Abstract:
A traffic radar utilizes digital signal processing (DSP) to determine targets based on signal strength histories. From these histories, a target vehicle having the strongest Doppler return signal is identified and its speed is displayed, and a target vehicle having the highest frequency return signal is identified and its speed is displayed. The traffic radar may also display the relative strength of the strongest return signal and the relative strength of the highest frequency return signal, thereby showing a comparison of the strengths of the return signals from the target vehicles.

Description:
This invention relates to improvements in traffic radar systems for law enforcement and related applications in which the speed of a target vehicle, or several such vehicles in the range of the radar, is detected from a stationary or moving transmitter platform and, in particular, to a traffic radar system which determines and displays information concerning the signal strength of selected targets. 
     BACKGROUND OF THE INVENTION 
     Traffic radar systems utilizing digital signal processing (DSP) have been in use for a number of years. Such a DSP radar is disclosed, for example, in U.S. Pat. No. 5,528,246 (“&#39;246”) owned by Kustom Signals, Inc. In the &#39;246 patent, the radar transforms target return information into the frequency domain and compares the magnitude of a tested target bin to the magnitude of a moving average of a number of bins surrounding the target bin. Typically, a target is qualified if the tested peak is greater than a threshold significantly higher in magnitude than a surrounding moving average. In this manner of processing, radar return signals from very strong targets qualify at the same time as much weaker return signals as long as the returns are above the target threshold level. If too low a threshold level is chosen, noise signals may be processed as targets. If too high a threshold is chosen, a low level target may not be found. After initial processing, information concerning the signal-to-noise levels of the respective bins is not retained. 
     It would be advantageous in traffic radar systems to determine targets such that strong targets with high magnitude returns qualify very quickly, whereas weak, but consistent, targets qualify more slowly. Another desired improvement in a traffic radar system would be to display signal strength information to the operator regarding the target being tracked, either the accumulated signal-to-noise ratio or the instantaneous signal strength. Capabilities of an improved system could include creating a target signal history for each target, validating the target based on its signal history, correlating the target with other targets in the operator&#39;s field of view, and displaying the magnitude of the target signal in association with the target&#39;s speed. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, the aforesaid is addressed by providing a traffic radar utilizing digital signal processing (DSP) to determine targets based on signal strength history. The DSP digitally samples and transforms the Doppler return signals to frequency bins by the fast Fourier transform (FFT) algorithm. A variable array of current target peaks is created by comparing the magnitude of a particular bin to a weighted moving average threshold. When a bin has been determined to be a target peak, a calculation is made to determine the relative strength of the target peak. This relative strength value is stored in a separate variable array, and sequentially associated with the array of current target peaks. 
     In another aspect of the invention, improved target tracking provides a history of signal-to-noise ratio values kept to form a running accumulated signal-to-noise array. 
     In still another aspect of the invention, a target peak found to be valid may be displayed as a speed when the accumulated signal-to-noise ratio is greater than a predetermined threshold. 
     In yet another aspect of the invention, the accumulated strength of each target (strongest or fastest) is displayed to enhance the operator&#39;s correlation of the radar system&#39;s displayed targets with the targets in his field of view. Preferably, this relative speed information is displayed in bar graphs of similar readout so that the operator can visually correlate vehicle speed and return signal strength utilizing, for example, a bar graph adjacent each digital display of an associated target&#39;s speed. 
     Other advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is flow chart of the DSP software routines of the radar system that gather the input Doppler samples and perform moving average calculations. 
         FIG. 2  is a flow chart of the target qualifying routines. 
         FIG. 3  is a flow chart of the DSP software routine for speed calculations and displaying the signal-to-noise ratio (SNR) of the selected targets. 
         FIG. 4  is an example of a front panel display and controls. 
         FIG. 5  is a simplified view illustrating the display in operation, showing the speeds of the strongest and fastest targets and respective accumulated signal-to-noise ratios. 
         FIGS. 6 and 7  illustrate Doppler and windowed Doppler waveforms respectively. 
         FIG. 8  shows a frequency spectrum resulting from a fast Fourier transform. 
         FIG. 9  shows a frequency spectrum resulting from a moving average target search routine. 
         FIG. 10  shows a comparison of the two arrays of  FIGS. 8 and 9  (FFT_Results with weighted moving average overlay). 
         FIG. 11  is an exemplary listing of the two target arrays. 
     
    
    
     DETAILED DESCRIPTION 
     Turning more particularly to the drawings,  FIGS. 1-3  depict a flowchart showing processing by the DSP software of the radar system of the present invention in response to input from the received Doppler signals through a moving average routine. The DSP software gathers analog input data at Block  110 , converts the data from analog-to-digital and stores this digital data in a Doppler memory array  112 . In a standard radar system this could be a single channel signal stream from a mixer signal path. In a preferred embodiment of a directional radar system, this may be a dual channel of two separate signals from the I/Q mixer signal path. For example, in the preferred embodiment, 1024 time domain samples for each signal channel are stored. ( FIG. 6  shows an example of Doppler input samples in memory array  112 .) 
     To reduce the end effect of sampling discontinuities, a windowing function may be performed on the Doppler array at Block  120 . The results of the windowing function may be stored in bit-reverse address form in memory array Windowed Doppler  122 .  FIG. 7  shows an example of a windowed Doppler array. This windowed Doppler array, along with a constants table of sine and cosine values, becomes the input data for the frequency transformation routine FFT, Block  130 . The output from the FFT routine is the transformed frequency domain results FFT_Results, Block  132 . In the embodiment illustrated, these FFT results are stored in 512 bins with each bin represented by real and imaginary components.  FIG. 8  shows an example of the FFT_Results array. The lower and upper bin limit area, in which to make the target search is set at 140. 
     In the preferred embodiment, the area of the moving average filter is 33 bins, with the center bin and the two adjacent bins on each side of the center bin not used in the average calculation (Block  150 ). An example of the arrays from the Target Moving Average routine is shown in  FIG. 9 . 
     The DSP routine determines current target peaks at Block  160 . First, the magnitudes of the elements in the FFT_Results  161  are processed using a second order slope derivative method to determine bin maxima. The magnitude of these bin maxima may be compared to the magnitude of the associated bin in the moving average array  163 . The comparison of the two arrays is shown in  FIG. 10 . If the test bin is larger than the moving average bin, the index of the bin is stored in memory array Tgt_Index  162 , and the signal-to-noise ratio may then be calculated from (test bin/moving average) and stored in memory array Tgt_Mag  164 , and the target direction may be stored in memory array Tgt_Dir  166 . In the example shown in  FIG. 10 , bins  174  and  185  qualify as current target peaks. At this point the elements of the three memory arrays  162 ,  164  and  166  are sequentially associated together.  FIG. 11  shows an example listing of arrays  162  and  164 . For clarity the directional arrays are not shown in  FIG. 11 . 
       FIG. 2  is a flowchart of the DSP software through the target qualifying routines. Each element Tgt_Index  211  of the current target bin array may be checked at Block  210  for a matching element from a recent history array Tgt_Peak_Hist  212 . During this check for a matching element, a margin may be given to account for slight variances in target speed over time. If a match is found within this tolerance, then the target direction of the current test target Tgt_Dir  215  may be compared to the direction of the target in history array Tgt_Dir_Hist  216 . For elements that were found to match, the historical signal-to-noise ratio array Tgt_Dura_Hist  224  is updated at Block  220  to the accumulated sum of the current target signal-to-noise element Tgt_Mag  213  and the historical signal-to-noise element Tgt_Dura_Hist  214 . When the update is complete at Block  220  the associated historical arrays Tgt_Peak_Hist  222 , Tgt_Dura Hist  224 , and Tgt_Dir Hist  226  have been updated and are in order of the current targets&#39; largest to smallest magnitude.  FIG. 11  shows an example of the arrays before and after updating. 
     At Block  230  each element of the accumulated signal-to-noise array Tgt_Dura_Hist  233  is checked to determine if the level has reached the threshold of a qualified target. If the element is above the target quality threshold, the associated elements from the arrays Tgt_Peak_Hist  231 , Tgt_Dura_Hist  233  and Tgt_Dir_Hist  235  may be copied to the target qualified arrays Targets  232 , Targets_Dura  234  and Targets_Direction  236  respectfully. This provides improved signal tracking and the ability to qualify returns with a high magnitude quickly, as the signal-to-noise accumulation reaches the target quality threshold. Accordingly, weak targets qualify more slowly due to the slower accumulation of the signal-to-noise values. 
     When the processing at Block  230  is complete the associated qualified target arrays Targets  232 , Targets_Dura  234 , and Targets_Direction  236  are in order of the current targets from the largest to the smallest magnitude.  FIG. 11  sets forth an example of listings of arrays  232  and  234  to show qualified targets as described above. 
       FIG. 3  shows the DSP software for the speed calculations and display routines. First, variables for the strongest target are determined at Block  310 . Typically, since the qualified target elements are listed in order of largest to smallest magnitude, the strongest target would be the represented by the first elements in the qualified target arrays. However, the radar system may have target restrictions such as target directionality. In cases such as this, the strongest target may be the first target in the qualified target arrays that meets this restriction. When the strongest target is determined, the associated element from the array Targets  311  may be copied into memory variable Strong_Target  312 , the associated element from array Targets_Direction  315  may be copied into memory variable Strong Direction  314 , and the associated element from array Targets_Dura  313  may be copied into memory variable Strong_SNR  316 . The variable Strong_Target  312  now contains the number of the frequency bin for the strongest target. From this bin number the strongest target speed based on the units of measure set for the radar system may be calculated and then stored in memory variable Strong_Speed  318 . In the preferred embodiment, the units of measure may be miles per hour (mph) or kilometers per hour (km/h). The variable Strong_SNR  316  now contains the accumulated signal-to-noise ratio for the strongest target speed. 
     The DSP software at Block  320  determines the fastest target variables. Typically, the qualified targets array Targets  321  is searched and the fastest target is represented by the highest value element in this array, but it should be understood that the radar system may have target restrictions such as target directionality. In such case the fastest target may be the highest frequency target in the qualified target arrays that meets this restriction. When the fastest target is determined, the associated element from the array Targets  321  is copied into memory variable Fastest_Target  322 , the associated element from array Targets_Direction  325  is copied into memory variable Fastest Direction  324 , and the associated element from array Targets_Dura  323  is copied into memory variable Fastest_SNR  326 . The variable Fastest_Target  322  now contains the number of the frequency bin for the fastest target. From this bin number the software calculates the fastest target speed based on the units of measure set for the radar system. The calculated speed is then stored in a memory variable Fastest Speed  328  in miles per hour (mph) or kilometers per hour (km/h), for example. The variable Fastest_SNR  326  now contains the accumulated signal-to-noise ratio for the fastest target speed. 
     Software Block  330  displays the strongest and fastest target information to the radar system operator. The radar system display driven by block  330  may comprise a graphical LCD type illustrated in  FIG. 4 , which shows a front panel display  400  and associated controls. The display  400  shown is a programmable liquid crystal display (LCD) such as an Optrex F-51852 transflective monochrome-graphic module having, for example, a 128×64 dot format. The module  400  has a display area  410  and indicates the active radar antenna in use (FRONT) and displays a direction indicator  412  which, in the illustrated example, indicates an oncoming target (closing on the police car). A primary target speed area  414  displays the speed of the strongest target vehicle in stationary and moving modes. A second target speed area  416  displays the speed of the fastest target, and the speed of the patrol car is displayed in the patrol area  418  therebelow. Indicators  419  show the mode of operation of each antenna. 
     A target signal traffic bar which, in the illustrated embodiment, has eight segments, for example, is shown at  420  alongside the display  414 , and a similar six-segment tracking bar  422  for the fastest target is alongside the FAST area  416 . Other features may include a hold switch  424  used to toggle the microwave transmitter on and off, a lock/release switch  426  which alternately locks and releases the target and patrol speeds, enter switch  428  for placing the unit in the menu screen, test switch  430  for initiating the display and internal self-test and tuning fork test mode, and power switch  432  for power on/off. 
       FIG. 5  shows a simplified, detailed view of the speed display. The strongest target speed value Strong_Speed  332  is, for example, displayed in numeric form at display window  414  and the accumulated strongest target signal-to-noise ratio Strong_SNR  334  is scaled and displayed in bar graph form alongside the strongest target speed display by the target signal tracking bar  420 . In the example shown, the speed in display window  414  is 70 mph and the vertical bar graph  420  shows seven blocks and thus indicates a relatively high accumulation of Doppler return. The fastest target speed value Fastest Speed  336  is displayed to the right in window  416  of the LCD in numeric form (80 mph) in the illustrated embodiment and the accumulated fastest target signal-to-noise ratio Fastest_SNR  338  is scaled and displayed in bar graph form by tracking bar  422  alongside the fastest target speed display. The three segments illustrated show in this example that the fastest vehicle has a lesser accumulation of Doppler return.  FIG. 4  shows an example of a preferred display, it being appreciated that other types of displays which provide speed and strength information may be employed as desired.  FIG. 11  shows the determination of the target speed variables  312 ,  322 ,  332 ,  336 ,  334  and  338  as described above. 
     In a typical speed enforcement application, a police officer directs the traffic radar at moving vehicles which, in the present example, are assumed to comprise oncoming traffic comprising several vehicles. The radar of the present invention determines the speed of the fastest oncoming vehicle, and the speed of the oncoming vehicle that presents the strongest target. For example, referring to  FIG. 5 , the display for the strongest speed indicates 70 miles per hour and the display for the fastest speed indicates 80 miles per hour. In addition, the display  414  (strongest speed) shows that the corresponding oncoming vehicle is either relatively close or comprises a target (such as a large truck) that presents a large surface area as indicated by the seven segments  420  of the tracking bar which, in the illustrated embodiment, displays a maximum of eight segments. In contrast, the six-segment tracking bar  422  for the fastest target vehicle displays three segments thereof, clearly indicating that it is likely the more distant target. The officer, therefore, is instantly advised of the probable relative positions of the two vehicles, as well as the speed of each vehicle. In the present invention, the tracking continues as the target vehicles close on the patrol car and both the speed display and the tracking bars remain active and continuously update the officer regarding both the speed and relative radar return strength of the two vehicles. Therefore, the officer is advised of the relative positions of the two target vehicles as well as their speeds. 
     From the foregoing it may be appreciated that, at a glance, the officer may read the speed of both the strongest and the fastest target vehicles. Additionally, as the strength of each target radar return is readily available to the officer in the form of a bar graph or other strength indicator, the officer can with confidence be certain as to the accuracy of the displayed speeds and affirm his or her observations. Alternatively, if desired, the signal strength may be displayed by other indicia or various colors to indicate relative signal strength. 
     It should be understood that while certain forms of this invention have been illustrated and described, it is not limited thereto except insofar as such limitations are included in the following claims.