Abstract:
A spread spectrum radar system, characterized in that the receive radar signal is mixed down to baseband and applied to leak cancellation means for subtracting therefrom an attenuated delayed version of the transmit radar signal to provide a reflected receive radar signal with improved signal-to-noise ratio for further processing and to prevent receiver saturation. The radar allows the capture of the range and speed of identified targets, and facilitates .a determination of the target&#39;s direction. It exhibits an inherent immunity to electromagnetic interference, and is relatively undetectable by radar detectors. The system is easily reprogrammable for different range resolutions and permits Doppler processing to be independent of the angle between radar signal and the track of the target.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to radar systems in general, and in particular to such systems utilizing spread spectrum (SS) and pseudo-noise (PN), maximal length sequence technologies. More particularly still, it is directed to a radar system where compensation (partial cancellation) of the leaked transmit signal is accomplished at baseband of the PN code sequence. As such, the radar system is particularly useful in police radar gun, traffic monitoring and automotive collision avoidance applications, where the use of a single antenna for both transmit and receive is desirable.  
           [0003]    2. Prior Art of the Invention  
           [0004]    U.S. Pat. No. 5,657,021 (commonly owned by the present assignee) for a SYSTEM AND METHOD FOR RADAR VISION FOR VEHICLES IN TRAFFIC, issued Aug. 12, 1997 discloses interference-free radar systems utilizing PN waveforms for sequential transmission by radar, wherein the PN waveform is tapped and adjustably attenuated to cancel leakage within the system prior to correlation of the received echo.  
           [0005]    A paper titled A COLLISION AVOIDANCE RADAR USING SIX-PORT PHASE/FREQUENCY DISCRIMINATOR (SPFD) by Ji LI et al published May 23, 1994 in 1994 IEEE MTT-S Digest, pp. 1553-1556, proposed a novel technique for collision avoidance radar used in automobiles, in which a new six-port microwave/millimeter wave digital phase/frequency discriminator (SPFD) is used to measure Doppler frequency shifts. Both relative speed and moving direction of the target are readily obtained. Ranging is implemented by the measurement of phase difference at two adjacent frequencies.  
           [0006]    In this paper by LI et al state:  
           [0007]    “In CW type radars, one of the most serious problems is to achieve sufficient isolation between transmission and reception. To prevent the receiver from saturation, separate transmitting and receiving antennas are often used. This results in unwanted larger volume and higher cost. Some other solutions such as Reflected Power Canceller (RPC) [6] are proposed and implemented, however the cost and complexity are still high. In contrast, by using new SPFD it is very easy to integrate a RPC into the sensor at the expense of only a vector modulator (phase shifter and attenuator). The single antenna scheme is shown in FIG. 3. In the six-port PFD, the leakage of the transmitted signal yields a deviation of the detected vector from the origin. A feedback algorithm can be adopted to control the loop to realign the vector to the origin, such that the leakage power is canceled out”.  
           [0008]    In UK patent application GB 2,268,350, published May 1, 1994, for HIGH RANGE RESOLUTION RADAR a phase-coded signal is transmitted by one antenna and the reflections received by another. Both the outward and return signals are mixed in a quadrature mixer to produce a baseband replica of the coded signals, which are then filtered and, amplified before being applied to a correlator. Internal signal leakage in this system does not appear to be a problem.  
           [0009]    In U.S. Pat. No. 5,134,411, issued Jul. 28, 1992, a NEAR RANGE OBSTACLE DETECTION AND RANGING AID apparatus is disclosed. Range measurement signals are produced by means of phase comparison of signals in two paths. The subject of “Leakage Correction” is discussed as follows:  
           [0010]    “In a practical system one or more leakage paths may exists between the RF and LO ports of the mixer. When measuring a target with a weak echo signal, a stronger leakage signal may cause significant errors. Since the transformation  13  has a commutative property, we can generate a corrected signal u corr (i)=u(i)−u cal (i), which is to be used in equation 13. The signal u ca (i) is measured when no targets are present. Alternatively, we can measure u ca (i) even in the presence of targets, if both antennas are replaced by a matched load. In this case, however, the external leakage between the antennas cannot be corrected and therefore will limit the useful dynamic range of the target echo”.  
           [0011]    The issue of leakage in the circulator in FIG. 7, where a single antenna is used, is not addressed.  
           [0012]    In a paper by Yukiko HANADA et al titled VEHICULAR SPREAD SPECTRUM RADAR FOR MULTIPLE TARGETS DETECTION USING MULTI-BEAM ANTENNA (IEIC TRANS. FUNDAMENTALS, VOL. E-80, NO. 12 DECEMBER 1997), the author propose and investigate a vehicular radar system that can measure the distance to, the relative speed of and the direction of arrival (DOA) of the reflected waves from multiple targets or vehicles using the direct-sequence spread spectrum (DS-SS) technique. In particular, they propose a DOA estimation scheme using a multi-beam antenna. In order to show that the proposed system can accurately measure the above-mentioned quantities, the performance is evaluated numerically in a multi-path environment. Moreover, the optimal multi-beam pattern is derived to minimize error probability of DOA estimation. The author state that they use several antennas which form sharp multiple beams, which can be implemented by using several types of antennas such as phased array antenna and a combination of directional antennas.  
           [0013]    In a paper titled 76 GHZ AUTOMOTIVE MILLIMETER-WAVE RADAR USING SPREAD SPECTRUM TECHNIQUE by Hiroshi ENDO et al, published in SAE TECHNICAL PAPER SERIES 1999-0102923, the author state:  
           [0014]    “In SS radar, transmission signals are modulated using PN codes, and then transmitted through the transmission antenna. The signal reflected from a target located ahead of the radar equipped vehicle has a time delay that corresponds with the two-way range delay, the Doppler shift corresponds with the range rate between the radar equipped vehicle and a target ahead; and that signal that is received by the reception antenna. The PN sequences have an auto-correlation function as shown in FIG. 1 [2]. Utilizing these characteristics, SS radar can measure range from the phase difference of PN sequences. The range rate can be measured by frequency analysis when the correlation peak is detected. In this method, accurate raging and multiple target separation are possible due to the detection method using the auto-correlation characteristics of PN sequences. Moreover, SS modulation has excellent interference capabilities since the demodulation process using PN sequence spreads undesired signals or interference in the channel and thus suppresses those signals”.  
           [0015]    Finally, in a paper titled SYSTEM ASPETS AND DESIGN OF AN AUTOMOTIVE COLLISION WARNING PN CODE RADAR USING WAVEFRONT RECONSTRUCTION By Jürgen DETLEFSON et al, published in 1992 IEEE MTT-S Digest, pp.625-628, the author disclose a 61 GHZ radar system with the following parameters:  
                                                                                                                           Carrier frequency   61   GHz           subcarrier frequency   1.2   GHz           range resolution   0.75   m           unambiguous range   767   m           maximum range   150   m                Modulation   BPSK           Code   maximal length PN sequence                code length   1023   chips           Chiprate   200   Mchips/s           code repetition frequency   196   kHz                angular resolution   wavefront reconstruction by FFT           angular resolution cells   4/8           angular resolution cell width     3°/1.5°                field of view   12° × 150   m           rf-power   1.6   mW           maximum Doppler frequency   +/−20   KHz                      
 
         SUMMARY OF THE INVENTON  
         [0016]    The radar systems of the present invention have some of the features of prior art systems. In a preferred implementation of the present invention, the radar is based on a CW carrier phase modulated with a maximal length PN code sequence providing a low power spread spectrum signal. Target range is determined by correlating the radar return signal with a delayed copy of the transmitted PN code.  
           [0017]    An important feature of the present invention is signal leakage compensation by means of signal feedthrough cancellation techniques. Such compensation, while generally useful, is particularly desirable for compact radar systems, whether for law enforcement applications (police radar gun) or for automotive and similar applications. However, while the system of the present invention is particularly suitable for single antenna radars, it is still applicable where separate transmit and receive antennas are used. In such applications, it would permit improved performance, for example, range increase and/or decreased receiver dynamic range requirements.  
           [0018]    A brief general description of the theory of operation of the spread spectrum system provides a better understanding of the basis for the unique features of the system. The following features are directly derived from the exploitation of PN maximal sequences as the class of modulation waveforms that are used in the present radar. These features are:  
           [0019]    Direct, simple and accurate range measurements to a target;  
           [0020]    Immunity to electromagnetic interference;  
           [0021]    Range resolution is easily changed;  
           [0022]    Lower probability of detection;  
           [0023]    Immunity from mutual interference in the presence of other spread spectrum radars which are in close proximity; and  
           [0024]    Implementation is independent of the frequency band of the carrier signal.  
           [0025]    The following properties of PN maximal length sequences contribute to these features.  
           [0026]    A PN maximal length sequence has an autocorrelation function which is a periodic triangle of height equal to N and a base 2/f o wide, and a level of −1 between the triangles, where f o  is the code clock frequency and N is the length of the code. The width of the auto correlation function as defined by the clock frequency enables accurate range measurements. Maximal PN sequences may be generated from an n-stage shift register with feedback points. The register length n determines the number of different maximal PN codes generated as per Table 3.3 on page 72 of SPREAD SPECTRUM SYSTEMS, Third Edition, by Robert C. Dixon (John Wiley &amp; Sons, Inc.); this book is incorporated herein by reference.  
           [0027]    External interference that does not auto-correlate with the code has its energy spread out over a large bandwidth by being a cross-correlation with the code, the result is that interference is attenuated by a factor of “N”. This is an important consideration because of the current proliferation of mobile communications emissions that can interfere with a radar&#39;s operation.  
           [0028]    For each length of code there is a large number of codes available to be used and each has the same optimal autocorrelation function property. Since the cross-correlation between the codes is very low and of the order of 1/N, therefore, in a situation where there are a number of radars working in close proximity of each other, if each radar uses a different code, then there is minimal interference between these radars. The selection of codes can be implemented in a very convenient manner. This property is very important when considering applications such as automobile collision avoidance, traffic detection/management, perimeter surveillance, etc where a multiplicity of radars are used in close quarters.  
           [0029]    In a spread spectrum radar the transmitter output power is spread over a large bandwidth determined by the code generator clock frequency. The net result is that the power spectral density is considerably reduced (approximately by a factor of 1/N) to such an extent that current state-of-the-art radar detectors cannot detect such low powers per unit frequency. This is an important consideration for police radar guns.  
           [0030]    Preferably, the spread spectrum radar measures both the Doppler velocity and range to a target. The measured Doppler velocity is a function of target speed and the angle between the radar and the track of the target. Normally, the angle is limited less than 10 degrees to minimize the measurement error. Beyond 10 degrees the error becomes unacceptable. By measuring the Doppler velocity and range of a target at two points on the target&#39;s track, the target&#39;s velocity can be computed accurately up to an angle of at least 40 degrees. This is particularly relevant to the police radar gun application. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The preferred exemplary embodiments of the present invention will now be described in detail in conjunction with the annexed drawing, in which:  
         [0032]    [0032]FIG. 1 is a system block diagram of a radar according to the present invention;  
         [0033]    [0033]FIG. 2 is a high level flow-chart for operating the system of FIG. 1;  
         [0034]    [0034]FIG. 3 is a flowchart explaining processing steps for leak-compensation adjustment;  
         [0035]    [0035]FIG. 4 shows a circuit implementation of the blocks labeled LEAK CANCELLER and CORRELATOR in FIG. 1; and  
         [0036]    [0036]FIG. 5 illustrates use of target range gates to exclude interfering Doppler signals. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Referring to FIG. 1, the functions and operation of the various circuit blocks will now be briefly described.  
         [0038]    The RF Portion  
         [0039]    Gunn Oscillator  10 : Converts DC power into microwave at carrier frequency, e.g., 35 GHz in this preferred embodiment.  
         [0040]    Isolating Divider  11 : Divides Gunn oscillator  10  output to feed phase modulator  12  and mixer  15 .  
         [0041]    Bi-phase modulator  12 : Phase modulates the carrier by 0 or 180 degrees according to baseband digital modulating code input of 0 or 1.  
         [0042]    Circulator  13 : Directs input from modulator  12  to antenna  14  only. Directs return signal from antenna  14  to mixer  15  only.  
         [0043]    Antenna  14 : Radiates input from modulator via circulator  13  out to targets. Captures target returns and feeds them to mixer  15  via circulator  13 .  
         [0044]    Mixer  15 : Strips carrier by mixing target return signal with carrier to output time-delayed, Doppler shifted code. Time delay depends on target-to-radar distance, Doppler shift dependent on relative velocity of target with respect to radar.  
         [0045]    The Transmit Code Generator and Associated Blocks  
         [0046]    Code clock oscillator  22 : Provide time base (chip clock) for transmit code generator  23 .  
         [0047]    Transmit code generator  23 : Maximal length PN bit sequence generator. E.g. 1023 bits in this preferred embodiment.  
         [0048]    Level shifter  24 : Converts TTL level binary bit stream into +/−2.5 Volt format Modulation drive waveform.  
         [0049]    Level shifter  25 : Converts TTL level binary bit stream into +/−1 Volt format correlator  18  drive waveform.  
         [0050]    Multiplier  26 : Microcontroller  31  controlled Four-quadrant multiplier  26  for amplitude control of leak subtraction.  
         [0051]    The Receive Code Generator and Associated Blocks  
         [0052]    Multiplexer  27 : Selects one of two possible inputs for receive code generator  28  under microcontroller  31  control  31 .  
         [0053]    Receive code generator  28 : Maximal length pseudo-random bit sequence generator. E.g. 1023 bit in this preferred embodiment.  
         [0054]    Pulse skipper  29 : Inhibits the passage of one and only one clock pulse for each asynchronous command pulse from the microcontroller.  
         [0055]    The Analog Signal Processing Blocks  
         [0056]    Pre-amp  16 : Low-noise amplifier to amplify the mixer  15  output for subsequent processing.  
         [0057]    Leak canceller  17 : Analog differential amplifier for removal of leaked code by subtraction of a replica of equal amplitude under control of microcontroller  31 .  
         [0058]    Correlator  18 : Combination analog multiplier/integrator for bit-serial correlation of receive code and target return signal.  
         [0059]    Amplifier  19 : Variable gain amplifier to match signal strength with dynamic range of A/D converter  20  under microcontroller  31  control.  
         [0060]    A/D converter  20 : Converts analog signal to digital words for subsequent digital signal processing (spectrum analysis and/or frequency measurement) by the microcontroller  31 .  
         [0061]    The Digital Processing Blocks  
         [0062]    Clock oscillator  30 : Provides system clock for microcontroller/DSP.  
         [0063]    Microcontroller/DSP  31 : Is the system controller and digital signal processor. Provides self-test, then random sequence seed generation via signal line  40 . Performs synchronization of the transmit/receive code generators via signal line  41 , and range bin control via control line  42 . Leak cancellation is controlled via control line  43 . Gain control of amplifier  19  is via control line  44 . The following application-specific (radar-gun) tasks are also performed:  
         [0064]    Determine target existence and speed on a per-range basis by spectrum analysis and/or frequency measurement;  
         [0065]    True target speed computation (cosine correction);  
         [0066]    Display control;  
         [0067]    User interface; and  
         [0068]    Response to external commands.  
         [0069]    While digital processing could be performed by a separate DSP, microprocessors are now available to perform DSP as well as controller functions.  
         [0070]    Display/control panel  32 : Programmable operator interface as per end-use requirements.  
         [0071]    Serial I/O interface  33 : Programmable RS-232 Data output as per end-use requirements. Provides remote display/operator interface.  
         [0072]    In a typical implementation of the circuit of FIG. 1 for a radar gun the operational parameters, for example, are as follows:  
                                           carrier transmit frequency f c  =   35   GHz       clock frequency f o  of the modulating PN code u(t) =   50   MHz       PN code u(t) length =   1023   bits                  
 
         [0073]    The transmit code generator  23  output is a PN code u(t). This PN maximal length sequence code has an autocorrelation function R (τ).  
           R        (   τ   )       =       1   T            ∫   o   T            u        (   t   )            u        (     t   +   τ     )               t             ,                         
 
         [0074]    where the code period T=Nf o . The code being a balanced square-wave signal oscillating between ±1 normalized levels. The resulting ideal autocorrelation is a triangle with height N and a base  
       =     2     f   o                             
 
         [0075]    wide and a constant sidelobe level of −1.  
       DESCRIPTION OF SYSTEM OPERATION  
       [0076]    The operation of the present radar system comprises a plurality of simultaneous processes, generally coordinated by the microcontroller/DSP  31  according to firmware. Processes after correlation and return signal spectrum analysis are application-specific in nature. A preferred embodiment of a police radar gun is now described with reference to the drawing figures. With modified firmware, the requirements of many other applications, such as traffic management sensors and automotive anti-collision radar sensors can be readily satisfied.  
         [0077]    The major processes are the following. (The reference numbers in the flow-charts of FIGS. 2 and 3 begin with 100 and 200, respectively).  
         [0078]    CARRIER GENERATION: The microwave carrier is generated by the CW Gunn oscillator  10  and fed to the modulator  12  and mixer  15 .  
         [0079]    TRANSMIT CODE GENERATION: A maximal length pseudo-random bit stream of length 1023 bits is generated by generator  23 . Upon powering ( 100 ) the microcontroller/DSP  31  initiates the code generation ( 101 ) by injecting, via control line  40 , a seed stream of bits to guard against the generation of the undesirable all-zero bit stream. The TTL level bit stream is level shifted by level shifters  24  and  25 .  
         [0080]    BI-PHASE MODULATION: The transmit code, in the form of a bipolar pseudo-random bit stream from level shifter  24 , modulates the microwave carrier at the bi-phase modulator  12 . The modulated carrier is fed via circulator  13  to the antenna  14  to form the out-going radar search beam.  
         [0081]    TARGET INTERACTION: Targets within the radiated beam will reflect microwave energy. Each reflected signal, still modulated with the transmit code, acquires a Doppler shift dependent on the relative velocity of each target (e.g. V 1  &amp; V 2  in FIG. 5) relative to the transmitting antenna  14 . The captured return signal at the antenna  14  therefore consists of a multitude of target reflections in the form of time delayed copies of the transmitted code, the time delay of each reflection is dependent on the distance of the target relative to the antenna  14 .  
         [0082]    TARGET RETURN DEMODULATION: Target return signal is mixed with the transmitting carrier and demodulated at the mixer  15 . The target return signal is now a multitude of time-delayed, Doppler shifted copies of the original transmitted code at baseband. This signal, together with the leak, (see Leak Cancellation section) is amplified at the pre-amp  16  for further processing.  
         [0083]    LEAK CANCELLATION: Due to the imperfection of the circulator  13  and/or impedance mismatch of the antenna  14 , a portion of the out-going modulated carrier (the leak) finds a direct path into the mixer  15  instead of being radiated out via the antenna  14 . This leakage has the undesirable effect of reducing the overall signal-to-noise ratio and dynamic range of the signal processing chain. Rather than perform leak cancellation in the RF domain, it is performed here at base band, resulting in a simpler arrangement. Since the leak is characterized by a near-zero time-delay relative to the transmit code sequence, a copy of the transmit code is used for the cancellation process. The transmit code from transmit code generator  23  is level shifted ( 24 ) to drive the modulator  12 . Another level shifter  25  provides the same code to the leak canceller  17  via four-quadrant multiplier  26 . The gain of multiplier  26  is controlled by the microcontroller/DSP  31  via control line  43 . With specific reference to FIG. 3, for optimum leak cancellation, the following steps are carried out:  
         [0084]    (a) The microprocessor/DSP  31  starts ( 200 ) by setting ( 201 ): the target range R=0; (which means that the delay of the receive code generator  20  signal applied to the correlator  18  is zero); the pointer or index LCI to a leak compensation voltage table to the first (Ø) of Ø to 225 incremental values; and the minimum leak value V to the end of the range of the (normalized) measured leak value C, which range of C varies between Ø and 4095 and is the measure of the value at the output of the correlator  18 . After this initialization, the leak compensation signal level is set ( 202 ) by reading the value in the table at LCI and by controlling the multiplier  26  gain via control line  43  accordingly.  
         [0085]    (b) The microcontroller/DSP  31  commands the mux  27  to feed the transmit code as a seeding bit-stream into the receive code generator  28 . Since both code generators are identical, the transmit and receive codes become identically synchronized after ten or more clock periods;  
         [0086]    (c) The analog chain consisting of correlator  18 , amplifier  19  and A/D converter  20  is now effectively measuring the auto-correlation function of the leak ( 203 ). The microcontroller/DSP  31  varies the gain of the multiplier  26  while monitoring the A/D converter  20  output and seeks a minimum by scanning through all possible values (255 values in this example) ( 202 ,  203 ,  204 ,  205  &amp;  209 ). Other methods (e.g., successive approximation) of scanning through the LCI table may be used in operation.  
         [0087]    (d) Optimum leak cancellation occurs when the auto-correlation function is a minimum ( 206 ); and  
         [0088]    (e) The gain of the multiplier  26  for minimum leak is stored ( 207  &amp;  208 ) in the flash memory in the microcontroller/DSP  31 . This gain setting is maintained for all subsequent operations until the next optimum leak cancellation search is performed.  
         [0089]    RECEIVE CODE GENERATION: The receive code is generated by receive code generator  28 . The core of this generator is of identical design as the transmit code generator  23 . A multiplexer  27  is added to allow the selection of bit stream feedback from either the receive code generation itself in normal operation or from the transmit code generator  23  (synchronization). The multiplexer  27  is controlled by the microcontroller  31  via control line  41 . Further, a pulse skipper  29  is used to allow the inhibition of one and only one clock pulse in response to each asynchronous command pulse from the microcontroller  31  via control line  42 .  
         [0090]    By means of the multiplexer  27  and the pulse skipper  29  as described above, the microcontroller  31  now can synchronize the transmit and receive codes, and precisely control the time delay of the receive code with respect to the transmit code at a time resolution of one code clock period. In the preferred embodiment of this invention, the code clock period is 20 nS for a 50 MHz clock. Therefore, by issuing a series of M pulse-skip commands to the pulse skipper  29 , the microcontroller  31  can cause the receive code to lag the transmit code in time by M times 20 nS, thereby moving the range gate/bin by M multiples of three meters. (While 20 nS is six meters in radar propagation distance, due to reflection the six meters are folded in half.)  
         [0091]    CORRELATION: After leak cancellation, the amplified return signal is bit-wise serially correlated with the receive code at the correlator  18 . The correlation process, as explained under Summary of the Invention, has the net effect of suppressing all return signals (delayed copies of the transmit code) that are of time delay different from that of the receive code by more than one clock period. Return signals that are of the same time delay or within +/− half chip period as the receive code are, on the other hand, enhanced by the correlation process. As illustrated in FIG. 5, since the time delay of each return signal is dependent on the distance of the target from the antenna  14 , the correlation process is effectively enhancing only signal from targets inside the “target range bin” of interest, such as vehicles V 1  and V 2 , and suppressing all other signals, e.g. from V 3  and V 4 , and noise. For the preferred embodiment with a 50 MHz chip clock, the range bin resolution W RB  is 3 meters. Range resolution W RB  can be varied in inverse proportion to the frequency of the chip clock.  
         [0092]    RETURN SIGNAL ANALYSIS: The output of the correlator  18  is a sum of sinusoidal waveforms each resulting from the reflection of a target within the range bin of interest. The amplitude of each sinusoid is proportional to the radar cross-section of the corresponding target, while the frequency of the sinusoid is proportional to the speed of the target. A target of zero velocity relative to the antenna  14  will return a signal of zero frequency, i.e., DC.  
         [0093]    The sum of waveforms is amplified by amplifier  19  and converted to digital form by the A/D converter  20 . The gain of the amplifier  19  is controlled by the microcontroller  31  such that the amplifier  19  signal is close to, but never exceeds the dynamic range of the A/D converter  20 . The digitized data stream is fed into the microcontroller/DSP  31  for spectrum analysis ( 104 ), which is accomplished by, for example, well known FFT techniques.  
         [0094]    TARGET DETECTION: Target existence and speed in each range bin can be determined by the following process:  
         [0095]    (1) Set the time delay of the receive code relative to the transmit code ( 102 ) according to the range bin of interest by the method as described in the “correlation” section.  
         [0096]    (2) Apply spectrum analysis to return signal after a finite dwell time ( 103  &amp;  104 ). The theoretical minimum dwell time per range bin is dependent on the minimum speed of interest for the intended target type. In general, the dwell time per range bin should be no less that one period of the Doppler Frequency of interest.  
         [0097]    (3) Determine the speed of target(s) inside the range bin of interest according to its Doppler frequency ( 105 ).  
         [0098]    By progressively changing the range bin setting and repeating the above process ( 106  &amp;  107 ), all targets and their respective speed within the search beam can be mapped. This process is termed full-range scanning. The time for a complete range scan is equal to the dwell time per range bin multiplied by the number of range bins covered in the scan.  
         [0099]    It is worthy of note that after a complete range scan is performed and targets mapped, subsequent scans can be drastically abbreviated by scanning only the target-bearing range bins and their immediate neighbors, plus the “entrance range bin”, where new targets may first enter the area covered by the search beam. This process is termed skip-scan.  
         [0100]    It is therefore possible to measure the target&#39;s velocity using only the elapsed time during which a target traverses two range gates. Relying only on the elapsed time would obviate the need for Doppler measurements and processing steps associated therewith, such as FFT (Fast Fourier Transform) or generally digital signal processing via a DSP, whether it is separate from or integral with the microprocessor.  
         [0101]    TARGET DIRECTION DETERMINATION: The direction of target movement cannot be derived from its Doppler frequency. It is possible to determine target direction of movement from the result of successive skip scan (as described above) and analyzing the pattern of target occupation of range bins. In this process the speed of each target can be tracked and used as its identifying parameter for tracking its movement.  
         [0102]    TARGET TRUE SPEED CALCULATION (COSINE CORRECTION): This is a desirable function to correct for the fact that existing radar speed measuring devices, such as police radar guns, particularly those relying on Doppler shift measurement, will indicate an incorrect speed if the vehicle&#39;s velocity vector makes an angle θ with the radar gun line-of sight to the vehicle that is larger than zero. The error may be tolerable for θ up to, say 10°. Note that θ increases as the vehicle approaches the radar gun, when the echo signal becomes stronger and reliable. In the present system, since the radar measures both Doppler shift and range of approaching vehicles by taking two sets of measurements, f 1 , R 1 , and f 2 , R 2  and two intersecting angles, θ 1  and θ 2 , the velocity measurement is made independent of the “cosine error” by performing the following computation in the microcontroller/DSP  31 :  
         V   =       λ   2          f   2                  (       f   1       f   2       )     2     -       (       R   2       R   1       )     2             1   -       (       R   2       R   1       )     2               ,   where                         
 
         [0103]    f 1 =doppler shift at range (distance) R 1 ,  
         [0104]    f 2 =doppler shift at range (distance) R 2 ,  
         [0105]    V=velocity of vehicle, and  
         [0106]    λ=radar wavelength  
         [0107]    Once the cosine correction has been completed the range and speeds of targets are displayed ( 108  &amp;  109 ).