseline-crossing target (1 14), and any relevant further analysis may be performed. In Figure 10(a) it is evident that the scan angle configuration (T_1: R-,) plot would trigger the threshold for the time window (t , t₁₂), while the scan angle configuration (T₃, R₃), indicated by time window (t₃₁, t₃₂), would not trigger the threshold. It may thereby be determined whether a target is moving parallel to the baseline, or through the baseline, of the link. Determining whether the target is moving parallel to, but not through, the baseline reduces the likelihood of potential false alarm events due to the movement of large reflectors, such as cars, parallel to the baseline of the link.

In Figure 2(a)-(c) the scanned link is scanned through multiple scan angle configurations with focus regions 46, 48, 50 in a locus running parallel to the baseline. For all three scan angle configurations shown in Figures 2(a)-(c) the amplitude envelope of the corresponding time domain signatures shown in Figure 6(a) (for an intruder crossing the baseline at a point perpendicular to the focal point of scan angle configuration (T_a, R_a)) increases in magnitude as the intruder moves closer to the baseline. This is due to the intruder's FS RCS increasing considerably here due to Babinet's principle (assuming their cross section is electrically large at the radar operating frequency). The time domain signature of the target 20 becomes visible in each plot at different times, i.e. for the scan angle configuration focussed closer to the transmitter node 12, in Figure 2(a), the time domain target signature is visible at an earlier time compared with the plots of Figure 2(b) or 2(c). This is due to the varying shape of the detection zone 16 for each scan angle configuration, as shown in Figures 2(a)-(c), whereby the transmit or receive phased array antennas18 focus, or are more sensitive to, signals in particular regions of space for each configuration. Hence an intruder passing close to the transmitter node 12 is visible at an earlier time for the scan angle configuration focussed close to the transmitter node 12 and so on. In order to determine a baseline crossing point and baseline crossing angle for the target 20, an autocorrelation process may be employed, typically by the processor 26, to correlate an expected phase variation in the received signal as a target 20 moves through the detection zone 16 with a given speed and baseline crossing point/angle (as predicted in equation [1]) with that observed. Expected phase variation may be obtained from any one or more of a plurality of reference signal data. This pre-processing process can only determine that an intrusion occurred at a certain distance from the midpoint of the baseline of the link, as the phase variation in the time domain for targets moving symmetrically with respect to the midpoint is identical. However by comparing the time domain signatures of the scanned link as it is scanned along a locus of points running substantially parallel to the baseline, for example as shown in Figures 2(a)-(c), it is possible to evaluate which side of the link the intrusion occurred and thereby determine the exact baseline crossing point. This may use a threshold process, as described above, to evaluate when the RSSI variation in the received signal increases beyond a preset RMS value, indicating detection of a target 20. In Figure 6(a) it is evident that the scan angle configuration (T_a, R_a) plot would trigger the threshold for the time window (t_a1, t_a2), which occurs before the threshold is triggered for scan angle configurations (T_b, R_b) or (T_c, R_c), indicated by time windows (t_b1, t_b2) and (t_c1, t_c2) respectively. Hence it may be determined that the intruder has crossed the baseline closer to the transmitter 12 side of the link, allowing determination of the exact baseline crossing point. Knowledge of the exact baseline crossing point is useful with regard to interception of the target 20 by on-site security as it pinpoints the exact location/direction of movement of the intruder, which may be especially useful for links that have a long baseline length. Figure 8 shows a block diagram that illustrates this process, as may be performed by the processor 26. At 801 an evaluation of the distance of an intrusion from the baseline is made using a first technique, namely correlating an expected phase variation in the received signal as a target 20 moves through the detection zone 16 with a given speed and baseline crossing point/angle (as predicted in equation [1]) with that observed. Then, at 802, threshold analysis of the signatures captured for the respective scan angle configurations of the parallel scanned link is performed to determine where the intrusion was detected first with respect to the focus regions of the scanned link. This allows the processor 26 to determine at which side of the baseline midpoint the intruder is, which in turn allows a baseline crossing point to be determined (803). Figure 9 shows a flow chart of a process that may be used, conveniently by processor 26, to evaluate the target's direction of movement through the link. At 901 the time domain signatures of the electronically scanned link as it is scanned along a locus of points running substantially perpendicular to the baseline, for example as shown in Figures 1 (a)-(c), are compared with one another. This may be achieved by use of a threshold process, similar to that described above, to evaluate when the RSSI variation in the received signal increases beyond a preset RMS value, indicating detection of a target. In Figure 5(a) it is evident that the scan angle configuration (T_1: R-,) plot would trigger the threshold for the time window (t , t₁₂), which occurs before the threshold is triggered for scan angle configurations (T₂, R₂) or (T₃, R₃), indicated by time windows (t₂₁, t₂₂) and ½ί, £32) respectively. The target's direction of movement through the link baseline may therefore be determined (downwards as viewed in Figure 1 , in this example) (902). Determining the target's direction of movement reduces the likelihood of potential false alarm events due to movement or people or vehicles within the perimeter being protected.

It will be apparent from the foregoing that preferred embodiments of the invention reduce the incidence of false alarm detection, and gather more target information than a conventional system, through the implementation of an electronically scanned bistatic radar system and method, which involves using a phased antenna array for transmission/reception at both sides of each link being protected. The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention.

Claims

CLAIMS:

1. An intruder detection system comprising at least one pair of detection nodes, each node comprising wireless communication means for supporting a wireless communication link between paired nodes, wherein at least one node of the, or each, pair is capable of sending a wireless signal to the other node of the respective pair, said other node being capable of receiving said wireless signal, said wireless signal creating, in use, an electromagnetic field defining a detection zone between the respective pair of nodes, and wherein the system further comprises analysing means for analysing said wireless signal received from said at least one node of the pair to detect one or more characteristics of said received signal that is indicative of a disturbance in said electromagnetic field caused by the presence of a target in said detection zone, and wherein the wireless communication means of said at least one node is configurable to direct said wireless signal selectably in any one of a plurality of transmit directions, and the wireless communication means of said other node is configurable to receive said wireless signal selectably from any one of a plurality of receive directions, and wherein said system further comprises control means for controlling the paired nodes such that said wireless signal is directed between the paired nodes along a plurality of different main signal paths in succession, each signal path corresponding to a respective one of said transmit directions and a corresponding respective one of said receive directions.

2. The system of claim 1 , wherein a notional baseline is defined as the shortest distance between paired nodes, and wherein each of said transmit directions is angularly displaced from said base line by a respective different transmit angle, and each of said receive directions is angularly displaced from said base line by a respective different receive angle.

3. The system of claim 2, wherein in respect of one of said signal paths, said transmit angle and said receive angle are zero such that said wireless signal is sent along said baseline.

4. The system of claim 1 or 2, wherein, for each of said signal paths, the respective transmit direction and receive direction are selected to intersect at a respective focus region in said detection zone, said respective focus regions typically being located between the respective paired nodes.

5. The system of claim 4, wherein a notional baseline is defined as the shortest distance between paired nodes, and wherein said control means is configured to select said plurality of signal paths such that the location of the respective focus region moves in a direction substantially perpendicular with said baseline.

6. The system of claim 4 or 5, wherein a notional baseline is defined as the shortest distance between paired nodes, and wherein said control means is configured to select said plurality of signal paths such that the location of the respective focus region moves in a direction substantially parallel with said baseline.

7. The system as claimed in any preceding claim, wherein analysing means is configured to perform said analysis in respect of at least some and preferably all of said signal paths.

8. The system as claimed in any preceding claim, wherein said analysing means is configured to determine if a target is present in said detection zone by analysing the respective received wireless signal of at least some and preferably all of said signal paths.

9. The system as claimed in any preceding claim, wherein said analysing means is configured to determine at least one aspect of a target's movement in the detection zone by analysing the respective received wireless signal of at least some and preferably all of said signal paths.

10. The system as claimed in any preceding claim, wherein said analysing means is configured to determine a direction of movement of said target from said analysis performed in respect of at least some and preferably all of said signal paths.

1 1. The system of claim 10 when dependent on claim 5, wherein said analysing means is configured to determine a direction of movement of said target from said analysis performed in respect of at least some and preferably all of said signal paths wherein said location of the respective focus region of said signal paths moves in a direction substantially perpendicular with said baseline.

12. The system of claim 1 1 , wherein said direction of movement is determined by determining an order in which said target is detected in said respective focus regions.

13. The system as claimed in any preceding claim, wherein a notional baseline is defined as the shortest distance between paired nodes, and wherein said analysing means is configured to determine location at which said target crosses said baseline from said analysis performed in respect of at least some and preferably all of said signal paths.

14. The system of claim 13 when dependent on claim 6, wherein said analysing means is configured to determine location at which said target crosses said baseline from said analysis performed in respect of at least some and preferably all of said signal paths, and wherein the location of the respective focus region moves in a direction substantially parallel with said baseline.

15. The system of any one of claims 6 to 14, wherein said analysing means is configured to determine at which side of the midpoint of the baseline said target is located from said analysis performed in respect of at least some and preferably all of said signal paths wherein the location of the respective focus region moves in a direction substantially parallel with said baseline.

16. The system of claim 15, wherein said side of the midpoint is determined by determining the relative proximity of the or each focus region in which the target is detection to the respective detection nodes.

17. The system as claimed in any preceding claim, wherein said wireless communication means comprise a respective phased antenna array at each node of a pair.

18. The system as claimed in any preceding claim, wherein said control means is configured to cause said paired nodes to switch from any one of said signal paths to the next in succession at a frequency greater than twice the maximum frequency of said received signal.

19. The system as claimed in claim 18, wherein, when said target is in said detection zone, said received signal includes a multipath signal reflected from said target, and wherein said control means is configured to cause said paired nodes to switch from any one of said signal paths to the next in succession at a frequency greater than twice the maximum frequency of said multipath signal.

20. The system as claimed in claim 16, wherein said control means is configured to cause said paired nodes to switch from any one of said signal paths to the next in succession with a frequency greater than 2n times that of the maximum frequency content of the received multipath signal, where n is the number of signal paths.

21. The system as claimed in any preceding claim, wherein the analysing means is configured to measure the strength of said received signal and to determine if said target is detected depending on said measurement, said measurement preferably being made against one or more threshold value.

22. The system as claimed in claim 21 , wherein, when said target is in said detection zone, said received signal includes a multipath signal reflected from said target, and wherein the analysing means is configured to measure the strength of said multipath signal and to determine if said target is detected depending on said measurement.

23. The system as claimed in claim 21 or 22, wherein, in response to, and preferably only in response to, detection of a target by said measurement of received signal strength, the analysing means and/or other system components, are configured to perform analysis of said received signal.

24. The system as claimed in any preceding claim, wherein the analysing means is configured to compare at least one characteristic of said received signal with one or more corresponding characteristics of a plurality of reference signals, and to match said received signal to one or more of said reference signals based on said comparison.

25. The system as claimed in claim 24, wherein the analysing means is configured to classify the received signal as one or more of a plurality of intrusion types based on said matching.

26. The system as claimed in claim 24 or 25, wherein said comparison and matching involves application of one or more pattern recognition algorithms.

27. The system as claimed in any one of claims 24 to 26, wherein prior to said comparison, said analysing means is configured to normalise said received signal to a reference target speed and preferably also to the maximum power level in the received signal.

28. The system as claimed in any one of claims 24 to 27, wherein prior to said comparison said analysing means is configured to determine baseline crossing data, preferably comprising a baseline crossing point and a baseline crossing angle.

29. The system as claimed in claim 28, wherein said analysing means is configured only to performed said comparison for a subset of said reference signals corresponding to said determined baseline crossing data.

30. The system as claimed in claim 28 or 29, wherein determining said baseline crossing data involves comparing the phase variation in the received time domain signal with a respective expected phase variation for one or more reference target speed, baseline crossing point and/or baseline crossing angle.

31. The system as claimed in any one of claims 24 to 30, wherein said analysing means is configured to operate on, and create as necessary, a respective frequency representation of said received signal, preferably a Doppler signature.

32. The system as claimed in any one of claims 24 to 31 including means for storing said reference signals, preferably respective frequency representations and more preferably a respective Doppler signature.

33. The system as claimed in any one of claims 24 to 32, wherein said reference signals include respective reference signals representing a plurality of target types, and optionally one or more anticipated false alarm types, respective such reference signatures preferably being provided or respective intervals of baseline crossing data.

34. The system as claimed in any preceding claim, wherein said wireless communication means is configured to support radar communication link, said wireless signals comprising radar signals.

35. The system as claimed in any preceding claim, wherein said radar link is a forward scatter radar link.

36. The system as claimed in any preceding claim, wherein said wireless communication means is configured to transmit continuous wave wireless signals.

37. The system as claimed in any preceding claim, wherein, when said target is in said detection zone, said received signal includes a multipath signal reflected from said target, and wherein the analysing means, and/or other system components, is configured to analyse one or more characteristics of said multi-path signal.

38. The system as claimed in any preceding claim wherein said control means is configured to cause said paired nodes to repeat implementation of said succession of signal paths continuously.

39. The system of any one of claims 5 to 38, wherein said analysing means is configured to determine whether or not a detected object is moving parallel with said notional baseline by comparing the respective wireless signal received in respect of at least two of said focus regions.

40. The system of claim 39, wherein said analysing means is configured to determine that said detected object is moving parallel with said notional baseline if the amplitude of the wireless signal received in respect of at least one focus region on one side of said notional baseline is greater than the amplitude of the wireless signal received in respect of at least one other focus region on the opposite side of said notional baseline, or closer to said baseline than said at least one focus region, by more than a threshold amount.

41. A method of operating an intruder detection system comprising at least one pair of detection nodes, each node comprising wireless communication means for supporting a wireless

communication link between paired nodes, wherein at least one node of the, or each, pair is capable of sending a wireless signal to the other node of the respective pair, said other node being capable of receiving said wireless signal, said wireless signal creating, in use, an electromagnetic field defining a detection zone between the respective pair of nodes, and wherein the system further comprises analysing means for analysing said wireless signal received from said at least one node of the pair to detect one or more characteristics of said received signal that is indicative of a disturbance in said electromagnetic field caused by the presence of a target in said detection zone, and wherein the wireless communication means of said at least one node is configurable to direct said wireless signal selectably in any one of a plurality of transmit directions, and the wireless communication means of said other node is configurable to receive said wireless signal selectably from any one of a plurality of receive directions, the method comprising:

controlling the paired nodes to direct said wireless signal between the paired nodes along a plurality of main signal paths in succession, each signal path corresponding to a respective one of said transmit directions and a corresponding respective one of said receive directions.