Patent Publication Number: US-7723990-B2

Title: Method of displaying digital image for digital locating system and device for underground object detection

Description:
PRIORITY CLAIM 
   This is a divisional of and claims the benefit of priority from U.S. patent application Ser. No. 12/016,139, filed Jan. 17, 2008, now U.S. Pat. No. 7,605,590 which in turn is a divisional and claims the benefit of U.S. patent application Ser. No. 11/060,053, filed Feb. 16, 2005, now U.S. Pat. No. 7,372,276, the full disclosures of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The field of this invention relates generally to digital locating systems, and more specifically, to underground digital locating systems that are capable of identifying the position, orientation and depth of an underground object, such as a utility line, a pipe line or a sonde. 
   BACKGROUND OF THE INVENTION 
   Many common construction and utility operations that require soil excavation depend on the operator&#39;s knowledge of the orientation and depth of underground utility lines, pipelines and cable lines. Because contact by excavating equipment is almost invariably damaging to underground lines, it is very important to know the exact position of those lines prior to commencing digging activities. Knowledge of underground line position allows the operator to avoid coming in contact with and damaging such lines. 
   There are several known locating systems that are currently used to locate underground lines. Most of the known locators involve receiver detection of a magnetic field derived from electrical current directly fed or induced onto an underground line. 
   The magnetic field lines emanating from a line are essentially cylindrical in shape with the center of the cylinder being the current-carrying line itself. As the current flows along the line, losses occur as a result of displacement and induction of currents into the soil. When the rate of loss along the line length is not great, depth can be computed through the use of a signal strength ratio. For lines that run straight underground along a certain depth, the magnetic field strength is inversely proportional to the distance from the line to the receiver. Depth is typically determined by taking two signal strength readings at different locations directly above the line. 
   U.S. Pat. No. 6,756,784 to Mercer et al. describes a locator/monitor that is capable of locating a boring tool and monitoring the progress of the tool for control purposes. The locator/monitor described in Mercer achieves its goals through the operation of an antenna assembly that features one cluster of two orthogonal antennas that are in spatial proximity to each other and not a fixed distance apart. 
   The locator/monitor described in Mercer operates in the following way: (1) an operator locates the underground transmitter (mounted on a boring tool); (2) operator deploys the receiver/locator at a first height above the transmitter location and measures the magnetic field strength emanating from the transmitter; and (3) deploys the receiver at a second height to measure the magnetic field strength emanating from the transmitter. Although this device may be accurate, its use is time-consuming, as the operator has to take measurements at two different heights. 
   U.S. Pat. No. 6,768,307 to Brune et al. describes a system for flux plane locating that includes a boring tool with a transmitter that transmits a locating field such that the locating field exhibits a pair of locate points in relation to the surface to the ground and a portable locator that is used to measuring the intensity of the locating field. The locator contains one antenna cluster that contains three orthogonally disposed antennas, two in a horizontal plane, and one disposed in a vertical plane. As with the previous device, the use of this locator suffers from the disadvantage of being time consuming, as it forces the operator to move the locator to several positions before arriving at the final determination of line location or depth. 
   There are also presently known line-locating devices that employ two antennas and logic circuitry to determine depth. The antennas are separated by a fixed distance. Because the separation distance is known, cable depth can be computed by interpreting the magnetic field strength. 
   U.S. Pat. No. 5,920,194 to Lewis et al. describes a locator that has spaced antennas that detect electromagnetic signals from an underground line. The locator contains a processor that analyzes the electromagnetic signals and determines the separation of the locator and the underground line, both in terms of the direction corresponding to the spacing of the antennas and the perpendicular direction to the underground line. The display on the device shows the separation of the locator and the object. 
   This device suffers from several disadvantages. First, because the device determines the separation between itself and the underground line by measuring the angles between its antennas and the surface of the underground solenoid, the angle of the positioning of the locator becomes critical. Lewis et al. attempts to solve that problem by including a tilt sensor in its device, which in all likelihood makes the device more expensive. 
   Also, the locator is preferably a ground penetration probe that is driven into the ground to maintain a constant angular position during measurements. This may be impractical in certain locations where the ground is difficult to penetrate. A further disadvantage of this locator is that it provides a relatively precise measurement only after requiring the operator to measure the separation of the locator and the underground line at two different locator positions and comparing the two results. Thus, the operation of the device in Lewis is time consuming and prone to unnecessary errors if the locator is not identically positioned at both measurement locations. 
   Measurement accuracy in devices of the prior art is often affected by differential drifting of the electronics associated with the antennas as well as by differential responses of the antennas themselves. To increase sensitivity, ferrite rods are sometimes employed to enhance the effective capture area of the antennas. As a result of the antenna separation, both antennas may not experience the same thermal environment. This can be a problem because the characteristics of ferrite vary measurably with temperature and are not consistent between rods. 
   There are other problems to be overcome when measuring AC flux fields using multiple ferrite antennas. For example, synchronization is a major problem. Because AC signals are characterized by amplitude and phase, both must be accounted for to get accurate results. If the phases of signals being measured on separate antennas are somehow shifted relative to each other without account, then there is no way of knowing at any given instance what the actual relative responses of those antennas are. Often, amplifying and filtering circuits will by their very nature introduce phase shifts that are not controllable enough to maintain accurate resulting signals on the back end. A second problem of using ferrite antennas is measurement accuracy and calibration. This is because of variations in ferrite permeability, winding anomalies, etc. 
   Therefore, a need exists for a digital locating system that can not only accurately predict the location, orientation and depth of an underground line using simple and quick procedure, but one that can also adequately account for anomalies in the ferrite antennas and component tolerance errors in the processing circuitry itself. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The invention satisfies this need. According to the invention, there is provided a digital locating system for identifying the position, orientation and depth of underground objects, such as underground lines and transmitters known as sondes. The locating system is compact and lightweight, and is designed for easy transportation and easy use by a single operator. 
   In the preferred embodiment of the invention, the system of the invention comprises a digital receiver for location of underground lines and a digital transmitter case, adapted for housing the digital receiver and incorporating a digital transmitter therein. The digital receiver is adapted for receiving a locating signal from an underground object, processing the information, and producing a video and audio output for the user regarding the predicted location of the underground object. The underground object can be a utility line, a pipeline, or a sonde. 
   In another embodiment, the system of the invention includes a connection cable for connecting the digital transmitter to an underground object. Connection of the digital transmitter to the underground object via the connection cable allows the transmitter to energize the underground object of interest. 
   In yet another embodiment, the system further includes a ground rod that provides grounding to the connection cable. In yet another embodiment of the invention, the system is capable of energizing the underground object of interest by induction. In this embodiment, the digital transmitter of the system does not have to be directly connected to the underground object. 
   In the preferred embodiment of the invention, the digital receiver comprises a main body that includes a microcontroller and circuitry for processing locating signals, a display screen for providing visual feedback to a user during operation, a pistol grip and trigger to select and execute menu choices for receiver operation, an antenna arm containing an antenna set at one end and swivelably connected to the receiver body at its other end, another antenna set housed within the main body, and a speaker for providing audible signals to the user during locating operations. 
   In the preferred embodiment, the carrying case of the invention comprises two halves connected in such a way as to permit the opening and closing of the case. One of the halves comprising the case contains a cavity designed to accommodate the digital receiver of the invention. One of the halves also includes at least one latch for keeping the case closed. The carrying case further comprises a handle, designed for comfortable carrying of the case. The carrying case also houses a digital transmitter designed to energize underground objects of interest. 
   The system provides accurate underground line orientation detection, as well as depth measurement. The device can operate at different locating frequencies and offers several locating modes. 
   In the preferred embodiment of the invention, the digital display of the locator offers a precise projection of the predicted image of the underground line. The display is capable of also projecting the predicted depth of the underground line, along with the approximate accuracy of the calculated depth prediction. The receiver of the present invention achieves its goals through the operation of an assembly comprising two sets of orthogonal ferrite-wound loop antennas, each set having three antennas. 
   It is the object of the invention to also provide a method for measuring the flux field surrounding an underground sonde. Another object of the invention is to also provide a method for translating the flux field measurements from an underground sonde into a digital representation of the underground object on a display screen. 
   Yet another object of the invention is to also provide a method for measuring the flux field surrounding an underground line. Yet another object of the invention is to also provide a method for translating the flux field measurements from an underground line into a digital representation of the line on a display screen. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the system of the invention. 
       FIG. 2  is perspective view of the preferred embodiment of the digital locator of the invention. 
       FIG. 2A  is an expanded cross sectional view of the interior structure of the digital locator of the invention. 
       FIG. 3  is a perspective view of the two antenna sets utilized within the digital locator of the invention. 
       FIG. 4  is a close-up perspective view of the connection panel of the digital locator of the invention. 
       FIG. 5  is a perspective view of the preferred embodiment of the carrying case of the invention. 
       FIG. 6  is a close-up perspective view of the control panel of the carrying case of the invention. 
       FIG. 7  is a close-up perspective view of the output panel of the carrying case of the invention. 
       FIG. 8  is a perspective view of the attachments used in conjunction with the carrying case in another embodiment of the invention. 
       FIG. 9  is a close-up perspective view of the display screen of the locating receiver of the invention. 
       FIG. 10  is a perspective view of the typical operation of the system of the invention. 
       FIGS. 11A ,  11 B, and  11 C are all various close-up perspective views of the display screen during line tracing operations. 
       FIGS. 12A ,  12 B, and  12 C are all various close-up perspective views of the display screen during sonde tracing operations. 
       FIG. 13  is a perspective view of the typical sonde-locating operations using the system of the invention. 
       FIG. 14  is a perspective view of the direct connection method of energizing a line using the system of the invention. 
       FIG. 15  is a perspective view of the induction method of energizing a line using the system of the invention. 
       FIG. 16  is a schematic diagram illustrating the operation of the various part of the digital locator in combination with the microcontroller and the circuitry, according to the invention. 
       FIG. 17A  provides a diagrammatic illustration of the line locating operation, illustrating a situation when the field vectors are misaligned. 
       FIG. 17B  provides a diagrammatic illustration of the line locating operation, illustrating a situation when the field vectors are aligned. 
       FIG. 17C  provides a diagrammatic illustration of the sonde locating operation, illustrating a situation when the field vectors are misaligned. 
       FIG. 17D  provides a diagrammatic illustration of the sonde locating operation, illustrating a situation when the field vectors are aligned. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description of the preferred embodiments reference is made to the accompanying drawings, which are shown by way of illustration of specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope of the present invention. 
   The invention is a system for identifying the orientation and depth of underground objects, such as lines and sondes. Referring to  FIG. 1 , the system  10  is comprised of a receiver  12  and a transmitter case  14 . A variety of accessories (not shown) can be used in conjunction with the system of the invention. 
   The preferred embodiment of the receiver  12  is shown in  FIG. 2 . The receiver  12  has a front end  16  and a rear end  18 . The receiver  12  is comprised of a main body  20  and a top arm  22 . The main body  20  and the top arm  22  of the locator  12  are integral to each other and the locator  12  is typically made from a single mold. The main body  20  has a top surface  21  and a bottom surface  23 . The top arm  22  has a top surface  25  and a bottom surface  27 . In the embodiment shown in the drawings, the bottom surface  27  of the top arm  22  and the top surface  21  of the main body  20  together form a U-shaped cavity  29  in the receiver  12 . 
   A handle  24  connects the main body  20  and the top arm  22 . The handle  24  is oriented approximately perpendicularly to the main body  20  and to the top arm  22 . The handle  24  is shaped to comfortably accommodate a human hand and has a top end  28  and a bottom end  30 . The handle  24  is in the general form of a pistol grip. A trigger  26  is positioned proximally to the top end  28  of the handle  24 . The handle  24  and the trigger  26  are positioned for comfortable gun-style operation. 
   In the illustrated embodiment, the receiver  12  contains a display screen  32 . The display screen  32  is located on the top arm  22  and is proximal to the rear end  18  of the receiver  12 . A typical sharp, high contrast LCD screen can be used as the display screen  32 . The display screen  32  provides visual feedback to a user during operation of the receiver  12 . 
   Referring to  FIG. 2 , the receiver further contains an antenna arm  34 . The antenna arm  34  has a first end  40  and a second end  42 . The first end  42  of the antenna arm  34  is swivelably connected to the main body  20  of the receiver  12 . When receiver  12  is not in use, the antenna arm  34  is typically in a folded position, as shown in  FIG. 1 . When receiver  12  is operated, the antenna arm  34  is in an unfolded position, as shown in  FIG. 2 . To move the antenna arm  34  from folded to unfolded position and vice versa, the antenna arm  34  is swung by a user approximately 180° around a swivel point  36 . Referring to  FIG. 2A , the swivel point  36  is located on the front end  16  of the receiver  12 . 
   Further referring to  FIG. 2 , the second end  42  of antenna arm  34  contains a pointer antenna  38 . The pointer antenna  38  points directly to the object being located, such as an underground line. In the illustrated embodiment, the pointer antenna  38  looks like a rounded arrowhead. A second antenna, called a chassis antenna  39 , is housed within the main body  20  of the receiver  12 . The chassis antenna  39  is located a predetermined distance away from the pointer antenna  38  along the Z-axis, as shown in  FIG. 3 . In the preferred embodiment illustrated in  FIG. 3 , the distance (D) between the pointer antenna  38  and the chassis antenna  39  is 21 inches, but other distances between the antennas can be used, as long as the exact distance is known. 
   Referring to  FIG. 2A , the receiver  12  of the invention measures a magnetic flux field using a total of six directional ferrite-wound loop antennas  41 , each antenna  41  having a center tap (ground) and with an equal number of turns on either side of the center tap. In the illustrated embodiment, the antennas  41  are arranged in two sets of three antennas each. One set  43  of three antennas  41  is housed within and makes up the pointer antenna  38 . The other set  45  of three antennas  41  is housed within the main body  20  and makes up the chassis antenna  39 . The chassis antenna  39  within the main body  20  is enclosed in a chassis antenna cover  39 ′. 
   Each set  43 ,  45  has the antennas  41  arranged as shown in  FIG. 3 , with the antennas  41  of one set  43  positioned so that they have the same overall orientation in space as the antennas  41  of the other set  45 . The antennas  41  are oriented along mutually orthogonal axes, X, Y, Z. Therefore, each antenna is capable of detecting some distinct portion of a 3D magnetic flux field vector. 
   The goal of having this arrangement is to measure two 3D magnetic flux vectors at two points in space, separated by some predetermined distance. Once these two vectors have been determined, they may be used in various calculations to determine the location of the target that is generating the magnetic field. The magnetic field is typically the result of an AC current flowing either along a straight line (e.g. a cable or a pipe that has been “energized”), or an AC current flowing in a solenoid (an energized coil often referred to as a “sonde” in the underground locating industry). 
   Referring to  FIG. 2A , the receiver  12  further contains a speaker  42 . In the illustrated embodiment, the speaker  42  is housed within the top arm  22  of the receiver  12 , and is located proximally to the front end  16  of the receiver  12 . The speaker  42  provides audible sound feedback to a user during operation. The volume of the speaker can be adjusted by a user, as needed. 
   Further referring to  FIG. 2A , the receiver  12  also contains at least one custom circuit board  47 , which contains at least one microcontroller  112  and logic circuitry (not shown). The microcontroller  112  and circuitry of the receiver  12  are designed to interpret the locating signals acquired by the antenna sets  43  and  45 , perform calculations with the acquired signals, and produce a result output on the display screen  32 . The result output is also preferably audible, and can be heard through the speaker  42  of the receiver  12 . 
   Further referring to  FIG. 2 , the receiver  12  also contains a battery compartment  44 . In the illustrated embodiment, the battery compartment  44  is proximal to the rear end  18  of the receiver  12  and is located on the bottom surface  23  of the main body  20 . 
   In the embodiment illustrated in the drawings, the receiver  12  further contains a connection panel  46 , located on the bottom surface  21  of the main body  20 , between the battery compartment  44  and the front end  16  of the receiver  12 . The connection panel  46  provides plug-in access for various standard and optional accessories (not shown). 
   The connection panel  46  of the preferred embodiment is illustrated in more detail in  FIG. 4 . The connection panel  46  contains a headset jack  48  for accommodating a headset plug (not shown). When a headset (not shown) is plugged into the headset jack  48 , the receiver  12  transmits sound into the headset and the speaker  42  no longer emits audible sounds. The connection panel  46  further contains an auxiliary antenna port  50 . The port  50  allows a user to connect up to two external antennas (not shown) into the receiver  12 . 
   By way of example, a generally available enhanced-sensitivity detection antenna (not shown) and a cable clamp (not shown) may be plugged into the auxiliary antenna port  50 . In the illustrated embodiment, the connection panel  46  further contains a small pinpoint aperture  52 . The aperture  52  acts as a reset button for the circuitry of the circuit board  47  housed inside of the receiver  12 . A small object, such an end of a regular paper clip may be inserted into the aperture  52  to reset the internal circuitry of the receiver  12 . The connection panel  46  also contains a communications port  54 . The communications port  54  allows the receiver  12  to communicate with external devices (not shown). 
   The second major component of the system of the invention is the transmitter case  14 , illustrated in detail in  FIG. 5 . One of the functions of the transmitter case  14  is to house the receiver  12 . The inside of the case  14  includes a cavity  15  that is shaped to accommodate the receiver  12 . The case  14  also includes a built in line-energizing transmitter (not shown). A Model 23X transmitter offered by Goldak, Inc. of Glendale, Calif. is compatible with the transmitter case  14 . 
   In the embodiment illustrated in the drawings, the case  14  is generally trapezoidal in shape and is made out of two halves, the top half  56  and the bottom half  58 . Alternatively, any other suitable shape may be used. The case  14  contains a handle  60 , designed to accommodate the hand of a user. The handle  60  makes the carrying of the case more convenient. In the illustrated embodiment, the bottom half  58  of the case  14  contains a control panel  62  on one side  64  and an output panel  66  on its other side  68 . The control panel  62  contains a plurality of buttons and is used to operate the transmitter. 
   In the illustrated embodiment, the transmitter case  14  also contains two latch covers—a left cover,  70 , and a right cover  72 . The latch covers  70 ,  72  secure the case  14  when it is in a closed position (not shown). When the case  14  in the closed position, the left latch cover  70  is positioned over the control panel  62 , and the right latch cover  72  is positioned over the output panel  66 . 
   The control panel  62 , illustrated in more detail in  FIG. 6 , comprises a display  74  that reports information regarding the operation of the transmitter, such as mode, frequency, output level, etc. An LCD display is typically used, but other display types are acceptable. The control panel  62  comprises the following buttons: “ON”, “OFF”, “Select”, “Up”, “Down”, “PULSE”. These buttons allow a user to operate the transmitter. 
   The output panel  66 , illustrated in  FIG. 7 , comprises three jacks, which are normally used for signal output and power input. Specifically, the output panel  66  contains two “Direct Output” jacks  76 , a “Signal ( 1 )” jack and a “Signal ( 2 )” jack. Each of the “Direct Output” jacks is made to accommodate a standard ¼″ mono phone plug (not shown). An example of such a phone plug is the direct connection cable  82 , which is shown in  FIG. 8 . 
   Preferably, the “Signal ( 1 )” jack is configured to output twice the standard signal level, when used alone. When the two “Direct Output” jacks  76  are used together, each jack outputs a standard signal level. The output panel  66  further contains a “DC Power In” input  78 . This input  78  is configured to accommodate a standard 2.5 mm DC power plug, which is commonly available. The input works best when the rated voltage of the external supply is between 7.5V and 25V. 
   Referring once more to  FIG. 5 , the transmitter case  14  further contains a battery compartment  80 . In the illustrated embodiment, the battery compartment  80  is housed within the bottom half  58  and is located proximal to the handle  60  of the transmitter case  14 . 
   In one embodiment, the system of the invention further comprises a direct connection cable  82  and a ground rod  84 , shown in  FIG. 8 , both of which are typically housed within the transmitter case  14 . The direct connection cable  82  is used to inject a signal directly into a target line. The ground rod  84  is usable to establish good grounding for direct connection to an underground line  100 . 
   To operate the system of the invention, a user unlocks the two latch covers  70 ,  72 , opens the transmitter case  14  and removes the receiver  12 . When the receiver  12  is removed from the case  14 , the receiver  12  is in its folded configuration. To switch receiver  12  to its unfolded configuration, the antenna arm  34  is swung in a clockwise direction 180° about the swivel point  36  until the antenna arm  34  is fully extended. 
   To turn on the receiver  12 , the trigger  26  is clicked once. When the receiver  12  is turned on, the display screen  32  will display information, as shown by way of example in  FIG. 9 . The information displayed on the display screen  32  is in the form of a menu. The user can use the menu-style information shown by the display  32  to select options for operation of the receiver  12 . The trigger  26  is used to scroll between and select the menu choices. In the preferred embodiment, for example, the trigger  26  is clicked and released once to scroll between the menu choices. In order to select a highlighted menu choice, the trigger  26  is clicked and held for approximately 1-3 seconds. 
   Before an underground line can be located, it must carry a current at the frequency of interest. A line that carries a tracing current is an energized line. The system of the invention not only allows a user to trace a line that is already energized with current, but also allows the user to use several methods of introducing a current into a line. 
   In the embodiment illustrated in  FIG. 14 , the line  100  is energized via direct connection in which a signal is injected from the active transmitter output  76  of the transmitter case  14  into the line  100  using the direct connection cable  82 . To energize the line  100  by direct hookup, a user needs an accessible electrically exposed part of the line  100  to which to connect the connection cable  82 . This could be a metallic water riser or the bare end of a tracer wire, for example. 
   In another embodiment of the invention shown in  FIG. 15 , the line  100  can be energized by induction. Referring to  FIG. 15 , when the signal jacks  76  of the transmitter case  14  are not being used, the transmitter case  14  emits an inductive field, which is capable of energizing an underground line  100  when placed over it. This method requires none of the underground line to be exposed. However, this method is best used on isolated lines and away from areas congested with many other lines and metallic objects. 
   Some lines are by their nature always energized. An example of such a line would be a power line. Currently used power lines that are actively in service constantly carry AC current. This current is typically at a frequency of 60 Hz with harmonic components at multiples of 60 Hz (i.e., 120 Hz, 180 Hz, 240 Hz etc.). In Europe this frequency is 50 Hz (100 Hz, 150 Hz, etc.). With such lines, the step of energizing the line is not necessary. 
   Once the target underground line  100  has been energized, the operator (not shown) is ready to use the receiver  12  to trace the line  100  and identify its orientation and depth. 
   As discussed earlier, the display  32  of the receiver  12  provides operation-related information to a user in menu-style format. Scrolling between the choices and selection of choices is achieved by operation of the trigger  26 . The menu-style software of the receiver  12  provides a user with three different ways to set the locating frequency of the receiver  12 . The user can use a “BYSCAN” option in the menu, which automatically tunes the receiver  12  to the exact frequency of the transmitting source. Also, the user could use a “BYCATALOG” option in the menu, which allows the user to choose the desired receiver frequency manually from a list of pre-catalogued selections. Also, the user could use a passive mode option in the menu, which scans at the frequency of 60 Hz. 
   Prior to initiating line locating, the user sets the locating mode using the menu on the display screen  32 . In the preferred embodiment of the invention, there are four modes available: Line, Sonde, Peak and Null. The Peak and Null modes are conventional and are well-known in the art. 
   The Line mode, generally shown in  FIG. 10 , is unique to the locator of the invention. If this mode is selected, a graphic representation of the target line is displayed on the display screen  32  while the underground line  100  is being traced. This mode is fully automatic, requiring no user intervention after setup. This mode displays line position and direction, and automatically calculates the depth and depth accuracy when the pointer antenna is positioned over the buried line. A sample output of the display screen  32  during the Line mode is shown in  FIGS. 11A ,  11 B, and  11 C. 
   Referring, to  FIG. 10 , to begin the searching of line  100 , a user walks with the locator  12  in hand away from the hookup point  104 , at which the transmitter case  14  connects to the line  100 . Once the user is several feet from the hookup point  104 , the user typically walks around the hookup point  104  in a circular direction. When the pointer antenna  38  crosses over the line  100 , the speaker  42  of the receiver  12  will notify the user by making a distinct sound. 
   Referring to  FIG. 11A , while searching for line  100  with the receiver  12  in hand, the user is guided by the audible signal generated through the speaker  42  of the receiver  12 , and by the arrow  86  on the display screen  32  that indicates the direction from which the signal generated by the line  100  is coming. In  FIG. 11A , the display screen  32  is shown at a time when a user approaches an energized line  100 . The screen  32  displays an arrow  86 . The circle  88  with a cross-hair  89  in the middle represents the pointer antenna  38  of the receiver  12 . The arrow  86  indicates the direction from which the signal is coming. For instance, in  FIG. 11A , the energized underground line  100  is to the right of the pointer antenna  38 . 
   Referring to  FIG. 11B , as the user moves closer to the underground line  100 , a digital image  101  of the line is projected on the screen  32 , indicating the orientation of the line  100  with respect to the pointer antenna  38  of the receiver  12 . 
   Referring to  FIG. 11C , once the line image  101  is centered under the crosshairs  89  inside of the circle  88 , the depth reading  98  of the line will appear in the upper right corner of the screen  32 . As the depth reading  98  is displayed, depth accuracy reading  97  also appears on the screen  32 . The strength of the signal coming from the line  100  is represented in the upper left corner of the display screen  32  by signal strength reading  99 . 
   There are many instances when a user needs to locate and trace a line that is not capable of carrying electric current. This kind of line is referred to as “nonconductive”. In the case of nonconductive lines, the user typically inserts a small transmitter that creates a locating field only around its own position. Such a transmitter is referred in the art as a “sonde.” Most often sondes are inserted inside a nonmetallic line or conduit in order to find specific locations of points within the line. 
   The receiver  12  of the invention employs a unique technology to determine the location of a sonde. The sonde mode is generally shown in operation in  FIG. 13 . This technology allows the user to know immediately the location depth and orientation of the sonde and the technique is not limited by unusual or unanticipated orientations of the sonde. 
   The sonde to be used needs to be activated. Typically, a battery is inserted into the sonde to activate it. Next, the receiver  12  is tuned to the frequency of the sonde  102 . To tune the receiver  12  to the sonde  102 , the user sets the frequency of the receiver  12  to that of the sonde  102  by using either the “BYSCAN” or the “BY CATALOG” in the menu, as explained above. Preferably, the scanning feature is to be used, as it enables the user to use and locate a sonde even if its frequency is unknown. Once the frequency of the receiver  12  is set, the Sonde Mode is selected from the menu, by using the trigger  26  as explained above. 
   The next step is to feed the sonde  102  into the underground pipe/conduit using techniques well known in the art. Once the sonde  102  is fed into the underground line, the user can begin locating the sonde using the Sonde locating mode of the invention. 
   The sonde mode, illustrated in  FIG. 13  and  FIGS. 12A ,  12 B,  12 C, is unique to the receiver  12  of the invention. This mode aids the user in locating a sonde  102  by displaying a graphic icon representation of its orientation and relative position. 
   Referring to  FIG. 12A , as the user approaches the sonde  102 , the display screen  32  of the receiver  12  will present an arrow icon  86 , which indicates the direction from which the signal is coming. As the user approaches the sonde,  102 , the receiver  12  will emit an audible sound through the speaker  42 , and the sound will gradually increase in pitch as the user gets closer to the sonde  102 . 
   Referring to  FIG. 12B , as the user moves closer to the sonde  102 , the screen  32  will display a sonde icon  103 , indicating its location and orientation of the sonde  102  with respect to the center cross-hair  94 . 
   Referring to  FIG. 12C , as the receiver  12  homes in on the sonde  102 , the centering circle  88  constricts about the sonde icon  103 . When the circle  88  fully constricts about the sonde icon  103 , and the pointer antenna  38  of the receiver  12  points directly at the sonde  102 , the approximate depth of the sonde  102  will be displayed by an icon  98  in the upper right corner of the display screen  32 , and the actual orientation of the sonde  102  will be evident from the sonde icon  103 . As in the line mode, the strength of the signal coming from the sonde  102  is represented in the upper left corner of the display screen  32  by icon  99  and the accuracy of the depth prediction is indicated by icon  97  in the upper right corner. 
   A novel method of measuring of flux field vectors is also provided by the invention. The receiver  12  of the invention employs a novel method of translating the flux field emitted by the underground object of interest into a digital projection of the underground object on the display screen  32 . 
   As explained above, the AC magnetic flux emitted by an underground line or a sonde is measured using a total of six directional ferrite-wound loop antennas  41 . A ferrite-wound antenna is simply a variation of a type of antenna more generally known as a “loop”. A loop is so called because it is usually a wire that is formed into some closed shape, typically a circle. A loop can have multiple iterations or winds, and a ferrite wound antenna is a loop having a multitude of winds. 
   Typically, the loop has the distinct feature of being directional, meaning that it has a single axis along which it is most receptive. The axis perpendicular or orthogonal to the first axis is not receptive at all, and is referred to as the “null” direction. So, in a set of three, as described herein, each antenna  41  responds to a component of the actual field vector. 
   Because the three antennas  41  are mutually orthogonal, the field detected by each can be summed using vector math to get a single, 3D resultant. The circuitry of the receiver  12  is responsible for performing the mathematical calculations. The three antennas  41  of the pointer antenna  38  and the three antennas  41  of the chassis antenna  39  are all connected to a network of analog switching circuitry. 
   Referring to  FIG. 16 , the pointer antenna  38  and the chassis antenna  39  are connected to an analog multiplier  118 , which is in turn controlled by a microcontroller  112 . The microcontroller  112  responds to signals from the trigger circuitry  144 , which is the intermediate between the trigger  26  and the microcontroller  112 . The low-level voltage responses of the individual antennas  41  of the pointer antenna  38  and the chassis antenna  39  to the flux field coming from the underground object of interest are then immediately combined through a summing amplifier  120  and a difference amplifier  122 . 
   Referring further to  FIG. 16 , and by way of example, the response signal from the pointer antenna  38  is called the “reference” (R or r), and the response from the second antenna is called the “test” (T or t). The signal that emerges from the summing amplifier  120  is G(R+T), where G is some known gain value, and is called the “sum”. 
   The signal that emerges from the difference amplifier  122  is G(R−T), and is called the “difference”. Having been combined at this low level, the sum and difference signals are then passed through matching signal processing paths illustrated in  FIG. 16 . By way of example, the travel paths of the sum signal (R+T) and the difference signal (R−T) are now described in more detail. 
   The processing paths of the sum and the difference signal through the circuitry of the receiver  12  involve the following intermediary steps. First, the signals pass through matched digital gain  124 ,  126 . The signals then travel through matched analog mixers/filters  128 ,  130 . Subsequently, the signals go through auxiliary matched DSP filters  132 ,  134  and through matched audio filters  136 ,  138 . 
   The matched circuitry simultaneously applies an absolute value function  140 ,  142 , rendering |R+T| for the sum signal and |R−T| for the difference signal. These results are then integrated by integrators  144 ,  146  over a predetermined time period. Special methods are applied during integration to reduce and/or nullify processing errors. 
   The integrated results are then digitized to produce two numerical results, S=∫|R+T|, and D=∫|R−T|. The signals then pass through converters  148 ,  150 , after which the signals travel to the microcontroller  112 . The microcontroller relays the signals incoming from the processing paths into a video output  114  on the Graphic LCD screen  32  and into an audio output  116  through the speaker  42  of the receiver  12 . The above-described processing paths essentially amplify and filter the signals under the control of the microcontroller  112  to produce back-end sum and back-end difference results. 
   The general method of arriving at a 3D vector measurement for an antenna set involves mathematical processing of multiple measurement pairs (S and D) together, which are generated by the circuitry as described above. Mathematically, several methods are provided by the present invention to arrive at usable results using mathematical calculations, depending on how antennas are switched and combined. Generally, each method relies on the ability of the circuitry of the receiver  12  to measure flux field strength using switched pairs of antennas in rapid succession. The microcontroller  112  is responsible for controlling the sequencing and duration of the measurements. Because the circuitry of the present invention provides matches signal processing paths, the S and D measurements occur simultaneously. 
   The preferred method of calculating flux field vectors using the three-antenna sets  43 ,  45  of the receiver  12  involves sequential measuring of the responses of the individual antennas  41  to the flux field emitted by the underground object. By way of example, the responses to the flux field of each of the three antennas  41  in the antenna set  43  of the pointer antenna  38  are called X, Y, and Z. 
   The microcontroller  112  sets the reference (R) and the test (T) to the same antenna (R=X and T=X), and makes the S and D measurements as described previously. Subsequently, the microcontroller  112  sets R=Y and T=Y, and makes the next set of S and D measurements. Finally, the microcontroller  112  sets R=Z and T=Z and makes the final S and D measurements. The microcontroller  112  can reiterate this sequence ad infinitum. 
   Mathematically, the measurement set is obtained in a total of three measurement cycles, and it looks like this:
 
Cycle 1: ( R=T=X )==&gt; S 1 =|X+X | and  D 1 =|X−X|= 0
 
Cycle 2: ( R=T=Y )==&gt; S 2 =|Y+Y | and  D 2 =|Y−Y| 
 
Cycle 3: ( R=T=Z )==&gt; S 3 =|Z+Z | and  D 3 =|Z−Z| 
 
   If this simple approach is used, X, Y, and Z are obtained from S 1 , S 2 , and S 3 . Actually, only the magnitudes of the three responses can be obtained using this approach:
 
| X|=S 1/2  |Y|=S 2/2  |Z|=S 3/2
 
The above example illustrates how three successive sets of simultaneous sum and difference measurements can be used to define a three-dimensional flux response vector.
 
   To fully define the measured flux vector, phase information must also be obtained. To do this, the microcontroller  112  pairs the antennas  41  in a staggered sequence, so that information can be inferred about the phase of each response with respect to the others. The preferred method of sequential pairing is:
 
Cycle 1: ( R=X, T=Y )==&gt; S 1 =|X+Y | and  D 1 =|X−Y| 
 
Cycle 2: ( R=Y, T=Z )==&gt; S 2 =|Y+Z | and  D 2 =|Y−Z| 
 
Cycle 3: ( R=Z, T=X )==&gt; S 3 =|Z+X | and  D 3 =|Z−X| 
 
From these six measurement results, a complete flux response vector can be calculated, including magnitude and sign (phase) of the three orthogonal components X, Y, and Z.
 
   The preferred method of the invention provides several ways of using the six measurements to get the correct vector result. As example, one way of calculating X and Y from S 1  and D 1  is illustrated below: 
   If it is known that X&gt;Y&gt;0, then S 1 =|X+Y|=X+Y, and D 1 =|X−Y|=X−Y. Therefore:
 
 S 1 +D 1 =X+Y+X−Y= 2 X  and
 
 S 1 −D 1 =X+Y −( X−Y )=2 Y  
 
   Generally, it can be shown that,
 
max(| X|, |Y |)= S 1 +D 1 and min(| X|, |Y |)=| S 1 −D 1|
 
   Thus, the magnitudes of X and Y are obtained. The magnitude of the larger of X and Y is assigned to the sum of S 1  and D 1 . The magnitude of the smaller is assigned the absolute value of the difference of S 1  and D 1 . 
   The magnitudes cannot be assigned unless the larger and the smaller of X and Y is first determined. To accomplish this, the microcontroller  112  compares all six measurements of {S 1 , S 2 , S 3 , D 1 , D 2 , D 3 } in order to rank the three vector components, X, Y, and Z. From the largest measurement, the microcontroller  112  deduces the top two ranked components, and hence the 3rd-ranked. From the values of other measurements, the microcontroller  112  then deduces the top ranked component. The remaining component is the 2nd-ranked one. 
   To obtain the relative signs of X and Y (phase), S 1  and D 1  are compared to each other:
         if S 1 &gt;D 1 , then sign(X)=sign(Y) (i.e., components share same polarity or phase)   if S 1 &lt;D 1 , then sign(X)=−sign(Y) (i.e., components have opposite polarity)       

   The final signed values of X and Y are obtained by first determining the overall significance ranking of X, Y, and Z. Then, the magnitude of each component is determined, for example, by summing and differencing S and D pairs. Then, the relative signs of components are determined by comparing S and D from the appropriate measurement pairs. 
   The invention also provides another method that can be used to obtain magnitude and phase relationships of the three vector components (X, Y, and Z). The mathematical calculations of this method are provided below:
 
Cycle 1: ( R=M, T=X )==&gt; S 1 =|M+X | and  D 1 =|M−X| 
 
Cycle 2: ( R=M, T=Y )==&gt; S 2 =|M+Y | and  D 2 =|M−Y| 
 
Cycle 3: ( R=M, T=Z )==&gt; S 3 =|M+Z | and  D 3 =|M−Z| 
 
   In this method of sequencing, M is a member of the set {X, Y, Z} and is assumed to be the maximally responding antenna of the three. If it is supposed that M=X, then:
 
 S 1 =|X+X|= 2 |X| D 1 =|X−X|= 0
 
 S 2 =|X+Y| D 2 =|X−Y| 
 
 S 3 =|X+z| D 3 =|X−Z| 
 
   This approach, “maximally-responding method” is very similar to the staggered-sequence method described above. The difference between this and the staggered-sequence method is that the reference antenna (R) does not change during the three cycles. The advantage to this approach is that the magnitude of the reference antenna response is always equal to S 1 . Therefore, S 1  (with S 2 , D 2 , S 3 , and D 3 ) can be used to easily obtain the magnitude and polarity of the two remaining antennas (i.e., the non-reference or “test” (T) antennas). 
   To obtain X and Y from the above measurements the following calculations are performed:
 
 X=|X|=S 1/2, where X is assumed to be always positive
 
 Y =(2 *S 2 −S 1)/2=(2 *|X+Y|− 2 |X |)/2 or
 
 Y =( S 1−2 *D 2)/2=(2 |X|− 2 *|X−Y |)/2
 
If Y&lt;0, then this is reflected in the above calculations as well. Thus, the magnitude and polarity of Y with respect to X are obtained. Z is calculated in similar manner to X and Y. From the above calculations, the microcontroller  112  can deduce the maximally-responding antenna, even if it was not selected to be the reference antenna R for the most recent cycles. The microcontroller  112  can then set M accordingly for the subsequent cycles.
 
   Because of the flux field processing by the antennas and the circuitry of the receiver  12 , the receiver  12  is capable of identifying the location of a transmitting target with a span of 360° around the receiver. As mentioned above, the processing of the multiple antenna responses enables the calculation of two 3D vectors, a V P  vector  108  associated with the pointer antenna  38 , and a Vc vector  106  associated with the chassis antenna  39 . How these vectors are interpreted depends on whether the target in question is an underground line  100  or a sonde  102 . 
   The invention herein also provides a method of converting the flux field vector measurements into a graphical representation of a sonde on the display screen  32  of the receiver  12 . This method is generally shown in operation in  FIGS. 17C and 17D . 
   If the target is a sonde, the magnetic field associated with an AC current flowing in the coil is toroidal in shape. Because the dipole field is more complex in its properties, the math required to determine the relative location of a sonde using the pointer and chassis vectors is also more complicated compared to that required for a current-carrying line. The 3D nature of the two determined vectors  106 ,  108  enable in an analogous way (compared to a current-carrying line) the translation to the 2D location on the display screen  32 , which displays a graphic icon  103  to represent the relative location of the sonde  102  in space. 
   The mathematical processing used to translate the results of the V P  vector  108  of the pointer antenna  38  and the V C  vector  106  of the chassis antenna into a representation of the location and orientation of the sonde in space is explained below. The inventor herein has found that the classic dipole field has properties that may be exploited to determine the center of the source of the field. Such a field source is classically known as a solenoid. 
   One of the properties of the solenoid field pertains to the relative directions of each of a pair of vectors that are located a predetermined distance apart along a straight line (ray) that emanates from the center of the solenoid. The inventor herein determined that all field vectors along this ray have the exact same direction. Essentially, the vectors share the exact same “unit vector”, although they differ in overall magnitude. 
   Furthermore, the inventor has determined that the magnitude of the flux vector B along this ray varies as the inverse of the cube of the distance away from the center along this ray. Furthermore, the inventor has found that if the two measurement points in space around the solenoid are not aligned along a ray that emanates from the center, then the unit vectors at these two points are not aligned either. The degree of their misalignment can be determined and used to calculate the location of the solenoid relative to the line of the two points at which the vectors were determined. 
   The receiver  12  of the invention uses two key quantities to identify the location and orientation of the sonde in space: (1) the unit vector cross product of the pointer antenna vector Vp and the chassis antenna vector Vc; and (2) the “vector gain” of the V P  and the V C  vectors. 
   To calculate the degree of vector misalignment, first the two measured vectors V C  and V P  are unitized and converted to vectors that each has a magnitude of 1. A third vector V CROSS  is calculated using the vector cross product of the two unit vectors V P  and V P . Thus, V CROSS =V P ×V P . Once V CROSS  is obtained, it is used to determine the approximate direction from which the dipole field originates. 
   V CROSS  can be re-written as follows:
 
 V   CROSS   =|V   P   |*|V   C |*( V   PU   ×V   CU )
 
in which the “U” subscript refers to the unit vector associated with the original vector from which it is derived. The unit vector, of course, always has a magnitude of 1. So, dividing the above equation by |V P | and by |V C |, the unit cross product of V P  and V C  is deduced:
 
 V   CROSS     —     U   =V   CROSS /(| V   P   |*|V   C |)= V   PU   ×V   CU  
 
   The unit cross product V CROSS     —     U  has a magnitude that varies from 0 to 1, and has the exact same direction as V CROSS . By the rules of vector math, if V CROSS     —     U =0, then V P  and V C  share the exact same line of direction. If V CROSS     —     U =±1, then V P  and V C  are orthogonal, having an angle of 90° between them. 
   The cross-product V CROSS  itself is a 3D vector whose components can be used to map a virtual location of the dipole source on the screen  32  of the receiver  12 . Also, the magnitude of V CROSS  reveals the degree of misalignment of the original measurement vectors. This degree is expressed in a number that ranges from 0 to 1, since the argument vectors themselves range from 0 to 1 in magnitude. 
   The magnitude of V CROSS  alone does not suffice to determine where on the 2D screen  32  the sonde icon  103  will appear. The reason is that the degree of misalignment of the measurement vectors also depends on the distance that the measurement points are away from the dipole source. 
   The further away the measurement points, the more aligned the unit vectors V P  and V C  will become. This means that the divergence of the flux field decreases as the distance away from the dipole source increases. 
   To account for this, the circuitry of the receiver  12  calculates a factor called the “vector gain.” The vector gain G V  is the ratio of the magnitude of the measured pointer vector to the magnitude of the measured chassis vector. Thus, G V =|V P |/|V C |. The gain value represents the intensity of the magnetic flux at the location of the pointer antenna set  43  compared to that at the location of the chassis antenna set  45 . 
   From V CROSS     —     U  and G V , the microcontroller  112  derives another factor, herein called Epsilon (E). In general, E is given by the equation,
 
 E=|V   CROSS     —     U |/ƒ( G   V )
 
where ƒ(G V ) is a function of the vector gain that typically increases as G V  increases and decreases as G V  decreases. The numerator is the magnitude of the unit cross product, and it varies from 0 to +1.
 
   In the preferred embodiment of the receiver  12  of the invention,
 
ƒ( G   V )=[ G   V −1] (1/2)  
 
However, this function, ƒ(G V ), can be defined a number of different ways in order to adjust the overall response characteristic. The purpose of ƒ(G V ) is to scale E to account for varying divergence in the magnetic flux field as the distance between the receiver and the sonde increases. The further away the receiver is from the sonde, the smaller the divergence of the flux field. Function ƒ(G V ) counter-balances this effect.
 
   In calculating E, the goal is to arrive at a single real scalar number that typically ranges from 0 to +1, and that approximately indicates the angle between two lines. Namely, the two lines in question are: (1) the line that radiates from the center of the sonde  102  to the center of the chassis antenna set  45 ; and (2) the line that may be drawn from the center of the chassis antenna set  45  to the center of the pointer antenna set  43 . Line ( 2 ) also happens to be the Z-axis of both the chassis and pointer antennas. 
   To translate the location results to the display screen  32 , the microcontroller  112  of the locator  12  uses the scalar number E and vector V CROSS     —     U  to interpret where in external 3D space the sonde  102  is located relative to the coordinate system of the receiver  12 . Referring to  FIG. 12B , this interpretation is drawn on the display screen  32 , representing the sonde  102  by a 2D projection  103  of a 3D cylinder, relative to a centering crosshair  89 . 
   Because V CROSS     —     U  is a vector cross product, its direction is by definition orthogonal to the plane formed by the argument vectors. Therefore, V CROSS     —     U  must be appropriately transformed to be useful in the coordinate space of the receiver  12 . In particular, V CROSS     —     U  must be transformed to the virtual coordinate space of the display screen  32 . The following equations perform this transformation:
 
 X   SCR =( Y   VCU   *e*K   SCR )/( X   VCU   2   +Y   VCU   2 ) (1/2)  
 
 Y   SCR =(− X   VCU   *e*K   SCR )/( X   VCU   2   +Y   VCU   2 ) (1/2)  
 
where (X SCR , Y SCR ) are 2D Cartesian coordinates on the display screen, K SCR =a scaling constant, and X VCU  and Y VCU  are coordinates of V CROSS     —     U .
 
   In sum, the locator  12  of the invention is able to calculate the location of a sonde in space by processing the following information: (1) the pointer and chassis vectors, (2) the unit vector cross-product (magnitude and orientation), and (3) the vector gain. This information is combined into an equation that renders a reasonably linear relationship between the measured/processed data and the angle between the line drawn between the two measurement points and the line drawn to the center of the dipole source from the chassis measurement point. This angle can then be used to determine where on the virtual 2D screen that the icon should be located. 
   Because the two measurement vectors have the exact same alignment when the two measurement points form a line passing through the center of the solenoid, their unit vector cross product in this state has a magnitude of zero. This is a special case of what has been described in the previous paragraph. In this special case, of course, the calculated angle is zero. This means that the sonde icon  103  will be drawn right at the origin, i.e., under the cross hair  89  of the display screen  32 . Thus, the user of the receiver  12  can perceive intuitively that the pointer antenna  38  is pointing directly at the sonde  102 , regardless of its orientation. 
   The orientation of the sonde that is reflected on the display screen  32  of the receiver  12  is computed internally based primarily on the pointer vector measurement, and may or may not be adjusted according to the vector cross product result discussed above. 
   The invention herein also provides a method of converting the flux field vector measurements into a graphical representation of an underground line  100  on the display screen  32  of the receiver  12 . This method is generally shown in operation in  FIGS. 17A and 17B . 
   If the target is a line  100 , the magnetic field associated with an AC current flowing along the line is cylindrical, with the axis of the cylinder being the current carrying line itself. Specifically, “cylindrical” refers to the shape of the equipotential flux field surrounding the line. The magnitude of the flux vector B varies as 1/r, where r is the perpendicular distance from the line. 
   Consequently, in a plane with a normal vector parallel to the current-carrying line, every point along a circle with radius r will have the same flux magnitude. Also, at every point along this circle, the magnetic flux vector B will be directed tangentially to the circle. Therefore, the direction of the flux vector at any given pair of points separated by a known distance can be read and mapped mathematically to render a virtual location that can be reported on the display screen  32  of the receiver  12 . 
   The determined vectors can be used to calculate line location, direction of travel, and depth. Because the calculated pointer vector V P    108  and chassis vector V C    106  are “true” 3D representations, the indicator arrow  86  and/or line graphic  101  can be drawn correspondingly in all four quadrants of the display screen  32  (a 360° range). 
   The method of translating a representation of a line  100  onto the display screen  32  is now described in more technical detail. As in the case of sonde locating, the receiver  12  of the invention uses V P  and V C  to determine line location relative to the receiver  12 . 
   As shown in  FIGS. 17A and 17B , V P  and V C  represent the flux vector measurements at two points in a cylindrical magnetic flux field. As illustrated in  FIG. 17A , V P  and V C  are misaligned when the Z-axis of the receiver is pointing away from the center of the current-carrying line. As the pointer is swung about the chassis toward the energized line, V P  and V C  become aligned as shown in  FIG. 17B , so that,
 
| V   P   ×V   C   |=|V   CROSS| =0
 
   Thus, again, V CROSS  vanishes when the virtual line radiating from the center of the energized line  100  is aligned with the Z-axis of the receiver  12 . The mathematic processing involved in making appropriate calculations is similar to that given for sonde locating earlier. The differences are found in how ƒ(G V ) is defined and in how E and V CROSS     —     U  are transformed to generate the display screen  32  coordinates and directions. 
   The preferred method for using V P  and V C  to locate the underground line  100  is to compare V P  to the virtual horizontal plane formed by the X and Y directions of the pointer antenna  38 . In general, the receiver  12  is pointing at the energized line  100  when V P  lies within this virtual horizontal plane. This is true only when the Z-component of V P  vanishes. This result is inferred directly from the cylindrical nature of the magnetic flux field around a current-carrying line. The cylindrical field also dictates the interpreted direction of the line. The line  100  is assumed by the microcontroller  112  of the receiver  12  to run in a direction that is orthogonal to the direction of V P . 
   Many modifications and variations are possible in light of the above teaching. The foregoing is a description of the preferred embodiments of the invention and has been presented for the purpose of illustration and description. It is not intended to be exhaustive and so limit the invention to the precise form disclosed. 
   The invention is to be determined by the following claims: