Abstract:
An apparatus and method for horizontally drilling provides for detecting subsurface features and avoiding such features during closed-loop control of an underground drilling machine. A horizontal drilling system includes a base machine capable of propelling a drill pipe rotationally and longitudinally underground. A cutting tool system is coupled to the drill pipe, and a control system controls the base machine. A detector is employed to detect a subsurface feature. A communication link is utilized for transferring data between the detector and the control system. The control system uses the data generated by the detector to modify control of the base machine in response to detection of the subsurface feature.

Description:
“This is a divisional of patent divisional application Ser. No. 09/676,730, which was filed on Sep. 29, 2000, U.S. Pat. No. 6,435,286, which is a division of Ser. No. 09/311,085 May 13, 1999, U.S. Pat. No. 6,161,630, which is a continuation of 08/784,061 Jan. 17, 1997, U.S. Pat. No. 5,904,210, which is a CIP of 08/587,832 Jan. 11, 1996, U.S. Pat. No. 5,720,534, issued Feb. 24, 1998, which are hereby incorporated by reference herein.” 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to the field of trenchless underground boring and, more particularly, to a system and method for horizontal drilling and subsurface object detection. 
   Utility lines for water, electricity, gas, telephone and cable television are often run underground for reasons of safety and aesthetics. In many situations, the underground utilities can be buried in a trench which is then back-filled. Although useful in areas of new construction, the burial of utilities in a trench has certain disadvantages. In areas supporting existing construction, a trench can cause serious disturbance to structures or roadways. Further, there is a high probability that digging a trench may damage previously buried utilities, and that structures or roadways disturbed by digging the trench are rarely restored to their original condition. Also, an open trench poses a danger of injury to workers and passersby. 
   The general technique of boring a horizontal underground hole has recently been developed in order to overcome the disadvantages described above, as well as others unaddressed when employing conventional trenching techniques. In accordance with such a general horizontal boring technique, also known as microtunnelling or trenchless underground boring, a boring system is situated on the ground surface and drills a hole into the ground at an oblique angle with respect to the ground surface. Water is typically flowed through the drill string, over the boring tool, and back up the borehole in order to remove cuttings and dirt. After the boring tool reaches a desired depth, the tool is then directed along a substantially horizontal path to create a horizontal borehole. After the desired length of borehole has been obtained, the tool is then directed upwards to break through to the surface. A reamer is then attached to the drill string which is pulled back through the borehole, thus reaming out the borehole to a larger diameter. It is common to attach a utility line or other conduit to the reaming tool so that it is dragged through the borehole along with the reamer. 
   In order to provide for the location of a boring tool while underground, a conventional approach involves the incorporation of an active beacon, typically in the form of a radio transmitter, disposed within the boring tool. A receiver is typically placed on the ground surface and used to determine the position of the tool through a conventional radio direction finding technique. However, since there is no synchronization between the beacon and the detector, the depth of the tool cannot be measured directly, and the position measurement of the boring tool is restricted to a two dimensional surface plane. Moreover, conventional radio direction finding techniques have limited accuracy in determining the position of the boring tool. These limitations can have severe consequences when boring a trenchless underground hole in an area which contains several existing underground utilities or other natural or man-made hazards, in which case the location of the boring tool must be precisely determined in order to avoid accidentally disturbing or damaging the utilities. 
   Recently the use of ground penetrating radar (GPR) for performing surveys along trenchless boring routes has been proposed. Ground-penetrating-radar is a sensitive technique for detecting even small changes in the subsurface dielectric constant. Consequently, the images generated by GPR systems contain a great amount of detail, much of it either unwanted or unnecessary for the task at hand. A major difficulty, therefore, in using GPR for locating a boring tool concerns the present inability in the art to correctly distinguish the boring tool signal from all of the signals generated by the other features, such signals collectively being referred to as clutter. Moreover, depending on the depth of the boring tool and the propagation characteristics of the intervening ground medium, the signal from the boring tool can be extremely weak relative to the clutter signal. Consequently, the boring tool signal may either be misinterpreted or undetectable. 
   It would be desirable to employ an apparatus for detecting a natural or man-made subsurface feature and controlling an underground excavator to avoid such subsurface feature with greater response time and accuracy than is currently attainable given the present state of the technology. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method of horizontally drilling and subsurface feature detection. According to one embodiment, a horizontal drilling system includes a base machine capable of propelling a drill pipe rotationally and longitudinally underground. A cutting tool system is coupled to the drill pipe, and a control system controls the base machine. A detector is employed to detect a subsurface feature. A communication link is utilized for transferring data between the detector and the control system. The control system uses the data generated by the detector to modify control of the base machine in response to detection of the subsurface feature. 
   The subsurface feature may be a geological or man-made obstruction, in which case the control system uses the data generated by the detector to modify control of the base machine to avoid contact between the cutting tool system and the obstruction. The subsurface feature may also comprise a transition between a first subsurface geology and a second subsurface geology, in which case the control system uses the data generated by the detector to modify control of the base machine to modify one or both of cutting tool system direction and base machine propulsion in response to the detected subsurface geology transition. Cutting tool system and/or subsurface feature location and depth may be computed. 
   The detector can be integral with the cutting tool system. In such a configuration, the cutting tool includes a cutting element, a power source, a transmitter, and a receiver. In another configuration, the detector is communicatively coupled to the cutting tool system. In a further configuration, the detector operates cooperatively with the cutting tool system to detect the subsurface feature. In yet another configuration, the detector is situated above ground. According to another configuration, elements of the detector are respectively situated at or proximate the cutting tool system and above ground. The detector can include a ground penetrating radar unit, a beacon or an acoustic wave detection unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a trenchless underground boring apparatus in accordance with an embodiment of the present invention; 
       FIG. 2  is a detailed schematic side view of the trenchless underground boring tool and a probe and detection unit shown in  FIG. 1 ; 
       FIG. 3  is a graph depicting time domain signature signal generation; 
       FIG. 4  is a graph depicting frequency domain signature signal generation; 
       FIGS. 5   a - 5   c  show three embodiments for passive microwave signature signal generation; 
       FIGS. 6   a - 6   d  show four embodiments for active microwave signature signal generation; 
       FIGS. 7   a - 7   b  show two embodiments for active acoustic signature signal generation; 
       FIG. 8  shows an embodiment of a cooperative target incorporating a signature signal generator and an orientation detector; 
       FIG. 9  is an illustration of an orientation detector for detecting an orientation of a cooperative target; 
       FIG. 10  is a block diagram of an orientation detector which, in accordance with one embodiment, detects an orientation of a cooperative target and produces an output indicative of such orientation, and, in accordance with another embodiment, produces an output signature signal that indicates both a location and an orientation of the underground boring tool; 
       FIGS. 11   a - 11   b  illustrate an embodiment of an orientation detecting apparatus which includes a number of passive signature signal generating devices that provide both boring tool location and orientation information; 
       FIG. 12  illustrates another embodiment of an orientation detector that produces an output indicative of an orientation of the underground boring tool; 
       FIGS. 13   a - 13   b  illustrate another embodiment of a passive orientation detector that produces a signature signal indicative of both the location and orientation of the underground boring tool; 
       FIG. 14  illustrates an embodiment of an orientation detector suitable for incorporation in an underground boring tool that produces an output indicative of the rotational orientation and pitch of the boring tool; 
       FIG. 15  shows an embodiment of a boring tool incorporating an active signature signal generator and an orientation detection apparatus; 
       FIG. 16  is a diagram of a methodology for determining the depth of an underground boring tool incorporating a cooperative target by use of at least two receive antennas and a single transmit antenna; 
       FIG. 17   a  is a depiction of an underground boring tool tracking methodology using an array of two receive antennas and a transmit antenna provided within the receive antenna array; 
       FIG. 17   b  is a graph illustrating signature signal detection by each of the antennae in the receive antenna array of  FIG. 17   a  which, in turn, is used to determine a location and deviation of an underground boring tool relative to a predetermined above-ground path; 
       FIG. 18   a  is a depiction of an underground boring tool tracking methodology using an array of four receive antennas and a transmit antenna provided within the receive antenna array; 
       FIG. 18   b  is a graph illustrating signature signal detection by each of the four antennae in the receive antenna array of  FIG. 18   a  which, in turn, is used to determine a location and deviation of an underground boring tool relative to a predetermined above-ground path; 
       FIG. 19  is an illustration of a single-axis antenna system typically used with a ground penetrating radar system for providing two-dimensional subsurface geologic imaging; 
       FIG. 20  is an illustration of an antenna system including a plurality of antennae oriented in an orthogonal relationship for use with a ground penetrating radar system to provide three-dimensional subsurface geologic imaging in accordance with one embodiment of the invention; 
       FIG. 21  illustrates an embodiment of a trenchless underground boring tool incorporating various sensors, and further depicts sensor signal information; 
       FIG. 22  illustrates an embodiment of a trenchless underground boring tool incorporating an active beacon and various sensors, and further depicts sensor signal information; 
       FIG. 23  is an illustration of a boring site having a heterogeneous subsurface geology; 
       FIG. 24  is a system block diagram of a trenchless boring system control unit incorporating position indicators, a geographical recording system, various databases, and a geological data acquisition unit; 
       FIG. 25  is an illustration of a boring site and a trenchless boring system incorporating position location devices; 
       FIG. 26  illustrates in flow diagram form generalized method steps for performing a pre-bore survey; 
       FIG. 27  is a system block diagram of a trenchless underground boring system control unit for controlling the boring operation; and 
       FIGS. 28-29  illustrate in flow diagram form generalized method steps for performing a trenchless boring operation. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Referring now to the figures and, more particularly, to  FIG. 1 , there is illustrated an embodiment of a trenchless underground boring system incorporating elements for controlling horizontal drilling and subsurface feature detection. In one embodiment, the detection system includes an above-ground probing and detection unit  28  (PDU) and a below-ground cooperative target  20  mounted to, contained in, or otherwise coupled to an underground boring tool  24 . 
   The PDU  28  and the target  20  operate in cooperation to provide reliable and accurate locating of an underground boring tool  24 . In addition, the orientation of the boring tool  24  during operation may also be provided. In terms of general operation, the PDU  28  transmits a probe signal  36  into the ground  10  and detects return signals reflected from the ground medium and the underground boring tool  24 . The return signals typically includes content from many different reflection sources, often rendering detection of the underground boring tool  24  unreliable or impossible using conventional techniques. Detecting an underground boring tool  24  is greatly enhanced by use of the cooperative target  20 , which, in response to the probe signal  36 , emits a signature signal that is readily distinguishable from the return signals reflected by the ground medium and the underground boring tool  24 . The cooperative target  20  may also include an orientation detection apparatus that senses an orientation of the boring tool  24 . Boring tool orientation information may be transmitted with the location information as a composite signature signal or as an information signal separate from the signature signal. As such, the presence, location, and orientation of an underground boring tool  24  is readily and reliably determined by employing the probing and detection system and method of the present invention. 
   It is well known in the field of subsurface imaging that conventional underground imaging techniques, such as those that employ GPR, detect the presence of many types of underground obstructions and structures. It is also well known in the art that detecting objects of interest, such as an underground boring tool  24 , is often made difficult or impossible due to the detection of return signals emanating from many sources not of interest, collectively known as clutter, associated with other underground obstructions, structures, and varying ground medium characteristics, for example. The clutter signal represents background noise in the composite return signal above which a return signal of interest must be distinguished. Attempting to detect the presence of the underground boring tool  24  using a conventional approach often renders the boring tool  24  undetectable or indistinguishable from the background noise. 
   It is understood that the return signal from an underground object of interest using conventional detection techniques may be weak relative to the clutter signal content. In such a case, the signal-to-clutter ratio would be low, which reduces the ability to clearly detect the return signal emanating from the underground object of interest. The probe and detection apparatus and method of the present invention advantageously provides for the production of a return signal from the cooperative target  20  provided at the underground boring tool  24  having a characteristic signature which can be more easily distinguished from the clutter. As will be discussed in detail hereinbelow, the generation of a signature signal containing either or both location and orientation information by the cooperative target  20  may be performed either passively or actively. 
     FIG. 1  illustrates a cross-section through a portion of ground  10  where the boring operation takes place, with most of the components of the detection system depicted situated above the ground surface  11 . The trenchless underground boring system, generally shown as the system  12 , includes a platform  14  on which is situated a tilted longitudinal member  16 . The platform  14  is secured to the ground by pins  18  or other restraining members in order to prevent the platform  14  from moving during the boring operation. Located on the longitudinal member  16  is a thrust/pullback pump  17  for driving a drill string  22  in a forward, longitudinal direction as generally shown by the arrow. The drill string  22  is made up of a number of drill string members  23  attached end-to-end. Also located on the tilted longitudinal member  16 , and mounted to permit movement along the longitudinal member  16 , is a rotating motor  19  for rotating the drill string  22  (illustrated in an intermediate position between an upper position  19   a  and a lower position  19   b ). In operation, the rotating motor  19  rotates the drill string  22  which has a boring tool  24  at the end of the drill string  22 . 
   A typical boring operation takes place as follows. The rotating motor  19  is initially positioned in an upper location  19   a  and rotates the drill string  22 . While the boring tool  24  is rotated, the rotating motor  19  and drill string  17  are pushed in a forward direction by the thrust-pullback pump  20  toward a lower position into the ground, thus creating a borehole  26 . The rotating motor  19  reaches a lower position  19   b  when the drill string  22  has been pushed into the borehole  26  by the length of one drill string member  23 . A new drill string member  23  is then added to the drill string  22  either manually or automatically, and the rotating motor  19  is released and pulled back to the upper location  19   a . The rotating motor  19  then clamps on to the new drill string member  23  and the rotation/push process is repeated so as to force the newly lengthened drill string  22  further into the ground, thereby extending the borehole  26 . Commonly, water is pumped through the drill string  22  and back up through the borehole to remove cuttings, dirt, and other debris. If the boring tool  24  incorporates a directional steering capability for controlling its direction, a desired direction can be imparted to the resulting borehole  26 . 
   In  FIG. 1 , there is illustrated a borehole  26  which bends in the vicinity of a point  31  after the initial oblique section becomes parallel to the ground surface  11 . Located above the surface  11 , and detachable from the trenchless underground boring system  12 , is a probing and detection unit  28  (PDU), mounted on wheels  29  or tracks in order to permit above-ground traversing of the PDU  28  along a path corresponding to the underground path of the boring tool  24 . The PDU  28  is coupled to a control unit  32  via a data transmission link  34 . 
   The operation of the PDU  28  is more clearly described in reference to FIG.  2 . The PDU  28  is generally used to transmit a probe signal  36  into the ground and to detect returning signals. The PDU  28  contains a generator  52  for generating the probe signal  36  which probes the ground  10 . A transmitter  54  receives the probe signal  36  from the generator  52 , which, in turn, transmits the probe signal  36  (shown as continuous lines in  FIG. 2 ) into the ground  10 . In a first embodiment, the generator  52  is a microwave generator and the transmitter  54  is a microwave antenna for transmitting microwave probe signals. In an alternative embodiment, the generator  52  is an acoustic wave generator and produces acoustic waves, and the transmitter  54  is typically a probe placed into the ground  10  to provide for good mechanical contact for transmitting the acoustic waves into the ground  10 . 
   The probe signal  36  is transmitted by the PDU  28 , propagates through the ground  10 , and encounters subsurface obstructions, one of which is shown as  30 , which scatter a return signal  40  (shown as dotted lines in  FIG. 2 ) back to the PDU  28 . A signature signal  38  (shown as dashed lines in  FIG. 2 ) is also returned to the PDU  28  from the boring tool  24  located in the borehole  26 . 
   The detection section of the PDU  28  includes a receiver  56 , a detector  58 , and a signal processor  60 . The receiver  56  receives the return signals from the ground and communicates them to the detector  58 . The detector  58  converts the return signals into electric signals which are subsequently analyzed in the signal processing unit  60 . In the first embodiment described hereinabove in which the probe signal  36  constitutes a microwave signal, the receiver  56  typically includes an antenna, and the detector  58  typically includes a detection diode. In another embodiment in which the probe signal  36  constitutes an acoustic wave, the receiver  56  typically is a probe in good mechanical contact with the ground  10  and the detector  58  includes a sound-to-electrical transducer, such as microphone. The signal processor  60  may include various preliminary components, such as a signal amplifier, a filtering circuit, and an analog-to-digital converter, followed by more complex circuitry for producing a two or three dimensional image of a subsurface volume which incorporates the various underground obstructions  30  and the boring tool  24 . The PDU  28  also contains a beacon receiver/analyzer  61  for detecting and interpreting a signal from an underground active beacon. The function of the beacon receiver/analyzer  61  will be described more fully hereinbelow. 
   The PDU  28  also contains a decoder  63  for decoding information signal content that may be encoded on the signature signal produced by the cooperative target  20 . Orientation, pressure, and temperature information, for example, may be sensed by appropriate sensors provided in the cooperative target  20 , such as a strain gauge for sensing pressure. Such information may be encoded on the signature signal, such as by modulating the signature signal with an information signal, or otherwise transmitted as part of, or separate from, the signature signal. When received by the receiver  56 , an encoded return signal is decoded by the decoder  61  to extract the information signal content from the signature signal content. It is noted that the components of the PDU  28  illustrated in  FIG. 2  need not be contained within the same housing or supporting structure. 
   Referring once again to  FIG. 1 , the PDU  28  transmits acquired information along the data transmission link  34  to the control unit  32 , which is illustrated as being located in proximity to the trenchless underground boring system  12 . The data transmission link  34  is provided to handle the transfer of data between the PDU  28  and the trenchless underground boring system  12 , and may be a co-axial cable, an optical fiber, a free-space link for infrared communication, or some other suitable data transfer medium or technique. A significant advantage of using a trenchless underground boring system  12  which employs the subsurface detection technique described herein concerns the detection of other important subsurface features which may purposefully be avoided by the boring tool  24 , particularly buried utilities such as electric, water, gas, sewer, telephone lines, cable lines, and the like. 
   Signature signal generation, in accordance with the embodiments of  FIGS. 3 and 4 , may be accomplished using temporal and frequency based techniques, respectively.  FIG. 3  is an illustration depicting the generation and detection of an underground boring tool signature signal in the time domain. Line A shows the emission of a probe signal  36   a  as a function of signal character plotted against time. Line B shows a return signal  62   a  detected by the PDU  28  in the absence of any signature signal generation. The return signal  62   a  is depictive of a signal received by the PDU  28  at a time ΔT 1  after emission of the probe signal  36   a , and is represented as a commixture of signals returned from the underground structure  22  and other scatterers. As previously discussed, a low signal-to-clutter ratio makes it very difficult to distinguish the return signal from the underground boring tool  24 . 
   Line C illustrates an advantageous detection technique in which cooperation between the cooperative target  22 , provided at the boring tool  24 , and the PDU  28  is employed to produce and transmit a signature signal at a certain time ΔT 2  following illumination with the probe signal  36   a . In accordance with this detection scheme, the return signal  40   a  received from the scatterers is detected initially, and the signature signal  38   a  received from the underground boring tool  24  is detected after a delay of ΔT 2 . The delay time ΔT 2  is established to be sufficiently long so that the signature signal produced by the cooperative target  20  is significantly more pronounced than the clutter signal at the time of detection. In this case, the signal-to-clutter ratio of the signature signal  38   a  is relatively high, thus enabling the signature signal  38   a  to be easily distinguished from the background clutter  40   a.    
     FIG. 4  is an illustration depicting the detection of a cooperative target signature signal emitted from an underground boring tool  24  in the frequency domain. Line A illustrates the frequency band  36   b  of the probe signal as a function of signal strength plotted against frequency. Line B shows a frequency band  62   b  of a return signal received from the underground boring tool  24  in the absence of any cooperative signal generation. It can be seen that the naturally occurring return signals from the underground boring tool  24  and other scatterers  30  share a frequency band  62   b  similar to that of the probe signal  36   b . Line C illustrates a case where cooperation is employed between the cooperative target  20  of the underground boring tool  24  and the PDU  28  to produce and transmit a signature signal which has a frequency band  38   b  different from that of the scattered return signal  40   b . The difference in frequency band, indicated as Δf, is sufficiently large to move the cooperative target signature signal out of, or at least partially beyond, the scattered signal frequency band  40   b . Thus, the cooperative target signature signal can be detected with relative ease due to the increased signal-to-clutter ratio. It is noted that high pass, low pass, and notch filtering techniques, for example, or other filtering and signal processing methods may be employed to enhance cooperative target signature signal detection. 
   It is an important feature of the invention that the underground boring tool  24  be provided with a signature signal-generating apparatus, such as a cooperative target  20 , which produces a signature signal in response to a probe signal transmitted by the PDU  28 . If no such signature signal was produced by the generating apparatus, the PDU  28  would receive an echo from the underground boring tool  24  which would be very difficult to distinguish from the clutter with a high degree of certainty using conventional detecting techniques. The incorporation of a signature signal generating apparatus advantageously provides for the production of a unique signal by the underground boring tool  24  that is easily distinguishable from the clutter and has a relatively high signal-to-clutter ratio. As discussed briefly above, an active or passive approach is suitable for generating the boring tool signature signal. It is understood that an active signature signal circuit is one in which the circuit used to generate the signature signal requires the application of electrical power from an external source, such as a battery, to make it operable. A passive circuit, in contrast, is one which does not utilize an external source of power. The source of energy for the electrical signals present in a passive circuit is the received probe signal itself. 
   In accordance with a passive approach, the cooperative target  20  does not include an active apparatus for generating or amplifying a signal, and is therefore generally less complex than an active approach since it does not require the presence of a permanent or replaceable power source or, in many cases, electronic circuitry. Alternatively, an active approach may be employed which has the advantage that it is more flexible and provides the opportunity to produce a wider range of signature response signals which may be more identifiable when encountering different types of ground medium. Further, an active approach reduces the complexity and cost of manufacturing the cooperative target  20 , and may reduce the complexity and cost of the signature signal receiving apparatus. 
   Three embodiments of a passive signature signal generating apparatus associated with a microwave detection technique are illustrated in FIG.  5 . Each of the embodiment illustrations shown in  FIG. 5  includes a schematic of a cooperative target including a microwave antenna and circuit components which are used to generate the signature signal. The three embodiments illustrated in  FIGS. 5   a ,  5   b , and  5   c  are directed toward the generation of the signature signal using a) the time domain, b) the frequency domain and c) cross-polarization, respectively. 
   In  FIG. 5   a , there is illustrated a cooperative target  20  which includes two antennae, a probe signal receive antenna  66   a , and a signature signal transmit antenna  68   a . For purposes of illustration, these antennae are illustrated as separate elements, but it is understood that microwave transmit/receive systems can operate using a single antenna for both reception and transmission. Two separate antennae are used in the illustration of this and the following embodiments in order to enhance the understanding of the invention and, as such, no limitation of the invention is to be inferred therefrom. The receive antenna  66   a  and the transmit antenna  68   a  in the physical embodiment of the signature signal generator will preferably be located inside the cooperative target  20  or on its surface in a conformal configuration. For antennae located entirely within the cooperative target  20 , it is understood that at least a portion of the cooperative target housing is made of a non-metallic material, preferably a hard dielectric material, thus allowing passage of the microwaves through at least a portion of the cooperative target housing. A material suitable for this application is KEVLAR®. Antennae that extend outside of the cooperative target housing may be covered by a protective non-metallic material. The antennae, in this configuration, may be made to conform to the housing contour, or disposed in recesses provided in the housing and covered with an epoxy material, for example. 
   The illustration of  FIG. 5   a  shows the signature signal generation apparatus for a microwave detection system operating in the time domain. In accordance with this embodiment, a receive antenna  66   a  receives a probe signal  70   a  from the PDU  28 , such as a short microwave burst lasting a few nanoseconds, for example. In order to distinguish a signature signal  74   a  from the clutter received by the PDU  28 , the received probe signal  70   a  passes from the receive antenna  66   a  into a time-delaying waveguide  72   a , preferably a co-axial cable, to a transmit antenna  68   a . The signature signal  74   a  is then radiated from the transmit antenna  68   a  and received by the PDU  28 . The use of the time-delay line, which preferably delays the response from the cooperative target  20  by about 10 nanoseconds, delays radiating the return signature signal  74   a  until after the clutter signal received by the PDU  28  has decreased in magnitude. 
   In accordance with another embodiment, a single antenna embodiment of the passive time domain signature generator could be implemented by cutting the waveguide at the point indicated by the dotted line  76   a  to form a termination. In this latter embodiment, the probe signal  70   a  propagates along the waveguide  72   a  until it is reflected by the termination located at the cut  76   a , propagates back to the receive antenna  66   a , and is transmitted back to the PDU  28 . The termination could be implemented either as an electrical short, in which case the probe signal  70   a  would be inverted upon reflection, or as an open circuit, in which case the probe signal  70   a  would not be inverted upon reflection. 
   The introduction of a time delay to create the signature signal  74   a  makes the underground boring tool  24  appear deeper in the ground than it is in actuality. Since microwaves are heavily attenuated by the ground, ground penetrating radar systems have a typical effective depth range of about 10 feet when employing conventional detection techniques, beyond which point the signal returns are generally too heavily attenuated to be reliably detected. The production of a time delayed signature signal return  74   a  from the underground boring tool  24  artificially translates the depth of the underground boring tool  24  to an apparent depth in the range of 10 to 20 feet, a depth from which there is generally no other strong signal return, thus significantly enhancing the signal-to-clutter ratio of the detected signature signal  74   a . The actual depth of the underground boring tool  24  may then be determined by factoring out the artificial depth component due to the known time delay associated with the cooperative target  20 . It is believed that the signature signal generated by a cooperative target  20  may be detectable at actual depths on the order of 100 feet. It is further believed that a signature signal generated by an active device will generally be stronger, and therefore more detectable, than a signature signal produced by a passive device. 
   The illustration of  FIG. 5   b  depicts a signature signal generating apparatus for a microwave detection system operating in the frequency domain. In accordance with this embodiment, a receive antenna  66   b , provided in or on the boring tool  24 , receives a microwave probe signal  70   b  from the PDU  28 . The probe signal  70   b  is preferably a microwave burst, lasting for several microseconds, which is centered on a given frequency, f, and has a bandwidth of Δf 1 , where Δf 1 /f is typically less than one percent. In order to shift a return signature signal  74   b  out of the frequency regime associated with the clutter received by the PDU  28 , the received probe signal  70   b  propagates from the receive antenna  66   b  along a waveguide  72   b  into a nonlinear device  78   b , preferably a diode, which generates harmonic signals, such as second and third harmonics, from an original signal. 
   The harmonic signal is then radiated from a transmit antenna  68   b  as the signature signal  74   b  and is received by the PDU  28 . The PDU  28  is tuned to detect a harmonic frequency of the probe signal  70   b . For a probe signal  70   b  of 100 MHz, for example, a second harmonic detector  58  would be tuned to 200 MHz. Generally, scatterers are linear in their response behavior and generate a clutter signal only at a frequency equal to that of the probe signal  70   b . Since there is generally no other source of the harmonic frequency present, the signal-to-clutter ratio of the signature signal  74   b  at the harmonic frequency is relatively high. In a manner similar to that discussed hereinabove with respect to the passive time domain embodiment, the passive frequency domain embodiment may be implemented using a single antenna by cutting the waveguide at the point indicated by the dotted line  76   b  to form a termination. In accordance with this latter embodiment, the probe signal  70   b  would propagate along the waveguide  72   b , through the nonlinear element  78   b , reflect at the termination  76   b , propagate back through the nonlinear element  78   b , propagate back to the receive antenna  66   b , and be transmitted back to the PDU  28 . The polarity of the reflection would be determined by the nature of the termination, as discussed hereinabove. 
   The illustration of  FIG. 5   c  depicts signature signal generation for a microwave detection system operating in a cross-polarization mode. In accordance with this embodiment, the PDU  28  generates a probe signal  70   c  of a specific linear polarity which is then transmitted into the ground. The clutter signal is made up of signal returns from scatterers which, in general, maintain the same polarization as that of the probe signal  70   c . Thus, the clutter signal has essentially the same polarization as the probe signal  70   c . A signature signal  74   c  is generated in the cooperative target  20  by receiving the polarized probe signal  70   c  in a receive antenna  66   c , propagating the signal through a waveguide  72   c  to a transmit antenna  68   c , and transmitting the signature signal  74   c  back to the PDU  28 . The transmit antenna  68   c  is oriented so that the polarization of the radiated signature signal  74   c  is orthogonal to that of the received probe signal  70   c . The PDU  28  may also be configured to preferentially receive a signal whose polarization is orthogonal to that of the probe signal  70   c . As such, the receiver  56  preferentially detects the signature signal  74   c  over the clutter signal, thus improving the signature signal-to-clutter ratio. 
   In a manner similar to that discussed hereinabove with respect to the passive time and frequency domain embodiments, the cross-polarization mode embodiment may be implemented using a single antenna by cutting the waveguide at the point indicated by the dotted line  76   c  to form a termination and inserting a polarization mixer  78   c  which alters the polarization of the wave passing therethrough. in this latter embodiment, the probe signal would propagate along the waveguide  72   c , through the polarization mixer  78   c , reflect at the termination  76   c , propagate back through the polarization mixer  78   c , propagate back to the receive antenna  66   c  and be transmitted back to the PDU  28 . The polarity of the reflection may be determined by the nature of the termination, as discussed previously hereinabove. It is understood that an antenna employed in the single antenna embodiment would be required to have efficient radiation characteristics for orthogonal polarizations. It is further understood that the cross-polarization embodiment may employ circularly or elliptically polarized microwave radiation. It is also understood that the cross-polarization embodiment may be used in concert with either the passive time domain or passive frequency domain signature generation embodiments described previously with reference to  FIGS. 5   a  and  5   b  in order to further enhance the signal-to-clutter ratio of the detected signature signal. 
   Referring now to  FIG. 6 , active signature signal generation embodiments will be described.  FIG. 6   a  illustrates an embodiment of active time domain signature signal generation suitable for incorporation in a boring tool  24 . The embodiment illustrated shows a probe signal  82   a  being received by a receive antenna  84   a  which is coupled to a delay-line waveguide  86   a . An amplifier  88   a  is located at a point along the waveguide  86   a , and amplifies the probe signal  82   a  as it propagates along the waveguide  86   a . The amplified probe signal continues along the delay-line waveguide  86   a  to the transmit antenna  90   a  which, in turn, transmits the signature signal  92   a  back to the PDU  28 .  FIG. 6   b  illustrates an alternative embodiment of the active time domain signature generator which incorporates a triggerable delay circuit for producing the time-delay, rather than propagating a signal along a length of time-delay waveguide. The embodiment illustrated shows a probe signal  82   b  being received by a receive antenna  84   b  coupled to a waveguide  86   b . A triggerable delay circuit  88   b  is located at a point along the waveguide  86   b . The triggerable delay circuit  88   b  operates in the following fashion. The triggerable delay circuit  88   b  is triggered by the probe signal  82   b  which, upon initial detection of the probe signal  82   b , initiates an internal timer circuit. Once the timer circuit has reached a predetermined delay time, preferably in the range 1-20 nanoseconds, the timer circuit generates an output signal from the triggerable delay circuit  88   b  which is used as a signature signal  92   b . The signature signal  92   b  propagates along the waveguide  86   b  to a transmit antenna  90   b  which then transmits the signature signal  92   b  to the PDU  28 . 
     FIG. 6   c  illustrates an embodiment of an active frequency domain signature generator suitable for incorporation in or on an underground boring tool  24 . The embodiment illustrated shows a probe signal  82   c  being received by a receive antenna  84   c  coupled to a waveguide  86   c  and a nonlinear element  88   c . The frequency-shifted signal generated by the nonlinear element  88   c  is then passed through an amplifier  94   c  before being passed to the transmit antenna  90   c , which transmits the signature signal  92   c  to the PDU  28 . The amplifier  94   c  may also include a filtering circuit to produce a filtered signature signal at the output of the amplifier  94   c . An advantage to using an active frequency domain signature signal generation embodiment over a passive frequency domain signature signal generation embodiment is is that the active embodiment produces a stronger signature signal which is more easily detected. 
   In a second embodiment of the active frequency domain signature signal generator, generally illustrated in  FIG. 6   c , a probe signal  82   c  passes through the amplifier  94   c  prior to reaching the nonlinear element  88   c . An advantage of this alternative embodiment is that, since the amplification process may take place at a lower frequency, the amplifier may be less expensive to implement. 
   A third embodiment of an active frequency domain signature generator suitable for use with an underground boring tool  24  is illustrated in  FIG. 6   d .  FIG. 6   d  shows a receive antenna  84   d  coupled through use of a waveguide  86   d  to a frequency shifter  88   d  and a transmit antenna  90   d . The frequency shifter  88   d  is a device which produces an output signal  92   d  having a frequency of f 2 , which is different from the frequency, f 1 , of an input signal  82   d  by an offset Δf, where f 2 =f 1 +Δf. In accordance with this embodiment, Δf is preferably larger than one half of the bandwidth of the probe signal  82   d , typically on the order of 1 MHz. The frequency shifter  88   d  produces a frequency shift sufficient to move the signature signal  92   d  out of, or at least partially beyond, the frequency band of the clutter signal, thereby increasing the signal-to-clutter ratio of the detected signature signal  92   d . For purposes of describing these embodiments, the term signature signal embraces all generated return signals from the cooperative target  20  other than those solely due to the natural reflection of the probe signal off of the underground boring tool  24 . 
     FIG. 7  illustrates an embodiment of a signature signal generator adapted for use in a cooperative target  20  provided on or within an underground boring tool  24  where the probe signal is an acoustic signal. In an acoustic time-domain embodiment, as illustrated in  FIG. 7   a , an acoustic probe signal  98   a , preferably an acoustic impulse, is received and detected by an acoustic receiver  100   a  mounted on the inner wall  96   a  of the boring tool  24 . The acoustic receiver  100   a  transmits a trigger signal along a trigger line  102   a  to a delay pulse generator  104   a . After being triggered, the delay pulse generator  104   a  generates a signature pulse following a triggered delay. The signature pulse is passed along the transmitting line  106   a  to an acoustic transmitter  108   a , also mounted on the inner wall  96   a  of the boring tool  24 . The acoustic transmitter  108   a  then transmits an acoustic signature signal  110   a  through the ground for detection by the PDU  28 . 
   In accordance with an acoustic frequency-domain embodiment, as is illustrated in  FIG. 7   b , an acoustic probe signal  98   b , preferably an acoustic pulse having a given acoustic frequency f 3 , is received and detected by an acoustic receiver  100   b  mounted on the inner wall  96   b  of the boring tool  24 . The acoustic receiver  100   b  transmits an input electrical signal corresponding to the received acoustic signal  98   b  at a frequency f 3  along a receive line  102   b  to a frequency shifter  104   b . The frequency shifter  104   b  generates an output electrical signal having a frequency that is shifted by an amount Δf 3  relative to the frequency of the input signal  98   b . The output signal from the frequency shifter  104   b  is passed along a transmit line  106   b  to an acoustic transmitter  108   b , also mounted on the inner wall  96   b  of the boring tool  24 . The acoustic transmitter  108   b  then transmits the frequency shifted acoustic signature signal  110   b  through the ground for detection by the PDU  28 . 
   In  FIG. 8 , there is illustrated in system block diagram form another apparatus for actively generating in a cooperative target  20  a signature signal that contains various types of information content. In one configuration, the signature signal generating apparatus of the cooperative target  20  includes a receive antenna  41 , a signature signal generator  43 , and a transmit antenna  45 . In accordance with this configuration, a probe signal  37  produced by the PDU  28  is received by the receive antenna  41  and transmitted to a signature signal generator  43 . The signature signal generator  43  alters the received probed signal  37  so as to produce a signature signal that, when transmitted by the transmit antenna  45 , is readily distinguishable from other return and clutter signals received by the PDU  28 . Alternatively, the signature signal generator  43 , in response to the received probe signal  37 , generates a signature signal different in character than the received probe signal  37 . The signature signal transmitted by the transmit antenna  45  differs from the received probe signal  37  in one or more characteristics so as to be readily distinguishable from other return and clutter signals. By way of example, and as discussed in detail hereinabove, the signature signal produced by the signature signal generator  43  may differ in phase, frequency content, polarization, or information content with respect to other return and clutter signals received by the PDU  28 . 
   Additionally, as is further illustrated in  FIG. 8 , the cooperative target  20  may include an orientation detector  47 . The orientation detector  47  is a device capable of sensing an orientation of the cooperative target  20 , and provides an indication of the orientation of the underground boring tool  24  during operation. 
   It may be desirable for the operator to know the orientation of the boring tool  24  when adjusting the direction of the boring tool  24  along an underground pathway, since several techniques known in the art for directing boring tools rely on a preferential orientation of the tool. If the boring tool  24  orientation is not known, the boring tool  24  cannot be steered in a preferred direction in accordance with such known techniques that require knowledge of boring tool  24  orientation. It may not be possible to determine the orientation of the boring tool  24  simply from a knowledge of the orientation of the members  23  of the drill string  22 , since one or more members  23  of the drill string  22  may twist or slip relative to one another during the boring operation. Since the boring operation takes place underground, the operator has no way of detecting whether such twisting or slipping has occurred. It may, therefore, be important to determine the orientation of the boring tool  24 . 
   The orientation detector  47  produces an orientation signal which is communicated to an encoder  49 , such as a signal summing device, which encodes the orientation signal produced by the orientation detector  47  on the signature signal produced by the signature signal generator  43 . 
   The encoded signature signal produced at the output of the encoder  49  is communicated to the transmit antenna  45  which, in turn, transmits the encoded signature signal  39  to the PDU  28 . Various known techniques for encoding the orientation signal on the signature signal may be implemented by the encoder  49 , such as by modulating the signature signal with the orientation signal. It is noted that other sensors may be included within the apparatus illustrated in  FIG. 8  such as, for example, a temperature sensor or a pressure sensor. The outputs of such sensors may be communicated to the encoder  49  and similarly encoded on the signature signal for transmission to the PDU  28  or, alternatively, may be transmitted as information signals independent from the signature signal. 
   Referring to  FIG. 9 , there is illustrated an embodiment of an orientation detecting apparatus which may include up to three mutually orthogonally arranged orientation detectors. The orientation detectors  210 ,  212 , and  214  are aligned along the x-axis, y-axis, and z-axis, respectively. In accordance with this embodiment, the orientation detector  210  detects changes in orientation with respect to the x-axis, while the orientation detector  212  senses changes in orientation with respect to the y-axis. Similarly, the orientation detector  214  detects changes in orientation with respect to the z-axis. Given this arrangement, changes in pitch, yaw, and roll may be detected when the cooperative target  20  is subject to positional changes. It is noted that a single orientation detector, such as detector  210 , may be used to sense changes along a single axis, such as pitch changes in the boring tool  24 , if multiple axis orientation changes need not be detected. Further, depending on the initial orientation of the cooperative target  20  when mounted to the underground boring tool  24 , two orthogonally arranged orientation detectors, such as orientation detectors  210  and  212  aligned respectively along the x and y-axes, may be sufficient to provide pitch, yaw, and roll information. 
   Referring now to  FIG. 10 , there is illustrated an embodiment of an apparatus for detecting an orientation of an underground boring tool  24 . In accordance with this embodiment, the cooperative target  20  provided on or within the underground boring tool  24  includes a tilt detector  290  that detects changes in boring tool orientation during boring activity. The cooperative target  20 , in addition to producing a signature signal for purposes of determining boring tool location, may include an orientation detector, such as that illustrated in  FIG. 10 , for purposes of producing and orientation signal representative of an orientation of the cooperative target  20  and, therefore, the underground boring tool  24 . 
   In one embodiment, as is illustrated in  FIG. 8 , the cooperative target  20  includes an orientation detecting apparatus, which produces an orientation signal, and a separate signature signal generator, which produces a signature signal. The signature signal and the orientation signal may be transmitted by the transmit antenna  45  of the cooperative target  20  as two separate information signals or, alternatively, as a composite signal which includes both the signature and orientation signals. Alternatively, the orientation detecting apparatus may produce a single signature signal that is indicative of both the location and the orientation of the cooperative target  20 . 
   Referring in greater detail to  FIG. 10 , there is illustrated a tilt detector  290  coupled to a selector  291 . The tilt detector  290  detects tilting of the cooperative target  20  with respect to one or more mutually orthogonal axes of the boring tool  24 . It is believed that the tilt detector  290  illustrated in  FIG. 10  is useful as a sensor that senses the pitch of the boring tool  24  during operation. The range of tilt angles detectable by the tilt detector  290  may be selected in accordance with the estimated amount of expected boring tool tilting for a given application. For example, the tilt detector  290  may detect maximum pitch angles in the range of ±45° relative to horizontal in one application, whereas, in another application, the tilt detector  290  may detect pitch angles in the range of ±90° relative to horizontal, for example. It is to be understood that the tilt detector  290 , as well as other components illustrated in  FIG. 10 , may be active or passive components. 
   As is further illustrated in  FIG. 10 , a probe signal  235  is received by the receive antenna  234  which, in turn, communicates the probe signal  235  to a selector  291 . The tilt detector  290  and selector  291  cooperate to select one of several orientation signal generators depending on the magnitude of tilting as detected by the tilt detector  290 . In one embodiment, the probe signal  235  is coupled to each of the orientation signal generators P 1    292  through P N    297 , one of which is selectively activated by the tilt detector  290  which incorporates the function of the selector  291 , such as the embodiment illustrated in FIG.  12 . In another embodiment, the probe signal  235  is coupled to the selector  291  which activates one of the orientation signal generators P 1    292  through P N    297  depending on the magnitude of tilting detected by the tilt detector  290 . 
   By way of example, and in accordance with a passive component implementation, each of the orientation signal generators P 1    292  through P N    297  represent individual transmission lines, each of which produces a unique time-delayed signature signal which, when transmitted by the transmit antenna  244 , provides both location and orientation information when received by the PDU  28 . As such, the orientation detection apparatus in accordance with this embodiment provides both location and orientation information and does not require a separate signature signal generator  43 . In another embodiment, each of the orientation signal generators, such as orientation signal generator P 3    294 , produces a unique orientation signal which is transmitted to an encoder  49 . A signature signal  299  produced by a signature signal generator  43  separate from the orientation detection apparatus may be input to the encoder  49 , which, in turn, produces a composite signature signal  301  which includes both signature signal and orientation signal content. The composite signal  301  is then transmitted to the PDU  28  and decoded to extract the orientation signal content from the signature signal content. 
   As discussed previously, the range of tilt angles detectable by the tilt detector  290  and the resolution between tilt angle increments may vary depending on a particular application or use. By way of example, it is assumed that the tilt detector  290  is capable of detecting maximum tilt angles of ±60°. The selector  291  may select orientation signal generator P 1    292  when the tilt detector  290  is at a level or null state (i.e., 0° tilt angle) relative to horizontal. When selected, orientation detector P 1    292  generates a unique orientation signal which is indicative of an orientation of 0°. As previously discussed, the orientation signal may be combined with a signature signal produced by a separate signature signal generator  43  or, alternatively, may provide both signature signal and orientation signal information which is transmitted to the PDU  28 . 
   In the event that the tilt detector  290  detects a positive 5° tilt angle change, for example, orientation signal generator P 2    293  is selected by the selector  291 . The orientation signal generator P 2    293  then produces an orientation signal that indicates a positive 5° tilt condition. Similarly, orientation signal generators P 3    294 , P 4    295 , and P 5    296  may produce orientation signals representing detected tilt angle changes of positive 10°, 15°, and 20°, respectively. Other orientation signal generators may be selected by the selector  291  to produce orientation signals representing tilt angle changes in five degree increments between 25° and 60°. Negative tilt angles between 0° and −60° in 5° increments are preferably communicated to the PDU  28  by selection of appropriate orientation signal generators corresponding to the magnitude of negative tilting. It will be appreciated that the range and resolution between tilt angle increments may vary depending on a particular application. 
   In  FIGS. 11   a  and  11   b , there is illustrated another embodiment of an underground boring tool  500  equipped with a signature signal generating apparatus which, in addition to providing location information, provides boring tool orientation information. Referring to  FIG. 11   a , the boring tool  500  includes a longitudinal axis  501  about which the boring tool  500  rotates during boring activity. Distributed about the periphery of the boring tool  500  are a number of a signature signal generating devices, such as devices  504  and  508 . In accordance with this embodiment, the signature signal generating devices operate passively and, as such, do not require an external power supply. Each of the signature signal generating devices distributed about the boring tool  500  produces a unique signature signal in response to a received probe signal generated by the PDU  28 . 
   As is further illustrated in  FIG. 11   b , the boring tool  500  includes a number of elongated recesses or channels within which signature signal generating devices are disposed. In  FIG. 11   b , there is shown a cross-sectional view of the boring tool  500  illustrated in  FIG. 11   a . A signature signal generating device  504 , such as a co-axial transmission line, for example, is disposed in a recess  502  and encased in a protective material  505  which permits passage of electromagnetic signals therethrough. The protective material  505  fixes the signature signal generating device  504  within the channel  502 . Also shown in  FIG. 11   b  is a second signature signal generating device  508  similarly disposed in a recess  506  and encased in a protective material  505 . A hard dielectric material, such as KEVLAR®, is a material suitable material for this application. 
   During operation, the boring tool  500  is rotated at an appropriate drilling rate which, assuming a full 360° rotation, exposes each of the signature signal generating devices to a probe signal  36  produced by the PDU  28 . When exposed to the probe signal  36  during rotation, each of the signature signal generating devices will emit a characteristic or signature signal  38  in response to the probe signal  36 . As a particular signature signal generating device rotates beyond a reception window within which the probe signal  36  is received and a signature signal  38  generated, the bulk metallic material of the boring tool  500  shields such a signature signal generating device from the probe signal  36 . It may be desirable to situate the signature signal generating devices about the periphery of the boring tool  500  such that the signature signal produced by the signature signal detecting device exposed to the probe signal  36  produces the predominant signature signal  38  received by the PDU  28 . It may further be desirable to provide for a null or dead zone between adjacent signature signal generating devices so that the only signature signal  38  received by the PDU  28  is that produced by a single signature signal generating device currently exposed to the probe signal  38 . 
   The type of signature signal generating device, configuration of the boring tool recesses, such as recess  502 , the type of protective material  505  employed, the number and location of signature signal generating devices used, and the rotation rate of the boring tool  500  will typically impact the ability of the PDU  28  to detect the signature signal  38  produced by each of the signature signal generating devices during boring tool rotation. 
   Turning now to  FIG. 12 , there is illustrated an embodiment of an orientation detector suitable for use in both active and passive signature signal generating apparatuses. In one embodiment, a mercury sensor  220  may be constructed having a bent tube  221  within which a bead of mercury  222  moves as the tube  221  tilts within a plane defined by the axes  223  and  225 . Pairs of electrical contacts, such as contacts  227  and  229 , are distributed along the base of the tube  221 . As the tube  221  tilts, the mercury bead  222  is displaced from an initial or null point, generally located at a minimum bend angle of the tube  221 . As the bead  222  moves along the tube base, electrical contact is made between electrical contact pairs  227  and  229  distributed along the tube base. As the amount of tube tilting increases, the mercury bead  222  is displaced further from the null point, thus completing electrical circuit paths for contact pairs located at corresponding further distances from the null point. As such, the incremental change in tilt magnitude may be determined by detecting continuity in the contact pair over which the mercury bead  222  is situated. 
   In one embodiment, sixty-four of such contact pairs are provided along the base of the tube  221  to provide 64-bit tilt resolution information. An electrical circuit or logic (not shown) is coupled to the pairs of electrical contacts  227  and  229  which provides an output indicative of the magnitude of tube tilting, and thus an indication of the magnitude of the cooperative target orientation with respect to the plane defined by axes  223  and  225 . It is appreciated that use of a mercury sensor  220  in accordance with this embodiment may require a power source. As such, this embodiment of an orientation detector is appropriate for use in active signature signal generating circuits. It is noted that the range of tilt angles detectable by the mercury sensor  220  is dependent on the bend angle α provided in the bent tube  221 . The bend angle α, as well as the length of the tube  221 , will also impact the detection resolution of mercury bead displacement within the tube  221 . 
   In accordance with another embodiment of an orientation detector suitable for use in passive signature signal generating circuits, reference is made to  FIGS. 12 and 13   a - 13   b . The illustration of the apparatus depicted in  FIG. 12  may be viewed in a context other than that previously described in connection with a mercury sensor embodiment. In particular, a metallic ball or other metallic object  222  is displaced within a tube  221  in response to tilting of the tube  221  within the plane defined by the axes  223  and  225 . The movable contact  222  moves along a pair of contact rails  235   a  and  235   b  separated by a channel  237 . The rails  235   a  and  235   b  include gaps  233  which separate one contact rail circuit from an adjacent contact rail circuit. As is illustrated in detail in  FIGS. 13   a - 13   b , each of the contact rail circuits is coupled to a pair of contacts  227  and  229  which, in turn, are coupled to a transmission line capable of producing a unique signature signal. 
   By way of example, and with particular reference to  FIGS. 13   a - 13   b , movable contact  222  is shown moving within the tube  221  between a first position P a  and a second position P b  in response to tilting of the tube  221 . When the movable contact  222  is at the position P a , continuity is established between contact  227 , contact rail  235   a , movable contact  222 , contact rail  235   b , and contact  229 . As such, the circuit path including the transmission line T 4    230  is closed. A probe signal  235  produced by the PDU  28  is received by the receive antenna  234  which communicates the probe signal along an input waveguide  232  and through the circuit path defined by contact  227 , rail contact  235   a , movable contact  222 , rail contact  235   b , and contact  229 . The received probe signal  235  transmitted to the time-delaying waveguide T 4    230  produces a time-delayed signature signal which is communicated to an output waveguide  242  and to a transmit antenna  244 . The signature signal produced by the waveguide T 4    230  is then received by the PDU  28 . The PDU  28  correlates the signature signal  245  with the selected signature signal waveguide, such as transmission line T 4    230 , and determines the magnitude of tube  221  tilting. Those skilled in the art will appreciate that various impedance matching techniques, such as use of quarter wavelength matching stubs and the like, may be employed to improve impedance matching within the waveguide pathways illustrated in  FIGS. 13   a - 13   b.    
   Referring now to  FIG. 14 , there is illustrated another embodiment of an orientation detection apparatus suitable for detecting an orientation of an underground boring tool  510 . In accordance with this embodiment, a number of rotation detectors, such as R 1    512  and R 2    514 , are disposed at various radial locations about the periphery of the boring tool  510 . The rotation detectors detect radial displacement of the boring tool  510  as the boring tool  510  rotates about its longitudinal axis  501 . A pitch detector  516 , oriented parallel with the longitudinal axis  501  of the boring tool  510 , is susceptible to changes in boring tool pitch. In one embodiment, the rotation detectors, such as R 1    512  and R 2    514 , and the pitch detector  516  are accelerometer-type sensors. Alternatively, the rotation and pitch detectors may constitute spring or strain gauge style sensors. Various other known displacement sensor mechanisms may also be employed. 
   The magnitude of the responsive of each rotation detector, such as detector R 1    512 , is typically dependent on the radial location of a particular rotation detector relative to earth&#39;s gravity vector as the boring tool  24  rotates about the longitudinal axis  501 . The magnitude of the output produced by the pitch detector  516  is typically dependent on the degree of a boring tool pitch off of horizontal relative to the ground surface  11 . The output signal produced by each of the rotation detectors and the pitch detector may be encoded onto the signature signal produced by the signature signal generating apparatus provided on the boring tool  510  or, alternatively, transmitted to the PDU  28  as a separate information signal. 
     FIG. 21   a  illustrates yet another embodiment of an orientation sensing apparatus suitable for use with a boring tool  400 . The boring tool  400  incorporates a passive time domain signature signal circuit including a single antenna  402 , connected via a time delay line  404  to a termination  406 , as discussed hereinabove with respect to  FIG. 5   a . The circuit illustrated in  FIG. 21   a  also includes a mercury switch  408  located at a point along the delay line  404  close to the termination  406 . The termination  406  also includes a dissipative load. When the boring tool  400  is oriented so that the mercury switch  408  is open, the time domain signature signal is generated by reflecting an incoming probe signal  407  at the open circuit of the mercury switch  408 . When the boring tool  400  is oriented so that the mercury switch  408  is closed, the circuit from the antenna  402  is completed to the dissipative load  406  through the delay line  404 . The probe signal  407  does not reflect from the dissipative load  406  and therefore no signature signal is generated. The generation of the signature signal  409  received by the PDU  28  is shown as a function of time in  FIG. 21   b . The top trace  407   b  shows the probe signal  407 , I p , plotted as a function of time. 
   As the boring tool  400  rotates and moves along an underground path, the resistance, Rm, of the mercury switch  408  alternates from low to high values, as shown in the center trace  408   b . The regular opening and closing of the mercury switch  408  modulates the signature signal  409   b , I s , received at the surface. The modulation maintains a constant phase relative to a preferred orientation of the boring tool  24 . The lower trace does not illustrate the delaying effects of the time delay line  404  since the time scales are so different (the time delay on the signature signal  409  is of the order of 10 nanoseconds, while the time taken for a single rotation of the boring tool  24  is typically between 0.1 and 1 second). Detection of the modulated signature signal  409  by the PDU  28  allows the operator to determine the orientation of the boring tool head. It is understood that the other embodiments of signature signal generation described hereinabove can also incorporate a mercury switch  408  and, preferably, a dissipative load  406  in order to generate a modulated signature signal  409  for purposes of detecting the orientation of the boring tool  24 . 
   In  FIG. 15 , there is illustrated an apparatus for actively generating a signature signal and an orientation signal in an underground boring tool  24 . There is shown the head of a boring tool  24   a . At the front end of the boring tool  24   a  is a cutter  120  for cutting through soil, sand, clay, and the like when forming an underground passage. A cut-away portion of the boring tool wall  122  reveals a circuit board  124  which is designed to fit inside of the boring tool  24   a . Attached to the circuit board  124  is a battery  126  for providing electrical power. Also connected to the circuit board  124  is an antenna  128  which is used to receive an incoming probe signal  36  and transmit an outgoing signature signal  38 . The antenna  128  may be located inside the boring tool  24   a  or may be of a conformal design located on the surface of the boring tool  24   a  and conforming to the surface contour. The boring tool  24   a  may also contain one or more sensors for sensing the environment of the boring tool  24   a . Circuitry is provided in the boring tool  24   a  for relaying this environmental information to the control unit  32  situated above-ground. The sensors, such as an orientation sensor  131 , may be used to measure, for example, the orientation of the boring tool  24   a , (pitch, yaw, and roll) or other factors, such as the temperature of the cutting tool head or the pressure of water at the boring tool  24   a.    
   In  FIG. 15 , there is illustrated a sensor  130 , such as a pressure sensor, located behind the cutter  120 . An electrical connection  132  runs from the sensor  130  to the circuit board  124  which contains circuitry for analyzing the signal received from the sensor  130 . The circuit board  124  may modulate the signature signal  38  to contain information relating to the sensor output or, alternatively, may generate separate sensor signals which are subsequently detected and analyzed above-ground. Also depicted is an orientation sensor  131  which produces an orientation signal indicative of an orientation of the boring tool  24 , such as the lateral position or deviation of the boring tool  24  relative to a predefined underground path or, by way of further example, the pitch of the boring tool  24  relative to horizontal. 
   A methodology for detecting the depth of a boring tool  24  incorporating a cooperative target  20  in accordance with one embodiment is illustrated in FIG.  16 . In accordance with this embodiment, the PDU  28  includes a transmit antenna  250  and two receive antennas, AR 1    252  and AR 2    254 . Each of the receive antennas AR 1    252  and AR 2    254  is situated a known distance 2 m from the transmit antenna AT  250 . It is assumed for purposes of this example that the propagation rate K through the ground medium of interest is locally constant. Although this assumption may introduce a degree of error with respect to actual or absolute depth boring tool, any such error is believed to be acceptable given the typical application or use of the boring tool cooperative detection technique described here. In other applications, absolute depth determinations may be desired. In such a case, the local propagation rate K, or dielectric constant, may be empirically derived, one such procedure being described hereinbelow. 
   Returning to  FIG. 16 , the time-of-flight, t 1 , of the signal traveling between the cooperative target  20  of the boring tool  24  and the receive antenna AR  252 , and between the transmit antenna  250  and the cooperative target  20  of the boring tool  24  is determined when the cooperative target  20  is positioned below the centerline of the antennas AR 1    252  and AT  250 . The travel time of the signal traveling between the cooperative target  20  and the receive antenna AR 2    254  is indicated as the time t 2 . The depth d of the boring tool  24  that incorporates the cooperative target  20  may then be determined by application of the following equations:
 
 d   2   =K   2 ( t   1   2 )− m   2   [1]
 
 d   2   =K   2 ( t   2   2 )−9 m   2   [2]
 
 K   2 ( t   2   2 )− K   2 ( t   1   2 )=8 m   2   [3]
 
  K   2 ( t   2   2   −t   1   2 )=8 m   2   [4]
 
 K   2 =[8 m   2 /( t   2   2   −t   1   2 )]  [5]
 
 d   2 =[(8 m   2 /( t   2   2   −t   1   2 )]( t   1   2 )− m   2   [6]
 
 d=m [(8 t   1   2 /( t   2   2   −t   1   2 ))−1] 2   [7]
 
   In accordance with an alternative approach for determining the depth d of a cooperative target  20 , depth calculations may be based on field-determined values for characteristic soil properties, such as the dielectric constant and wave velocity through a particular soil type. A simplified empirical technique that may be used when calibrating the depth measurement capabilities of a particular GPR system involves coring a sample target, measuring its depth, and relating it to the number of nanoseconds it takes for a wave to propagate through the core sample. 
   For an embodiment of the invention which uses a microwave probe signal, a general relationship for calculating the depth or dielectric constant from the time of flight measurement is described by the following equation: 
               T   ⁢           ⁢   E     =         T   ⁢           ⁢   F     -     T   ⁢           ⁢   D       =     ∑         d   j     ⁢       ɛ   j         c                 [   8   ]             
 
where, TE is an effective time-of-flight, which is the duration of time during which a probe signal or signature signal is traveling through the ground; TF is the measured time-of-flight; TD is the delay internal to the cooperative target between receiving the probe signal and transmitting the signature signal; d j  is the thickness of the jth ground type above the cooperative target; M j  is the average dielectric constant of the jth ground type at the microwave frequency; and c is the speed of light in a vacuum. It is important to know the dielectric constant since it provides information related to the type of soil being characterized and its water content. Having determined the dielectric constant of a particular soil type, the depth of the boring tool  24  traversing through similar soil types can be directly derived by application of the above-described equations.
 
   A methodology for detecting the location of an underground boring tool  24  as the boring tool  24  creates or otherwise travels along an underground path is illustrated in  FIGS. 17   a - 17   b  and  18   a - 18   b . With reference to these figures and to  FIG. 1 , an underground boring operation is depicted in which a boring tool  24  is shown excavating the ground  10  so as to create an underground path or borehole  26 . The drill string  22  is increased in length during the boring operation typically by adding individual drill string members  23  to the drill string  22  in a manner previously discussed. As the drill string length is increased, and the boring tool  24  forced further into the ground  10 , the PDU  28  is moved along a preferred above-ground path  41  at a speed approximately equal to the horizontal speed component of the boring tool  24 . 
   In one embodiment, the PDU  28  repeatedly transmits a probe signal  36  into the ground  10  when moved along the path  41 , which is received by the signature signal generating apparatus provided on or within the boring tool  24 . In response to the probe signal  36 , a signature signal  38  is transmitted at the boring tool  24  and received is by the PDU  28 . Any deviation taken by the boring tool  24  from the preferred above-ground path  41  is detected by the PDU  28 . An appropriate course correction may be effected either manually or automatically by the trenchless underground boring system  12  in response to such a deviation, as will be discussed hereinbelow. While effecting a boring tool course change, the PDU  28  is moved along the path  41  so as to continue tracking the progress and direction of the boring tool  24  through the ground  10 . In this manner, cooperation between the PDU  28 , the boring tool  24 , and the above-ground portion of the trenchless underground boring system  12  provide for reliable and accurate navigating and tracking of an underground boring tool  24  during excavation. 
     FIGS. 17   a  and  17   b  illustrate one embodiment of a detection methodology employing an antenna array  37  coupled to the PDU  28 . The antenna array  37  includes a left receive antenna A L  and a right receive antenna A R  which are respectively positioned on either side of a transmit antenna (not shown) situated at a mid-point between the two receive antennas A L  and A R . The dashed line  41  shown in  FIG. 17   a  depicts a preferred above-ground path under which a borehole  26  is to be created, or has been created, by a boring tool  24  equipped with a cooperative target  20 . At a first location L 1 , it can be seen that the underground boring tool  24  is located immediately beneath the transmit antenna positioned in the center of the antenna array  37 . A probe signal  36  emitted by the transmit antenna at a time t 0  is received by the cooperative target  20  in the boring tool  24 , which, in turn, produces a signature signal  38  that is received by the left receive antenna A L  and the right receive antenna A R  at approximately the same time, as is illustrated in the graph G 1  of  FIG. 17   b.    
   Referring to the graph G 1  of  FIG. 17   b , it is assumed that the probe signal  36  produced by the PDU  28  is transmitted at a time t 0 . Because the two receive antennas A L  and A R  of the antenna array  37  are substantially equidistant relative to the cooperative target  20 , the signature signal produced by the cooperative target  20  is received by the two antennas A L  and A R  at substantially the same time, t 1 , after transmission of the probe signal at time t 0 . Concurrent reception of the signature signal by the two receive antennas A L  and A R  is depicted in the graph G 1  of  FIG. 17   b  as detected signals S R  and S L , respectively, at a time t 1 . 
   At a second location L 2  along the preferred or predetermined above-ground path  41 , it can be seen that the boring tool  24  has deviated in a direction left (L) of the center (C) of the predetermined path  41 . This deviation of the boring tool  24  is detected by the PDU  28  as a time delay between a time the signature signal  38  is received by the left and right receive antennas A L  and A R , respectively. This time delay results from a difference in the separation distance between the boring tool  24  with respect to the left and right receive antennas A L  and A R . It can be seen that the separation distance between the left receive antenna A L  and the boring tool  24  is less than the separation distance between the right receive antenna A R  and the boring tool  24 . The boring tool deviation from the center of path  41  is reflected in the graph G 2  of  FIG. 17   b  as a delay between reception of the signature signal S L  by the left receive antenna A L  at a time t 2  and reception of the signature signal S R  by the right receive antenna A R  at a later time t 3 . 
   At a third location L 3  further along the preferred path  41 , it can be seen that the boring tool  24  has deviated to the right (R) of the center (C) of the preferred path  41 . Such a deviation may result from overcompensation when effecting a course change from a left-of-center location, such as from the second location L 2 . The right-of-center drift of the boring tool  24  is detected by the PDU  28  as the relative time delay between signature signal reception by the left and right receive antennas A L  and A R , respectively. At the location L 3 , it can be seen that the distance between the boring tool  24  and the right receive antenna A R  is less than the distance between the boring tool  24  and the left receive antenna A L . Accordingly, as is indicated in the graph G 3  of  FIG. 17   b , the signature signal S R  is received by the right receive antenna A R  in advance of the signature signal S L  received by the left receive antenna A L , thereby resulting in a time delay defined as (t 3 -t 2 ). This relative time delay may be used to determine the magnitude of boring tool deviation from the predetermined path  41 . 
   At a fourth location L 4  along the predefined-above-ground path  41 , it can is be seen that the boring tool  24  has been directed to the desired center point location along the path  41  after having deviated to the right of the path center point at the previously discussed location L 3 . As is shown at location L 4 , the boring tool  24  is again orientated immediately below the center point of the antenna array  37 . The signature signal  38  produced by the cooperative target  20  in response to a probe signal  36  emitted from the transmit antenna situated within the antenna array  37  is received substantially concurrently by the left and right receive antennas A L  and A R . The graph G 4  of  FIG. 17   b  demonstrates that the boring tool  24  is once again progressing as desired along the center line of the predetermined path  41 , as evidenced by contemporaneous reception of the signature signal  38  by the left and right receive antennas A L  and A R , respectively. It is noted that the depth of the boring tool, d, may be determined by any of the approaches discussed herein above. In addition, orientation of the boring tool  20  may also be detected and determined in a manner previously discussed above. 
     FIGS. 18   a - 18   b  illustrate another embodiment of an antenna array configuration which may be employed in combination with the PDU  28  to accurately track the progress of the underground boring tool  24  along an underground path. With reference to  FIG. 18   a , an antenna array  37  includes four receive antennas A 1 , A 2 , A 3 , and A 4 . The antenna array  37  also includes a transmit antenna (not shown) situated at a location within the array  37 , typically at a center location. In accordance with this embodiment, the four receive antennas are distributed about the circular array  37  at 0°, 90°, 180°, and 270° positions, respectively. It is to be understood that the configuration of the antenna array  37  need not be circular as is illustrated in the figures, but may instead be arranged in any suitable geometric configuration. Also, the distribution of receive antennas about the antenna array may be different from that illustrated in the figures. 
     FIG. 18   a  is a depiction of the antenna array  37  having its center transmit antenna orientated co-parallel with a predetermined above-ground path  41 . Superimposed in  FIG. 18   a  is an underground boring tool  24  equipped with a cooperative target  20  depicted at three different locations L 1 , L 2 , and L 3  along the predetermined path  41 . At the location L 1 , it can be seen that the boring tool  24  is properly aligned co-parallel with the preferred path  41 . The signature signal produced by the cooperative target  20 , in response to a probe signal produced by the transmit antenna at a time t o , is received at substantially the same time, t 4 , by each of the four receive antennas A 1 , A 2 , A 3 , and A 4 . The in-phase relationship of the signature signals S 1 , S 2 , S 3 , and S 4  respectively received by receive antennas A 1 , A 2 , A 3 , and A 4  is depicted in the graph G 1  of  FIG. 18   b.    
   At a location L 2 , it can be seen that the boring tool  24  has deviated right-of-center with respect to the path  41 . This course deviation taken by the boring tool  24  is detected by the PDU  28  as an out-of-phase signature signal response within the antenna array  37 . The right-of-center deviation is demonstrated in the graph G 2  of  FIG. 18   b  by the signature signal reception relationship associated with each of the four receive antennas A 1 , A 2 , A 3 , and A 4 . It can be seen that the distance between the boring tool  24  at location L 2  and the receive antenna A 2  is less than the distance between the boring tool  24  and the other receive antennas A 1 , A 3 , and A 4 . As is depicted in the graph G 2  of  FIG. 18   b , the signature signal S 2  is received at a time t 2  by the receive antenna A 2  earlier than the reception times associated with the other receive antennas. By way of further example, the relative distances between the cooperative target  24  and the receive antennas A 1  and A 4  at the previous location L 1  have effectively increased when the boring tool  24  deviates to the location L 2 , thereby increasing the delay time of signature signal reception by receive antennas A 1  and A 4 . As such, reception of the signature signal S 1  by antenna A 1  at a time t 7  and the signature signal S 4  by receive antenna A 4  at a time t 8  is delayed with respect to the reception of signature signal received by receive antennas A 2  and A 3  at times t 2  and t 5  respectively. 
   At a location L 3 , the graph G 3  of  FIG. 18   b  demonstrates that the boring tool  24  has deviated to a left-of-center position relative to the path  41 . The magnitude of the relative time delay within the antenna array  37  indicates the magnitude of off-of-center boring tool deviations as is illustrated by the signature signal response graph of  FIG. 18   b . It is noted that the boring tool  24  may deviate beyond the periphery of the antenna array  37 . Such a deviation will result in a more pronounced reduction in the signal-to-noise ratio with respect to receive antennas situated furthest away from the boring tool location. It is understood that an increase in the number of receive antennas within the antenna array  37  provides for a concomitant increase in boring tool detection resolution. It is believed that an antenna array  37  having a diameter ranging between approximately 2 feet and 5 feet is sufficient for detecting the location of the boring tool  24  at depths of approximately 10 to 15 feet or less. 
   In order to obtain three-dimensional data, a GPR system employing single-axis antenna must make several traverses over the section of ground or must use multiple antennae. The following describes the formation of two and three dimensional images in accordance with another embodiment of an antenna configuration used in combination with the PDU  28 . In  FIG. 19 , there is shown a section of ground  500  for which a PDU  28 , typically including a GPR forms an image, with a buried hazard  502  located in the section of ground  500 . The ground surface  504  lies in the x-y plane formed by axes x and y, while the z-axis is directed vertically into the ground  500 . Generally, a single-axis antenna, such as the one illustrated as antenna-A  506  and oriented along the z-axis, is employed to perform multiple survey passes  508 . The multiple survey passes  508  are straight line passes running parallel to each other and have uniform spacing in the y direction. The multiple passes shown in  FIG. 19  run parallel to the x-axis. 
   Generally, as discussed previously, a GPR system has a time measurement capability which allows measuring of the time for a signal to travel from the transmitter, reflect off of a target, and return to the receiver. After the time function capability of the GPR system provides the operator with depth information, the radar system is moved laterally in a horizontal direction parallel to the x-axis, thus allowing for the construction of a two-dimensional profile of a subsurface. By performing multiple survey passes  508  in a parallel pattern over a particular site, a series of two-dimensional images can be accumulated to produce an estimated three-dimensional view of the site within which a buried hazard may be located. It can be appreciated, however, that the two-dimensional imaging capability of a conventional antenna configuration  506  may result in missing a buried hazard, particularly when the hazard  502  is parallel to the direction of the multiple survey passes  508  and lies in between adjacent survey passes  508 . 
   A significant advantage of a geologic imaging antenna configuration  520  of the present invention provides for true three-dimensional imaging of a subsurface as shown in  FIG. 20. A  pair of antennae, antenna-A  522  and antenna-B  524 , are preferably employed in an orthogonal configuration to provide for three-dimensional imaging of a buried hazard  526 . Antenna-A  522  is shown as directed along a direction contained within the y-z axis and at +45° relative to the z-axis. Antenna-B  524  is also directed along a direction contained within the y-z plane, but at −45° relative to the z-axis, in a position rotated 90° from that of antenna-A  522 . It is noted that the hyperbolic time-position data distribution typically obtained by use of a conventional single-axis antenna, may instead be plotted as a three-dimensional hyperbolic shape that provides width, depth, and length dimensions of a detected buried hazard  526 . It is further noted that a buried hazard  526 , such as a drainage pipeline, which runs parallel to the survey path  528  will readily be detected by the three-dimensional imaging GPR system. Respective pairs of orthogonally oriented transmitting and receive antennae may be employed in the transmitter  54  and receiver  56  of the PDU  28  in accordance with one embodiment of the invention. 
   Additional features can be included on the boring tool  24 . It may be desired, under certain circumstances, to make certain measurements of the boring tool  24  orientation, shear stresses on the drill string  22 , and the temperature of the boring tool  24 , for example, in order to more clearly understand the conditions of the boring operation. Additionally, measurement of the water pressure at the boring tool  24  may provide an indirect measurement of the depth of the boring tool  24  as previously described hereinabove. 
     FIG. 21   c  illustrates an embodiment which allows sensors to sense the environment of the boring tool  410 . The figure shows an active time domain signature signal generation circuit which includes a receive antenna  412  connected to a transmit antenna  414  through an active time domain circuit  416 . A sensor  418  is connected to the active time domain circuit  416  via a sensor lead  420 . In this embodiment, the sensor  418  is placed at the tip of the boring tool  410  for measuring the pressure of water at the boring tool  410 . The reading from the sensor  418  is detected by the active time domain circuit  416  which converts the reading into a modulation signal. The modulation signal is subsequently used to modulate the actively generated signature signal  415 . This process is described with reference to  FIG. 21   d , which shows several signals as a function of time. The top signal  413   d  represents the probe signal, I p , received by the receive antenna  412 . The second signal,  415   d , represents the actively generated signature signal I a , which would be generated if there were no modulation of the signature signal. The third trace,  416   d , shows the amplitude modulation signal, I m , generated by the active time domain circuit  416 , and the last trace,  422   d , shows the signature signal, I s , after amplitude modulation. The modulated signature signal  415  is detected by the PDU  28 . Subsequent determination of the modulation signal by the signal processor  60  in the PDU  28  provides data regarding the output from the sensor  418 . 
   Modulation of the signature signal is not restricted to the combination of amplitude modulation of a time domain signal as shown in the embodiment of FIG.  21 . This combination was supplied for illustrative purposes only. It is understood that other embodiments include amplitude modulation of frequency domain signature signals, and frequency modulation of both time and frequency domain signature signals. In addition, the boring tool  24  may include two or more sensors rather than the single sensor as illustrated in the above embodiment. 
     FIG. 22   a  illustrates another embodiment of the invention in which a separate active beacon is employed for transmitting information on the orientation or the environment of the boring tool  430  to the PDU  28 . In this embodiment, shown in  FIG. 22   a , the boring tool  430  includes a passive time domain signature circuit employing a single antenna  432 , a time delay line  434 , and an open termination  436  for reflecting the electrical signal. The single antenna  432  is used to receive a probe signal  433  and transmit a signature/beacon signal  435 . An active beacon circuit  438  generates a beacon signal, preferably having a selected frequency in the range of 50 KHz to 500 MHz, which is mixed with the signature signal generated by the termination  436  and transmitted from the antenna  432  as the composite signature/beacon signal  435 . A mercury switch  440  is positioned between the active beacon circuit  438  and the antenna  432  so that the mercury switch  440  operates only on the signal from the active beacon circuit  438  and not on the signature signal generated by the termination  436 . 
   When the boring tool  430  is oriented so that the mercury switch  440  is open, the beacon signal circuit  438  is disconnected from the antenna  432 , and no signal is transmitted from the active beacon circuit  438 . When the boring tool  430  is oriented so that the mercury switch  440  is closed, the active beacon circuit  438  is connected to the antenna  432  and the signal from the active beacon circuit  438  is transmitted along with the signature signal as the signature/beacon signal  435 . The effect of the mercury switch on the signature/beacon signal  435  has been described previously with respect to  FIG. 21   b . The top trace  438   b , in  FIG. 22   b , shows the signal, I b , generated by the active beacon circuit  438  as a function of time. As the boring tool  430  rotates and moves along an underground path, the resistance, Rm, of the mercury switch  440  alternates from low to high values, as shown in the center trace  440   b . The continual opening and closing of the mercury switch  440  produces a modulated signature/beacon signal  435   b , I m , which is received at the surface by the PDU  28 . Only a beacon signal component, and no signature signal component, is shown in signal I m    435   b . The modulation of signal I m    435   b  maintains a constant phase relative to a preferred orientation of the boring tool  430 . Analysis of the modulation of the beacon signal by a beacon receiver/analyzer  61  on the PDU  28  allows the operator to determine the orientation of the boring tool head. 
     FIG. 22   c  illustrates an embodiment which allows sensors to sense the environment of the boring tool  450  where an active beacon is used to transmit sensor data. The figure shows an active time domain signature signal generation circuit including a receive antenna  452 , a transmit antenna  454 , and an active time domain signature signal circuit  456 , all of which are connected via a time delay line  457 . An active beacon circuit  460  is also connected to the transmit antenna  454 . A sensor  458  is connected to the active beacon circuit  460  via a sensor lead  462 . In this embodiment, the sensor  458  is placed near the tip of the boring tool  450  and is used to measure the pressure of water at the boring tool  450 . The sensor reading is detected by the active beacon circuit  460  which converts the signal from the sensor  458  into a modulation signal. The modulation signal is subsequently used to modulate an active beacon signal generated by the active beacon circuit  460 . 
   To illustrate the generation of the signature/beacon signal  455  transmitted to the PDU  28 , several signals are illustrated as a function of time in  FIG. 22   d . The signal  453   d  represents the probe signal, I p , received by the receive antenna  452 . The second signal  456   d  represents the time-delayed signature signal, I s , generated by the active time domain circuit  456 . The third signal  460   d , I c , represents a combination of the time-delayed signature signal I s    456   d  and an unmodulated signal produced by the active beacon circuit  460 . The last trace,  455   d , shows a signal received at the surface, I m , which is a combination of the time-delayed signature signal I s    456   d  and a signal produced by the active beacon circuit  460  which has been modulated in accordance with the reading from the sensor  458 . Detection of the modulated active beacon signal by the beacon signal detector  61  in the PDU  28 , followed by appropriate analysis, provides data to the user regarding the output from the sensor  458 . 
   In  FIG. 23 , there is illustrated an embodiment for using a detection system to locate an underground boring tool and to characterize the intervening medium between the boring head and the PDU  28 . In this figure, there is illustrated a trenchless underground boring system  12  situated on the surface  11  of the ground  10  in an area in which the boring operation is to take place. A control unit  32  is located near the trenchless underground boring system  12 . In accordance with this illustrative example, a boring operation is taking place under a roadway. The ground  10  is made up of several different ground types, the examples as shown in  FIG. 23  being sand (ground type (GT 2 ))  140 , clay (GT 3 )  142  and native soil (GT 4 )  144 . The road is generally described by the portion denoted as road fill (GT 1 )  146 .  FIG. 12  illustrates a drill string  22  in a first position  22   c , at the end of which is located a boring tool  24   c . The PDU  28   c  is shown as being situated at a location above the boring tool  24   c . The PDU  28   c  transmits a probe signal  36   c  which propagates through the road fill and the ground. 
   In the case of the boring tool at location  24   c , the probe signal  36   c  propagates through the road fill  146  and the clay  142 . The boring tool  24   c , in response, produces a signature signal  38   c  which is detected and analyzed by the PDU  28   c . The analysis of the signature signal  38   c  provides a measure of the time-of-flight of the probe signal  36   c  and the signature signal  38   c . The time-of-flight is defined as a time duration measured by the PDU  28   c  between sending the probe signal  36   c  and receiving the signature signal  38   c . The time-of-flight measured depends on a number of factors including the depth of the boring tool  24   c , the dielectric conditions of the intervening ground medium  146  and  142 , and any delay involved in the generation of the signature signal  38   c . Knowledge of any two of these factors will yield the third from the time-of-flight measurement. 
   The depth of the boring tool  24   c  can be measured independently using a mechanical probe or sensing the pressure of the water at the boring tool  24   c  using a sensor  130  located in the boring tool head  24   c  as discussed hereinabove. For the latter measurement, the boring operation is halted, and the water pressure measured. Since the height of the water column in the drill string  22  above the ground is known, the depth of the boring tool  24   c  can be calculated using known techniques. For an embodiment of the invention which uses a microwave probe signal, a general relationship for calculating the depth or dielectric constant from the time of flight measurement is given by Equation [8] discussed previously hereinabove. 
   For the case where the boring tool is located at position  24   c  as shown in  FIG. 23 , and with the assumption that the road fill has a negligible thickness relative to the thickness of clay, the relationship of Equation [8] simplifies to: 
               T   ⁢           ⁢   E     =         T   ⁢           ⁢   F     -     T   ⁢           ⁢   D       =     ∑         d   3     ⁢       ɛ   3         c                 [   9   ]             
 
where, the subscript “3” refers to GT 3 . Direct measurement of the time-of-flight, TF, and the depth of the boring tool  24   c , d 3 , along with the knowledge of any time delay, TD, will yield the average dielectric constant, M 3 , of GT 3 . This characteristic can be denoted as GC 3 .
 
   Returning to  FIG. 23 , there is illustrated an embodiment in which the boring tool  24  has been moved from its first location  24   c  to another position  24   d . The drill string  22   d  (shown in dashed lines) has been extended from its previous configuration  22   c  by the addition of extra drill string members in a manner as described previously hereinabove. The PDU  28  has been relocated from its previous position  28   c  to a new position  28   d  (shown in dashed lines) in order to be close to the boring tool  24   d . The parameter GC 4 , which represents the ground characteristic of the native soil GT 4 , can be obtained by performing time-of-flight measurements as previously described using the probe signal  36   d  and signature signal  38   d . Likewise, ground characteristic GC 2  can be obtained from time-of-flight measurements made at the point indicated by the letter “e”. The continuous derivation of the ground characteristics as the boring tool  24   d  travels through the ground results in the production of a ground characteristic profile which may be recorded by the control unit  32 . The characteristics of the intervening ground medium between the PDU  28  and the cooperative target  20  may be determined in manner described herein and in U.S. Pat. No. 5,553,407, which is assigned to the assignee of the instant application, the contents of which are incorporated herein by reference. 
   It may be advantageous to make a precise recording of the underground path traveled by the boring tool  24 . For example, it may be desirable to make a precise record of where utilities have been buried in order to properly plan future excavations or utility burial and to avoid unintentional disruption of such utilities. Borehole mapping can be performed manually by relating the boring tool position data collected by the PDU  28  to a base reference point, or may be performed electronically using a Geographic Recording System (GRS)  150  shown generally as a component of the control unit  32  in FIG.  24 . In one embodiment, a Geographic Recording System (GRS)  150  communicates with a central processor  152  of the control unit  32 , relaying the precise location of the PDU  28 . Since the control unit  32  also receives information regarding the position of the boring tool  24  relative to the PDU  28 , the precise location of the boring tool  24  can be calculated and stored in a route recording database  154 . 
   In accordance with another embodiment, the geographic position data associated with a predetermined underground boring route is acquired prior to the boring operation. The predetermined route is calculated from a survey performed prior to the boring operation. The prior survey includes GPR sensing and geophysical data in order to estimate the type of ground through which the boring operation will take place, and to determine whether any other utilities or buried hazards are located on a proposed boring pathway. The result of the pre-bore survey is a predetermined route data set which is stored in a planned route database  156 . The predetermined route data set is uploaded from the planned route database  156  into the control unit  32  during the boring operation to provide autopilot-like directional control of the boring tool  24  as it cuts its underground path. In yet another embodiment, the position data acquired by the GRS  150  is preferably communicated to a route mapping database  158  which adds the boring pathway data to an existing database while the boring operation takes place. The route mapping database  158  covers a given boring site, such as a grid of city streets or a golf course under which various utility, communication, plumbing and other conduits may be buried. The data stored in the route mapping database  158  may be subsequently used to produce a survey map that accurately specifies the location and depth of various utility conduits buried in a specific site. The data stored in the route mapping database  158  also includes information on boring conditions, ground characteristics, and prior boring operation productivity, so that reference may be made by the operator to all prior boring operational data associated with a specific site. 
   An important feature of the novel system for locating the boring tool  24  concerns the acquisition and use of geophysical data along the boring path. A logically separate Geophysical Data Acquisition Unit  160  (GDAU), which may or may not be physically separate from the PDU  28 , may provide for independent geophysical surveying and analysis. The GDAU  160  preferably includes a number of geophysical instruments which provide a physical characterization of the geology for a particular boring site. A seismic mapping module  162  includes an electronic device consisting of multiple geophysical pressure sensors. A network of these sensors is arranged in a specific orientation with respect to the trenchless underground boring system  12 , with each sensor being situated so as to make direct contact with the ground. The network of sensors measures ground pressure waves produced by the boring tool  24  or some other acoustic source. Analysis of ground pressure waves received by the network of sensors provides a basis for determining the physical characteristics of the subsurface at the boring site and also for locating the boring tool  24 . These data are processed by the GDAU  160  prior to sending analyzed data to the central processor  152 . 
   A point load tester  164  may be employed to determine the geophysical characteristics of the subsurface at the boring site. The point load tester  164  employs a plurality of conical bits for the loading points which, in turn, are brought into contact with the ground to test the degree to which a particular subsurface can resist a calibrated level of loading. The data acquired by the point load tester  164  provide information corresponding to the geophysical mechanics of the soil under test. These data may also be transmitted to the GDAU  160 . 
   The GDAU  160  may also include a Schmidt hammer  166  which is a geophysical instrument that measures the rebound hardness characteristics of a sampled subsurface geology. Other geophysical instruments may also be employed to measure the relative energy absorption characteristics of a rock mass, abrasivity, rock volume, rock quality, and other physical characteristics that together provide information regarding the relative difficulty associated with boring through a given geology. The data acquired by the Schmidt hammer  166  are also stored in the GDAU  160 . 
   In the embodiment illustrated in  FIG. 24 , a Global Positioning System (GPS)  170  is employed to provide position data for the GRS  150 . In accordance with a U.S. Government project to deploy twenty-four communication satellites in three sets of orbits, termed the Global Positioning System (GPS), various signals transmitted from one or more GPS satellites may be used indirectly for purposes of determining positional displacement of a boring tool  24  relative to one or more known reference locations. It is generally understood that the U.S. Government GPS satellite system provides for a reserved, or protected, band and a civilian band. Generally, the protected band provides for high-precision positioning to a classified accuracy. The protected band, however, is generally reserved exclusively for military and other government purposes, and is modulated in such a manner as to render it virtually useless for civilian applications. The civilian band is modulated so as to significantly reduce the accuracy available, typically to the range of one hundred to three hundred feet. 
   The civilian GPS band, however, can be used indirectly in relatively high-accuracy applications by using one or more GPS signals in combination with one or more ground-based reference signal sources. By employing various known signal processing techniques, generally referred to as differential global positioning system (DGPS) signal processing techniques, positional accuracies on the order of centimeters are now achievable. As shown in  FIG. 24 , the GRS  150  uses the signal produced by at least one GPS satellite  172  in cooperation with signals produced by at least two base transponders  174 , although the use of one base transponder  174  may be satisfactory in some applications. Various known methods for exploiting DGPS signals using one or more base transponders  174  together with a GPS satellite  172  signal and a mobile GPS receiver  176  coupled to the control unit  32  may be employed to accurately resolve the boring tool  24  movement relative to the base transponder  174  reference locations using a GPS satellite signal source. 
   In another embodiment, a ground-based positioning system may be employed using a range radar system  180 . The range radar system  180  includes a plurality of base radio frequency (RF) transponders  182  and a mobile transponder  184  mounted on the PDU  28 . The base transponders  182  emit RF signals which are received by the mobile transponder  184 . The mobile transponder  184  includes a computer which calculates the range of the mobile transponder  184  relative to each of the base transponders  182  through various known radar techniques, and then calculates its position relative to all base transponders  182 . The position data set gathered by the range radar system  180  is transmitted to the GRS  150  for storing in route recording database  154  or the route mapping system  158 . 
   In yet another embodiment, an ultrasonic positioning system  190  may be employed together with base transponders  192  and a mobile transponder  194  coupled to the PDU  28 . The base transponder  192  emits signals having a known clock timebase which are received by the mobile transponder  194 . The mobile transponder  194  includes a computer which calculates the range of the mobile transponder  194  relative to each of the base transponders  192  by referencing the clock speed of the source ultrasonic waves. The computer of the mobile transponder  194  also calculates the position of the mobile transponder  194  relative to all of the base transponders  192 . It is to be understood that various other known ground-based and satellite-based positioning systems and techniques may be employed to accurately determine the path of the boring tool  24  along an underground path. 
     FIG. 25  illustrates an underground boring tool  24  performing a boring operation along an underground path at a boring site. An important advantage of the novel geographic positioning unit  150 , generally illustrated in  FIG. 25 , concerns the ability to accurately navigate the boring tool  24  along a predetermined boring route and to accurately map the underground boring path in a route mapping database  158  coupled to the control unit  32 . It may be desirable to perform an initial survey of the proposed boring site prior to commencement of the boring operation for the purpose of accurately determining a boring route which avoids difficulties, such as previously buried utilities or other obstacles, including rocks, as is discussed hereinbelow. 
   As the boring tool  24  progresses along the predetermined boring route, actual positioning data are collected by the geographic recording system  150  and stored in the route mapping database  158 . Any intentional deviation from the predetermined route stored in the planned path database  156  is accurately recorded in the route mapping database  158 . Unintentional deviations are corrected so as to maintain the boring tool  24  along the predetermined underground path. Upon completion of a boring operation, the data stored in the route mapping database  158  may be downloaded to a personal computer (not shown) to construct an “as is” underground map of the boring site. Accordingly, an accurate map of utility or other conduits installed along the boring route may be constructed from the route mapping data and subsequently referenced by those desiring to gain access to, or avoid, such buried conduits. 
   Still referring to  FIG. 25 , accurate mapping of the boring site may be accomplished using a global positioning system  170 , range radar system  180  or ultrasonic positioning system  190  as discussed previously with respect to  FIG. 24. A  mapping system having a GPS system  170  includes first and second base transponders  600  and  602  together with one or more GPS signals  606  and  608  received from GPS satellites  172 . A mobile transponder  610 , coupled to the control unit  32 , is provided for receiving the GPS satellite signal  606  and base transponder signals  612  and  614  respectively transmitted from the transponders  600  and  602  in order to locate the position of the control unit  32 . As previously discussed, a modified form of differential GPS positioning techniques may be employed to enhance positioning accuracy to the centimeter range. A second mobile transponder  616 , coupled to the PDU  28 , is provided for receiving the GPS satellite signal  608  and base transponder signals  618  and  620  respectively transmitted from the transponders  600  and  602  in order to locate the position of the PDU  28 . 
   In another embodiment, a ground-based range radar system  180  includes three base transponders  600 ,  602 , and  604  and mobile transponders  610  and  616  coupled to the control unit  32  and PDU  28 , respectively. It is noted that a third ground-based transponder  604  may be provided as a backup transponder for a system employing GPS satellite signals  606  and  608  in cases where GPS satellite signal  606  and  608  transmission is temporarily terminated, either purposefully or unintentionally. Position is data for the control unit  32  are processed and stored by the GRS  150  using the three reference signals  612 ,  614 , and  622  received from the ground-based transponders  600 ,  602 , and  604 , respectively. Position data for the PDU  28 , obtained using the three reference signals  618 ,  620 , and  624  received respectively from the ground-based transponders  600 ,  602 , and  604 , are processed and stored by the local position locator  616  coupled to the PDU  28  and then sent to the control unit  32  via a data transmission link  34 . An embodiment employing an ultrasonic positioning system  190  would similarly employ three base transponders  600 ,  602 , and  604 , together with mobile transponders  610  and  616  coupled to the control unit  32  and PDU  28 , respectively. 
   Referring now to  FIG. 26 , there is illustrated in flowchart form generalized steps associated with the pre-bore survey process for obtaining a pre-bore site map and determining the optimum route for the boring operation prior to commencing the boring operation. In brief, a pre-bore survey permits examination of the ground through which the boring operation will take place and a determination of an optimum route, an estimate of the productivity, and an estimate of the cost of the entire boring operation. 
   Initially, as shown in  FIG. 26 , a number of ground-based transponders are positioned at appropriate locations around the boring site at step  300 . The control unit  32  and the PDU  28  are then situated at locations L 0  and L 1  respectively at step  302 . The geographical recording system  150  is then initialized and calibrated at step  304  in order to locate the initial positions of the control unit  32  and PDU  28 . After successful initialization and calibration, the PDU  28  is moved along a proposed boring route, during which PDU data and geographical location data are acquired at steps  306  and  308 , respectively. The data gathered by the PDU  28  are preferably analyzed at steps  306  and  308 . The acquisition of data continues at step  312  until the expected end of the proposed boring route is reached, at which point data accumulation is halted, as indicated at step  314 . 
   The acquired data are then downloaded to the control unit  32 , which may be a personal computer, at step  316 . The control unit  32 , at step  318 , then calculates an optimum pre-determined path for the boring operation, and does so as to avoid obstacles and other structures. If it is judged that the pre-determined route is satisfactory, as is tested at step  320 , the route is then loaded into the planned path database  156  at step  322 , and the pre-bore survey process is halted at step  324 . If, however, it is determined that the planned route is unsatisfactory, as is tested at step  320  because, for example, the survey revealed that the boring tool  24  would hit a rock obstacle or that there were buried utilities which could be damaged during a subsequent boring operation, then the PDU  28  can be repositioned, at step  326 , at the beginning of the survey route and a new route surveyed by repeating steps  304 - 318 . After a satisfactory route has been established, the pre-bore survey process is halted at step  324 . 
   In another embodiment, the pre-bore survey process includes the collection of geological data along the survey path, concurrently with position location and PDU data collection. This collection activity is illustrated in  FIG. 26  which shows an initialization and calibration step  328  for the geophysical data acquisition unit  160  (GDAU) taking place concurrently with the initialization and calibration of the geographical recording system  150 . The GDAU  160  gathers geological data at step  330  at the same time as the PDU  28  and position data are being acquired in steps  306  and  308 , respectively. The inclusion of geological data gathering provides for a more complete characterization of the ground medium in the proposed boring pathway, thus allowing for more accurate productivity and cost estimates to be made for the boring operation. 
   In a third embodiment, the survey data are compared with previously acquired data stored in the route mapping database  158  in order to provide estimates of the boring operation productivity and cost. In this embodiment, historical data from the route mapping database are loaded into the control processor  152  at step  332  after the survey data have been downloaded to the control unit  32  in step  316 . The data downloaded from the route mapping database  158  include records of prior surveys and boring operations, such as GPR and geological characterization measurements and associated productivity data. The pre-planned route is calculated at step  334  in a manner similar to the calculation of the route indicated at step  318 . By correlating the current ground characterization, resulting from the PDU  28  and GDAU  160  data, with prior characterization measurements and making reference to associated prior productivity results, it is possible to estimate, at step  336 , productivity data for the planned boring operation. Using the estimated production data of step  336 , it is then possible to produce a cost estimate of the boring process at step  338 . In the following step  320 , a determination is made regarding whether or not the pre-planned route is satisfactory. This determination can be made using not only the subsurface features as in the first embodiment, but can be made using other criteria, such as the estimated duration of the boring process or the estimated cost. 
   Referring now to  FIG. 27 , there is illustrated a system block diagram of a control unit  32 , its various components, and the functional relationship between the control unit  32  and various other elements of the trenchless underground boring system  12 . The control unit  32  includes a central processor  152  which accepts input data from the geographic recording system  150 , the PDU  28 , and the GDAU  160 . The central processor  152  calculates the position of the boring tool  24  from the input data. The control processor  152  records the path taken by the boring tool  24  in the route recording database  154  and/or adds it to the existing data in the route mapping database  158 . 
   In an alternative embodiment, the central processor  152  also receives input data from the sensors  230  located at the boring tool  24  through the sensor input processor  232 . In another embodiment, the central processor  152  loads data corresponding to a predetermined path from the planned path database  156  and compares the measured boring tool position with the planned position. The position of the boring tool  24  is calculated by the central processor  152  from data supplied by the PDU input processor  234  which accepts the data received from the PDU  28 . In an alternative embodiment, the central processor  152  also employs data on the position of the PDU  28 , supplied by the Geographic Recording System  150 , in order to produce a more accurate estimate of the boring tool location. 
   Corrections in the path of the boring tool  24  during a boring operation can be calculated and implemented to return the boring tool  24  to a predetermined position or path. The central processor  152  controls various aspects of the boring tool operation by use of a trenchless ground boring system control (GBSC)  236 . The GBSC  236  sends control signals to boring control units which control the movement of the boring tool  24 . These boring control units include the rotation control  238 , which controls the rotating motor  19  for rotating the drill string  22 , the thrust/pullback control  242 , which controls the thrust/pullback pump  20  used to drive the drill string  22  longitudinally into the borehole, and the direction control  246 , which controls the direction activator mechanism  248  which steers the boring tool  24  in a desired direction. The PDU input processor  234  may also identify buried features, such as utilities, from data produced by the PDU  28 . The central processor  152  calculates a path for the boring tool  24  which avoids any possibility of a collision with, and subsequent damage to, such buried features. 
   In  FIGS. 28 and 29 , there are illustrated flow charts for generalized process and decision steps associated with boring a trenchless hole through the ground. Initially, as shown in FIG.  28  and at step  350 , a number of ground-based transponders are positioned at appropriate locations around a boring site. The trenchless underground boring system  12  is then positioned at the appropriate initial location, as indicated at step  352 , and the transponders and geographic recording system are initialized and calibrated, at step  354 , prior to the commencement of boring, at step  356 . After boring has started, the PDU  28  probes the ground at step  358  and then receives and analyzes the signature signal at step  360 . Independent of, and occurring concurrently with, the probing and receiving steps  358  and  360 , the GRS receives position data at step  362  and determines the position of the PDU  28  at step  364 . After steps  362  and  364  have been completed, the central processor  152  then determines the position of the boring tool  24  at step  366 . 
   The central processor  152  then compares the measured position of the boring tool  24  with the expected position, at step  368 , as given in the planned path database  156  and calculates whether or not a correction is required to the boring tool direction, at step  370 , and provides a correction at step  372 , if necessary. The trenchless underground boring system  12  continues to bore through the ground at step  374  until the boring operation is completed as indicated at steps  376  and  378 . If, however, the boring operation is not complete, the central processor  152  decides, at step  380 , whether or not the PDU  28  should be moved in order to improve the image of the boring tool  24 . The PDU  28  is then moved if necessary at step  382  and the probing and GRS data reception steps  358  and  362  recommence. The operation is halted after the boring tool  24  has reached a final destination. 
   In an alternative embodiment, shown in dashed lines in  FIGS. 28 and 29 , the central processor  152  records, at step  384 , the calculated position of the boring tool  24  in the route mapping database  158  and/or the route recording database  154 , after determining the position of the boring tool at step  366 . In another embodiment, the steps of comparing (step  368 ) the position of the boring tool  24  with a pre-planned position and generating any necessary corrections (steps  370  and  372 ) are omitted as is illustrated by the dashed line  386 . 
   It will, of course, be understood that various modifications and additions can be made to the preferred embodiments discussed hereinabove without departing from the scope or spirit of the present invention. Accordingly, the scope of the present invention should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.