Abstract:
A method for determining an angle of incidence of periodic optical signals is provided. The method includes detecting whether the periodic optical signals are present; generating a pulsed output when the periodic optical signals are present, the pulsed output corresponding to peaks in the periodic optical signals; predicting a timing of the periodic optical signals from the pulsed output; controlling a gated detector array to take a first reading and a second reading of the periodic optical signals based upon the timing, the first reading being out of phase with the timing and the second reading being in phase with the timing; and generating the angle of incidence by filtering the first reading from the second reading.

Description:
FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0001] This invention was funded with government support under contract No. DAA-B07-99-D-B032 awarded by the Department of the Army. The government may have certain rights in this invention as provided for by the terms of such contract. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is directed to a method for determining the angle of incidence of low level, periodic optical signals.  
           [0004]    2. Description of Related Art  
           [0005]    Military ordinance and weaponry have been used in conjunction with guidance systems using optical signals. The optical signals enable the guidance system to guide the weapon to that target (e.g., a beam rider). The optical signals are typically emitted, in the form of low power, low signal-to-noise laser signals. Weapons having such a guidance system (hereinafter referred to as “guided weapons”) have proven useful in increasing the effectiveness of offensive military operations.  
           [0006]    The proliferation of guided weapons has given rise to the need for systems that can detect when an object has been targeted by such a weapon. For example, warning systems have been developed to detect the presence of the optical signals emitted to guide a weapon. Early detection of the optical signals can prove useful in preventing strikes by such guided weapons. For example, early detection of an optical signal from a guided weapon can allow a potential target to take evasive action to avoid the weapon. Alternately, early detection of an optical signal from a guided weapon can allow the potential target to enact countermeasures, such as emitting signals configured to confuse the guidance system of the incoming weapon, releasing weapons configured to destroy the incoming weapon, and combinations thereof.  
           [0007]    In addition to merely detecting the presence of optical signals from guided weapons, it can be important for the warning system to determine where such signals are originating, i.e., the angle of incidence or the angle of arrival of such signals. However, prior art warning systems that detect both the presence of optical signals emitted by guided weapons and their angle of incident have proven to be less than optimal. For example, some warning systems present a video image of an incoming optical signal that requires further processing for detection and location. Here, the image can require the operator to process the signal in order to remove or filter out other non-laser sources (e.g., sun, light, etc.). Further, some prior art warning systems require the use of a gated image intensifier to minimize background noise, which has the unintended effects of limiting the spectral response of the system and increasing the cost of the system.  
           [0008]    Due to the high cost of such prior art gated image intensifiers, the prior warning systems have used a moving or gimbaled sensor. However, the gimbaled sensor must first be moved to the sector where an incoming optical signal is detected, before further processing of the signal can occur. Thus, such gimbaled sensors can cause a delay in location determination. Moreover, if more than one incoming weapon is present, the gimbaled sensor can not simultaneously detect the multiple weapons. Further, the gimbaled sensor requires moving mechanical components. These moving mechanical components can decrease reliability of the system and/or can increase the weight of the system, which can be undesired aspects in military vehicles, such as airplanes, helicopters, armored personnel vehicles, and the like.  
           [0009]    Accordingly, there is a continuing need for improved warning systems and methods of operation that address one or more of the aforementioned and other deficiencies in the prior art.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    A method of determining the angle of incidence of low level, periodic optical signals is provided.  
           [0011]    A method is provided for gating a detector to minimize interference of ambient or incident light on a low level optical signal and, thus, allow the angle of incidence of the optical signals to be easily determined.  
           [0012]    A method is provided for determining an angle of incidence of periodic optical signals including: detecting whether the periodic optical signals are present; generating a pulsed output when the periodic optical signals are present, the pulsed output corresponding to peaks in the periodic optical signals; predicting a timing of the periodic optical signals from the pulsed output; controlling a gated detector array to take a first reading and a second reading of the periodic optical signals based upon the timing, the first reading being out of phase with the timing and the second reading being in phase with the timing; and generating the angle of incidence by filtering the first reading from the second reading.  
           [0013]    A method for determining an angle of incidence of periodic optical signals is provided. The method includes: detecting whether said periodic optical signals are present; sending a pulsed output when said periodic optical signals are present, said pulsed output corresponding to peaks in the periodic optical signals; generating a synchronization pulse from said pulsed output, said synchronization pulse being indicative of an anticipated period of said periodic optical signals; gating a stationary optical detector array to take a first reading and a second reading of said periodic optical signals, said first reading being out of phase with said anticipated period and said second reading being in phase with said anticipated period; and generating said angle of incidence by filtering said first reading from said second reading.  
           [0014]    Yet another aspect of the present invention is provided by a warning system having a first portion and a second portion. The first portion has a first optical detector and a pulse interval correlator, where the first optical detector sends an input to the pulse interval correlator. The second portion has a stationary gated optical detector, a predictive repeater, and a processor.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0015]    [0015]FIG. 1 illustrates an exemplary embodiment of a warning system according to the present disclosure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring now to FIG. 1, an exemplary embodiment of a warning system  10  according to the present disclosure is illustrated. Warning system  10  detects periodic, low-level optical signals and determines the angle incidence of these signals, a pair of parameters that have previously been considered mutually exclusive. Warning system  10  comprises a first channel  12  and a second channel  14 . First channel  12  is preferably a high sensitivity channel and second channel  14  is preferably a high angular resolution channel.  
         [0017]    First and second channels  12 ,  14  work in conjunction with one another to provide warning system  10  with the aforementioned low-level optical signal detection and angle of incidence capabilities. First channel  12  detects the presence or absence of the signals, and triggers second signal channel  14  to determine the angle of incidence of the detected signals.  
         [0018]    First channel  12  includes a first optical detector  16 , a filter  18 , a signal amplifier  20 , and a pulse interval correlator  22 . First channel  12  is configured to control first optical detector  16  to detect the presence of periodic, low-level optical signals  24  originating in a detection range  26 . Detection range  26 , preferably, provides about ninety (90) degrees of azimuth and elevation detection.  
         [0019]    First channel  12  processes signals  24  detected by detector  16  through filter  18  and amplifier  20  to provide a filtered and amplified input  28  to correlator  22 . Filter  18  compensates signals  24  for interference from normal ambient sources, such as the sun, that are detected by detector  16 . For example, filter  18  can apply a DC current load to signals  24 , while amplifier  20  can amplify and/or boost signals  24 .  
         [0020]    Correlator  22  provides a warning output  30  to an operator indicative that signals  24  have been detected. Correlator  22  also converts inputs  28  into a series of digital pulse outputs  32  that correspond to peaks in periodic signals  24 . Pulse outputs  32  are provided to second channel  14  for further processing. Thus, first channel  12  detects the presence of signals  24 , alerts the operator, and sends pulse outputs  32 .  
         [0021]    First channel  12  continuously monitors range  26  for the presence of signals  24 . Conversely, second channel  14  is normally dormant, i.e., is not monitoring range  26  for the presence of signals  24 . However, second channel  14  is activated by the receipt of pulse outputs  32  from first channel  12 . In essence, warning system  10  comprises two sub-systems where the first system detects the presence of incoming signals  24  and triggers the second system to determine the angle of incidence of such signals  24 . Thus, first channel  12  alarms the operator of the detection of incoming signals  24  and sends output pulses  32  to trigger second channel  14 , which then determines the angle of incidence of these signals.  
         [0022]    Second channel  14  includes a predictive repeater  34 , a gated optical detector  36 , and a processor  38 . Gated optical detector  36  is preferably an electronically gated optical detector array, and more preferably is an array of gated cameras, such as an array of charge-coupled device (CCD) cameras.  
         [0023]    Predictive repeater  34  receives pulse outputs  32  from correlator  22 , then analyzes and deciphers pulse outputs  32  to generate synchronization pulses  42 . Synchronization pulses  42  are indicative of the anticipated timing of the next pulse of signal  24 . Predictive repeater  34  is adapted to decipher both pulsed and chopped periodic, low-level optical signals, a capability that was previously unavailable. For example, signals  24  can be emitted in several different forms. Some signals  24  are emitted as pulsed signals, others are emitted as chopped signals, and still others are emitted as a combination of pulsed and chopped signals. Importantly, predictive repeater  34  is configured to detect both pulsed and chopped waveforms.  
         [0024]    It has been determined that synchronizing the gating of detector  36  with the anticipated timing of the next pulse of signal  24  can be used to provide the angle of incidence of signal  24 . Thus, second channel  14  uses synchronization pulses  42  to control the gating of gated detector  36  to send a plurality of gated inputs  44  to processor  38 . Specifically, second channel  14  uses synchronization pulses  42  to electronically gate (i.e., switch on and off) detector  36  in a synchronous relationship with the arrival of signals  24  detected by first channel  12  to cause detector  36  to generate gated inputs  44 .  
         [0025]    Gated inputs  44  includes a first input that is out of phase with the arrival of signals  24  and a second input that is in phase with the arrival of signals  24 . Thus, the first input is representative of the ambient radiation detected by detector  36  without signals  24  being present. Conversely, the second input is representative of the radiation detected by detector  36  with signals  24  being present. Processor  38  compares gated inputs  44  to filter ambient radiation from signals  24 , which then allows the processor to generate an angle of incidence output  40  therefrom.  
         [0026]    The synchronous gating of detector  36  with respect to the periodic rate of signals  24  minimizes interference in the detection of signal  24  by gated detector  36 . Thus, second channel  14  is a temporal filtering means, which minimizes interference in the detection of signal  24 . The reduced interference allows gated detector  36  to be less sensitive and hence inexpensive, yet still provide a higher resolution of gated inputs  44  than conventional systems. For example, gated detector  36  does not require an image intensifier used by conventional systems. Accordingly, warning system  10  uses low-cost digital electronics to precisely synchronize gated detector  36  with incoming radiation signal  24 . This allows gated detector  36  to provide gated inputs when incoming signal  24  is both present and absent, which allows the summation of these inputs by processor  38  (e.g., cancellation of the first input from the second input) to minimizes the interference from surrounding illumination.  
         [0027]    In the illustrated embodiment, warning system  10  provides about ninety (90) degrees of azimuth and elevation detection. Here, four warning systems  10  would be needed to provide three hundred and sixty (360) degrees of azimuth and elevation. Of course, it is contemplated by the present disclosure for warning system  10  to provide a larger or smaller detection range  26  and, thus, it is contemplated that the aforementioned 360 degrees of azimuth and elevation detection be provided by more or less than four warning systems  10 .  
         [0028]    In contrast to gimbaled sensors, optical detectors  16 ,  36  remain stationary. Thus, each warning system  10  provides 90 degrees of detection without moving mechanical components. The elimination of moving components can increase the reliability and reduce the weight of warning system  10  as compared to prior systems. By way of example only, a known prior gimbaled sensor system providing 360 degrees of detection has a weight of about eighty (80) pounds. In contrast, four warning systems  10 , which provide the same 360 degrees of detection, have an overall weight of about forty (40) pounds. Thus, warning system  10  provides about a fifty percent (50%) reduction in weight as compared to conventional systems for the same range of detection.  
         [0029]    It should also be noted that the terms “first”, “second”, and “third” and the like may be used herein to modify elements performing similar and/or analogous functions. These modifiers do not imply a spatial, sequential, or hierarchical order to the modified elements unless specifically stated.  
         [0030]    While the invention has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.