Abstract:
A system for reducing CFAR loss due to sea clutter is disclosed. The system includes a first channel tuned for CFAR gain in a spatially correlated background and a second channel tuned for low CFAR loss in spatially uncorrelated backgrounds. Each of the channels employs a distribution free CFAR using rank ordered statistics to establish a constant false alarm rate. The output of each channel is fused by a hit correlation function and the stream of combined hits is processed by a target centroiding function.

Description:
[0001]     This application claims priority on U.S. Provisional Patent Application Ser. No. 60/627,652 filed Nov. 12, 2004 the disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is directed to improvements in radar and in particular improvements in reducing CFAR loss due to sea clutter.  
       BACKGROUND OF THE INVENTION  
       [0003]     Radar backscatter from an ocean surface is commonly referred to as sea clutter. Any radar backscatter not due to the scattering from an ocean surface can be considered a potential target. The amplitude statistics of clutter have been modeled by Rayleigh, log-normal, contaminated-normal, Weibull, log-Weibull and K-distributions. Maritime surveillance radar relies on non-coherent processing techniques and the amplitude statistics of the sea clutter backscatter determines radar sensitivity and false alarm performance.  
         [0004]     In one type of surveillance radar system designed to detect low radar cross section targets such as periscopes or small watercraft against a background of sea clutter, high range resolution is applied to minimize sea clutter competing with the target. The scan rate is chosen to be sufficiently high to provide multiple observations while the target is present. The system sensitivity is set to provide a high probability of detecting small targets of interest while accepting a high likelihood of also detecting tens of thousands of false plot detections per radar scan from noise and sea clutter. The false plot detections are filtered out over the observation time using integration along-a-path techniques such as retrospective track-before-detect processing that are capable of operating in a high false activity rate environment without incurring miss-association errors that are typically encountered in Track-While-Scan systems. The retrospective track-before-detect process maintains a history of scan level detections over the observation time and integrates along prior velocity trajectories searching for persistent returns that are indicative of a real target. The plots from sea clutter and noise will have little correlation in their position from scan to scan, being “noise like” and will have a low probability of integrating up and passing the retrospective process. The end result is a track picture that is sufficiently clean of false and clutter tracks that it can be readily understood.  
         [0005]     A key factor that determines the detection performance is the ability to provide a first threshold on each range/azimuth cell that maintains a near Constant False Alarm Rate to fit within the radars processing resources in the presence of an unknown statistical background. There are numerous Constant False Alarm Rate techniques in the literature that develop thresholds based on a measure of the background region around the cell under test using both parametric or non-parametric algorithms. The parametric CFAR requires knowledge of the underlying statistics while the non-parametric CFAR is distribution free. CFAR techniques are based on an interference background (noise and sea clutter) that is assumed to be spatially uncorrelated. Large background regions are typically used to minimize the CFAR loss (which is the loss between the ideal threshold and the one computed by the CFAR). A guard band is typically employed that separates the cell under test from the background region and prevents extended targets from entering the background region and affecting sensitivity. CFAR designed for spatially uncorrelated backgrounds will introduce a higher threshold in the upwind direction to adapt to the higher clutter levels returned.  
         [0006]     Recent studies of sea clutter phenomenology by Watts and Ward suggest that at certain aspect angles relative to the wind the sea clutter is spatially correlated (upwind and downwind). CFAR techniques have been developed to take advantage of this property and they result in a significant CFAR gain (&gt;5 dB) when compared to standard techniques that have a large background region. These CFAR gain techniques require that the background estimate be derived from a region very close to the cell under test and also that the background size be on the order of the spatial correlation interval. Since spatial correlations are on the order of 10 meters, the number of background cells is much less than what is typically used in traditional CFAR designed for low CFAR loss. Therefore, CFAR designed for CFAR gain in correlated backgrounds will have high CFAR loss in uncorrelated backgrounds such as crosswind and noise limited conditions. Another problem is that CFAR gain techniques have problems with extended targets since the background measurement is taken with virtually no guard band.  
         [0007]     To mitigate this problem, the current method is to build a clutter map where the statistics of each region are measured over time. The appropriate CFAR parameters are applied in each region. The problem with this approach is that it takes time to develop the clutter map since sufficient averaging is required to accurately characterize the clutter backscatter; the CFAR loss is higher because clutter variations within a clutter map cell are not tolerated, there are discontinuities at the clutter map cell boundaries and the clutter map solution still does not address the detection of extended targets in regions where CFAR gain techniques are applied.  
       OBJECTS OF THE INVENTION  
       [0008]     It is an object of the present invention to provide improvements in radar systems.  
         [0009]     It is also an object of the present invention to provide improvements in CFAR radar systems.  
         [0010]     It is another object of the invention to provide additional target sensitivity in connection with current radar systems particularly upwind, in higher sea states and from higher platform altitudes.  
         [0011]     It is a further object of the invention to eliminate most CFAR loss in a radar system so that enhanced target detection performance can be achieved at all aspect angles relative to the wind direction.  
         [0012]     It is a still further object of the invention to provide a dual channel spatially adaptive constant false alarm rate detection method, system and apparatus.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is directed to a dual channel spatially adaptive constant false alarm rate detection method, apparatus and system. The present invention provides CFAR gain in spatially correlated sea clutter and low CFAR loss in uncorrelated sea clutter. The present invention is capable of providing a means of detecting targets of a plurality of sizes from small point targets to large extended targets. Small point targets include but are not limiter to targets of the size of a periscope, raft and person in the water, etc. Larger extended targets include but are not limited to large surface ships, commercial vessels and combatants, etc. Optimum detection sensitivity in sea clutter environments is obtained in the present invention without sacrificing the ability to simultaneously detect small point target and large extended targets. The present invention has the ability to adapt automatically to sea clutter variations with look angle relative to the wind/swell direction.  
         [0014]     The dual channel spatially adaptive CFAR has one channel (CFAR Gain Channel) tuned for CFAR gain and this channel uses a small guard band (1 to about 10 cells) and a small background region (2 to about 10 cells). The second channel (Main Channel) is tuned for low CFAR loss in uncorrelated backgrounds and uses a large guard band (about 100 to about 400 cells) and a large background region (about 100 to about 1000 cells).  
         [0015]     The mean level subtraction in the CFAR gain channel follows the spatial correlation and establishes low thresholds (high sensitivity) in regions where there is low sea clutter backscatter over a spatially correlated interval. Each channel employs a distribution free (DF) CFAR using rank ordered statistics to establish a constant false alarm rate while the probability-of-detection (Pd) varies in each channel depending on the background statistics.  
         [0016]     For the case where the background is spatially correlated, the CFAR Gain Channel provides the best probability of detection (Pd). For the case of uncorrelated backgrounds and extended targets, the Main Channel provides the best Pd. The output of each channel is fused by a hit correlation function resulting in a stream of combined hits to be processed by the target centroiding function. With the DF property holding a constant false alarm rate, the hit correlation function is performed using a simple range association process where detections from either channel are “or&#39;ed” to form combined hits. The incorporation of parallel channels has a small impact on overall false alarm rate however since the false alarms from the two channels are highly correlated so the combined false alarm rate after the fusion process is much less than two times the single channel false alarm rate. Therefore, the loss associated with the parallel channels is low.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic Dual Channel Spatially Adaptive CFAR 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The following is an example of one embodiment of the present invention. The following describes a method, system and apparatus for implementing CFAR gain in correlated sea clutter while maintaining low CFAR loss in uncorrelated background areas. The embodiment also maintains the ability to simultaneously detect large targets and small targets as well as detect anomalies in the background region caused by land returns and nearby target returns. The method is applicable towards a wide range of maritime surveillance radars as long as the range resolution is sufficiently small relative to the sea clutter spatial correlation.  
         [0019]     An example of the present invention is illustrated in  FIG. 1 .  FIG. 1  shows two parallel processing channels performed in XILFNX on the Signal Processor Interface (SPI) card. The input radar video signal  1  is applied to two or more shift registers  10  and  20 . One shift register  10  is sized to cover a large regional area while a second shift register  20  is sized to cover the local region. Shift register  10  is preferably divided into  5  segments. Segment # 1   12  is the leading background region. Segment# 2  is the leading guard band region. Segment# 3   16  is the cell under test. Segment# 4  is the trailing guard band region and Segment# 5   11  is the trailing background region. Typically the background regions are sized large enough to provide low CFAR Loss and small enough to control contamination from large targets that may affect sensitivity. The guard region is sized to fit a large surface target to avoid self-cancellation. For example a guard region of 192 cells and a background region of 128 cells on each side with a 5 ft range cell avoids self cancellation for targets up to 960 ft long (192×5′); maintains full sensitivity in dense target environments as long as two extended targets are separated by at least 2565 ft (192+1+192+128×5′) and results in a CFAR loss of less than 1 dB.  
         [0020]     The cells in each background region  11  and  12  are independently averaged  13 . The averages are tested by the Edge Detection function  14  and if there is a large imbalance the background region then the lower average is selected  13 . If both averages are within a range dependent limit then they are both selected and averaged together by  13  to produce the Regional Mean. A bottom clip function  17  subtracts the Regional Mean from the cell under test  16  and clips negative numbers to a zero value producing the Main Channel Video  18 .  
         [0021]     Shift register  20  is sized to cover a local area that is on the order of the sea clutter spatial correlation. Shift register  20  is divided into preferably 6 segments. Segment # 1   22  is the leading background region. Segment # 2  is the leading guard band region. Segment # 3   26  is the cell under test. Segment # 4  is the trailing guard band region and Segment # 5   21  is the trailing background region. Typically the background regions are range from 2 to 10 cells depending on radar resolution and the guard region is set to insure that background region is within the clutter spatial correlation distance of the cell under test. It will be appreciated by those skilled in the art that other numbers of cells are possible. Segment # 6  is set so that the cell under test  26  is time aligned with  16 .  
         [0022]     The cells in each background region  21  and  22  are averaged  23  to produce a Local Mean. A bottom clip function  27  subtracts the Local Mean from the cell under test  26  and clips negative numbers to a zero value producing the CFAR Gain Channel Video  28 .  
         [0023]     The Main Channel Video  18  is applied to Shift Register  30  and the CFAR Gain Channel Video  28  is applied to Shift Register  40 . Both Shift Registers  30  and  40  are sized the generally the same and each preferably has  5  Segments. Segment # 1   32  &amp;  42  is the leading background region. Segment # 2  is the leading guard band region. Segment # 3   36  &amp;  46  is the cell under test. Segment # 4  is the trailing guard band region and Segment# 5   31  &amp;  41  is the trailing background region. Typically the background regions are 256 cells each but other numbers of cells are possible. The guard region is sized to fit a large surface target to avoid self-cancellation. Each cell in the background regions of shift register  30  are compared to the cell under test  36  and the number of background cells that are greater than the cell under test  36  are counted separately  33  for each background region. Each cell in the background region is also averaged  39  and the averages are tested by the Edge Detection function  34 . The Edge Detection function tests for a large imbalance (about 1 to about 3 dB) between the two background regions and when the condition is detected, the lower count  33  is selected  35 . This insures that sensitivity is not reduced in the presence of land or a large target. The Edge Detection function includes a range dependent compensation factor for the R 4  variation between the leading and trailing averages. If both averages are within a range dependent limit then both counts  33  are selected and summed to produce the Regional Rank. Therefore for the case when there is no edge detect, the Regional Rank is the number of background cells that are greater than the cell under test. A Main Channel Hit is declared if the Regional Rank is less than the Rank Threshold  37 . The Rank Threshold and the Background size establish the false alarm rate. For example a Rank Threshold of  6  and a total Background size of 512 produces a constant false alarm rate of 6/512=1.2%. A wide range of other values will also provide the desired false alarm rate. For the case when there is an edge detect, the background  31  or  32  associated with the lower average  39  is selected and the Regional Rank is the number of background cells that are greater than the cell under test. When an edge is detected  34 , the Rank Threshold is divided by  2  to maintain a constant false alarm rate when detecting targets close to land and in the vicinity of large surface ships.  
         [0024]     In the CFAR Gain channel, each cell in the background regions of shift register  40  are compared to the cell under test  46  and the number of background cells that are greater than the cell under test  46  are counted separately  43  for each background region. Each cell in the background region is also averaged  49  and the averages are tested by the Edge Detection function  44 . If there is a large imbalance between the two background regions, then the lower count  43  is selected  45 . If both averages are within a range dependent limit then both counts  43  are selected to produce the Regional Rank. A CFAR Gain Channel Hit is declared if the Regional Rank is less than the Rank Threshold  47 . The Rank Threshold  47  is generally set to the same value as Rank Threshold  37  however each can be adjusted independently to set the sensitivity of each channel.  
         [0025]     Main Channel Hits and CFAR Gain Channel Hits are combined  50  preferably using an “or-ing” function to produce the final Combined Hits output  51 . The Target Extraction and Centroiding function then typically processes the Combined Hits to produce Radar Plot data for the Track-Before-Detect function. Each channel provides a constant false alarm rate due to the Distribution Free (DF) property of the rank ordered detector  37  &amp;  47 . The combined false alarm rate is somewhere between 1× to 2× the single channel false alarm rate depending on the correlation between the channels. A potential 2:1 variation in false alarm rate is manageable by the downstream processing. By holding the total false alarm rate within a 2:1 range, the target detections produced by each channel can be simply or&#39;ed by correlating each channel on a range cell basis  50 . The channel with the best sensitivity for the background therefore, provides the detection. If the target is detected in both channels, the correlation process combines them into a single hit  50 .  
         [0026]     The detection capabilities of each channel is summarized below:  
         [0027]     Main Channel detects the following:  
         [0028]     Large targets  
         [0029]     Small targets in uncorrelated backgrounds  
         [0030]     Small targets in low sea state correlated backgrounds  
         [0031]     Sea Spikes  
         [0032]     Noise  
         [0033]     CFAR Gain Channel detects the following:  
         [0034]     Small targets in correlated backgrounds  
         [0035]     Sea Spikes  
         [0036]     Noise  
         [0037]     Portions of large targets  
         [0038]     Portions of large targets from the CFAR Gain Channel that are within the same range interval as the Main Channel are fused into a single hit  50 . Likewise for small targets, sea spikes and noise detections. Each hit message  51  to the Target Extractor includes a label to identify which channel the detection was made. This label is used to set appropriate parameters in subsequent software processing steps.  
         [0039]     The preferred parameters for each channel are as follows:  
         [0000]     Main Channel  
         [0040]     Mean Level Subtract Background 128 cells on each side of the cell under test  
         [0041]     DF Background is 256 cells on each side of the cell under test  
         [0042]     Guard band set to pass large targets in both mean removal and DF segments  
         [0043]     Provides detection of small to large targets  
         [0044]     Background and guard band set for regional processing  
         [0045]     Small targets detected in moderate sea clutter and under noise limited conditions  
         [0046]     Edge detect function maintains sensitivity near lass mass returns and large surface targets  
         [0000]     CFAR Gain Channel  
         [0047]     Mean Level Subtract Background up to 2 cells on each side of the cell under test depending on the radar resolution  
         [0048]     DF Background is 256 cells on each side of the cell under test  
         [0049]     Guard band set the same as main channel DF  
         [0050]     Provides detection of small targets  
         [0051]     Background and guard band set for local processing  
         [0052]     Provides CFAR gain in correlated sea clutter  
         [0053]     Higher CFAR loss than main channel in uncorrelated backgrounds  
         [0054]     Mean subtraction edge detect is not required due to small background size  
         [0000]     Hit Correlation Function  
         [0055]     The Hit Correlation combines the detections from the two channels into a composite hit.