RELATED APPLICATION 0001 This application claims priority to U.S. Provisional Patent Application Serial No. 60/152,607, entitled GROUND PENETRATING RADAR ARRAY AND TIMING CIRCUIT, filed Sep. 8, 1999, which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 0002 1. Field of the Invention 0003 The invention relates to Ground Penetrating Radar (GPR), and more specifically to a GPR antenna array and timing circuit. 0004 2. Description of the Related Art 0005 Unlike upward-looking radar used for air traffic control and meteorology, the antenna array in a GPR is directed toward the ground. For example, GPR is used for geophysical applications such as mapping subsurface strata, locating toxic waste sites for remediation, and detecting of unexploded subsurface ordinance. 0006 A GPR system comprises at least one transmitter that transmits an electromagnetic impulse, usually in the frequency range of 1 MHz to 10 GHz. The system also comprises at least one receiver that receives a reflected waveform. The length of the impulse is adjusted to match the desired frequency range. The desired impulse duration may be expressed in nanoseconds (ns) as 1/f, where f is a center frequency in Gigahertz (GHz). Therefore, a 1 GHz antenna is fed with an impulse of 1 ns duration, a 500 MHz antenna is fed with an impulse of 2 ns duration, and a 100 MHz antenna is fed with an impulse of 10 ns duration. Ideally, this gives the transmitted waves very broad frequency content, centered around the frequency f. In practice, the impulse is between one to two cycles of the frequency. Therefore, GPR systems are sometimes referred to as impulse or ultra-wide band (UWB) radars. 0007 Subsurface industries such as construction, utility location, environmental remediation, and unexploded-ordnance detection have long sought safe, reliable, cost-effective methods for seeing into the ground. The utility location market suffers greatly from inadequate location technologies that result in hundreds of millions of dollars in damages, delays, and lost revenue for utility companies and contractors every year, losses that can be reduced significantly by use of GPR. Three utility locating market segments, other than GPR, can be distinguished by their accuracy and price: (1) One Call; (2) private locating; and (3) subsurface utility engineering (SUE). 0008 One Call is a nationwide clearinghouse that provides an alert to all public and private utilities of when and where construction may impact their lines. By law, contractors must register their site with One Call, which in turn contacts all the relevant utilities so they can mark their utility lines. One Call locating systems are based on electromagnetic induction technology that sense current passing through a conductor attached to the underground utility. Utility companies, responding to a One Call work order, guarantee accuracy on conductive lines within twenty-four inches horizontally on either side, with no guarantee of depth. With One Call, utility line locations are simply painted temporarily on the ground, easily subject to erosion or destruction. This poor accuracy results in broken utility lines and revenue loss. 0009 Construction, utility, and industrial companies often rely on private locating. Private locating provides a greater degree of accuracy than is delivered by One Call. These companies often hire a utility locating company or a geophysics company to apply more expensive and time-consuming locating techniques. Private locating companies typically use electromagnetic induction technology, GPR, and magnetometry. Often this includes excavation, the most reliable and expensive method for determining the exact location of utilities. 0010 Industrial and utility companies, however, frequently require more accurate maps of the subsurface than One Call or private locating can provide. For instance, extra accuracy may be needed while excavating near an oil pipeline because it may be too dangerous to break the pipe. Or, it may be too costly to accidentally cut an interstate fiber optic cable carrying important communications. In such situations, excavators perform a total cost/value analysis, including consideration of risk/cost avoidance. Often, they are more willing to pay higher fees to ensure greater accuracy. 0011 SUE can provide more accuracy than One Call or private locating. SUE is a rapidly growing specialty service offered by geophysical and engineering companies. It entails planning and designing utility arrangements before highway or other large infrastructure construction. SUE engineers painstakingly map all discernible utilities at a given site using a variety of traditional and advanced geophysical methods. SUE uses electromagnetic induction technology, GPR, and magnetometry. It is generally more costly than private locating services because it uses computer aided design to produce a permanent record of the location of utilities. Even this premium service often only identifies 80% of utilities with certainty, frequently less when unknown non-conductive utilities are present. Further, SUE is very expensive. 0012 An advanced GPR system may overcome the disadvantages of One Call, private locating, and SUE by providing a cost effective method to locate and image conductive and non-conductive utilities, vertically and horizontally, with a margin of error to satisfy any excavating needs. An advanced GPR system may also provide a permanent record of images of the excavation site that can be used in the future. 0013 There are technical difficulties that must be address to implement such a GPR system, however. As mentioned above, for instance, GPR antennae may transmit an impulse signal that lasts for a very short time. Because the center frequency of a GPR system may exceed 10 MHz, there may be no sampling circuit whose sampling and digitizing rate is fast enough to sample the whole received waveform at once with a high enough dynamic range. In order to solve this timing problem, it is common to transmit a plurality of impulses, each having the same waveform. Instead of sampling a received waveform multiple times, each of the plurality of received waveforms is sampled only once, but at a different point along the waveform. A signal processor acts upon these sampled points. It is very difficult, however, to accurately schedule the time when each transmitter transmits an impulse signal and when each receiver samples the received waveform. Typical GPR systems cannot accurately schedule the time when each transmitter transmits and when each receiver samples the received waveform in a way optimized for an antenna array. 0014 Currently available systems capable of handling multiple antennas use one digitizing circuit (one A/D converter), and one impulse generation circuit. These systems thus select one pair of antennas and route the generated high-voltage impulse to the transmitter, and the received sampled, analog, value to the single A/D converter. Because of the difficulty of creating high-voltage impulses at a higher rate than approximately 100 KHz, and due to the limited speed of the existing A/D converters with sufficient dynamic range, the data acquisition rate is effectively limited to approximately 100-150 kHz, independent of the number of receiving and transmitting antennas used. 0015 Furthermore, existing systems use combined receiving and transmitting antennas, without the possibility of individually positioning each antenna element to form a suitable antenna array. If the receiving and transmitting antennas are not separate, the array may not have suitable coverage or sufficiently different polarization schemes. Horn antennas may be separable, but may also be unsuitable for GPR applications. 0016 Further, for a GPR system to be practical, it should easily fit onto a moving vehicle, trailer, or portable housing so that subsurface images can be formed as the system is moving. This requirement introduces width and length requirements on the shape, arrangement, and number of transmit and receive antenna. 0017 Thus, there is a need for a highly accurate timing circuit capable of timing multiple transmit and receive antenna to accurately image the subsurface. Further, it is necessary to have a transmit and receive antennas that meet the necessary physical design constraints for a mobile system.

SUMMARY OF THE INVENTION 0018 This summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show several embodiments of the invention and, together with the description, explain the principles of the invention. 0019 Methods and systems consistent with this invention control an impulse radar having a plurality of transmit antennas and a plurality of receive antennas, wherein a control circuit of the radar receives a transmit timing input signal and a receive timing input signal. Such methods and systems delay the transmit timing input signal and generate a number of intermediate transmit timing signals delayed with respect to each other by a delay time, select either the transmit timing input signal or a corresponding one of the intermediate transmit timing signals as a corresponding output transmit timing signal, delay the receive timing input signal and generate a number of intermediate receive timing signals delayed with respect to each other by the delay time, add the delay time to the intermediate receive timing signals, and select either the receive timing input signal or a corresponding one of the intermediate receive timing signals as a corresponding output receive timing signal. 0020 A system consistent with this invention comprises an antenna array. Such an antenna array comprises a plurality of transmit antennas, a plurality of receive antennas, and means for selectively enabling the transmit and receive antennas to allow each of the receive antennas to receive energy from any one of the transmit antennas. In such a system, the plurality of transmit antennas may be linearly arranged, and the plurality of receive antennas may be linearly arranged and parallel to the transmit antennas. 0021 A system consistent with this invention provides a high voltage generator and a high-voltage impulse generator for each transmit antenna, and a sampler and analog to digital converter for each receive antenna.

BRIEF DESCRIPTION OF THE DRAWINGS 0022 The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings, 0023 FIG. 1 is a diagram of a mobile vehicle with a GPR system, consistent with this invention; 0024 FIG. 2 is a diagram, consistent with this invention, of a mobile vehicle with a trailer having a GPR system; 0025 FIG. 3 is a diagram, consistent with this invention, of a portable housing with a GPR system; 0026 FIG. 4 is a block diagram of a system, consistent with this invention, comprising an antenna array, a trig box, a computer, a control unit, and a positioning device; 0027 FIG. 5 is a block diagram of components in the control unit of FIG. 4; 0028 FIG. 6 is a block diagram of the control unit computer of FIG. 5 comprising a central processing unit (CPU), a timing board interface, a RAM, an EPROM, one or more serial/parallel interfaces, a personal computer interface, and one or more pulse decoders; 0029 FIG. 7 is a block diagram of the timing board of FIG. 5 comprising a transmitter trig generator, a receiver trig generator, a time base generator, and a sweep control; 0030

FIG. 8

a is a signal diagram, consistent with this invention, of three threshold signals; 0031

FIGS. 8

b-c are diagrams of a transmit timing input signal generator and receive timing input signal generator; 0032 FIG. 9 shows a circuit in the trig box of FIG. 4 for scheduling the triggering of transmitting antenna and receive antenna; 0033 FIG. 10 is a signal diagram of transmit timing input signal and receive timing input signal, consistent with this invention; 0034 FIG. 11 is a block diagram of a receiving antenna comprising a receive balun, a pre-amplifier, a first and second sample and hold circuits, an analog to digital converter, and a sync-timer; 0035 FIG. 12 is a block diagram of transmitter, consistent with this invention, comprising a transmit balun, antenna elements, an impulse generator, a trig shaping network, and a high voltage generator; 0036 FIG. 13 is a diagram of one possible layout of an antenna array, consistent with this invention, comprising nine transmit antennas and eight receive antennas; 0037 FIGS. 14(a)-14(d) are diagrams, consistent with this invention, of examples of possible antenna pairing scheme for transmit antenna and receive antenna for monostatic radar measurement. 0038 FIGS. 15(a)-15(d) are diagrams, consistent with this invention, of examples of possible antenna pairing scheme for transmit antenna and receive antenna for bistatic or multistatic measurement; and 0039 FIGS. 16(a)-16(c) are diagrams, consistent with this invention, of examples of possible antenna configurations.

DETAILED DESCRIPTION 0040 The following description of embodiments of this invention refers to the accompanying drawings. Where appropriate, the same reference numbers in different drawings refer to the same or similar elements. 0041 FIG. 1 is a diagram of a mobile vehicle 104 with a GPR system, consistent with this invention. A radar array 106 attaches to an arm 105, which attaches to the back a vehicle 104, as shown in FIG. 1. Radar array 106 may comprise a plurality of transmit antennas and a plurality of receive antennas. Vehicle 104 may move in direction shown by arrow 102. Radar array 106 transmits impulses into ground 108. The impulses may reflect off of a subterranean pipe 112 and radar array 106 may receive reflected waveforms. Module 114 on the back of vehicle 104 may comprise electronics that control array 106 and process signals received by array 106. It may also display images on display 116 for the operator. 0042 FIG. 2 is a diagram, consistent with this invention, of mobile vehicle 104 with a trailer 202 having a GPR system. In the embodiment shown in FIG. 2, radar array 106 (not shown in FIG. 2) is within trailer 202, which is attached to vehicle 104. Trailer 202 moves in the direction of arrow 102 with vehicle 104. FIG. 3 is a diagram, consistent with this invention, of a portable housing 304 with a GPR system. In the embodiment shown in FIG. 3, radar array 106 (not shown in FIG. 3) is within portable housing 304. A user 312 may guide portable housing 304 over ground 108 using a handle 316. Portable housing 304 may have wheels 308. It is possible, however, that portable housing 304 is sufficiently light to omit wheels 308. 0043 FIG. 4 is a block diagram of a system 400, consistent with this invention, comprising antenna array 106, a trig box 422, a control unit 404, a first positioning device 405, a second positioning device 406, a computer 402, and a display 116. Antenna array 106 may comprise a plurality of receive antenna R1-R8 and a plurality of transmit antennas T1-T9. Antenna array 106 transmits electromagnetic impulses into the ground and receives reflected electromagnetic waveforms. Trig box 422 outputs trigger signals TT1-TT9 that trigger, i.e. activate, transmit antennas T1-T9 to transmit an impulse and trigger signals TR1-TR8 that trigger receive antennas R1-R8 to sample a received waveform. For example, signal TR1 triggers when receive antenna R1 samples a received waveform. Signal TR8 triggers when receive antenna R8 samples a received waveform. Likewise, signal TT1 triggers when T1 transmits an impulse. Signal TT9 triggers when antenna T9 transmits an impulse. Similar trigger signals exist for R2-R7 and T2-T8 but are not shown. Trig box 422 is described below in more detail. 0044 Control unit 404 may output timing signals to trig box 422, which trig box 422 uses to create trigger signals TT1-TT9 and TR1-TR8, as explained below. Computer 402 sends and receives signals, including commands, to and from control unit 404 and performs the digital signal processing on received signals and displays images on display 116. 0045 First positioning device 405 may attach to a wheel 110 of vehicle 104, similar to an odometer in an automobile. First positioning device 405 allows computer 402 to determine the distance vehicle 104 has traveled, as well as speed, velocity, and acceleration. Second positioning device 406 may attach to a different wheel than first positioning device 405. For example, first positioning device 405 may attach to a left rear wheel and second positioning device 406 may attach to a right rear wheel. In this case, the current direction of movement of the antenna array may be determined, with respect to a start direction, by calculating the difference in traveled distance between first and second positioning devices 405, 406. 0046 As described above, antenna array 106 may comprise eight receive antennas R1-R8, and nine transmit antennas T1-T9. In FIG. 4, only receive antenna R1 and R8 and transmit antenna T1 and T9 are shown. The configuration of array 106 is described in more detail below, along with alternative configurations. 0047 Trig box 422 may also input sampled waveforms on lines 420 and 421 from receive antennas R1-R8, which information will eventually be passed to computer 402 via control unit 404. For example, waveform on line 420 is a sampled waveform from receive antenna R1 that feeds into trig box 422. Waveform on line 421 is a sampled waveform from receive antenna R8 that feeds into trig box 422. Other receive antenna R2-R8 similarly have signals that feed into trig box 422 but are not shown in FIG. 2. Sample waveform on lines 420 and 421 are described in more detail below. 0048 FIG. 5 is a block diagram of components in control unit 404 of FIG. 2. Control unit 404 comprises a control unit computer 510 and a timing board 512. Control unit computer 510 controls timing board 512 that generates a transmit timing input signal 514 and a receive timing input signal 516 that are fed into trig box 422. Trig box 422 uses these signals 514, 516 to create trigger signals TT1-TT9 and TR1-TR8. Control unit computer 510 also sends and receives data to and from personal computer 402. Control unit computer 510 also receives signals from positioning devices 405, 406 and other serial data 508. Other serial data 508 may include sampled waveforms received by trig box 422 and passed to control unit 404. 0049 FIG. 6 is a block diagram of control unit computer 510 comprising a central processing unit (CPU) 602, a timing board interface 608, a RAM 606, an EPROM 605, one or more serial/parallel interfaces 612, a personal computer interface 614, and first and second pulse decoders 610, 611. RAM 606 and EPROM 605 store applications and data structures necessary to run programs in CPU 602. Timing board interface 608 interfaces control unit computer 510 with timing board 512. Personal computer interface 614 interfaces control unit computer 510 with personal computer 502. First pulse decoder 610 decodes two pulse trains output from first positioning device 405, one for forward movement and the second for backward movement. By subtracting the backward counted pulses from the forward counted pulses, an absolute position of the device may be calculated. Pulse decoder 611 may perform the same function for second positioning device 406. 0050 FIG. 7 is block diagram of timing board 512 including a transmitter trig generator 704, a receiver trig generator 706, a time base generator 708, and a sweep control 710. FIGS. 8b-c is a diagram of trig drive circuitry 702 including transmit timing input generator 704 and receive timing input signal generator 706. A system consistent with this invention generates a saw-tooth triangular signal S, a transmit threshold signal TL, and a receive threshold signal RL. FIG. 8a is a signal diagram, consistent with this invention, of saw-tooth triangular signal S, transmit threshold signal TL, and receive threshold signal RL. Waveforms S, TL, and RL may be easily generated by a combination of operational amplifiers and discrete components, as readily known to one of ordinary skill in the art. Transmit threshold TL may be a constant value, as shown in FIG. 8a. Receive threshold RL may step from a high level down to a low level by use of a fast D/A converter controlled by computer 510 via interface 608. Sweep control 710 controls the slope of saw-tooth signal S and time base generator 708 controls the period (time base) of saw-tooth signal S. 0051 Transmitter trig generator 704 may comprise a first comparator 804. First comparator 804 compares transmit threshold signal TL and saw-tooth signal S. When transmit threshold TL is less than saw-tooth signal S, then comparator 804 outputs a high voltage as transmit timing input signal 514, as shown in FIG. 10. When transmit threshold TL is greater than triangular signal S, then comparator 804 outputs a low voltage signal as transmit timing input signal 514, also as shown in FIG. 10. Thus, transmit timing input signal 514 is a periodic square wave. 0052 Receiver trig generator 706 may comprise a second comparator 802. Second comparator 802 compares receive threshold signal RL and saw-tooth signal S. When receive threshold RL is less than saw-tooth signal S, then comparator 802 outputs a high voltage as receive timing input signal 516, as shown in FIG. 10. When receive threshold signal RL is greater than saw-tooth signal S, then comparator 802 outputs a low voltage signal as receive timing input signal 516, also as shown in FIG. 10. Thus, receive timing input signal 516 is a square wave that has a varying width. The width of receive timing input signal 516 is narrow and then gradually become wider, only to repeat itself. 0053 The period of transmit timing input signal 514 is dependent on the slope and time period (time base) of saw-tooth signal S. As mentioned above, the slope of saw-tooth signal S is controlled by control unit computer 510 by sweep control 710. The period (time base) of saw-tooth signal S is controlled by control unit computer 510 and time base generator 708. 0054 FIG. 9 shows a trig box 422 circuit, consistent with this invention, for scheduling the triggering of transmitting antenna T1-T9 and receive antenna R1-R8. Trig box 422 receives a transmit timing input signal 514 and a receive timing input signal 516. Trig box 422 splits the transmit timing input signal 514 and receive timing input signal 516 and distributes the signal among transmit antenna T1-T9 and receive antenna R1-R8. For example, trigger signals TR1-TR8 are split from receive timing input signal 514. Trigger signals TT1-TT9 are split from transmit timing input signal 516. In this embodiment, trigger signals TR1-TR8 have the same shape as receive timing input signal 516, except with a possible delay. Likewise, trigger signals TT1-TT9 have the same shape as transmit timing input signal 514, except with a possible delay. 0055 Trigger signals TT1-TT9 trigger when a pulse is transmitted from antenna array 106 by transmit antennae T1-T9, respectively. Trigger signals TR1-TR8 trigger when a sample is taken from the waveform received in array 106 by receive antennae R1-R8, respectively. For example, transmitting antennas T1-T9 may transmit at the falling edge of transmit timing output signal TT1-TT9. Receiving antennas R1-R8 may sample received waveforms at the falling edge of trigger signals TR1-TR8. 0056 Trig box circuit 422 comprises a first delay circuit comprising delay elements 920-936 for receiving the transmit timing input signal 514 and generating a number of intermediate transmit timing signals 952-968 delayed with respect to each other by a delay time (D). Delay elements 920-936 may be very stable. For example, intermediate transmit timing signal 952 is transmit timing input signal 514, delayed by delay time D; intermediate transmit timing signal 954 is transmit timing input signal, delayed by a delay time 2D; intermediate transmit timing signal 956 is transmit timing input signal, but delayed by a delay time 3D; etc. 0057 Trig box circuit 422 also comprises a transmit output switch circuit ST1-ST9 to select either the transmit timing input signal 514 or a corresponding one of the intermediate transmit timing signals 952-968 as corresponding trigger signals TT1-TT9. For example, trigger signal TT1 may be transmit timing input signal 514 when switch ST1 is in position 0. Or, trig signal TT1 may be first intermediate transmit timing signal 952 when switch ST1 is in position 1. Trigger signal TT2 be transmit timing input signal 514 when switch ST2 is in position 0. Or, trig signal TT2 may be second intermediate timing signal 954 when switch ST1 is in position 1, etc. This allows any transmitting antenna to be first in line when transmitting, as explained below. 0058 Trig box circuit 422 also comprises a second delay circuit 904-918 for receiving receive timing input signal 516 and generating a number of intermediate receive timing signals 938-951 delayed with respect to each other by the delay time (D). Delay circuit 904-918 may be very stable. For example, intermediate receive timing signal 938 is receive timing input signal 516, but delayed by delay time D; intermediate receive timing signal 954 is transmit timing input signal, but delayed by a delay time 2D; etc. The above example assumes that a double pole double throw switch SR0 is in position 0. 0059 Trig box circuit 422 also comprises a shift-delay circuit 902 coupled to the second delay circuit 904-918 and receive timing input signal 516 to add the delay time (D) to the intermediate receive timing signals 938-951. For example, when switch SR0 is in position 0, then intermediate receive timing signals 938-950 are as in the above example. When switch SR0 is in position 1, then intermediate receive timing signal 938 is receive timing input signal 516, but delayed by delay time 2D; intermediate receive timing signal 954 is transmit timing input signal, but delayed by a delay time 3D; etc. 0060 Trig box circuit 422 also comprises a receive output switch circuit SR1-SR8 to select either the receive timing input signal 516 or a corresponding one of the intermediate receive timing signals 938-951 as corresponding trig signals TR1-TR8. For example, output receive timing signal TR1 may be either transmit timing input signal 516, or first intermediate transmit timing signal 938; trigger signal TR2 may be either transmit timing input signal 516, or second intermediate transmit timing signal 940; etc. This allows any receiving antenna to be first in line when transmitting, as explained below. 0061 FIG. 10 is a signal diagram of transmit timing input signal 514 and receive timing input signal 516. Transmit timing input signal 514 and receive timing input signal 516 are each generated from a saw-tooth signal S. Transmit timing input signal 514 in this example is a periodic square wave, as described above with respect to FIG. 8 and shown in FIG. 10. Receive timing input signal 514, on the other hand, is a square wave with a varying period. Transmit trigger and receive trigger may occur at the falling edge of signals 514, 516 shown in FIG. 10. 0062 Transmit timing input signal 514 feeds into trig box 422. If switch ST1 is in position 1, then transmit antenna T1 transmits an impulse at time 0D, i.e., at the falling edge of transmit timing input signal 514 delayed by a time D. If switch SR0 is in position 0 and switch SR1 is in position 1, then receive antenna R1 samples a value of the received waveform at time 0Dt1. Thus, a sample is taken by receive antenna R1 at time t1 before the impulse is transmitted. 0063 In this example, transmit antenna T1 also transmits an impulse at time 0TD, i.e., at the falling edge of transmit timing input signal 514 delayed by a time D. Receive antenna R1 samples a value of the received waveform at time 0TDt2. Thus, a sample is taken at time t2 before the impulse is transmitted, etc. The time between the receive trig and transmit trig become smaller and time t4 in FIG. 10 is the first event when a value of the received waveform is sampled after the impulse is transmitted. The time between the transmit trig and receive trig then increases. 0064 FIG. 10 may be more easily understood if the falling edge of 514 is defined as 0 in each period. The times t1 through t5 then start negative, become smaller, then become positive, and finally increase. In this manner, the received waveforms are sampled at different points along the waveform. The same pattern results with respect to antenna pairs R2 and T2, R3 and T3, etc., on down the line. 0065 In the above example, R1 is paired with T1. If switch SR0 is in position 1, however, then R1 is paired with T2. In this case, R1 is paired with T2 because there is a delay of 2D before both intermediate trig signal 938 and intermediate trig signal 954. Thus, R1-R8 are paired with T1-T8 when switch SR0 is in position 0. Alternatively, R1-R8 are paired with T2-T9 when switch SR0 is in position 1. 0066 Switches SR1-SR8 and switches ST1-ST2 also play a role in pairing. Any or all signals TR1-TR8 can be the receive timing input signal 516 without any delay. This allows any or all receiving antennas to be first in line when receiving. Further, any or all signals TT1-TT9 can be the transmit timing input signal 514 without any delay. This allows any transmitting antenna to be first in line when transmitting. Thus, any or all receivers R1-R8 can be paired with any transmitter T1-T9. In other words, methods or systems consistent with this invention provide means for selectively enabling each of the receive antennas to receive reflected energy from any one of the transmit antennas. Methods or systems consistent with this invention may pair any transmitter to any receiver. 0067 FIG. 11 is a block diagram of receiving antenna R1, consistent with this invention, comprising a receive balun 1110, a pre-amplifier 908, first and second sample and hold (S/H) amplifiers 1104 and 1114, an analog to digital (A/D) converter 1112, and a sync-timer 1102. Antenna elements 1116 receive reflected waveforms that are amplified by pre-amplifier 1108. Receive balun 1110 may match the impedance of the antenna elements to the coaxial feed-lines (not shown). The received waveform is then sampled by first S/H amplifier 1104 at a time specified by sync-timer 1102. Sync-timer 1102 specifies when to sample the received waveform at, for example, the falling edge of trigger signal TR1. Because of the high frequency of the received waveform, it may be necessary to use two S/H amplifiers to preserve dynamic range. Thus, the output of first S/H amplifier 1104 is fed into second S/H amplifier 1114. Second S/H amplifier 1114 samples the output of first S/H 1104 at a time shortly after first S/H 1104 sampled the received waveform, as specified by sync-timer 1102. Sync-timer 1102 specifies when second S/H amplifier should sample at, for example, a small time after the falling edge of trigger signal TR1. The output of second S/H amplifier 1114 is fed into A/D converter 912 and output to trig box 422 in a serial format. The A/D converter 912 may also use the output of sync-timer 1102. 0068 FIG. 12 is a block diagram of transmitter T1 comprising a transmit balun 1206, antenna elements 1212, an impulse generator 1204, a trig shaping network 1202, and a high voltage generator 1208. Trig shaping network 1202 and impulse generator 1204 create a well shaped impulse that is fed through to radiating antenna elements 1212. For example, trig shaping network 1202 forms a trig signal with sharp edges and sufficient electrical current. Transmitting balun 1206 matches the impedance between the coaxial line (not shown) and antenna elements 1212. Impulse generator 1204 may be powered by a high voltage generator 1208 of approximately 600V. The radiating element of transmit antenna T1 may be a bow-tie type antenna, which is well known in the art. Other types of antennas that may be used include resistively loaded dipoles and cavity backed dipoles. The same types of antennas may be used for receiving and transmitting elements. Transmit antenna T1-T9 may transmit with a center frequency of 200 MHz and a bandwidth of 300 MHz. Other frequencies may be possible, including at least 300 MHz, 400 MHz, and 500 Mz. 0069 The timing circuit described above may be optimized for use with an antenna array. This means that the timing circuit controls the antenna array in a way that enables each antenna to work at a higher firing/digitizing rate, i.e., approximately 100 kHz. The individual transmit antennas may each have its own high-voltage impulse generator and the receiving antenna may each have its own digitizer (sampler head and A/D-converter) in order to support high-speed operation of the array. 0070 FIG. 13 is a diagram of one possible layout of antenna array 106, consistent with this invention, comprising nine transmit antennas T1-T9 and eight receive antennas R1-R8. Referring to FIG. 13, transmit antennas T1-T9 may be linearly arranged. Further, receive antennas R1-R8 may also be linearly arranged and parallel to the transmit antennas. Also, receive antennas R1-R8 and transmit antennas T1-T9 may be offset from each other in the linear direction one half the width of the antennas. 0071 In systems consistent with this invention, the length of antenna array 106 is approximately 2.4 meters. This length allows array 106 to easily fit on the back of vehicle 104. The length of array 106 and the motion of vehicle 104 allow a large area of ground to be covered by the GPR system. 0072 In systems consistent with this invention, one transmitting antenna may transmit at a given time and one or more receive antenna may receive at a given time. FIGS. 14(a)-14(d) are diagrams, consistent with this invention, of a possible antenna pairing scheme for transmit antenna and receive antenna for monostatic radar measurement. In FIGS. 14(a)-14(d), T indicates a transmitting antenna and R indicates a receiving antenna. The antennae shaded in black are active while those not shaded are inactive. FIGS. 12(a)-12(d) show the progression of pairings. In FIGS. 14(a)-14(d) there is one receive antenna paired to every transmit antenna. 0073 FIGS. 15(a)-15(d) are diagrams, consistent with this invention, of another possible antenna pairing for bistatic or multistatic measurement. Again, T indicates a transmitting antenna and R indicates a receiving antenna, and antennae shaded in black are active while those not shaded are inactive. FIGS. 15(a)-15(d) show the progression of pairings. In FIGS. 15(a)-15(d) there are a plurality of receive antennas paired to every transmit antenna. 0074 The antenna array configuration shown in FIGS. 14(a)-14(d) and FIGS. 15(a)-15(d) is different than that shown in FIG. 12, but is an alternative to that in FIG. 13. 0075 FIGS. 16(a)-16(c) are diagrams, consistent with this invention, of other possible antenna configurations. In FIG. 16(a), the transmit and receive antenna alternate and are linearly arranged. In FIG. 16(b), the transmit antenna and receive antenna are as shown in FIG. 11, except the receive antenna are not offset in the parallel direction from the transmit antenna and there is an equal number of transmit antenna and receive antenna. FIG. 16(c) is similar to FIG. 13. 0076 The description of the invention does not limit the invention. Instead, it provides examples and explanations to allow persons of ordinary skill to appreciate different ways to practice the invention. The following claims define the true scope and spirit of the invention.