Abstract:
A radar for locating and tracking objects based on the use of a pulsed waveform, each pulse of the pulsed waveform being made up of a plurality of spectral components having different frequencies, including an antenna. The radar further includes a transmitter operatively coupled to the antenna for generating the plurality of spectral components that make up each pulse of the pulsed waveform and a receiver operatively coupled to the antenna for receiving signals at the frequencies of the plurality of spectral components. The radar also includes a signal processor operatively coupled to the receiver for processing the received signals in order to generate and output a radar presentation and to detect the presence of other signals at particular frequencies, a display operatively coupled to the signal processor for displaying the radar presentation, and finally a controller operatively coupled to the transmitter and the signal processor for varying the frequencies at which the plurality of spectral components are generated, such that the transmitter generates spectral components at frequencies different from the frequencies of other signals detected by the signal processor.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to a radar system and a method of generating waveforms for use by the radar system. More particularly, the present invention relates to an impulse radar system that generates individual pulses of a pulsed waveform from spectral components having frequencies that vary between individual pulses.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radar systems generally require bandwidth in order to resolve targets, i.e., the larger the bandwidth, the higher the range resolution. Conventional radar systems use waveforms with long pulse width and typically have an instantaneous bandwidth on the order of 100 MHz. To improve the instantaneous bandwidth, exploration has been done in connection with impulse radars. Impulse radars use a train of short pulses on the order of 200 picoseconds and have been shown to have an instantaneous bandwidth on the order of 5 GHz.  
           [0003]    In the past, impulse radars have taken the approach of switching the RF transmit signal on and off in picoseconds in order to generate the train of extremely short pulses. However, such systems generally require the impulse generator to have a peak power on the order of several megawatts due to the fact that it has a low duty factor in that the pulse width of the impulse generator is extremely short when compared to the required interpulse period.  
           [0004]    In an effort to ameliorate these problems, the inventor of the present invention explored an ultra-wide bandwidth radar that used a specified set of narrow band spectral components to synthesize a waveform with very high range resolution. This concept, which was embodied in U.S. Pat. No. 5,146,616 (the &#39;616 Patent) and U.S. Pat. No. 5,239,309 (the &#39;309 Patent), was implemented by combining (summing) multiple continuous wave sources having frequencies that were equally spaced. This superposition of continuous wave sources resulted in the desired repeating pulse train without the need for fast switching circuits. However, the waveform described in the aforementioned patents required that the transmitted sources be evenly spaced across at least a portion of the available frequency spectrum.  
           [0005]    Recently, a need has been expressed for a radar system that could operate in the communication bands, e.g., from 3 Mhz to 1 GHz (covering HF, VHF, and UHF bands). Such a radar would be quite useful, particularly since it would have superior foliage penetration to radars operating at microwave frequencies and above. Unfortunately, the impulse radars of the prior art, including those covered by the &#39;616 Patent and the &#39;309 Patent, would not be suitable for such operation. Specifically, the prior art impulse radar systems are likely to interfere with communication signals being transmitted in the band of operation of the radar.  
           [0006]    Therefore, it would be advantageous to have a radar system that could operate in the communication bands without interfering with other users transmitting within these bands.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a radar system that uses a wide bandwidth pulsed signal that is composed of spectral components having frequencies spaced at irregular intervals. Specifically, the present invention provides a radar system that is capable of varying the frequencies of the spectral components composing individual pulses of the pulsed signal so as to avoid interfering with ongoing communications within the radar&#39;s transmission band.  
           [0008]    In accordance with one aspect of the present invention, a radar for locating and tracking objects based on the use of a pulsed waveform, each pulse of the pulsed waveform being made up of a plurality of spectral components having different frequencies is provided. The radar includes an antenna and a transmitter operatively coupled to the antenna for generating the plurality of spectral components that make up each pulse of the pulsed waveform. The radar further includes a receiver operatively coupled to the antenna for receiving signals at the frequencies of the plurality of spectral components and a signal processor operatively coupled to the receiver for processing the received signals in order to generate and output a radar presentation and to detect the presence of other signals at particular frequencies. The signal processor is operatively coupled to a display for displaying the radar presentation. Finally, the radar includes a controller operatively coupled to the transmitter and the signal processor for varying the frequencies at which the plurality of spectral components are generated, such that the transmitter generates spectral components at frequencies different from the frequencies of other signals detected by the signal processor.  
           [0009]    In accordance with another aspect of the present invention, a radar is provided wherein the controller suppresses the generation of those spectral components having frequencies that are the same as the frequencies of the other signals detected by the signal processor.  
           [0010]    In accordance with still another aspect of the present invention, a radar is provided wherein the spectral components are produced at frequencies within a frequency band of between approximately 20 MHz and approximately 600 MHz.  
           [0011]    In accordance with still a further aspect of the present invention, a method of generating a pulsed waveform having a plurality of spectral components is provided. The method includes the steps of listening across a predetermined frequency band in order to determine which frequencies within the frequency band are available for transmission and generating for a finite period of time a plurality of spectral components having frequencies corresponding to at least a portion of the frequencies available for transmission. The method further includes the steps of combining the plurality of spectral components into a pulse of the pulsed waveform, transmitting the pulse of the pulsed waveform, and repeating the prior steps to generate and transmit a plurality of subsequent pulses of the pulsed waveform.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a simplified block diagram illustrating a radar system in accordance with the present invention.  
         [0013]    [0013]FIG. 2A is a frequency domain representation of uniformly frequency spaced continuous wave sources, which when summed together create the pulsed waveform illustrated in FIG. 2B.  
         [0014]    [0014]FIG. 2B is a time domain representation of the pulsed waveform created by summing the continuous wave sources represented in FIG. 2A.  
         [0015]    [0015]FIG. 2C is a frequency domain representation of eleven logarithmically frequency spaced continuous wave sources, which when summed together create the pulsed waveform illustrated in FIG. 2D.  
         [0016]    [0016]FIG. 2D is a time domain representation of the pulsed waveform created by summing the continuous wave sources represented in FIG. 2C.  
         [0017]    [0017]FIG. 2E is a frequency domain representation of sixteen logarithmically frequency spaced continuous wave sources, which when summed together create the pulsed waveform illustrated in FIG. 2F.  
         [0018]    [0018]FIG. 2F is a time domain representation of the pulsed waveform created by summing the continuous wave sources represented in FIG. 2E.  
         [0019]    [0019]FIG. 3 is a simplified block diagram of a digital implementation of a radar system in accordance with the present invention.  
         [0020]    [0020]FIG. 4 is a schematic illustration of an analog implementation of one channel of a radar system in accordance with the present invention.  
         [0021]    [0021]FIG. 5 is a flow chart illustrating the steps performed by a radar system in accordance with the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention will now be described in detail with reference to the drawings. In the drawings, like reference numerals are used to refer to like elements throughout.  
         [0023]    [0023]FIG. 1 is a block diagram representation of a radar system  10  in accordance with the present invention. The radar system  10  includes an antenna  12  coupled to a plurality of switches  14   a - 14   n.  The switches  14   a - 14   n  are of the single-pole double-throw variety and operate to connect electrically the antenna  12  to both a series of transmitters  16   a - 16   n  and a series of receivers  18   a - 18   n.  The transmitters  16  are driven by a common master oscillator  20  and are connected to a master controller  22 , the function of which will be described in more detail below. The master controller  22  is also connected to the switches  14  and a signal processor  24 . In turn, the signal processor is connected to both the receivers  18  and a display  26 .  
         [0024]    Referring now to FIGS.  2 A- 2 F, the waveforms generated by the radar system  10  will be discussed in more detail. For simplicity, the discussion will be confined to a frequency band between 50 MHz and 550 MHz, although any frequency band could be used without departing from the scope of the present invention. FIG. 2A is representative of 11 continuous wave (CW) sources  40  of equal amplitude but uniformly spaced in frequency across the frequency band. These 11 CW sources  40  make up the spectral components of the waveform represented in FIG. 2B. As discussed previously, it was found that when these 11 sources were summed together, the result was a waveform having a strong central peak  42  with noisy time-domain side lobes  44  mirrored about the central peak  42 .  
         [0025]    As is shown in FIGS.  2 C- 2 H, a similar waveform is generated when summing or combining CW sources that are not uniformly spaced in frequency. FIG. 2C illustrates 11 CW sources  46 , which are logarithmically spaced in frequency. As is illustrated in FIG. 2D, when these sources  46  are combined, the resulting waveform contains a strong central peak  48  with noisy time-domain lobes  50  mirrored about the central peak, although the time-domain side lobe structure does differ from the side lobe structure illustrated in FIG. 2B. Similarly, when 16 tones or CW sources are combined that are logarithmically spaced in frequency (See FIG. 2E), a waveform similar to the waveform illustrated in FIG. 20 (see FIG. 2F) is generated. As is readily seen, the time-domain side lobes  52  are of lesser amplitude than the time-domain side lobes  50  illustrated in FIG. 2D. It should be noted that as the number of tones or CW sources used to generate the transmitted waveform is increased, the relative strength (amplitude) of the time-domain side lobes decreases as compared to the central peak (See FIGS. 2D and 2F).  
         [0026]    Ultimately, the inventor of the present invention determined that there need not even be a mathematical correlation for the frequencies of the tones or CW sources combined to generate a waveform that could be used by the radar system  10 . The tones could be randomly spaced in frequency and the resulting waveform would still contain a strong central peak with noisy time-domain side lobes mirrored thereabout. The only requirement is that the tones used to generate the waveform be derived from a common master oscillator, i.e., that the tones be mutually coherent.  
         [0027]    Referring back to FIG. 1, the basic operation of the radar system  10  will be described. As is the case with all radar systems, radar system  10  operates in both a transmission mode and a receive mode. To transmit a signal, the master controller  22  places switches  14   a - 14   n  in an appropriate position to connect electrically the antenna  12  and the transmitters  16   a - 16   n.  The transmitters  16  each act as a single CW source. Each signal produced by the transmitters  16  is coherently generated from the master oscillator  22  and provided to the antenna  12 . In this embodiment of the present invention, each signal is generated for a period of 0.33 milliseconds, although other generation time periods could be used if an application required a longer pulse train. In other words, the “on” time of the transmitter  16  corresponds to the pulse duration for each individual pulse in the pulsed waveform.  
         [0028]    The antenna  12 , which is preferably a broadband multiplexing antenna, receives the signals generated by the transmitters  16  and combines them into a high gain beam. The master controller  22  controls the “on” time of the transmitters  16 . After the “on” time has expired, the master controller  22  shuts down the transmitters  16  and shifts the switches  14   a - 14   n  into the appropriate position for the radar system  10  to act in a receive mode.  
         [0029]    On receive, the antenna  12  separates all of the spectral components of the incoming waveforms. The spectral components are then coupled to the plurality of receivers  18   a - 18   n.  The receivers  18 , the operation of which will be described in more detail below, each provide an output to the signal processor  24 , which coherently combines and processes the outputs in order to produce a signal that is provided to the display  26 , thereby creating a radar presentation. In this embodiment, the radar system  10  functions in the receive mode for a period of 50 milliseconds. Generally, the “off” time for the transmitters will correspond to the range of the radar system  10 . Specifically, the “off” time should be sufficient to ensure that all return pulses have been received, thereby negating the potential for antenna  12  to receive and transmit simultaneously.  
         [0030]    As was discussed above, the number or density of the spectral components combined in order to create the pulsed waveform influences the strength of the time domain side lobes of the pulsed waveform in comparison to the central peak. Therefore, if the spectral components are densely frequency spaced, the pulsed waveform reduces to a single transmitted impulse without side lobes. Although such a waveform may be ideal, it is not necessary to achieve the benefits of the present invention. For example, the present radar system  10  can be effective when using 20 to 40 spectral components.  
         [0031]    Turning now to FIG. 3, a digital implementation of the radar system  10  is illustrated. The radar system  10  includes a broadband multiplexing antenna  60  electrically connected to a single-pole double-throw switch  62 . The switch  62  is illustrated electrically connected to a receive path  64 . However, the switch  62  will toggle between the receive path  64  and a transmit path  66  in response to commands from a master controller  68 .  
         [0032]    When toggled into connection with the transmit path  66 , the switch  62  couples the antenna  60  to a digital transmitter  70 . In the illustrated embodiment, the digital transmitter  70  is in communicative relation with both a memory  72 , which stores digitally synthesized waveforms, and a master oscillator  74 , which functions as a master clock for the radar system  10 . In response to commands from the master controller  68 , the digital transmitter  70  selects the appropriate waveform for transmission.  
         [0033]    As will be discussed in more detail by reference to FIG. 5, the waveform will be selected based upon the spectral components available for transmission, i.e., those spectral components that will not interfere with other communication ongoing within the transmission band of the radar system  10 . The memory  72  may contain digital representations of the actual waveforms to be transmitted. Alternatively, the memory  72  may contain digital representations of individual spectral components. In this case, the digital transmitter  70  would select the appropriate spectral components from the memory  72  and digitally synthesize therefrom the waveform to be transmitted.  
         [0034]    When toggled into connection with the receive path  64 , the switch  62  couples the antenna  60  to a direct sampling receiver  76 . The direct sampling receiver  76  samples received signals in order to generate data that will be used by a digital signal processor  78  which is coupled to the direct sampling receiver  76 . In this embodiment of the present invention, a sample rate of 1 gigasample per second would be sufficient to capture information on the received signals.  
         [0035]    As is the case with conventional radar systems, the digital signal processor  78  processes the information provided by the direct sampling receiver  76  in order to generate a radar presentation that the digital signal processor  78  then provides to a display  80 .  
         [0036]    Referring now to FIG. 4, the present invention, if desired, could also be implemented in analog circuitry. FIG. 4 represents an analog implementation of one channel or tone of the present invention. One skilled in the art will appreciate that this implementation will be repeated for each channel of the radar system  10 . To the extent practical, certain of the components may be common to each such channel.  
         [0037]    As with the digital implementation described above, an antenna  90  is coupled via a switch  92  to both a transmit path  94  and a receive path  96 . When connected to the transmit path  94 , the switch  92  couples the antenna  90  to a transmitter  98  that is controlled by a controller  99 . The transmitter  98  is driven by a frequency synthesizer  100  so as to create a spectral component having a particular frequency. As was discussed previously, it is desirable that each CW source be coherently generated. Accordingly, the frequency synthesizer  100  is connected to a master oscillator  102 , which synchronizes the generation of the CW sources for all channels of the radar system  10 .  
         [0038]    When connected to the receive path  96 , the antenna  90  is coupled to an RF amplifier  104  to detect and amplify spectral components of the received signals. The RF amplifier  104  is connected to a mixer  106 , which mixes the output signal of the RF amplifier  104  with a signal from the frequency synthesizer  100 . The signal from the frequency synthesizer provided the mixer  106  is offset in frequency from the signal the RF amplifier  104  provides the mixer  106  by an amount equal to the frequency of the master oscillator  102 .  
         [0039]    The mixer  106  outputs a signal to an intermediate frequency amplifier  108 , which provides an amplified output to both in-phase mixer  110  and quadrature mixer  112 . In-phase mixer  110  and quadrature mixer  112  mix the amplified output with a signal from the master oscillator  102  and provide respective outputs to an inphase A/D converter  114  and a quadrature A/D converter  116 .  
         [0040]    The in-phase A/D converter  114  and the quadrature A/D converter sample the outputs from mixers  110  and  1   12  and provide I and Q data to a digital signal processor  118  for use in creating a radar presentation. In accordance with the Nyquist criterion, the A/D converters  114  and  116  must sample at a sufficient rate to capture available information from the received signals. Generally, a sampling rate of 8 kHz would be adequate in the present embodiment of this invention.  
         [0041]    Referring now to FIG. 5, the operation of a radar system in accordance with the present invention will be described. In step  200 , the system commences operation and, in step  202 , initially determines the frequencies within the frequency band of the spectral components that will form a pulse of the pulsed waveform. The frequencies could be static or dynamic. In other words, the system could be built such that it included a plurality of transmitters (on the order of 20 to 40), each transmitter designed to generate a continuous wave signal at a predetermined frequency. Alternatively, the system could be designed such that the frequencies at which the transmitters generate the signal vary based upon information received from other components in the system.  
         [0042]    In step  204 , the system is set to operate in the receive mode, and listens across at least a portion of the frequency band in which the system is designed to operate in order to detect the presence of signals at the same frequencies as the desired frequencies for the spectral components. If the system detects the presence of signals at the desired frequencies, the controller will send a signal to the applicable transmitters, thereby suppressing the generation of that spectral component (see step  206 ). Then, as indicated in step  208 , the system is switched to the transmit mode and the remaining spectral components, i.e., the spectral components having frequencies not conflicting with other signals within the operational range of the radar system, are transmitted.  
         [0043]    As discussed previously, eliminating one or more of the spectral components that make up a pulse results in an increase of the relative strength of the time-domain side lobes as compared to the main lobe, thereby degrading the “quality” of the pulse. This degradation is generally quite slight and should not impact adversely the operation of the radar system. However, as opposed to suppressing one or more of the spectral components, the system could be configured to provide a predetermined number of spectral components, the frequencies of which vary from pulse to pulse based upon the frequencies within the band available for transmission. This “frequency hopping” would reduce both the likelihood of repetitively being unable to transmit and the ability of a third party to jam this radar system.  
         [0044]    In step  210 , the signal that is transmitted by the antenna is recorded and stored for use by the signal processing electronics. In step  212 , the system switches back to the receive mode and listens for the return signals. The return signals that are received are provided to the signal processor and correlated against the transmitted signal, as recorded. The basic purpose of the correlation function is to match the received signals to the transmitted signal. As is indicated in steps  214  and  216 , the information generated by this “matching” is used to create the impulse response or “A-scope” response of the radar system, which is in turn used in a conventional manner to generate the radar presentation or display.  
         [0045]    The correlation of the received to the transmitted signal may be complicated by the Doppler shift created in the returned signals. One potential method of addressing such complication would be to correlate the received waveform against a pluralityof trial Doppler-shifted transmitted waveforms, using the results of such correlations to create the impulse response of the radar system.  
         [0046]    Steps  202  through  216  are then repeated to create subsequent pulses of the pulsed waveform. Generally, it is anticipated that each pulse will be made up of a superposition of spectral components having frequencies that vary from the frequencies of the spectral components making up one or more of the previous pulses. In this manner, a radar system is provided which employs a signal having spectral components that will not interfere with other communication signals being transmitted. Thus, the present system can be employed in any frequency bands including communication frequency bands. Furthermore, because the spectral components of each pulse will likely vary, a radar system is provided which is very difficult to jam since any jamming scheme will need to know exactly which frequencies will be received by the system at a precise point in time.  
         [0047]    Although the invention has been shown and described with respect to certain embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. What is claimed is: