Abstract:
A method is disclosed for calibrating a multistatic array having a plurality of transmitter and receiver pairs spaced from one another along a predetermined path and relative to a plurality of bin locations, and further being spaced at a fixed distance from a stationary calibration implement. A clock reference pulse may be generated, and each of the transmitters and receivers of each said transmitter/receiver pair turned on at a monotonically increasing time delay interval relative to the clock reference pulse. Ones of the transmitters and receivers may be used such that a previously calibrated transmitter or receiver of a given one of the transmitter/receiver pairs is paired with a subsequently un-calibrated one of the transmitters or receivers of an immediately subsequently positioned transmitter/receiver pair, to calibrate the transmitter or receiver of the immediately subsequent transmitter/receiver pair.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/821,277, filed on May 9, 2013. The entire disclosure of the above application is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT RIGHTS 
       [0002]    The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory. 
     
    
     FIELD 
       [0003]    The present disclosure relates to multistatic arrays, and more particularly to a system and method for calibrating multistatic arrays without the need for human involvement in manually placing an external antenna in a measurement field and making manual measurements with electronic instruments to time-align the various pairs of arrays to remove time skewing. 
       BACKGROUND 
       [0004]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0005]    Multistatic arrays are a set of transmitters and receivers that can be used to measure a “location” of an object. Such arrays typically operate by emitting a wave from a transmitter and measuring the response received by all the receivers in the array. The response allows the location of the target as well as its physical shape to be characterized. 
         [0006]    Multistatic arrays require that all the transmitters and receivers be timed in such a way that a target in the field of view of the array appears to be in the same “location” no matter what transmitter/receiver pair is used to measure the location of the target. At high speed data collection, varying cable lengths and capacitance result in time skewing from the ideal. 
         [0007]    Previously, the needed time calibration was accomplished by a person manually moving an extra antenna in front of the array and observing the signal on an oscilloscope and manually adjusting the timing to put a well-known portion of the pulse at the proper time location on the oscilloscope. As one can imagine, this process of manually adjusting the timing introduces the possibility of human error into the calibration process. Requiring the use of a person to manually position an external antenna in front of the array also may introduce additional cost, time and complexity into the calibration process. 
       SUMMARY 
       [0008]    In one aspect the present disclosure relates to a method for calibrating a multistatic array having a plurality of transmitter and receiver pairs spaced from one another along a predetermined path and relative to a plurality of bin locations, and further being spaced at a fixed distance from a stationary calibration implement. The method may comprise generating a clock reference pulse and turning on each of the transmitters and receivers of each said transmitter/receiver pair at a monotonically increasing time delay interval relative to the clock reference pulse. Ones of the transmitters and receivers may be used such that a previously calibrated transmitter or receiver of a given one of the transmitter/receiver pairs is paired with a subsequently un-calibrated one of the transmitters or receivers of an immediately subsequently positioned transmitter/receiver pair, to calibrate the transmitter or receiver of the immediately subsequent transmitter/receiver pair. 
         [0009]    In another aspect the present disclosure relates to a method for calibrating a multistatic array having a plurality of transmitter and receiver pairs spaced from one another along a predetermined path and positioned relative to a plurality of bin locations, and further being spaced at a fixed distance from a stationary calibration implement. The method may comprise generating a clock reference pulse and turning on a transmitter of the first transmitter/receiver pair to generate a first transmitted signal. The receiver of the first transmitter/receiver pair may alternately be turned on and off at monotonically increasing time delay intervals to obtain a plurality of first data samples of a first reflected signal portion of the first transmitted signal. The first data samples may be used to determine in what specific one of the bins a peak of the first reflected signal portion fell. The receiver of the first transmitter/receiver pair may be calibrated by setting a turn on delay for the receiver of the first transmitter/receiver pair sufficient to skew a turn on time for the receiver of the first transmitter/receiver pair to cause the peak of the first reflected signal portion to fall within a first specific predetermined bin. A transmitter of a second transmitter/receiver pair may be used to generate a second transmitted signal. The receiver of the first transmitter/receiver pair may alternately be turned on and off at increasing time delay intervals to obtain a plurality of second data samples of a second reflected signal portion of the second transmitted signal. The second data samples may be used to determine in what specific one of the bins a peak of the second reflected signal portion fell. The transmitter of the second transmitter/receiver pair may be calibrated by setting a turn on delay for the transmitter of the second transmitter/receiver pair sufficient to skew a turn on time for the transmitter of the second transmitter/receiver pair to cause the peak of the second reflected signal portion to fall within a second predetermined bin. 
         [0010]    In still another aspect the present disclosure relates to a system for calibrating a multistatic array having a plurality of transmitter and receiver pairs spaced from one another along a predetermined path and positioned relative to a plurality of bin locations, and further being spaced at a fixed distance from a stationary calibration implement. The system may comprise a controller and a timing subsystem for controllably turning on select ones of the transmitters and the receivers in response to signals from the controller. The controller may be being configured to perform several operations including generating a clock reference pulse and turning on each of the transmitters and receivers of each said transmitter/receiver pair at a monotonically increasing time delay interval relative to the clock reference pulse. The controller may also be configured to collect data samples obtained by the receivers using signals reflected from the calibration implement. In this manner a previously calibrated transmitter or receiver of a given one of the transmitter/receiver pairs is paired with a subsequently un-calibrated one of the transmitters or receivers of an immediately subsequently positioned transmitter/receiver pair, so that the transmitter or receiver of the immediately subsequent transmitter/receiver pair may be calibrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0012]      FIG. 1  is a highly simplified block diagram showing a plurality of transceiver pairs that may be used in carrying out a method of the present disclosure; 
           [0013]      FIG. 2  is a waveform illustrating the first 150 of 512 bins in which signal measurements are obtained to determine in which bin the peak of a reflected pulse is being received; 
           [0014]      FIG. 3  is a flowchart illustrating major operations in use in calibrating the multistatic array of  FIG. 1 ; 
           [0015]      FIG. 4  is a flowchart setting forth detailed operations that may be performed during the data acquisition phase of the calibration methodology set forth in  FIG. 3 ; 
           [0016]      FIG. 5  is a flowchart setting forth detailed operations that may be performed during the skew adjustment phase of the methodology mentioned in connection with  FIG. 3 ; and 
           [0017]      FIG. 6  is a flowchart setting forth detailed operations that may be performed during the error calculation phase of the methodology mentioned in connection with  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0019]    Referring to  FIG. 1  there is shown one example of a multistatic array  10  (hereinafter “array”  10 ) that may be calibrated using the teachings of the present disclosure. The array  10  may comprise a plurality of transceiver pairs  12   1 - 12   16 , in this example 16 such transceiver pairs, although it will be appreciated that a greater or lesser number of transmitters and receivers may be used to meet the needs of a specific application. A Field Programmable Gate Array (“FPGA”) controller  16  may be used to control a timing board  14 , which turns on and off each of the transmitters and receivers of each of the transceiver pairs  12   1 - 12   16 . The FPGA controller  16  may have an electrically erasable programmable read only memory (EEPROM)  16   a  or any other suitable memory for storing collected data values pursuant to carrying out a calibration methodology in accordance with the present disclosure. A conventional calibration bar  18  may be used as the stationary target or object which is being detected during the calibration process of the present disclosure. 
         [0020]    It will be understood that multistatic array “timing” is the setting of delay triggers on the timing board  14  using the FPGA  16  digital-to-analog converter (“DAC”) outputs in such a way that individual transmitters “fire”, in turn, at the same delay time after a clock reference pulse and the receivers “latch” (i.e., turn on) at the same delay time (different than the transmitters) after the clock reference pulse. The goal is to be able to align the transmit and receive times without the use of an outside reference or the need for an oscilloscope. However, because of variables such as capacitance, cable lengths and so forth, previous calibration techniques have required the use of a separate antenna and an oscilloscope which an individual uses to time align the transceiver pairs. This has previously been required so that the time delay associated with the turn on of each transmitter and receiver can be precisely taken into account. 
         [0021]    It will be appreciated that it is critical that when properly calibrated, each of the receivers of the transceiver pairs  12   1 - 12   16  will be turned on at a precise time, and the precisely determined turn on time must take into account the time delay that may be introduced by the variables described above, as well as the distance between the first transmitter (or transceiver pair  12   1 ) and each of the other transmitters and receivers. Briefly, each transmitter and receiver must turn on such that the peak of the transmitted pulse falls within a specific predetermined bin, where each bin defines a time slot of predetermined duration taking into account the time delay introduced by variables such as varying capacitance and cable lengths. In this example, the predetermined duration is about 40 picoseconds.  FIG. 2  illustrates a portion of a transmitted pulse  21  and the first 150 of 512 bins to illustrate how the reflected peak  22  of the pulse  21  is detected. In this instance the peak  22  of the pulse  21  reflected back to the specific receiver being used is received by the specific receiver in bin  90 . Short horizontal lines  24  represent specific readings taken by the specific receiver at each of the first 150 bins. As an example, it may be that mathematical calculations have determined that the reflected peak  22  should fall within bin  85 , taking into account the time delay introduced by cabling, capacitance, spacing of the specific receiver, distance to the calibration bar, etc. Accordingly, in this example the turn on time of the receiver would need to be adjusted so that the peak of the reflected signal received by the receiver falls in bin  85  (i.e., the reflected peak is received by the receiver in bin  85 ). Referring to  FIG. 3 , a high level flowchart is shown of one example of a calibration methodology  50  in accordance with the present disclosure. The calibration methodology  50  may be viewed as executing a plurality of major operations  100 ,  200  and  300 , which will each be described in greater detail in the following paragraphs and with reference to  FIGS. 4-6 . But at a high level, operation  100  initially involves obtaining a desired number of data samples from a transmitter and receiver being calibrated  12   1 - 12   16 . The data samples may then be used to adjust the transmit (Tx) or receive (Rx) skew  12   2 - 12   16 , as indicated at operation  200 . This pair of operations,  100  and  200 , are repeated sequentially down the array until all components have been calibrated. At operation  300  the error between where the calculated peak locations and the obtained data samples for all combinations of transmitters and receivers  12   1 - 12   16  is determined and the root means square error (RMSE) is calculated for each transmitter and receiver  12   1 - 12   16 . Referring to  FIG. 4 , a more detailed flowchart of the various operations that may be performed during the data collection operation  100  described in  FIG. 3 . The RF design of the array  10  is configured so that the cross coupling waveform is normally not visible during operation. By “not visible” it is meant that the cross coupling waveform is shifted outside the viewing window (as represented by the bins) of the receivers  12   1 - 12   16 . The calibration routine  100  begins by querying the FPGA EEPROM  16   a  to collect the current unit settings at operation  102  to allow the system to reset to current state in the case of a failed or aborted calibration. At operation  104  the transmit (Tx) range is set to 1000 for this system to allow an operationally unused, but useful to calibration, part of the transmitted waveform to be viewed. This may be understood to be the viewing “window” of the entire array of transceivers  12   1 - 12   16  and defines the outer boundaries of the viewing window. At operation  106  the obtained settings from the FPGA  16  and the desired target bins for the calibration are verified by the operator. The calibration routine  100  then waits to see if it is to be continued, as indicated at operation  108 . If the user selects not to continue, then the EEPROM values (i.e., reset values) may be read, as indicated at operation  110 . This results in resetting all timing parameters to whatever is in the FPGA&#39;s EEPROM  16   a ). The transmitters are all turned on, as indicated at operation  112 , to ensure that the system  10  is operating. 
         [0022]    If at operation  108  the user selects to continue with the calibration routine  100 , then the skew for Tx 1  is set to a designated initial value, in this example 16150 at operation  114 . At operation  116  Tx 1  of transceiver pair  12   1  is turned on and all other transmitters are turned off. At operation  118  a software component is used to set Tx=1 and Rx=0, meaning that Rx 1  is turned off. At operation  120  the receiver is incremented to “1”, meaning that the Rx 1  receiver is turned on. At operation  122  data is obtained for a predetermined number of samples (i.e., pulses). More particularly, Tx 1  is pulsed a predetermined number of times, in this example 512 times, and for each pulse emitted by Tx 1 , Rx 1  of transceiver pair  12   1  is turned on at predefined and monotonically increasing delay intervals and measurements are taken of the signal received by Rx 1 . These measurements are used to construct a reflected waveform  21  as seen in  FIG. 2 . The peak of the reflected waveform as seen by Rx 1  is denoted by reference numeral  22 . In this example one hundred of these reflected waveforms  21  are collected and averaged to use for processing. The data obtained is then used to adjust the timing skew of the receiver RX 1  to align the peak  22  of the reflected pulse  21  at the correct (i.e., desired) bin location. At operation  124  a check is made to see if the skew needed to be changed. If the skew did not need to be changed, then the receiver Rx 1  is consider calibrated and the process continues to operation  126 . If the receiver Rx 1  needed the skew adjusted, then operation  122  is repeated. 
         [0023]    When the check at operation  124  indicates that the receiver being checked is calibrated, a check is made at operation  126  to see if this is the last receiver needing calibration (Rx 16 ). If not, then the next transmitter is calibrated beginning at operation  128 . 
         [0024]    At operation  128  the transmitter is incremented, meaning that Tx 2  is selected for use. At operation  130  Tx 2  is turned on and a check is made to ensure that all the other transmitters are turned off. At operation  132 , 512 pulses are generated by Tx 2  and received by Rx 1 , the previously calibrated receiver, at predefined and monotonically increasing delay interval, which generates 512 data samples, just as was done with Rx 1  using the pulses from Tx 1  in operation  122 . This produces the waveform  21  as seen in  FIG. 2 . One hundred of these waveforms are collected and averaged to use for processing. The data obtained is then used to adjust the timing skew of the Tx 2  to align the reflected pulse  22  at the correct bin location. At operation  134  a check is made to see if the skew needed to be changed. If the skew did not need to be changed, then the transmitter is consider calibrated and the process returns to operation  120 . If the transmitter needed the skew adjusted, then operation  132  is repeated. It will be appreciated then that Rx 1  is being used as the receiver to obtain the data sample associated with each pulse from Tx 2 . Thus, it will be appreciated that Rx 1  is being used to help calibrate the delay time for Tx 2 . 
         [0025]    When the test at operation  134  indicates that transmitter Tx 2  has been calibrated, then operations  120 - 124  are repeated using the Rx 2 /Tx 2  pair. Operation  120  increments the Rx flag to 2, indicating that Rx 2  is now being calibrated using Tx 2 . Operations  120 - 134  are thus repeated until all 16 transmitters and receivers have been calibrated. When the check at operation  126  indicates that these conditions have been satisfied (i.e., that the last receiver has been calibrated, in this example Rx 16 ) then the process loop ends and the process continues at operation  136 . 
         [0026]    Operation  136  collects and averages 100 waveforms for each Tx and Rx pair in the system (256 waveforms in our example of 16 transmitters and 16 receivers) in the same manner as in operations  122  and  132 . This data is passed to operation  138  to calculate the error. The error calculation in this example is a Root Mean Square Error (RMSE) between the expected location of the return pulse based on mechanical separation and the measured value&#39;s return pulse gathered in Operation  136 . There are 32 RMSE calculations, one for each transmitter to all the receivers and one for each receiver to all the transmitters. All 32 RMSE measurements must be below a desired factor (i.e., a predetermined RMSE value) for the system to be considered calibrated. In this example, the predetermined RMSE value is 2.1. 
         [0027]    At operation  140  a check is made to determine if the user wants to view the data or not. If so, the data is displayed at operation  142  on a suitable display. If the check at operation  140  determines that the user does not wish to view the data, then a check is made at operation  144  to determine if the data is to be saved. If so, then the Tx range is set to 900 (the appropriate value for this system to return it to the operational range) and the timing skew data is written to the FPGA&#39;s EEPROM  16   a , as indicated at operation  146 . If not, then the EEPROM  16   a  is reset at operation  148 . Operation  112  (described above) executes to ensure that all transmitters are operating and the system is ready for operation. 
         [0028]      FIG. 5  illustrates in greater detail various operations that may be performed as part of operation  200  of  FIG. 3  relating to adjusting the skew of the transmitters and receivers of the transceiver pairs  12   1 - 12   16 . More particularly,  FIG. 5  shows in greater detail various operations that may be performed in finding the peak location of the waveform as seen by one specific transmitter and receiver. The averaging of 100 samples for each of the 512 data bins at operation  202  results in a waveform with limited noise effects. Operation  204  is a mathematical process of creating an envelope  20  in ( FIG. 2 ) of the waveform. This is important for consistently finding the reference signal peak  23  ( FIG. 2 ) and the reflection return peak  22  ( FIG. 2 ) as they are consistently separated by the same number of bins. At operation  206  the fractional peak location is obtained for the reflection return signal  21 . A mathematical algorithm that fits a quadratic polynomial to sequential groups of data points is used to determine the location of the peak (in one example peak  22  in  FIG. 2 ), even if it would occur between the data points. At operation  208  a check is made if the current peak location is within 1 ps of the target location (i.e., the mathematically determined bin that the peak should fall within). If the current peak is not within 1 ps of the target bin, then the time window is skewed 80% of the difference, or a maximum of 20 bins, as indicated at operation  210 . This value is converted into a timing delay change and added/subtracted from the current delay of the channel under calibration. This delay can include fractional bin sizes since the timing board  14  has greater resolution than the receiver bin size. To provide for extreme accuracy, the return peak is placed on the boundary of the 99 and 100 bin. The receiver delay offset is adjusted until the reflected waveform peak is on the boundary. A time delay of 20 ms may then be implemented to allow the system to stabilize and stale data to be flushed before further operations may be performed, as indicated at operation  212 . 
         [0029]    At  FIG. 6  a flowchart is shown that presents in greater detail various operations that may be performed in carrying out the error calculation methodology  300  mentioned in  FIG. 3 . At operation  302  the expected peak locations for all Tx/Rx combinations are determined using the same methodology as shown in  FIG. 5 , operations  202 ,  204  and  206 . At operation  304  the methodology finds the peak near the expected location for each Tx/Rx pair. These locations are compared with the theoretical locations created by the geometry of the system. At operation  306 , the root means square error (RMSE) for a Tx to all 16 Rx is determined, which involves a comparison of the collected versus theoretical reflection return for each Tx/Rx pair. At operation  308  the transmitter is incremented and operation  306  is re-performed until all 16 transmitters have had a comparison run against all 16 receivers receiver. At operations  310  and  312  the same comparison is performed for each receiver to all 16 transmitters  12   1 - 12   16 . The Root Mean Square Error is created for each transmitter and receiver, 32 RMSE values in total. If the Mean Square Error of any element is greater than 2.1, then the system fails the calibration, as indicated at operation  314 . 
         [0030]    The present system and method thus enables calibration of a multistatic array without the need for an individual to manually place an external antenna in front of each array. By calibrating each transmitter and receiver relative to the transmitter or receiver immediately adjacent to it that was just calibrated, the present system and method can properly determine the appropriate turn on time for the receiver and transmitter of each transceiver pair. 
         [0031]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.