Abstract:
The present disclosure relates to unattended ground sensors for detecting the presence of a pedestrian or vehicle in a monitored area using seismic sensors. Networks of simple and inexpensive sensors are disclosed that may be rapidly deployed. The networks may be formed from a dense array of low cost ground sensors having low sensitivity.

Description:
PRIORITY APPLICATION 
       [0001]    This application claims the benefit of priority of European Patent Application Serial No. 10382321.7, filed Nov. 29, 2010, entitled “Unattended Ground Sensors”, which is incorporated herein by reference. 
       BACKGROUND INFORMATION 
       [0002]    1. Field: 
         [0003]    The present disclosure relates to unattended ground sensors for detecting movement of a pedestrian or vehicle in a monitored area using seismic sensors. In particular, the present disclosure relates to a network of simple and inexpensive sensors that may be rapidly deployed, collected and redeployed. 
         [0004]    2. Background: 
         [0005]    Networks of unattended ground sensors have been developed to make use of wireless sensor networks for surveillance applications. For example, networks of unattended ground sensors may be used in wooded zones where surveillance using radar systems is not viable. 
         [0006]    A network typically comprises battery-powered devices that use various sensors to monitor physical or environmental properties, such as temperature, seismic waves or magnetic fields. In this way, the ground sensors may detect movement of pedestrians and vehicles. The ground sensors send data over the wireless network to a base station that processes the data provided by each ground sensor. 
         [0007]    To date, unattended ground sensors have been sophisticated, intelligent devices. Typically, each sensor will process data it receives to identify events corresponding to the passage of a pedestrian or vehicle. Further data processing may be performed by the ground sensor, such as discriminating between types of events (e.g., pedestrian, light vehicle, heavy vehicle, etc) and determining a position of the pedestrian or vehicle. As a result, each ground sensor is relatively expensive. In addition, the size of the ground sensors is relatively large and they are designed for long endurance in semi-permanent deployment. Thus, generally much effort is required to plan the deployment. 
         [0008]    The sensors themselves are either merely dropped into position or placed on the ground, where their size makes them relatively easy to detect. Where covertness is required, a hole must be dug so that the ground sensor can be at least partially buried, or other effort must be made to camouflage the ground sensor such as locating it within bushes or piles of stones if the required site is fortunate enough to have such features. 
       SUMMARY 
       [0009]    Against this background, and from a first aspect, an illustrative embodiment resides in an unattended ground sensor comprising a seismic sensor, a transmitter, a controller and a power supply. 
         [0010]    The seismic sensor is operable to detect seismic waves incident on the ground sensor and to generate seismic data therefrom. For example, the seismic sensor may provide an electrical signal with a quantity, such as voltage or current, that varies according to the strength of the seismic waves detected. The variation of the electrical signal will provide the data that corresponds to the detected seismic waves. The seismic sensor may be a geophone. 
         [0011]    The controller is operable to receive signals from the seismic sensor corresponding to the seismic data. The signals may be received directly from the seismic sensor or they may be received indirectly. For example, the ground sensor may further comprise an analogue to digital converter to digitize the seismic data provided by the seismic sensor. One or more amplifiers may be provided to amplify the signal, and one or more filters may be provided to condition the signal provided by the seismic sensor. For example, a bandpass filter may be provided such that only frequencies known to contain data indicative of a passing pedestrian or vehicle are provided to the controller. In this way, noise or events caused by small animals passing nearby may be filtered out. Other forms of commonly employed signal conditioning will be apparent to those skilled in the art. 
         [0012]    The controller may process the signal it receives that corresponds to the seismic data. For example, any of the signal conditioning steps like filtering and amplifying that were described above may be performed by the controller. The controller is operable to send signals to the transmitter for onward transmission. The signals include the signals that correspond to the seismic data, or are derived therefrom (e.g., after signal conditioning such as noise suppression). The signals may be passed directly or indirectly to the transmitter. 
         [0013]    The transmitter is operable to transmit signals from the ground sensor wirelessly, including signals that are derived from the seismic data. The wireless transmission is likely to use electromagnetic radiation. The transmitter may transmit at radio frequencies, although other methods are possible such as microwave transmission. The transmission may be directional, for example towards the known location of a base station. The transmitter may include components for converting the signals provided by the controller into a form suitable for driving an antenna. 
         [0014]    A power supply is arranged to provide power to the ground sensor. The power supply will provide power to at least the controller, and may also provide power to the transmitter and the seismic sensor. 
         [0015]    In contrast to prior art systems, an illustrative embodiment provides a small, simple and inexpensive ground sensor that may be used in a network of such sensors. The simplified design of the sensors allows rapid deployment, collection and subsequent redeployment. The lesser expense of the sensors means that loss of individual sensors, either through damage or by inadvertently being left behind, is tolerable. 
         [0016]    The outer body of the ground sensor is shaped to make rapid deployment as easy as possible. In particular, the outer body is shaped to aid pushing the ground sensor into the ground. In this respect, the ground sensor is provided with an elongate outer body that is longer than it is wide. The seismic sensor, the controller and the power supply are housed within the elongate outer body. The transmitter may also be housed within the elongate outer body. The seismic sensor, the controller, the power supply, the transmitter (when present within the outer body) and any other components may be stacked one above the other, thus allowing a narrower, longer outer body. Preferably, the transmitter is located at the top of the stack, e.g., at the top of the outer body. Preferably, the seismic sensor is located at the bottom of the stack. 
         [0017]    The outer body is conveniently provided with a narrowing base to help drive the ground sensor into the ground. The elongate outer body has a shape that tapers to an end. The end may take the form of a spike. The elongate outer body may taper to a point. Ribs may extend towards a point at the base of the ground sensor. For example, four ribs may extend around the ground sensor. Preferably the ribs are narrow to ease insertion into the ground. The ribs may extend from a base of a main body section of the outer body. This base may itself taper to a point. Thus, the ribs first penetrate the ground, and then the sloping surface of the base of the main section will push into the ground. Preferably, the main section of the body has a constant cross section. This cross section may be circular, such that the main section is cylindrical, and the base may be conical. 
         [0018]    With this design, the ground sensors may be pushed easily into the ground. To ensure the ground sensor is inserted into the ground to a desired depth, the elongate body may be provided with a collar at an end of ground sensor remote from base. The collar presents a widened portion, preferably with a flat underside. Thus the collar will provide a stop that prevents the ground sensor being pushed too far into the ground. The transmitter may be positioned at the top of the ground sensor adjacent to the collar such that the transmitter is located above ground, provided with a suitable field of view for transmission (e.g., such that the transmission may radiate across the ground rather than be directed mostly upwards). 
         [0019]    If the ground is soft enough, a person may simply push down on the top of the sensor and it will be driven into the ground. Where more force is required, this may be simply administered by standing on the sensor or by tapping the sensor into the ground using a mallet or the like. To aid insertion, the top may be provided with a flat upper surface. The outer body, and the top in particular, must be robust enough to withstand the physical insertion of the ground sensor into the ground, particularly bearing in mind this action will be repeated. 
         [0020]    Preferably, the ground sensor has a top that is spaced from the collar so as to form an air gap to admit air into interior of elongate outer body. This allows for ventilation and cooling of the components within the ground sensor. The top may be supported above the collar by supports equally spaced around the collar. The height of the ground sensor from the collar upwards, i.e., the portion of the ground sensor exposed above ground when deployed, may have a low profile thereby increasing covertness. For example, this height may be made to be less than 30 mm, preferably less than 20 mm, with a height of 16 mm being particularly preferred. 
         [0021]    Overall, the height of the ground sensor is preferably less than 200 mm, more preferably less than 150 mm and most preferably 148 mm. The height of the tapering end is preferably less than 100 mm, more preferably less than 65 mm, more preferably 60 mm or less, and most preferably 60 mm. The height of the main section of the outer body with constant cross section is preferably less than 100 mm, more preferably less than 80 mm and most preferably 76 mm. The width of the ground sensor at the collar is preferably less than 100 mm, more preferably less than 75 mm, most preferably less than 60 mm or 60 mm. The width of the constant cross section of the outer body is preferably less than 50 mm, more preferably 35 mm or less and most preferably 35 mm. 
         [0022]    As is described more fully below, it is preferred that relatively dense networks are formed from simple, cheap unattended ground sensors. Consequently, each ground sensor need not have a large detection range, i.e., the area within which the passage of a pedestrian or vehicle may be detected. Seismic sensors having a sensitivity in the range of 10 to 20 V/m/s are preferred, the range 12 to 18 V/m/s being further preferred, and the range 14 to 16 V/m/s most preferred. 
         [0023]    From a second aspect, an illustrative embodiment resides in a network of unattended ground sensors and a base station. Each ground sensor comprises a seismic sensor, a transmitter and a controller. The seismic sensor is operable to detect seismic waves incident upon the ground sensor and to generate seismic data therefrom. The transmitter is operable to transmit wirelessly signals from the ground sensor to the base station. The controller is operable to receive signals from the seismic sensor corresponding to the seismic data and to send signals to the transmitter for onward transmission to the base station. The base station comprises a receiver operable to receive the signals transmitted by the ground sensors. The network of ground sensors is arranged with a spacing between adjacent ground sensors of no more than 30 m, no more than 20 m, no more than 15 m or no more than 10 m. 
         [0024]    This arrangement provides a denser network of ground sensors. This may be illustrative where low cost sensors are being used. For example, each ground sensor may have a sensitivity in the range of 10 to 20 V/m/s, 12 to 18 V/m/s or 14 to 16 V/m/s. The network may comprise more than 25, 50, 100 or 250 ground sensors. This enables a less expensive network to be created with a greater number of low cost ground sensors. As well as reducing overall cost, this arrangement has a further benefit in providing a network with greater resolution due to the greater number of nodes. Moreover, better opportunities are provided in combining data produced from many more ground sensors, as will be described below. 
         [0025]    From a third aspect, an illustrative embodiment resides in a network of unattended ground sensors and a base station. 
         [0026]    Each ground sensor comprises a seismic sensor, a transmitter and a controller. The seismic sensor is operable to detect seismic waves transmitted to the ground sensor and to generate seismic data therefrom. The transmitter is operable to transmit wirelessly signals from the ground sensor to the base station. The controller is operable to receive signals from the seismic sensor corresponding to the seismic data and to send signals corresponding to the seismic data to the transmitter for onward transmission to the base station. 
         [0027]    The base station comprises a receiver and a processor. The receiver is operable to receive the signals corresponding to the seismic data transmitted by the ground sensors. The processor is operable to process the received signals corresponding to the seismic data and to identify events corresponding to the movement of a pedestrian or vehicle across the ground recorded by the ground sensor that provided the signal being processed. 
         [0028]    This arrangement sees responsibility for processing and analyzing the seismic data pushed to the base station, and not imposed upon the ground sensors. This simplifies the requirements for the ground sensors, thus allowing their cost to be reduced. Essentially, data corresponding to the seismic signals is sent from the ground sensors to the base station, where a processor performs the analysis. It is to be understood that the ground sensors may perform some manipulation of the seismic data, for example to reduce noise, to amplify the signal, or to select frequency bands of interest. In any event, the signal transmitted by the ground sensor still corresponds to the seismic signal recorded by the seismic sensor. No analysis has been performed at the ground sensor in the sense that no attempt has been made to determine whether an event corresponding to the movement of a pedestrian or vehicle has occurred. 
         [0029]    Thus, the processor at the base station collects seismic data from the ground sensors. The processor may be operable to analyze the received signals to characterize the pedestrian or vehicle detected. Preferably, the processor performs analysis of the seismic data from the ground sensors in a comparative sense, as well as analyzing the data from each ground sensor in isolation. For example, the data received may be fused, as is well known in the art. Comparative analysis may be used to verify the detection of events and/or to reduce the instances of false detections. Comparative analysis may be used track a pedestrian or vehicle. 
         [0030]    Features of the networks of ground sensors according to the second and third aspects of the illustrative embodiment may be freely mixed and used in combination. Also, any of the networks described above may include one or more of the ground sensors according to the first aspect of the illustrative embodiments and as modified according to any of the optional features described above. 
         [0031]    The illustrative embodiments also extend to methods associated with any of the ground sensors and networks described above. 
         [0032]    For example, the illustrative embodiments extend to a method of manufacturing an unattended ground sensor by installing into an elongate outer body, having a shape that tapers to an end, a seismic sensor operable to detect seismic waves incident on the ground sensor and to generate seismic data therefrom, a transmitter operable to transmit wirelessly signals from the ground sensor, a controller operable to receive signals from the seismic sensor corresponding to the seismic data and to send signals to the transmitter for onward transmission, and a power supply arranged to provide power to the ground sensor. 
         [0033]    The illustrative embodiments also extend to a method of deploying an unattended ground sensor comprising a seismic sensor operable to detect seismic waves transmitted to the ground sensor and to generate seismic data therefrom, a transmitter operable to transmit wirelessly signals from the ground sensor, a controller operable to receive signals from the seismic sensor corresponding to the seismic data and to send signals to the transmitter for onward transmission, a power supply arranged to provide power to the ground sensor, and an elongate outer body within which the seismic sensor, the controller and the power supply are housed. The elongate outer body has a shape that tapers to an end. The method comprises pushing the ground sensor into the ground by exerting a force on the top of the ground sensor such that the tapered end of the ground sensor is driven into the ground. 
         [0034]    The illustrative embodiments also extend to a method of forming a network of unattended ground sensors and a base station. Each ground sensor comprises a seismic sensor operable to detect seismic waves transmitted to the ground sensor and to generate seismic data therefrom, a transmitter operable to transmit wirelessly signals from the ground sensor to the base station, and a controller operable to receive signals from the seismic sensor corresponding to the seismic data and to send signals to the transmitter for onward transmission to the base station. The base station comprises a receiver operable to receive the signals transmitted by the ground sensors. The method comprises installing the ground sensors with a spacing between adjacent ground sensors of no more than 30 m. 
         [0035]    The illustrative embodiments also extend to a method of operating a network of unattended ground sensors and a base station, the method comprising: detecting seismic waves incident on the ground sensors and generating seismic data therefrom; transmitting signals corresponding to the seismic data from the ground sensors to the base station; receiving at the base station the signals corresponding to the seismic data transmitted by the ground sensors; and processing the received signals corresponding to the seismic data to identify events corresponding to the movement of a pedestrian or vehicle across the ground recorded by the ground sensor that provided the signal being processed. 
         [0036]    The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
           [0038]      FIG. 1  is a schematic representation of an unattended ground sensor according to an illustrative embodiments; 
           [0039]      FIG. 2  is a schematic representation of an unattended ground sensor according to another illustrative embodiments; 
           [0040]      FIG. 3  is a perspective view of an unattended ground sensor according to yet another illustrative embodiments; 
           [0041]      FIG. 4  is a partial cut-away perspective view of the unattended ground sensor of  FIG. 3 ; 
           [0042]      FIG. 5  is a side view of the unattended ground sensor of  FIG. 3 ; 
           [0043]      FIG. 6  is a longitudinal section through the unattended ground sensor of  FIG. 3  when deployed in the field; 
           [0044]      FIG. 7  is a schematic representation of a network of unattended ground sensors like those of  FIGS. 1 to 6 ; 
           [0045]      FIG. 8  is a schematic representation of another network of unattended ground sensors like those of  FIGS. 1 to 6 ; and 
           [0046]      FIG. 9  is a schematic plan view of a part of a network of unattended ground sensors like those of  FIGS. 1 to 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    An unattended ground sensor  10  is shown schematically in  FIG. 1  such that the main components of the ground sensor  10  may be seen clearly. The ground sensor  10  comprises a transmitter  12 , a controller  14  and a sensor  16 . In this embodiment, sensor  16  is a seismic sensor, such as a geophone. The seismic sensor  16  detects vibrations transmitted to the ground sensor  10  when the ground sensor  10  is deployed in the field. The ground sensor  10  is driven into the ground. Any movement across the ground close to the ground sensor  10  will cause vibrations to travel through the ground. These seismic vibrations impinging on the ground sensor  10  will be detected by the seismic sensor  16 , for example the vibrations may be reproduced as a voltage output of the seismic sensor  16 . A battery  18  provides power to the ground sensor  10 . 
         [0048]    The controller  14  manages operation of the ground sensor  10 . The controller  14  receives signals from the seismic sensor  16 , as shown by arrow  20 . The signals may correspond to the voltage output of a geophone. In addition, controller  14  may send signals to the seismic sensor  16 , as shown by arrow  22 , for example to set the frequency with which the seismic sensor  16  collects data. The controller  14  processes the signal received from the seismic sensor  16 . For example, the controller  14  may filter the signal to reduce noise, and/or may amplify the signal. The controller  14  may perform more sophisticated processing of the signal received from the seismic sensor  16 , for example according to algorithms to discriminate between actual movements of people or vehicles as opposed to other ambient vibrations. However, in preferred embodiments, the ground sensor  10  is simple and a relatively unsophisticated controller  14  is used that provides just conditioning of the signal received from the seismic sensor  16 . 
         [0049]    The controller  14  passes signals to the transmitter  12 , as shown by arrow  24 , for onward transmission to a base station or the like. For example, the controller  14  may provide the signal from the seismic sensor  16 , after conditioning, to the transmitter  12 . The controller  14  may send signals whenever they are available for transmission, or may store data in memory such that batches of data may be sent periodically. The transmitter  12  may be a transceiver capable of receiving signals that are passed to the controller  14 , as indicated by arrow  26 . For example, diagnostic signals may be sent to the ground sensor  10  to elicit a response thereby indicating that the ground sensor  10  is functioning correctly. 
         [0050]    The battery  18  provides power to the controller  14 , as indicated by the arrow  28 . The controller  14  may distribute power to the other components of the ground sensor  10  that require power, for example the transmitter  12  and/or the sensor  16 . Alternatively, power may be supplied direct from the battery  18 , as indicated by broken arrows  30  and  32 . The battery  18  may be rechargeable, and may be recharged from a solar cell provided on the ground sensor  10 . Other power sources may be used in place of the battery  18 , for example a fuel cell. 
         [0051]    The seismic sensor  16  may gather data continuously, or it may gather data periodically. How often the seismic sensor  16  gathers data may be set by the controller  14 , and may vary. For example, the frequency may increase at certain times of day, or after receiving a signal indicating movement nearby. The seismic sensor  16  may gather data continuously, but the controller  14  may accept a signal from the seismic sensor  16  only when the signal varies above a threshold. This allows general background noise to be ignored, and data to be collected only when a larger amplitude signal arises that is more indicative of movement nearby. 
         [0052]      FIG. 2  shows an alternative embodiment of a ground sensor  10 . This embodiment generally corresponds to the embodiment of  FIG. 1 , and the optional features described above with respect to  FIG. 1  apply to the embodiment of  FIG. 2  also. 
         [0053]    In the embodiment of  FIG. 2 , the battery  18  supplies electricity directly to the controller  14 . The controller  14  provides a regulated power supply to the remaining components of the ground sensor  10 , as indicated by the double-headed arrows  21 ,  25  and  46 . As noted above, the battery  18  may alternatively supply power directly to the other components of the ground sensor  10 . 
         [0054]    The transmitter  12  is a transceiver, although may be implemented to transmit alone. 
         [0055]    The ground sensor  10  of  FIG. 2  is provided with three sensors  16 ,  40  and  42 . A seismic sensor  16  like that of  FIG. 1  is included. In this embodiment, the ground sensor  10  further comprises a temperature sensor  40  and a magnetic field sensor  42 . Either or both of these additional sensors may be present in the illustrative embodiments. Temperature sensor  40  provides a signal that varies according to the ambient temperature, and the magnetic field sensor  42  provides a signal that varies with the sensed magnetic field. Variations in the temperature sensed and the magnetic field sensed may be indicative of a pedestrian or vehicle passing by the ground sensor  10 . 
         [0056]    In this embodiment, the seismic sensor  16 , the temperature sensor  40  and the magnetic field sensor  42  produce analogue signals. These analogue signals are passed to an ADC as shown by the arrows labeled  21 . ADC  44  is preferably a sixteen-bit ADC that digitizes the signals provided by the three sensors  16 ,  40  and  42 . The ADC  44  may provide further functionality, for example amplification and filtering. The ADC  44  provides the digitized signals from the three sensors  16 ,  40  and  42  to the controller  14 , as shown by arrow  46 . The signals may be passed in parallel or in series, for example after multiplexing. The controller  14  processes the digital signals received from the ADC  44 , and sends the results to the transmitter  12  for onward transmission, as previously described. 
         [0057]      FIGS. 3 to 6  show another embodiment of an unattended ground sensor  10 . The ground sensor  10  of  FIGS. 3 to 6  may conform to the schematic representations of  FIGS. 1 and 2 , and to the accompanying description above. 
         [0058]    As will be seen, the ground sensor  10  has a generally elongate body  100  comprising a generally cylindrical main section  102  that extends between a pointed base  104  and a collar  106 . The elongate body  100  is generally circular in cross-section. A cap  108  is supported above the collar  106  by six legs  110 . The cap  108  is solid and provides protection for the components that are housed within the elongate body  100 . An air passage  120  extends between the collar  106 , cap  108  and legs  110  and communicates with an aperture  122  provided in the top of the collar  106  to allow air to pass into the interior of the elongate body  100 . 
         [0059]    The pointed base  104  of the elongate body  100  comprises a conical part  112  and four ribs  114  that extend downwardly from the conical part  112  to meet at a sharp point  116 . The pointed base  104  is provided to aid insertion of the ground sensor  10  into the ground  200 , as shown best in  FIG. 6 . The sharp point  116  may be placed onto the ground  200 . The flat surface  118  provided on the top of the cap  108  allows weight to be exerted on the ground sensor  10  such that the sharp point  116  penetrates the ground  200 . The weight may be applied using a hand, foot or a tool such as a hammer. To this end, the elongate body  100  is constructed so as to be robust and not to break under such repeated action. 
         [0060]    As the sharp point  116  penetrates the ground  200 , the ground  200  is pushed aside by the four ribs  114  and the conical part  112  until a hole is formed in the ground  200  with sufficient size to provide clearance for the main section  102 . The ground sensor  10  is driven into the ground  200  until the underside of the collar  106  makes contact with the top surface of the ground  200 , as shown in  FIG. 6 . As will be appreciated, the majority of the ground sensor  10  resides within the ground  200  and so will pick up seismic vibrations travelling through the ground  200 . Only a small part of the ground sensor  10  projects above the ground  200 , meaning that the ground sensor is well hidden and less likely to be discovered. 
         [0061]      FIG. 5  shows dimensions of this particular embodiment of the ground sensor  10 . The ground sensor  10  has an overall height h 1  of 148 mm. The height h 2  of the ground sensor  10  above ground  200  is 16 mm, with the height h 5  of the air passage  120  being 5 mm. The main section  102  of the elongate body  100  has a height h 3  of 76 mm. The height h 4  of the pointed base  104  is 60 mm. The width w 1  of the ground sensor  10  at its widest point (the cap  108  and collar  106 ) is 50 mm. The main section  102  of the elongate body  100  has a width w 2  of 35 mm. 
         [0062]    The elongate body  100  is provided with a hollow center that houses several components. In contemplated embodiments, the internal diameter of the hollow center of the elongate body  100  varies in accordance with the components&#39; sizes. However, it is preferred not to form the elongate body  100  in this way, but instead to form the hollow center of the main section  102  with a constant internal diameter. A shaped sleeve  140  is then inserted into the hollow center of the elongate body  100 . The sleeve  140  has an outer diameter that matches the internal diameter of the elongate body  100  to ensure a snug fit. The internal diameter of the sleeve  140  varies to match the sizes of the components. This is shown in  FIGS. 4 and 6 . 
         [0063]    The elongate body  100  houses the components in a stack, one above the other. A geophone  150  is provided at the bottom of the stack. The geophone  150  is firmly supported by the sleeve  140 , and the snug fit of the sleeve  140  within the elongate body  100  ensures strong coupling of seismic waves from the ground  200  to the geophone  150 . The geophone  150  is small, having a diameter of 22.2 mm and a height of 25.4 mm. The geophone may be a type SM-7m miniature geophone, produced by Sensor Nederland b.v., Rouwkooplaan 8, 2251 AP Voorschoten, The Netherlands. 
         [0064]    Above the geophone  150 , there is a battery  160 . A lithium ion battery  160  is preferred, with a battery life of 19 Ah or 1.2 Ah or any value in between. A 1.2 Ah battery has been found to produce a typical working life of 60 days. The battery  160  may have any of the following properties: a rated voltage of 1.8 to 3.6 V (3.6 V being preferred), a maximum recommended continuous current of 50 mA, a maximum pulse current of 100 mA, and an operating range of −55° C. to +85° C. Such batteries are commonly available. 
         [0065]    Above the battery  160  there is a controller board  170 . The controller board  170  provides the controller  14  and may also provide the ADC  44 . The controller board  170  may be any commonly available electronics circuit board, such as an application specific integrated circuit (ASIC) board. The functions that may be implemented by the controller board  170  have been described above. The controller  14  may operate at 16 MIPS throughput up to 16 MHz. 
         [0066]    Above the controller board  170 , a radio board  180  is sandwiched between the controller board  170  and an antenna  190 . The radio board  180  converts the signals provided by the controller  14  into a form suitable for radio transmission, and provides this as a driving signal to the antenna  190 . In this particular embodiment, a single board is used to replace both the controller board  170  and the radio board  180 . An Atmel ATmega128RFA1 is used that is a single component based on the combination of an ATmega1281 microcontroller and an AT86RF231 radio transceiver. The component is available from Atmel Corporation, 2325 Orchard Parkway, San Jose, Calif. 95131, USA. In this embodiment, the antenna  190  is a 2 dBi chip antenna available from Fractus, Avda. Alcalde Barnils, 64-68, Sant Cugat del Vallés, 08174 Barcelona, Spain (part number FR05-SI-N-0-001). This antenna has an operating frequency range of 2400 to 2500 MHz, a peak gain of more than 2 dBi, a standing wave ratio (VSWR) less than 2:1, an impedance of 50Ω and can operate in the range −40° C. to +85° C. Frequencies of 868/915 MHz or 2.4 GHz may also be used. 
         [0067]    The electrical connections between the components are not shown in the  FIGS. 3 to 6 . Air passages, not shown in the figures, may be provided between the components to aid cooling. 
         [0068]    The ground sensor  10  may be taken apart to allow components to be renewed, such as the battery  160 . For example, the collar  106  may couple to the elongate body  100  via a screw thread. The collar  106  and cap  108  may then be unscrewed to allow access to the interior of the elongate body  100 . Where a sleeve is provided, this may be slid out from the elongate body  100 . The sleeve may be split longitudinally to allow access to the components. 
         [0069]      FIGS. 7 to 9  show a network  300  of unattended ground sensors  10 . The individual ground sensors  10  may be like any of those described above. Each ground sensor  10  within the network  300  need not be the same. The ground sensors  10  are spread out to form an array. Although the figures show fifteen to twenty ground sensors  10  in a network  300 , typically a network  300  would comprise a hundred or more ground sensors  10 . The ground sensors communicate with a base station  250  provided with a transceiver  252 . The base station  250  may be located remote from the network  300  of ground sensors  10 , as shown in  FIG. 7 . Alternatively, the base station  250  may be located within the network  300  of ground sensors  10 , as shown in  FIG. 8 . 
         [0070]    The base station  250  receives signals from each of the ground sensors  10 . The signals correspond to the data provided by the sensors  16 ,  40 ,  42  of the ground sensors  10 . 
         [0071]    As discussed above, it is preferred for the bulk of the data analysis to be performed centrally at the base station  250 . Accordingly, each ground sensor  10  performs only minimal processing of data received from the sensors  16 ,  40 ,  42  before transmitting the data to the base station  250 . 
         [0072]    Processors are provided at the base station  250  to analyze the incoming data. The data from each ground sensor  10  are analyzed to determine occurrences where a passing pedestrian or vehicle has been detected, as is well known in the art. Such an occurrence may cause an alarm to be generated. Further analysis may be able to characterize the occurrence, for example the movement of a pedestrian will provide a different signature to movement of a vehicle. Similarly, movement of heavy and light vehicles provides different signatures, as do wheeled vehicles compared to tracked vehicles. 
         [0073]    Data from several sensors may be fused, for example to allow determination of the position of a moving object through triangulation, for example. Moreover, an object may be tracked, either merely by following the occurrences as they are detected by one ground sensor  10 , followed by the next ground sensor  10 , and so on, or by determining successive positions of the object using multiple ground sensors  10  as described above. Fusing data from multiple ground sensors  10  also allows more reliable detection of events, i.e., allows better discrimination of false alarms. The ability to fuse data is an inherent advantage of the network arrangement described herein where data processing is handled centrally at the base station  250  rather than pushing processing out to more sophisticated ground sensors  10 . 
         [0074]      FIG. 9  shows an example of how the ground sensors  10  may be positioned within part of a network  300 . The ground sensors  10  need not be precisely positioned, as shown in  FIG. 9 . This conforms with the desire to provide a simple network of sensors that may be rapidly deployed. Thus, operators may simply distribute the ground sensors  10  quickly so that they adopt approximate spacings and such that best use can be made of the landscape (e.g., to avoid trees or hard, impenetrable ground). 
         [0075]    As shown in  FIG. 9 , the ground sensors  10  are placed to adopt an approximate array of rows and columns. The ground sensors  10  of any row (and column) are placed approximately midway between adjacent ground sensors  10  in the neighboring rows (or columns) to form a staggered pattern. 
         [0076]    In the example of  FIG. 9 , each ground sensor  10  has a nominal range r of 15 m thereby providing an effective area of coverage as shown by the dashed circles  202 . By nominal range, it is meant that the ground sensors have been found reliable to detect passing vehicles within 15 m of the detector. Of course, the precise range is a function of the ground in which they are sited and the success rate of identifying movements decays with distance from the ground sensor. Put simply, the ground sensors  10  of  FIG. 9  are assumed to be reliable to detect moving vehicles within 15 m of the sensor  10 , and are reliable to detect moving pedestrians within 10 m of the sensor  10 . 
         [0077]    The spacing d 1  between adjacent rows and columns of ground sensors  10  is set to be approximately 7.5 m. Thus, there is a spacing d 2  of 15 m between ground sensors  10  in every other row or column. Such an arrangement provides strong overlap between the areas of coverage as shown by dashed circles  202 . The spacings may be varied, for example to make a more dense network  300  to allow more accurate positioning through triangulation. Where triangulation is not required, a less dense configuration may be adopted as the need for overlap in the areas of coverage as shown by dashed circles  202  is less. Spacings may also be varied with respect to the environment. For example, a denser spacing may be used at and around paths and roads where traffic us expected to be more likely. 
         [0078]    It will be clear to the skilled person that variations may be made to the above embodiments without necessarily departing from the scope of the illustrative embodiments that are defined by the appended claims. 
         [0079]    The description of the different illustrative embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different illustrative embodiments may provide different features as compared to other illustrative embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.