Abstract:
A multi-function, range-Doppler, synthetic aperture and micro-Doppler, coherent laser radar system having improved spatial resolution and immunity to undesired platform motion utilizing two or more simultaneous, spatially offset transceiver apertures.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     Remote sensing coherent ladar (laser radar) imaging systems can provide images at ranges far beyond the useful range of a traditional diffraction limited imaging systems. Coherent ladar relies on phase sensitive measurements to resolve an object and provide information about its motion. To overcome the diffraction limitations of a finite aperture system, synthetic aperture ladar (SAL) systems use multiple spatial samples from a single small aperture in motion to synthesize a larger effective aperture. This results in higher spatial resolution, but only in the along-track dimension (i.e., in the direction of flight) in which the samples were collected. Conventional SAL imaging has been demonstrated both in the laboratory as well as in flight, but is sensitive to undesired platform motions. 
     Apertures separated in the cross-track dimension (i.e., the direction perpendicular to both the range and direction of flight) can be used to implement support interferometric SAL (IFSAL). IFSAL can provide enhanced cross-track resolution and can be implemented in the along-track dimension to provide differential SAL (DSAL) that is substantially insensitive to platform motion. 
     Interferometric synthetic aperture radar (IFSAR) is a frequently used technique developed to realize enhanced resolution in the cross-track, or vertical, dimension. IFSAR utilizes the phase between each resolution cell of two synthetic aperture images collected from two closely spaced apertures to make an estimate of the cross-track position, or height, of an object. Implementing IFSAR techniques at optical wavelengths is extremely challenging and can be impacted by phase variations from a variety of sources, including variations between the two apertures, target and atmospheric de-correlation, and target motion. 
     Synthetic aperture ladar is extremely sensitive to uncompensated motion due to the short wavelength of the transmit laser. Along-track Differential SAL (DSAL) has been proposed as a way to produce SAL images that are immune to both platform translation and vibration. A CDMA multi-aperture ladar can implement DSAL using a single transmit and receive channel and slightly modified image formation processing. In DSAL, two apertures are separated in the along-track direction. 
     Range-Doppler and Micro-Doppler imaging detects and characterizes the macro-translational velocity and micro-velocity of a target. Use of multiple transceiver apertures can help to isolate and characterize these motions. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus and method for multiple aperture coherent ladar using periodic pseudo noise (PPN) waveforms and code division multiple access (CDMA) transmission and reception. Improved spatial resolution and immunity to undesired platform motion utilizing two or more simultaneous, spatially offset transceiver apertures is provided. In addition, implementation of the apertures with minimal increase in transmitter or processing complexity can be achieved. The invention will support multiple types of imaging systems, including synthetic aperture imaging, interferometric synthetic aperture imaging, differential synthetic aperture imaging, range-Doppler imaging, and micro-Doppler imaging. 
     A multi-function, synthetic aperture coherent ladar system having improved spatial resolution and immunity to undesired platform motion utilizing two or more simultaneous, spatially offset transceiver apertures is described. 
     The use of CDMA transmission and reception to enable multiple apertures coherent laser radar can provide a coherent ladar having multiple, simultaneous apertures. The phase of each aperture of the multiple aperture ladar system can be detected, monitored, and related to the phase of the other apertures within the multiple aperture ladar system. Phase, consequently, can be measured to a fraction of a wavelength between each aperture transmit/receive combination. Cross-track height estimation in synthetic aperture ladar can also be accomplished. When the ladar apertures are oriented in the cross-track dimension, the multi-aperture phase retrieval required for interferometric synthetic aperture ladar height estimate can be provided. 
     The present invention can also provide synthetic aperture imaging, which is substantially immune to undesired radial platform motions. When the ladar apertures are oriented in the along-track dimension, the invention enables the multi-aperture phase retrieval required for differential synthetic aperture ladar (DSAL). Multiple apertures within the along-track dimension reduce conventional synthetic aperture imaging times, or baseline motion, when the multiple apertures are oriented in the along-track dimension. System complexity can be reduced and can be implemented with relatively simple robust modulators. Implementation can be accomplished with multiple transmitters or using an optical delay for code generation. 
     The use of an optical delay to transmit, detect, and process PPN waveforms in CDMA ladar can reduce the complexity of multi-aperture coherent ladar hardware. A single laser and modulator can be used to produce orthogonal signals transmitted from each aperture. Phase measurement accuracy can be improved since exact waveform copies are transmitted with delay from each aperture. Also improved is the accuracy of phase measurement since all signals received from all transmit apertures are processed on a single receiver. The use of CDMA reduces hardware complexity since a single receiver channel is used to detect the signal from multiple apertures. The use of CDMA reduces the signal processing load since a single matched filter operation produces the output for all transmit paths received by an aperture. 
     The use of continuous wave PPN waveforms in coherent ladar provides a high duty cycle waveform and increases performance by pseudo noise waveform. Because of its binary phase, a PPN waveform is a high time bandwidth waveform enabling multiple imaging modes by simply changing post processing algorithms. The waveform allows adaptable Doppler sensitivity by selectively processing different sub-code lengths. The PPN waveform also allows adaptable energy usage by selectively processing long waveform sequences at long ranges to increase the energy per measurement and short sequences at short range to increase imaging rates. This allows high resolution imaging to be maintained over a wide range of operating conditions as well as increasing areas rates at shorter ranges. 
     According to one aspect of the present invention, there is provided a multi-function, synthetic aperture coherent laser radar system, having improved spatial resolution and immunity to undesired platform motion, to provide an image of a target. The laser radar system includes a first transceiver aperture and a second transceiver aperture, spatially offset from the first transceiver aperture. A signal generator is adapted to generate a multiple sub-code PPN waveform. The signal generator can be coupled to the first transceiver. An orthogonalization device can be coupled to the signal generator and to the first transceiver. The orthogonalization device provides an orthogonal PPN waveform orthogonal to the generated PPN waveform. A demodulation device, coupled to at least one of the first and second transceiver apertures, demodulates reflected PPN waveforms received from the target. A signal processing unit, coupled to the demodulation device, provides the image of the target. 
     According to another aspect of the present invention there is provided a method of improving the spatial resolution and immunity to undesired platform motion in a synthetic aperture coherent ladar system. The method includes the steps of generating a first PPN waveform having multiple sub-codes, generating an orthogonal PPN waveform orthogonal to the first PPN waveform, transmitting the first PPN waveform and the orthogonal PPN waveform, respectively, through a first and second aperture to a target, receiving a composite waveform reflected from the target, and demodulating the composite waveform to determine a phase history with spectra indicative of target characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conceptual diagram of use of CDMA transmission and reception via optical delay to enable multiple apertures coherent ladar. 
         FIGS. 2A and 2B  respectively illustrate cross-track and along-track interferometric synthetic aperture ladar. 
         FIG. 3  illustrates a block diagram of one embodiment of a multiple aperture coherent ladar system of the present invention. 
         FIG. 4  illustrates a block diagram of another embodiment of a multiple coherent ladar system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Each of the above described systems of coherent ladar require detailed and accurate measurements of the phase of the individual return signals as well as the phase between the signals being received by the apertures. The present invention includes a method and apparatus to measure the required return phase of signals and the phase between the signals returned by respective apertures using PPN waveforms in a CDMA architecture. The technique uses a multiple sub-code PPN waveform, as shown in Table 1, to phase two or more physical apertures and allows multi-input, multi-output (MIMO) operation. 
     
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Composite binary phase code consisting of C Ncode  sequential 
               
               
                 orthogonal sub-codes. 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
       FIG. 1  illustrates one conceptual example of a CDMA implementation in a multiple aperture ladar system  100 . By introducing a one sub-code delay between the apertures, orthogonal sub-codes may be transmitted and received from each of the apertures simultaneously, and a multiple-input, multiple-output synthetic aperture ladar based on periodic pseudo noise waveforms with code division multiple access can be achieved. This CDMA implementation allows each receiving aperture to detect and process the codes from all of the transmitting apertures. Because a single transmit waveform and receiver are used, the relative phase between the apertures can be measured and monitored. 
     To implement the multiple aperture system, the orthogonality of the PPN sub-codes is exploited. As shown if  FIG. 1 , an outgoing composite code is divided into a first leg  102  and a second leg  104 . A Transmit/Receive Switch  106  receives a generated signal (to be described later in more detail), such as signal  108 , and transmits the signal to a 50/50 fiber splitter  110 . The split signal is transmitted to the second leg  104 , which is an un-delayed leg and to an aperture U. The first leg  102  receives the same signal, but the signal is delayed by one sub-code length via a fiber delay line  112 . Signal  114  is transmitted along the first leg  102  to an aperture D. Each signal  108  and  114  is transmitted by respective apertures U and D to a target A, where the signals  108 ,  114  are reflected and return to the apertures U and D. 
     Upon reflection, both the undelayed signal from aperture U and the delayed signal from aperture D are collected by both apertures U, D resulting in three copies of the signal as shown in Table 2. 
     
       
         
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Reflected Signals from Target A 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
     In the first path, denoted as UU, the code is transmitted and received from the undelayed aperture U and experiences no delay. In the second path, denoted UD, the code is transmitted and received from either the delayed aperture D or undelayed aperture U and experiences a one sub-code delay. Finally, in the third path, denoted DO, the code is transmitted and received from the delayed aperture D and experiences a two sub-code delay. All paths are recombined at the fiber splitter  110  and processed as a single return signal with delayed components. Applying the matched filter for the undelayed composite code to the multi-aperture return results in three delayed signals notionally shown in Table 3. 
     
       
         
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Three Delayed Signals 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
             
          
         
       
     
     Synthetic aperture imaging is a geometric imaging approach, which localizes targets as a function of range and cross range. Targets are localized in range utilizing high bandwidth waveforms for good range resolution. The cross range localization exploits the phase variation of the signal due to the controlled relative motion between the aperture phase center and the target. For simplicity, it is assumed the aperture moves in a straight line perpendicular to the target. 
     In interferometric SAL (IFSAL) imaging geometry there are two vertically offset apertures traveling perpendicularly to the target, where y A  is the range to target, d is the aperture separation, R DA  and R UA  the distances from the undelayed and delayed aperture to the target respectively, and λ is the transmitter wavelength. 
     The two cross-track apertures D, U are shown in  FIG. 2A  as traveling in direction V P  and having a vertical separation d, which provides three separate paths to the target. The measurements made from the two apertures D, U are the range and phase from each of the apertures to the target A and can be combined to estimate the height of the target z A  (not shown in  FIGS. 2A and 2B ), and is written as 
     
       
         
           
             
               z 
               A 
             
             = 
             
               
                 
                   
                     λ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       y 
                       A 
                     
                   
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     d 
                   
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       
                         
                           2 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           π 
                         
                         λ 
                       
                       ⁢ 
                       
                         R 
                         DA 
                       
                     
                     - 
                     
                       
                         
                           2 
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                           ⁢ 
                           π 
                         
                         λ 
                       
                       ⁢ 
                       
                         R 
                         UA 
                       
                     
                   
                   ) 
                 
               
               = 
               
                 
                   
                     λ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       y 
                       A 
                     
                   
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     d 
                   
                 
                 ⁢ 
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ϕ 
                   . 
                 
               
             
           
         
       
     
     As shown in  FIG. 2A , the UU and DD paths measure the round trip path from aperture U and D to the target, denoted as 2R UA  and 2R DA , respectively. The third path is similar whether beginning at the un-delayed or delayed aperture (UD or DU) and measures the sum of the path from the un-delayed aperture U to the target A and back to the delayed aperture D and is denoted as R UA +R DA . Since the measurements are all based on a single, common waveform and are processed in a single receiver, the phase relationships between each of the paths can be monitored. 
     The present invention is also equally applicable to along-track differential SAL. Differential SAL (DSAL) exploits the same PPN/CDMA transceiver construct, but with the apertures aligned in the along-track dimension. The two apertures moving along the same path are used to produce a SAL difference signal that is relatively insensitive to common aperture translation and vibration. It has been shown that the DSAL phase difference can be written as 
                 Δ   ⁢           ⁢   ϕ     =     exp   ⁡     (     j   ⁢           ⁢       2   ⁢           ⁢   π     λ     ⁢   d   ⁢           ⁢       (       x   A     -   u     )       (     y   -     y   A       )         )         ,         
where x A  and y A  are the coordinates of the target, the phase difference is proportional to the range difference R DA -R UA  between the apertures and the target, d is the aperture separation, and λ is the transmitter wavelength.  FIG. 2B  illustrates a configuration of the along-track DSAL having apertures traveling in direction V P .
 
       FIG. 3  illustrates one embodiment of the present invention in a multiple aperture ladar system  300 . A master oscillator laser  302  generates an unmodulated single frequency square wave laser signal having a predetermined frequency. The laser signal is transmitted along a signal line to a first aperture system  304  and a second aperture system  306 . The first and second aperture systems  304 ,  306  include like components but are individually adapted to provide first signal and a second signal delay by one subcode from the first as previously described. 
     The signal generated by the master oscillator laser  302  is transmitted to a first binary waveform generator  308  and a second binary waveform generator  310 . Each of the generators  308 ,  310  is respectively programmed to modulate the received signal to provide a square wave signal having predetermined pseudo-code with a specified number of chips. In one embodiment the pseudo-code could include twenty thousand chips. The first generated signal, having a code 1, and the second generated signal, having a code 2, are created to be orthogonal with respect to each other. It is also possible to generate completely random signals as long the signal at code 1 and at code 2 are orthogonal with respect to each other. 
     Each of the signals is transmitted to respective optical amplifiers  312 ,  314  to provide amplification thereof, as would be understood by one skilled in the art. The outputs of each respective amplifier  312 ,  314  are respectively coupled to first and second transmit-receive (TR) switches  316 ,  318  and to respective first and second IQ Demodulator and Detection devices  320 ,  322 . Each of the IQ demodulation and detection devices  320 ,  322  also receives the original maser oscillator signal generated by master oscillator laser  302 . The outputs of respective TR switches  316 ,  318  are also coupled to the IQ demodulation and detection devices  320 ,  322 . 
     An input/output interface  324  of TR switch  316  is coupled to aperture  326 , which produces a code 1 output beam,  327  for transmission to the target  323 . Likewise, an input/output interface  328  of TR switch  318  is coupled to aperture  330 , which produces a code 2 output beam  331  for transmission to the target  323 . The target  323  reflects the output beams  327 ,  331 , as previously described, back to each of the apertures  326 ,  330 . These reflected signals are transmitted back through respective TR switches  316 ,  318 , IQ demodulation and detection devices  320 , and to a signal processing unit  332  for processing. The IQ demodulation and detection devices  320 ,  322  mix the reflected signals with both the master oscillator laser signal and a copy of the master oscillator laser signal that is delayed by 90 degrees to produce the inphase (I) and quadrature (Q) components of the return signals. The IQ components of the transmitted, or monitor, waveforms are generated in similar fashion. These signals are then detected by photodetectors and digitized to produce the raw phase history data for subsequent processing. 
     The signal processing unit  332  includes signal processing software to analyze the phase history data. Mode specific algorithms can be applied to the phase history to produce information about the target  323  including its location, speed, or micro-motions. This approach to implementing multiple apertures via CDMA reduces the signal processing load since multiple paths are processed with a single matched filter operation. This single operation can provide phase history data sufficient for traditional range-Doppler, synthetic aperture, and micro-Doppler imaging, but also provides the multiple phase histories needed for interferometric and differential SAL. Once reduced to phase history data, standard signal processing techniques can be applied. Additional apertures can be added by adding additional systems, such as those described for apertures  304 ,  306 . 
     Key aspects of PPN waveforms in CMDA/MIMO ladar, including PPN waveform generation, optically delayed transmission and reception, CDMA multi-code compression, and multi-aperture phase retrieval, have been demonstrated using a vibrating target rather than a moving aperture. This technique is directly transferable to inverse synthetic aperture ladar (ISAL) and differential synthetic aperture ladar (DSAL) systems, as further illustrated in  FIG. 4 . 
       FIG. 4  illustrates another embodiment of the present invention ladar system  400  to generate, transmit, detect, and record long sequence PPN waveforms in a MIMO architecture using CMDA encoding. The system  400  includes a fiber delay to provide a phase difference between first and second generated signals and operates at a wavelength of 1.5 micrometers. A stable master oscillator (MO)  402  generates an unmodulated, continuous square wave, such as is available with a Koheras Adjustik laser with nominal spectral line width less than 1 kHz. The Koheras Adjustik laser is available from NKT Photonics, Morganville, N.J. The MO  402  is isolated from the remainder of the system  400  by an Optics for Research fiber isolator (OFR IOT-F-1550) (not shown). The laser output of the MO  402  is split with a 96/4 fiber splitter  404 . The high energy leg is coupled to and injected in to a phase modulator (PM)  406 . One suitable phase modulator is the PhotLine MPZ-LN-10 10 GHz phase modulator, which is available from Photline Technologies, Besancon, France. The phase modulator  406  was adjusted to condition the 1-volt binary signal from an arbitrary waveform generator (AWG)  408  to produce the V π  required to produce a phase shift in the phase modulator  406 . The output of the modulator  406  is coupled through a variable attenuator  410  and a 96/4 fiber splitter  412  to a free space transmit/receive (TR) switch  414 . A power meter  416  can be coupled to the fiber splitter  412  to monitor the signal power. The output of the TR switch  410  is then split into two legs by a 50/50 fiber splitter  418 . The first undelayed leg U is coupled to a transceiver aperture  420 , such as a telescope or, more specifically, a fiber collimator, and directed to the target  425 . The second leg D is delayed by a single sub-code length using a 200 m polarization maintaining fiber delay  422 , before coupling to an identical telescope or collimator  424  directed to the target  425 . While transceiver apertures  420 ,  424  are illustrated, it is within the scope of the present invention to use separate transmitters and receivers. 
     The length of the fiber I fiber  is 
                 l   fiber     =         cN   c     ⁢     T   c         n   fiber         ,         
where n fiber  is the index of refraction of the fiber, N c  is the number of chips in a sub-code, T c  is the chip width, and c is the speed of light. The delay line length is 200 meters for a sub-code with 1000 chips 1 nsec in duration. The delay line length is 200 m for a sub-code with 1000 chips 1 nanosecond in duration. The range resolution of the PPN waveform is proportional to the chip width and, with T c  1 nsec, is limited to approximately 0.166 m. But since the relative phase between the apertures  420 ,  424  can now be measured, the relative motion between the apertures  420 ,  424  and the target  425  can be measured to a fraction of the optical wavelength.
 
     The fiber collimators  420 ,  424  each produce a 3.4 mm output beam  419 ,  423  and can be co-aligned by contact mount to a support structure (not shown). The return signals (light) from the target  425  include signals transmitted from both apertures  420 ,  424 . Both signals  419 ,  423  are collected by both the un-delayed and delayed apertures  420 ,  424  creating the UU, DD, and UD paths described previously. The signals  419 ,  423  from the apertures  420 ,  424  are recombined and passed though the optical return path  426  of the TR switch  414  and mixed with the un-modulated fraction of the MO  402  from the fiber splitter  404  in an IQ demodulator  428  or free space quadrature mixer. The resulting phase modulated signal is detected at baseband on two matched fiber coupled 10 GHz detectors  430 . One suitable detector  430  is a PicoSecondPulse Laboratory DC-10 GHz detector available from Picosecond Pulse Labs of Boulder, Colo. An arbitrary waveform generator  432  can be used to generate the phased codes to drive the phase modulator  406 . One suitable phase modulator  406  is a Tektronix 7052 arbitrary phase modulator, available from Tektronix Inc., Beaverton Oreg. The present invention can be incorporated by using twenty binary PPN sub-codes each with 1000, 1-nanosecond phase chips generated and transmitted continuously from each aperture  420 ,  424 . Both the in-phase and quadrature outputs of the IQ demodulator  428  can be digitized and stored on two channels of a digitizer (not shown) as would be understood by one skilled in the art. One suitable digitizer is an Acqiris 582 digitizer at 4 GS/second, available from Agilent Technologies of Santa Clara, Calif. The resulting waveforms were then transferred to a personal computer for post processing and analysis, as would be understood by one skilled in the art. The multicode-PPN waveform can be generated, optically delayed, and transmitted from two apertures via CDMA, as described in  FIG. 4 . 
     While this invention has been described with specific embodiments thereof, alternatives, modifications and variations may be apparent to those skilled in the art. For instance, the present invention can be used with many different types of ladar systems, including those with apertures separated in the cross-track dimension and the along-track dimension. In addition, the present invention is not limited to ladar systems having two apertures, but can include systems having more than two apertures. Such multiple aperture systems include the necessary related circuitry to enable a multiple aperture system as would be understood by one skilled in the art. For instance, when adding additional apertures in the  FIG. 4  configuration, each additional aperture would include a delay such that the signal to each additional aperture is delayed by a fiber delay such that the signal is orthogonal to that of an adjacent aperture. Additional fiber splitters can be added to provide the appropriate signals. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.