Abstract:
An optical SAR transmits toward a target an amplitude modulated optical signal. Modulation of optical signals may be performed using light emitting devices such as semiconductor laser diodes driven by a modulation signal so that the emitted optical signal intensity is amplitude modulated. Transmitted optical signals are reflected from a target, and reflected optical signals are detected by light detecting devices such as photodiodes that detect and automatically demodulate the reflected optical signals. Optical elements such as a polarizer, a lens, and a frequency filter such as a color filter may optically process the amplitude modulated optical signal before transmission and detection. This technique achieves the potential benefits of an optical SAR, such as high resolution, better image quality, and elimination of electromagnetic interference, while circumventing many of the problems traditionally associated with optical SARs, such as the requirement for optical coherence and extremely accurate platform motion measurements.

Description:
BACKGROUND 
       [0001]    Synthetic Aperture Radar (SAR) is a radar technique that achieves the effect of a large aperture antenna by using a relatively small aperture antenna that is physically moved along a path. By combining the information from many pulses transmitted along the path, a SAR can synthesize the performance of a single large aperture antenna. SAR has been effectively used in airplanes and satellites to generate images of ground targets, for example. 
       SUMMARY 
       [0002]    An optical SAR is disclosed that transmits an amplitude modulated optical signal toward a target. Forming a SAR image directly at optical wavelengths is difficult because platform motion must be measured with accuracy related to the wavelength of the optical signal, which is in the micron range. However, transmitting an optical signal that is modulated with a much lower frequency SAR waveform takes advantage of benefits of optical signals such as small aperture size and low weight while avoiding drawbacks such as mentioned above. 
         [0003]    Modulation of optical signals may be performed using light emitting devices such as one or more light emitting diodes (LEDs) or laser diodes, for example. An LED or laser diode may be driven by a modulation signal via an amplifier so that the emitted optical signal intensity is amplitude modulated accordingly. The transmitted optical signals are reflected from a target, and the reflected optical signals are detected by light detecting devices such as photodiodes, for example. Photodiodes receive and automatically demodulate the reflected amplitude modulated optical signals. The optical SAR requires simple hardware to amplitude modulate an optical signal, and to recover the modulation from an amplitude modulated optical signal without the necessity of a coherent optical receiver. 
         [0004]    The optical SAR may comprise a transmitter and a receiver. The transmitter is capable of generating an optical amplitude modulated signal by amplitude modulating the optical signal with the SAR waveform modulation signal and transmitting the amplitude modulated signal. The transmitted optical signal travels out to the target, and a portion of the transmitted optical signal reflects off the target and is received by the receiver. The receiver detects the received optical signal and demodulates it, thus recovering the original SAR waveform modulation signal. The optical SAR also may include a SAR computer that processes the received and demodulated SAR-waveform to generate an image and information such as target recognition, and moving-target information, for example. 
         [0005]    The optical signal source may be any light-generating device that can be amplitude modulated such as the LED or laser diode examples mentioned above. The modulation waveform used to modulate the optical signal can be a linear chirp, for example. In general, any modulation waveform can be used, as long as its autocorrelation function is high at zero shift, and low everywhere else. Optical elements such as a polarizer, a lens, and a frequency filter such as a color filter may optically process the amplitude modulated optical signal before transmission and/or detection. The target may be interrogated with different optical signal properties that separate materials having different responses to the different optical signal properties, such as color or polarization. In this way, more accurate target recognition may be achieved. 
         [0006]    Additionally, the bandwidth of the amplitude modulation signal may be set to be much higher than the bandwidth of conventional SAR signals, so that a dramatic increase of image resolution can be achieved. The optical SAR avoids object image variations resulting from capturing reflected light radiated from the sun, for example, that adversely affects optical imaging techniques. Sun angle, cloud cover, etc. all affect image properties that makes automated image interpretation difficult for optical imaging, but does not affect SAR imaging. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Exemplary embodiments are described in detail below with reference to the accompanying drawings wherein like numerals reference like elements, and wherein: 
           [0008]      FIG. 1  shows an exemplary optical SAR operation; 
           [0009]      FIG. 2  shows an exemplary block diagram of an optical SAR apparatus; 
           [0010]      FIG. 3  shows an exemplary block diagram of a SAR computer; 
           [0011]      FIG. 4  shows an exemplary frequency domain diagram of an amplitude modulated signal having a carrier and 2 side bands; 
           [0012]      FIG. 5  shows an exemplary frequency domain diagram of a carrier suppressed modulated signal with only 2 side bands; 
           [0013]      FIG. 6  shows an exemplary block diagram of a transmitter and a receiver; 
           [0014]      FIG. 7  shows an exemplary circuit diagram of a transmit-transducer; 
           [0015]      FIG. 8  shows an exemplary circuit diagram of a receive-transducer; 
           [0016]      FIG. 9  shows an exemplary block diagram of a waveform generator; 
           [0017]      FIG. 10  shows exemplary linear chirp waveform diagrams; 
           [0018]      FIG. 11  shows exemplary pseudo-random code waveform diagrams; 
           [0019]      FIG. 12  shows an exemplary block diagram of a receiver; 
           [0020]      FIG. 13  shows an exemplary block diagram of a transmit/receive (T/R) controller; 
           [0021]      FIG. 14  shows an exemplary flow chart for a T/R controller process; 
           [0022]      FIG. 15  shows an exemplary flow chart of a timer process; 
           [0023]      FIG. 16  shows an exemplary flow chart of a transmitter process; and 
           [0024]      FIG. 17  shows an exemplary flow chart of a receiver process. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 1  shows an airplane  100  flying along a path  102 . An optical SAR  106  mounted in airplane  100  transmits an amplitude modulated optical signal in the form of a beam  108  toward target  110  of ground area  112 , and subsequently receives a reflected optical signal. The transmitted beam  108  has an illumination angle  116  designed to illuminate the target area of interest  110 . Optical SAR  106  performs the above transmit and receive operations repeatedly at points  114  along path  102  and processes the reflected optical signals resulting from a set number of points  114  to form an image and/or to generate other information such as moving target indications. 
         [0026]    Optical SAR  106  forms beam  108  using optical signals instead of radio frequency (RF) signals to benefit from optical signal properties such as physical size and weight of required hardware. For example, as is well known in the art, the size of an antenna required to transmit a signal of a particular beamwidth is proportional to the wavelength of the transmitted signal. Because optical signals have a much smaller wavelength than that of RF signals, a transducer that transmits beam  108  can be physically much smaller than if RF signals were transmitted. Reduction in size leads naturally to reduction in weight. 
         [0027]    Other benefits of using optical signals can also be obtained such as better image quality, improvement in moving target indication (MTI), avoidance of bandwidth interference with other communication and navigation systems operating in particular geographical areas, fully polarimetric SAR capability, and multi-spectral images. Each of these is briefly discussed below. 
         [0028]    Images in a SAR that operate using RF signals (RF-SAR) suffer from image degradation known as Pulse Repetition Frequency Ambiguity (PRFA) which are artifacts caused by a lack of beam pattern control resulting in illumination of objects outside the desired image area. For optical signals, beam  108  can be precisely controlled and thus PRFA may be virtually eliminated. 
         [0029]    Further, image quality may be degraded by scintillation caused by multiple scattering centers in a resolution cell that come in and out of phase over an aperture of the RF-SAR. This scintillation degradation may be reduced or eliminated by using a 50% bandwidth, i.e., a bandwidth that spans a full octave frequency range. This technique is difficult to achieve for RF-SAR due to antenna design and spectrum allocations. However, octave bandwidth is easily achieved for the modulation waveform of an optical SAR. 
         [0030]    Another source of image degradation is “flashes” caused by dihedral or trihedral reflections from sides of buildings and then off the ground. Such reflections are possible for RF SARs because the buildings and ground act as smooth, mirror-like surfaces at RF wavelengths. At optical frequencies, very few things appear smooth, thus “flashes” may be significantly avoided and image quality improved. 
         [0031]    RF-SAR may be used to detect moving targets. However, an illumination angle  116  for RF-SARs is relatively large, which only allows motion detection for targets moving at relatively high speeds. An illumination angle  116  for optical SAR  106  may be made much smaller than for an RF SAR, and thus provide detection of targets moving at much lower velocities. 
         [0032]    RF signals transmitted by RF-SAR may interfere with communications and navigation equipment either onboard the platform operating the RF-SAR or operating in the geographical area near the target of interest. For example, FCC and/or FAA permission may be required before an RF-SAR can even be operated. In some situations, an RF-SAR is required to have gaps in their transmission spectrum, which degrades image quality. Using optical signals of optical SAR  106  eliminates this interference problem. 
         [0033]    Polarities of optical signals may be easily controlled, so that beam  108  may be controlled to be horizontally, vertically or circularly polarized, for example, and light reflected from a target area may be received in specified polarities. Since polarization characteristics of various objects are different at different frequencies, optical SAR  106  may provide rich information regarding targets that cannot be obtained at RF frequencies. 
         [0034]    In addition to polarization, multi- or hyper-spectral SAR may be used. Such a SAR would transmit optical signals at multiple wavelengths over the same aperture and directed toward the same target, thus producing a SAR image at each wavelength. This may be used, for example, to discriminate natural foliage from a camouflage net based on reflectivity differences at different frequencies. If red, green and blue optical frequencies are used, a true-color optical SAR image may be generated. 
         [0035]    Optical SAR  106  also may take advantage of SAR properties to overcome electro-optics (EO) imaging drawbacks. For example, PO imaging forms images depending on reflected light or, in the case of InfraRed (IR) imaging, black body radiation. The appearance of objects in such images varies due to the angle of illumination, cloud cover, temperature variations, shadows, range, atmospheric conditions, etc. These changes in appearance make it difficult to implement automated image interpretation. However, SAR images do not suffer from the above variations. Thus, optical SAR  106  generated images are suitable for automatic target recognition and automatic change detection. 
         [0036]    Advantageously, as is discussed below, optical SAR  106  receives the reflected signals and generates received data that conventional SAR systems could process. Thus, existing SAR processing may be applied to the received data generated by optical SAR  106  including large suits of exploitation tools already developed for SAR. For example, tools such as Coherent Change Detection (CCD), red-blue multi-view, moving target indication (MTI), automatic target detection (ATD), and various mensuration tools may be applied to the received data generated by optical SAR  106 . 
         [0037]      FIG. 2  shows an exemplary block diagram of optical SAR  106  that may include a SAR computer  200 , a transmit/receive (T/R) controller  202 , transmitter  204  and receiver  206 . SAR computer  200  may be implemented by a general purpose computer or a specialized computer designed specifically for efficient processing of SAR related algorithms such as SAR image generation, automatic target recognition, movement target indication, etc. as mentioned above. 
         [0038]    SAR computer  200  interfaces with an operator who commands an optical SAR operation by specifying beam aim-point, image size, and image resolution parameters, for example. SAR computer  200  generates operational parameters for T/R controller  202  to execute the commands and then waits to receive SAR-data from T/R controller  202 . T/R controller  202  initializes transmitter  204  and receiver  206  and starts the commanded SAR operation. Transmitter  204  transmits an amplitude modulated optical signal  208  as beam  108  that is directed at target  110 , for example. Receiver  206  receives reflected optical signals  210 , converts reflected optical signals  210  into digital SAR data, and forwards the SAR-data to SAR computer  200  via T/R controller  202 . When received, SAR computer  200  processes the SAR-data based on the operator commands. For example, the operator commands may have specified various types of displays to be generated and specific detection processes to be performed such automatic target recognition or motion target indication. 
         [0039]      FIG. 3  shows an exemplary block diagram of SAR computer  200  that may include a SAR processor  300 , a SAR memory  302 , a T/R controller interface (I/F)  304  and an operator I/F  306 . These components are connected together by a bus  308  configured in a bus architecture. The bus architecture is shown as an example for ease of explanation. Other types of connections may be used as is well known in the computer architecture art. For example, depending on bandwidth requirements, SAR memory may require a dedicated direct memory access (DMA) port to support high data rates from T/R controller I/F  304 . SAR processor  300  may be implemented by using a general-purpose computer or specialized computers implemented by hardware components such as Application Specific Integrated Circuits (ASICs), PLAs, PALs, FPGAs, etc as is well known in the computer art. The functions needed for SAR processing may be implemented using software programs executing in either the general purpose or the special purpose computers. These programs may be stored in a computer readable medium and loaded into the computer for execution. 
         [0040]    Additionally, SAR memory  302  may be any type of memory that can support the data rates that are required for a SAR operation. For example, SAR memory  302  can be implemented by using dynamic or static solid-state memory, programmable read/write memory and/or disk memory, for example. If the speeds of a mass storage is not fast enough, a form of cache using high speed memory can be used. 
         [0041]    In the example shown in  FIG. 3 , SAR memory  302  stores the SAR-data received from receiver  206 . When the SAR operation is completed for all points  114  of the commanded synthetic aperture, T/R controller  202  sends a completion message to SAR processor  300  through T/R controller I/F  304 . SAR processor  300  then begins to process the SAR-data to generate the commanded displays and other processing operations. Although the above example contemplates processing the SAR-data after the data is collected from all points  114 , SAR processor  300  may process SAR-data on the fly after sufficient data is collected. In this way, results of the SAR operation may be made available faster than if SAR processor  300  waited until data is collected from all points  114 . Thus, in the example shown in  FIG. 3 , SAR memory  302  stores the SAR-data received from receiver  206 . As the SAR pulses are collected from points  114  of the synthetic aperture, the T/R controller  202  notifies SAR processor  300  through T/R controller IF  302 . SAR processor  300  then begins to process the SAR-data to generate the image and other SAR products. 
         [0042]    Optical SAR  106  transmits an amplitude-modulated optical signal as beam  108 . As shown in  FIG. 4 , amplitude modulation of a carrier signal generates 2 side bands one above and one below a carrier signal frequency, f c . Each of the side bands is separated by a modulation frequency, f m , from the carrier signal frequency f c . This double side band amplitude modulation (DSBAM) is different from other types of modulation such as double side band suppressed carrier (DSBSC) where the carrier signal is missing, as shown in  FIG. 5 . This is a significant difference, because the DSBAM optical signal can be more easily generated in the transmitter, and more easily recovered in the receiver compared to DSBSC. In particular, the DSBAM signal can be generated by, for example, modulating the power that drives the light emitting device, without the need for components such as optical mixers. Also, the modulation can be recovered from a DSBAM signal using a simple envelope detector, without the need for a coherent local oscillator to regenerate the carrier as is required in DSBSC. Thus, the amplitude modulated optical signal permits relatively simple designs for transmitter  204  and receiver  206 . 
         [0043]    The frequency of the optical signal is much higher than a frequency of SAR signals that modulate the optical signal. The resolution of SAR images is determined by the bandwidth of the transmitted SAR signal. The resolution of SAR images generated by optical SAR  106  can be superior to that of RF-SAR because the optical signal can be modulated at an extremely high rate. For example, optical signals generated by a laser can be modulated by a signal sweeping from between about 1 GHz to 6 GHZ, achieving a range resolution of about 3 centimeters. 
         [0044]    An example of the transmitter and receiver is shown in  FIG. 6 . Here, transmitter  204  generates an amplitude modulated optical signal by driving transmit-transducer  602  with a modulation signal output from a waveform generator  600 . Transmit-transducer  602  includes one or more devices such as semiconductor laser diodes  606  that are driven by amplifier  604 . Thus, light that is emitted by laser diodes  606  is a carrier signal that is amplitude modulated in accordance with the modulation signal and transmitted through one or more optical elements  608  as optical signals  610  to form beam  108 . Other types of light emitting devices may be used such as Light Emitting Diodes (LEDs), solid-state lasers, gas lasers, fiber lasers, etc. In fact, any light-emitting device in which the emitted light intensity can be controlled by the modulation signal at a desired SAR frequency can be used. The use of lasers may have some advantages, because they can be modulated at very high rates, and because they lend themselves to precise control of the optical beamwidth. However, it is not necessary that the lasers have long-term coherency, and if many laser devices are employed simultaneously for the purpose of increasing the transmitter power, the individual lasers do not need to be coherently synchronized to each other. 
         [0045]    Reflected optical signals  622  may be detected by a receive-transducer  614  that includes one or more light sensitive devices  618  such as photodiodes, phototransistors, Charge-Coupled Devices, photo-multiplier tubes, etc.  FIG. 6  shows photodiodes  618  receiving reflected optical signals  622  through one or more optical elements  620 . The intensity of reflected optical signals  622  is directly translated into output electrical signals by photodiodes  618  without the need for a mixer, optical down-converter, or a separately generated carrier signal. The output electrical signals are amplified by receive amplifiers  616  into received signals and then forwarded to waveform receiver  612  for conversion into digital signals as SAR-data for storing into SAR memory  302  via T/R controller  202 . 
         [0046]      FIG. 7  shows a specific circuit example of one or more laser diodes  606  being driven by an amplifier  604 . Multiple laser diodes can be connected to a single amplifier to increase power of the transmitted beam.  FIG. 8  shows a photodiode  618 , biased by biasing circuit  800 , outputting electrical signals to receive amplifier  616 . Here also, multiple photodiodes  618  may be used where multiple output electrical signals may be summed using a receive summing amplifier  616  and a summed value is output as the received signals to waveform receiver  612 . 
         [0047]    Returning to  FIG. 6 , optical elements  608  and  620  may include:
       1. One or more lenses to focus beam  108  and/or reflected optical signals,   2. one or more polarization filters to generate beam  108  of a specific polarity or to receive reflected optical signals of a specific polarity, and   3. one or more spectrum filters to irradiate target  110  with one or more specific frequencies and/or to receive reflected optical signals at one or more specific frequencies that are matched to the transmit frequencies.       
 
         [0051]    As discussed above, depending on the information needed regarding a target, specific optical properties of beam  108  may be required. Such properties are obtained by one or more optical elements  608 . For example, if multiple frequency ranges are desired to be simultaneously transmitted, transmitter  204  may use multiple amplifier/light emitters to concurrently transmit multiple frequency optical signals in beam  108 , such as red, blue, and green to form a color image, for example. In this case, light emitting devices having substantial energy in wavelengths different from each other may be used. Alternatively or in addition, optical elements  620  may receive reflected optical signals  622  through red, blue, and green filters, for example. 
         [0052]    Optical elements  608  and  620  may be implemented by disposing frequency filters and/or light polarizers in the path of optical signals  610  emitted from light emitting devices such as laser diodes  606  or in the path of reflected optical signals  622  before reaching light sensitive devices  618  such as photodiodes  618 . For example, a specific frequency filter may be placed over each light emitting device. Although, for laser diodes  606 , filters are most likely not needed because these devices emit light in a specific frequency range. However, if light emitters that emit white light are used, then light filters could be used to filter the emitted light. The light filters can be placed over light sensitive devices  618  such as photodiodes to select a specific frequency range. If lenses are needed to set a beamwidth, for example, these may also be similarly disposed in the path of emitted optical signals  610 . Lenses may be used to receive reflected optical signals  622 . As is well known, lenses may have actuating elements such as motors to change focal lengths and other optical properties if needed. 
         [0053]    Light emitting devices  606  and associated optical elements  608  may be disposed relative to a mirror that is mounted with motorized gimbals for the vertical and azmuth directions so that a launch direction of beam  108  may be set to aim at target  110 . Similarly, light sensitive devices  618  and optical elements  620  may be disposed relative to a mirror that is mounted with a second set of gimbals so that light from a specific direction may be received. As is well known, the gimbals may be set based on a platform position and orientation, and a location that beam  108  is to be aimed or a direction from which reflected optical signals  622  are to be received. As an example, U.S. Pat. No. 8,040,525, herein incorporated by reference, discloses a laser tracking device having a mirror mounted on motorized gimbals. 
         [0054]    T/R controller  202  may control optical elements  608  and  620  by setting one or more values in optical-element-control-registers  607  and  619 , for example. Optical-element-control-registers  607  and  619  control optical treatments (focus, polarity, frequency range, e.g., red, blue or green) of beam  108  prior to transmission and a treatment of reflected optical signals  622  before detection by light sensitive devices  618 . T/R controller  202  may set values in optical-element-control-registers  607  and  619  while initializing transmitter  204  and receiver  206  prior to starting transmission of beam  108 . 
         [0055]      FIG. 9  shows an exemplary block diagram of waveform generator  600  that may include a transmit-control-register  900 , a waveform source  902 , and a waveform selector  912 . Waveform source  902  includes signal generators  904 - 910  such as one or more of linear chirp generators  904 , pseudo-random code generators  906 , Barker code generators  908 , etc. Any type of signal generators may be used that is compatible with SAR applications such as a signal that has a high degree of auto-correlation, for example. Waveform generators  904 - 910  may be implemented by storing data representing a waveform in a memory and reading from the memory at a desired rate and starting at a desired location, for example. 
         [0056]      FIG. 10  shows a linear chirp signal that is a signal whose frequency increases monotonically from a first frequency f 1  to a second frequency f 2 , as shown on the left side of the figure. The top right of  FIG. 10  again shows a linear chirp modulation signal. The middle right of  FIG. 10  shows an optical signal&#39;s amplitude after modulating it with the linear chirp. The optical signal appears dark because the frequency of the optical signal is much higher than a frequency of the SAR modulation signals. Thus, there are many cycles of the optical signal for each cycle of the modulation signal. The bottom right of  FIG. 10  shows an optical signal that is modulated with an alternative binary version of the linear chirp. This type of modulation has similar performance in terms of SAR processing as the linear chirp, but may have the advantage of being easier to produce with some types of optical emitters. 
         [0057]      FIG. 11  shows an alternative SAR waveform. A pseudo-random code example is shown where each bit of the pseudo-random code is allocated a symbol space of T time. Each symbol is represented by a fixed number of pulses. In  FIG. 11 , an example of 5 pulses per symbol is shown, where each pulse has a period of t time. The pulses of each symbol modulate the optical signal as shown by the bottom waveform where each pulse includes many cycles of the optical signal because the frequency of the optical signal is far greater than that of the modulation signal. Thus, when the pseudo-random code is a 1, five pulses of the optical signal are transmitted, and when the pseudo-random code is a 0, no pulses are transmitted for 5 t pulse time periods. 
         [0058]    Yet another alternative SAR waveform uses Barker codes, which are transmitted similarly as for the pseudo-random code but the 1s and 0s are arranged according to the Barker code scheme. The Barker code is defined as a sequence of N values of +1 and −1 as follows: 
         [0000]    a j  for j=1, 2, . . . , N such that 
         [0000]    
       
         
           
             
                
               
                 
                   ∑ 
                   
                     j 
                     = 
                     1 
                   
                   
                     N 
                     - 
                     v 
                   
                 
                  
                 
                     
                 
                  
                 
                   
                     a 
                     j 
                   
                    
                   
                     a 
                     
                       j 
                       + 
                       v 
                     
                   
                 
               
                
             
             ≤ 
             1 
           
         
       
     
         [0000]    for all 1≦ν≦N. This means that the autocorrelation of a barker code is between −1 and +1 for all non-zero shifts. Some known Barker codes are listed in the table below. If a binary 1 represents +1, a binary 0 represents −1, then the amplitude modulation using Barker codes is similar to the amplitude modulation using pseudo-random code except that the code bits are specified according to the Barker code scheme. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Table of known Barker codes 
               
             
          
           
               
                 Length 
                 Code 1 
                 Code2 
               
               
                   
               
             
          
           
               
                 2 
                 +1 −1 
                 +1 +1 
               
               
                 3 
                 +1 +1 −1 
               
               
                 4 
                 +1 +1 −1 +1 
                 +1 +1 +1 −1 
               
               
                 5 
                 +1 +1 +1 −1 +1 
               
               
                 7 
                 +1 +1 +1 −1 −1 +1 −1 
               
               
                 11 
                 +1 +1 +1 −1 −1 −1 +1 −1 −1 +1 −1 
               
               
                 13 
                 +1 +1 +1 +1 +1 −1 −1 +1 +1 −1 +1 −1 +1 
               
               
                   
               
             
          
         
       
     
         [0059]    Returning to  FIG. 9 , T/R controller  202  initializes transmit-control-register  900  by loading a bit pattern that arms one or more of signal generators  904 - 910  so that they are ready to start outputting modulation signals when a start command is received from T/R controller  202 . Similarly, another bit pattern is loaded that controls waveform selector  912  to connect the armed signal generators to desired amplifiers  604  in transmit-transducer  602 . For example, if transmitter  204  has a single amplifier  604  and if only linear chip is desired to be used as a modulation signal, linear chirp generator  904  is armed and waveform selector  912  is set to connect an output of linear chirp generator  904  to one amplifier  604 . When the start command is received, linear chirp generator  904  begins outputting the linear chirp modulation signal, and transmit-transducer  602  begins to emit optical signals  610  directed at target  110 . 
         [0060]    If multiple signal generators  904 - 910  and/or multiple amplifiers  604  are desired to be used concurrently, then waveform selector  912  may be a M×N signal switch that is capable of connecting selected signal generators to appropriate selected amplifiers  604 . For example, if red, blue, and green frequency bands are all desired, these colors are to be concurrently transmitted, and each of the colors are to be modulated with a different modulation signal, then three of signal generators  904 - 910  and three amplifiers  604  may be selected. 
         [0061]      FIG. 12  shows an exemplary block diagram of waveform receiver  612  that may include a receive-control-register  1200 , a frequency-down-converter  1202 , a signal selector  1204 , and an analog-to-digital converter  1206 . Received signals from amplifiers  616  may be connected directly to analog-to-digital converter  1206  for conversion into received SAR-data if the frequency range of the received signals is low enough. Current analog-to-digital converters may have speeds sufficient to process signal frequencies of up to about 1. GHz. If received signal frequency exceeds 1 GHz, then it may be necessary to first down-convert the received signals to a lower frequency more compatible with analog-to-digital converter  1206 . If down-converting is required, an output of frequency-down-converter  1202  is connected to analog-to-digital converter  1206  instead of the received signals. 
         [0062]    Frequency-down-converter  1202 , signal selector  1204 , and analog-to-digital converter  1206  are controlled by data stored in receive-control-register  1200 . Bit fields may be dedicated to each of these components to control their functions. For example, frequency-down-converter  1202  may have capabilities to down-convert in different frequency ranges. Contents of receive-control-register  1200  may select a specific down-convert frequency amount. Additionally, frequency-down-converter  1202  may be turned off or put on stand-by mode if not needed to conserve power and reduce ambient noise. Signal selector  1204  is controlled by contents of receive-control-register  1200  to select the received signals or the output of frequency-down-converter  1202  as input to analog-to-digital converter  1206 . Analog-to-digital converter  1206  may be controlled by a start/stop signal to time the SAR-data output so that SAR-data of a desired range is obtained. 
         [0063]    Waveform receiver  612  may include multiple frequency-down-converters  1202 , signal selectors  1204  and analog-to-digital converters  1206  to service multiple receive amplifiers  616 . For example, if red, blue and green reflected signals are to be detected concurrently, each of the colors is received by a different receive amplifier  616 . Received signals from amplifiers  616  are concurrently frequency down-converted, if needed, and converted to digital SAR-data by separate analog-to-digital converters  1206 . As a second example, two analog-to-digital converters  1206  may be used concurrently to capture the real and imaginary channels of a complex signal from amplifiers  616  or frequency-down-converter  1202 . 
         [0064]      FIG. 13  shows an exemplary block diagram of T/R controller  202  that may include a T/R processor  1300 , a local memory  1302 , a timer  1304 , a DMA controller  1306 , a transmitter/receiver I/F  1308 , an optical elements I/F  1310 , and a SAR computer I/F  1312 . These components are coupled together by a bus  1314  configured in a bus architecture. As in  FIG. 3  discussed in connection with SAR computer  200 , the bus architecture is shown as an example for ease of explanation. Other types of connections may be used as is well known in the computer architecture art. T/R processor  1300  may be implemented by using a general-purpose computer or specialized computers implemented by hardware components such as Application Specific Integrated Circuits (ASICs), PLAs, PALs, FPGAs, etc. as is well known in the computer art. The functions performed by T/R processor  1300  may be implemented using software programs executing in either the general purpose or the special purpose computers. 
         [0065]    SAR computer  200  and T/R controller  202  may be implemented by a single hardware unit that performs all the required functions with the beneficial result of fewer components and fewer interfaces. The functions of the SAR computer  200  and the T/R controller  202  may be software implemented with hardware interfaces to transmitter  204  and receiver  206 . 
         [0066]    An example of a SAR operation in T/R controller  202  may be as follows. After receiving a command for a SAR operation from SAR computer I/F  1312 , T/R processor  1300  initializes waveform generator  600  and waveform receiver  612  by loading appropriate instructions in the form of bit patterns into transmit-control-register  900  and receive-control-register  1200 . The instructions may be retrieved from a table stored in local memory  1302 . The table may be indexed by possible commands that may be received from SAR processor  300 . T/R processor  1300  similarly initializes optical-element-control-registers  607  and  619  to aim or focus beam  108  if needed and to set optical elements  608  and  620  to the various other optical treatment settings as may be commanded by SAR processor  300 . 
         [0067]    T/R processor  1300  determines memory allocations in SAR memory  302  that is reserved for the upcoming SAR operation. The memory allocations may be determined by the command sent from SAR processor  300 , or determined by a generalized memory usage scheme. For example, a dedicated portion of SAR memory  302  may be allocated to operate in a swing buffer manner so that new SAR data is uploaded into one buffer while the another buffer is being read by the SAR processor  300 . T/R processor may initialize DMA controller  1306  with addresses and buffer sizes. These parameters may be stored in local memory  1302 , for example, so that SAR data received from waveform receiver  612  may be streamed into SAR memory  302  via SAR computer I/F  1312 . 
         [0068]    T/R processor  1300  initializes timer  1304  by setting timer-values that is determined by a range of interest. A first timer-value sets a time from a start of transmission of beam  108  to a start of analog-to-digital converter  1206  to begin generating SAR-data. A second timer-value sets a time between the start of transmission to a stop of transmission of beam  108 . A third timer-value sets a time between the start of transmission to a stop of analog-to-digital converter  1206  to stop generating further SAR-data. 
         [0069]    After timer  1304  is initialized, T/R processor  1300  issues a start command to timer  1304 , and waveform generator  600 . At this time, transmitter  204  may also respond to the start command by applying power to transmit-transducer  602  to begin transmitting beam  108 . Transmit-transducer  602  may have been turned off or placed in standby mode to save power, for example. But it is not necessary to place the transmit-transducer  602  in an off or standby mode because receiver  206  can be controlled to receive only the signals of interest regardless of a state of transmitter  204 . When the first timer-value expires, a start command is issued to analog-to-digital converter  1206  to begin generating the SAR-data. As analog-to-digital converter  1206  outputs portions of the SAR data, DMA controller  1306  uploads the outputted portions into SAR memory. 
         [0070]    When the second timer-value expires, a stop transmission command is issued to waveform generator  600  to stop generating modulation signals. At this time, transmit-transducer  602  may be returned to off or standby mode or simply permitted to continue transmitting. Receiver  206  is independently controlled to receive the desired reflected optical signals. Analog-to-digital converter  1206  continues to output SAR-data portions because reflected optical signals  622  resulting from a desired portion of transmitted beam  108  may still be received. When the third timer-value expires, a stop command is issued to analog-to-digital converter  1206  because SAR data generation is completed. DMA controller  1306  also receives this stop command and stops streaming data from analog-to-digital converter  1206  to SAR memory  302 . When the commanded SAR operation is completed, T/R processor  1300  returns an operation-completed message to SAR processor  300 . After receiving the operation completed message, SAR processing may proceed to process the SAR-data to obtain desired information such as images and target data such as motion detection and automatic target recognition, for example. 
         [0071]    The above example relates to a single transmission of a single beam. However, as discussed above, transmitter  204  and receiver  206  are capable of more complex SAR operations. For example, concurrent or fast sequential transmissions of multiple beams  108  of different optical frequency ranges and/or different optical properties such as different polarizations are possible. For fast sequential operation, the transmit-control-register  900 , receive-control-register  1200 , optical-element-control-registers  607  and  619  and DMA controller  1306  may be capable of instruction stacks arranged in a FIFO (first-in-first-out) configuration, for example. Similarly, timer-values in timer  1304  may be in multiple groups where each group controls one transmission event. In this way, a rapid sequence of beams  108  may be transmitted corresponding to each point  114  of a synthetic aperture. 
         [0072]      FIG. 14  shows a flow-chart  1400  of an exemplary process for T/R controller  202 . In step  1402 , the process checks if a SAR command has been received from SAR processor  300 . If a SAR command has been received, the process goes to step  1404 . Otherwise, the process returns to step  1402 . In step  1404 , the process initializes DMA controller  1306  to upload SAR-data to allocated address locations in SAR memory  302 , and goes to step  1406 . DMA controller  1306  may be designed to follow a predetermined memory usage scheme, in which case initializing address allocation would not be necessary and the process arms DMA controller  1306  to be prepared for the upcoming SAR-data. 
         [0073]    In step  1406 , the process initializes timer  1304  for sequencing transmitter  204  and receiver  206  to transmit beam  108  and receive reflected optical signals  622  for a range of interest, and the process goes to step  1408 . For example, first, second and third timer-values of timer  1406  may be set to the desired values. These timer-values determine the time to start analog-to-digital converter  1206 , the time to stop waveform generator  600  and transmit-transducer  602 , and the time to stop analog-to-digital converter  1206  relative to the start of transmission of beam  108 . In step  1408 , the process initializes transmit-control-register  900 , optical-element-control-register  607  of transmitter  204 , receive-control-register  1200 , and optical-element-control-register  619  of receiver  206 , and the process goes to step  1410 . 
         [0074]    In step  1410 , the process issues a start command to timer  1304 , DMA controller  1306  and waveform generator  600  to begin a SAR operation, and the process goes to step  1412 . In step  1412 , the process checks if the SAR operation is completed. If the SAR operation is completed, the process goes to step  1414 . Otherwise, the process returns to step  1412 . In step  1414 , the process reports To SAR processor  300  that the SAR operation has completed, goes to step  1416  and ends. 
         [0075]      FIG. 15  shows a flow-chart  1500  of an exemplary process of timer  1304  for transmission of a single beam  108  for a single frequency as opposed to a sequence of beams  108  of multiple frequencies. In step  1502 , the process checks if an initialize command has been received. If an initialize command has been received, the process goes to step  1504 . Otherwise, the process returns to step  1502 . In step  1504 , the process initializes all timer values such as first, second and third timer-values, for example, and the process goes to step  1506 . In step  1506 , the process checks if a start command has been received. If a start command has been received, the process goes to step  1508 . Otherwise, the process returns to step  1506 . In step  1508 , the process starts required timers, and the process goes to steps  1510  and  1514  concurrently. For example, for a single transmission of beam  108  of a single frequency, first, second and third timers (corresponding to the first, second and third timer values) are started. Also, here it is assumed that the third timer value is larger than the first and second timer values, because, normally, analog-to-digital converter  1206  should not be turned off before it was started and before the transmission of beam  108  has ended. 
         [0076]    In step  1510 , the process checks if the first timer has expired. If the first time has expired, the process goes to step  1512 , stops transmission of beam  108  and then goes to step  1518 . Otherwise, the process returns to step  1510 . In step  1514  and concurrently with steps  1510  and  1512 , the process checks if the second timer has expired. If the second timer has expired, the process goes to step  1516  and starts analog-to-digital converter  1206  and goes to step  1518 . Otherwise the process returns to step  1514 . 
         [0077]    In step  1518 , the process checks if the third timer has expired. If the third timer has expired, then the process goes to step  1520 . Otherwise, the process returns to step  1518 . In step  1520 , the process stops analog-to-digital converter  1206  and goes to step  1522 . In step  1522 , the process sends a message to T/R controller  202  that the SAR operation is completed and goes to step  1524  and ends. 
         [0078]      FIG. 16  shows a flow-chart  1600  of an exemplary process for transmitter  204 . For ease of explanation, it is assumed that only a single beam  108  is being transmitted at a single frequency range. As noted above, if a more complex transmission is desired such as rapid sequential transmission of multiple beams at multiple frequencies, transmit-control-register  900  and optical-element-control-register  607  may in fact be stacks operating like FIFOs, for example. In such cases, a sequencer may be added to sequence the actions of transmitter  204 . 
         [0079]    In step  1602 , the process checks if an initialization value has been received. If an initialization value has been received, the process goes to step  1604 . Otherwise, the process returns to step  1602 . In step  1604 , the process initializes the transmit-control-register  900  and optical-element-control-register  607  based on the initialization value and goes to step  1606 . The optical elements  608  changes to the settings loaded into the optical-element-control-register  607 , and waveform generator  600  selects the specific ones of amplitude modulation waveform generators  904 - 910 , and changes these components into active mode from off or standby mode if not already in the active mode. Also, waveform selector  912  is set to the selection indicated in transmit-control-register  900 . In step  1606 , the process checks if a start command has been received. If a start command has been received, the process goes to step  1608 , starts the selected one of amplitude modulation generators  904 - 910  and goes to step  1610 . Otherwise, the process returns to step  1606 . The amplitude modulation signal generated by the selected amplitude modulation generator  904 - 910  is connected to a selected amplifier  604  which modulates the amplitude of the optical signal emitted by one or more optical light elements  606  to generate an amplitude modulated optical signal. The amplitude modulated optical signal is transmitted through optical elements  608  to form beam  108  directed at target  110 . 
         [0080]    In step  1610 , the process checks if a stop command has been received. If a stop command has been received, the process goes to step  1612 , places waveform generators  904 - 910  and transmit-transducer  602  to standby or off mode, goes to step  1614  and ends. As noted above, transmitter  204  may continue to function without affecting the SAR operation. However, in the interest of conserving power, the various components of transmitter  204  may be turned off or placed in a power saving mode such as a standby mode, for example. If a stop command has not been received, the process returns to step  1610 . 
         [0081]      FIG. 17  shows a flow-chart  1700  of an exemplary process for receiver  206 . For ease of explanation, it is assumed that only a single beam  108  has been transmitted at a single frequency range. As noted above in connection the transmitter, if a more complex transmission is desired such as rapid sequential transmission of multiple beams at multiple frequencies, receive-control-register  1200  and optical-element-control-register  619  may in fact be stacks operating like FIFOs, for example. In such cases, a sequencer may be added to sequence the actions of receiver  206 . 
         [0082]    In step  1702 , the process checks if an initialization value has been received. If an initialization value has been received, the process goes to step  1704 . Otherwise, the process returns to step  1702 . In step  1704 , the process sets receive-control-register  1200  and optical-element-control-register  619  based on the initialization value and goes to step  1706 . Analog-to-digital converter  1206 , signal selector  1204  and frequency-down-shifter  1202  are set according to the contents of receive-control-register  1200 . Optical elements  620  are set to states according to the contents of optical-element-control-register  619 . These components may have been turned off or placed in a standby mode to conserve power. In such a case, these components are placed in an active mode ready to immediately process reflected optical signals  622 . In step  1706 , the process checks if a start command has been received. If the start command has been received, the process goes to step  1708 . Otherwise, the process returns to step  1706 . 
         [0083]    In step  1708 , the process starts analog-to-digital converter  1206  to begin generating SAR-data and outputting the SAR-data to DMA controller  1306  for storing in SAR memory  302 , and the process goes to step  1710 . In step  1710 , the process checks if a stop command has been received. If a stop command has been received, the process goes to step  1712 , stops analog-to-digital converter  1206  from generating further SAR-data, goes to step  1714  and ends. Otherwise, the process returns to step  1710 . Again, when the stop command is received, various components of receiver  612  may be turned off or placed in a standby mode to save power. 
         [0084]    While the invention has been described in conjunction with exemplary embodiments, these embodiments should be viewed as illustrative, not limiting. Various modifications, substitutes, or the like are possible within the spirit and scope of the invention.