Abstract:
A method for developing a map of objects in a region surrounding a location is disclosed. The method includes interrogating the region along a detection axis with a series of optical pulses and detecting reflections of the optical pulses that originate at objects located along the detection axis. A multi-dimensional map of the region is developed by scanning the detection axis about the location in at least one dimension. The reflections are detected via a single-photon detector that is armed using a sub-gating scheme such that the single-photon detector selectively detects photons of reflections that originate only within each of a plurality of zones that collectively define the detection field. In some embodiments, the optical pulses have a wavelength within the range of 1350 nm to 1390 nm, which is a spectral range having a relatively high eye-safety threshold and a relatively low solar background.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This case is a continuation of co-pending U.S. patent application Ser. No. 14/147,478 (Attorney Docket: 293-033US1) filed Jan. 3, 2014, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to laser range finding in general, and, more particularly, to LiDAR. 
       BACKGROUND OF THE INVENTION 
       [0003]    Light Detection And Ranging (LiDAR) systems are attractive for use in many applications, such as driverless automobiles, farm equipment, and the like. Laser range finding is used to determine the range from a source to an object by sending a pulse of light in the direction of the object, detecting a reflection of the pulse, and determining the time required for the light to travel to and return from the object (i.e., its time-of-flight). 
         [0004]    A typical prior-art LiDAR system creates a local map around a vehicle by performing laser range finding in several directions and elevations around the vehicle. Prior-art systems accomplish this in different ways, such as using an array of laser sources, rotating a single laser source about an axis through the vehicle, or directing the output signal from a single source about the vehicle using a rotating mirror or prism, or a stationary reflective cone. For example, U.S. Patent Publication No. 20110216304 describes a LiDAR system based on a vertically oriented array of emitter/detector pairs that are rotated about an axis to provide a 360° horizontal field-of-view (FOV) and vertical FOV of several tens of degrees. This system emits multiple pulses at a high repetition rate while the emitter/detector assembly is scanned about the vehicle. The resultant distance measurements form the basis for a three-dimensional simulated image of the scene around the vehicle. 
         [0005]    The requirements for a LiDAR system used in automotive applications are quite challenging. For instance, the system needs to have a large FOV in both the horizontal and vertical directions, where the FOV is supported over a distance that ranges from approximately 5 meters (m) to approximately 300 m. Further, the system must have high resolution, as well as an ability to accommodate a changing environment surrounding a vehicle that could be travelling at relatively high speed. As a result, the system needs to be able to update the simulated environment around the vehicle at a high rate. In addition, an automotive LiDAR system needs to be able to operate at both day and night. As a result, the system needs to accommodate a wide range of ambient light conditions. 
         [0006]    The need for high-resolution performance would normally dictate the use of high laser power to ensure sufficient return signal from objects as far away as 300 m. Unfortunately, eye safety considerations limit the laser power that can be used in a LiDAR system. The safety threshold for the human eye is a function of wavelength, with longer wavelengths being, in general, safer. For example, the Maximum Permissible Energy (MPE) for a nanosecond-range light pulse is approximately six orders of magnitude higher at 1550 nanometers (nm) than at 980 nm. Unfortunately, the solar background is quite high at 1550 nm and can degrade measurement sensitivity in this wavelength regime. As a result, many prior-art LiDAR systems operate in the wavelength range from approximately 800 nm to approximately 1050 nm at low optical power levels, thereby sacrificing system performance. 
         [0007]    Some prior-art LiDAR systems do operate 1550 nm; however, FOV is normally restricted to mitigate the effects of the solar background radiation. Unfortunately, such an operational mode reduces the update rate for the system. To develop a sufficiently large image of the surrounding region, therefore, multiple systems having overlapping fields of view are required, which increases overall cost and system complexity. 
         [0008]    For these reasons, a low-cost, high-performance LiDAR system suitable for vehicular applications would represent a significant advance in the state-of-the-art. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention enables a LiDAR system without some of the costs and disadvantages of the prior art. Embodiments of the present invention are particularly well-suited for use in applications such as autonomous vehicles, adaptive automotive cruise control, collision-avoidance systems, and the like. 
         [0010]    An illustrative embodiment of the present invention is a LiDAR system comprising a transmitter and a single-photon detector, where transmitter provides nanosecond-scale optical pulses that are in a wavelength range from approximately 1350 nm to approximately 1390 nm. Operation in this wavelength regime exploits an MPE that is several orders of magnitude higher than at the 905 nm wavelength typically used in prior-art LiDAR systems. Also, by operating in this wavelength range, embodiments of the present invention can also take advantage of a narrow wavelength band in which the solar background is relatively low. In order to enable an FOV that extends to a distance of 500 m, the single-photon detector is gated at a frequency of approximately 3.3 MHz, yielding a series of 0.3 microsecond detection frames. 
         [0011]    To further reduce the impact of solar-background-induced noise, in some embodiments, the single-photon detector is operated in a “range-gated” mode. In such operation, each detection frame includes a plurality of sub-gate periods, each including a different portion of the detection frame. The single-photon detector is armed at the beginning of each sub-gate period and disarmed at the end of each sub-gate period. As a result, each of the sub-gate periods corresponds to a longitudinal slice of the detection field. By cycling through a series of detection frames, each including the plurality of sub-gate periods, the present invention enables high signal-to-noise operation with high resolution over the entire field-of-view. By using short sub-gate periods, the probability of detecting a noise photon associated with the solar background in any individual sub-gate period is dramatically reduced. 
         [0012]    In some embodiments, the results from multiple detection frames are processed using statistical analysis techniques to reduce the impact of the receipt of noise photons. In some embodiments, the results from multiple detection frames are collected in a dataset and digital thresholding is applied to the dataset to mitigate the effects of background noise. 
         [0013]    An embodiment of the present invention comprises a method for developing a map of objects relative to a first location, the method comprising: transmitting a first optical pulse at a first time, the first optical pulse having a wavelength within the range of approximately 1350 nm to approximately 1390 nm; detecting a first reflection of the first optical pulse from a first object within the detection field, wherein the first reflection is detected at a second time; and computing a distance between the first location and the first object based on the difference between the first time and the second time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  depicts a schematic drawing of a LiDAR system in accordance with the illustrative embodiment of the present invention. 
           [0015]      FIG. 2  depicts operations of a method for developing a map of objects in a detection field in accordance with the illustrative embodiment of the present invention. 
           [0016]      FIG. 3  depicts a plot of Maximum Permissible Energy (MPE) versus wavelength for pulses having different pulse widths. 
           [0017]      FIG. 4  depicts a plot of solar power density as a function of wavelength. 
           [0018]      FIG. 5  depicts a circuit diagram of a portion of a receiver in accordance with the illustrative embodiment of the present invention. 
           [0019]      FIG. 6  depicts a timing diagram for receiver operation of a LiDAR system in accordance with the illustrative embodiment of the present invention. 
           [0020]      FIG. 7  depicts a schematic drawing of a LiDAR system in accordance with a first alternative embodiment of the present invention. 
           [0021]      FIG. 8  depicts operations suitable for developing a map of a detection field in accordance with the first alternative embodiment of the present invention. 
           [0022]      FIG. 9  depicts a sub-method suitable for interrogating a detection field along a detection axis in accordance with the first alternative embodiment of the present invention. 
           [0023]      FIG. 10  depicts a timing diagram for interrogating a detection field along a detection axis in accordance with the first alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  depicts a schematic drawing of a LiDAR system in accordance with the illustrative embodiment of the present invention. System  100  includes transmitter  102 , receiver  104 , and processor  106 . 
         [0025]      FIG. 2  depicts operations of a method for developing a map of objects in a detection field in accordance with the illustrative embodiment of the present invention. Method  200  begins with operation  201 , wherein transmitter  102  transmits a train of pulses  110  along detection axis  112 . 
         [0026]    Transmitter  102  is an optical transmitter suitable for emitting a train of optical pulses having a wavelength within the range of approximately 1350 nm to approximately 1390 nm. Typically transmitter  102  generates the optical pulses using a laser source, such as a diode laser. 
         [0027]    One skilled in the art will recognize that the performance of a LiDAR system is based on several factors, such as available optical power, the sensitivity of the detector used to detect reflected pulses, and the desired sensitivity of the system. The present invention employs single-photon detectors, which greatly reduces the amount of optical power required in pulses  110 . Significant optical power is still required to enable a suitable signal-to-noise ratio (SNR) for longer detection distances, however. Unfortunately, the amount of optical power that can be used in a LiDAR system is constrained by eye-safety issues. These are particularly stringent at shorter wavelengths, such as at 905 nm, a typical operating wavelength of prior-art LiDAR systems. 
         [0028]    It is an aspect of the present invention that operation at a wavelength within the 1350-1390 nm-wavelength range enables embodiments of the present invention to take advantage of a relatively narrow intensity dip in the solar background spectrum. This wavelength range also exploits an eye-safety threshold that is more than an order of magnitude higher than at 905 nm. 
         [0029]      FIG. 3  depicts a plot of Maximum Permissible Energy (MPE) versus wavelength for pulses having different pulse widths. One skilled in the art will recognize that the MPE denotes an eye-safety threshold for the power at which a LiDAR system can safely operate. 
         [0030]    It is apparent from plot  300  that, for nanosecond pulse widths, MPE is approximately 1 microJoule/cm 2  at a wavelength of 900 nm; however, it increases to approximately 50, 5000, and 10 6  microJoules/cm 2  at the wavelengths of 1350, 1450, and 1550 nm, respectively. By operating at longer wavelengths, therefore, system  100  can provide higher laser power without exceeding the eye safety threshold. 
         [0031]    It should be noted that plot  300  also suggests that operation at 1550 nm should enable even longer ranging distance, as well as improved system performance. Unfortunately, the solar background radiation is also higher at 1550 nm wavelengths, thereby degrading the signal-to-noise ratio and negating many of the benefits of long-wavelength operation. 
         [0032]      FIG. 4  depicts a plot of solar power density as a function of wavelength. 
         [0033]    While plot  400  evinces that the power level of the solar background is quite high at 1550 nm, it also shows that it dips significantly within the range of approximately 1340 nm to approximately 1500 nm. Trace  402  indicates a sliding 40 nm-wide average of the solar energy. Window  404  denotes a 40 nm-wide wavelength window suitable for vehicular LiDAR operation, which is centered at 1370 nm in accordance with the illustrative embodiment of the present invention. It should be noted that other operational windows are in accordance with the present invention. For example, plot  400  also depicts window  406 , centered at approximately 1455 nm, which represents another possible window of operation for system  100 . Operation in this wavelength regime allows pulse  110  to be at a higher energy level by virtue of a higher MPE at this higher wavelength. 
         [0034]    Transmitter  102  transmits each of pulses  110  such that it has a duration of approximately 5 nanoseconds at a repetition rate of approximately 300 kHz. In some embodiments, each of pulses  110  is transmitted with a duration that is less than 5 ns. In some embodiments, each of pulses  110  is transmitted with a duration that is greater than 5 ns. One skilled in the art will recognize that the pulse width of pulse  110  and the repetition rate of transmitter  102  are matters of design and can that pulse  110  can have any suitable values. 
         [0035]    In the illustrative embodiment, the repetition rate of transmitter  102  is based on the desired maximum range of detection, Lmax, in detection field  114  (i.e., the size of detection field  114 ). In the illustrative embodiment, transmitter  102  transmits a train of pulses  110  at a repetition rate of 3.3 microseconds, which is based on the time-of-flight for a photon travelling to and from an object that is at a maximum detection distance that is approximately 500 meters from receiver  104 . This time-of-flight also determines the duration of each of a plurality of detection frames for system  100 , as discussed below and with respect to  FIG. 5 . 
         [0036]    It should be noted that, in the illustrative embodiment, system  100  is mounted on vehicle  108  such that transmitter  102  and receiver  104  are substantially co-located. In some embodiments, transmitter  102  and receiver  104  are not co-located and are separated by some separation distance and the repetition rate of transmitter  102  is based on the total time for a photon to travel from transmitter  102  to an object located at Lmax and be reflected back to receiver  104 . 
         [0037]    At operation  202 , processor  106  enables receiver  104  to detect reflection  116 . 
         [0038]      FIG. 5  depicts a circuit diagram of a portion of a receiver in accordance with the illustrative embodiment of the present invention. Receiver  104  comprises single-photon avalanche diode (SPAD)  502  and load  504 . In some embodiments, receiver  104  includes a negative-feedback avalanche diode (NFAD), a SPAD operatively coupled with a switching element (e.g., a transistor) operative for discharging the SPAD after an avalanche event, a photodiode operating in linear mode, or another suitable photodiode. SPADs and NFADs suitable for use in the present invention are described in detail in U.S. Pat. Nos. 7,378,689 and 7,719,029, each of which is incorporated herein by reference. 
         [0039]      FIG. 6  depicts a timing diagram for receiver operation of a LiDAR system in accordance with the illustrative embodiment of the present invention. Each detection frame depicted in timing diagram  600  (i.e., detection frame  602 - 1 ) begins at time t 0 , when transmitter  102  transmits pulse  110 . 
         [0040]    At time t 1 , processor  106  arms SPAD  502  by applying a voltage bias, V 2 , at terminal  506 , where V 2  exceeds breakdown voltage Vbr of SPAD  502 . For the purposes of this Specification, including the appended claims, “arming” a SPAD is defined as putting the diode into Geiger mode by biasing it with a bias voltage that exceeds the breakdown voltage of the SPAD. One skilled in the art will recognize that when armed, receipt of a single photon will give rise to an avalanche event that results in a macroscopically detectable current through the SPAD. In similar fashion, “disarming” a SPAD is defined taking it out of Geiger mode by reducing its bias voltage below the breakdown voltage of the SPAD. It should be noted that time t 1  is delayed slightly from time t 0  to account for the transmission duration of pulse  110 , during which detection of reflection  116  is disabled. In some embodiments, SPAD  502  is armed simultaneously with, or during, the transmission of pulse  110 . 
         [0041]    Prior to absorbing a photon, the entirety of Vbias is developed across SPAD  504  since the magnitude of avalanche current, i, is zero and there is no voltage drop across load  504 . As a result, the magnitude of Vout (provided to processor  106  at terminal  508 ) is equal to zero. Although the illustrative embodiment comprises a load that is a resistive load, in some embodiments of the present invention, load  504  comprises a circuit element in addition to or other than a resistor, such as a capacitor, inductor, or reactive element (e.g., a transistor, etc.). 
         [0042]    At operation  203 , receiver  104  detects reflection  116 , which originates from object  118 . Reflection  116  is detected at receiver  104  at time tr. 
         [0043]    Upon absorbing a photon, avalanche current, i, builds through SPAD  502  and load  504 . As a result, a voltage drop equal to i·R will appear across load  504 , where R is the resistance of load  504 . As a result, processor  106  detects a change in the magnitude of voltage Vout, at time tr. 
         [0044]    At operation  204 , processor  106  computes the distance between vehicle  108  and object  118  based on the delay between times t 0  and tr. 
         [0045]    It should be noted that the use of an NFAD in receiver  104  enables detection of multiple reflections within a single detection frame, since an NFAD can be rapidly quenched and re-armed at rates higher than the typical repetition rate of transmitter  102 , as discussed in detail in U.S. Pat. No. 7,719,029. 
         [0046]    At operation  205 , processor  106  disarms SPAD  502  at time t 2  by reducing its bias voltage below Vbias. It should be noted that time t 2  in detection frame  602 - 1  is also time t 0  of detection frame  602 - 2 . 
         [0047]    In some embodiments, operations  201  through  205  are performed for each of detection frames  602 - i , where i=1 through N and N is a desired number of detection frames over which it is desired to interrogate detection field  114  along detection axis  112 . Repeated interrogation of the same detection axis enables the use of statistical methods to increase system tolerance for false counts at SPAD  502 . In some embodiments, digital thresholding is used to increase system tolerance for false counts, as discussed below and with respect to  FIGS. 7-10 . 
         [0048]    At operation  206 , system  100  scans detection axis  112  horizontally such that system  100  interrogates a different portion of detection field  114 . In some embodiments, system  100  scans detection axis  112  vertically. Typically, detection axis  112  is scanned via a rotating element, such as a mirror, prism, and the like. In some embodiments, transmitter  102  and receiver  104  are mounted on a scanning element, such as a turntable, gimbal element, etc., which enables detection axis  112  to be scanned. 
         [0049]    In some embodiments, system  100  includes a one- or two-dimensional array of transmitters and/or receivers that are arranged to simultaneously interrogate detection field  114  along a plurality of detection axes arranged horizontally and/or vertically. 
         [0050]    At operation  207 , processor  106  develops a map of detection field  114 . 
         [0051]    It should be noted that even when system  100  operates at a wavelength within the range of 1350-1390 nm, the power level of the solar background is still quite high and can lead to false counts at receiver  104 , as shown in Table 1 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Maximum  
                   
               
               
                   
                 Solar-Power  
                 False Photon  
               
               
                 Wavelength 
                 Density on  
                 Counts per Nanosecond 
               
             
          
           
               
                 Range 
                 Ground Level 
                 High FOV Res. 
                 Low FOV Res. 
               
               
                 (nm) 
                 (W/m 2 /nm) 
                 0.075 deg 
                 0.15 deg 
               
               
                   
               
             
          
           
               
                  905 ± 20 
                 0.7 
                 12  
                 (2500 pW) 
                 46  
                 (100000 pW) 
               
               
                 1370 ± 20 
                 10 −4   
                 0.0025  
                 (0.4 pW) 
                 0.01  
                 (1.6 pW) 
               
               
                 1420 ± 20 
                 2 × 10 −2   
                 0.5  
                 (72 pW) 
                 2  
                 (290 pW) 
               
               
                   
               
             
          
         
       
     
         [0052]    Although the false-count rate at 1370 nm is much lower than at other wavelengths, even at 1370 nm, 6-30 false counts will occur in a detection frame of approximately 3 microseconds. 
         [0053]    It is another aspect of the present invention that the negative impact of solar background on system performance is mitigated by apportioning each detection frame into a plurality of sub-gate periods. In other words, by dividing each detection frame into smaller time intervals in which SPAD  502  is armed, the probability of a false count during each sub-gate period can be greatly reduced, as shown in Table 2 below. 
         [0000]    
       
         
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
                 False Photon 
               
               
                   
                 Counts per Sub-gate 
               
             
          
           
               
                 Sub-gate Periods Per 
                 High FOV Res. 
                 Low FOV Res. 
               
               
                 1 microsecond-long Detection Frame 
                 0.075 deg 
                 0.15 deg 
               
               
                   
               
             
          
           
               
                 20 (50 ns/sub-gate) 
                 0.1 
                 0.5 
               
               
                 50 (20 ns/sub-gate) 
                 0.05 
                 0.2 
               
               
                   
               
             
          
         
       
     
         [0054]      FIG. 7  depicts a schematic drawing of a LiDAR system in accordance with a first alternative embodiment of the present invention. System  700  is analogous to system  100 ; however, in operation, processor  702  is operative for logically separating detection field  114  into detection zones  704 - 1  through  704 - 3  (referred to, collectively, as zones  704 )—each of which corresponds to a different sub-gate period within each detection frame. In the first alternative embodiment, detection field  114  is separated into three zones; however, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use embodiments of the present invention wherein detection field  114  is separated into any practical number of zones. 
         [0055]      FIG. 8  depicts operations suitable for developing a map of a detection field in accordance with the first alternative embodiment of the present invention. Method  800  begins with operation  801 , wherein, for each of i=1 through N, system  700  interrogates detection field  114  along detection axis  112  during detection frame  1002 - i . N is the number of times interrogation of detection axis  112  is repeated before rotating detection axis  112  to enable system  100  to interrogate a different portion of detection field  114 . 
         [0056]    The value of N is based on a number of factors. For example, in any LiDAR system, it is important to be able to distinguish detection of a signal photon (i.e., a photon arising from reflection from an object in detection field  114 ) from a detected “noise photon” due to the solar background, a dark count, and the like. For proper system operation, a Probability of Detection (PD) of a signal photon is preferably at least 99%, while the false alarm rate (FAR) due to noise photons is preferably &lt;1%. It is yet another aspect of the present invention that, by interrogating detection axis  112  repeatedly to collect data from multiple detection frames, statistical methods and/or digital thresholding can be employed to increase PD and decrease FAR, as discussed below. 
         [0057]      FIG. 9  depicts a sub-method suitable for interrogating a detection field along a detection axis in accordance with the first alternative embodiment of the present invention. Operation  801  begins with sub-operation  901 , wherein transmitter  102  transmits pulse  110  along detection axis  112 . 
         [0058]      FIG. 10  depicts a timing diagram for interrogating a detection field along a detection axis in accordance with the first alternative embodiment of the present invention. In similar fashion to timing diagram  600  described above and with respect to  FIG. 6 , each measurement frame  1002 - i  begins at time t 0 - i , when transmitter  102  transmits pulse  110 - i . 
         [0059]    For each of j=1 through M, where M is the number of sub-gate periods within each detection frame, processor  106  enables receiver  104  to detect a reflection that originates only within zone  704 - j.    
         [0060]    Detection of a reflection originating within each zone  704 - j  is enabled at sub-operation  902 , wherein SPAD  502  is armed at time ta-i-j. Time ta-i-j corresponds to the time-of-flight for a photon between vehicle  108  and point  706 - j  (i.e., from transmitter  102  to point  706 - j  and back to receiver  104 ), which is the point in zone  704 - j  nearest vehicle  108  on detection axis  112 . 
         [0061]    At sub-operation  903 , reflection  712 - j  is detected at SPAD  502  at detection time tr-i-j. It should be noted that a reflection is not always detected from each zone  704 . For example, in the example depicted in  FIGS. 7-10 , objects are located only in zones  704 - 2  and  704 - 3 . As a result, no reflection is originated in zone  704 - 1  and Vout remains at zero volts throughout sub-gate period  1004 - i - 1 . 
         [0062]    At sub-operation  904 , detection time tr-i-j is saved in memory at processor  702 . 
         [0063]    As discussed above and with respect to system  100 , the use of an NFAD in receiver  104  enables detection of multiple reflections within a single detection frame, or in the case of system  700 , within a single sub-gate period. 
         [0064]    At sub-operation  905 , SPAD  502  is disarmed at time td-i-j. Time td-i-j corresponds to the time-of-flight for a photon between vehicle  108  and point  708 - j , which is the point in zone  704 - j  furthest from vehicle  108  on detection axis  112 . 
         [0065]    In some embodiments, SPAD  502  is not armed in each sub-gate period within each detection frame. For example, in some embodiments, a SPAD is armed only during a subset of the sub-gate periods (e.g., only during sub-gate period  1004 - i - 1 ) for a first plurality of detection frames and armed during a different subset of the sub-gate periods (e.g., only during sub-gate period  1004 - i - 2 ) for a second plurality of detection frames, and so on. 
         [0066]    By dividing each detection frame into a plurality of sub-gate periods, embodiments of the present invention—particularly those embodiments that operate at a wavelength within the range of 1350 nm to 1390 nm—are afforded advantages over prior-art LiDAR systems that operate at other wavelengths because of a significant reduction in the probability of detecting a photon due to the solar background. For example, prior-art LiDAR systems operating at a wavelength of 905 nm struggle to overcome the effects of the solar background since their detectors are blinded by solar-based photons on a substantially continuous basis. Further, prior-art LiDAR systems that operate at longer wavelengths, such as 1550, typically must deal with solar background by restricting FOV of each pixel and/or by using very narrow bandwidth filters. For vehicular LiDAR, however, FOV requirements are high. As a result, more restricted-FOV pixels must be included to cover the same system-level FOV. In addition, it is challenging to provide cost-effective wavelength filters suitable for filtering out solar background, since the filter bandwidth cannot be narrower than the spectrum of the laser used to transmit pulses  110 . Embodiments of the present invention avoid the need for restricted FOV as well as wavelength filters and, therefore, afford better system performance and lower cost. 
         [0067]    Returning now to method  800 , at operation  802 , digital thresholding is applied to dataset  714 . Digital thresholding enables an improvement in the Probability of Detection (PD) of a signal photon while also reducing the impact of “noise photons, ”such as detected photons arising from events other than reflection from an object (e.g., a solar background photon, dark count, and the like). The negative impact of noise photons on system performance is mitigated by setting a threshold value for the number of reflections detected from a zone during a plurality of detection frames (e.g., detections frame-1; through detection frame-N), and establishing a position for an object in that zone only when the number of reflections detected is equal to or greater than the threshold value. 
         [0068]    The positive impact of applying digital thresholding to dataset  714  can be readily seen in Table 3 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Data Collected Over 20 Detection Frames to Enhance SNR 
               
             
          
           
               
                   
                 Threshold # of 
                 Maximum # of 
                 Minimum # of 
                 Minimum 
               
               
                   
                 Frames Having 
                 Noise Photons 
                 Signal Photons 
                 Required 
               
               
                   
                 a Photon Count 
                 (FAR = 0.1) 
                 (PD = 0.99) 
                 SNR 
               
               
                   
                   
               
             
          
           
               
                   
                 4 
                 0.2 
                 3 
                 15 
               
               
                   
                 5 
                 0.4 
                 4 
                 10 
               
               
                   
                 6 
                 0.8 
                 4 
                 6.7 
               
               
                   
                 7 
                 1.0 
                 5 
                 6.3 
               
               
                   
                 8 
                 1.0 
                 5 
                 5 
               
               
                   
                 9 
                 1.2 
                 5.7 
                 4.8 
               
               
                   
                 10 
                 1.4 
                 6.5 
                 4.6 
               
               
                   
                 11 
                 1.75 
                 7.3 
                 4.2 
               
               
                   
                 12 
                 2.05 
                 8.2 
                 4 
               
               
                   
                 13 
                 2.42 
                 9.2 
                 3.8 
               
               
                   
                 14 
                 2.85 
                 10.5 
                 3.7 
               
               
                   
                 15 
                 3.3 
                 12 
                 3.6 
               
               
                   
                   
               
             
          
         
       
     
         [0069]    Careful examination of Table 3 shows that, for a solar background of approximately 0.5 photons, a threshold value of about 4 signal photons is required at receiver  104 . Further, approximately 1 photon due to solar background can be tolerated with 5 signal photons at a threshold level of 8 detection frames where N=20. One skilled in the art will recognize that the threshold value is consideration of system design and desired performance and that the threshold value can be any value sufficient to achieve a desired system performance. 
         [0070]    At operation  803 , processor  106  computes the distance between vehicle  108  and objects located along detection axis  112 . 
         [0071]    At operation  804 , system  700  rotates detection axis  112  to interrogate a different portion of detection field  114 . 
         [0072]    At operation  805 , processor  106  develops a map of detection field  114 . 
         [0073]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.