Radar system and radar signal processing method

Abstract

To detect a target in long range high resolution and Doppler high resolution.SOLUTION: According to an embodiment, the range resolution and the Doppler resolution are limited by the narrow band of the range frequency axis and the slow-time axis limitation, the effects of the range walk and the Doppler walk are reduced, the target is tentatively detected by CFAR or the like, and the search range by the tentative detection is limited using the wide-band range high-resolution data and the Doppler high-resolution data, so that the processing scale is reduced. Then, the range walk (range cell deviation due to velocity) and the Doppler walk (Doppler cell deviation due to acceleration) are corrected, and FFT processing is performed in the slow-time axis, so that the target is detected in range high resolution and Doppler high resolution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present embodiment relates to a radar system and a radar signal processing method for detecting small targets at long distances. [Background technology]

[0002] Conventional radar systems have had the problem of integration loss due to range walk and Doppler walk when there are a large number of integration hits or when long-term integration is performed with a long PRI (Pulse Repetition Interval) and a long CPI (Coherent Pulse Interval).

[0003] To address this issue, for example, there are integrated sequence maximization methods disclosed in Patent Documents 1 and 2. The method disclosed in Patent Document 1 utilizes the entire chirp band, achieves pseudo-high resolution on the range frequency axis by zero-padding, and maximizes the integrated sequence on the slow-time axis using a search method. However, with this method, when the range walk is large, the application range of the search method increases, resulting in an increase in the processing scale. Meanwhile, the method disclosed in Patent Document 2 maximizes the integrated sequence using a velocity and acceleration search method. However, even with this method, when the range walk or Doppler walk is large, the range of the search method increases, resulting in an increase in the processing scale. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4881239 [Patent Document 2] Patent No. 5025403 [Patent Document 3] Patent No. 5072694 [Non-patent literature]

[0005] [Non-Patent Document 1] SAR (Synthetic Aperture Radar) Method (Range Compression), Ouchi, 'Fundamentals of Synthetic Aperture Radar for Remote Sensing', Tokyo Denki University Press, pp.131-149 (2003) [Non-patent document 2] CFAR (Constant False Alarm Rate), Yoshida, 'Revised Radar Technology', Institute of Electronics, Information and Communication Engineers, pp.87-89 (1996) [Non-patent document 3] PGA (Phase gradient autofocus), Charles V. Jakowatz, 'Spotlight-Mode Synthetic Aperture Radar: A Signal Processing Approach', Springer, pp.251-256(1996) [Non-patent document 4] Window Functions, Takebe, 'Design of Digital Filters', Tokai University Press, pp.62-65(1985) Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, conventional radar systems have a method for maximizing the integral sequence in order to reduce integration losses due to range walk and Doppler walk. However, in the case of long-term integration, it is necessary to process a long-term FFT over the entire search range, which poses a problem of increasing the overall processing scale.

[0007] An object of this embodiment is to provide a radar system and a radar signal processing method that can reduce integration loss due to range walk and Doppler walk with a small processing scale even during long-term integration. [Means for solving the problem]

[0008] In order to solve the above problem, a radar system according to an embodiment includes: At PRI (Pulse Repetition Interval) intervalsIt receives the reflected wave of a transmitted single pulse or modulated pulse signal of N (N≧2) hits and extracts the target signal from the received signal. A CPI (Coherent Pulse Interval) signal, which is two-dimensional data on the slow-time axis between the PRIs and the fast-time axis for each range cell, is generated from the received signal, and the CPI signal is distributed to the first and second systems. In the first system, the fast-time axis of the distributed CPI signal is converted into a range-frequency axis by FFT (Fast Fourier Transform), the range-frequency axis is divided by Mf (Mf≧2) to generate Mf band-limited divided signals, the Mf divided signals are sequentially selected and zero-filled outside the band of the selected divided signals and then subjected to inverse FFT processing to convert them into narrowband range-compressed signals, the slow-time axis of the CPI signal is divided by Ms (Ms≧2) to generate Ms time-limited divided signals, and the Ms divided signals are sequentially selected and subjected to FFT processing to convert them into a Doppler axis, the narrowband range compressed and Doppler axis converted Mf×Ms signals are each amplitude-integrated, and reflection points with amplitudes equal to or greater than a predetermined value are provisionally detected from the amplitude-integrated Mf×Ms signals to extract a provisional detection range cell Rt (t=1 to Pt: Pt is the provisional detection number). In the second system, the distributed CPI signal is wideband range compressed over the entire receiving band, and for each range of the provisional detection range cell Rt, Sv×Sa×Sb integral series with Sv velocities, Sa accelerations, and Sb range biases are set as the range walk search range for each provisional detection, the maximum integral series with the maximum value is searched for from the Sv×Sa×Sb integral series, the signal of the provisional detection range cell is replaced with the signal of the searched maximum integral series to correct the range walk and Doppler walk, and the signal is converted into a range-Doppler signal by slow-time axis FFT processing, and the signal of a reflection point above a predetermined value from the range-Doppler signal is output as a target signal. That is, frequency band limiting and slow-time axis limiting are performed, reflection points are tentatively detected, range compression is performed across the entire band, range walk and Doppler walk are corrected, and FFT processing of the entire slow-time axis is performed to detect the target signal.

[0009] As described above, in the embodiment, the range resolution and Doppler resolution are limited by narrowing the range frequency axis and limiting the slow-time axis, reducing the effects of range walk and Doppler walk, and then tentative detection is performed using CFAR. Wideband high-resolution range data and high-resolution Doppler data are used to limit the search range for tentative detection and reduce the processing scale, and the range walk (range cell shift due to velocity) and Doppler walk (Doppler cell shift due to acceleration) are corrected, and an FFT is performed on the slow-time axis, making it possible to detect targets with high range resolution and high Doppler resolution.

[0010] A radar system according to an embodiment receives a reflected wave of a single pulse or a modulated pulse signal with N (N≧1) hits transmitted from a transmission system and extracts a target signal from the received signal. The radar system includes a first FFT processing means, a second FFT processing means, and a detection means. The first FFT processing means sequentially inputs a signal having Ls (Ls≧1) cells on the slow-time axis and Nf (Nf≧1) cells on the fast-time axis as division units, and performs a first FFT process on each unit on the slow-time axis to obtain Doppler cells (ls=1 to Ls). The second FFT processing means performs a second FFT process of Ms points for each Doppler cell using Ms (Ms≧2) division units that have already been processed. The detection means detects targets by long-term integration of the signals processed by the second FFT processing means, which are arranged for each original Doppler cell (ls=1 to Ls).

[0011] That is, by performing the first FFT process on the division units that are input sequentially and the second FFT process using a plurality of division units that have already been processed, it is possible to reduce the processing scale and realize FFT processing that takes a long time.

[0012] Furthermore, the radar system according to the embodiment receives the reflected wave of a single pulse or a modulated pulse signal with N (N≧1) hits transmitted from a transmission system and extracts a target signal from the received signal. First, a signal with Ns (N≧1) cells on the slow-time axis and Nf (N≧1) cells on the fast-time axis is used, and an FFT is performed on the fast-time axis to generate a range frequency axis. The range frequency is then divided into Mf (M≧1) parts, and the slow-time axis is divided into Ms (Ms≧1) parts. Of the Mf×Ms divided signals, signals with zero padding on the fast-time axis and no zero padding on the slow-time axis are used within the remaining Nf×Ns regions. The fast-time axis is range compressed, and the FFT-processed Mf×Ms signal on the slow-time axis is amplitude integrated, and provisional detection is performed to extract Pt (Pt≧1) range cells Rt and Doppler cells Ft. Next, the range is compressed using signals from the entire range frequency band, and the slow-time axis is divided into Ms divisions, after which a second FFT is performed Ms times for each division unit. For each Doppler cell (ls = 1 to Ls) in each division unit, an integral series (Sv x Sa x Sb) along the slow-time axis is set for the Ms points of slow-time axis data, centered on the provisionally detected Pt cells, using the velocity (Sv ways) and acceleration (Sa ways) in a predetermined search range and the range cell bias (Sb ways). The range shift amount is set by the phase gradient of the range-frequency axis so that each integral series becomes the Rt cell on the range axis, and range walk correction is performed by rearranging the results on the slow-time axis.Then, the Ms-point integral series (before the slow-time axis FFT) with the largest value of the Sv × Sa × Sb results obtained by performing a second FFT (Ms points) on the slow-time axis is selected, and the results of the second FFT on the slow-time axis at Ms points are replaced with Doppler cells (ls = 1 to Ls).The Doppler cells (ls = 1 to Ls) are then detected using range-Doppler data repeated for each provisional detection Pt.

[0013] In other words, by narrowing the range frequency axis and limiting the slow-time axis, the range resolution and Doppler resolution are limited, reducing the effects of range walk and Doppler walk, and then tentative detection is performed using CFAR. Using wideband high-resolution range data and high-resolution Doppler data from a two-stage FFT, the search range for tentative detection is limited to reduce the processing scale, and the range walk (range cell shift due to velocity) and Doppler walk (Doppler cell shift due to acceleration) are corrected, allowing targets to be detected with high range resolution and high Doppler resolution.

[0014] By using the above method, even when integration is performed for a long time, it is possible to realize it with a small processing scale and also to reduce losses due to range walk and Doppler walk. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a block diagram showing the configuration of a transmission system and a reception system of a radar system according to the first embodiment. [Figure 2A] FIG. 2A is a flowchart showing the flow of processing in the entire reception system of the first embodiment. [Figure 2B] FIG. 2B is a flowchart showing the flow of processing in the entire reception system of the first embodiment. [Figure 3] FIG. 3 is a diagram illustrating the processing state of the reception system of the first embodiment. [Figure 4] FIG. 4 is a diagram showing the narrowband range compression process used in the first embodiment. [Figure 5] FIG. 5 is a diagram showing the time-division FFT processing and amplitude integration processing used in the first embodiment. [Figure 6] FIG. 6 is a diagram showing the wideband range compression process used in the first embodiment. [Figure 7A] FIG. 7A is a diagram showing the range walk and Doppler walk correction processing used in the first embodiment. [Figure 7B]FIG. 7B is a diagram showing the range walk and Doppler walk correction processing used in the first embodiment. [Figure 8] FIG. 8 is a diagram showing the slow-time axis FFT processing used in the first embodiment. [Figure 9A] FIG. 9A is a flowchart showing the flow of processing in the entire reception system of the second embodiment. [Figure 9B] FIG. 9B is a flowchart showing the flow of processing in the entire reception system of the second embodiment. [Figure 10] FIG. 10 is a flowchart showing the flow of processing for preventing false detection using the integral sequence search method in the second embodiment. [Figure 11] FIG. 11 is a diagram showing the process of preventing the occurrence of false detection shown in FIG. [Figure 12A] FIG. 12A is a flowchart showing the flow of processing in the entire reception system of the third embodiment. [Figure 12B] FIG. 12B is a flowchart showing the flow of processing in the entire reception system of the third embodiment. [Figure 13] FIG. 13 is a flowchart showing the flow of a countermeasure process for preventing erroneous detection by PGA in the third embodiment. [Figure 14] FIG. 14 is a diagram showing the process of preventing the occurrence of false detection by the PGA shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments will be described with reference to the drawings.

[0017] (First embodiment) The first embodiment will be described with reference to FIGS.

[0018] FIG. 1 is a block diagram showing the configuration of a radar system according to the first embodiment, where (a) is a block diagram showing the configuration of a transmission system, and (b) is a block diagram showing the configuration of a reception system.

[0019] In the transmission system shown in Figure 1(a), a signal generator 11 generates a transmission seed signal, a modulator 12 modulates and multiplexes transmission information onto the transmission seed signal, a frequency converter 13 converts the modulated signal into a high-frequency signal, a pulse modulator 14 pulse-modulates the high-frequency signal to generate a transmission pulse train, and N (N≧2) hit pulses are transmitted from a transmission antenna 15 at PRI (Pulse Repetition Interval) intervals.

[0020] In the receiving system shown in FIG. 1(b), the reflected wave of the pulse signal transmitted from the transmitting antenna 15 is received by the receiving antenna 21, and the received signal is frequency converted to baseband by the frequency converter 22 and converted to a digital signal by the AD converter 23 to obtain a CPI (Coherent Pulse Interval) signal (fast-time and slow-time signal).

[0021] Next, the CPI signal output from the AD converter 23 is distributed to two systems. In one system, narrowband range compression is performed by a narrowband range compressor 24, the slow-time axis is divided by a slow-time axis divider 25, the slow-time axis is FFT-processed by a slow-time axis FFT processor 26, and the reflection point observation value is provisionally detected by a CFAR (Constant False Alarm Rate; see Non-Patent Document 2) provisional detector (not necessarily a CFAR) 27. The narrowband signal is generated to reduce the range resolution and suppress the effects of range walk, and the slow-time axis is divided to reduce the Doppler resolution and suppress the effects of Doppler walk. Note that dividing the range frequency band and dividing the slow-time axis reduces the signal-to-noise ratio (SN), so the amplitude integration of the processing results for each division unit prevents this reduction in SN and enables provisional detection.

[0022] In the other system, wideband range compressor 28 performs wideband range compression in the transmitted chirp band and inputs the result to range walk / Doppler walk corrector 29. This corrector 29 corrects the range walk and Doppler walk of the wideband range compressed output using the observation values ​​of the reflection points provisionally detected by CFAR provisional detector 27, and slow-time axis FFT processor 30 performs slow-time axis FFT processing to replace it with a slow-time axis signal passing through the provisional detection point. After that, CFAR detector (or a non-CFAR) 31 detects the observation values ​​of the reflection points, performs target determination, and performs distance, speed, and angle measurements, outputting them as target information.

[0023] The transmission system and the reception system may be integrated or may be installed at separate locations.

[0024] In the radar system configured as described above, the processing operation after the reception system acquires received data will be described with reference to Fig. 2 to Fig. 8. Here, Fig. 2A and Fig. 2B are a flowchart showing the overall processing flow of the reception system of the first embodiment, Fig. 3 is a diagram showing the range frequency band and slow-time axis division processing of the reception system of the first embodiment and the processing up to tentative detection, Fig. 4 is a diagram showing the narrowband range compression processing used in the first embodiment, Fig. 5 is a diagram showing the time-division FFT processing and amplitude integration processing used in the first embodiment, Fig. 6 is a diagram showing the wideband range compression processing used in the first embodiment, Fig. 7A and Fig. 7B are diagrams showing the range walk and Doppler walk correction processing used in the first embodiment, and Fig. 8 is a diagram showing the slow-time axis FFT processing used in the first embodiment.

[0025] First, the signal reflected by a target or the like from the N (N≧2) hits of transmission pulses transmitted from the transmission system shown in Figure 1(a) is received by the receiving antenna 21, frequency converted to baseband by the frequency converter 2, and converted to a digital signal by the AD converter 23, thereby obtaining the received data. In this way, the received data obtained by receiving the N (N≧2) hits of transmission pulses is called CPI (Coherent Pulse Interval) data based on PRI (Pulse Repetition Interval) data. This CPI data is two-dimensional data with a fast-time axis for each range cell and a slow-time axis between the PRIs.

[0026] Here, the long-term integration method used in this embodiment will be described. Data is acquired for each range cell within the PRI for each pulse transmitted at the PRI interval. This acquired data is used for long-term integration processing. In the case of long-term integration, the range cells and Doppler cells move relative to the slow-time axis due to the target's speed and acceleration. Therefore, when high-resolution range compression and slow-time axis FFT processing are performed, the target SN (signal-to-noise ratio) decreases. To address this issue, this embodiment considers reducing the range resolution and Doppler resolution and performing provisional detection.

[0027] That is, in the radar system according to this embodiment, in the transmission system, a transmission seed signal is generated by a signal generator 11, a modulated signal is generated by a modulator 12, this is converted into a high-frequency signal by a frequency converter 13, pulse-modulated by a pulse modulator 14, and N (N≧2) hit pulses are transmitted from a transmission antenna 15.

[0028] In the receiving system, the signal received by the receiving antenna 21 is frequency-converted by a frequency converter 22, converted to a digital signal by an AD converter 23, and output as a received signal. Next, if the received signal is a pulse-compressed signal (see Non-Patent Document 1), the pulse is compressed (range-compressed) in a band-limited narrow band (24), the slow-time axis (PRI axis with N hits) is divided (25), FFT processing is performed on the divided slow-time axis (26), and reflection points are tentatively detected using CFAR or the like (27). The narrow-band signal is used to reduce the range resolution and suppress the effects of range walk, and the slow-time axis is divided to reduce the Doppler resolution and suppress the effects of Doppler walk. Note that dividing the range frequency band and dividing the slow-time axis reduces the signal-to-noise ratio, so the processing results for each division are amplitude-integrated to prevent this reduction in signal-to-noise ratio and perform tentative detection.

[0029] Meanwhile, the digital signal from the AD converter 23 is wideband range compressed using the transmitted chirp band (28), and then range walk and Doppler walk corrections are made based on the provisionally detected reflection point, replacing it with the slow-time axis signal passing through the provisionally detected point (29), after which slow-time axis FFT processing is performed (30), and the target is detected using CFAR or the like to obtain target information (31).

[0030] In the above configuration, the processing of the reception system of this embodiment will be described with reference to Fig. 2A, Fig. 2B, and Fig. 3. Fig. 2A and Fig. 2B are flowcharts showing the flow of processing of the reception system of this embodiment. Fig. 3 is a diagram showing the state of processing of the reception system of this embodiment.

[0031] In the radar system of this embodiment, as shown in Fig. 2A, a wideband pulse signal is transmitted and received (step S11), and an FFT is performed on the fast-time axis using a CPI (Coherent Pulse Interval) signal (fast-time and slow-time signal, Fig. 3(a)) from the AD converter 23, converting it to the range-frequency axis (step S12, Fig. 3(b)). Using this signal, the signals on the range-frequency axis and the slow-time axis are divided by Mf and Ms, respectively, to obtain Mf × Ms signals (steps S13 and S14, Fig. 3(c)). On the range-frequency axis, signals other than the divided signals are zero-padded. On the other hand, on the slow-time axis, zero-padding is not performed to reduce the number of FFT points. For each of these Mf×Ms signals, pulse compression is performed on the fast-time axis (step S15) and FFT is performed on the slow-time axis (step S16), to obtain low-resolution signals on the range axis and Doppler axis.Furthermore, the Mf×Ms signals are amplitude integrated (steps S17 to S21), and a tentative detection point is determined using CFAR or the like (step S22).

[0032] Let us formulate this part. For ease of understanding, we will explain range compression and slow-time axis FFT separately. In practice, the processing is performed in the order of range compression followed by slow-time axis FFT, or vice versa.

[0033] First, narrowband range compression will be described with reference to Fig. 4 (see Non-Patent Document 1). Fig. 4 is a diagram showing the narrowband range compression process used in this embodiment. Range compression is a correlation process between an input signal and a range compression signal, and can be formulated as follows when performed in the frequency domain:

[0034] First, when the input signal (Fig. 4(a1)) and the reference signal (Fig. 4(a2)) are subjected to FFT on the fast-time axis, equations (1) (Fig. 4(b1)) and (2) (Fig. 4(b2)) are obtained. When these are band-limited, equations (3) (Fig. 4(c1)) and (4) (Fig. 4(c2)) are obtained. When the band-limited signal and the reference signal are conjugate-multiplied, equations (5) (Fig. 4(d1) and (Fig. 4(d2)) are obtained. do.

[0035]

number

[0036]

number

[0037] As a result, the resolution of the range cells in narrowband range compression and wideband range compression can be made the same, and cells provisionally detected in narrowband range compression can be directly used in wideband range compression.

[0038] Next, the time-division FFT processing will be formulated with reference to Fig. 5. Fig. 5 is a diagram showing the time-division FFT processing used in this embodiment.

[0039] First, the input signal (Fig. 5(a)) is expressed as equation (7), and when the band is limited and the slow-time axis is divided (Fig. 5(b)), and FFT processing is performed on the slow-time axis, equation (8) is obtained (Fig. 5(c)).

[0040]

number

[0041] As mentioned above, when moving from range compression to slow-time axis FFT, by replacing equation (6) with the input signal of equation (7) and performing the processing of equations (7) and (8), a signal that has undergone fast-time axis pulse compression and slow-time axis FFT processing for the signal in Figure 5(c) is obtained. If amplitude integration is then further performed, the signal in Figure 5(d) is obtained, and reflection points can be tentatively detected using CFAR or the like.

[0042] The processing after the provisional detection will be described with reference to FIG. 2B and FIG. 2B, after the provisional detection, the fast-time axis is subjected to wideband range compression (step S23), the slow-time axis is divided and subjected to FFT processing (step S24), the range and Doppler integral series are corrected using the integral series search method (step S25), slow-time axis FFT processing is performed (step S26), and the results are saved (step S27). At this point, the velocity number is determined (step S28). If the predetermined velocity number is not reached, the velocity is changed (step S29) and the processes of steps S25 to S28 are repeated. If the predetermined velocity number is exceeded, it is determined whether the predetermined acceleration has been reached (step S30). If not, the acceleration is changed (step S31), and the processes of steps S25 to S30 are repeated. If the predetermined acceleration has been reached, it is determined whether the range bias adjustment has been completed (step S32). If the adjustment has not been completed, the range bias is changed (step S33), and the processes of steps S25 to S32 are repeated. If range bias adjustment is complete, the maximum sequence is extracted from the FFT processing result (step S34), the maximum sequence is replaced (step S35), and it is determined whether the number of provisional detections has reached the maximum value (step S36). If not, the number of provisional detections is changed and the processing of steps S25 to S36 is repeated. If the number of provisional detections has reached the maximum value, FFT processing on the slow-time axis is performed (step S38), and the target cell is detected using CFAR or the like (step S39).

[0043] Fig. 6 shows the wideband range compression process used in this embodiment. That is, after tentative detection, wideband range compression is performed on the fast-time axis using the original input signal (Fig. 6(a1)) and the reference signal (Fig. 6(a2)), and an FFT is performed on the fast-time axis (Fig. 6(b1) and (Fig. 6(b2))), and then complex conjugate multiplication of both signals is performed (Fig. 6(c)). The process at this time is as follows:

[0044]

number

[0045]

number

[0046] Next, range walk and Doppler walk correction will be described with reference to Figures 7A and 7B, which are diagrams showing the range walk and Doppler walk correction processes used in this embodiment.

[0047] First, the processing results of the narrowband range compression and the divided slow-time axis are tentatively detected using CFAR or the like, and range cells are extracted for each tentative detection to obtain range cells Rt (t = 1 to Pt: Pt is the number of tentative detections). As the unit of correction, signals divided on the slow-time axis are generated, and FFT processing is performed on the slow-time axis for each divided signal (Figure 7A(a)).

[0048] Next, wideband range compression is performed, and the slow-time axis uses the signal before FFT (RDwide), and the velocity (Sv) and acceleration (Sa) of the specified search range and the range cell bias (Sb) are used to set an integral series (Sv × Sa × Sb) along the slow-time axis, centered on the provisionally detected Rt cell (Fig. 7A(b)). This integral series is expressed as follows:

[0049]

number

[0050] Each integral sequence is range-walk corrected to form the Rt cell on the range axis and rearranged on the slow-time axis. This range-walk correction involves shifting the sequence on the range-frequency axis (Fig. 7A(c)), which will be described later. The integral sequence (before slow-time axis FFT processing) with the largest value of the Sv × Sa × Sb results obtained by FFT processing this sequence on all slow-time axes is selected (Fig. 7B(d)). For each range Rt, the data of N cells on the slow-time axis is replaced with the maximum integral sequence, and this process is repeated for the Pt provisionally detected cells to obtain RDcal data (Fig. 7B(e)). This RDcal data is then subjected to slow-time axis FFT processing and detection using CFAR or other methods (Fig. 7B(f)). The input signal (Fig. 8(a)) at this time is transformed into the signal shown in Fig. 8(b) by slow-time axis FFT processing.

[0051] The above processing procedure limits the search range through tentative detection to reduce the processing scale, and then performs range walk correction and Doppler walk correction, enabling processing with a high SN ratio, making it possible to detect targets while suppressing false detections.

[0052] As described above, in this embodiment, a single pulse or modulated signal having Ns (Ns≧1) cells on the slow-time axis and Nf (Nf ≧1) cells on the fast-time axis is used, and first, FFT processing is performed on the fast-time axis to create a range frequency axis. After that, the range frequency is divided into Mf (Mf≧1) parts, and the slow-time axis is divided into Ms (Ms≧1) parts. The Mf×Ms divided signals are used, and the remaining Nf×Ns regions are filled with zeros. The fast-time axis is range compressed, and the FFT processed Mf×Ms signal on the slow-time axis is amplitude integrated, and provisional detection is performed using CFAR or the like to extract Pt (Pt≧1) range cells Rt and Doppler cells Ft. Next, range compression is performed using signals across the entire range frequency band. For the slow-time axis, the pre-FFT signal (RDbef) for the entire slow-time axis is used. The velocity (Sv) and acceleration (Sa) within a predetermined search range, along with the range cell bias (Sb), are used to define an integral sequence (Sv × Sa × Sb) along the slow-time axis, centered on the provisionally detected Pt cells. Range walk correction is then performed on each integral sequence to create the Rt cell on the range axis. After rearranging the integral sequence on the slow-time axis, the FFT processing is performed on the Sv × Sa × Sb sequence. The integral sequence with the largest peak value (local maximum) is selected and used to replace the pre-FFT signal (RDbef) on the slow-time axis. This process is repeated for each provisionally detected Pt, and the RDaft signal is used to perform a slow-time axis FFT. The results are used for detection using CFAR or other methods.

[0053] In other words, by narrowing the range frequency axis and limiting the slow-time axis, the range resolution and Doppler resolution are limited, reducing the effects of range walk and Doppler walk, and then tentative detection is performed using CFAR or the like. Wideband high-resolution range data and high-resolution Doppler data are used to limit the search range for tentative detection and reduce the processing scale, and then range walk (range cell shift due to velocity) and Doppler walk (Doppler cell shift due to acceleration) are corrected, and FFT processing is performed on the slow-time axis, making it possible to detect targets with high range resolution and high Doppler resolution.

[0054] (Second embodiment) In the first embodiment, a method for range walk and Doppler walk correction was described. Here, a method for performing range walk correction on the range frequency axis will be described. To perform range walk and Doppler walk correction for each candidate (Sv × Sa × Sb) integral sequence for each target Pt, it is necessary to correct the range axis shift for each cell on the slow-time axis, as shown in FIG. 7A(c). In this case, if the shift amount is determined on a range cell basis, quantization errors will occur in the shift amount, degrading the Doppler axis side lobes during FFT processing on the slow-time axis and reducing the signal-to-noise ratio (SN), resulting in false detections. In this embodiment, a solution to this problem will be described with reference to FIGS. 9A and 9B, 10, and 11.

[0055] Figures 9A and 9B are flowcharts showing the flow of processing in the entire receiving system of the second embodiment, Figure 10 is a flowchart showing the flow of processing to prevent false detection in the second embodiment, and Figure 11 is a diagram showing the state of the processing to prevent false detection shown in Figure 10. In Figures 9A and 9B, the same parts as in Figures 2A and 2B are denoted by the same reference numerals, and only different parts will be described here.

[0056] The receiving system of this embodiment differs from the first embodiment in the processing for preventing false detections by correcting the range and Doppler integral series of the integral series search method in step S40 in Figure 9B. This processing involves inputting range-compressed slow-time axis data (step S401), performing fast-time axis FFT processing, and converting it to the range-frequency-slow-time axis (step S402). Next, for each integral series (Sv x Sa x Sb combinations), the amount of range shift is calculated for each cell on the slow-time axis (step S403, Figure 11(a)). Furthermore, for each cell on the slow-time axis, the phase gradient corresponding to the amount of range shift is calculated using the following equation (step S404, Figure 11(b)).

number

[0057] The range frequency axis is corrected by the phase gradient for each cell on the slow-time axis (step S405), and the range frequency axis is subjected to inverse FFT to obtain a range compressed slow-time axis signal (step S406).

[0058]

number

[0059] This result is repeated for each integral series (Sv × Sa × Sb ways) and the integral series that maximizes the result of the slow-time axis FFT processing is extracted, thereby obtaining the range walk and Doppler walk correction results (Figure 11(c)).

[0060] As described above, in this embodiment, the range walk correction is performed by setting the range shift amount using the phase gradient of the range frequency axis. In other words, the range walk is corrected with a high accuracy of less than a range cell unit using the phase gradient of the range frequency axis, so that targets can be detected with high range resolution by performing FFT processing on the slow-time axis.

[0061] (Third embodiment) In the first and second embodiments, we described methods for correcting range shifts on each slow-time axis due to range walk and Doppler walk (velocity and acceleration). In this case, even if range cell shifts are corrected, phase correction due to velocity and acceleration within the range cells is not possible. In this embodiment, an example of applying a method similar to PGA (Phase Gradient Autofocus, see Non-Patent Document 3), an autofocus method for synthetic aperture processing (see Patent Document 3), will be described with reference to FIGS. 12 to 14. FIGS. 12A and 12B are flowcharts showing the overall processing flow of the receiving system of the third embodiment. FIG. 13 is a flowchart showing the flow of processing to prevent false detections by PGA in the third embodiment. FIG. 14 illustrates the processing to prevent false detections by PGA shown in FIG. 13. In FIGS. 12A and 12B, parts that are the same as those in FIGS. 2A and 2B and 9A and 9B are designated by the same reference numerals, and differences will be described here.

[0062] The difference between this embodiment and the first and second embodiments is that a false detection countermeasure process (step S41) using PGA is added after the maximum series replacement process (step S35) in FIG. 12B and before the determination of the end of the provisional detection count (step S36).

[0063] The process to prevent false detection by the PGA described above involves first inputting the range compression-slow-time (Doppler) axis signal of the range cell provisionally detected by range walk and Doppler walk correction (step S411, Figures 14(a1) and 14(a2)), and then extracting the peak value (maximum value) that exceeds a predetermined amplitude threshold value (step S412, Figures 14(b1) and 14(b2)).

[0064] Next, the signals on the Doppler axis are rearranged so that the peak value is shifted to zero frequency on the Doppler axis (step S413). This is to remove the phase gradient of the peak value relative to the Doppler axis and extract only the phase shift due to velocity and acceleration.

[0065] Next, a ±R (R≧1) cell centered around the peak value (0 Doppler) is multiplied by a window function (Non-Patent Document 4), and a signal s0 is generated by padding the area outside the window function with zeros (step S414, Fig. 14(b1), Fig. 14(b2)). This signal is then subjected to inverse FFT processing (step S415, Fig. 14(c)). This is to remove the phase-shifted vibration component and obtain a stable correction component, as shown in the following equation.

[0066]

number

[0067] Next, the correction amount Wc(ts), which has the inverse characteristics of this signal S0(ts), is calculated as the correction value for the signal on the slow-time axis as shown in the following equation, and the input signal is corrected (step S416, Figure 14(c)). After this correction, FFT processing is performed to obtain the signal on the Doppler axis.

[0068]

number

[0069] As described above, in this embodiment, the signal outputs of the ±R cells centered on the Doppler cell that exceeds a predetermined amplitude threshold are extracted, the Doppler frequency is shifted to 0, and the remaining signals are zero-filled, and the inverse FFT processing is performed on the signal, and the conjugate complex value of the result is used as a correction coefficient to add PGA processing to correct the signal on the slow-time axis. This extracts the phase change on the slow-time axis, corrects the Doppler walk by inverse correction, and then performs FFT on the slow-time axis, enabling high-sensitivity target detection.

[0070] In the embodiment, a system that transmits and receives radar waves has been described, but the present invention can also be applied to a passive radar that uses a transmission source of another system (including communications, etc.).

[0071] Furthermore, the present invention is not limited to the above-described embodiments, and the components can be modified and embodied in practice without departing from the spirit of the invention. Furthermore, various inventions can be formed by appropriately combining multiple components disclosed in the above-described embodiments. For example, some components may be omitted from all the components shown in the embodiments. Furthermore, components from different embodiments may be appropriately combined. [Explanation of symbols]

[0072] 11... signal generator, 12... modulator, 13... frequency converter, 14... pulse modulator, 15... transmitting antenna, 21... receiving antenna, 22... frequency converter, 23... AD converter, 24... narrow-band range compressor, 25... slow-time axis divider, 26... slow-time axis FFT processor, 27... CFAR temporary detector, 28...wideband range compressor, 29...range walk / Doppler walk corrector, 30...slow-time axis FFT processor, 31...CFAR detector.

Claims

1. A radar system that receives a reflected wave of a single pulse or a modulated pulse signal of N (N≧2) hits transmitted at PRI (Pulse Repetition Interval) intervals, and extracts a target signal from the received signal, A CPI (Coherent Pulse Interval) signal is generated from the received signal, which is two-dimensional data of a slow-time axis between the PRIs and a fast-time axis for each range cell; Distributing the CPI signal into a first system and a second system; a fast-time axis of the CPI signal distributed to the first system is subjected to FFT (Fast Fourier Transform) processing to convert it into a range frequency axis, the range frequency axis is divided by Mf (Mf≧2) to generate Mf band-limited divided signals, the Mf divided signals are sequentially selected and zero-filled outside the band of the selected divided signals and then subjected to inverse FFT processing to convert them into narrow-band range compressed signals, the slow-time axis of the CPI signal is divided by Ms (Ms≧2) to generate Ms time-limited divided signals, the Ms divided signals are sequentially selected and subjected to FFT processing to convert them into a Doppler axis, the narrow-band range compressed and Doppler axis converted Mf×Ms signals are amplitude-integrated respectively, reflection points having amplitudes equal to or greater than a predetermined amplitude are provisionally detected from the amplitude-integrated Mf×Ms signals, and a provisional detection range cell Rt (t=1 to Pt: Pt is the provisional detection number) is extracted; The CPI signal distributed to the second system is subjected to wideband range compression over the entire receiving band, and for each range of the provisional detection range cell Rt, Sv × Sa × Sb integral series with Sv velocities, Sa accelerations, and Sb range biases are set as the range walk search range for each provisional detection, a maximum integral series with a maximum value is searched for from the Sv × Sa × Sb integral series, the signal of the provisional detection range cell is replaced with the signal of the searched maximum integral series to correct the range walk and Doppler walk, and the signal is converted into a range-Doppler signal by slow-time axis FFT processing, and a signal of a reflection point having a predetermined value or more from the range-Doppler signal is output as a target signal. Radar system.

2. 2. The radar system according to claim 1, wherein the range walk correction is performed by converting the range axis into a range frequency axis by fast-time axis FFT processing, setting a phase gradient according to the amount of range shift, and then converting the range axis into a range frequency axis by inverse fast-time axis FFT processing.

3. A radar system as described in claim 1, wherein the range-Doppler signal, which has been wideband range compressed in the second system and range walk corrected to convert the slow-time axis to the Doppler axis, is shifted to 0 Doppler to suppress the phase gradient, a signal within a range of ±R cells centered on a Doppler cell that exceeds a predetermined amplitude threshold is extracted, and signals outside the range of ±R cells are zero-filled and subjected to inverse FFT processing, and the resulting conjugate complex signal is used as a correction coefficient to correct the phase of the range-Doppler signal.

4. A radar signal processing method for receiving a reflected wave of a single pulse or a modulated pulse signal of N (N≧2) hits transmitted at PRI (Pulse Repetition Interval) intervals, and extracting a target signal from the received signal, comprising: A CPI (Coherent Pulse Interval) signal is generated from the received signal, which is two-dimensional data of a slow-time axis between the PRIs and a fast-time axis for each range cell; Distributing the CPI signal into a first system and a second system; a fast-time axis of the CPI signal distributed to the first system is subjected to FFT (Fast Fourier Transform) processing to convert it into a range frequency axis, the range frequency axis is divided by Mf (Mf≧2) to generate Mf band-limited divided signals, the Mf divided signals are sequentially selected and zero-filled outside the band of the selected divided signals and then subjected to inverse FFT processing to convert them into narrow-band range compressed signals, the slow-time axis of the CPI signal is divided by Ms (Ms≧2) to generate Ms time-limited divided signals, the Ms divided signals are sequentially selected and subjected to FFT processing to convert them into a Doppler axis, the narrow-band range compressed and Doppler axis converted Mf×Ms signals are amplitude-integrated respectively, reflection points having amplitudes equal to or greater than a predetermined amplitude are provisionally detected from the amplitude-integrated Mf×Ms signals, and a provisional detection range cell Rt (t=1 to Pt: Pt is the provisional detection number) is extracted; The CPI signal distributed to the second system is subjected to wideband range compression over the entire receiving band, and for each range of the provisional detection range cell Rt, Sv × Sa × Sb integral series with Sv velocities, Sa accelerations, and Sb range biases are set as the range walk search range for each provisional detection, a maximum integral series with a maximum value is searched for from the Sv × Sa × Sb integral series, the signal of the provisional detection range cell is replaced with the signal of the searched maximum integral series to correct the range walk and Doppler walk, and the signal is converted into a range-Doppler signal by slow-time axis FFT processing, and a signal of a reflection point having a predetermined value or more from the range-Doppler signal is output as a target signal. Radar signal processing method.

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