Abstract:
An incoherent ladar transmitter ( 12 ) adapted for use with synthetic aperture processing. The system ( 12 ) includes a first mechanism ( 44, 48, 50 ) for generating a laser beam ( 18 ). A second mechanism ( 44, 68 ) records phase information pertaining to the laser beam ( 18 ) and subsequently transmits the laser beam ( 18 ) from the system in response thereto. A third mechanism ( 40 ) receives a reflected version ( 20 ) of the laser beam and provides a received signal in response thereto. A fourth mechanism ( 72 ) corrects the received signal based on the phase information recorded by the second mechanism ( 44, 68 ). In a more specific embodiment, the ladar system ( 12 ) includes a synthetic aperture processor ( 46 ) for correcting the received signal based on the phase information and providing a corrected laser signal in response thereto. The synthetic aperture processor ( 46 ) includes a mechanism ( 76 ) for applying, a Discrete Fourier Transform (DFT) to the corrected laser signal to obtain high frequency resolution and cross-range resolution. A fifth mechanism ( 48 ) constructs a range-Doppler image based on the corrected laser signal and the movement of the ladar system ( 12 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to laser radar systems (ladars). Specifically, the present invention relates to synthetic aperture ladar systems employing incoherent laser pulses. 
     2. Description of the Related Art 
     Ladar systems are employed in various applications including high-resolution 3-dimensional imaging, mapping, chemical analysis, and military targeting applications. Such applications require accurate, space-efficient, and cost-effective ladar systems. 
     Ladar systems are particularly applicable for long-range, high-resolution 3-dimensional imaging applications employed in terrain mapping and target imaging applications on satellites and missile systems. A ladar system often includes a sensor suite mounted on a satellite, missile system, or aircraft. The sensor suite has one or more fixed physical apertures through which a ladar system views a scene. A ladar system views a scene by transmitting a laser through the aperture toward the scene. The laser reflects off the scene, producing a laser return that is detected by the ladar system. Many conventional radar and ladar systems measure the intensity of the return beam and the round trip delay from transmission to detection, which yields the distance (range) to the scene. Laser return intensity and range information may be combined with other image information to facilitate target tracking, terrain mapping, and so on. 
     Ladar systems are either coherent or noncoherent. Coherent ladar systems transmit a laser beam with a predetermined phase and frequency. Knowledge of the spectral characteristics of the transmitted laser beam enables coherent ladar systems to record additional information about the scene, such as target movement, and to further improve Signal-to-Noise Ratio (SNR) over corresponding noncoherent ladar systems. The velocity of a target may be determined from the frequency spectrum of the laser return. 
     Conventional noncoherent ladar systems typically lack phase and frequency information pertaining to the transmitted laser beam. A noncoherent detector combines various wavelengths of the laser return and converts them into corresponding electrical signals. Consequently, without laser spectrum information, certain types of noise filtering, which would increase SNR, are difficult or impossible to implement. 
     Generally, coherent ladar systems have several advantages over noncoherent ladar systems. For example, coherent ladar systems generally have better SNR&#39;s than corresponding noncoherent systems. Unlike incoherent ladar systems, coherent ladar systems may reach Shot Noise Limited (SNL) sensitivity to maximize the SNR. SNL sensitivity is achieved by scaling up the power of a local oscillator aimed on the detector surfaces. 
     Typically, a coherent ladar system receiver detector is illuminated by a laser return and a local oscillator reference beam. The detector outputs a cross-product of the laser return and local oscillator optical fields. The desired information about a scene is contained in the portion of the detector&#39;s output that oscillates at the frequency difference between the local oscillator reference beam and the laser return. This output is often narrow-band filtered to eliminate noise in frequency regions outside predicted signal locations. This noise filtering is enabled by the preservation of the spectrum information pertaining of the transmit laser by an optical heterodyne or homodyne detection process. Noncoherent ladar systems generally do not perform this noise filtering, since they lack requisite spectrum information pertaining to the transmitted laser beam. Unfortunately, coherent ladar systems are generally more sensitive to misalignments and beam distortions. 
     In a conventional ladar imaging system not employing synthetic aperture methods, image cross-resolution is limited by the size of the ladar system aperture. Very large and expensive apertures are required to obtain sufficient resolution for many current long-range imaging and mapping applications. This is particularly problematic for ladar systems employed in satellites or missile systems, which have prohibitive space constraints and require long-range viewing capabilities. 
     To reduce aperture-size requirements, synthetic aperture radar and ladar systems are employed. In a synthetic aperture ladar system, additional information about the scene is obtained by changing the viewing angle of the scene. This additional information, called cross-range information, is contained in Doppler frequency shifts detected in the laser return caused by the transmit laser striking various features of the scene at different angles. Cross-range information indicates the relative angular position of certain scene features associated with a given range or distance from the ladar system. The cross-range information is combined with range information to yield an accurate scene profile to enhance the image of the scene. 
     High resolution topography applications operating at a range of approximately 100 kilometers, an eye-safe laser wavelength of 1.5×10 −6  m, and a typical cross resolution of 20 cm, require a conventional aperture of approximately 75 cm, which is prohibitively large and expensive for many applications. The large apertures are also undesirably sensitive to thermal and gravitational distortions. An analogous synthetic aperture ladar system on a platform travelling at, for example, 100 m/s would require 7.5 milliseconds (ms) to cover the required 75 cm aperture. In traditional ladar, this requires that the laser transmitter produce a high-power waveform that is coherent for the full 7.5 ms. The high power is often required to reach long ranges of interest. Typically, coherent waveforms longer than a fraction of a millisecond are difficult to achieve, especially at high power levels. In addition to coherence time and high power, the transmitted waveform requires high bandwidth to achieve high down-range resolution, yielding typical bandwidth-time products (BT) greater than 300,000. This implies that the transmitted waveform must be accurate (phase coherent) to {fraction (1/300,000)} (1/BT). Consequently, conventional synthetic aperture ladar systems have generally been unsuccessful in achieving this bandwidth time product. 
     Previous synthetic aperture ladar systems could not maintain transmitter coherence for sufficient duration to accurately measure a scene. Accurate synthetic aperture measurements require relatively high beam pulse energy for which coherence is difficult to maintain. For example, synthetic aperture ladar systems employing trains of FM chirped signals are employed on some mobile ladar systems. Unfortunately, these systems have difficulty maintaining laser beam coherence, yielding inferior imaging capabilities. 
     Generally, conventional synthetic aperture ladar systems require a coherent waveform throughout the measuring time during which the laser return is detected. This severely limits waveform selection, preventing use of otherwise more desirable waveforms, such as high-energy Q-switched pulses. 
     Hence, a need exists in the art for an efficient synthetic aperture ladar system that does not require transmission of a coherent laser beam yet maintains the advantages of coherent ladar systems over those of conventional noncoherent ladar systems while maintaining beam alignment advantages of noncoherent systems. There exists a further need for a synthetic aperture ladar system that employs Q-switched laser pulses and an accompanying receiver for detecting a Q-switched laser return. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the synthetic aperture ladar system using incoherent laser pulses of the present invention. In the illustrative embodiment, the inventive system is adapted for use with synthetic aperture ladar systems employed in military targeting and imaging applications. The ladar system includes a first mechanism for generating a laser beam. A second mechanism records phase information pertaining to the laser beam and subsequently transmits the laser beam from the system in response thereto. A third mechanism receives a reflected version of the laser beam and provides a received signal in response thereto. A fourth mechanism corrects the received signal based on the phase information recorded by the second mechanism. 
     In a specific embodiment, the ladar system is a synthetic aperture ladar system that further includes a fifth mechanism for moving the ladar system while the ladar system operates. The fourth mechanism includes a synthetic aperture processor for correcting the received signal in accordance with the phase information and providing a corrected ladar signal in response thereto. The synthetic aperture processor includes a mechanism for applying a Discrete Fourier Transform (DFT) to the corrected ladar signal to obtain high-resolution frequency and cross-range information. A fifth mechanism constructs a coherent range-Doppler scene profile based on the corrected ladar signal and the movement of the ladar system. 
     The first mechanism includes an Er:Yb:Glass Q-switched laser or an Er:Yb:YAG high-power laser for generating the transmitted laser beam. The second mechanism includes a digitizer for recording the phase information and frequency information. The phase information includes waveform information about the transmitted laser beam including measured phase jumps, phase offsets, frequency hops, and frequency offsets. The transmitted laser beam comprises Q-switched or Q-switched mode locked ladar pulses having random phase (incoherent) from shot to shot. 
     The third mechanism includes an In-phase (I) and Quadrature (Q) receiver for implementing I and Q detection and outputting the received signal having I and Q electrical signal components in response thereto. The I and Q receiver is an optical heterodyne receiver that includes a local oscillator for generating reference beam. An optical retarder shifts the reference beam. An l-detector and a Q-detector detect a combination of the reference beam and the reflected version of the laser beam and a combination of the shifted reference beam and the reflected version of the laser beam, respectively. The I and Q heterodyne receiver further includes one or more beam splitters having reflectivities specified to equalize intensities of the reflected version of the laser beam, the reference beam, and the shifted reference beam at the I and Q detectors. 
     In a more specific embodiment, the third mechanism further includes a digitizer for converting the received signal from an analog signal to a digital received signal with I and Q components. A range demultiplexer isolates portions ((r l +i*r Q ) n ,) of the digital received signal which represent laser returns, each associated with a range bin (n). 
     The fourth mechanism maintains detected phases (θ 1 , θ 2 , θ m , . . . θ M ) and frequency offset (f 1 , f 2 , f m, . . . f   M ) associated with each of the M transmitted laser pulses. Another mechanism corrects the digital received signal ((r l +i*r Q ) n,m ) based on the detected phases and frequency offsets that were measured on the outgoing pulses and provides the corrected signal in response thereto in accordance with the following, equation: 
     
       
         Corrected Signal= Re {( r   l   +i·r   Q   n,m   ·e   (−i(θ     m     +2πf     m     τ))   
       
     
     where (r l +i*r Q ) n,m  represents a portion of the digital received signal associated with an n th  range bin and the m th  pulse having an in-phase component r l  and a quadrature component r Q ; θ m  represents a phase correction term associated with one of the detected phases that is associated with the m th  pulse; f m  represents a frequency correction term associated with the m th  pulse; and τ is a digital time variable. 
     The fourth mechanism further includes a mechanism for computing centroids, one centroid for each n th  portion of the received digital signal, based on the square of the magnitude of a DFT of each n th  portion of the received digital. Another mechanism extracts peak intensity information and range Doppler information from the centroids and image information about the scene. 
     The novel design of the present invention is facilitated by the second mechanism for recording phase information about the transmitted laser beam and by the fourth mechanism for correcting the laser return based on the recorded phase information. This relieves coherence requirements on the transmitted laser, thereby enabling use of very desirable transmit waveforms, such as high-energy Q-switched beams, for which coherence is difficult to maintain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an aircraft employing a ladar system constructed in accordance with the teachings of the present invention and illustrating general ladar principles of operation. 
     FIG. 2 is a graph showing an exemplary ladar pulse train transmitted by the ladar system of FIG. 1 and a received pulse train after reflection off three different surfaces. 
     FIG. 3 is a more detailed diagram of the ladar system of FIG. 1 employing an In-phase (I) and Quadrature (Q) laser detector. 
     FIG. 4 is more detailed diagram of an alternative embodiment of the I and Q laser detector of FIG.  3 . 
     FIG. 5 is an amplitude versus time graph of Doppler tones detected in a train of coherent ladar pulses and incoherent pulses via a conventional ladar system (not shown). 
     FIG. 6 is an amplitude versus range bin graph juxtaposing Doppler tones obtained from an exemplary received signal with and without phase correction by the ladar system of FIG.  3 . 
     FIG. 7 is an intensity versus frequency graph juxtaposing the frequency responses of an exemplary received signal with and without correction by the ladar system of FIG.  3 . 
     FIG. 8 is a frequency versus range graph illustrating exemplary image information output by the ladar system of FIG.  3 . 
    
    
     DESCRIPTION OF THE INVENTION 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1 is a diagram showing an aircraft  10  employing a ladar system  12  constructed in accordance with the teachings of the present invention and illustrating general ladar principles of operation. The aircraft  10  has a velocity vector (V)  14  as it flies by a building  16 . The ladar system  12  is mounted on the front of the aircraft  10  and transmits a laser beam  18  toward the building  16 . 
     In the present embodiment, the transmitted laser beam  18  is a high-energy eye-safe Q-switched pulsed laser beam comprising a sequence of high-energy pulses. The Q-switched pulsed laser beam  18  reflects from the building  16 , producing three laser returns  20  from three different surfaces of the building  16  for each pulse of the laser beam  18 . By measuring the time between transmission of a given pulse and the detection of the corresponding laser returns by the ladar system  12 , the distance to the building  16  and its various surfaces is determined. 
     As the aircraft  10  passes over or by the building  16 , it fires the laser  18  for a predetermined time, called the dwell time or the measuring time (T meas ). Throughout the measuring time T, the angle (θ) at which the transmitted laser beam  18  and the returns  20  strike and reflect from the building  16  changes (Δθ). As the angle θ changes, each surface of the building  16  yields a return at a slightly different frequency than returns from the other surfaces due to Doppler frequency shifts in the returns from the radial motion of the aircraft  10  relative to the building  16 . The Doppler frequency shifts depend on the angle at which the transmitted laser beam  18  strikes the different surfaces of the building  16 . 
     Each return pulse  20  effectively samples the Doppler tones that are present due to the radial motion of the aircraft  10  relative to the building  16 . Each return pulse is sampled several times (N times), with each sample being stored in a range bin corresponding to the time at which the sample was taken. The time at which the sample was taken represents the distance or range corresponding to the sample. Over the measuring time T meas , several return pulses are sampled by a high-speed A/D converter, as discussed more fully below. After all the samples have been taken, the phase and frequency correction process may be performed. The sampled data in N range bins and M pulses is output to Discrete Fourier Transform (DFT) modules. The DFT modules extract data from the individual range bins to compute frequency spectrum associated with each range bin. The frequency spectrum represent cross-range information, which indicate the relative angular position associated with the sampled data from each range bin. Consequently, the angular position associated with each range bin yields range and cross-range information for each surface of the building  16 . This range and cross-range information may then be plotted to yield an accurate profile of the building  16  in the direction of travel  14  of the aircraft  12 . 
     The measuring time (T meas ) multiplied by the velocity (V) of the aircraft is proportional to the synthetic aperture, which is inversely proportional to the cross-range resolution of the ladar system  12 . Generally, the Doppler frequency shift (Doppler velocity) (f d ) associated with a return  20  due the motion of the aircraft  10  relative to the building  16  for a small angle θ is given by the following equation:                  f   d     =             2      V     λ     ·   sin                     (   θ   )       ≅         2      V     λ     ·   θ         ,           [   1   ]                                
     where λ is the wavelength of the transmitted laser beam  18 , and V is the velocity of the aircraft  10 . 
     The angle θ corresponding to the Doppler shift f d  given by the following equation, which is obtained by rearranging equation (1):                θ   =           λ                   f   d         2      V       ⇒     δ                 θ       =         λ   ·   δ                     f   d         2      V           ,           [   2   ]                                
     where δθ is a small change in angle θ and represents the cross-range resolution, while δf d  is a corresponding small change in the Doppler shift f d  and corresponds to the accuracy with which the Dopplered f d  shift can be measured. 
     The best frequency resolution given by the following equation, which is obtained from Fourier theory:                  δ                   f   d       ≈     1     T   meas         ,           [   3   ]                                
     where T meas  is the measuring time or dwell window during which scene measurements are performed by the ladar system  12 . 
     The cross-range resolution δθ, also called the resultant Field Of View (FOV) is given by the following equation:                  δ                 θ     =       λ     2        V   ·     T   meas           =     λ     2   ·     D   synth             ,           [   4   ]                                
     where D synth =VT meas , which is the synthetic aperture size. 
     Using the synthetic aperture approach, one could use a relatively small physical aperture and set the measuring time T meas  such that 2VT meas =75 cm. In coherent ladar systems, T meas  represents the time during which the transmitted laser must remain coherent. The ladar system  12  of the present invention does not require that the transmitted laser beam  18  be coherent. 
     For a typical cross-range resolution of 20 cm (required for long-range high-resolution topography) operating 100 km from scene  16  at an eye-safe laser wavelength of 1.5×10 −6  m, from a plane flying at 200 m/s, the measuring time T meas  is approximately 1.875 milliseconds. The bandwidth required for the line of sight resolution is approximately 300 MHz. The figure of merit, the bandwidth-time product, BT, of laser beam  18  is approximately 560,000. 
     FIG. 2 is a graph  30  showing an exemplary Q-switched laser pulse train  18  transmitted by the ladar system  12  of FIG. 1 and a received pulse train  20  detected by the ladar system  12  after reflection off three different surfaces of the building  16  of FIG.  1 . The graph  30  is shows pulse intensity versus time. 
     Each pulse (n→n+3) of the transmitted pulse train  18  yields, a short time later, a corresponding set of three returns in the received pulse train  20 , one return for each surface reflected by the building  16  of FIG.  1 . For illustrative purposes, the return pulse train  20  has three distinct returns for each transmitted pulse of the pulse train  18 . In practice, each set of three returns in the pulse train  20  are typically closer together and may blend into a single return pulse, with different peaks, each peak corresponding to a surface of the building  16 . 
     The transmitted pulse train  18  is incoherent from pulse to pulse, and consequently has a random phase and random frequency offsets. Each pulse of the transmitted pulse train  32  is associated with a frequency offset (f n ) and random phase (θ n ). The ladar system  12  of FIG. 1 measures the frequency offsets f n  and phases θ n  of the outgoing noncoherent pulse train  18  to compensate the return pulse train  20  for frequency offset f n  and phases θ n , as discussed more fully below. 
     FIG. 3 is a more detailed diagram of the ladar system  12  of FIG. 1 employing an In-phase (I) and Quadrature (Q) laser receiver  40 , which is an optical heterodyne detector. For clarity, various well-known components, such as power sources, signal amplifiers, noise filters, and focusing optics have been omitted from FIG.  3 . However those skilled in the art with access to the present teachings will know which components to implement and how to implement them to meet the needs of a given application. 
     The ladar system  12  includes a common aperture  42 , which communicates with a Q-switched laser transmitter  44  and the optical heterodyne receiver detector  40 . The transmitter  44  and the heterodyne receiver  40  communicate with a synthetic aperture processor  46 , which communicates with la computer  48 , such as a display, target tracking, or fire control computer. The Q-switched transmitter  44  receives input from a signal generator  49 , which receives input from al transmit controller  50 . 
     In the present embodiment, the ladar system  12  is a monostatic ladar system since the transmitter  44  and the receiver  40  share the common aperture  42 . The Q-switched transmitter  44  includes a transmit laser that may be implemented as an Er:Yb:Glass Q-switched laser or an Er:Yb:YAG high-power laser. Those skilled in the art will appreciate that the ladar system  12  may be another type of ladar system, such as a bistatic ladar system employing a different type of transmit laser, without departing from the scope of the present invention. 
     In operation, the transmit controller  50  provides control signals to the signal generator  49  which specify waveform details, such as pulse width, energy per pulse, pulse spacing, and so on. The signal generator  49  generates an electrical signal according to the waveform details. The electrical signal drives the Q-switched laser transmitter  44 , which produces a Q-switched laser beam  18  characterized by the waveform details. The Q-switched laser transmitter  44  determines I and Q components of the Q-switched transmitted laser beam  18  before transmission from the ladar system  12 . The I and Q components of the outgoing Q-switched laser beam  18  are determined similarly to the optical heterodyne receiver  40 , as discussed more fully below. The I and Q components of the outgoing Q-switched laser beam  18  are input to a phase and frequency offset detector  68  of the synthetic aperture processor  46 . The transmit controller  50  and/or the signal generator  49  may be implemented via software running on the synthetic aperture processor  46  without departing from the scope of the present invention. 
     The transmitted laser beam  18  reflects from the scene, such as the building  16  of FIG. 1, yielding the laser return signal  20 , which is detected by the optical heterodyne receiver  40 . The optical heterodyne receiver  40  includes a local oscillator laser  52 , a first beam splitter arrangement  54 , a second beam splitter arrangement  56 , a quarter-wave retarder  58 , an in-phase (I) optical detector  60 , a quadrature (Q) optical detector  62 , and 1 GHz analog-to-digital converters  64 . 
     In operation, the local oscillator  52  transmits a coherent Continuous Wave (CW) reference laser beam, in the form of a sine wave, to the first beam splitter arrangement  54 . The first beam splitter arrangement  54  transmits the local oscillator reference signal to the I-detector  60  and the quarter-wave retarder  58 . The quarter-wave retarder  58  converts the input sine wave into a laser beam having a cosine waveform and transmits the cosine waveform to the surface of the Q-detector  62 . Similarly, the first beam splitter arrangement  56  splits the laser return beam  20  received from the common aperture  42  into two sinusoidal beams. The two sinusoidal beams strike the I-detector  60  and the Q-detector  62  coincident with the beams output by the first beam splitter arrangement  54  and the quarter-wave optical retarder  58 , respectively. 
     The I-detector  60  and the Q-detector  62  detect beat frequencies, called Doppler tones, corresponding to I and Q received signal components, respectively. The I-detector  60  and the Q-detector  62  convert respective the I and Q received signal components into corresponding analog I and Q electrical signals. The I and Q analog electrical signals are then sampled at 1 GHz Analog-to-Digital (A/D) converters  64 . The A/D converters output a 1 GHz digital received signal having I and Q components. The 1 GHz digital received signal is then input to a range demultiplexer  66  implemented in the synthetic aperture processor  46 . 
     The synthetic aperture processor  46  includes the range demultiplexer  66 , which provides input to a full phase circuit  70 . The full phase circuit  70  provides input to a phase corrector  72 . The phase corrector  72  also receives input from the phase and frequency offset detector  68  from the M transmitted pulses. The phase corrector  72  provides output to a set of N range bins  74  and M pulses per range bin. Each n th  range bin of the N range bins  74  provide output to N corresponding Discrete Fourier Transform (DFT) circuits  76 . The N DFT modules  76  provide input to N corresponding centroid detectors  78 . The N centroid detectors  78  provide Doppler information to the computer  48  and to N corresponding peak intensity detectors  80 . The peak intensity detectors  80  provide intensity input to the computer  48 . 
     In operation, the various modules  66 - 80  of the synthetic aperture processor  46  are implemented in software. The synthetic aperture processor  46  may be implemented by the computer  48  without departing from the scope of the present invention. 
     The A/D converters  64  sample the analog signals output from the I-detector  60  and the Q-detector  62  at predetermined intervals based on a priori knowledge of the pulse spacing of the transmitted laser beam  18 . The A/D converters  64  may be preset with pulse spacing information or may obtain the pulse spacing information via a connection (not shown) to the Q-switched laser transmitter  44 . The intervals at which the A/D converters  64  sample the received signal coincide with pulses the received analog I and Q signal output from the detectors  60  and  62 . The A/D converters take N samples per pulse. The number of samples N taken per pulse depends on the pulse width of the transmitted laser  18  and the sampling rate (1 GHz) of the A/D converters  64  and the range window that is desired. In the present specific embodiment, the sampling rate is approximately three nanoseconds. The exact pulse width, intensity, frequency, and other waveform characteristics of the transmitted laser beam  18  are application-specific and may be determined by one skilled in the art to meet the needs of a given application. 
     Each of the N digital I and Q samples output by the A/D converters  64  per pulse are demultiplexed onto a single path by the range demultiplexer  66  and input to the full phase circuit  70 . The full phase circuit  70  expresses the sampled demultiplexed I and Q signal output from the range demultiplexer  66  in imaginary form, r l +ir Q , where r l  is the in-phase component of the received signal and ir Q  is the imaginary component. Expressing the return signal in this form (r l +ir Q ) facilitates phase correction by the phase corrector  72 . 
     Frequency and phase information about the transmitted laser beam  18  is provided to the phase and frequency offset detector  68 . I and Q components, t l  and it Q , respectively, of the transmitted laser beam  18  are forwarded to the phase and frequency offset detector  68  by the Q-switched laser transmitter  44 . The Q-switched laser transmitter  44  determines the I and Q components of the transmitted laser beam  18  similar to the way the optical heterodyne receiver  40  determines I and Q components of the received signal  20 . The phase and frequency offset detector  68  extracts frequency and phase information ([θ 1 , θ 2 , θ 3 , . . . , θ M ], [f 1 , f 2 , f 3 , . . . , f M ]) from the transmit signal (t l +it Q ) via methods known in the art. The phase and frequency information includes measured phase offsets and frequency offsets that occur from one Q-switched pulse to the next. 
     The relative phase θ n  of each transmitted pulse of the transmitted beam  18  is detected and recorded by the transmitter  44  of the ladar system  12 . A measured phase array ([θ 1 , θ 2 , θ 3 , . . . , θ M ]) and a frequency offset (frequency hop) array ([f 1 , f 2 , f 3 , . . . , f M ]) computed by the phase and frequency offset detector  68  are used to correct the received signal in preparation for a subsequent Fourier transform operation, as discussed more fully below. 
     In this mode, the pulses of the received signal are not necessarily evenly spaced. However, the reference beam output by the local oscillator  52  is coherent throughout the measuring time, T meas . The local oscillator  52  may be implemented with a standard laser usually of the same base material as the transmitter, such as Er:Yb:Glass or Er:YAG in the current embodiment. As is known in the art, the coherence of the local oscillator  52 , which is relatively low-power and runs in CW mode, is easier to maintain than a high energy pulsed transmit laser, such as the Q-switched laser transmitter  44 . 
     After the I and Q received signals are obtained via the optical heterodyne receiver  40 , they are digitized by the A/D converters  64 . To reduce computational requirements, the range demultiplexer  66  performs range demultiplexing. The range demultiplexer  66  adjusts the input bit stream so that the subsequent phase correction is only performed at range bins associated with expected returns. After the received signal r n  for a range bin n is collected, the phase corrector  72  corrects it. 
     The phase corrector  72  employs the phase and frequency information ([θ 1 , θ 2 , θ 3 , . . . , θ M ], [f 1 , f 2 , f 3 , . . . , f M ]) of all M pulses, to apply a phase correction term (e (−i(θ     m     +2πf     m     τ)) ) to the received signal r l +ir Q . The phase corrector  72  then outputs a corrected signal at range bin n (S n ) given by the following equation: 
     
       
           S   n   =Re {( r   l   +i·r   Q ) n,m   ·e   (−i(θ     m     +2πf     m     τ)) ,  [5] 
       
     
     where (r l +ir Q ) n,m  represents a portion of digital received signal associated with an n th  range bin and the m th  pulse, and having an in-phase component r l  and a quadrature component r Q ; θ m  represents a phase correction term associated with the m th  pulse; F n  represents a frequency correction term associated with the m th  pulse; and τ is a digital time variable. 
     The phase-corrected signal S n  is then clocked into the range bins  74 , which may be implemented via a software register. After the N range bins have been filled by S l  through S N , which represents the portion of the received signal corresponding to a single set of returns, the range bins  74  are cleared in parallel as the contents of the range bins  74  are clocked into the N corresponding DFT modules  76 . The DFT modules  76  compute the square of the magnitude of the DFT of the signal corresponding to each range bin. For example, the DFT operation for the signal in the first range bin S l (m), where m is an integral time variable, involves accumulating S l (m) according to the following equation:                           DFT   1          (   ω   )            2     =              ∑     m   =   0       M   -   1                S   1          (   m   )                   -   j                   ω                 m                2       ,           [   6   ]                                
     where M is the number of samples taken during the measuring time T meas ; ω=2πk/L, k=0, 1, 2, . . . , M−1; S l (m)=0 for 0&gt;m&gt;M. DFT l (ω) represents the frequency response associated with the first range bin of the range bins  76 . The magnitude squared of DFT n (ω) will preferably have one or more peaks at the frequency corresponding to the Doppler tone associated with the n th  range bin as discussed more fully below. This frequency peak represents cross-range information associated with the n th  range bin. 
     The DFT modules  76  compute the DFT l (ω) via Fast Fourier Transform (FFT) algorithms, which are well known in the art. The DFT modules  76  output frequency responses (spectrums) to the corresponding centroid detectors  78 . The centroid detectors  78  compute the centroids of the frequency responses, via methods known in the art, yielding center frequencies. The center frequencies output by the centroid detectors  78  represent cross-range information in the form of Doppler frequencies. This cross-range information is input to the computer  48 . The computer  48  may then generate a range versus cross-range plot based on the cross-range information and the range information. The range information is indicated by the number of the range bin associated with cross-range information output by each centroid detector  78 . Furthermore, the cross-range information from the centroid detectors  78  is input to corresponding peak intensity detectors  80 . The peak intensity detectors  80  compute intensity information corresponding to the magnitudes of the peaks of the centroids and not just the frequency locations of the centroids. The intensity information corresponding to each range bin is also input to the computer  48 . 
     The computer  48  may include a display, tracking software, fire control software, chemical analysis software, and so on. In the present embodiment, the computer  48  runs software for displaying a cross-range versus range plot, which is indicative of a profile of the scene, such as the building  16  of FIG. 1 being imaged. 
     FIG. 4 is more detailed diagram of an alternative embodiment  40 ′ of the I and Q laser receiver  40  of FIG.  3 . The I and Q laser detector  40 ′ is an optical heterodyne detector that includes a first beam splitter  90 , a quarter wave retarder  58 , a mirror  94 , a second beam splitter  96 , a third beam splitter  98 , the I-detector and pre-amplifier  60 , and the Q-detector and pre-amplifier  62 . 
     In operation, the sinusoidal local oscillator reference beam is split by the first beam splitter  90 , which directs a first portion of the reference beam to the third beam splitter  98  and a second portion of the reference beam to the quarter wave retarder  58 . The quarter wave retarder  58  converts the sine-wave input to a cosine-wave output, which reflects off the mirror  94 , passes through the second beam splitter  96  and onto the detecting surface of the Q-detector and pre-amplifier  62 . The second portion of the sine-wave reference beam passes through the third beam splitter  98  and onto the detecting surface of the I-detector and pre-amplifier  60 . 
     The transmissivity (coefficient of transmission) and reflectivity (coefficient of reflection) of the various beam splitters  90 - 98  are adjusted to equalize the total energies at the surfaces of the detectors  60  and  62 . Consequently, the first detector  60  will receive equivalent percentages of the first and second beams as received by the second detector  62 . In the present specific embodiment, the first beam splitter  90  is a 5% beam splitter; the mirror  94  is a 100% beam splitter, the second beam splitter  96  is a 95% beam splitter, and the fourth beam splitter  98  is a 50% beam splitter. 
     The received laser return signal received is split into a first and second portion by the 50% beam splitter  96 . The first portion is directed to the detecting surface of the I-detector and pre-amplifier  60 , where it mixes with the local oscillator sine-wave signal, yielding a beat or Doppler tone corresponding to an in-phase (I) signal component of the received laser return signal. The second portion of the received laser return signal reflects from the 95% beam splitter  98  onto the detecting surface of the Q-detector and pre-amplifier  62 . It then mixes with the cosine-wave derived from the local oscillator reference signal, yielding a beat or Doppler tone corresponding to a Quadrature (Q) signal component of the received laser return signal. 
     Converting the received signal into I and Q components via the optical heterodyne receiver  40 ′ facilitates recovering Doppler information from the received signal via phase correction operations. The Doppler information may be recovered by using the recorded phase and frequency offset measured of each individual transmitted pulse. To perform phase correction operation, the received signal phase must be also known unambiguously, which is enabled via I and Q detection implemented by the receiver  40 ′. 
     Generally, the return signal is split into two beams, one is mixed with an local oscillator laser beam that has a natural optical oscillation (sin(ωt)), and the other return beam is mixed with a version of the local oscillator reference beam that has been shifted in phase to have an optical oscillation cos(ωt). This shift is obtained by employing the optical retarder  58  of λ/4. 
     FIG. 5 is an amplitude versus time graph  100  of first and second Doppler tones  102  and  104  detected in a train of coherent ladar pulses and incoherent pulses, respectively, via a conventional ladar system (not shown). The sinusoidal form of the first Doppler tone  102  is clear due to the coherence of the received pulses and is obtained by sampling a train of received coherent pulses at the pulse repetition rate. The second Doppler tone  102  is obtained by sampling a train of received noncoherent (incoherent) pulses at the pulse repetition rate. However, the second Doppler tone (shown dotted)  104  is unclear, and the Doppler tone information appears lost due to the incoherence (random phase and frequency offsets) of the received pulses corresponding to the Doppler tone  104 . 
     The present invention employs measured frequency and phase offset information of the transmitted waveform and employs this information to restore the Doppler tone information in the received signal. This relieves previous coherence requirements placed on the transmitted laser, thereby enabling use of various types of desirable transmit laser beam waveforms, such as high-energy Q-switched pulses. 
     FIG. 6 is an amplitude versus range bin graph  110  juxtaposing Doppler tones  104 ′ and  106  obtained from exemplary received signal with and without phase correction, respectively, by the ladar system  12  of FIG.  3 . With reference to FIGS. 3 and  6 , without phase correction by the phase corrector  72 , the contents of the range bins  46  might have an exemplary random pattern (shown dotted)  104 ′, corresponding to the Doppler tone  104  of FIG.  5 . After phase correction by the phase corrector  72 , the contents of the range bins  46  contain clear Doppler tone information  106 . 
     FIG. 7 is an intensity versus frequency graph  120  juxtaposing the frequency responses  122  and  124  of an exemplary received signal with and without phase correction, respectively, by the ladar system  12  of FIG.  3 . The uncorrected frequency response  122  appears as background noise. The corrected frequency response  124 , which corresponds to a particular range bin n, has a clear peak at a particular frequency. The location of the peak in terms of frequency corresponds to the relative angular position of the surface that produced the set of returns corresponding to the peak. This relative angular position represents cross-range information. 
     The intensity of the peak of the corrected frequency response  124  may be employed by the computer  48  of FIG. 3 to approximate the reflectivity of the surface that produced the return. The intensity information may be employed do differentiate the various detected surfaces by reflectivity. 
     FIG. 8 is a frequency versus range graph  130  illustrating exemplary image information output by the ladar system  12  of FIG.  3 . With reference to FIGS. 3 and 8, the graph  130  shows a profile  132  based on the cross-range information (Doppler frequency information) extracted from the range bins  74  via the DFT modules  76  and the centroid detectors  78 . The profile  132  depicts the profile of the building  16  of FIG. 1 in the direction of flight of the aircraft  12 . Target cross-sections, such as the profile  132  may greatly improve the accuracy of automatic target recognition systems 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications, and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,