Abstract:
An object detection system ( 24 ) is disclosed having a transducer ( 40, 40 ′) for detecting buried objects ( 26 ). The transducer is encapsulated within a robust, electromagnetically transparent construction ( 42 ).

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates to a system and method for communicating with ground penetrating radar. More particularly, the present disclosure relates to a system and method for ground penetrating radar communication utilizing antenna crosstalk in the detection of buried objects. 
         [0003]    2. Description of the Related Art 
         [0004]    Many excavations are performed in well-developed, utility-congested areas. The congestion of underground space in many urban areas, combined with poor record keeping and difficulties in accurately locating buried utilities from the surface, has led to many inadvertent utility strikes during mechanical excavations. Utility strikes may lead to work-stop orders and delays, mechanical damage to buried utilities, and numerous costs associated with litigation, insurance, downtime, and repair. 
       SUMMARY 
       [0005]    According to an exemplary embodiment of the present disclosure, a method of communicating with ground penetrating radar is provided. The method includes the steps of providing a transmitter and a detector each coupled to a tool configured to penetrate the ground, communicating a ground-penetrating signal with the transmitter, and detecting crosstalk from the ground-penetrating signal with the detector. The method further includes the step of determining a position of at least one of the detector and the transmitter relative to the ground based on a characteristic of the crosstalk. 
         [0006]    According to another exemplary embodiment of the present disclosure, a method of communicating with ground penetrating radar is provided. The method includes the step of providing a tool and a detection system coupled to the tool. The detection system includes a transmitter and a receiver, and the tool is configured to penetrate the ground. The method further includes the steps of communicating a ground-penetrating signal with the transmitter and determining a position of the detection system relative to the ground based on signal coupling between the transmitter and the receiver. The method further includes the step of increasing a magnitude of the ground-penetrating signal upon a determination that the detection system is in contact with the ground. 
         [0007]    According to yet another exemplary embodiment of the present disclosure, a method of communicating with ground penetrating radar is provided. The method includes the step of providing a tool and a detection system coupled to the tool. The detection system includes a transmitter and a receiver and is configured to detect an object positioned in the ground. The method further includes the steps of penetrating the ground with the tool to create a penetration, communicating a signal between the transmitter and the object, and identifying crosstalk from the signal with the receiver. The method further includes the step of determining a dielectric property of the ground based on the crosstalk of the signal. 
         [0008]    According to still another exemplary embodiment of the present disclosure, a construction vehicle is provided including a chassis, a plurality of traction devices positioned to support the chassis, and a work tool supported by the chassis and configured to penetrate the ground. The vehicle further includes a detection system including a transmitter configured to communicate a ground-penetrating signal and a detector configured to detect crosstalk from the ground-penetrating signal. The transmitter and the detector are mounted to the work tool. The detection system is configured to determine a position of at least one of the detector and the transmitter relative to the ground based on a characteristic of the crosstalk from the ground-penetrating signal. 
         [0009]    According to another exemplary embodiment of the present disclosure, a detector assembly is provided that is configured to detect an object positioned in the ground. The detector assembly includes a housing structure having an interior region. The detector assembly further includes an antenna positioned in the interior region of the housing structure. The antenna is configured to communicate a ground-penetrating signal. A dielectric medium is molded around the housing structure. The housing structure and the dielectric medium cooperate to substantially reduce signal loss during a communication of the ground-penetrating signal between the antenna and the ground. 
         [0010]    According to yet another exemplary embodiment of the present disclosure, a method of communicating with ground-penetrating radar is provided. The method includes the step of providing a work tool, a transducer, and a housing structure having an interior region for receiving the transducer. The method further includes the steps of molding a dielectric medium around the housing structure, securing the transducer in the interior region of the housing structure, and coupling the housing structure and the transducer to the work tool. The method further includes the steps of penetrating the ground with the work tool to create a penetration and communicating a ground-penetrating signal with the transducer during the penetrating step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above-mentioned and other features of the present disclosure will become more apparent and the present disclosure itself will be better understood by reference to the following description of embodiments of the present disclosure taken in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a side elevation view of an excavator illustrating the excavator excavating an area and having a ground penetrating radar positioned towards a tip of the excavator bucket to detect objects located in the ground; 
           [0013]      FIG. 2  is a schematic diagram of ground penetrating radar illustrating the radar including a transmitting antenna and a receiving antenna that transmit and detect objections located in the ground; 
           [0014]      FIG. 3  is a perspective view of a portion of an excavator bucket tooth including an encapsulated transceiver antenna, shown in phantom; 
           [0015]      FIG. 4  is a plan view of the metallization layers of the encapsulated antenna of  FIG. 3 ; 
           [0016]      FIG. 5  is a view of the antenna of  FIG. 3  mounted on an excavator bucket; 
           [0017]      FIG. 6A  is a graphical representation of the signal detected by the antenna of  FIG. 2  with the transmitting and receiving antennas positioned above the ground consisting of soil; 
           [0018]      FIG. 6B  is a view similar to  FIG. 6A  showing the signal detected by the antenna with the transmitting and receiving antennas positioned in contact with the ground and illustrating a peak indicative of a plastic pipe located in the ground; 
           [0019]      FIG. 7A  is a graphical representation of the signal detected by the antenna of  FIG. 3  with the transmitting and receiving antennas positioned above the ground consisting of sandy soil with no object in the sandy soil; 
           [0020]      FIG. 7B  is a view similar to  FIG. 7A  illustrating the signal detected by the antenna with the transmitting and receiving antennas positioned in contact with the ground and illustrating a peak indicative of a steel pipe located in the sandy soil; 
           [0021]      FIG. 8A  is a graphical representation of the signal detected by the antenna of  FIG. 3  with the transmitting and receiving antennas positioned above the ground consisting of sandy soil with no object in the sandy soil; 
           [0022]      FIG. 8B  is a view similar to  FIG. 8A  illustrating the signal detected by the antenna with the transmitting and receiving antennas positioned in contact with the ground and illustrating a peak indicative of a polyethylene pipe located in the sandy soil; 
           [0023]      FIG. 9A  is a graphical representation of a signal detected by the antenna of  FIG. 3  without an object located in the soil; 
           [0024]      FIG. 9B  is a view similar to  FIG. 9A  illustrating a graphical representation of a detected signal with a steel pipe located about 6 inches (152 millimeters) deep in the soil; 
           [0025]      FIG. 9C  is a view similar to  FIG. 9A  illustrating a graphical representation of a detected signal with the steel pip located about 10 inches (254 millimeters) deep in the soil; 
           [0026]      FIG. 10  is a perspective view of an exemplary bucket tooth illustrating the tooth including four discone antennas and a Vivaldi antenna; 
           [0027]      FIG. 11  is an end view of the bucket tooth of  FIG. 10 ; 
           [0028]      FIG. 12  is an end view of an array of discone antennas; 
           [0029]      FIG. 13  is a top view of a combination of discone antenna arrays; 
           [0030]      FIG. 14  is a view of an excavator bucket showing discone antenna arrays mounted thereon; 
           [0031]      FIG. 15A  is a graphical representation of a signal detected by a first detector of an antenna array positioned in contact with the ground with two objects located in the ground; 
           [0032]      FIG. 15B  is a view similar to  FIG. 15A  illustrating a graphical representation of the signal transmitted and detected by a transceiver of the antenna array positioned in contact with the ground with two objects located in the ground; 
           [0033]      FIG. 15C  is a view similar to  FIG. 15A  illustrating a graphical representation of the signal detected by a second detector in the antenna array positioned in contact with the ground with two objects located in the ground; 
           [0034]      FIG. 16A  is a graphical representation of a cross-correlation of the signals illustrated in  FIGS. 15A and 15B ; 
           [0035]      FIG. 16B  is a graphical representation of a cross-correlation of the signals illustrated in  FIGS. 15A and 15C ; 
           [0036]      FIG. 16C  is a graphical representation of a cross-correlation of the signals illustrated in  FIGS. 15B and 15C ; 
           [0037]      FIG. 17A  is an exemplary hyperbola illustrating potential locations of buried objects based on the cross-correlated signals of  FIGS. 15A and 15B ; 
           [0038]      FIG. 17B  is an exemplary hyperbola illustrating potential locations of buried objects based on the cross-correlated signals of  FIGS. 15A and 15C ; 
           [0039]      FIG. 17C  is an exemplary hyperbola illustrating potential locations of buried objects based on the cross-correlated signals of  FIGS. 15B and 15C ; 
           [0040]      FIG. 18  is a view of the hyperbolas of  FIGS. 17A-17C  superimposed showing estimated locations of two buried objects; 
           [0041]      FIG. 19A  is an exemplary graphical representation of a signal detected with the object detection system of  FIG. 2  with the transmitting and receiving antennas positioned above the ground; and 
           [0042]      FIG. 19B  is a view similar to  FIG. 19A  showing a signal detected with the object detection system with the transmitting and receiving antennas positioned in contact with the ground. 
       
    
    
       [0043]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the disclosure and such exemplifications are not to be construed as limiting the scope of the disclosure in any manner. 
       DETAILED DESCRIPTION 
       [0044]    The embodiments disclosed below are not intended to be exhaustive or limit the disclosure to the precise form disclosed in the following detailed description. Rather, the embodiments are described so that others skilled in the art may utilize its teachings. 
         [0045]    An excavator  10  is shown in  FIG. 1  that includes a chassis  12  and a plurality of fraction devices  14 , such as tracks, that support and propel chassis  12  over the ground  16 . Excavator  10  further includes a boom  18  supporting a work tool or bucket  20  that is configured to penetrate the ground  16  to create a trench, hole, pit, or other depression  22  in the ground  16 . Excavator  10  further includes an object detection radar system  24 , shown in  FIG. 2 , which is configured to detect objects  26 , such as a utility pipes and wires, in the ground  16 . Although an excavator  10  is shown in  FIG. 1  and discussed in the application, other construction vehicles, such as backhoes, loaders, bulldozers, graders, and other constructions vehicles may be provided with object detection system  24 . Further, although traction devices  14  are shown as tracks, other traction devices, such as wheels may be provided on construction vehicle  10 . 
         [0046]    Portions of object detection radar system  24  are mounted on bucket  20 . According to the preferred embodiment of the present disclosure, detection system  24  includes a transmitter  28  and/or a receiver/detector  30  mounted on bucket  20 . For example, according to the embodiment shown in  FIG. 1 , transmitter  28  and detector  30  are mounted on one or more teeth  32  of bucket  20 . Transmitter  28  and detector  30  may also be mounted on other construction equipment work tools, such as bulldozer or grader blades, loader or backhoe buckets, or other work tools. 
         [0047]    With transmitter  28  and detector  30  mounted on teeth  32 , transmitter  28  and detector  30  are in direct contact with the ground  16  during excavation of depression  22 . By placing transmitter  28  and detector  30  in direct contact with the ground  16 , signal losses are reduced during communication of the ground-penetrating signal between the transducer and the ground  16 . 
         [0048]    Transmitter  28  is configured to emit electromagnetic waves and receiver  30  is configured to detect electromagnetic waves. In one embodiment, transmitter  28  and detector/receiver  30  utilize ultra-wide band (UWB) communication. As shown in  FIG. 2 , detection system  24  includes a signal generator  34 , such as a Picosecond Pulse Labs Generator Model 4500D, and a signal detection monitor  36 , such as a Tektronix Oscilloscope Model DSA 8200). Signal generator  34  provides a signal to transmitter  28  that emits the ground-penetrating signal into the ground  16  and provides a trigger signal to monitor  36 . Objects  26 , such as a pipe, reflect the ground-penetrating signal and detector  30  detects signals reflected off of object  26 . Monitor  36  provides a visual representation of the reflected signal for visual analysis. A computer  37  with a processor  39  may also be used to analyze the signal provided with detector  30 . 
         [0049]    One embodiment of transmitter  28  and detector  30  is shown in  FIG. 3  as a Vivaldi antipodal antenna. Each of transmitter  28  and detector  30  includes a Vivaldi antenna  40  and body  42  that encapsulates antenna  40 . Antenna  40  is an electromagnetic transducer that detects/converts electromagnetic waves into signals useable for analysis. As discussed below, other types of antennas and other transducers may also be used according to the present disclosure. 
         [0050]    After fabrication of antenna/transducer  40 , antenna/transducer  40  is encased in one or more materials that form body  42  to provide a protective casing or shell around antenna  40 . As illustrated in  FIG. 3 , body  42  illustratively includes a base end  66  and a cutting end  68  opposite base end  66 . Cutting end  68  is illustratively wider than base end  66 , although other configurations of body  42  may be provided. A cable  48  coupled at base end  66  of body  42  provides an electrical connection between antenna  40  and signal generator  34  ( FIG. 2 ). As illustrated in  FIG. 5 , cutting end  68  includes a cutting edge  50 , as described herein. 
         [0051]    According to the preferred embodiment of the present disclosure, body  42  of  FIG. 3  is made of a high strength dielectric medium. The dielectric material may be a polymer or a ceramic material that may include fiber reinforcements, such as micro-fibers or nano-fibers, for example, to enhance the durability of body  42 . For example, according to one embodiment, body  42  is made of high modulus polyurea with a dielectric constant of approximately 4. Other exemplary materials include rigid polyurethane, epoxy, other thermoplastic or thermoset materials, and other non-conductive materials. Body  42  may also be coated with materials to increase its durability and/or its abrasion resistance. Body  42  may also be coated with carbon or other electromagnetic insulating materials to insulate antenna  40  from adjacent conductive surfaces to reduce or prevent signal leakage, ringing, or other interference. Preferably, the dielectric medium has a dielectric constant about equal to the ground  16 . According to the present disclosure, the dielectric medium has a dielectric constant ranging from about 1 to about 20, but may have other values. In one embodiment, the dielectric medium serves to reduce signal loss during signal communication between antenna  40  and the ground  16 . 
         [0052]    In one embodiment, body  42  is molded around an insert to form a pocket for receiving antenna  40 . See, for example, insert  49  illustrated in  FIG. 3 . Insert  49  includes an interior region sized to receive antenna  40 . After molding the dielectric material of body  42  around insert  49 , antenna  40  may be inserted within the interior region of insert  49  in a secondary assembly operation. Alternatively, antenna  40  may be secured within insert  49  prior to molding body  42  around insert  49 . In one example, body  42  is made of a cast polyurethane formed around insert  49 . Other types of thermoset or thermoplastic materials and processes may be used for molding body  42  around insert  49 . In one embodiment, insert  49  is comprised of a high strength dielectric material having similar dielectric properties to body  42 . In one embodiment, insert  49  is comprised of a rigid plastic or other polymer material providing a high strength housing structure around antenna  40 . Antenna  40  may be preassembled before being secured within insert  49  or may be assembled within insert  49 . An insert structure, such as insert  49 , may be used to house other types of antennas or antenna arrays, such as discone antennas  40 ′ illustrated in  FIGS. 10-11  and described herein. 
         [0053]    As shown in  FIGS. 3 and 4 , antenna  40  includes three planes of material, which include upper and lower ground plates  44  with a conductive plate  46  sandwiched between ground plates  44 . Portions of conductive plate  46  positioned directly between ground plates  44  are shown in phantom in  FIG. 4 . The conductive plates  46  are preferably made of copper, but may be made of other metals, and other conductive materials. The dielectric/ground plates  46  may be made of epoxy, ceramic, Teflon®-brand polytetrafluoroethylene (PTFE) or other materials. In one embodiment, antenna  40  is about 135 millimeters (5.2 inches) long and about 45 millimeters (1.8 inches) high as shown in  FIG. 4 . 
         [0054]    In operation, antenna/transducer  40  and body  42  are mounted or otherwise coupled to tooth  32  as shown in  FIG. 5 . In one embodiment, a shank mount is used to couple body  42  to tooth  32 . A signal from signal generator  34  is provided to antenna  40  through cable  48 . During excavating, as shown in  FIG. 1 , antenna  40  and body  42  are repeatedly positioned in ground  16  as dirt and other materials are excavated. As a result, antenna  40  is often positioned below the lowest portions of tracks  14 . Further, antenna  40  is positioned into penetrations, such as depression  22 , created by excavator  10  during the excavation process. As shown in  FIG. 1 , antenna  40  of transmitter  28  and detector  30  are simultaneously positioned in the soil  16  as teeth  32  create penetrations in the soil  16 . In one embodiment, an antenna  40  is coupled to each tooth  32  of bucket  20 . In one embodiment, a transmitter  28  and several detectors  30  are coupled to teeth  32  of bucket  20 . 
         [0055]    While positioned in the penetrations, signals are transmitted and detected by antennas  40  of transmitter  28  and detector  30 . Because antenna  40  and dielectric body  42  are mounted on teeth  32 , they cooperate to define cutting elements of teeth  32  with portions of body  42  defining cutting edge  50  of tooth  32 . Thus, simultaneously with excavation, objects  26  are being detected. Further, because bodies  42  and antennas  40  are able to be lowered into penetrations  22  and assists in creating penetrations  22 , objects  26  are closer to antenna  40  and more easily detected than if one was attempting to detect objects  26  before any excavation started. In one embodiment, body  42  is positioned between antenna  40  and the soil to protect antenna  40  during excavation. As a result, the signals transmitted and received by antennas  40  pass through body  42  on their way from and antenna  40  during respective transmission of the signal and receipt of the reflected signal. 
         [0056]    Example outputs from detectors  30  are provided in  FIGS. 6A-9C . In  FIG. 6A , a signal is shown when antennas/transducers  40  of transmitter  28  and detector  30  are positioned above ground  16  without direct contact between the respective antennas  40  and ground  16 . A peak  52  is shown that indicates crosstalk between antenna  40  of transmitter  28  and antenna  40  of detector  30 . In  FIG. 6B , antennas  40  of respective transmitter  28  and detector  30  are placed in direct contact with ground  16 . In addition to showing a crosstalk peak  52 , a second peak  54  is shown indicating the presence of a 2 inch (51 millimeters) diameter polyethylene pipe that was buried 4 inches (102 millimeters) in the test soil. As a result, a perceptible indication is provided indicating that an object  26 , such as a plastic natural gas pipe, is in the path of bucket  20 . A trained operator of excavator  10  can notice this indication to avoid striking pipe  26 . Similarly, computer  37  can be programmed to recognize any peak after crosstalk peak  52  that satisfies a predetermined characteristic, such as slope. If computer  37  detects such a peak, or other predetermined characteristic, it can send an alarm, stop further movement of bucket  20 , or otherwise attempt to avoid bucket  20  striking pipe  26 . 
         [0057]    In addition to detecting objects  26 , the reflections detected by detector  30  can also be used to determine characteristics of objects  26  buried within the ground  16 . For example,  FIGS. 7A and 7B  illustrate the output of detector  30  for a 2 inch (51 millimeters) metal pipe buried in sandy soil at a depth of 4 inches (102 millimeters). In  FIG. 7A , antennas/transducers  40  of transmitter  28  and detector  30  are above the ground  16 . In  FIG. 7B , antennas  40  of transmitter  28  and detector  30  are in direct contact with the ground and provide a distinctive, “cursive v”  53  pattern indicative of the metal pipe.  FIGS. 8A and 8B  illustrate the output of detector  30  for a 1 inch (25 millimeters) polyethylene pipe buried in sandy soil at a depth of 2 inches (51 millimeters). In  FIG. 8A , antennas  40  of transmitter  28  and detector  30  are above the ground  16 . In  FIG. 8B , antennas  40  of transmitter  28  and detector  30  are in direct contact with the ground and provide a distinctive, “w” pattern  55  indicative of the plastic pipe. A trained operator of excavator  10  can notice the distinctive patterns  53 .  55  of metal, polyethylene, and other pipes to determine the type of pipe. Similarly, computer  37  can be programmed to recognize any peak after crosstalk peak  52  that satisfies a predetermined characteristic, such as the shape of patterns  53 ,  55 . If computer  37  detects such a pattern, or other predetermined characteristic, it can send an indication of the type of pipe, such as metal or plastic. 
         [0058]    In addition to determining the presence and type of object  26 , the reflections detected by detector  30  can also be used to determine the distance of object  26  from bucket  20  (or any other portion of excavator  10 ). Additional representations of the reflections detected by detector  30  are provided in  FIGS. 9A-9C . In  FIG. 9A , no object  26  is placed in the test soil so that no object  26  is detected when antennas  40  are placed in contact with ground  16 . In  FIG. 9B , a 2 inch (51 millimeters) diameter steel pipe was placed 6 inches (152 millimeters) deep in sandy soil and in  FIG. 9C , the same pipe was placed 10 inches (254 millimeters) deep in the sandy soil. As shown by the circled region in  FIGS. 9B and 9C , “cursive v” pattern  53  of the steel pipe occurs later in time in  FIG. 9B  than in  FIG. 9C  because the reflection took longer to reach detector  30  after being sent by transmitter  28 . A trained operator of excavator  10  can notice the gap in time between a feature, such as crosstalk peak  52 , and distinctive pattern  53  to determine the distance from object  26 . Similarly, computer  37  can be programmed to recognize the time delay and calculate the distance of tooth  32  of bucket  20  from object  26  and provide an indication to the operator of the distance and/or use the distance as a trigger for an alarm or otherwise. The operator may use this distance information when performing fine movements around objects  26 , such as known utility pipes or cables. 
         [0059]    Another embodiment of transmitter  28 ′ and detectors  30 ′ is shown in  FIGS. 10 and 11  that includes four discone antennas/transducers  40 ′ performing as detectors  30 ′ and a Vivaldi antipodal antenna  40  performing as a transmitter  28 ′. Combined transmitter/detector  56  includes body  42 ′ that encapsulates antennas  40 ,  40 ′ in a manner similar to body  42 , as described herein. An exemplary body  42 ′ includes a Swampers bucket tooth available from John Deere Company. To enhance the directionality of discone antennas  40 ′, if used as transmitters, antennas  40 ′ may be aligned in an array  58  as shown in  FIG. 12 . To further improve the directionality of antennas  40 ′, an array reflector may be positioned behind array  58 . In one embodiment, a reflective metal plate, such as plate  60  illustrated in  FIG. 13 , for example, is placed at the back of array  58 . In one embodiment, the array reflector is positioned between about 6 mm and 8 mm behind array  58 , although other suitable distances may be used. 
         [0060]    In  FIG. 13 , several arrays  58  with discone antennas  40 ′ are provided as detectors and a transmitter to detect objects  26 . In the illustrated embodiment, arrays  58  have differing numbers of discone antennas  40 ′. As illustrated in  FIG. 14 , arrays  58  may be placed on bucket  20  in locations other than on tooth  32 . For example, arrays  58  may be mounted to a side wall  62  or a front wall  64  near an edge of bucket  20 , although arrays  58  may be placed at other suitable locations. In one embodiment, discone antennas  40 ′ are embedded in a dielectric medium, such as a dielectric shield or casing. In the illustrated embodiment, the dielectric medium has a dielectric constant of about 4 or 5. In one embodiment, the dielectric medium may have a dielectric constant ranging from about 1 to 20, but may have other values. 
         [0061]    As described herein, object detection system  24  of  FIG. 2  may include a transmitter  28  that emits a ground-penetrating signal and several detectors  30  that detect the reflections of the ground-penetrating signal from one or more buried objects  26 . For example, bucket  20  of  FIG. 5  may include at least one transmitter/transceiver  28  and several detectors  30  mounted to teeth  32 . Similarly, arrays  58  of  FIGS. 13-14  may include a transmitter/transceiver  28  and multiple detectors  30 . In one embodiment, object detection system  24  includes at least three detectors/receivers  30 . 
         [0062]      FIGS. 15A-15C  provide exemplary outputs of multiple detectors  30  in an object detection system  24 . In  FIGS. 15A-15C , a transceiver  28  and two detectors  30  of  FIG. 2  are positioned in contact with the ground to detect two objects located in the ground. Transceiver  28  is positioned between the two detectors  30 , although other antenna arrangements may be provided. The signals of  FIG. 15A  correspond to a first detector  30 , the signals of  FIG. 15B  correspond to transceiver  28 , and the signals of  FIG. 15C  correspond to a second detector  30 . Transceiver  28  provides a ground-penetrating signal, as represented by pulse  70  of  FIG. 15B . After detecting crosstalk, shown at peaks  74  and  76  of  FIGS. 15A and 15C , each detector  30  detects a reflection from each of the two objects. Due to the position and spacing of the particular detectors  30  relative to the transceiver  28  and the buried objects, the reflections from the objects are detected at different times by each detector  30 . Peaks  78  of  FIG. 15A  represent the reflection from a first object received by the first detector  30 , and peaks  80  of  FIG. 15C  represent the reflection from the first object received later by the second detector  30 . Similarly, in  FIG. 15A , peaks  82  represent the reflection from a second object received by the first detector  30 , and peaks  84  of  FIG. 15C  represent the reflection from the second object received earlier by the second detector  30 . In the illustrated embodiment, transceiver  28  also detects reflections from the buried objects, as represented by peaks  86  in  FIG. 15B . 
         [0063]    As illustrated in  FIGS. 15A-15C , the reflections detected by each detector  30  have various magnitudes, times of arrival, curve signatures, and other properties that can be analyzed to determine the location and other characteristics, such as size or type, of the detected objects. Signal processor  39  of computer  37  may be programmed to analyze the reflection signals detected with detectors  30  and transceiver  28  to determine the position and other characteristics of the detected objects. 
         [0064]    In one embodiment, a time difference of arrival (TDOA) method is used to determine the relative location of the detected objects. In the TDOA method, processor  39  calculates the difference in the arrival times of the reflected signals at the different receivers of the antenna array. In the illustrated embodiment, these time delays are determined by calculating the pair-wise cross-correlation of the signal reflections measured at different receivers, and, based on the calculated time delays and dielectric properties of the soil, determining the relative positions (in a two- or three-dimensional space) of the target objects. The cross-correlation between two signal reflections from two different receivers may be represented as: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     12 
                   
                   = 
                   
                     
                       ∫ 
                       
                         - 
                         ∞ 
                       
                       ∞ 
                     
                      
                     
                       
                         
                           s 
                           1 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                        
                       
                         s 
                         2 
                       
                       * 
                       
                         ( 
                         
                           t 
                           - 
                           τ 
                         
                         ) 
                       
                        
                       
                           
                       
                        
                       
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein s 1  is the signal reflection detected at a first antenna, s 2  is the signal reflection detected at a second antenna, and τ is the time delay between the two signals. The time difference of the arrival of the reflected signals corresponds to the peak or maxima in the cross-correlation of the reflected signals. Using the exemplary signals of  FIGS. 15A-15C , exemplary maxima or peaks of the calculated correlation function (1) above are illustrated in  FIGS. 16A-16C . The maxima  100  shown in  FIG. 16A  represent the time delay between reflection signals received at the first detector  30  and the transceiver  28 . The maxima  102  shown in  FIG. 16B  represent the time delay between reflection signals received at the first detector  30  and the second detector  30 . The maxima  104  shown in  FIG. 16C  represent the time delay between reflection signals received at the transceiver  28  and the second detector  30 . 
         [0065]    The position of a target object is determined in a two-dimensional space based on the cross-correlation function (1) for each receiver pair. For example, the position of the object in a two-dimensional space may be determined by solving the following equation: 
         [0000]        d   ij =√{square root over (( X   i   −x ) 2 +( Y   i   −y ) 2 )}{square root over (( X   i   −x ) 2 +( Y   i   −y ) 2 )}−√{square root over (( X   j   −x ) 2 +( Y   j   −y ) 2 )}{square root over (( X   j   −x ) 2 +( Y   j   −y ) 2 )}  (2)
 
         [0000]    wherein (x, y) are the coordinates of the target, (X i , Y i ) and (X j , Y j ) are the coordinates of the transceiver  28  and/or receivers  30 , and d ij  is a difference in the target distance determined by the difference in the time of flight and the velocity of the signal propagation. 
         [0066]    The differences in the arrival times of the signal reflections determined from the cross-correlation function (1) are used to define hyperbolas for the cross-correlated signals of each receiver pair. In particular, each peak or maxima illustrated in  FIGS. 16A-16C  is used to calculate hyperbolas for each receiver pair. Each derived hyperbola represents estimated areas where the target object or objects may exist. The densities and widths of the hyperbolas are based on the height and the width of the maxima (peaks) of the cross-correlation function (1). Thus, a larger peak of the cross-correlation function (1) results in a greater density and width of the hyperbola. The position of the target object is determined by overlapping the hyperbolas from each receiver pair and identifying the areas of high density where the hyperbolas intersect. Thus, the denser and wider hyperbolas provide a greater indication of the location of the detected object(s). See, for example, the hyperbolas  150 - 154  illustrated separately in  FIGS. 17A-17C  and overlapped in  FIG. 18 . Hyperbolas  150 - 154  of  FIGS. 17A-17C  each represent areas where one or more objects may exist. Hyperbola  150  shown in  FIG. 17A  is based on the difference in the arrival times of the reflection signals received at the first detector  30  and the transceiver  28  (i.e., based on maxima  100  of  FIG. 16A ). Hyperbola  152  shown in  FIG. 17B  is based on time differences in the arrival of the reflection signals between the first detector  30  and the second detector  30  (i.e., based on maxima  102  of  FIG. 16B ). Hyperbola  154  shown in  FIG. 17C  is based on the time differences in the arrival of the reflection signals between the transceiver  28  and the second detector  30  (i.e., based on maxima  104  of  FIG. 16C ). 
         [0067]    The hyperbolas of  FIGS. 17A-17C  are shown superimposed in  FIG. 18 . The estimated positions of two detected objects correspond to the high-density areas  156 ,  158  where the hyperbolas intersect. As illustrated, area  156  indicates that a corresponding object is located about 35 cm in front of transceiver  28  and about 2 cm to the left of transceiver  28 . Area  158  indicates that a corresponding second object is located about 30 cm in front of transceiver  28  and about 5 cm to the right of transceiver  28 . In one embodiment, the estimated object positions determined in  FIG. 18  are obtained by sampling the reflection signals in time intervals of about 1 picoseconds. 
         [0068]    The position of a target object in a three-dimensional space is also based on the cross-correlation function (1) for each receiver pair. The position of the object in a three-dimensional space may be determined by introducing a third coordinate in equation (2) as follows: 
         [0000]        d   ij =√{square root over (( X   i   −x ) 2 +( Y   i   −y ) 2 +( Z   i   −z ) 2 )}{square root over (( X   i   −x ) 2 +( Y   i   −y ) 2 +( Z   i   −z ) 2 )}{square root over (( X   i   −x ) 2 +( Y   i   −y ) 2 +( Z   i   −z ) 2 )}−√{square root over (( X   j   −x ) 2 +( Y   j   −y ) 2 +( Z   j   −z ) 2 )}{square root over (( X   j   −x ) 2 +( Y   j   −y ) 2 +( Z   j   −z ) 2 )}{square root over (( X   j   −x ) 2 +( Y   j   −y ) 2 +( Z   j   −z ) 2 )}  (3)
 
         [0000]    wherein (x, y, z) are the coordinates of the target, (X i , Y i , Z i ) and (X j , Y j , Z j ) are the coordinates of the transceiver  28  and/or receivers  30 , and d ij  is a difference in the target distance determined by the difference in the time of flight and the velocity of the signal propagation. In the illustrated embodiment, the detection of a signal reflection from an object by a minimum of three detectors  30  is required to determine the three-dimensional position of the object with the TDOA method. In one embodiment, to determine the three-dimensional position of the object(s), the antennas of transceiver  28  and receivers  30  do not lie in the same line, i.e., the alignment of the antennas is offset to some extent. 
         [0069]    Knowledge of the dielectric properties of the soil may further be used in determining the location of detected objects. For example, the distance r of an object from a receiver is related to the time of arrival of the signal t (calculated as described above) and the velocity of the signal propagation v through the ground by the following equation: 
         [0000]        r=vt   (4)
 
         [0000]    The velocity v of the signal in the ground depends on the dielectric property of the ground or soil, as illustrated by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   v 
                   = 
                   
                     c 
                     ɛ 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein ∈ is the dielectric property of the ground and c is the speed of light in a vacuum. By knowing the dielectric property ∈ of the soil, the velocity v of the signal in the soil is determined with Equation (5). As such, the distance r of the object may be determined with Equation (4) based on the calculated velocity v and the time of arrival t measured at transceiver  28  and receivers  30 . In one embodiment, the dielectric property ∈ of the ground is determined based on crosstalk signals between the receivers, as described herein. 
         [0070]    Other methods of detecting the locations of objects  26  ( FIG. 1 ) may also be used, such as known methods including the time of arrival (TOA) method, the roundtrip time of flight (RTOF) method, the angle of arrival (AOA) method, and the received signal strength (RSS) method. 
         [0071]    Referring again to  FIGS. 1-2 , when transmitter  28  is positioned above ground  16 , the signals provided with transmitter  28  may interfere with other nearby radio frequency devices. To reduce the risk of signal interference, the magnitude and frequency of the signals provided with transmitter  28  are limited when antenna  40  of transmitter  28  is operating above ground  16 . In some areas, a regulation agency, such as the Federal Communications Commission, may impose restrictions on ground-penetrating radar communication by setting limits on the available bandwidth and power density of ultra-wide band signals or other types of signals. In one embodiment, transmitter  28  and detector  30  operate in a range of about 3.1 GHz to 10.6 GHz when positioned above ground  16 . 
         [0072]    By positioning antennas  40  of transmitter  28  and detector  30  in contact with the ground, the risk of signal interference with other nearby radio frequency devices is reduced. Further, government regulations may be less restrictive or inapplicable to underground radio frequency or ultra-wide band signal communication. Accordingly, the magnitude and frequency of the signal provided with transmitter  28  may be increased when antennas  40  of transmitter  28  and detector  30  are in contact with the ground, providing a greater penetration depth of the generated signal. As a result, objects at a greater distance or depth from transmitters  28  and detectors  30  are detectable by objection detection system  24 . With improved detection capability, object detection system  24  is able to provide greater advance warning upon detecting objects to allow for the avoidance of the detected objects. For example, the increased penetration depth of the generated signal provides additional time for signal processing and for an operator or a control system to react to avoid the detected object. 
         [0073]    In the illustrated embodiment, the crosstalk or signal coupling between transmitter  28  and detector  30  is used to determine whether transmitter  28  is in contact with the ground so that the power and/or pulse duration of the generated signal may be increased. With antennas  40  of transmitter  28  and detector  30  in close proximity, some radiating energy transmitted from transmitter  28  is received by antenna  40  of detector  30  directly without first reflecting off a target object, resulting in crosstalk detected at detector  30 . Soil or other ground medium between transmitting and receiving antennas  40  interferes with the crosstalk detected by the receiving antenna  40 . For example, the interference of the soil may result in a reduction in magnitude, a phase change, a change in slope, or another characteristic change of the crosstalk signal. As a result, a dynamic change in the crosstalk signal level or signature between adjacent or closely mounted transmitting and receiving antennas  40  may be used to detect when antennas  40  of transmitter  28  and detector  30  are in contact with the soil or ground. Further, the crosstalk signal may arrive later at detector  30  when transmitter  28  is placed in contact with the ground due to the ground interference. 
         [0074]    Referring to  FIGS. 19A and 19B , peaks  90  and  92  illustrate crosstalk between detector  30  and transmitter  28  after transmission of a signal pulse with transmitter  28 . In  FIG. 19A , antennas/transducers  40  of transmitter  28  and detector  30  are positioned above ground  16  without direct contact between the respective antennas  40  and ground  16 . In  FIG. 19B , antennas  40  of respective transmitter  28  and detector  30  are placed in direct contact with ground  16 . As illustrated, the slope of crosstalk peak  90  in  FIG. 19A  is steeper than the slope of crosstalk peak  92  in  FIG. 19B . Similarly, the magnitude of crosstalk peak  90  is greater than the magnitude of crosstalk peak  92 , illustrating a reduction in the crosstalk effect when antennas  40  of transmitter  28  and detector  30  are positioned in the ground. In one embodiment, the arrival time of the crosstalk signal at detector  30  after generation of the ground-penetrating signal pulse varies according to whether antennas  40  of transmitter  28  and detector  30  are positioned in the ground. In particular, the presence of soil between antennas  40  of transmitter  28  and detector  30  will cause a delay in the arrival of the crosstalk signal at detector  30  after transmission of the signal pulse by transmitter  28 . Accordingly, the characteristics and/or time delay of the crosstalk peak of the reflected signal provide a perceptible indication of whether antennas  40  of transmitter  28  and detector  30  are in contact with the ground. 
         [0075]    Based on the detected crosstalk response between transmitter  28  and detector  30 , the power level, pulse width, and/or wavelength of signals emitted from transmitter  28  may be automatically adjusted upon detection of transmitter  28  and detector  30  contacting the ground. In one embodiment, processor  39  of computer  37  analyzes the crosstalk response of the signals detected with detector  30  and initiates a control event upon detection that transmitter  28  is in contact with the ground. Based on the control event provided with processor  39 , signal generator  34  may automatically increase the magnitude and/or frequency of the ground-penetrating pulse from transmitter  28 . In one embodiment, computer  37  includes an analog-to-digital (A/D) converter  35  (see  FIG. 2 ) that provides a digital output to processor  39  that is representative of the magnitude of the crosstalk detected with detector  30 . Based on the magnitude or the time delay of the output of A/D converter  35 , processor  39  determines when transmitter  28  and detector  30  are positioned in the ground and causes signal generator  34  to increase the power and/or frequency of the ground penetrating signal provided with transmitter  28 . 
         [0076]    In one embodiment, the control event provided with processor  39  may enable or prompt an operator to manually increase the magnitude and/or frequency of the generated signal pulse. For example, detection system  24  may provide an audio or visual indication to an operator that transmitter  28  and/or detector  30  are in contact with the ground. In one embodiment, monitor  36  may provide a visual representation of the reflected signal for visual analysis by an operator. Upon observing a change in the characteristics of the crosstalk peak of the reflected signal, an operator may manually increase the power and/or frequency of the transmitted signal to increase the penetration depth of the transmitted signal. 
         [0077]    In one embodiment, other characteristics of the ground-penetrating signal provided with transmitter  28  may be altered upon detection that antennas  40  of transmitter  28  and/or detector  30  contact the ground. Exemplary characteristics include the directionality of the signal, the pulse duration, or other signal level or signature-related characteristics. 
         [0078]    The crosstalk response detected with detector  30  is also used to determine the dielectric properties of the soil, aggregate, or other ground material surrounding the transmitter  28  and detector  30 . In one embodiment, the dielectric properties of the soil are determined based on the arrival time of the crosstalk signal at detector  30  after transmission of the signal pulse with transmitter  28  when the antennas  40  are positioned in contact with the ground. In particular, based on the known distance between antennas  40  of transmitter  28  and detector  30  and the time between transmission of the signal pulse with transmitter  28  and detection of crosstalk with detector  30 , the dielectric properties of the ground are estimated. For example, using Equation (4) above, by knowing a distance r between two antennas  40  and measuring the time t between transmission of the signal pulse with transmitter  28  and detection of the cross talk with a detector  30 , the velocity v of the signal may be determined. By plugging the determined velocity v into Equation (5), the dielectric property ∈ of the surrounding ground material may be determined. Alternatively, the magnitude and/or slope of the crosstalk peak, such as crosstalk peak  92  of  FIG. 19B , for example, may also be used to estimate the dielectric properties of the soil or ground. The dielectric properties of the ground material may be used in calculating the location of the target object, as described herein. For example, the time difference of arrival (TDOA) method calculates the relative position of object  26  based on, among other parameters, the dielectric properties of the surrounding ground or soil. By analyzing the crosstalk between transmitter  28  and detector  30 , the dielectric properties of the soil are determined and considered in calculating the position of object  26 . 
         [0079]    While this invention has been described as having preferred designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.