A target detection method for helicopter collision avoidance radar to suppress fuzzy false alarms

Abstract

The invention discloses a target detection method for suppressing fuzzy false alarms by a helicopter anti-collision radar. When the anti-collision radar detects a stationary target on the ground, the invention fully considers the characteristic that the target Doppler position can be obtained under the condition that the helicopter speed is known when the target is stationary, and increases the frequency offset between different pulses so that the difference in frequency between the fuzzy target echo and the unambiguous target echo is used to distinguish the unambiguous target, thereby solving the false alarm problem caused by the fuzzy echo.

Description

Technical Field

[0001] The invention relates to the field of radar technology, in particular to a target detection method for suppressing fuzzy false alarms by a helicopter anti-collision radar. Background Art

[0002] Millimeter wave radar is widely used in helicopter collision avoidance. The radar calculates the distance of the object by emitting electromagnetic waves and reflecting the total path back. Millimeter wave radars commonly used in helicopter collision avoidance usually have two signal systems. One is the frequency-modulated continuous wave (FMCW) waveform, which emits a continuous wave signal that changes linearly with time, and receives the reflected echo and mixes it with the transmitted signal to calculate the difference frequency after mixing. It is difficult to ensure the isolation between transmission and reception in this scheme, resulting in the transmission power not being too large and the detection distance being limited. The other is the pulse Doppler (PD) system, which uses time-sharing transmission and reception, and does not receive during transmission, which allows the transmission power to be very large and the detection distance to be longer. Since there is no reception during transmission, the blind area of ​​reception is related to the transmission pulse width. Therefore, the transmission pulse width cannot be too wide, otherwise the blind area is very large. However, under the condition of narrow pulse width, in order to ensure the average radiated power, the pulse repetition interval (PRI) must be very small to ensure the duty cycle and maintain the advantage of long detection distance. If the PRI is too small, the fuzzy distance will be very short. For example, when the PRI is 50us, the corresponding fuzzy distance is a multiple of 7.5km. The radar will mistakenly think that the target echo that falls 8km in front of the helicopter is a 0.5km target. This will cause the helicopter radar to believe that there is a collision risk at 500m and may issue an alarm, resulting in reduced radar credibility. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a target detection method for helicopter anti-collision radar to suppress fuzzy false alarms. The present invention solves the target false alarm problem caused by the fuzziness of the existing helicopter millimeter-wave anti-collision radar.

[0004] The present invention adopts the following technical solutions to solve the above technical problems:

[0005] A target detection method for suppressing fuzzy false alarms by a helicopter collision avoidance radar proposed in the present invention comprises the following steps:

[0006] Step 1: Add frequency offset to the radar transmitting pulse signal;

[0007] Assume that the radar transmits N pulse signals and the baseband of the pulse signals transmitted by the radar is s T(t), the initial carrier frequency is f0, the frequency offset of the carrier frequency as the pulse signal changes is df, and the i-th pulse signal is The carrier frequency at this time is Among them, e is the natural base, j is the imaginary unit, t is the time, A t represents the signal modulation amplitude, T p Represents the pulse repetition interval, i = 1, 2, 3…, N;

[0008] Step 2: The radar target reflected echo is received, mixed and low-pass filtered; the details are as follows:

[0009] After the ith pulse signal is transmitted, the echo signal of the ith pulse signal is obtained. s r (t) is the baseband echo with Doppler offset, s r The carrier frequency of the i-th pulse signal (t) is s r (t) The result after mixing and low-pass filtering is Among them, j' is the j'th pulse signal, A r is the echo amplitude, j′≥i;

[0010] Step 3: Sampling and pulse Doppler processing;

[0011] Will Sampling is performed to obtain a sampling signal S with N pulses and M sampling points for each pulse. M,N , M is the number of sampling points at each pulse moment, and N is the number of pulse signals;

[0012] For S M,N After pulse compression processing, fast Fourier transform FFT processing is performed. The number of fast Fourier transform FFT points is N', and B is obtained. B is S M,N Pulse pressure and FFT data;

[0013] Step 4: Select a limited frequency gate range in B for target detection;

[0014] The Doppler offset center of the target in the N′ Doppler gates is D, and the target is detected in the pulse with a frequency gate range of [D-M1, D+M2]. M1 and M2 are the minimum and maximum values ​​of the preset detection frequency, respectively.

[0015] As a further optimization scheme of the target detection method for suppressing fuzzy false alarms of a helicopter collision avoidance radar described in the present invention, the relative speed between the carrier aircraft and the stationary obstacle on the ground is V. Under this condition, the calculation formula of the Doppler offset center D of the target is as follows:

[0016]

[0017] Wherein, Int() indicates rounding, PRF indicates pulse repetition frequency, and λ indicates the wavelength of carrier frequency f0.

[0018] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0019] (1) When the anti-collision radar detects a stationary target on the ground, the present invention fully considers the characteristic that the target Doppler position can be obtained when the helicopter speed is known when the target is stationary, and uses frequency offset to distinguish the unambiguous target based on the difference in frequency between the fuzzy target echo and the unambiguous target echo, thereby solving the false alarm problem caused by the fuzzy echo;

[0020] (2) Compared with the existing technology, the invention can maximize the duty cycle without reducing the radiation power, thereby making the radar radiation efficiency higher and suppressing the false alarm problem caused by fuzzy echoes, thereby improving the performance of helicopter millimeter-wave collision avoidance radar and having broad application prospects;

[0021] (3) The present invention can be used for helicopter millimeter-wave collision avoidance radars and can be extended to the millimeter-wave radar collision avoidance waveform design for low-altitude flights on platforms such as fixed-wing aircraft and unmanned aerial vehicles, which can effectively suppress the problem of false alarms for close-range collision avoidance caused by distance ambiguity. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the target scene of the pulse radar;

[0023] Figure 2 This is a schematic diagram of frequency change as the number of pulses increases;

[0024] Figure 3 The echo range-Doppler frequency diagram obtained by the traditional method;

[0025] Figure 4 Schematic diagram of echo distance-Doppler frequency and detection method after application of this method. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] The present invention considers that the helicopter anti-collision radar is superimposed with a progressive Doppler frequency deviation on the basis of the existing pulse transmission waveform, so that the unambiguous echo pulse has a fixed Doppler offset, and the signal Doppler offset of the fuzzy echo pulse is an integer multiple of the unambiguous target echo. Through FFT processing, the target and the fuzzy echo are distinguished to achieve the purpose of suppressing fuzzy false alarms. The advantage of this technology is that the accumulation of target signals can be achieved through coherent accumulation, while avoiding the fuzzy echo from entering the radar detection and being detected, thereby reducing the probability of false alarms of the helicopter anti-collision radar.

[0028] The radar operating parameters in the example are:

[0029] The sampling rate is 50MHz, the pulse width is 1us, and T p 20us, duty cycle 5%, s T (t) is a linear frequency modulation signal with a frequency modulation bandwidth of 40MHz, an operating frequency of 35GHz, 32 pulses, and a df of 5000Hz. There are two targets, 2000m away and 4050m away respectively. Figure 1 ).

[0030] Step 1: Add frequency offset to the radar transmitting pulse signal;

[0031] Assume the transmitted signal baseband s T (t) is a linear frequency modulation signal, the initial carrier frequency is 35 GHz, the carrier frequency changes with the frequency offset of the pulse is 5000 Hz, and each pulse frequency is increased by 5000×T compared with the previous pulse frequency. p , is 0.1Hz( Figure 2 ). Then the first pulse signal of the transmitted pulse is The carrier frequency is The second pulse signal of the transmitted pulse is A t = 1, T p Equal to 20us, aircraft speed V = 25m / s.

[0032] Step 2: The radar target reflected echo is received, mixed and low-pass filtered; the details are as follows:

[0033] The frequency of the receiving mixer is also the same as the transmitting carrier frequency, which is When the first target is mixed after the unambiguous pulse is transmitted, the result obtained after mixing and low-pass filtering of the unambiguous first target echo is A r s r (t). The echo of the blurred second target echo, after mixing and low-pass filtering, is Calculated as A r s r (t)e j2π(0.1)tThe second target frequency is 5000Hz offset from the first target, and the corresponding frequency gate is 3.2. Because the pulse repetition frequency (PRF) is the inverse of PRI, the PRF is 50KHz, which corresponds to 32 frequency gates. Therefore, it is 3.2 more frequency gates offset than target one.

[0034] In step 2 of the invention, is the mixing residue of the carrier frequency of the j′th pulse signal and the echo signal at the current moment, is the carrier frequency of the j′th pulse signal at the current moment, A r is the echo amplitude, j′≥i;

[0035] Step 3: Sampling and pulse Doppler processing;

[0036] The received signal is sampled and the sampling signal is S when M is 1000 range gates and N is 32 pulses. M,N After the 32 pulses are subjected to pulse compression processing, a fast Fourier transform (FFT) process is performed between pulses with N′=32, and B (such as Figure 3 );

[0037] Step 4: Select a limited frequency range in B for target detection;

[0038] The Doppler offset center of the target in the 32 Doppler gates is D. D is calculated according to formula (1). The relative speed between the aircraft and the target is 25 m / s. According to formula (1), the target Doppler offset center is D = 4.73. M1 is set to 2, M2 is set to 1, and rounded off to obtain the target detected in the pulse with a frequency gate range of [3,6].

[0039] 3 to 6 Doppler gates are the gates that can detect targets. Since the position of target 2 is 4.73+3.2=7.93 gates away, no target detection is performed in this area. Figure 4 ), to achieve the purpose of suppressing fuzzy false alarms.

[0040] The relative speed between the carrier aircraft and the stationary obstacle on the ground is V. Under this condition, the calculation formula of the Doppler offset center D of the target is as follows:

[0041]

[0042] Int() means rounding, PRF means pulse repetition frequency, and λ means the wavelength of carrier frequency f0.

[0043] The above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a technician familiar with the technical field within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A target detection method for helicopter collision avoidance radar to suppress fuzzy false alarms, characterized in that: The following steps are involved: Step 1: Add frequency offset to the radar transmitting pulse signal; Assume that the radar transmits N pulse signals and the baseband of the pulse signals transmitted by the radar is s T (t), the initial carrier frequency is f0, the frequency offset of the carrier frequency as the pulse signal changes is df, and the i-th pulse signal is The carrier frequency at this time is Among them, e is the natural base, j is the imaginary unit, t is the time, A t represents the signal modulation amplitude, T p Represents the pulse repetition interval, i = 1, 2, 3…, N; Step 2: The radar target reflected echo is received, mixed and low-pass filtered; the details are as follows: After the ith pulse signal is transmitted, the echo signal of the ith pulse signal is obtained. s r (t) is the baseband echo with Doppler offset, s r The carrier frequency of the i-th pulse signal (t) is s r (t) The result after mixing and low-pass filtering is Among them, j' is the j'th pulse signal, A r is the echo amplitude, j′≥i; Step 3: Sampling and pulse Doppler processing; Will Sampling is performed to obtain a sampling signal S with N pulses and M sampling points for each pulse. M,N , M is the number of sampling points at each pulse moment, and N is the number of pulse signals; For S M,N After pulse compression processing, fast Fourier transform FFT processing is performed. The number of fast Fourier transform FFT points is N', and B is obtained. B is S M,N Pulse pressure and FFT data; Step 4: Select a limited frequency gate range in B for target detection; The Doppler offset center of the target in the N′ Doppler gates is D, and the target is detected in the pulse with a frequency gate range of [D-M1, D+M2]. M1 and M2 are the minimum and maximum values ​​of the preset detection frequency, respectively.

2. The target detection method for helicopter collision avoidance radar suppressing fuzzy false alarm according to claim 1 is characterized in that: The relative speed between the carrier aircraft and the stationary obstacle on the ground is V. Under this condition, the calculation formula of the Doppler offset center D of the target is as follows: Wherein, Int() indicates rounding, PRF indicates pulse repetition frequency, and λ indicates the wavelength of carrier frequency f0.

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