Abstract:
An apparatus and method of dynamically optimizing an acquisition range of a radar system, including modulating a CW radar signal with a PN code that has an adjustable code frequency of modulation and an adjustable chip length. The method includes transmitting the modulated radar signal and receiving a return radar signal, based on the transmitted radar signal. The method continuously measures a SNR of the received return radar signal, and adaptively tunes the adjustable code frequency of modulation, and adaptively tunes the adjustable chip length, based on the continuously measured SNR. In this manner, the acquisition range of the radar system is optimized.

Description:
TECHNICAL FIELD 
     This invention relates, in general, to a radar system and, more specifically, to a ladar (laser radar) system having an adjustable, pseudo-noise (PN) coded transmitted signal that is adaptively tuneable to optimize target acquisition. 
     BACKGROUND OF THE INVENTION 
     Laser radar, optical radar and ladar are all names used for radar systems utilizing electromagnetic radiation at optical frequencies. The radiation used by a ladar system is at wavelengths which are 10,000 to 100,000 times shorter than that used by a microwave radar system. Radiation scattered by the target of interest is collected and processed to yield information about the target and range to the target. Early conventional radar and ladar systems observed the intensity of the collected radiation and the time delay from transmission to collection. 
     Ladar systems may be classified as continuous-wave (CW) or pulsed, as well as focused or collimated. CW ladar systems are generally used when the signal may be integrated over long time periods and/or when the target of interest is nearby. Focusing is mainly performed using CW ladar systems to permit them to make a more sensitive measurement over a smaller span of ranges. In contrast, pulsed ladar systems use much higher peak power levels during the laser pulse than can be maintained with a CW laser, producing higher signal-to-noise ratios for the collected radiation. Pulsed ladar systems are usually chosen for long-range remote sensing and when signal integration over a long time period is impractical. 
     A ladar system transmits light to a target of interest, the transmitted light interacts with and is changed by the target, and some of this light is reflected/scattered back (returned) to the ladar system where it can be analyzed. For example, the round trip time required for the light to travel to the target of interest and back to the ladar system is used to determine the range to the target. 
     As known in the art, CW radar systems have been used to acquire a target and measure range to the target. One such system, disclosed in U.S. Pat. No. 5,999,119, issued to Carnes et al. on Dec. 7, 1999, includes changing the phase of a transmitted CW signal by 180 degrees, in accordance with a binary, PN code. A code of N bits, for example, is stored in a re-circulating shift register memory. The bits are read out from memory at a predetermined rate, f s , and fed to a bi-phase modulator along with the CW signal. The phase of the CW signal changes 180 degrees in response to a logic 1 bit and remains unchanged in response to a logic 0 bit. The coded CW signal is transmitted through a transmit antenna. Radar returns from a target are received by a receive antenna. The received CW radar returns have the same code as the transmitted code; however, the received code pattern is time delayed with respect to the transmitted code, by an amount related to the target range. As the target radar returns are correlated with different time delayed replicas of the PN code, target range may be determined. 
     Another radar system, disclosed in U.S. Pat. No. 6,236,352, issued to Walmsley on May 22, 2001, includes an intermediate frequency (IF) signal used to modulate and demodulate transmitted and received signals, respectively. The IF signal is modulated by a PN coded sequence that has a predetermined number of digital bits. The patent discloses various embodiments, each having a PN coded sequence of different number of bits in length. For example, a PN sequence of one embodiment has 12-bits in length. Another embodiment has 32-bits in length, and yet another has 8-bits in length. The patent does not describe an embodiment having an adaptively tuneable PN sequence, in which the bit length may be adjustable to optimize a received signal to noise ratio (SNR) or acquisition range to a target. 
     This invention addresses a radar system and a ladar system each including an adjustable, PN coded transmitted signal that is adaptively tuneable to optimize the received SNR, target acquisition range and resolution. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a radar system for acquiring a target including a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length. Also included are a transmitter for transmitting a continuous wave (CW) radar signal modulated in accordance with the programmable PN code, a receiver adapted to receive a radar return based on the transmitted radar signal, and a processor for processing the received radar return and acquiring the target. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize target acquisition. 
     In an embodiment of the invention, the transmitter includes a laser diode for transmitting a light signal, the light signal modulated in accordance with the programmable PN code, and the receiver includes optics for receiving the light signal. 
     In one embodiment, the chip length of the programmable PN code varies from a 32 bit sequence to a 1024 bit sequence, and the code modulation frequency varies from 1 MHz to 1024 MHz. 
     The processor, during a first time period, provides the first tuning control signal to the waveform generator for adjusting the code modulation frequency, and during a second time period, provides the second tuning control signal to the waveform generator for adjusting the chip length, whereby the first time period occurs prior to the second time period. 
     In another embodiment, the invention includes a ladar system for acquiring an image of a target. The ladar system has a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length, an optical transmitter for transmitting a continuous wave (CW) laser signal modulated in accordance with the programmable PN code, an optical receiver adapted to receive an image of the target based on the transmitted laser signal, and a processor for processing the received image of the target. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize the received image of the target. 
     The invention also includes a method of dynamically optimizing an acquisition range of a radar system. The method modulates a CW radar signal with a PN code that has an adjustable code frequency of modulation and an adjustable chip length. The method also includes transmitting the modulated radar signal and receiving a return radar signal based on the transmitted radar signal. The method continuously measures a SNR of the received return radar signal and adaptively tunes the adjustable code frequency of modulation, and adaptively tunes the adjustable chip length, based on the continuously measured SNR. In this manner, the acquisition range of the radar system is optimized. 
     The method further includes determining if the code frequency of modulation is at a minimum value, and increasing the chip length, if the code frequency of modulation is at a minimum value. 
     The method further determines if the code frequency of modulation is at a maximum value, and decreases the chip length, if the code frequency of modulation is at a maximum value. 
     In yet another embodiment, the invention includes a ladar system for acquiring an image of a target. The ladar system has a waveform generator for generating a programmable pseudo-noise (PN) code of a variable code modulation frequency and a variable chip length, an optical transmitter for transmitting a continuous wave (CW) laser signal modulated in accordance with the programmable PN code, an optical receiver adapted to receive an image of the target based on the transmitted laser signal, and a processor for processing the received image of the target. The optical receiver includes a silicon detector array for dividing the image into a number of pixels. The received signal from each pixel is processed by autocorrelation of the received PN code to determine a time delay of the signal at a respective pixel. From the time delay, the distance to the corresponding point on the target is calculated. This pixel by pixel distance information is then used to derive the depth dimension of the target allowing improved target recognition. The processor includes a first tuning control signal provided to the waveform generator for adjusting the code modulation frequency, and a second tuning control signal provided to the waveform generator for adjusting the chip length, whereby the waveform generator adjusts the code modulation frequency and the chip length, in response to the first and second tuning control signals, to optimize the received image of the target. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
     FIG. 1 is a block diagram of a ladar system in accordance with an embodiment of the invention; 
     FIG. 2 is a block diagram of a master oscillator/PN waveform generator of the ladar system shown in FIG. 1, in accordance with an embodiment of the invention; 
     FIG. 3 is a flow diagram of a method executed by a radar system and/or the ladar system shown in FIG. 1, in accordance with an embodiment of the invention; 
     FIG. 4 is a flow diagram of a method of averaging SNR of bits received by a radar system and/or the ladar system shown in FIG. 1, in accordance with an embodiment of the invention; 
     FIG. 5 is a flow diagram of a method of computing a threshold dead band in a radar system and/or the ladar system shown in FIG. 1, in accordance with an embodiment of the invention; and 
     FIG. 6 is a block diagram of a receiver portion of a ladar system for obtaining a three dimensional (3-D) image of a target, in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A ladar system according to the present invention advantageously tunes the code modulation frequency and chip length of an agile PN coded transmitted signal, so that the received signal may be optimized for target acquisition over a wide dynamic range. The agile PN code balances acquisition range, range resolution and update time in a single system. 
     A ladar system (or a radar system) using this agile PN code may acquire a fast moving target at a distant range, as well as a slow moving target at a close range. For example, two outer space missions representing extreme differences in closing velocities and required ranges of operation may use the same ladar system (or radar system). One such mission, for example, may be an interplanetary asteroid flyby mission and the other mission, for example, may be a docking maneuver. These two missions represent extreme differences in closing velocities and required ranges of operation. The asteroid flyby mission may need to acquire the asteroid at hundreds of kilometers and may get no closer than thousands of meters. The closing velocity may be in the range of several kilometers per second. On the other extreme, the docking mission may not need to acquire a target (space station) much beyond thousands of meters in a low closing velocity of several centimeters per second. By advantageously tuning the modulation frequency and chip length of the agile PN coded transmitted signal, in accordance with the present invention, the ladar system (or radar system) may satisfy these mission extremes with a single system. 
     A ladar system, generally designated as  10 , is shown in FIG.  1 . As will be explained, the ladar system includes an agile PN code that may be programmable by tuning the code modulation frequency and code word size (or chip length). The programming, or tuning, may be executed in real time based on actual target conditions. 
     As shown, ladar system  10  includes transmitter  12  and receiver  22 . The transmitter illuminates a target under investigation with a programmable PN bit pattern. Laser light, used as the frequency carrier, is digitally modulated with coded bit patterns of variable length. Energy returned from the target is collected by the receiver, detected and processed to obtain range. Display  32  may be used to display range to the target. 
     As shown, transmitter  12  includes master oscillator/PN waveform generator  14 , power amplifier  16 , launcher  18  and optical lens  20 . Receiver  22  includes optical lens  28 , amplifier  26 , detector/correlator  24  and signal processor  30 . In transmitter  12 , master oscillator/PN waveform generator  14  may include a continuous wave (CW) laser operating at a 1.55 micron wavelength, which is eye-safe. The laser source may be a distributed feedback (DFB) laser diode that is modulated by a programmable PN code generator. 
     As best shown in FIG. 2, programmable PN code generator  40  is coupled to modulator  42 . The binary modulator, which may be an on/off modulator or a bi-phase modulator, receives a PN word or chip, corresponding to a pseudo-random sequence of ones and zeros from PN generator  40 . In general, the pseudo-random sequence may be generated by digital techniques to produce a coded word. The coded word may be produced repetitively, without hiatus, between the end of one word and the beginning of the next word. 
     Carrier source  41 , which may be a CW laser signal operating at an optical frequency (or a CW radar signal operating at a microwave frequency), is modulated by modulator  42  to produce the agile PN coded transmitted signal. The bits of the PN word are transmitted at a bit rate of f m , the code modulation frequency, or the rate at which the phase of the transmitted signal is bi-phase or binary encoded. 
     In accordance with an embodiment of the invention, PN generator  40  may receive control signals from processor  30  of receiver  22  (FIG.  1 ). The control signals may include two separate tuning commands to adjust the code modulation frequency, f m , and the chip length, or the number of bits in a PN word (FIG.  2 ). 
     Continuing the description of FIG. 1, power amplifier  16  may be a coherent array of optical fiber amplifiers, collectively forming a fiber optic power amplifier. The optical fiber may be an optical fiber having a core doped with one of more rare earth elements, such as an erbium doped fiber amplifier (EDFA) which amplifies the laser energy. The output of the EDFA may be coupled to transmitter optics (launcher  18  and lens  20 ) that produces the desired far field energy distribution. 
     The transmitter optics may be an optical lens (as shown), although a mirror, or an array of fibers in a phased array may also be used. The transmitter optics collimates the high power signal from the fiber optic power amplifier and directs the collimated transmit laser beam towards a distant target. The transmitter optics may be gimbal mounted, so as to permit the transmitted laser beam to sweep a target in accordance with a predetermined sweep pattern. Coverage of the target may occur several ways. For example, a fan beam, spot beam or flood beam may be used. 
     The receiver optics may be the same optics as the transmitter optics. Receive lens  28  may collect back scattered light from a target and couple the light into receive amplifier  26 , which may be a single mode diode pumped fiber optic preamplifier. For ease of explanation, the transmitter optics and the receiver optics are shown separately in FIG.  1 . The function of receive amplifier  26  is to amplify the received signal to a higher, more convenient optical signal level and to enhance the signal to noise ratio (SNR) of the receiver. 
     Detector/correlator  24  may include a photo-detector, such as an avalanche photo diode (APD) operating in the linear or Geiger mode. Two dimensional (2D) format arrays may be used consisting of 256×256 elements, for example. An APD 2D array may provide moderate gain at modest voltage levels and may have low noise. This structure may have an added benefit of not having sharp edges or corners that produce high potential fields resulting in failure from electrical breakdown. Each pixel of the array may further be processed by a push broom or starring system technique for obtaining a three dimensional (3D) image. 
     The detector/correlator, as shown, receives the output of the digital PN code, which is generated by master oscillator/PN waveform generator  14 , and the received optical signal, which includes a delayed version of the same digital PN code. The detector finds the product of these two signals. If the two PN codes arrive at the detector in unison, the output level is high. If the PN code sequence does not arrive at the detector input in synchronism, the magnitude of the detector output is low. 
     An auto correlation of the PN code sequence may be generated by delaying the PN code received from the waveform generator with respect to the PN coded waveform received from the target, then multiplying and integrating the two PN codes. When the two PN codes are aligned in time (or range), i.e. bit  1  with bit  1 , bit  2  with bit  2 , etc., throughout the code, the integrator output is at a maximum value. When the integrator output is at a maximum value, a conclusion may be drawn that the time required for the ladar signal to transverse the distance to a target and return is equal to the delay time in the delay unit. Range to the target may be calculated from the delay time. 
     Continuing the description of FIG. 1, processor  30 , which may include the auto correlation function described above, provides control signals to master oscillator/PN waveform generator  14 . These control signals, as shown in FIG. 2, are adjustment of the code modulation frequency, f m , and the code word length (or chip length). As will be explained, processor  30  executes an algorithm that measures the SNR of the received signal, compares the SNR to a threshold value, and provides adjustment controls to the transmitter for adaptively tuning the code modulation frequency and the chip length produced by the PN waveform generator. 
     Referring now to FIG. 3, there is shown a method of the invention, generally designated as  50 , which may be executed in processor  30 , in accordance with an embodiment of the invention. Starting at step  51 , the method measures the SNR of the received signal. The SNR is determined by measuring the SNR per bit (or pulse), and then integrating the measured SNR per bit over the chip length. This is the chip SNR. A predetermined number of chip SNR measurements (for example 10 chips) are then averaged to obtain the averaged SNR over the predetermined number of consecutive chips. 
     The signal per bit (S B ) may be obtained by using the following equation:          S   B     =     E   ·     P   π     ·     η   ch     ·     Ω   IFOV     ·     A   opt     ·     T   opt     ·     η   d     ·     λ   hc     ·     1     2   ·     f   m                                  
     Where: 
     S B =Signal per Bit 
     E=Irradiance on surface        E   =     P   ·     η   ch     ·     1       Ω   T     ·     R   2                                  
     P=Transmitter output laser power 
     η ch —channel transmission 
     Ω T —Transmitted beam width [sr] 
     R—Range from ladar system to target or object 
     ρ—object reflectivity 
     Ω IFOV —IFOV [sr]=Ad/EFL 2    
     A opt —Area of the receiver aperture 
     T opt —Receiver optical transmission 
     η d —APD gain, QE, and fill factor 
     f m —PN code modulation frequency 
     From the above equation, it may be seen that the signal is inversely proportional to the code modulation frequency, f m . 
     The noise (N) is given by the following equation:        Noise   =       (       S   B     +     id   ·     1   q     ·     1     f   m           )                              
     where: 
     id=detector dark current 
     q=1.6·10 −19  coulombs/electron (elementary charge) and the SNR, per bit, is:        SNR   =       [         (     N     S   B       )     2     +     1     S   B       +     1     M   speckle         ]         -   1     /   2                              
     To avoid oscillation due to measurement dynamics, the SNR per bit may be integrated over the chip length and the average SNR may be calculated over the chip length. This is the chip SNR. Speckle in the above equation may be ignored. 
     For a 1 MHz modulation frequency and a 1024 bit chip length, the integration time for computing one chip SNR is 1 msec. In an embodiment of the invention, 1 MHz modulation frequency is the slowest code modulation frequency and 1024 bit chip length is the longest chip length. These two values likely represent an extreme condition in which acquisition range may be at a maximum. 
     A predetermined number of consecutive chip SNR measurements may be averaged (for example, 10 consecutive chip SNR measurements) and the result may be compared to a threshold value in step  52 . It will be appreciated that other number of consecutive chip SNR measurements may be used for obtaining the average value. 
     A method of measuring SNR in accordance with an embodiment of the invention is illustrated in FIG. 4, and is generally designated as  70 . As shown, the method receives the PN coded bits detected in the receiver (steps  71  and  72 ). The SNR per bit is measured (step  73 ). The method next determines whether all the bits in the PN coded word (chip) have been received and measured (decision box  74 ). If all the bits in the word (chip) have not been received and measured, the method branches to step  72  and continues measuring the SNR of each bit. After all the bits in the word (adjustable size) have been received, the method branches to step  75  and measures the chip SNR. 
     The method next enters decision box  76  and determines whether a predetermined number of chips have been received (the number of chips may be adjusted in the method). If the predetermined number of chips have not been received, the method branches to step  72  and continues calculating each consecutive chip SNR. After the predetermined number of chip SNRs have been calculated, the method branches to step  77  and calculates the average SNR for the predetermined number of chips. 
     Returning to the description of FIG. 3, after calculating the average SNR over a consecutive number of chips (step  51 ), the method enters step  52  and compares the average SNR to a threshold value. The manner in which the threshold value is calculated is explained below. 
     The variance of the predetermined number of consecutive chip SNRs is σ 2  (SNR). The σ (SNR), which is the square root of the variance, is used to compute a threshold dead band (a range between T −  and T + ). 
     The threshold may be set such that for each SNR event (single bit) that exceeds the threshold level, a trigger is recorded and the event is said to have occurred. For SNR events below threshold, no trigger is recorded and the event is said to be absent. 
     The SNR and threshold levels determine the probability of detection (Pd). The SNR may be measured and the threshold may be set by the method or algorithm. The equation relating these parameters is as follows:        Pd   =       1   2     ·     [     1   +     erf        [       (     SNR   -   TNR     )     2     ]         ]                              
     The threshold to noise ratio (TNR) may be a function of laser pulse width (one bit width, Δt) and false alarm rate (FAR). The FAR may be set according to the user&#39;s needs. The TNR may be defined as follows: 
     
       
           TNR   ={square root over (−2·1 n ( 2·{square root}{square root over (3)})}     ·Δt·FAR ) 
       
     
     where: 
     
       
           T   −   =TNR −σ( SNR ), and 
       
     
     
       
           T   +   =TNR +σ( SNR ) 
       
     
     In an embodiment of the invention, when the SNR is within ±σ of the threshold (the dead band), no adjustment to code modulation frequency or chip length occurs. 
     Referring to FIG. 5, there is shown a method, generally designated as  90 , for computing the threshold dead band. After calculating the average SNR over a predetermined number of consecutive chips (step  77  in FIG.  4 ), the method computes σ(SNR), which is the square root of the variance (step  91 ). Step  92  computes TNR based on the false alarm rate (which may be adjustable) and the bit pulse width (Δt), which is a function of the code modulation frequency and the chip length (both adjustable). Step  93  then computes the threshold dead band based on the equations given above. 
     Returning to the description of FIG. 3, decision box  52  compares the SNR to the threshold (T −  and T + ). So long as the SNR is within ±σ of the threshold, no adjustment is made to the code modulation frequency or chip length. 
     If the SNR is less than T − , the method branches to decision box  53  and determines if the code modulation frequency (f m ) is at a minimum value. If it is not at a minimum value, the method branches to step  54  and reduces f m . After reducing f m , the method returns to step  51  to calculate the average SNR over the next predetermined number of chips (with the reduced f m  value). If decision box  53  determines that f m  is at a minimum value, the method branches to decision box  57 . 
     If the chip length is at a maximum value, the method branches to step  61  and may alert the operator of an extreme operating condition (f m  at a minimum and chip length at a maximum). The method then branches back to step  51 . If the chip length is not at a maximum value, however, the method branches to step  58  and increases the chip length. After increasing the chip length, the method returns to step  51  and computes a new SNR value. 
     Returning to decision box  52 , if the SNR value is greater than T + , the method branches to decision box  55  and determines if the code modulation frequency (f m ) is at a maximum value. If it is not at a maximum value, the method branches to step  59  and increases f m . After increasing f m , the method returns to step  51  to calculate the average SNR value over the next predetermined number of chips (with the increased f m  value). If decision box  55  determines that f m  is at a maximum value, the method branches to decision box  56 . 
     If the chip length is at a minimum value, the method branches to step  61  and may alert the operator of an extreme operating condition (f m  at a minimum) and may then branch back to step  51 . If the chip length is not at a minimum value, however, the method branches to step  60  and decreases the chip length. After decreasing the chip length, the method returns to step  51  and computes a new SNR value. 
     In an embodiment of the invention, the upper f m  limit may be set to a value of 1 GHz (1024 MHz) and the lower f m  limit may be set to a value of 1.0 MHz. It will be appreciated that these limits are not absolute limits and other f m  limits may be used. An initial condition of 512 MHz (which roughly corresponds to one foot range resolution, ignoring the benefit of processing the PN code chip length) may be initially used in the method. 
     If the average of the consecutive chip SNR value is greater than T +  then the code modulation frequency may be adjusted to the following frequency:          f   m     i   +   1       =       f   m   ′     +              f   m   ′     -     f   m     i   -   1         2                                   
     If the average of the consecutive chip SNR value is less than T −  then the frequency may be adjusted to the following frequency:          f   m     t   +   1       =       f   m   ′     -              f   m   ′     -     f   m     ′   -   1         2                                   
     It will be appreciated that f m   i  is the current f m  value, f m   i−1  is the previous f m  value, and f m   i+1  is the next f m  value. 
     As shown in FIG. 3, modulation frequency is adjusted first. Once the frequency reaches a minimum value (1 MHz) or a maximum value (1024 MHz), then the chip length is adjusted. The initial chip length may be set to a 32 bit chip length. 
     The equation governing range resolution is given by:          Δ                 R     =       α       f   m            N   bits                e       -   β     ·   SNR                                
     The adjustment to the chip length (number of bits) may be the same as the moving average adjustment of frequency, described above. 
     The inventor has discovered that it is preferable to adjust the frequency of modulation first, and then next the chip length. The reason the frequency is preferably adjusted first is because the frequency adjustment may act as a coarse adjustment and the chip length adjustment may act as a fine adjustment. 
     The table below shows combinations of extreme conditions and a description of conditions that may lead to these extreme combinations. The SNR may continue to be measured and adjustments may be made, in accordance with the method shown in FIG. 3, when warranted. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Description 
                 f m   
                 Chip length 
               
               
                   
                   
               
             
             
               
                   
                 Low power, cooperating target 
                 Min 
                 Min 
               
               
                   
                 Maximum acquisition range 
                 Min 
                 Max 
               
               
                   
                 High velocity, fast response 
                 Max 
                 Min 
               
               
                   
                 Highest range resolution 
                 Max 
                 Max 
               
               
                   
                   
               
             
          
         
       
     
     The method of the invention advantageously balances the probability of detecting the laser pulse (Pd), the signal-to-noise ratio (SNR) of a pulse, and the false alarm rate (FAR) of the system. In addition, the processing is tolerant of dynamic SNR due to temporal changes in surface reflectivity properties (glint, fade, drop out) resulting from viewing angle dynamics and changing range. 
     Referring last to FIG. 6, there is shown a block diagram of a receiver of a ladar system, generally designated  100 . As shown, receiver  100  includes optics  101 , focal plane detector array (FPA)  102 , read-out integrated circuit (ROIC)  103 , frame store  104 , processor/correlator  105  and 3-D image file  106 . FPA  102  includes an array of pixels, for example, 256×256 pixels, and ROIC  103  includes a unit cell for each corresponding pixel of FPA  102  for reading the image signal sensed by its corresponding pixel. The image signals sensed by FPA  102  are digitized and stored as frames in frame store  104 . The image signal sensed by each pixel includes the received PN code. This received PN code is auto correlated with the PN code modulation in processor/correlator  105  to obtain range (or time delay) between a point on the target and a respective pixel. By measuring relative time delays between the received signal of one pixel and the received signal of another pixel, depth dimension of the target may be obtained. The depth dimension of the target corresponding to each pixel in the array may be stored in 3-D image file  106 . 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.