Abstract:
Systems and methods for avoidance of partial pulse interference in radar. The systems and methods include a radar processor for generating control signals that direct the generation and transmission of two consecutive radar pulses using a waveform and pulse train generator and transmitter. The systems and methods also include receiving reflected echoes corresponding to the transmitted pulses using a receiver and processing the echoes using an analog to digital converter, filter, and digital signal processor to separate echoes from each pulse, process them, and combine the results to avoid partial pulse interference while maintaining pulse energy and an acceptable signal to noise ratio.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Pulse compression for airborne weather radar has several advantages. Among these is the ability to provide excellent radar detection performance using a low power transmitter, which allows much lower system cost. Although pulse compression is powerful, it has an inherent drawback because it requires longer pulses to achieve the equivalent peak power signal to noise ratio (SNR) of a system not using pulse compression. This can lead to severe partial pulse interference at close ranges. Partial pulse interference is caused by radar returns from ranges closer than a pulse length from the radar. The radar receiver is turned on after the pulse transmission, so close in returns do not contain complete pulse histories. When partial pulses are processed by the pulse compression portion of the receiver, it results in range side-lobes that can overwhelm data from valid ranges which are greater than the pulse length. The amount of interference is highly dependent on the environment of a particular application. For airborne weather radar, partial pulse interference can cause false detections.  
         [0002]     There is therefore a need for a technique to avoid partial pulse interference while maintaining pulse energy and an acceptable signal to noise ratio.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention includes systems and methods for avoidance of partial pulse interference in radar. The systems and methods include a radar processor for generating control signals that direct the generation and transmission of two consecutive radar pulses using a waveform and pulse train generator and transmitter. The systems and methods also include receiving reflected echoes corresponding to the transmitted pulses using a receiver and processing the echoes using an analog to digital converter, filter, and digital signal processor to separate echoes from each pulse, process them, and combine the results to avoid partial pulse interference while maintaining pulse energy and an acceptable signal to noise ratio. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0005]      FIG. 1  is a schematic diagram of one embodiment of the present invention.  
         [0006]      FIG. 2  is a schematic diagram showing additional detail of the system of  FIG. 1 .  
         [0007]      FIGS. 3-5  are flowcharts of a method of avoiding partial pulse interference in accordance with an embodiment of the present invention.  
         [0008]      FIG. 6  is an example showing transmitted and received pulses in accordance with an embodiment of the present invention.  
         [0009]      FIGS. 7 and 8  illustrate schematic diagrams of examples of embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]      FIG. 1  is a schematic illustration of a system  10  for avoiding partial pulse interference for pulse compressed radar in accordance with an embodiment of the present invention. The system  10  includes a radar processor  20  to generate two consecutive pulses which are transmitted by a transmitter and receiver device  40 . The generated pulses are each approximately of duration T/2, where T represents a pulse duration ordinarily used for pulse compressed radar at a given range. Echoes corresponding to the transmitted pulses are also received by the transmitter and receiver device  40  which are then processed into useable image data by the radar processor  20 . The system  10  can be used to improve the performance of airborne weather radar systems such as Honeywell&#39;s RDR-4000 and RDR-4000M products for example.  
         [0011]      FIG. 2  is a schematic diagram showing additional detail for the system  10  of  FIG. 1 . The radar processor  20  includes a control processor  22  in data communication with a Digital Signal Processor (DSP)  24  in data communication with a filter  28  linked to an analog to digital (A/D) converter  26 . The transmitter and receiver device  40  includes a waveform and pulse train generator  42  in data communication with a synthesizer  44  linked to a transmitter  46  which is connected to an antenna  54  via a device  50  that controls whether a receiver  48  or the transmitter  46  is currently connected to the antenna  54 . The synthesizer  44  is also linked to the receiver  48 . The receiver  48  to uses phase and timing information from the synthesizer  44  to coherently modulate received radar data. The transmitter and receiver device  40  also includes an antenna controller and drive component  52  which is connected to the waveform and pulse train generator  42  as well as the antenna  54 .  FIG. 2  also illustrates more detailed links between the radar processor  20  and the transmitter and receiver device  40 . The control processor  22  is in data communication with the waveform and pulse train generator  42 , and directs the transmitter and receiver device  40  via this linkage. The receiver  48  sends the received echoes to the A/D converter  26 . For example, the received echoes could enter the receiver  48  at 9.3 GHz analog and the receiver  48  would shift them in frequency to 48 MHz analog before sending them to the A/D converter  26 .  
         [0012]      FIG. 3  is a flowchart of a method  58  to avoid partial pulse interference in airborne weather radar. The method  58  begins at a block  60 , where two consecutive pulses separated in frequency by Δω&gt;bandwidth (BW) are generated and transmitted, where BW refers to the bandwidth of a single pulse. The pulses are each approximately of duration T/2, where T represents a pulse duration ordinarily used for pulse compressed radar at a given range. Next, at a block  70 , reflected echoes of the transmitted pulses are received by the transmitter and receive device  40  and a visual representation of weather data is produced by the radar processor  20  using the reflected signals received. Although not shown, these steps will be repeated in a continuous manner.  
         [0013]      FIG. 4  is a flowchart showing additional detail for the block  60  shown in  FIG. 3 . The block  60  includes a block  62  where a control signal is sent to the waveform and pulse train generator  42 . The control processor  22  sends a control signal to the waveform and pulse train generator  42 , which is linked to the synthesizer  44 . Next, at a block  64 , two pulses, each of duration T/2, separated by Δω&gt;BW are generated based on the control signal sent. The waveform and pulse train generator  42  generates a waveform based on the control signal sent and is responsible for the detailed modulation and timing of the pulses. Each pulse consists of a specific set of frequencies over the pulse width. The waveform and pulse train generator  42  creates pulses at a low center frequency, for example at 112 MHz. The waveform and pulse train generator  42  is also responsible for creating the sequence of precisely timed pulses that compose a pulse train. Each pulse in the pulse train may have a different center frequency, starting phase, modulation, and pulse width. Timing and phase jitter are also implemented using the waveform and pulse train generator  42 . The synthesizer  44  then shifts the low frequency pulses up to the corresponding center frequency in the radar band, for example at 9.3 GHz, in preparation for amplification and transmission. Then, at a block  66 , the two generated pulses are amplified and transmitted by the transmitter  46 . The generated pulses are transmitted using the transmitter  46  which is connected to the antenna  54  via the device  50  during transmission of the pulses.  
         [0014]      FIG. 5  is a flowchart showing additional detail for the block  70  illustrated in  FIG. 3 . First, at a block  72 , the reflected echoes of the transmitted pulses are received by the receiver  48 . The reflected echoes are received using the receiver  48  which is connected to the antenna  54  via the device  50  during reception of the reflected echoes. Next, at a block  74 , the received signal is converted to digital form by the A/D converter  26 . Then, at a block  75 , the signal is filtered to obtain relevant frequencies. This is followed by a block  76  where the filtered signal is processed. Finally, at a block  78 , the processed signal is converted to a visual representation. The filter  28  performs the filtering described in the block  75  and the DSP  24  processes the signal and prepares it for conversion to a visual representation. These five more detailed blocks will not necessarily be conducted in the order shown in all embodiments. Some of the blocks may happen in parallel or in orders other than those shown.  
         [0015]      FIG. 6  is an example of a time frequency diagram showing both transmitted and reflected pulses in accordance with an embodiment of the present invention. The time frequency diagram illustrates a first transmitted pulse P 1 (t) and a second transmitted pulse P 2 (t) which are separated by a frequency of Δω. The diagram also shows reflected pulses r 1 , r 2 , r 3 , r 4 , and r 5  for the P 1 (t) transmitted pulse and reflected pulses r 6 , r 7 , r 8 , r 9 , r 10 , r 11 , and r 12  for the P 2 (t) transmitted pulse. The reflected pulses indicate point scatter down range from the radar as follows: r 1  (3T/4), r 2  (T), r 3  (5T/4), r 4  (3T/2), r 5  (7T/4), r 6  (T/4), r 7  (T/2), r 8  ( 3 T/ 4 ), r 9  (T), r 10  (5T/4), r 11  (3T/2), and r 12  (7T/4). The diagram also illustrates that each transmitted pulse is of duration T/2 and that a receive start time t 1  begins after the P 2 (t) pulse has finished transmitting at time T.  
         [0016]     Although receiving begins at time t 1 , the reflections of interest received between time t 1  and time t 2  are affected by partial pulse interference because echoes from partial pulses are not separated by more than T/2 from the echoes of interest in this range. For example, the partial pulse reflection r 1  will interfere with the reflection r 2 . The reflections received between time t 1  and t 2  are still processed by pulse compression matched filter detectors, such as are shown in  FIGS. 7 and 8 , so that the reflections received between t 2  and t 3  will not suffer from partial pulse interference, but the processed data for the portion between t 1  and t 2  is discarded after the filtration.  
         [0017]     Beginning at time t 2 , reflections from both the P 1 (t) data and P 2 (t) data are used, but reflections from the P 1 (t) data must be delayed by T/2 in order to align them with the reflections from the P 2 (t) data. This results in valid data for the closest point scatter ranges of T to 3T/2 coming only from reflections for P 2 (t). The processed signals for this range are amplified by a factor of 2 because data from only one of the transmitted radar pulses is being used. This amplification does not negatively affect later processing because the SNR is high at this range. For point scatter farther down range than 3T/2, reflections from both P 1 (t) and P 2 (t) are used, which allows the energy of a T second pulse to be maintained. For example, processed data from reflections r 4  and r 11  will be summed or combined by the radar processor and processed data from reflections r 5  and r 12  will be summed or combined by the radar processor. This allows imaging starting at T down-range of the radar and avoids partial pulse interference while maintaining the energy of a T second pulse.  
         [0018]      FIG. 7  illustrates an example of signal processing steps  90  for a non-coherent receiver approach. The steps  90  are performed using a filter  28   a  and a digital signal processor  24   a , which are located within the overall system in the same locations as the filter  28  and the DSP  24 . A signal, which includes echoes corresponding to both a first and a second transmitted pulse, enters from the A/D converter  26  into the filter  28   a . The signal could include 4 MHz complex baseband digital radar data, for example. In other embodiments, the function performed in the filter  28   a  may be performed by the DSP  24   a . Also, an additional filter (not shown) may perform a previous filtering step resulting in the entering signal coming from the additional filter rather than the A/D converter  26 .  
         [0019]     The filter  28   a  includes a first multiplier  100  and a second multiplier  102  in parallel. The signal from the A/D converter  26  is received by the multipliers  100  and  102 . The multipliers  100 ,  102  are used to separate the received echoes into a first echo signal corresponding to the first transmitted pulse and a second echo signal corresponding to the second transmitted pulse. The first multiplier  100  is used to filter the received signal so that only the first echo signal corresponding to the first transmitted pulse such as P 1 (t) shown in  FIG. 6  is processed further in this branch of the signal processing logic. After the first echo signal has been separated, a first pulse compression matched filter detector  104  is used to properly detect the echoes corresponding to the first transmitted pulse and convert them into a first filtered echo signal. Next, the first T/2 duration of the first filtered echo signal is discarded at a truncating element  109  to produce a truncated first filtered echo signal. Then, at a box  112 , the magnitude of the first filtered echo signal is taken to produce a non-complex truncated first filtered echo signal. Finally, at a box  113 , the non-complex truncated first filtered echo signal is delayed by T/2 to produce a delayed non-complex truncated first filtered echo signal. The delaying which occurs at the box  113  can be performed by appending zeros to the beginning portion of the data, for example. The delayed non-complex truncated first filtered echo signal then passes to a summing element  114 .  
         [0020]     In like manner, the second multiplier  102  separates out a second echo signal corresponding to the second transmitted pulse such as that shown as P 2 (t) in  FIG. 6 . After the second echo signal has been separated, a second pulse compression matched filter detector  106  is used to properly detect the echoes corresponding to the second transmitted pulse and convert them into a second filtered echo signal. After this step has been performed, the first T/2 duration of the second filtered echo signal is discarded at a truncating element  108  to produce a truncated second filtered echo signal that can later be summed with the processed echoes corresponding to the transmitted pulses from P 1 (t) to produce a full power signal. Then, at a block  110 , the magnitude is taken of the truncated second filtered echo signal to produce a non-complex truncated second filtered echo signal.  
         [0021]     A switch  115  remains in a first switch position for T/2 seconds before changing to a second switch position. When the switch  115  is in the first switch position, the non-complex truncated second filtered echo signal from the block  110  next passes through a block  111  that amplifies the non-complex truncated second filtered echo signal by a factor of two to produce an amplified non-complex truncated second filtered echo signal which then passes to the summing element  114 . When the switch  115  is in the second switch position, the non-complex truncated second filtered echo signal passes to the summing element  114  without passing through the block  111 . Depending on the position of the switch  115 , the delayed non-complex truncated first filtered echo signal is then combined with either the amplified non-complex truncated second filtered echo signal or the non-complex truncated second filtered echo signal to produce an output signal with greater pulse energy than could be obtained by using only one of the T/2 duration pulses for times after T/2. During the first T/2 seconds, the switch  115  will be in the first switch position and the delayed non-complex truncated first filtered echo signal will not contribute to the output signal due to its delay, so the output signal will simply be the amplified non-complex truncated second filtered echo signal. The output signal can then be used in further display processing. The display processing could include scaling the data to fit a display resolution for visual representation of weather date, for example. The scaling may include log compression of the magnitude and interpolation or decimation in time in some embodiments.  
         [0022]     The description above and  FIG. 7  assume there is no processing delay through the first pulse compression matched filter detector  104  or the second pulse compression matched filter detector  106 . This is accomplished, for example, by using a Fast-Fourier Transform (FFT) to implement the convolution performed by the first pulse compression matched filter detector  104  and the second pulse compression matched filter detector  106  and applying a phase term across matched filter coefficients. However, in embodiments where the implementation incurs a processing delay of a, the amount of signal data discarded by the truncating element  108  and the truncating element  109  is T/2+τ rather than T/2.  
         [0023]      FIG. 8  illustrates an example of signal processing steps  92  for a coherent receiver approach. The steps  92  are implemented using a filter  28   b  and a digital signal processor  24   b , which are located within the overall system in the same locations as the filter  28  and the DSP  24 . The figure shows a signal, which includes echoes corresponding to both a first and a second transmitted pulse, entering from the A/D converter  26  into the filter  28   b . The signal could include 4 MHz complex baseband digital radar data, for example. In other embodiments, the function performed in the filter  28   b  may be performed by the DSP  24   b . Also, an additional filter (not shown) may perform a previous filtering step resulting in the entering signal coming from the additional filter rather than the A/D converter  26 .  
         [0024]     The first portion of the signal processing is the same as with the non-coherent receiver approach described for  FIG. 7 . The differences begin at the point where the truncated first filtered echo signal exits the truncating element  109  and the truncated second filtered echo signal exits the truncating element  108 . In the coherent receiver approach, the truncated first filtered echo signal next passes into the box  113  where it is delayed by T/2 to produce a delayed truncated first filtered echo signal. The delaying which occurs at the box  113  can be performed by appending zeros to the beginning portion of the data, for example. The delayed truncated filtered echo signal then passes to the summing element  114 .  
         [0025]     After the truncated second filtered echo signal exits the truncating element  108 , the next processing step is determined by the position of the switch  115 . When the switch  115  is in the first switch position, the truncated second filtered echo signal from the block  110  next passes through a block  111  that amplifies the truncated second filtered echo signal by a factor of two to produce an amplified truncated second filtered echo signal which then passes to the summing element  114 . When the switch  115  is in the second switch position, the truncated second filtered echo signal passes to the summing element  114  without passing through the block  111 . Depending on the position of the switch  115 , the delayed truncated first filtered echo signal is then combined with either the amplified truncated second filtered echo signal or the truncated second filtered echo signal to produce a combined signal with greater pulse energy than could be obtained by using only one of the T/2 duration pulses for times after T/2. During the first T/2 seconds, the switch  115  will be in the first switch position and the delayed truncated first filtered echo signal will not contribute to the combined signal due to its delay, so the combined signal will simply be the amplified truncated second filtered echo signal. The combined signal then enters a block  122  where the magnitude is taken of the combined signal to produce a non-complex output signal. The non-complex output signal can then be used in further display processing. The display processing could include scaling the data to fit a display resolution for visual representation of weather date, for example. The scaling may include log compression of the magnitude and interpolation or decimation in time in some embodiments.  
         [0026]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, preliminary filtering could be performed using the DSP rather than by using a separate filter. In like manner, other processing steps implemented in separate devices could also be combined into single components and processing steps performed in single components could be divided among multiple components. Additionally, the receiver and transmitter could be integrated into a single transceiver device rather than being separate components. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.