Radio altimeter ranging method based on digital frequency modulation continuous wave system

Abstract

The invention discloses a radio altimeter distance measurement method based on a digital frequency modulation continuous wave system, and the method comprises the following specific steps: S100, generating a linear frequency modulation continuous wave signal through a direct digital frequency synthesis technology, and precisely controlling the frequency modulation slope and the center frequency through a digital control word; s200, converting the digital frequency modulation signal into a radio frequency signal, transmitting the radio frequency signal, receiving an echo signal reflected by the ground, and converting the echo signal into a digital signal; s300, performing digital down-conversion processing on the digital echo signal, and converting the digital echo signal into a baseband signal; s400, carrying out fast Fourier transform processing on the baseband signal to obtain a distance-Doppler two-dimensional frequency spectrum; s500, separating and processing the multipath signals by adopting a spatial spectrum estimation technology; s600, adaptively adjusting a detection threshold according to the signal environment and the noise level; and S700, calculating the height of the aircraft according to the processed main path signal, and compensating atmospheric refraction, temperature and air pressure factors.

Description

Technical Field

[0001] The invention belongs to the technical field of avionics equipment, and in particular relates to a radio altimeter ranging method based on a digital frequency modulation continuous wave system. Background Art

[0002] A radio altimeter, a crucial navigational aid for aircraft, primarily measures the true altitude of an aircraft above the ground or sea. It operates by transmitting a radio signal to the ground, receiving an echo reflected from the ground, and calculating altitude using the time delay or frequency difference between the echo and the transmitted signal. Radio altimeters are widely used in aerospace applications, including civil and military aircraft, helicopters, and spacecraft, providing critical altitude information for takeoff, landing, low-altitude flight, and terrain following.

[0003] Traditional radio altimeter technologies primarily include pulse and frequency modulated continuous wave (FMCW) systems. Pulse altimeters calculate altitude by measuring the time difference between the transmitted and received pulses. They offer advantages such as high accuracy and strong anti-interference capabilities. However, they suffer from significant range blind spots, bulk, and high power consumption, making them unsuitable for low-altitude flight applications. FMCW altimeters calculate altitude by measuring the frequency difference between the transmitted and received signals. They offer advantages such as no range blind spots, compact size, and low power consumption, making them widely used in low-altitude flight applications.

[0004] Patent application CN117590387A discloses a digital altimeter transceiver and altimeter measurement method based on frequency modulated continuous wave technology. The digital altimeter transceiver includes a transceiver assembly, including a transmitting antenna and a receiving antenna; a microwave assembly electrically connected to the transceiver assembly, which transmits a transmit signal via the transmitting antenna and receives an echo signal via the receiving antenna; and a low-frequency board electrically connected to the microwave assembly, which controls the modulation period of the microwave assembly's radio frequency signal based on the transmit and echo signals. This patented technical solution reduces the difficulty of implementing digital ranging in radio altimeters.

[0005] The altitude measurement method provided by the aforementioned patent utilizes a frequency modulated continuous wave (FMCW) technology. However, existing digital FMCW altimeters typically require extensive computation in their signal processing algorithms, such as fast Fourier transforms (FFTs) and correlation operations. This places high demands on processor performance, making real-time processing difficult in resource-constrained embedded systems. Furthermore, during low-altitude flight, radio signals can be affected by multipath effects, such as ground and building reflections. This can cause the echo signal to contain signal components from multiple different paths, thus affecting ranging accuracy.

[0006] In summary, a radio altimeter ranging method based on a digital frequency modulated continuous wave system is needed, which can improve the ranging accuracy, anti-interference ability and dynamic range, while reducing the complexity of the signal processing algorithm and meeting the real-time requirements. Summary of the Invention

[0007] The purpose of the present invention is to provide a radio altimeter ranging method based on a digital frequency modulated continuous wave system to solve the problems of the existing radio altimeter ranging method proposed in the above background technology, such as complex signal processing algorithm and poor anti-interference ability.

[0008] To achieve the above purpose, the present invention provides the following technical solutions A radio altimeter ranging method based on a digital frequency modulated continuous wave system, the specific steps of the ranging method are as follows: S100 uses direct digital frequency synthesis technology to generate linear frequency modulation continuous wave signals, and accurately controls the frequency modulation slope and center frequency through digital control words; S200, converting the digital FM signal into a radio frequency signal and transmitting the signal, receiving the echo signal reflected by the ground and converting the signal into a digital signal; S300, performing digital down-conversion processing on the digital echo signal to convert it into a baseband signal; S400, performing fast Fourier transform processing on the baseband signal to obtain a range-Doppler two-dimensional spectrum; S500, using spatial spectrum estimation technology to separate and process multipath signals; S600, adaptively adjusting the detection threshold according to the signal environment and noise level; S700: Calculate the aircraft altitude based on the processed main path signal and compensate for atmospheric refraction, temperature, and pressure factors.

[0009] Preferably, in step S100, the frequency modulation slope and center frequency of the linear frequency modulation continuous wave signal generated by the direct digital frequency synthesis technology can be dynamically adjusted according to different application scenarios; the mathematical expression of the linear frequency modulation signal is: ; in, is the signal amplitude, For time, In order to accurately control the FM slope, is the center frequency; The frequency modulation slope of the linear frequency modulation continuous wave signal generated by the direct digital frequency synthesis technology The calculation formula is: ; in, is the FM bandwidth, is the modulation period.

[0010] Preferably, in step S300, a field programmable gate array (FPGA) is used to implement high-speed digital signal processing, including digital mixing and low-pass filtering; the received signal With local reference signal After mixing, the baseband signal The expression is: ; in, is the Doppler shift, is the echo delay, represents the complex conjugate.

[0011] Preferably, in step S400, the Doppler FFT processing method is used, and the distance dimension resolution is and Doppler velocity resolution They are: , ; in, is the speed of light, is the duration of the chirp signal.

[0012] Preferably, in step S500, the multipath signal is separated and processed by using a multiple signal classification (MUSIC) algorithm; the signal arrival angle is estimated by using a multiple signal classification (MUSIC) algorithm. , its spatial spectrum function is: ; in, is the array steering vector, is the noise subspace matrix, represents the conjugate transpose.

[0013] Preferably, in step S600, a constant false alarm rate (CFAR) detection algorithm is used to adaptively adjust the detection threshold according to the local noise level, and a unit average constant false alarm rate (CA-CFAR) detection algorithm in the constant false alarm rate (CFAR) detection algorithm is used, and the detection threshold The calculation formula is: ; in, is the threshold factor, which is determined by the false alarm probability Decide: ; is the signal amplitude of the reference unit, is the number of reference units.

[0014] Preferably, in step S700, a standard atmospheric model or real-time atmospheric data is used to compensate for atmospheric refraction, temperature and pressure factors, and the true altitude of the aircraft is The calculation formula is: ; in, is the slope distance, is the pitch angle, is the antenna height, is the atmospheric refraction compensation, calculated using the standard atmospheric model: ; in, is the sea level height, and are the refractive indices of sea level and measurement point, respectively.

[0015] Preferably, the method further includes step S800: evaluating the reliability of the measurement result by analyzing the signal-to-noise ratio, correlation peak and spectrum purity of the signal; The assessment process steps include: S810, performing time domain windowing processing on the baseband signal after digital down conversion to suppress spectrum leakage, performing sliding average filtering to reduce the influence of random noise, and extracting a valid signal segment within a specified range gate as an evaluation object; S820. Perform a fast Fourier transform (FFT) on the preprocessed signal to convert it to the frequency domain, calculate the main lobe width, evaluate whether the frequency resolution meets the design requirements, measure the power ratio of the main lobe to the first side lobe, and detect whether there are spurious signals in the spectrum. S830. Use a segmented threshold method to distinguish signal segments from noise segments; perform statistical analysis on the noise segments, calculate the noise power spectrum density, and calculate the SNR using the power ratio of the signal segment to the noise segment. The formula is: ; in, is the signal power, The noise power is estimated through spectrum analysis; the SNR threshold is set, and when it is lower than the threshold, the signal enhancement or resampling strategy is triggered; S840. Perform cross-correlation calculation on the transmitted signal and the echo signal; detect the main peak position of the correlation function, which corresponds to the target distance; calculate the peak-to-sidelobe ratio (PSLR) to evaluate the impact of multipath interference; measure the peak width to determine whether the range resolution meets the theoretical value; S850. Perform phase tracking on the same range gate for consecutive chirp signals; calculate the phase standard deviation to evaluate the degree of phase jitter; use Kalman filtering to predict the phase change trend and detect abnormal jumps; mark signals with phase jitter exceeding the threshold as low-quality signals; S860. Real-time monitor the amplitude change range of the received signal; calculate the power difference between the maximum and minimum signals to evaluate the system dynamic range; trigger automatic gain control (AGC) adjustment when the signal approaches the ADC full scale; S870. Assign weights to the above各项指标 (should be "above-mentioned indicators" in English) and calculate the comprehensive quality score. The calculation formula is: Score 指标 (should be "indicator" in English) ; Make decisions based on the scoring results: High quality (Score > 80): Directly adopt the current measurement results; Medium quality (60 < Score ≤ 80): Enable multi-frame averaging or Kalman filtering fusion; Low quality (Score ≤ 60): Trigger re-sampling or alarm mechanism; S880. Dynamically adjust system parameters according to the evaluation results: Increase the transmit power or integration time at low SNR; Optimize the DDS frequency synthesis parameters at high spurs; Adjust the clock synchronization strategy when the phase is unstable; Feed back the evaluation results to the altitude calculation module for error compensation.

[0016] Preferably, it further includes step S900: Dynamically adjust the transmit power, modulation mode or operating frequency by monitoring the external electromagnetic environment to suppress external interference.

[0017] Preferably, it further includes step S1000: Form an array by arranging multiple receiving antennas and use spatial diversity technology to improve the angle measurement accuracy and anti-interference ability.

[0018] Compared with the prior art, the beneficial effects of the present invention are: 1. By adopting the digital frequency modulation continuous wave system and combining with DDS technology to generate linearly frequency modulated signals, the present invention ensures the frequency modulation linearity and improves the ranging accuracy.

[0019] 2. Through range-Doppler two-dimensional processing and multipath signal separation technology, the present invention effectively suppresses the influence of multipath effects and improves the low-altitude measurement performance.

[0020] 3. By adopting the adaptive threshold detection technology, the present invention improves the weak signal detection ability and anti-interference ability and expands the dynamic range.

[0021] 4. The signal processing algorithm of the present invention has low complexity and good real-time performance, and can meet the application requirements of embedded systems.

[0022] 5. The system architecture of the present invention is flexible and can dynamically adjust working parameters according to different application scenarios and requirements, and has strong adaptability and scalability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Flow chart of the radio altimeter ranging method of the present invention; Figure 2 This is a flowchart of a specific evaluation process of step S800 of the present invention; Figure 3 A flowchart of the detailed operation of step S900 of the present invention; Figure 4 This is a flowchart of the detailed work of step S1000 of the present invention. DETAILED DESCRIPTION

[0024] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0025] Example 1 See also Figure 1 A radio altimeter ranging method based on a digital frequency modulated continuous wave system, the specific steps of the ranging method are as follows: S100 uses direct digital frequency synthesis technology to generate linear frequency modulated continuous wave signals, and the frequency modulation slope and center frequency are precisely controlled by digital control words. The frequency modulation slope and center frequency of the linear frequency modulated continuous wave signal generated by direct digital frequency synthesis technology can be dynamically adjusted according to different application scenarios. Direct digital frequency synthesis technology has the advantages of high frequency resolution, fast switching speed, and phase continuity, and can generate high-quality linear frequency modulation signals. The chip used for direct digital frequency synthesis is AD9959, with an operating clock of 300MHz. It can generate linear frequency modulation signals with a frequency range of 10MHz-150MHz, and the frequency modulation slope can be dynamically adjusted within the range of 1MHz / ms-10MHz / ms. The mathematical expression of the linear frequency modulation signal is:

[0026] in, is the signal amplitude, For time, In order to accurately control the FM slope, is the center frequency; Frequency modulation slope of the linear frequency modulated continuous wave signal generated by direct digital frequency synthesis technology The calculation formula is:

[0027] in, is the FM bandwidth, is the modulation period.

[0028] S200: Convert the digital FM signal into a radio frequency signal and transmit it. Receive the echo signal reflected from the ground and convert it into a digital signal. Specifically, the generated digital FM signal is converted into an analog radio frequency signal using a digital-to-analog converter (DAC), amplified by a power amplifier, and then transmitted. Simultaneously, the receiving antenna receives the echo signal reflected from the ground, amplifies it using a low-noise amplifier, mixes it using a mixer, and samples it using an analog-to-digital converter (ADC) before converting it into a digital signal. The center frequency of the transmitted signal is 4.3 GHz, the transmit power is 20 dBm, the noise figure of the receiving channel is 3 dB, the ADC sampling rate is 100 MHz, and the bit count is 14 bits.

[0029] S300 performs digital down-conversion on the digital echo signal and converts it into a baseband signal. First, the echo signal is digitally mixed and multiplied with the local reference signal. Then, the high-frequency component is filtered out through a low-pass filter to obtain the baseband signal. A field programmable gate array (FPGA) is used to implement high-speed digital signal processing, including digital mixing and low-pass filtering. With local reference signal After mixing, the baseband signal The expression is:

[0030] in, is the Doppler shift, is the echo delay, represents the complex conjugate.

[0031] S400, perform fast Fourier transform on the baseband signal to obtain the range-Doppler two-dimensional spectrum; first perform time domain FFT processing on the baseband signal in each range gate to obtain the range information; then perform Doppler FFT processing on the frequency domain signals of multiple range gates to obtain the velocity information. Using 1024-point FFT processing, the range resolution is 150m and the velocity resolution is 0.3m / s. Using Doppler FFT processing method, the range resolution is 150m and the velocity resolution is 0.3m / s. and Doppler velocity resolution They are:

[0032]

[0033] in, is the speed of light, is the duration of the chirp signal.

[0034] S500 uses spatial spectrum estimation technology to separate and process multipath signals. First, the signal covariance matrix is ​​constructed. Then, the covariance matrix is ​​eigen-decomposed to obtain the signal subspace and noise subspace. Finally, the orthogonality of the signal subspace and the noise subspace is used to estimate the arrival angle and amplitude of the multipath signal. The multiple signal classification (MUSIC) algorithm is used to separate and process the multipath signal. The signal arrival angle is estimated using the multiple signal classification (MUSIC) algorithm. , its spatial spectrum function is:

[0035] in, is the array steering vector, is the noise subspace matrix, represents the conjugate transpose.

[0036] S600, adaptively adjust the detection threshold according to the signal environment and noise level; use the constant false alarm rate (CFAR) detection algorithm to adaptively adjust the detection threshold according to the local noise level, first estimate the local noise power; then calculate the corresponding detection threshold based on the preset false alarm probability; finally, compare the signal amplitude with the detection threshold to determine whether there is a target signal; and use the unit average constant false alarm rate (CA-CFAR) detection algorithm in the constant false alarm rate (CFAR) detection algorithm to adjust the detection threshold. The calculation formula is

[0037] in, is the threshold factor, which is determined by the false alarm probability Decide:

[0038] is the signal amplitude of the reference unit, is the number of reference units.

[0039] S700, calculate the aircraft altitude based on the processed main path signal and compensate for atmospheric refraction, temperature and pressure factors; first calculate the slant distance between the aircraft and the ground based on the distance information; then calculate the true altitude based on the pitch angle information; finally, use the atmospheric model to compensate for atmospheric refraction, temperature, pressure and other factors to obtain a more accurate altitude value. And use the standard atmospheric model or real-time atmospheric data to compensate for atmospheric refraction, temperature and pressure factors to obtain the true altitude of the aircraft. The calculation formula is:

[0040] in, is the slope distance, is the pitch angle, is the antenna height, is the atmospheric refraction compensation, calculated using the standard atmospheric model:

[0041] in, is the sea level height, and are the refractive indices of sea level and measurement point, respectively.

[0042] Example 2 See also Figure 1 A radio altimeter ranging method based on a digital frequency modulated continuous wave system, the specific steps of the ranging method are as follows: S100 uses direct digital frequency synthesis technology to generate linear frequency modulated continuous wave signals, and the frequency modulation slope and center frequency are precisely controlled by digital control words. The frequency modulation slope and center frequency of the linear frequency modulated continuous wave signal generated by direct digital frequency synthesis technology can be dynamically adjusted according to different application scenarios. Direct digital frequency synthesis technology has the advantages of high frequency resolution, fast switching speed, and phase continuity, and can generate high-quality linear frequency modulation signals. The chip used for direct digital frequency synthesis is AD9959, with an operating clock of 300MHz. It can generate linear frequency modulation signals with a frequency range of 10MHz-150MHz, and the frequency modulation slope can be dynamically adjusted within the range of 1MHz / ms-10MHz / ms. The mathematical expression of the linear frequency modulation signal is:

[0043] in, is the signal amplitude, For time, In order to accurately control the FM slope, is the center frequency; Frequency modulation slope of the linear frequency modulated continuous wave signal generated by direct digital frequency synthesis technology The calculation formula is:

[0044] in, is the FM bandwidth, is the modulation period.

[0045] S200: Convert the digital FM signal into a radio frequency signal and transmit it. Receive the echo signal reflected from the ground and convert it into a digital signal. Specifically, the generated digital FM signal is converted into an analog radio frequency signal using a digital-to-analog converter (DAC), amplified by a power amplifier, and then transmitted. Simultaneously, the receiving antenna receives the echo signal reflected from the ground, amplifies it using a low-noise amplifier, mixes it using a mixer, and samples it using an analog-to-digital converter (ADC) before converting it into a digital signal. The center frequency of the transmitted signal is 4.3 GHz, the transmit power is 20 dBm, the noise figure of the receiving channel is 3 dB, the ADC sampling rate is 100 MHz, and the bit count is 14 bits.

[0046] S300 performs digital down-conversion on the digital echo signal and converts it into a baseband signal. First, the echo signal is digitally mixed and multiplied with the local reference signal. Then, the high-frequency component is filtered out through a low-pass filter to obtain the baseband signal. A field programmable gate array (FPGA) is used to implement high-speed digital signal processing, including digital mixing and low-pass filtering. With local reference signal After mixing, the baseband signal The expression is:

[0047] in, is the Doppler shift, is the echo delay, represents the complex conjugate.

[0048] S400, perform fast Fourier transform on the baseband signal to obtain the range-Doppler two-dimensional spectrum; first perform time domain FFT processing on the baseband signal in each range gate to obtain the range information; then perform Doppler FFT processing on the frequency domain signals of multiple range gates to obtain the velocity information. Using 1024-point FFT processing, the range resolution is 150m and the velocity resolution is 0.3m / s. Using Doppler FFT processing method, the range resolution is 150m and the velocity resolution is 0.3m / s. and Doppler velocity resolution They are:

[0049]

[0050] in, is the speed of light, is the duration of the chirp signal.

[0051] S500 uses spatial spectrum estimation technology to separate and process multipath signals. First, the signal covariance matrix is ​​constructed. Then, the covariance matrix is ​​eigen-decomposed to obtain the signal subspace and noise subspace. Finally, the orthogonality of the signal subspace and the noise subspace is used to estimate the arrival angle and amplitude of the multipath signal. The multiple signal classification (MUSIC) algorithm is used to separate and process the multipath signal. The signal arrival angle is estimated using the multiple signal classification (MUSIC) algorithm. , its spatial spectrum function is:

[0052] in, is the array steering vector, is the noise subspace matrix, represents the conjugate transpose.

[0053] S600, adaptively adjust the detection threshold according to the signal environment and noise level; use the constant false alarm rate (CFAR) detection algorithm to adaptively adjust the detection threshold according to the local noise level, first estimate the local noise power; then calculate the corresponding detection threshold based on the preset false alarm probability; finally, compare the signal amplitude with the detection threshold to determine whether there is a target signal; and use the unit average constant false alarm rate (CA-CFAR) detection algorithm in the constant false alarm rate (CFAR) detection algorithm to adjust the detection threshold. The calculation formula is

[0054] in, is the threshold factor, which is determined by the false alarm probability Decide:

[0055] is the signal amplitude of the reference unit, is the number of reference units.

[0056] S700, calculate the aircraft altitude based on the processed main path signal and compensate for atmospheric refraction, temperature and pressure factors; first calculate the slant distance between the aircraft and the ground based on the distance information; then calculate the true altitude based on the pitch angle information; finally, use the atmospheric model to compensate for atmospheric refraction, temperature, pressure and other factors to obtain a more accurate altitude value. And use the standard atmospheric model or real-time atmospheric data to compensate for atmospheric refraction, temperature and pressure factors to obtain the true altitude of the aircraft. The calculation formula is:

[0057] in, is the slope distance, is the pitch angle, is the antenna height, is the atmospheric refraction compensation, calculated using the standard atmospheric model:

[0058] in, is the sea level height, and are the refractive indices of sea level and measurement point, respectively.

[0059] S800: Evaluate the reliability of the measurement results by analyzing the signal-to-noise ratio, correlation peak, and spectral purity of the signal. Evaluation process steps: S810 , performing time domain windowing processing on the baseband signal after digital down conversion to suppress spectrum leakage, performing sliding average filtering to reduce the influence of random noise, and extracting a valid signal segment within a specified range gate as an evaluation object.

[0060] S820. Perform a fast Fourier transform (FFT) on the preprocessed signal, convert it to the frequency domain, calculate the mainlobe width (3dB bandwidth), evaluate whether the frequency resolution meets the design requirements, measure the power ratio of the mainlobe to the first sidelobe (usually required to be >20dB), and detect whether there are spurious signals (unexpected frequency components exceeding -60dBc) in the spectrum.

[0061] S830. Use a segmented threshold method to distinguish signal segments from noise segments; perform statistical analysis on the noise segments, calculate the noise power spectrum density, and calculate the SNR using the power ratio of the signal segment to the noise segment. The formula is:

[0062] in, is the signal power, = is the noise power, estimated by spectrum analysis. Set an SNR threshold (usually 10dB). When the SNR falls below the threshold, a signal enhancement or resampling strategy is triggered.

[0063] S840. Perform cross-correlation calculations on the transmitted signal and the echo signal; detect the main peak position of the correlation function, which corresponds to the target distance; calculate the peak-to-sidelobe ratio (PSLR) to assess the impact of multipath interference; measure the peak width to determine whether the range resolution meets the theoretical value.

[0064] S850: Phase tracking of multiple consecutive chirp signals with the same range gate is performed; the phase standard deviation is calculated to assess the degree of phase jitter; a Kalman filter is used to predict phase change trends and detect abnormal jumps; and a low-quality signal is marked when the phase jitter exceeds a threshold (usually 5°).

[0065] S860. Monitor the amplitude change range of the received signal in real time; calculate the power difference between the maximum signal and the minimum signal to evaluate the system dynamic range; when the signal approaches the ADC full scale, trigger the automatic gain control (AGC) adjustment.

[0066] S870. Assign weights to the above各项指标 (for example: SNR 40%, PSLR 30%, phase stability 20%, spectral purity 10%).

[0067] Calculate the comprehensive quality score, with the formula: Score 指标 ; Make a decision based on the scoring result: High quality (Score > 80): Directly adopt the current measurement result; Medium quality (60 < Score ≤ 80): Enable multi-frame averaging or Kalman filter fusion; Low quality (Score ≤ 60): Trigger re-sampling or alarm mechanism.

[0068] S880. Dynamically adjust the system parameters according to the evaluation result: increase the transmit power or integration time at low SNR; optimize the DDS frequency synthesis parameters at high spurs; adjust the clock synchronization strategy when the phase is unstable; feedback the evaluation result to the altitude calculation module for error compensation.

[0069] S900. Dynamically adjust the transmit power, modulation mode or operating frequency by monitoring the external electromagnetic environment to suppress external interference; The following is the detailed work process: S910. Interference monitoring and feature extraction; First, analyze the multi-domain signals, including time domain, frequency domain and time-frequency domain; Time domain: Calculate the signal amplitude distribution, pulse width and repetition frequency; Frequency domain: Analyze the spectral characteristics through FFT to identify narrowband / wideband interference; Time-frequency domain: Use wavelet transform or short-time Fourier transform (STFT) to detect pulse interference; Second, perform interference classification and identification: Identify common interference types (such as CW, pulse, swept-frequency interference) based on feature library matching; and calculate key parameters such as interference intensity, center frequency, bandwidth, etc.; Finally, spatial spectrum analysis (multi-channel system): Estimate the interference direction (DOA) through beamforming technology; and construct a spatial interference distribution map.

[0070] S920. Interference evaluation and threat level determination; First, evaluate the interference intensity: Calculate the interference-to-signal power ratio (ISR): Then, the duration and frequency of interference are counted, and the threat level is classified as follows: low threat: ISR < 10dB and occurs intermittently; medium threat: 10dB ≤ ISR < 20dB or occurs continuously; high threat: ISR ≥ 20dB or causes signal distortion; finally, an impact assessment is conducted to analyze the impact of interference on SNR, correlation peak and altitude measurement accuracy; and the probability of false alarms or missed alarms that may be caused by interference is predicted.

[0071] S930, Anti-interference Strategy Generation and Decision-making. First, a strategy library is matched: candidate solutions are selected from a predefined library based on the interference type and threat level. Typical strategies include: pulse cancellation and time-domain threshold detection in the time domain; frequency agility and notch filtering in the frequency domain; adaptive beamforming and null steering in the spatial domain; and modulation parameter modification and spread spectrum technology in the modulation domain. Parameter optimization is then performed, specifically by dynamically calculating the optimal anti-interference parameters (such as notch filter bandwidth and beamforming weight vector). The optimal configuration is then predicted based on historical data and machine learning algorithms. Finally, multi-strategy fusion is performed: for complex interference scenarios, a combination of various anti-interference technologies is employed, for example, simultaneously applying frequency-domain notching and time-domain pulse cancellation.

[0072] S940, implementation of anti-interference measures; first, adjustments are made to the transmitter, including power control, frequency agility and waveform design; power control: dynamically adjust the transmit power according to the interference intensity; frequency agility: jump the operating frequency to avoid narrowband interference; waveform design: change the chirp slope or bandwidth to reduce the correlation with interference; secondly, processing the receiver; including digital filtering: applying an adaptive notch filter to suppress narrowband interference; beamforming: forming a null in the interference direction to improve spatial anti-interference capability; threshold adjustment: increasing the CFAR detection threshold to reduce the false alarm rate; finally, system-level collaboration: collaborating with other subsystems (such as the navigation system) to use location information to assist in anti-interference decision-making; and switching to backup frequencies or channels to achieve redundant design.

[0073] S950, effect evaluation and feedback optimization; First, evaluate the anti-interference effect: compare the signal quality indicators (SNR, PSLR, etc.) before and after the implementation of anti-interference measures; calculate the interference rejection ratio (IRR): Next, closed-loop feedback is performed: if the results do not meet expectations, strategy parameters are adjusted or an alternative strategy is switched. Interference mitigation process data is recorded, and the interference signature library and strategy library are updated. Finally, adaptive learning is performed: machine learning models are trained based on historical data to optimize interference identification and strategy selection algorithms, enabling progressive learning and adaptation to unknown interference types.

[0074] S960, anti-interference resource management; first, resource allocation: balance anti-interference performance and system resource consumption (such as computing power and power consumption); and reserve anti-interference resources for critical tasks; second, switching mechanism: design a smooth switching algorithm to avoid system instability caused by parameter mutations; realize hot switching of primary / backup anti-interference channels; finally, fault recovery: real-time monitoring of the working status of the anti-interference module, automatically restoring the default configuration in the event of an abnormality; record fault logs for subsequent analysis.

[0075] S970 collaborates with other functional modules; first, it collaborates with signal quality assessment: sharing interference feature data to assist in signal quality scoring; dynamically adjusting the strength of the anti-interference strategy based on signal quality; second, it collaborates with altitude calculation: providing interference correction parameters to compensate for the impact of residual interference on ranging; marking measurement points that are severely interfered with, reducing their weight or eliminating them; finally, it collaborates with system control: when the interference exceeds the system's processing capacity, it triggers an alarm or switches to backup mode; and it participates in the overall system resource scheduling and task priority allocation.

[0076] S1000, by arranging multiple receiving antennas to form an array, uses spatial diversity technology to improve angle measurement accuracy and anti-interference capabilities; the following is a detailed workflow: S1100, multi-channel hardware architecture configuration; first, antenna array layout: 4-8 receiving antennas are arranged to form a uniform linear array (ULA) or planar array; antenna spacing is set to half a wavelength (to avoid spatial aliasing); typical array forms include linear array, L-shaped array, and circular array; second, RF front-end synchronization: all channels use the same clock source to ensure sampling synchronization; at the same time, calibrate the amplitude and phase inconsistency between channels (typical indicators: amplitude <0.5dB, phase <1°); finally, analog-to-digital conversion (ADC) configuration: the ADC sampling rate of each channel matches the signal bandwidth (usually >2 times the highest signal frequency); the bit number is selected as 12-16 bits to balance dynamic range and data volume.

[0077] S1200, raw data acquisition and preprocessing; first, synchronous sampling: trigger all channel ADCs to sample simultaneously to obtain the time domain signal sequence; the typical sampling rate during sampling is 10-100MSPS, depending on the signal bandwidth; second, channel equalization: apply pre-measured calibration coefficients to compensate for inconsistencies between channels; digital domain correction formula: Finally, data caching is performed: multi-channel data is stored in a high-speed cache (such as FPGA internal BRAM or external DDR); the cache depth meets the data storage requirements of a complete chirp cycle.

[0078] S1300, channel-level signal processing; first, digital down conversion (DDC): each channel independently performs mixing, filtering, and decimation; and converts the RF signal into baseband I / Q signals. Next, range dimension processing: a fast Fourier transform (FFT) is performed on each channel data. The typical FFT point number is 1024-4096 points, depending on the range resolution required. Finally, target detection is performed: a constant false alarm rate (CFAR) detection algorithm is applied to identify targets within the range gate; the range, amplitude, and phase information of the target in each channel are recorded.

[0079] S1400, spatial domain processing and beamforming. First, spatial snapshot data is constructed: frequency domain data of multiple channels within the same distance is extracted to form a spatial snapshot vector. The snapshot vector dimension is equal to the number of antenna channels (e.g., an M×1 vector). Next, adaptive beamforming is performed: the received signal covariance matrix is ​​calculated: ; Calculate the weight vector based on the minimum variance distortionless response (MVDR) criterion:

[0080] in, The weight vector is applied to each channel data to achieve spatial filtering; a null is formed in the interference direction to enhance the target signal.

[0081] S1500, angle of arrival (DOA) estimation; first, high-resolution spectrum estimation: using the multiple signal classification (MUSIC) algorithm:

[0082] in, is the noise subspace matrix; or the rotation invariance technique (ESPRIT) algorithm:

[0083] in, and The signal subspace matrix is ​​then used; angle search and peak detection are then performed: the spatial spectrum peak is searched within the angle range of interest (e.g., -60° to +60°); the typical angular resolution is 1° to 5°, depending on the array aperture and signal-to-noise ratio; finally, multi-target resolution is performed: spatial smoothing technology is applied to process coherent signal sources; and targets in different directions are distinguished based on eigenvalue decomposition.

[0084] S1600, multi-channel data fusion and altitude calculation: First, angle-range correlation is performed: each angle estimation result is matched with the corresponding range gate; the target's three-dimensional position information (range, azimuth, pitch angle) is constructed; then, altitude resolution is performed: combining the platform attitude information (pitch angle, roll angle) to calculate the true altitude:

[0085] in, is the slope distance, is the pitch angle, is the roll angle; finally, the data fusion algorithm is performed: Kalman filtering is used to fuse multi-frame angle and distance data; the confidence interval and error covariance matrix of the height estimate are calculated.

[0086] The S1700 features enhanced anti-interference and performance. First, spatial nulling technology creates a deep null (typical value > -30dB) in the interference direction. Dynamic nulling position adjustment tracks interference changes. Second, digital beamforming (DBF) simultaneously forms multiple receive beams for full spatial coverage. Multi-beam parallel processing improves target detection probability. Finally, inter-channel mutual coupling compensation estimates and compensates for electromagnetic mutual coupling between antennas, and updates the steering vector model. , where C is the mutual coupling compensation matrix; S1800, System Monitoring and Calibration. First, channel consistency monitoring is performed: a known reference signal is periodically transmitted to assess inter-channel amplitude and phase deviations. Calibration is triggered when the deviation exceeds a threshold (e.g., amplitude > 1dB, phase > 5°). Next, an adaptive calibration algorithm is implemented: calibration coefficients are updated using a least mean square error (LMS) or recursive least squares (RLS) algorithm. The calibration process does not interrupt normal operation, utilizing blind calibration techniques. Finally, fault detection and isolation is performed: signal quality of each channel is monitored to identify the faulty channel. In the event of a fault, the array configuration is reconfigured to maintain partial system functionality.

[0087] S1900, real-time processing implementation and resource optimization; first, a parallel processing architecture: using FPGA to implement parallel processing of multi-channel data streams; typical resource allocation: 30-50% of DSP modules are used for FFT, and 20-30% for beamforming; second, data transmission optimization: using high-speed serial interfaces (such as RapidIO and Aurora) to transmit processed data; implementing on-chip cache management to reduce data handling overhead; finally, algorithm simplification and quantization: using fixed-point arithmetic instead of floating-point operations to reduce resource consumption; optimizing FFT and matrix operation algorithms to reduce the use of multipliers.

[0088] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A radio altimeter ranging method based on a digital frequency modulated continuous wave system, characterized in that: The specific steps of the ranging method are as follows: S100. Generate a linear frequency modulation continuous wave signal using direct digital frequency synthesis technology, and precisely control the frequency modulation slope and center frequency through a digital control word; S200. Convert the digital frequency modulation signal into a radio frequency signal and transmit it, receive the echo signal reflected by the ground and convert it into a digital signal; S300. Perform digital down-conversion processing on the digital echo signal to convert it into a baseband signal; S400. Perform fast Fourier transform processing on the baseband signal to obtain a range-Doppler two-dimensional spectrum; S500. Use spatial spectrum estimation technology to separate and process multipath signals; S600. Adaptively adjust the detection threshold according to the signal environment and noise level; S700. Calculate the altitude of the aircraft according to the processed main path signal, and compensate for factors such as atmospheric refraction, temperature, and air pressure.

2. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S100, the frequency modulation slope and center frequency of the linear frequency modulation continuous wave signal generated by direct digital frequency synthesis technology can be dynamically adjusted according to different application scenarios; the mathematical expression of the linear frequency modulation signal is: ； in, is the signal amplitude, For time, In order to accurately control the FM slope, is the center frequency; The frequency modulation slope of the linear frequency modulation continuous wave signal generated by the direct digital frequency synthesis technology The calculation formula is: ； in, is the FM bandwidth, is the modulation period.

3. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S300, a field programmable gate array is used to implement high-speed digital signal processing, including digital mixing and low-pass filtering; the received signal With local reference signal After mixing, the baseband signal The expression is: ； in, is the Doppler shift, is the echo delay, represents the complex conjugate.

4. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S400, the Doppler FFT processing method is used, and the range dimension resolution and Doppler velocity resolution They are: ， ； in, is the speed of light, is the duration of the chirp signal.

5. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S500, a multiple signal classification algorithm is used to separate and process the multipath signal; a multiple signal classification algorithm is used to estimate the signal arrival angle. , its spatial spectrum function is: ； in, is the array steering vector, is the noise subspace matrix, represents the conjugate transpose.

6. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S600, a constant false alarm rate detection algorithm is used to adaptively adjust the detection threshold according to the local noise level, and a unit average constant false alarm rate detection algorithm in the constant false alarm rate detection algorithm is used to detect the threshold. The calculation formula is: ； in, is the threshold factor, which is determined by the false alarm probability Decide: ； is the signal amplitude of the reference unit, is the number of reference units.

7. The method for measuring distance using a radio altimeter based on a digital frequency modulated continuous wave system according to claim 1, wherein: In step S700, the standard atmospheric model or real-time atmospheric data is used to compensate for atmospheric refraction, temperature and pressure factors, and the actual altitude of the aircraft is The calculation formula is: ； in, is the slope distance, is the pitch angle, is the antenna height, is the atmospheric refraction compensation, calculated using the standard atmospheric model: ； in, is the sea level height, and are the refractive indices of sea level and measurement point, respectively.

8. A radio altimeter ranging method based on a digital frequency modulated continuous wave system according to any one of claims 1 to 7, characterized in that: It further includes step S800: Evaluate the reliability of the measurement result by analyzing the signal-to-noise ratio, correlation peak, and spectrum purity of the signal; The evaluation process steps include: S810. Perform time-domain windowing processing on the baseband signal after digital down-conversion to suppress spectrum leakage, perform moving average filtering to reduce the influence of random noise, and extract the effective signal segment within the specified range gate as the evaluation object; S820. Perform fast Fourier transform on the preprocessed signal to convert it to the frequency domain, calculate the main lobe width, evaluate whether the frequency resolution meets the design requirements, measure the power ratio of the main lobe to the first side lobe, and detect whether there are spurious signals in the spectrum; S830. Use the segmented threshold method to distinguish the signal segment and the noise segment; perform statistical analysis on the noise segment, calculate the noise power spectral density, and calculate the SNR through the power ratio of the signal segment to the noise segment. The formula is: ； in, is the signal power, The noise power is estimated through spectrum analysis; the SNR threshold is set, and when it is lower than the threshold, the signal enhancement or resampling strategy is triggered; S840. Perform cross-correlation calculation on the transmitted signal and the echo signal; detect the main peak position of the correlation function, corresponding to the target distance; calculate the peak-to-side lobe ratio to evaluate the influence of multipath interference; measure the peak width to judge whether the range resolution meets the theoretical value; S850. Perform phase tracking on the same range gate of multiple consecutive chirp signals; calculate the phase standard deviation to evaluate the phase jitter degree; use Kalman filtering to predict the phase change trend and detect abnormal jumps; when the phase jitter exceeds the threshold, mark it as a low-quality signal; S860. Real-time monitor the amplitude change range of the received signal; calculate the power difference between the maximum signal and the minimum signal to evaluate the system dynamic range; when the signal approaches the full scale of the ADC, trigger automatic gain control adjustment; S870. Assign weights to the above各项指标 (I'm not sure what this specific term means in Chinese, assuming it's "each index"); and calculate the comprehensive quality score. The calculation formula is: Score index ; Make a decision according to the scoring result: High quality (Score > 80): Directly adopt the current measurement result; Medium quality (60 < Score ≤ 80): Enable multi-frame averaging or Kalman filtering fusion; Low quality (Score ≤ 60): Trigger re-sampling or alarm mechanism; S880: Dynamically adjust system parameters based on evaluation results: increase transmit power or integration time when SNR is low; optimize DDS frequency synthesis parameters when spurious signals are high; adjust clock synchronization strategy when phase is unstable; and feed back evaluation results to the height calculation module for error compensation.

9. A radio altimeter ranging method based on a digital frequency modulated continuous wave system according to any one of claims 1 to 7, characterized in that: The method further includes step S900: dynamically adjusting the transmission power, modulation mode or operating frequency by monitoring the external electromagnetic environment to suppress external interference.

10. A radio altimeter ranging method based on a digital frequency modulated continuous wave system according to any one of claims 1 to 7, characterized in that: The method further includes step S1000: arranging multiple receiving antennas to form an array and utilizing spatial diversity technology to improve angle measurement accuracy and anti-interference capability.

Images