Apparatus and method for detecting a location and an orientation of an underground boring tool

An apparatus and method for determining a location and an orientation of an underground boring tool by employment of a radar-like probe and detection technique. The boring tool is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperation between the probe signal transmitter at ground level and the signature signal generating device provided at the underground boring tool results in accurate detection of the boring tool location and, if desired, orientation, despite the presence of a large background signal. Precision detection of the boring tool location and orientation enables the operator to accurately locate the boring tool during operation and, if provided with a directional capacity, avoid buried obstacles such as utilities and other hazards. The signature signal produced by the boring tool may be generated either passively or actively, and may be a microwave or an acoustic signal. Further, the signature signal may be produced in a manner which differs from that used to produce the probe signal in one or more ways, including timing, frequency content, information content, or polarization.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of trenchless 
underground boring and, more particularly, to a system and process for 
acquiring positional and orientation data on an underground boring tool. 
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 determining a location and 
an orientation of an underground boring tool with higher accuracy than is 
currently attainable given the present state of the technology. 
SUMMARY OF THE INVENTION 
The present invention relates to an apparatus and method for determining a 
location and an orientation of an underground boring tool by employment of 
a radar-like probe and detection technique. The boring tool is provided 
with a device which generates a specific signature signal in response to a 
probe signal transmitted from above the ground. Cooperation between the 
probe signal transmitter at ground level and the signature signal 
generating device provided at the underground boring tool results in 
accurate detection of the boring tool location and, if desired, 
orientation, despite the presence of a large background signal. Precision 
detection of the boring tool location and orientation enables the operator 
to accurately locate the boring tool during operation and, if provided 
with a directional capacity, avoid buried obstacles such as utilities and 
other hazards. 
The signature signal produced by the boring tool may be generated either 
passively or actively, and may be a microwave or an acoustic signal. 
Further, the signature signal may be produced in a manner which differs 
from that used to produce the probe signal in one or more ways, including 
timing, frequency content, information content, or polarization. 
In accordance with one embodiment, surveying the boring site, either prior 
to or during the boring operation, provides for the production of data 
associated with the characteristics of the ground medium subjected to the 
survey. The ground characteristic data acquired during the survey may be 
correlated with historical data which relate ground types to boring 
productivity, hence enabling estimates of boring productivity and overall 
cost to be made for the site subjected to the survey. Accurate surveys of 
planned boring pathways can be made and the position of the boring tool 
accurately measured during a boring operation for contemporaneous or 
subsequent comparison with the planned pathway. The direction of the 
boring tool may be adjusted in response to the measured position in order 
to maintain the boring tool along the planned pathway.

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 a detection system for detecting a location and an 
orientation of an underground boring tool. 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 19a and a lower position 19b). 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 19a 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 19b 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 19a. 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 10 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 36a as a function of 
signal character plotted against time. Line B shows a return signal 62a 
detected by the PDU 28 in the absence of any signature signal generation. 
The return signal 62a is depictive of a signal received by the PDU 28 at a 
time .DELTA.T1 after emission of the probe signal 36a, 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 .DELTA.T2 following illumination with the probe signal 36a. In 
accordance with this detection scheme, the return signal 40a received from 
the scatterers is detected initially, and the signature signal 38a 
received from the underground boring tool 24 is detected after a delay of 
.DELTA.T2. The delay time .DELTA.T2 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 38a is relatively high, thus enabling the signature signal 38a to 
be easily distinguished from the background clutter 40a. 
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 36b of the probe 
signal as a function of signal strength plotted against frequency. Line B 
shows a frequency band 62b 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 62b similar to that of the probe signal 36b. 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 38b different from 
that of the scattered return signal 40b. The difference in frequency band, 
indicated as .DELTA.f, is sufficiently large to move the cooperative 
target signature signal out of, or at least partially beyond, the 
scattered signal frequency band 40b. 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 20 including a microwave antenna and circuit 
components which are used to generate the signature signal. The three 
embodiments illustrated in FIGS. 5a, 5b, and 5c 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. 5a, there is illustrated a cooperative target 20 which includes two 
antennae, a probe signal receive antenna 66a, and a signature signal 
transmit antenna 68a. 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 66a and the 
transmit antenna 68a 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.RTM.. 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. 5a shows the signature signal generation apparatus 
for a microwave detection system operating in the time domain. In 
accordance with this embodiment, a receive antenna 66a receives a probe 
signal 70a from the PDU 28, such as a short microwave burst lasting a few 
nanoseconds, for example. In order to distinguish a signature signal 74a 
from the clutter received by the PDU 28, the received probe signal 70a 
passes from the receive antenna 66a into a time-delaying waveguide 72a, 
preferably a co-axial cable, to a transmit antenna 68a. The signature 
signal 74a is then radiated from the transmit antenna 68a 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 74a 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 76a to form a 
termination. In this latter embodiment, the probe signal 70a propagates 
along the waveguide 72a until it is reflected by the termination located 
at the cut 76a, propagates back to the receive antenna 66a, and is 
transmitted back to the PDU 28. The termination could be implemented 
either as an electrical short, in which case the probe signal 70a would be 
inverted upon reflection, or as an open circuit, in which case the probe 
signal 70a would not be inverted upon reflection. 
The introduction of a time delay to create the signature signal 74a 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 74a 
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 74a. 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. 5b depicts a signature signal generating apparatus 
for a microwave detection system operating in the frequency domain. In 
accordance with this embodiment, a receive antenna 66b provided in or on 
the boring tool 24, receives a microwave probe signal 70b from the PDU 28. 
The probe signal 70b is preferably a microwave burst, lasting for several 
microseconds, which is centered on a given frequency, f, and has a 
bandwidth of .DELTA.f1, where .DELTA.f1/f is typically less than one 
percent. In order to shift a return signature signal 74b out of the 
frequency regime associated with the clutter received by the PDU 28, the 
received probe signal 70b propagates from the receive antenna 66b along a 
waveguide 72b into a nonlinear device 78b, 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 68b as the 
signature signal 74b and is received by the PDU 28. The PDU 28 is tuned to 
detect a harmonic frequency of the probe signal 70b. For a probe signal 
70b 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 70b. Since there is generally no other source of the harmonic 
frequency present, the signal-to-clutter ratio of the signature signal 74b 
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 
76b to form a termination. In accordance with this latter embodiment, the 
probe signal 70b would propagate along the waveguide 72b, through the 
nonlinear element 78b, reflect at the termination 76b, propagate back 
through the nonlinear element 78b, propagate back to the receive antenna 
66b 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. 5c 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 70c 
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 70c. 
Thus, the clutter signal has essentially the same polarization as the 
probe signal 70c. A signature signal 74c is generated in the cooperative 
target 20 by receiving the polarized probe signal 70c in a receive antenna 
66c, propagating the signal through a waveguide 72c to a transmit antenna 
68c, and transmitting the signature signal 74c back to the PDU 28. The 
transmit antenna 68c is oriented so that the polarization of the radiated 
signature signal 74c is orthogonal to that of the received probe signal 
70c. The PDU 28 may also be configured to preferentially receive a signal 
whose polarization is orthogonal to that of the probe signal 70c. As such, 
the receiver 56 preferentially detects the signature signal 74c 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 76c to form a 
termination and inserting a polarization mixer 78c which alters the 
polarization of the wave passing therethrough. In this latter embodiment, 
the probe signal would propagate along the waveguide 72c, through the 
polarization mixer 78c, reflect at the termination 76c, propagate back 
through the polarization mixer 78c, propagate back to the receive antenna 
66c 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. 5a and 5b 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. 6a 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 82a being received by 
a receive antenna 84a which is coupled to a delay-line waveguide 86a. An 
amplifier 88a is located at a point along the waveguide 86a, and amplifies 
the probe signal 82a as it propagates along the waveguide 86a. The 
amplified probe signal continues along the delay-line waveguide 86a to the 
transmit antenna 90a which, in turn, transmits the signature signal 92a 
back to the PDU 28. FIG. 6b 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 82b being received by a receive antenna 84b coupled 
to a waveguide 86b. A triggerable delay circuit 88b is located at a point 
along the waveguide 86b. The triggerable delay circuit 88b operates in the 
following fashion. The triggerable delay circuit 88b is triggered by the 
probe signal 82b which, upon initial detection of the probe signal 82b, 
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 88b which is used as a signature signal 92b. The signature signal 
92b propagates along the waveguide 86b to a transmit antenna 90b which 
then transmits the signature signal 92b to the PDU 28. 
FIG. 6c 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 82c being received by 
a receive antenna 84c coupled to a waveguide 86c and a nonlinear element 
88c. The frequency-shifted signal generated by the nonlinear element 88c 
is then passed through an amplifier 94c before being passed to the 
transmit antenna 90c, which transmits the signature signal 92c to the PDU 
28. The amplifier 94c may also include a filtering circuit to produce a 
filtered signature signal at the output of the amplifier 94c. An advantage 
to using an active frequency domain signature signal generation embodiment 
over a passive frequency domain signature signal generation embodiment 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. 6c, a probe signal 82c passes 
through the amplifier 94c prior to reaching the nonlinear element 88c. 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. 
6d. FIG. 6d shows a receive antenna 84d coupled through use of a waveguide 
86d to a frequency shifter 88d and a transmit antenna 90d. The frequency 
shifter 88d is a device which produces an output signal 92d having a 
frequency of f2, which is different from the frequency, f1, of an input 
signal 82d by an offset .DELTA.f, where f2=f1+.DELTA.f. In accordance with 
this embodiment, .DELTA.f is preferably larger than one half of the 
bandwidth of the probe signal 82d, typically on the order of 1 MHz. The 
frequency shifter 88d produces a frequency shift sufficient to move the 
signature signal 92d 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 92d. 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. 7a, an acoustic 
probe signal 98a, preferably an acoustic impulse, is received and detected 
by an acoustic receiver 100a mounted on the inner wall 96a of the boring 
tool 24. The acoustic receiver 100a transmits a trigger signal along a 
trigger line 102a to a delay pulse generator 104a. After being triggered, 
the delay pulse generator 104a generates a signature pulse following a 
triggered delay. The signature pulse is passed along the transmitting line 
106a to an acoustic transmitter 108a, also mounted on the inner wall 96a 
of the boring tool 24. The acoustic transmitter 108a then transmits an 
acoustic signature signal 110a through the ground for detection by the PDU 
28. 
In accordance with an acoustic frequency-domain embodiment, as is 
illustrated in FIG. 7b, an acoustic probe signal 98b, preferably an 
acoustic pulse having a given acoustic frequency f3, is received and 
detected by an acoustic receiver 100b mounted on the inner wall 96b of the 
boring tool 24. The acoustic receiver 100b transmits an input electrical 
signal corresponding to the received acoustic signal 98b at a frequency f3 
along a receive line 102b to a frequency shifter 104b. The frequency 
shifter 104b generates an output electrical signal having a frequency that 
is shifted by an amount .DELTA.f3 relative to the frequency of the input 
signal 98b. The output signal from the frequency shifter 104b. is passed 
along a transmit line 106b to an acoustic transmitter 108b, also mounted 
on the inner wall 96b of the boring tool 24. The acoustic transmitter 108b 
then transmits the frequency shifted acoustic signature signal 110b 
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 
an 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.degree. relative to horizontal in one application, 
whereas, in another application, the tilt detector 290 may detect pitch 
angles in the range of .+-.90.degree. 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.sub.1 
292 through P.sub.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.sub.1 292 through P.sub.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.sub.1 292 
through P.sub.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.sub.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 .times.60.degree.. The selector 291 may select orientation 
signal generator P.sub.1 292 when the tilt detector 290 is at a level or 
null state (i.e., 0.degree. tilt angle) relative to horizontal. When 
selected, orientation detector P.sub.1 292 generates a unique orientation 
signal which is indicative of an orientation of 0.degree.. 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.degree. tilt 
angle change, for example, orientation signal generator P.sub.2 293 is 
selected by the selector 291. The orientation signal generator P.sub.2 293 
then produces an orientation signal that indicates a positive 5.degree. 
tilt condition. Similarly, orientation signal generators P.sub.3 294, 
P.sub.4 295, and P.sub.5 296 may produce orientation signals representing 
detected tilt angle changes of positive 10.degree., 15.degree., and 
20.degree., 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.degree. and 
60.degree.. Negative tilt angles between 0.degree. and -60.degree. in 
5.degree. 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. 11a and 11b, 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. 11a, 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. 11b, the boring tool 500 includes a 
number of elongated recesses or channels within which signature signal 
generating devices are disposed. In FIG. 11b, there is shown a 
cross-sectional view of the boring tool 500 illustrated in FIG. 11a. 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. 11b 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.RTM., 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.degree. 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 
.alpha. provided in the bent tube 221. The bend angle .alpha., 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 13a-13b. 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 
235a and 235b separated by a channel 237. The rails 235a and 235b include 
gaps 233 which separate one contact rail circuit from an adjacent contact 
rail circuit. As is illustrated in detail in FIGS. 13a-13b, 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. 13a-13b, movable 
contact 222 is shown moving within the tube 221 between a first position 
P.sub.a and a second position P.sub.b in response to tilting of the tube 
221. When the movable contact 222 is at the position P.sub.a, continuity 
is established between contact 227, contact rail 235a, movable contact 
222, contact rail 235b, and contact 229. As such, the circuit path 
including the transmission line T.sub.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 235a, movable contact 
222, rail contact 235b, and contact 229. The received probe signal 235 
transmitted to the time-delaying waveguide T.sub.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.sub.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.sub.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. 
13a-13b. 
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.sub.1 512 and R.sub.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.sub.1 512 and R.sub.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.sub.1 512, is typically dependent on the radial location of a particular 
rotation detector relative to earth'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. 21a 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. 5a. The circuit 
illustrated in FIG. 21a 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. 21b. The top trace 407b shows the probe signal 407, I.sub.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 408b. The regular opening and closing 
of the mercury switch 408 modulates the signature signal 409b, I.sub.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 24a. At the front end of the 
boring tool 24a 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 24a. 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 24a or may be of a conformal design 
located on the surface of the boring tool 24a and conforming to the 
surface contour. The boring tool 24a may also contain one or more sensors 
for sensing the environment of the boring tool 24a. Circuitry is provided 
in the boring tool 24a 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 24a, (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 24a. 
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.sub.1 252 and AR.sub.2 
254. Each of the receive antennas AR.sub.1 252 and AR.sub.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.sub.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.sub.1 252 and AT 
250. The travel time of the signal traveling between the cooperative 
target 20 and the receive antenna AR.sub.2 254 is indicated as the time 
t.sub.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: 
EQU d.sup.2 =K.sup.2 (t.sub.1.sup.2)-m.sup.2 1! 
EQU d.sup.2 =K.sup.2 (t.sub.2.sup.2)-9m.sup.2 2! 
EQU K.sup.2 (t.sub.2.sup.2)-K.sup.2 (t.sub.1.sup.2)=8m.sup.2 3! 
EQU K.sup.2 (t.sub.2.sup.2 -t.sub.1.sup.2)=8m.sup.2 4! 
EQU K.sup.2 =8m.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2)! 5! 
EQU d.sup.2 =8m.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2)!(t.sub.1.sup.2)-m.sup.2 
6! 
EQU d=m(8t.sub.1.sup.2 /(t.sub.2.sup.2 -t.sub.1.sup.2))-1!.sup.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: 
##EQU1## 
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.sub.j is the thickness of the jth ground type above 
the cooperative target; .di-elect cons..sub.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. 17a-17b and 18a-18b. 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 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. 17a and 17b 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.sub.L and a right receive antenna 
A.sub.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.sub.L and A.sub.R. The dashed line 41 shown in FIG. 17a 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 L1, 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.sub.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.sub.L 
and the right receive antenna A.sub.R at approximately the same time, as 
is illustrated in the graph G.sub.1 of FIG. 17b. 
Referring to the graph G.sub.1 of FIG. 17b, it is assumed that the probe 
signal 36 produced by the PDU 28 is transmitted at a time to. Because the 
two receive antennas A.sub.L and A.sub.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.sub.L and A.sub.R at substantially the same time, t.sub.1, 
after transmission of the probe signal at time to. Concurrent reception of 
the signature signal by the two receive antennas A.sub.L and A.sub.R is 
depicted in the graph G.sub.1 of FIG. 17b as detected signals S.sub.R and 
S.sub.L, respectively, at a time t.sub.1. 
At a second location L2 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.sub.L and A.sub.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.sub.L and 
A.sub.R. It can be seen that the separation distance between the left 
receive antenna A.sub.L and the boring tool 24 is less than the separation 
distance between the right receive antenna A.sub.R and the boring tool 24. 
The boring tool deviation from the center of path 41 is reflected in the 
graph G.sub.2 of FIG. 17b as a delay between reception of the signature 
signal S.sub.L by the left receive antenna A.sub.L at a time t.sub.2 and 
reception of the signature signal S.sub.R by the right receive antenna 
A.sub.R at a later time t.sub.3. 
At a third location L3 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 L2. 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.sub.L and 
A.sub.R, respectively. At the location L3, it can be seen that the 
distance between the boring tool 24 and the right receive antenna A.sub.R 
is less than the distance between the boring tool 24 and the left receive 
antenna A.sub.L. Accordingly, as is indicated in the graph G.sub.3 of FIG. 
17b, the signature signal S.sub.R is received by the right receive antenna 
A.sub.R in advance of the signature signal S.sub.L received by the left 
receive antenna A.sub.L, thereby resulting in a time delay defined as 
.DELTA.(t.sub.3 -t.sub.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 L4 along the predefined-above-ground path 41, it can 
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 L3. As is shown at 
location L4, the boring tool 24 is again oriented 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.sub.L 
and A.sub.R. The graph G.sub.4 of FIG. 17b 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.sub.L and 
A.sub.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. 18a-18b 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. 18a, an antenna array 37 includes 
four receive antennas A.sub.1, A.sub.2, A.sub.3, and A.sub.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.degree., 90.degree., 180.degree., and 
270.degree. 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. 18a is a depiction of the antenna array 37 having its center transmit 
antenna oriented co-parallel with a predetermined above-ground path 41. 
Superimposed in FIG. 18a is an underground boring tool 24 equipped with a 
cooperative target 20 depicted at three different locations L1, L2, and L3 
along the predetermined path 41. At the location L1, 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.sub.0, is received at substantially the same time, t.sub.4, by each of 
the four receive antennas A.sub.1, A.sub.2, A.sub.3, and A.sub.4. The 
in-phase relationship of the signature signals S.sub.1, S.sub.2, S.sub.3, 
and S.sub.4 respectively received by receive antennas A.sub.1, A.sub.2, 
A.sub.3, and A.sub.4 is depicted in the graph G.sub.1 of FIG. 18b. 
At a location L2, 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.sub.2 of FIG. 18b by the 
signature signal reception relationship associated with each of the four 
receive antennas A.sub.1, A.sub.2, A.sub.3, and A.sub.4. It can be seen 
that the distance between the boring tool 24 at location L2 and the 
receive antenna A.sub.2 is less than the distance between the boring tool 
24 and the other receive antennas A.sub.1, A.sub.3, and A.sub.4. As is 
depicted in the graph G.sub.2 of FIG. 18b, the signature signal S.sub.2 is 
received at a time t.sub.2 by the receive antenna A.sub.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.sub.1 and A.sub.4 at the previous location L1 
have effectively increased when the boring tool 24 deviates to the 
location L2, thereby increasing the delay time of signature signal 
reception by receive antennas A.sub.1 and A.sub.4. As such, reception of 
the signature signal S.sub.1 by antenna A.sub.1 at a time t.sub.7 and the 
signature signal S.sub.4 by receive antenna A.sub.4 at a time t.sub.8 is 
delayed with respect to the reception of signature signal received by 
receive antennas A.sub.2 and A.sub.3 at times t.sub.2 and t5 respectively. 
At a location L3, the graph G.sub.3 of FIG. 18b 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. 18b. 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.degree. relative to the z-axis. Antenna-B 524 is also 
directed along a direction contained within the y-z plane, but at 
-45.degree. relative to the z-axis, in a position rotated 90.degree. 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. 21c 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. 21d, which shows several signals as a function of time. The top 
signal 413d represents the probe signal, I.sub.p, received by the receive 
antenna 412. The second signal, 415d, represents the actively generated 
signature signal Ia, which would be generated if there were no modulation 
of the signature signal. The third trace, 416d, shows the amplitude 
modulation signal, I.sub.m, generated by the active time domain circuit 
416, and the last trace, 422d, shows the signature signal, I.sub.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. 22a 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. 22a, 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. 21b. The top trace 438b, in FIG. 22b, shows the 
signal, I.sub.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 440b. The continual opening and 
closing of the mercury switch 440 produces a modulated signature/beacon 
signal 435b, I.sub.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 Im 435b. The modulation of signal I.sub.m 435b 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. 22c 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. 22d. The signal 453d represents the probe signal, I.sub.p, received 
by the receive antenna 452. The second signal 456d represents the 
time-delayed signature signal, I.sub.s, generated by the active time 
domain circuit 456. The third signal 460d, I.sub.c, represents a 
combination of the time-delayed signature signal I.sub.s, 456d and an 
unmodulated signal produced by the active beacon circuit 460. The last 
trace, 455d, shows a signal received at the surface, I.sub.m, which is a 
combination of the time-delayed signature signal I.sub.s 456d 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 (GT2)) 140, clay (GT3) 142 and native soil (GT4) 144. 
The road is generally described by the portion denoted as road fill (GT1) 
146. FIG. 12 illustrates a drill string 22 in a first position 22c, at the 
end of which is located a boring tool 24c. The PDU 28c is shown as being 
situated at a location above the boring tool 24c. The PDU 28c transmits a 
probe signal 36c which propagates through the road fill and the ground. 
In the case of the boring tool at location 24c, the probe signal 36c 
propagates through the road fill 146 and the clay 142. The boring tool 
24c, in response, produces a signature signal 38c which is detected and 
analyzed by the PDU 28c. The analysis of the signature signal 38c provides 
a measure of the time-of-flight of the probe signal 36c and the signature 
signal 38c. The time-of-flight is defined as a time duration measured by 
he PDU 28c between sending the probe signal 36c and receiving the 
signature signal 38c. The time-of-flight measured depends on a number of 
factors including the depth of the boring tool 24c, the dielectric 
conditions of the intervening ground medium 146 and 142, and any delay 
involved in the generation of the signature signal 38c. Knowledge of any 
two of these factors will yield the third from the time-of-flight 
measurement. 
The depth of the boring tool 24c can be measured independently using a 
mechanical probe or sensing the pressure of the water at the boring tool 
24c using a sensor 130 located in the boring tool head 24c 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 24c 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 24c 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: 
##EQU2## 
where, the subscript "3" refers to GT3. Direct measurement of the 
time-of-flight, TF, and the depth of the boring tool 24c, d.sub.3, along 
with the knowledge of any time delay, TD, will yield the average 
dielectric constant, .di-elect cons..sub.3, of GT3. This characteristic 
can be denoted as GC3. 
Returning to FIG. 23, there is illustrated an embodiment in which the 
boring tool 24 has been moved from its first location 24c to another 
position 24d. The drill string 22d (shown in dashed lines) has been 
extended from its previous configuration 22c 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 28c to a new position 
28d (shown in dashed lines) in order to be close to the boring tool 24d. 
The parameter GC4, which represents the ground characteristic of the 
native soil GT4, can be obtained by performing time-of-flight measurements 
as previously described using the probe signal 36d and signature signal 
38d. Likewise, ground characteristic GC2 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 
24d 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 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 L0 and 
L1 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/pullack 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.