JP7735199B2 - Radar system and radar signal processing method - Google Patents

Description

This embodiment relates to a radar system and radar signal processing method for detecting small targets at long distances.

Conventional radar systems have had issues with integration loss due to range walk and Doppler walk when there are a large number of integration hits or when performing long integration times with a long PRI (Pulse Repetition Interval) and CPI (Coherent Pulse Interval).

One solution to this problem is the integral sequence maximization method shown in Patent Documents 1 and 2, for example. The method in Patent Document 1 uses the entire chirp band, artificially increasing the resolution of the range frequency axis by zero-padding, and maximizes the integral sequence of 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 processing scale. Furthermore, the method shown in Patent Document 2 maximizes the integral 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 processing scale.

Patent No. 4881239 Patent No. 5025403 Patent No. 5072694

SAR (Synthetic Aperture Radar) Method (Range Compression), Ouchi, 'Fundamentals of Synthetic Aperture Radar for Remote Sensing', Tokyo Denki University Press, pp.131-149(2003) CFAR (Constant False Alarm Rate), Yoshida, 'Revised Radar Technology', Institute of Electronics, Information and Communication Engineers, pp.87-89(1996) PGA (Phase gradient autofocus), Charles V. Jakowatz, 'Spotlight-Mode Synthetic Aperture Radar: A Signal Processing Approach', Springer, pp.251-256(1996) Window Functions, Takebe, 'Design of Digital Filters', Tokai University Press, pp.62-65(1985)

As mentioned above, conventional radar systems have a method for maximizing the integral sequence in order to reduce integration loss due to range walk and Doppler walk. However, in the case of long-term integration, a long-term FFT must be processed across the entire search range, which poses a problem of increasing the overall processing scale.

The objective of this embodiment is to provide a radar system and 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.

In order to solve the above problems, a radar system according to an embodiment receives a reflected wave of a single pulse or a modulated pulse signal with N (N≧2) hits transmitted at PRI (Pulse Repetition Interval) intervals , extracts a target signal from the received signal, generates a CPI (Coherent Pulse Interval) signal from the received signal, which is two-dimensional data on a slow-time axis between the PRIs and a fast-time axis for each range cell, and distributes the CPI signal to a first system and a second system. In the first system, the fast-time axis of the distributed CPI signal 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 it into a narrow-band range compressed signal, 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 it into a Doppler axis, the narrow-band 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 amplitude 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). The second system performs wideband range compression on the distributed CPI signal over the entire reception band, sets Sv × Sa × Sb integral series with Sv velocities, Sa accelerations, and Sb range biases as the range walk search range for each provisional detection for each range of the provisional detection range cell Rt, searches for the maximum integral series with the maximum value from the Sv × Sa × Sb integral series, replaces the signal of the provisional detection range cell with the signal of the searched maximum integral series to correct the range walk and Doppler walk, converts the signal into a range-Doppler signal by slow-time axis FFT processing, and outputs a signal of a reflection point with a predetermined value or greater from the range-Doppler signal as a target signal. That is, frequency band limiting and slow-time axis limiting are performed, provisional detection of reflection points is performed, full band range compression is performed, range walk and Doppler walk are corrected, and FFT processing of all slow-time axes is performed to detect the target signal.

As described above, in this embodiment, the range frequency axis is narrowband and the slow-time axis is limited to limit the range resolution and Doppler resolution, 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 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, enabling targets to be detected with high range resolution and high Doppler resolution.

The radar system according to the embodiment receives reflected waves of a single pulse or modulated pulse signal with N (N≧1) hits transmitted from a transmission system and extracts a target signal from the received signal. It comprises a first FFT processing means, a second FFT processing means, and a detection means. The first FFT processing means sequentially inputs a signal with Ls (Ls≧1) cells on the slow-time axis and Nf (Nf≧1) cells on the fast-time axis as division units, and performs first FFT processing on each unit on the slow-time axis to obtain Doppler cells (ls = 1 to Ls). The second FFT processing means performs second FFT processing 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).

In other words, by performing a first FFT process on sequentially input division units and a second FFT process using multiple division units that have already been processed, the processing scale can be reduced and long-term FFT processing can be achieved.

In addition, the radar system of the embodiment receives reflected waves of a single pulse or modulated pulse signal with N (N≧1) hits transmitted from a transmission system and extracts a target signal from the received signal. First, using a signal with Ns (N≧1) cells on the slow-time axis and Nf (N≧1) cells on the fast-time axis, an FFT is performed on the fast-time axis to create a range frequency axis, and then the range frequency is divided into Mf (M≧1) parts and the slow-time axis is divided into Ms (M≧1) parts. The Mf×Ms divided signals are then range compressed on the fast-time axis and amplitude integrated on the slow-time axis after FFT processing to extract Pt (Pt≧1) range cells Rt and Doppler cells Ft using signals from the remaining Nf×Ns regions, with the fast-time axis being zero-filled and the slow-time not being zero-filled. Next, range compression is performed using signals of the entire range frequency band, and the slow-time axis is divided into Ms divisions, and then a second FFT is performed Ms times on each division unit. For each Doppler cell (ls = 1 to Ls) of each division unit, an integral series (Sv x Sa x Sb) along the slow-time axis is set for the slow-time axis data at Ms points, centered on the provisionally detected Pt cells, using the speed (Sv ways) and acceleration (Sa ways) of a predetermined search range and the range cell bias (Sb ways). The range shift amount is set using 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. After rearranging on the slow-time axis, a second FFT (Ms points) is performed on the slow-time axis. The Ms-point integral series (before the slow-time axis FFT) with the largest maximum value of the Sv x Sa x Sb results 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). Next, the Doppler cells (ls = 1 to Ls) are detected using range-Doppler data repeated for each provisional detection Pt.

In other words, by narrowing the range frequency axis and limiting the slow-time axis, 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 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.

Using the above method, even long-term integration can be achieved with a small processing scale, and losses due to range walk and Doppler walk can be reduced.

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

The following describes the embodiments with reference to the drawings.

(First embodiment)
The first embodiment will be described with reference to FIGS. 1 to 5. FIG.

Figure 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 the transmission system, and (b) is a block diagram showing the configuration of the reception system.

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.

In the receiving system shown in Figure 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).

Next, the CPI signal output from the AD converter 23 is distributed into two systems. In one system, narrowband range compression is performed by the narrowband range compressor 24, the slow-time axis is divided by the slow-time axis divider 25, the slow-time axis is subjected to FFT processing by the slow-time axis FFT processor 26, and the reflection point observation value is provisionally detected by the CFAR (Constant False Alarm Rate; see Non-Patent Document 2) provisional detector (not necessarily 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 (SNR), so the amplitude integration of the processing results for each division unit prevents this reduction in SNR and enables provisional detection.

In the other system, wideband range compressor 28 performs wideband range compression using the transmitted chirp band, and the result is input 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 point provisionally detected by the 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, a CFAR detector (not necessarily CFAR) 31 detects the observation values of the reflection point and performs target determination, and performs distance, speed, and angle measurements, outputting them as target information.

The above-mentioned transmission system and reception system may be integrated, or may be installed in separate locations.

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

First, the signal reflected by a target or the like from N (N≧2) hits of transmitted 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 N (N≧2) hits of transmitted 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 PRIs.

Here, we will describe the long-term integration method used in this embodiment. Data is acquired in range cell units within the PRI for each pulse transmitted at PRI intervals. This acquired data is used to perform long-term integration processing. With long-term integration, the range cells and Doppler cells move relative to the slow-time axis due to the target's speed and acceleration. For this reason, performing high-resolution range compression and slow-time axis FFT processing reduces the target SN (signal-to-noise ratio). As a countermeasure, this embodiment considers reducing the range resolution and Doppler resolution and performing provisional detection.

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.

In the receiving system, the signal received by the receiving antenna 21 is frequency-converted by the frequency converter 22, converted to a digital signal by the 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 of 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 similar (27). The narrow-band signal is used to reduce range resolution and suppress the effects of range walk, and the slow-time axis is divided to reduce 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 amplitude integration of the processing results for each division unit prevents this reduction in signal-to-noise ratio and allows tentative detection.

Meanwhile, the digital signal from the AD converter 23 undergoes wideband range compression using the transmitted chirp band (28), and then range walk and Doppler walk correction is performed based on the provisionally detected reflection point to replace 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 similar to obtain target information (31).

In the above configuration, the processing of the reception system of this embodiment will be explained with reference to Figures 2A, 2B, and 3. Figures 2A and 2B are flowcharts showing the processing flow of the reception system of this embodiment. Also, Figure 3 is a diagram showing the processing state of the reception system of this embodiment.

As shown in Figure 2A, the radar system of this embodiment transmits and receives wideband pulse signals (step S11), and then performs an FFT on the fast-time axis using the CPI (Coherent Pulse Interval) signals (fast-time and slow-time signals, Figure 3(a)) from the AD converter 23, converting them to the range-frequency axis (step S12, Figure 3(b)). Using these signals, the signals on the range-frequency axis and slow-time axis are divided into Mf and Ms, respectively, to obtain Mf x Ms signals (steps S13 and S14, Figure 3(c)). On the range-frequency axis, signals other than the divided signals are zero-padded. On the slow-time axis, on the other hand, zero-padding is not performed to reduce the number of FFT points. For each of these Mf x Ms signals, pulse compression is performed on the fast-time axis (step S15) and FFT is performed on the slow-time axis (step S16), resulting in low-resolution signals on the range and Doppler axes. Furthermore, the Mf x Ms signals are amplitude integrated (steps S17-S21), and a tentative detection point is determined using CFAR or similar (step S22).

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

First, narrowband range compression will be described with reference to Figure 4 (see Non-Patent Document 1). Figure 4 shows 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:

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.

To put it on the time axis, this s can be inverse Fourier transformed as shown in the following equation (FIG. 4(e1) and (FIG. 4(e2)).

The above results are amplitude integrated to extract tentatively detected cells (FIG. 4(f)).

This allows the range cell resolution of narrowband range compression and wideband range compression to be the same, and cells provisionally detected using narrowband range compression can be directly used with wideband range compression.

Next, we will formulate the time-division FFT processing with reference to Figure 5. Figure 5 is a diagram showing the time-division FFT processing used in this embodiment.

First, the input signal (Figure 5(a)) becomes equation (7). After band-limiting and dividing the slow-time axis (Figure 5(b)), FFT processing on the slow-time axis results in equation (8) (Figure 5(c)).

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. Further amplitude integration from this gives the signal in Figure 5(d), which allows tentative detection of reflection points using CFAR, etc.

The processing after the provisional detection will be described with reference to FIG. 2B and FIG.
In FIG. 2B , after the provisional detection, wideband range compression is performed on the fast-time axis (step S23), the slow-time axis is divided and FFT processing is performed (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 by CFAR or the like (step S39).

Figure 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 (Figure 6(a1)) and the reference signal (Figure 6(a2)), an FFT is performed on the fast-time axis (Figures 6(b1) and 6(b2)), and complex conjugate multiplication of the two is performed (Figure 6(c)). The process at this time is as follows:

To put it on the time axis, this s can be inverse Fourier transformed as shown in the following equation (FIG. 6(d)).

Next, range walk and Doppler walk correction will be explained using Figures 7A and 7B. Figures 7A and 7B are diagrams showing the range walk and Doppler walk correction processes used in this embodiment.

First, the results of narrowband range compression and the divided slow-time axis are tentatively detected using CFAR or other methods, and range cells are extracted for each tentative detection to obtain range cell Rt (t = 1 to Pt: Pt is the number of tentative detections). Signals divided on the slow-time axis are generated as units for correction, and FFT processing is performed on the slow-time axis for each divided signal (Figure 7A (a)).

Next, wideband range compression is performed, and the slow-time axis uses the pre-FFT signal (RDwide). Using the provisionally detected Rt cell as the center, the speed (Sv) and acceleration (Sa) of the specified search range and the range cell bias (Sb) are used to set an integral series (Sv x Sa x Sb) along the slow-time axis (Figure 7A (b)). This integral series is expressed as follows:

Each integral series is range-walk corrected to form Rt cells on the range axis and rearranged on the slow-time axis. This range-walk correction involves shifting on the range-frequency axis (Figure 7A(c)), which will be described later. This series is subjected to FFT processing on all slow-time axes, resulting in Sv x Sa x Sb combinations. The integral series (before slow-time axis FFT processing) with the largest maximum value is selected (Figure 7B(d)). For each range Rt, the process of replacing N cells' data on the slow-time axis with the maximum integral series is repeated for Pt provisionally detected cells to obtain RDcal data (Figure 7B(e)). This RDcal data is used to perform slow-time axis FFT processing and detection using CFAR or similar (Figure 7B(f)). The input signal (Figure 8(a)) at this time (Figure 8(b)) is transformed into the signal shown in Figure 8(b) after slow-time axis FFT processing.

The above processing procedure limits the search range through tentative detection, reducing the processing scale, and then performs range walk and Doppler walk corrections, allowing processing with a high S/N ratio, making it possible to detect targets while suppressing false detections.

As described above, in this embodiment, a single pulse or modulated signal with 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.The range frequency is then divided into Mf (Mf ≧ 1) and the slow-time axis is divided into Ms (Ms ≧ 1).The Mf × Ms divided signals are then zero-filled in the remaining Nf × Ns regions, and the fast-time axis is range compressed.The FFT-processed Mf × Ms signal is amplitude integrated on the slow-time axis, and provisional detection is performed using CFAR, etc. 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. A predetermined search range is used to determine an integral sequence (Sv x Sa x Sb) along the slow-time axis, centered on the provisionally detected Pt cells, using the velocity (Sv patterns), acceleration (Sa patterns), and range cell bias (Sb patterns). Each integral sequence is then range-walk corrected to result in Rt cells on the range axis. After rearranging the sequences on the slow-time axis, the Sv x Sa x Sb results are subjected to FFT processing on the slow-time axis. The integral sequence (slow-time axis pre-FFT) with the largest peak value (local maximum value) is selected and used to replace the slow-time axis pre-FFT signal (RDbef). This is repeated for each provisionally detected Pt, and the RDaft signal is used for slow-time axis FFT processing. The results are used for detection using CFAR or similar techniques.

In other words, by narrowing the range frequency axis and limiting the slow-time axis, range resolution and Doppler resolution are limited, reducing the effects of range walk and Doppler walk, and then tentative detection is performed using CFAR, etc., and 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. Range walk (range cell shift due to velocity) and Doppler walk (Doppler cell shift due to acceleration) are then corrected, and FFT processing is performed on the slow-time axis, allowing targets to be detected with high range resolution and high Doppler resolution.

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 integral sequence (Sv × Sa × Sb) 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-by-range 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. This embodiment describes a solution to this problem with reference to FIGS. 9A and 9B, 10, and 11.

Figures 9A and 9B are flowcharts showing the overall processing flow of the receiving system in the second embodiment, Figure 10 is a flowchart showing the flow of the processing to prevent false detection in the second embodiment, and Figure 11 is a diagram showing the processing to prevent false detection shown in Figure 10. In Figures 9A and 9B, parts that are the same as those in Figures 2A and 2B are designated by the same reference numerals, and only the differences will be explained here.

The difference between the receiving system of this embodiment and the first embodiment is the countermeasure for false detections by correcting the range and Doppler integral sequences of the integral sequence search method in step S40 of FIG. 9B. This process 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 sequence (Sv × Sa × Sb), the range shift amount is calculated for each cell on the slow-time axis (step S403, FIG. 11(a)). Furthermore, for each cell on the slow-time axis, the phase gradient corresponding to the range shift amount is calculated using the following equation (step S404, FIG. 11(b)).

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

By repeating this process for each integral series (Sv x Sa x Sb combinations) and extracting the integral series that maximizes the result of the slow-time axis FFT processing, the range walk and Doppler walk correction results (Figure 11(c)) can be obtained.

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 degree of precision of less than a range cell using the phase gradient of the range frequency axis, so targets can be detected with high range resolution by performing FFT processing on the slow-time axis.

(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 the PGA in the third embodiment. FIG. 14 illustrates the processing to prevent false detections by the PGA shown in FIG. 13 . In FIGS. 12A and 12B, parts identical to those in FIGS. 2A and 2B and 9A and 9B are designated by the same reference numerals, and differences will be described here.

This embodiment differs from the first and second embodiments in that a PGA-based countermeasure process (step S41) is added after the maximum series replacement process (step S35) in Figure 12B and before the determination of the completion of the provisional detection count (step S36).

The PGA's process for preventing false detections begins by 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 extracting peak values (maximum values) that exceed a predetermined amplitude threshold value (step S412, Figures 14(b1) and 14(b2)).

Next, the Doppler axis signal is 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.

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, Figures 14(b1) and 14(b2)). This signal is then subjected to inverse FFT processing (step S415, Figure 14(c)). This is done to remove the phase-shift vibration component and obtain a stable correction component, as shown in the following equation.

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.

By using this Fs0(fs), a target can be detected with a high signal-to-noise ratio even when there is a phase change due to a change in speed or acceleration.

As described above, in this embodiment, the signal output of ±R cells centered around Doppler cells that exceed a predetermined amplitude threshold is extracted, the Doppler frequency is shifted to 0, and the remaining signals are zero-filled, followed by inverse FFT processing. The resulting conjugate complex value is used as a correction coefficient, and PGA processing is added to correct the signal on the slow-time axis. This extracts the phase change on the slow-time axis, corrects the Doppler walk through inverse correction, and then performs FFT on the slow-time axis, enabling highly sensitive target detection.

Note that while the embodiments have been described as systems that transmit and receive radar waves, the present invention can also be applied to passive radars that use transmission sources from other systems (including communications, etc.).

Furthermore, the present invention is not limited to the above-described embodiments, and in the implementation stage, the components can be modified and embodied without departing from the spirit of the invention. Furthermore, various inventions can be created by appropriately combining multiple components disclosed in the above-described embodiments. For example, some components may be deleted from all of the components shown in the embodiments. Furthermore, components from different embodiments may be appropriately combined.

11... signal generator, 12... modulator, 13... frequency converter, 14... pulse modulator, 15... transmitting antenna
21... receiving antenna, 22... frequency converter, 23... AD converter, 24... narrowband 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 (4)

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. 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 . 2. The radar system according to claim 1, wherein the range-Doppler signal, which has been subjected to wideband range compression and range walk correction in the second system and whose slow-time axis has been converted to a Doppler axis, is shifted to 0 Doppler to suppress a phase gradient, a signal within a range of ±R cells centered on a Doppler cell exceeding 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 . 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.