Abstract:
In one aspect, a method includes representing a range of Doppler frequency offsets as a local oscillator waveform comprising a plurality of digital waveform samples, selecting a portion of the plurality of digital waveform samples using a Doppler value to form an optical heterodyne; and generating a signal associated with a target within a bandwidth of a receiver using the optical heterodyne.

Description:
BACKGROUND 
       [0001]    While collecting heterodyned data in a coherent LADAR (laser detection and ranging) system, velocity differences between a sensor and an object the sensor is evaluating cause large Doppler shifts in a received signal. These Doppler shifts cause the frequency of the heterodyned signal to vary. Thus, the bandwidth of the receiver must be sufficiently large to account for this variation in the frequency and, in most cases, an excessively large receiver bandwidth may be required. 
       SUMMARY 
       [0002]    In one aspect, a method includes representing a range of Doppler frequency offsets as a local oscillator waveform comprising a plurality of digital waveform samples, selecting a portion of the plurality of digital waveform samples using a Doppler value to form an optical heterodyne; and generating a signal associated with a target within a bandwidth of a receiver using the optical heterodyne. 
         [0003]    In another aspect, an article includes a non-transitory machine-readable medium that stores executable instructions. The instructions cause a machine to represent a range of Doppler frequency offsets as a local oscillator waveform comprising a plurality of digital waveform samples, select a portion of the plurality of digital waveform samples using a Doppler value to form an optical heterodyne and generate a signal associated with a target within a bandwidth of a receiver using the optical heterodyne. 
         [0004]    In a further aspect, an apparatus, includes circuitry to represent a range of Doppler frequency offsets as a local oscillator waveform comprising a plurality of digital waveform samples; select a portion of the plurality of digital waveform samples using a Doppler value to form an optical heterodyne; and generate a signal associated with a target within a bandwidth of a receiver using the optical heterodyne. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a block diagram of a LADAR environment. 
           [0006]      FIG. 2A  is a graph of frequency over time of one period of a linear chirped waveform that, while not to scale, compares the magnitude of the receiver bandwidth and potential Doppler frequency shifts a LADAR sensor will encounter in operation. 
           [0007]      FIG. 2B  is a graph of frequency versus time of a received signal with respect to the transmitted signal. 
           [0008]      FIG. 2C  is a graph of frequency versus time of another received signal with respect to the transmitted signal. 
           [0009]      FIG. 3  is a flowchart of an example of a process to determine appropriate waveform samples. 
           [0010]      FIG. 4  is a computer on which the process of  FIG. 3  may be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Described herein is an approach to select appropriate digital waveforms samples and timing of a local oscillator (LO) waveform in order to mix the LO waveform with a target echo and acquire target information inside a frequency range of a receiver. The techniques described herein allow a system to accommodate large Doppler shifts in signal without having to increase the receiver bandwidth required. 
         [0012]    For example, the techniques described herein are applicable to a coherent LADAR (laser detection and ranging), which uses a linear frequency modulated (LFM) chirp optical transmit signal. In a coherent heterodyne system using LFM signals, an intermediate signal is formed by transmitting a LFM signal to an object, and optically heterodyning or mixing the received signal from the object with a local LFM signal at the receiver. The frequency of this intermediate signal formed after mixing is referred to as the intermediate frequency (IF). The local LFM signal is referred to as the local oscillator (LO) signal. The intermediate frequency produced from the mixed LO and received signal shifts in frequency with both range to object and relative velocity between sensor and object. The techniques described herein represent the entire range of Doppler frequency shifts as a LO signal comprised of digital waveform samples, and adjust the start and stop of the frequency modulation of the LO signal to compensate for target Doppler shifts, while simultaneously adjusting the timing of the LO relative to the transmit signal to account for IF frequency shifts due to range, enabling the receiver to accommodate very large target Doppler shifts without having to increase the receiver bandwidth necessary to capture signal information from the target. The frequency range over which the LO signal is modulated is adjusted independently of the frequency range of the transmit signal. In one example, the techniques described herein allows for systems using 100% duty cycle to maintain nearly complete overlap of the received and LO chirps, while accommodating a range of Doppler shifts limited only by the variability of the LO start and stop frequency. 
         [0013]    Referring to  FIG. 1 , a LADAR environment  100  includes a LADAR sensor  102  at a location, L S , to detect a target  104  at a location, L T  with a range to target, R T . The range to target, R T , is a length of a vector pointing from the LADAR sensor  102  to the target  104 . The LADAR sensor  102  is disposed on a sensor platform  106  traveling at a velocity, V P . A line  108  between the LADAR sensor  102  and the target  104  and a nadir axis  110  form a squint angle, θ S . The nadir axis corresponds to an axis where the Doppler shift with respect to the LADAR sensor  102  is zero. For example, a target above the nadir axis  110  (i.e., in front of the sensor  102  or where the sensor  102  is moving towards) would have a blue Doppler shift while a target below the nadir axis  110  (behind the sensor  102  or where the sensor  102  is moving away from) would have a red Doppler shift. The nadir axis  110  is 90 degrees (orthogonal) to the sensor velocity (velocity of the platform) vector, V P . For example, if you have a sensor on an aircraft, the nadir axis  110  will change as the aircraft turns or changes its flight profile or directional heading. In one example as described herein, the sensor platform velocity vector, V P  is determined first and then the nadir axis  110  is determined from the sensor platform velocity vector, V P . The squint angle, θ S  relative to the nadir vector is measured and a Doppler value of the target is determined using the sensor platform velocity, V P  and the squint angle, θ S . 
         [0014]    A GPS sensor  112  and a high precision angular resolver  114  are also disposed on the sensor platform  106 . The angular resolver measures the angle between the nadir axis  110 , and the range to target vector  108 . 
         [0015]    Referring to  FIG. 2A , the LADAR sensor,  102 , is required to accommodate a very large range of target signal Doppler offsets,  220 , and minimize the amount of intermediate frequency (IF) bandwidth,  224 , that must be acquired and processed. For the technique described herein, a linear frequency modulation (FM) chirp signal or LO waveform  202  may be represented by a set of digital samples,  204 , that spans the entire range of target Doppler frequency offsets,  220 . Given a target echo  206 , a desired LO waveform  208  is determined. The desired LO waveform  208  has a corresponding subset of digital samples  210  and is synthesized by clocking the digital samples  210  through a high speed digital to analog converter. 
         [0016]    The LO waveform  202  has a chirp slope of t, which is the change in frequency, Δ F , per unit of time, Δ T . A Doppler estimate of the target echo, f DE , is used to determine the subset of digital samples  210  corresponding to the desired LO waveform  208  starting at a time, two. The time, t WO  is the Doppler estimate, f DE , divided by the chirp slope, μ. A LO waveform bandwidth, BW LO  is also used to determine the subset of digital samples  210  of the desired LO waveform  208 . The BW LO  is controlled by a number of waveform samples clocked from memory to a digital to analog converter (DAC). 
         [0017]    Referring to  FIGS. 2B and 2C , a target signal  216   a  is a received optical signal plus a blue Doppler shift and has a potential Doppler offset  220 . The signal  216   a  has a desired optical LO waveform  218   a . A target signal  216   b  is a received optical signal less a red Doppler shift and has a potential Doppler offset  220 . The signal  216   b  has a desired LO waveform  218   b . As will be shown herein, knowledge of the sensor platform velocity V P  and the squint angle, θ S , enables a selection of the subset of digital samples  210   a  corresponding to the desired LO waveform  218   a  in order to generate the proper optical heterodyne to generate a radio frequency signal  230   a  inside a receiver radio frequency bandwidth  224  and enables selection of the subset of digital samples  210   b  corresponding to the desired LO waveform  218   b  in order to generate the proper optical heterodyne to generate a radio frequency signal  230   b  inside the receiver radio frequency bandwidth  224 . 
         [0018]    Referring to  FIG. 3 , an example of a process to determine appropriate digital waveforms samples is a process  300 . Process  300  determines an estimate of a position of a sensor ( 302 ). For example, an estimate of the position of the sensor  102 , L SE  is determined. 
         [0019]    For example, the GPS receiver  112  is used to determine an estimate of the position of the sensor  102 , L SE . An estimate of the position, L SE , is determined since the sensor  102  is traveling on the sensor platform  106 , and an exact position of the sensor L S  is not known. 
         [0020]    Process  300  determines a location of the target, L T  ( 304 ). For example, the sensor  102  determines the position of the target  104 , L T . 
         [0021]    Process  300  determines an estimate of the range to target, R TE  ( 306 ). For example, the estimate of the range to target, R TE , is the difference between the estimate of the location of the sensor, L SE , and the location of the target, L T . 
         [0022]    Process  300  converts the estimate of the range to target, R TE , to an estimate of the time to target, t RE  ( 308 ). For example, the t RE  is equal to two times the R TE  divided by the speed of light. 
         [0023]    Process  300  determines an estimate of the velocity of the sensor platform, V PE  ( 310 ). For example, the GPS receiver  112  is used to determine an estimate of the velocity of the platform  106 , L SE . 
         [0024]    Process  300  determines an estimate of a squint angle, θ SE  ( 312 ). For example, the squint angle, θ SE , is determined based on the estimate of the location of the sensor, L SE . 
         [0025]    Process  300  determines an estimate of the Doppler value, f DE  ( 314 ). For example, the estimate of the Doppler value, f DE , is determined from: 
         [0000]        f   DE =(2 V   PE /λ L )(cos θ SE ),
 
         [0000]    where λ L  is the laser wavelength of the LADAR sensor  102 . 
         [0026]    Process  300  determines the digital samples of the LO waveform to use to form optical heterodyne ( 316 ). For example, the digital samples of the LO waveform to use is based on the estimate of the Doppler value, f DE  and the LO waveform bandwidth, BW LO . Process  300  generates a signal with in a bandwidth of a receiver using the optical heterodyne ( 318 ). 
         [0027]    Referring to  FIG. 4 , a computer  400  includes a processor  402 , a volatile memory  404 , a non-volatile memory  406  and a user interface (UI)  408  (e.g., a mouse, a keyboard, a display, a touch screen and so forth). The non-volatile memory  406  stores computer instructions  412 , an operating system  416  and data  418  (e.g., digital samples of LO waveform  204 ). In one example, the computer instructions  412  are executed by the processor  402  out of volatile memory  404  to perform all or part of the processes described herein (e.g., the process  300 ). 
         [0028]    The processes described herein (e.g., the process  300 ) are not limited to use with the hardware and software of  FIG. 4 ; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a non-transitory machine-readable medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information. 
         [0029]    The system may be implemented, at least in part, via a computer program product, (e.g., in a non-transitory machine-readable storage medium), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se. 
         [0030]    The processes described herein are not limited to the specific examples described. For example, the process  300  is not limited to the specific processing order of  FIG. 3 . Rather, any of the processing blocks of  FIG. 3  may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
         [0031]    The processing blocks in  FIG. 3  associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as special purpose logic circuitry (e.g., an FPGA (field programmable gate array) and/or an ASIC (application-specific integrated circuit)). Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.