Abstract:
A communications and data link jamming system employs a fiber-optic RF delay line to provide rapid responses to threat signals. A sample of the RF signal threat environment is stored within the delay line, and a jamming video signal is added to the stored sample by modulation as it is being extracted from the delay line. The extracted signal is re-circulated back into the delay line, thereby effectively stretching the sample for highly efficient jamming. The jamming system is effective in countering burst communications and in defeating multiple simultaneous threat signals.

Description:
[0001]    This application claims the benefit, under 35 U.S.C. § 119(e), of co-pending Provisional Application No. 60/748,093, filed Dec. 7, 2005, the disclosure of which is incorporated herein by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates generally to electronic countermeasure systems. More specifically, the invention relates to a communications jamming system based on a Radio Frequency (RF) memory device using fiber-optic re-circulation technology. 
         [0005]    2. Description of the Related Art 
         [0006]    Modern military communication systems often employ short, burst type transmissions. These transmissions may occur at static frequencies or may constantly cycle through a secret sequence of frequencies in order to prevent detection and jamming. Typically, these systems only transmit on a particular frequency for at most a few milliseconds. Jamming such transmissions is often sought as a counter-measure, but the extremely short duration of such transmissions has made jamming difficult in practice. 
         [0007]    The continuing development of modern military communication systems requires the ability to detect and counter enemy communications in a specified sector of a battlefield, no matter how short these transmissions are or how fast the communications change frequencies to avoid detection. Furthermore, since the duration of the target transmissions is so short, it is impractical to evaluate signals, make a determination, and then direct the transmission of a jamming. There is simply not enough time to engage these signals before they cease transmission or have moved on to a new frequency. 
         [0008]    Conventional jamming systems attempt to solve this “short cycle” problem in one of two ways: (1) Barrage jamming, which involves “splashing” a segment of the radio frequency (RF) spectrum with random or distributed noise in order to jam frequency-hopping transmissions by brute force. Barrage jamming is impractical for several reasons, among them being the amount of power needed to apply sufficient RF energy to wash out all transmissions. (2) Responsive jamming, also called “fast-reaction” jamming, which requires the reception of signals and the automatic selective jamming of those signals soon thereafter, for as long as the enemy transmission is active. There are, in turn, two types of responsive jamming. The first type is “transponder” jamming, which uses a receiver to measure specific parameters of active signals that are necessary for constructing a jamming waveform. The second type is “follower” jamming, which captures or intercepts a sample of the active signals and applies a jamming modulation to this sample to create a jamming signal. 
         [0009]    A typical conventional transponder jammer  100  is shown in  FIG. 1 . It includes an antenna  102 , a transmit/receive (T/R) switch  104 , a receiver  106 , a controller  108 , and an exciter  110 . The transponder jammer  100  is programmed to intercept and respond to active signals from a potential target(s). During the signal detection period (or reception mode) of jammer operation, the controller  108  actuates the transmit/receive (T/R)  104  switch so as to allow external signals to enter the system through the antenna  102  for processing by the receiver  106 . Typically, the receiver  106  scans an instantaneous bandwidth window  116 , as shown in  FIG. 1A , across the expected threat operating frequency range (“Expected Target Range”  117  in  FIG. 1A ). 
         [0010]    Once a signal is detected, the controller  108  determines whether the signal should be disrupted or jammed. Following a positive determination, the controller  108  directs the exciter  110  to tune to the detected signal frequency and add a jamming waveform, such as noise, a continuous wave (CW) tone, or a swept tone. Then, the system  100  transmits the disruption or jamming signal, via the T/R Switch  104 , through the antenna  102 , and radiates it into the atmosphere. 
         [0011]    The size of the instantaneous bandwidth is dependent upon the specific receiver technology used. For example, a common receiver architecture (not shown here) employs a hybrid configuration, including a super-heterodyne receiver that performs the scan operation, followed by a digital receiver that implements a Fast Fourier Transform (FFT). The digital receiver converts the analog signal to digital data and then performs an FFT, resulting in the identification of the frequency and power level of all active signals within the instantaneous bandwidth. The processing time  118  in  FIG. 1A  includes the time necessary to change the receiver&#39;s frequency and to sample and process the signals within this bandwidth. 
         [0012]    There are several shortcomings associated with transponder type of jammer system. First, due to the scanning nature of the receiver, an undesirably long revisit time  119  may exist, as shown in  FIG. 1A . This may result in a long response time relative to the duration of the threat signal. In many instances, involving short or burst messages, the threat transmit time may be so short that the transponder jammer&#39;s response will arrive after the threat has completed its transmission. Similarly, for a frequency hopping threat signal, the signal may change or “hop” to another frequency so quickly that the conventional jammer is unable to perform its internal processing and adjustment tasks before the threat signal moves to another frequency. A second problem is that if many potential threat signals are simultaneously present, the transponder jammer may not be able to disrupt all of them in an efficient manner. Finally, if the transponder jammer limits its receiver scan to only a limited number of frequencies identified from previous experience or intelligence-gathering operations as threats, when the threat evolves into a different frequency, the transponder jammer will fail. 
         [0013]    An exemplary form of a conventional follower jamming system  100 ′, also known as a re-circulating follower, is shown in  FIG. 2 . The antenna assembly  102 ′ and T/R switch  104 ′ function as previously described for the transponder jamming system. The incoming signal that is intercepted by the antenna  102 ′ is routed through the T/R switch  104 ′, a first coupler  111   a , and an amplifier  112 . A portion of the signal is removed by a second coupler  111   b  and sent to a delay line  113  that acts as a storage medium. As the signal propagates thought the delay line  113 , it is reintroduced into the RF path by the first coupler  111   a . The amplifier  104 ′ compensates for the insertion losses associated with the couplers  111   a ,  111   b  and the delay line  113 . As the signal loops or re-circulates around the coupler-amplifier-delay line structure, a portion propagates through the second coupler  111   b . A jamming modulator  114  causes the signal to be modified in such a manner as to disrupt the threat communication link. A controller  108 ′ sets the timing and state of all switches in the system. 
         [0014]    The conventional follower jamming system contains several drawbacks associated with the delay line implementation. Those systems that incorporate surface acoustic wave or bulk acoustic wave technologies suffer from limited instantaneous RF bandwidth, since these devices are inherently narrow band. Delay lines consisting of coaxial cable overcome bandwidth limitations but exhibit high insertion losses, thus limiting maximum storage times. Reduced storage time causes increased spectral spreading due to the phase discontinuity that nearly always exists as the signal re-circulates. Excessive spectral spreading reduces the concentration of jamming power on the threat signal, reducing jamming effectiveness. 
         [0015]    Thus, there is a need in the communications jamming art for systems and techniques that effectively provide rapid wideband jamming effective against both short message threats and frequency hopping threats, as well as multiple simultaneous threats. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention overcomes the limitations of the prior art by using a wideband RF delay line. In a preferred embodiment, this delay line is a fiber-optic cable arranged to allow for recirculation of RF signals. In place of a conventional scanning receiver, the present invention provides instantaneous frequency coverage across the entire communications band of 20 MHz to over 2 GHz. Friendly or non-threat frequency ranges are excluded from processing. Fixed and tunable band-pass and band-reject filters are used during equipment setup to exclude these frequency ranges. All “active” signal samples (i.e., those that are not excluded by the filter assembly) are fed to a fiber-optic delay line (FODL) that stores an RF sample that is typically less than 1 millisecond in duration. The sample period is not adjustable and is determined by the length of fiber-optic cable. Once the sample is stored, RF switches within the jammer change the routing of the signal, so that external signals no longer enter the jammer. The contents of the FODL re-enter or re-circulate through the FODL a predetermined number of times, and then the FODL contents exit the FODL to combine with a jamming video waveform generated by a controller in the system. The combined signals are amplified and radiated into the environment. The re-circulation action continues for a defined number of re-circulations (e.g., ten to twenty) before a new RF sample is taken. Since the jamming signal is generated from an input sample, it does not require time-consuming scanning, frequency conversions, and analog-to-digital conversions or any digital computations. As a result, the jammer&#39;s response time is extremely short, thereby enabling the jammer to defeat short messages, as well as more complex communication systems, such as those employing frequency hopping transmissions. Furthermore, since all signals in the FODL are treated as threat signals, the jammer can defeat multiple simultaneous threats. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The foregoing features and other features of the present invention will now be described, with reference to the drawings of several preferred embodiments. In the drawings, the same components have the same reference numerals. The illustrated embodiments are intended to illustrate, but not to limit the invention. The drawings include the following figures: 
           [0018]      FIG. 1  is a general block diagram of a conventional prior art transponder jammer; 
           [0019]      FIG. 1A  is a diagram illustrating the relationships associated with searching a range of frequencies and the time necessary to perform this activity, as it applies to the conventional prior art transponder jammer; 
           [0020]      FIG. 2  is a general block diagram of a conventional prior art follower jammer; 
           [0021]      FIG. 3  is a general block diagram of a jamming system using RF delay line technology, in accordance with a first preferred embodiment of the present invention; 
           [0022]      FIGS. 4 ,  5 ,  6  and  7  are block diagrams of a front end assembly, a channel assembly, an AGC assembly and FODL assembly respectively; 
           [0023]      FIG. 8  is a timing diagram showing a representative timing relationship between the sampling and jamming periods in the present invention; 
           [0024]      FIG. 9  is a block diagram illustrating the switch configuration during the sampling mode in the operation of the present invention; 
           [0025]      FIG. 10  is block diagram illustrating the switch configuration during the jamming mode in the operation of the present invention; 
           [0026]      FIG. 11  is a block diagram illustrating a second preferred embodiment of the present invention, in which separate receiving and transmitting antennas are used; and 
           [0027]      FIG. 12  is a block diagram illustration of a third preferred embodiment of the present invention, in which multiple high power amplifiers are used. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIGS. 3 ,  4 ,  5 ,  6  and  7  are functional block diagrams of a communications jamming system  120 , in accordance with a first preferred embodiment of the present invention. The system  120  includes an antenna assembly  122  comprising one or more antenna elements (not shown), depending upon the frequency range of operation in intercepting electromagnetic signals from the surrounding physical environment for input into the system. A T/R (Transmit/Receive) switch assembly  124  allows individual elements within the antenna assembly  122  selectively to function either as signal sensors or signal radiators. Timing circuits (not shown) within a controller  144  (to be described in more detail below) provide appropriate timing signals that direct the flow of RF energy into and out of the jamming system  120 . 
         [0029]    A power supply  142  provides operational power to the system. The particular type of power supply will depend on the specific application and the operational environment of the system. For a mobile vehicle installation, the power source  142  may be either 12V DC (commercial automobile or truck) or 24V DC (military vehicle). For a stationary installation, such as protection of a building, roadway, entrance ramp, etc., the power source  142  may be 110V AC, 220V AC or 440V AC. Finally, for a man-portable application, such as a backpack, an assembly of primary or secondary batteries (e.g., 6 to 48V DC) would be appropriate. 
         [0030]    An RF front-end (RFFE) assembly  126  performs several important functions associated with signal processing prior to signal sample storage and re-circulation. These functions include the protection of internal electronic components against excessive RF power. As shown in  FIG. 4 , the RFFE  126  includes a power limiter  146  receiving the RF signal from the T/R switch  124 , a signal amplifier  148  receiving the power-limited output of the power limiter  146 , and a first RF switch  150  that receives the amplified signal from the amplifier  148  and a signal from a second RF power divider  170 , to be discussed below. 
         [0031]    A channel assembly  128  includes a first RF power divider circuit  152  (see  FIG. 5 ) that separates the incoming signals from the RFFE  126  into two or more RF channels (two of which are shown and labeled A and B in  FIG. 5 ), each having a pre-defined RF frequency range. The number of channels and their respective frequency ranges are set by the user during a system set-up operation. The system set-up operation may be performed, for example, by creating a system configuration file on a portable or remote computer, and then down loading the system configuration file to the controller  144  in the system  120 . 
         [0032]    The channel assembly  128  also includes an RF power combiner circuit  162  ( FIG. 5 ) that produces a single RF output for further processing. Each RF channel A and B includes a band pass filter  154  that defines the specific operating frequency range of the channel; at least one adjustable attenuator  156  for controlling the peak amplitude of the RF signals within the channel; a channel switch  158  that enables or disables the channel; a mixer/modulator circuit  163  that inserts a jamming video signal generated in, and received from the controller  144 ; and signal monitor  160  that monitors signal activity within the channel. The signal monitor  160  includes a directional detector and an analog-to-digital converter (not shown). The directional detector removes the RF carrier, leaving a video signal that is representative of signal amplitude. The video signal is sent to the controller  144  where it is converted to a digital word. Data provided by the directional detector are used by the controller  144  to calculate the settings of the adjustable attenuators  156  in each channel before the signal is fed to a fiber optic delay line (FODL) assembly  140  (to be described below), which performs optimally only when input signal levels are within a specific range. The settings of the adjustable attenuators  156  may be controlled in accordance with a program, stored in or downloaded to the controller  144 , that may take into account a number of operational parameters such as, for example, output signal power capacity, individual channel power capacity, the linearity limits of the FODL assembly  140 , the number and amplitudes of active threat signals, and a predetermined threat signal priority. 
         [0033]    The output signal from the channel assembly  128  is fed to an automatic gain control (AGC) assembly  130  and then to a high power amplifier (HPA) assembly  132 , which in a preferred embodiment of the invention, comprises a high-efficiency class AB amplifier having an operational frequency range that encompasses the entire frequency range of the system  120 . The AGC assembly  130 , illustrated in  FIG. 6 , substantially inhibits the overdriving of the HPA assembly  132 , and it protects the system from damage caused by high-reflected power. As shown in  FIGS. 5 and 6 , signals arriving from the mixer/modulator  163  in the channel assembly  128  are split into two signal paths by a first AGC RF power divider  165 . One path sends the signal to a second RF switch  166  in the channel assembly  128 , while the other path sends the signal to the HPA assembly  132  via an automatic gain control circuit  168  that is included in the AGC assembly  130 . The automatic gain control circuit  168  prevents a strong signal within any one or more channels from either driving HPA  132  beyond its recommended output power level, causing the generation of unwanted harmonics and spurious signals, or unduly consuming a large amount of the available power for the HPA  132 . 
         [0034]    A dual directional detector  172 , operatively associated with the HPA assembly  132 , enables the monitoring of either forward RF power or reverse reflected RF power for AGC purposes. High-reflected power is an indication that a component in the system, such as an element of the antenna assembly  122  a cable, or the T/R switch  124 , has failed, or that the antenna assembly  122  has been improperly installed. The controller  144  recognizes the possibility of any of these conditions and directs the HPA  132  to shut down, thus reducing the possibility of permanent damage to the system. 
         [0035]    The FODL assembly  140  ( FIG. 7 ) includes an RF-to-optical converter  174 , a length of single-mode fiber-optic cable  176  (advantageously provided on a spool, not shown), and an optical-to-RF converter  178 . The FODL assembly  140  receives the signal from the second RF switch  166  in the channel assembly  128  ( FIG. 5 ), and it provides an analog RF memory feature that expands a short time sample into a powerful and robust jamming signal by repetitively extracting the contents of the analog RF memory, so that a quasi-CW waveform is created. The length of the fiber-optic cable  176  is determined by the sampling time interval of jammer system  120 . For example, a sample time of 25 microseconds requires a fiber-optic cable length of approximately 5.14 km. The fiber-optic cable  176  is ideal for obtaining and repetitively extracting relatively long samples, due to its low insertion loss and time-dispersion characteristics. Other delay line technologies, such as those employing coaxial cables and surface or bulk acoustic-wave devices, are unable to match these performance qualities of the fiber-optic cable. 
         [0036]    The output of the optical-to-RF converter  178  is fed back to a second AGC RF power divider  170  in the AGC assembly  130 . The second AGC RF power divider  170  divides the signal into a first signal path that is input to the second RF switch  166  in the channel assembly  128 , and a second signal path that is input to the first RF switch  150  in the RFFE  126  ( FIG. 4 ). 
         [0037]    Referring again to  FIG. 3 , a global positioning system (GPS) Antenna  134  and a GPS Receiver/Time Reference  136  are used to allow multiple systems  110  to operate without interfering with each other. During normal operation, multi-system synchronization is based on a one-pulse-per-second timing from GPS receiver  136 . The look-through period is synchronized with this signal. This signal is used also to compensate for drift in a local time reference, thereby improving the ability to maintain synchronization when there is a loss of GPS signals. Failure to maintain GPS signal lock causes the internal time reference to become the system&#39;s timing signal. If necessary, the system can continue operation for over one hour in this clock “flywheeling” mode. The reference in this case is provided by an oven-stabilized, crystal-controlled oscillator (not shown). The time reference reverts to GPS once the GPS time reference signal is re-acquired. 
         [0038]    The controller  144  is a microprocessor-based system, located on the system backplane (not shown). The controller  144  performs a variety of functions, including system initialization and configuration, timing, operator interface, diagnostics, maintenance and GPS control. The controller  144  may advantageously include a variety of digital devices, such as a microprocessor, a random access memory (RAM), a read only memory (ROM) and a field programmable gate array (FPGA), as is well-known in the art. The microprocessor provides the decision making capability that is essential for real-time system operation, while the RAM is used to store temporary or changing data. The ROM is used to store operating system and application programs that provide the sequence of steps needed for the system  120  to perform its tasks. The FPGA is configured to generate a video signal that is fed to the mixer/modulator  163  as a jamming signal waveform, as mentioned above. The FPGA is also configured to perform all of the remaining specialized digital processing functions. For example, look-through timing uses a portion of the FPGA that has been configured as a counter to set the sample and transmit times of the system  120 . Additional counters are configured within the FPGA to provide control for internal switches (i.e., the T/R switch  124  and the switches in the RFFE  126  and the channel assembly  128 ) that are related to look-through timing. 
         [0039]    The controller  144  is also responsible for performing the calculations associated with the functioning of the AGC assembly  130 . This is accomplished by performing analog-to-digital conversions on the video pulse trains from the channel assembly  128  (each channel providing a separate pulse train) and calculating the maximum signal amplitude value emanating from the HPA  132  based on the combined input signal amplitudes plus the gain of the remaining RF path. The calculated maximum signal amplitude value is compared to the peak power capacity of the HPA  132 , and the RF path gain is adjusted so that the HPA  132  is not operating in saturation, which could cause excessive signal distortion and possibly unequal sharing of HPA power. Portions of the FPGA are configured to convert the amplitude from the dual directional detector  172  that monitors reverse power within the AGC assembly  130  into its digital equivalent, determines if this amplitude exceeds a specified limit and, if so, generates a sequence of commands to limit or reduce the possibility of damage to the system. Finally, the FPGA contains two serial data ports for controlling the GPS receiver and for providing an operator&#39;s interface (not shown). 
         [0040]    While operating, the system  120  alternates between Sample Mode and Jam Mode, as shown in the timing diagram of  FIG. 8 . A guard-band  139  surrounds each of these operation intervals. The guard band  139  is necessary to allow for internal switching, tuning, and other adjustments needed to optimize system performance. 
         [0041]    Jamming systems in accordance with the present invention generate jamming waveforms based on a relatively short sample time.  FIGS. 9 and 10  respectively show the key internal components within channel assembly  128 , the AGC assembly  130 , and the FODL assembly  140 , in respectively illustrating the sampling and jamming functions of the invention. 
         [0042]    As shown in  FIG. 9 , when the system  120  is in the sampling mode, the first RF Switch  150  is configured to allow the entry of signals from the external electromagnetic environment, via the antenna  122  assembly and the RFFE  126  assembly, into the channel assembly  128 . As described previously, the channel assembly  128  performs several signal conditioning processes, including dividing the incoming RF signal into two or more paths, removing unwanted signals that lie outside of a specific channel&#39;s operating frequency bandwidth, adjusting the amplitude of the in-range signals, and combining the processed signals of all channels into a single output. This output is then divided into two paths by the first AGC RF power divider  165 . One path is connected to the input of the HPA  132  although during the sampling period the HPA  132  output is disabled, so that it does not interfere with the sampling process. The other path encounters the second RF Switch  166 , which is configured so that the FODL assembly  140  receives and is filled with the sampled signals. For maximum jamming effectiveness, the length of the cable  176  in the FODL assembly  140  should coincide with the sampling interval. The sampling and delay filling operations occur automatically, regardless of whether weak signals, or even no signals, are present in the sample. Once filled, the sampling process is complete, and the system  120  is automatically reconfigured for jamming. 
         [0043]    The filling of the optical fiber cable  176  in the FODL assembly  140  is analogous to a liquid traveling through an empty open-ended pipe. When a sufficient quantity of liquid has entered the pipe, so that it is full, then the liquid begins to spill out on the other end. Similarly, the optical cable  176  of the FODL assembly  140  is also filled when a time sample of sufficient length is entered. Thereafter, the stored sample begins to appear at the delay line output. The output is split into two paths by the second AGC RF power divider  170 . The first path re-circulates or feeds the signal back to the FODL assembly  140  through the second RF switch  166 , which has changed its configuration so that it no longer inputs the signals from the channel assembly  128  to the FODL assembly  140 . In this manner, the contents of the FODL assembly  140  re-enter or re-circulate to the FODL assembly  140  to re-fill the fiber optic cable  176 . The re-circulation is performed a predetermined number of times (e.g., 10-20), as determined by the controller  144 , before a new RF sample is taken. 
         [0044]    The FODL assembly output signals are directed by the second AGC RF power divider  170  to a second signal path that is connected back to the first RF Switch  150 , which has changed its configuration, so that external signals are prevented from entering the channel assembly  128 . Instead, the first RF switch  150  allows the previously-stored signal to propagate through the channel assembly  128  and the first AGC RF power divider  165  to the HPA assembly  132 , which is now enabled. The stored signal (which has been modulated with a jamming video waveform in the channel assembly  128 , as described above) is then amplified and radiated to the environment through the antenna assembly  122 . Specifically the T/R Switch Assembly  124  is directed by the controller  144  to operate in a transmission mode in which external signals are prevented from entering the system, but in which the output of HPA assembly  132  is sent to the antenna assembly  122  for radiation into the environment. 
         [0045]    It can be seen from the foregoing that all signal processing, storage and re-circulation operations are performed at the original RF frequencies of the input signals which may be termed the “baseband” frequencies. Thus, unlike many typical prior art communication and data link jammers, RF frequency conversions are not necessary in the present invention. 
         [0046]      FIG. 11  shows a jammer system  180  in accordance with a second embodiment of the present invention. This implementation provides a separate receiving antenna  182  and transmission antenna  184 . While this configuration doubles the number of antenna elements relative to the previously described embodiment, it eliminates the T/R Switch. In some applications, this arrangement may improve operational reliability and decrease manufacturing costs. In addition, the use of separate reception and transmission antennas provides a physical separation that may improve the electromagnetic isolation between input and output assemblies and components. This will often have the effect of reducing the quantity and amplitude of spurious signals within the system, thereby improving the quality of the jamming signal. 
         [0047]      FIG. 12  shows a jammer system  190  in accordance with a third embodiment of the present invention, in which multiple high power amplifier (HPA) assemblies  132  are used (three being shown in the drawing). This embodiment may advantageously be employed when higher output powers are needed to increase jamming effectiveness. In some applications, each of the multiple HPA assemblies  132  may be operated in a narrower bandwidth. In other cases, the operating frequency ranges of the devices being jammed may be so wide that only a single HPA assembly cannot be employed, due to limitations in the power handling capability of its internal components. The use of multiple HPA assemblies may also assist in the disruption of multiple simultaneous threats, whereby the threat signals may be divided among the several amplifiers without exceeding the maximum output power capacity of a single amplifier. Finally, the use of multiple HPA assemblies may result in a lower overall system cost in some applications. 
         [0048]    While exemplary embodiments of the invention have been described herein, it is understood that a number of modifications and variations will suggest themselves to those skilled in the pertinent arts. These variations and modifications are may deemed to constitute equivalents to various aspects of the invention described herein, and are considered within the spirit and scope of the invention. Furthermore, the specific software and hardware that may be used to implement various aspects of the invention, as mentioned above, will readily suggest itself to those skilled in the art, and may take any number of equivalent forms that will provide the above-described functional aspects and advantages of the invention.