A Secondary Radar Signal Processing Method with Adaptive Threshold

Abstract

The present invention discloses a secondary radar signal processing method with an adaptive threshold, which is suitable for processing the target response information of a single pulse secondary radar MSSR. It includes: using an adaptive threshold to perform pulse detection on the sum channel video signal to obtain a processed sum video, called PSV; preprocessing the PSV to extract the pulse leading edge, including the real leading edge ALE, the pseudo leading edge PLE and the extra leading edge XLE; detecting the response frame; extracting the amplitude of the response code bit; allocating the detected response frame to N decoding channels, and decoding the response signal. The present invention solves the problem that the performance of the traditional pulse detection method is easily affected by the amplitude fluctuation in the pulse of the response signal, improves the detection performance under low signal-to-noise ratio, and effectively increases the detection probability of interleaved pulses.

Description

Technical Field

[0001] The invention relates to the processing of target response information of a single pulse secondary radar (MSSR), in particular to a secondary radar signal processing method with an adaptive threshold. Background Art

[0002] At present, the civil aviation traffic volume is large, there are many aerial targets, multipath reflection, asynchronous interference, and interleaving of reply signal pulses have put forward higher requirements on MSSR signal processing capabilities. The pulse detection module in MSSR signal processing determines the probability of correct decoding of the reply signal and is the basis for signal processing to correctly deinterleave the reply.

[0003] The MSSR response signal is a 1090MHz single-frequency coded pulse signal, which can be divided into two modes: mode AC and mode S. The response format is as follows: Figure 1 As shown. The response of mode AC consists of 16 information code bits, with pulses indicating "1" and no pulses indicating "0", and the standard pulse width is 0.45μs±0.1μs. The 1st and 15th pulses are frame pulses, which are always "1", indicating that there is a response. The 2nd to 14th bits are the response code, the 8th bit is an invalid bit, and the other 12 bits are valid codes. The 16th bit is the SPI code bit, which is a special position identification code. It is usually used to identify two aircraft that are close to each other or have the same response code, and is generally not used. Therefore, each response of mode AC corresponds to a 12-bit code, and the 12 bits are encoded as 4 octal numbers, ranging from 0000 to 7777. The response signal of mode S consists of two parts: 4 leading pulses and 56 or 112 response pulses. The leading pulse part is similar to mode AC, with a standard pulse width of 0.5μs±0.05μs. The response pulse part is high at the beginning and low at the end to indicate "1", and low at the beginning and high at the end to indicate "0". The response processing of mode S is similar to that of mode AC, with only differences in the leading pulse pair detection and code position discrimination parts. This description takes the response of mode AC as an example.

[0004] The pulse width of the reply signal is defined as the half-power point of the modulated RF output. The traditional reply signal pulse detection method uses the point where the pulse peak drops 6dB as the detection threshold. This method only requires simple amplitude comparison and less calculation. However, the performance of the traditional method is easily affected by the following factors:

[0005] (1) The traditional pulse detection method uses a simple threshold to detect pulses, which is easily affected by pulse amplitude fluctuations and has poor noise resistance.

[0006] (2) When the pulses of multiple response signals are intertwined and have different amplitudes, it is easy to cause intra-pulse fluctuations;

[0007] (3) Susceptible to interference from other signals in the frequency band;

[0008] (4) Waveform shaping before pulse detection will affect the actual width of the pulse.

[0009] In the case of low signal-to-noise ratio or large amplitude fluctuations within the pulse, the probability of correctly detecting the pulse position by traditional methods is low, which seriously affects the accuracy of decoding the response signal. Its shortcomings are particularly significant when processing multiple interleaved signals. Summary of the invention

[0010] In view of the problems existing in the prior art, the present invention provides a secondary radar signal processing method with an adaptive threshold that solves the problem that the performance of traditional pulse detection methods is easily affected by the amplitude fluctuation of the response signal pulse, improves the detection performance under low signal-to-noise ratio, and effectively improves the detection probability of interleaved pulses.

[0011] The purpose of the present invention is achieved through the following technical solutions.

[0012] A secondary radar signal processing method with an adaptive threshold, the steps comprising:

[0013] Step 1: The secondary radar obtains the logarithmic amplitude of the sum, difference and control channel video signals through front-end processing.

[0014] Step 2: Use adaptive threshold to perform pulse detection on the channel video signal to generate PSV. The specific steps are as follows:

[0015] S2-1: ICAO Annex 10 stipulates that all response pulses should meet the rise time of 0.05-0.1μs and the pulse fall time of 0.05-0.2μs. Therefore, the width of the sliding window can be set to the actual clock frequency * the rounded value of the rise time N1 and the actual clock frequency * the rounded value of the fall time N2, and the sliding window width can be fine-tuned according to actual use requirements. Set the sliding window width according to the above principles, and use the sliding window to segment the channel video signal;

[0016] S2-2: Take the maximum value of the amplitude of N1 sampling points after the current sampling point, which drops by 6dB, as the first set of thresholds;

[0017] S2-3: Take the maximum value of the amplitude of the N2 sampling points before the current sampling point, which is 6dB lower, as the second set of thresholds;

[0018] S2-4: Compare the amplitude of the sum channel video signal with two sets of thresholds to complete pulse detection and obtain PSV.

[0019] Step 3: PSV preprocessing, including anti-narrowing and pulse leading edge detection. When the pulse width is greater than the theoretical single response pulse width, it is considered to be the interweaving of multiple pulse signals. The steps are as follows:

[0020] S3-1: Suppress narrow pulses less than 0.3μs;

[0021] S3-2: When the pulse width is greater than a standard pulse width, it may be the interweaving of two pulse signals. According to the position of the pulse trailing edge, a standard response pulse width can be pushed forward to obtain the leading edge of the second pulse, that is, the pseudo leading edge PLE;

[0022] S3-3: When the pulse width is greater than two pulse widths, the pseudo leading edge PLE can be obtained according to step S2-2. According to the pulse leading edge position, m standard pulse widths are pushed back to obtain m additional leading edges XLE, where m is not greater than 2.

[0023] Step 4: Response frame detection.

[0024] Step 5: Mode S preamble pulse pair detection, no mode AC response is detected within 120μs after the mode S preamble pulse pair is detected.

[0025] Step 6: Sample the amplitude and sum-difference ratio (SDR) of the response pulse at the ALE / PLE position of the pulse leading edge.

[0026] Step 7: Mode AC special response processing, including interference response, phantom response, military response, etc.

[0027] Step 8: When the response frame is detected, any idle decoding channel among the N decoding channels is triggered. The decoding channel performs correlation processing on the response code bit pulse and makes a judgment. The steps are as follows:

[0028] S8-1: Channel status is set to busy;

[0029] S8-2: The main lobe is preferably received and the non-interleaved frame pulse F1 / F2 is used as the reference;

[0030] S8-3: If neither F1 nor F2 meets the conditions, then the answer code pulse that meets the conditions is selected as the reference pulse;

[0031] S8-4: statistically average the amplitude of the sum channel video signal of the frame pulse and code pulse received in the main lobe and not interleaved and the SDR value, and use the statistical average as a reference amplitude for correlation processing;

[0032] S8-5: Response code bit judgment and report the decoding result;

[0033] S8-6: The decoding channel returns to the idle state.

[0034] Compared with the prior art, the advantages of the present invention are: (1) only simple amplitude comparison is required for pulse detection, which results in a small amount of calculation and a small delay;

[0035] (2) When there is co-band interference or the response pulses are interleaved, the threshold value can be changed in real time according to the response pulse waveform and amplitude, greatly improving the decoding probability and accuracy of signal processing for multiple interleaved responses;

[0036] (3) Step S8: When the frame pulses F1 / F2 do not meet the conditions for being used as reference pulses, a response pulse that meets the conditions may be selected as the reference pulse;

[0037] (4) The statistical average of the channel video amplitude and SDR value of the main lobe received and non-interleaved code bit pulses in the same response is used as the reference value for correlation processing, which can reduce the impact of amplitude fluctuations on response decoding;

[0038] (5) Parallel processing of N decoding channels can meet the deinterleaving requirements to the greatest extent possible. This signal processing method uses four parallel decoding channels, so it can process multiple interleaved responses. However, this signal processing method is not limited to four parallel decoding channels if resources permit. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Secondary radar mode AC reply signal format.

[0040] Figure 2 Secondary radar Mode S reply signal format.

[0041] Figure 3 Block diagram of secondary radar signal processing flow.

[0042] Figure 4 Schematic diagram of the secondary radar pulse detection principle.

[0043] Figure 5 Schematic diagram of pulse detection when secondary radar reply pulses are multiplexed. DETAILED DESCRIPTION

[0044] The specific implementation of the present invention will be described below in conjunction with the accompanying drawings and examples, so that those skilled in the art can understand the present invention, but it should be clear that the present invention is not limited to the scope of the specific implementation. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

[0045] In this embodiment, the sampling clock frequency is 20 MHz. For example, when sampling a 1 μs pulse signal, the number of sampling points is 20 MHz*1 μs=20. Figure 3 The figure shows a secondary radar signal processing flow chart disclosed in the present invention, and the specific implementation steps are as follows:

[0046] Step 1: The secondary radar obtains the logarithmic amplitude of the sum, difference and control channel video signals through front-end processing.

[0047] Step 2: The width of the sliding window is set to 20 MHz * 0.1μs = 2 and 20MHz * 0.15μs = 3 sampling points respectively. The sliding window is used to segment the sum channel video signal to generate two sets of adaptive detection thresholds. The pulse detection module uses the adaptive threshold to perform pulse detection on the sum channel video signal to obtain PSV. The processed sum video is called PSV after pulse detection on the sum channel video signal using the adaptive threshold. The sliding window width can be fine-tuned according to actual usage requirements.

[0048] Step 3: Preprocess PSV and suppress narrow pulses with a pulse width less than 20 MHz * 0.3μs = 6 sampling points. When the pulse width is greater than 20 MHz * 0.45μs = 9 sampling points, it is considered to be the interweaving of multiple pulse signals. For example, when the pulse width is greater than 9, it may be the interweaving of two pulse signals. According to the position of the pulse trailing edge, the leading edge of the second pulse can be pushed forward by 9 sampling points, that is, the pseudo leading edge PLE; when the pulse width is greater than 18, according to the pulse leading edge position, push back m*9 sampling points to get m extra leading edges XLE, where m is not greater than 2.

[0049] Step 4: Response frame detection.

[0050] Step 5: Mode S preamble pulse pair detection, no mode AC response is detected within 120μs after the mode S preamble pulse pair is detected.

[0051] Step 6: Sample the channel video amplitude and SDR value at the ALE / PLE position at the leading edge of the pulse.

[0052] Step 7: Mode AC special response processing, including interference response, phantom response, military response, etc.

[0053] Step 8: When a response frame is detected, any idle decoding channel is triggered. The frame pulse F1 / F2 without main lobe interleaving is selected as the reference pulse. If F1 / F2 does not meet the conditions, the response code pulse that meets the conditions is selected as the reference pulse. The sum of the channel video amplitude and SDR value of the frame pulse and code pulse that meet the conditions is statistically averaged as the reference amplitude.

[0054] Step 9: After the decoding channel is triggered, the channel state is set to busy, and the pulses in the frame are processed in relation to the channel video amplitude and SDR value. After the decoding is completed, the decoding result is output and the idle state is restored. This signal processing uses four parallel decoding channels, so it can realize the processing of multiple interleaved responses.

[0055] Figure 4 The schematic diagram of the principle of the pulse detection method with adaptive threshold used in this patent. Two sliding windows are used to segment the video amplitude data of the sum channel to obtain two sets of thresholds, which are used to detect the leading edge and trailing edge of the sum channel video respectively, so as to realize pulse detection.

[0056] Figure 5 This is a schematic diagram of the adaptive threshold pulse detection method used in this patent to detect N-interleaved pulses, where N=4. In the figure, 4 response pulses of different amplitudes are intertwined together, and the adaptive threshold pulse detection method can be used to detect 4 response pulses. After the interleaved pulses are detected by the adaptive threshold pulse detection, they enter the subsequent response processing flow, thereby realizing the decoding of multiple interleaved responses. In order to show the effect, Figure 4 and Figure 5 The amplitude of the pulse detection result PSV shown is the effect after 10 times magnification.

Claims

1. A secondary radar signal processing method with an adaptive threshold, characterized in that: The following steps are involved: S1: The secondary radar obtains the logarithmic amplitude of the sum, difference and control channel three-way video signals after front-end processing; S2: Use a sliding window to segment the sum channel video signal to generate two sets of adaptive detection thresholds. The pulse detection module uses the adaptive threshold to perform pulse detection on the sum channel video signal to generate PSV; S3: PSV preprocessing, including anti-narrowing and pulse leading edge detection; S4: response frame detection; S5: Mode S preamble pulse pair detection, no mode AC response is detected within 120μs after the mode S preamble pulse pair is detected; S6: Sample the amplitude and sum-difference ratio SDR value of the response pulse at the ALE / PLE position of the pulse leading edge; S7: Mode AC special response processing, including interference response, phantom response, and military response; S8: When a response frame is detected, any idle decoding channel among the N decoding channels is triggered, and the decoding channel performs correlation processing on the response code bit pulse and makes a judgment; The specific steps of step S8 are: S8-1: Channel status is set to busy; S8-2: The main lobe is preferably received and the non-interleaved frame pulse F1 / F2 is used as the reference; S8-3: If neither F1 nor F2 meets the conditions, then the answer code pulse that meets the conditions is selected as the reference pulse; S8-4: statistically average the amplitude of the sum channel video signal of the frame pulse and code pulse received in the main lobe and not interleaved and the SDR value, and use the statistical average as a reference amplitude for correlation processing; S8-5: Response code bit judgment and report the decoding result; S8-6: The decoding channel returns to the idle state.

2. The method for processing secondary radar signals with an adaptive threshold according to claim 1, characterized in that The specific steps of step S2 are as follows: S2-1: The width of the sliding window is set to the integer value of the actual clock frequency * rise time and the integer value of the actual clock frequency * fall time, respectively. The sliding window width is fine-tuned according to actual use requirements. The sliding window width is set according to the above principles, and the sliding window is used to segment the channel video signal; S2-2: Take the maximum value of the amplitude of N1 sampling points after the current sampling point, which drops by 6dB, as the first set of thresholds; S2-3: Take the maximum value of the amplitude of the N2 sampling points before the current sampling point, which is 6dB lower, as the second set of thresholds; S2-3: Compare the amplitude of the sum channel video signal with two sets of thresholds to complete pulse detection and obtain PSV.

3. The method for processing secondary radar signals with an adaptive threshold according to claim 2, characterized in that The specific steps of step S3 are as follows: S3-1: Suppress narrow pulses less than 0.3μs; S3-2: When the pulse width is greater than a standard pulse width, it may be the interweaving of two pulse signals. According to the position of the pulse trailing edge, push forward a standard response pulse width to get the leading edge of the second pulse, that is, the pseudo leading edge PLE; S3-3: When the pulse width is greater than two standard pulse widths, a pseudo leading edge PLE is obtained according to step S2-2, and m additional leading edges XLE are obtained by pushing back m standard pulse widths according to the pulse leading edge position, where m is not greater than 2.

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