A signal processing method for Doppler frequency offset interference of height-finding radar

Abstract

The present application relates to radar detection technology field, especially to a kind of signal processing method of Doppler frequency offset interference of height-finding radar, the method is by monitoring the maneuverability of height-finding radar and dynamically adjusting frequency modulation period and frequency modulation bandwidth of symmetrical frequency modulation signal;Based on the frequency modulation period and frequency modulation bandwidth after adjustment generate positive slope frequency modulation signal and negative slope frequency modulation signal and carry out periodic alternate emission, collect and process and difference calculation first echo signal and second echo signal after reflection, obtain after eliminating Doppler frequency offset interference ranging frequency difference, further avoid the height-finding error caused by Doppler shift amount, to obtain accurate target height based on ranging frequency difference calculation.This application is by using the Doppler shift polarity opposite characteristics of positive and negative slope frequency modulation signal, by frequency spectrum line pairing and difference calculation offset Doppler shift component, finally based on the target height of corrected frequency information inversion, to realize high-precision height-finding.

Description

Technical Field

[0001] This invention relates to the field of radar detection technology, and in particular to a signal processing method for Doppler frequency offset interference in altimeter radar. Background Technology

[0002] Current altimeter radars are core sensors ensuring safe takeoff and landing and low-altitude flight for aircraft. They typically employ a single-slope frequency-modulated (FM) signal, measuring altitude by the frequency difference between the transmitted FM continuous wave signal and the ground echo signal. This method offers advantages such as simple structure, high resolution, and ease of integration. However, this altimeter method faces severe Doppler interference problems in real-world high-speed flight scenarios. Due to the high-speed relative motion between the aircraft and the ground, a Doppler frequency shift occurs between the radar and the ground. This shift is directly superimposed on the ranging frequency difference determined by altitude, thus introducing significant altimeter measurement errors.

[0003] For example, under typical high-speed flight conditions, even if the beam direction is perpendicular to the flight velocity direction, a significant Doppler frequency shift may still occur. This frequency shift, after being processed by the radar system, is converted into a corresponding time measurement deviation, resulting in a distance error on the order of several meters. Especially in the low-altitude flight phase, where the altitude is relatively low, this type of error accounts for a significant proportion of the altimeter results, seriously threatening flight safety.

[0004] To suppress the Doppler effect, existing technologies mainly focus on signal processing and system design, including: (1) Phase compensation method: relying on external or estimated speed information for frequency offset correction, but speed measurement error will be directly converted into residual height measurement error; (2) Doppler filtering method: using adaptive filtering algorithm to separate difference frequency and Doppler component, however, the algorithm is complex and computationally burdensome, making it difficult to meet the real-time requirements under high dynamic conditions; (3) Dual-frequency / multi-frequency radar method: transmitting multiple frequency signals and processing them together to cancel the Doppler term, but it requires increasing the cost of radio frequency channels and hardware, which is not conducive to equipment miniaturization and cost control. It can be seen that existing methods are difficult to achieve high-precision Doppler suppression over a wide speed range without additional hardware and while ensuring real-time processing.

[0005] Therefore, there is an urgent need for a high-precision altimeter radar signal processing method that can effectively combat Doppler frequency shift and adapt to high-speed flight conditions without significantly increasing system complexity and cost, while ensuring real-time processing, so as to improve the reliability of aircraft altimeter measurement across the entire speed range, especially in the low-altitude phase. Summary of the Invention

[0006] To address this issue, the present invention provides a signal processing method for Doppler frequency offset interference in altimeter radar, which solves the problem in the prior art that it is impossible to effectively counteract Doppler frequency offset under high-speed flight conditions without additional hardware, thus causing inaccurate altimeter measurement.

[0007] To achieve the above objectives, the present invention provides a signal processing method for Doppler frequency shift interference in altimeter radar, comprising:

[0008] Based on the mobility of the altimeter radar, the frequency modulation period and frequency modulation bandwidth used to generate the symmetrical frequency modulation signal can be dynamically adjusted, including extending the frequency modulation period and narrowing the frequency modulation bandwidth, or shortening the frequency modulation period and widening the frequency modulation bandwidth.

[0009] Based on the dynamically adjusted frequency modulation period and frequency modulation bandwidth, a symmetrical frequency modulation signal containing a positive slope segment and a negative slope segment is generated, and the frequency modulation signal of the positive slope segment is recorded as the positive slope frequency modulation signal, and the frequency modulation signal of the negative slope segment is recorded as the negative slope frequency modulation signal.

[0010] The positive slope frequency modulation signal and the negative slope frequency modulation signal are periodically and alternately transmitted;

[0011] After periodically transmitting the signal, the first echo signal reflected by the positive slope frequency modulation signal and the second echo signal reflected by the negative slope frequency modulation signal are collected.

[0012] The first echo signal and the second echo signal are preprocessed to obtain the corresponding positive slope echo signal and negative slope echo signal;

[0013] Spectral analysis and feature extraction were performed on the positive slope echo signal and the negative slope echo signal respectively to obtain the corresponding positive slope peak frequency and negative slope peak frequency;

[0014] The ranging frequency difference is obtained by performing a difference calculation based on the peak frequency of the positive slope and the peak frequency of the negative slope;

[0015] Based on the stability of the ranging frequency difference, the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal is dynamically adjusted, including extending or shortening the alternating transmission period;

[0016] The altitude is determined by measuring the distance frequency difference obtained after adjusting the alternating transmission cycle.

[0017] Furthermore, the process of dynamically adjusting the frequency modulation period and frequency modulation bandwidth for generating the symmetrical frequency-modulated signal includes:

[0018] The acceleration of the platform equipped with the altimeter radar is obtained in several stages of movement, and the maneuverability value is calculated.

[0019] The mobility of the altimeter is determined based on the mobility value, and the frequency modulation period and frequency modulation bandwidth are dynamically and collaboratively adjusted based on the determination result.

[0020] Furthermore, based on the comparison result between the mobility value and the mobility threshold, the frequency modulation period is extended and the frequency modulation bandwidth is narrowed, or the frequency modulation period is shortened and the frequency modulation bandwidth is widened.

[0021] Furthermore, the frequency modulation period is extended and the frequency modulation bandwidth is narrowed based on the comparison result between the first maneuver difference and the preset first maneuver difference;

[0022] Wherein, the first mobility difference is the difference between the lower limit of the mobility threshold and the mobility value.

[0023] Furthermore, the frequency modulation period is shortened and the frequency modulation bandwidth is widened based on the comparison result between the second maneuver difference and the preset second maneuver difference;

[0024] The second mobility difference is the difference between the mobility value and the upper limit of the mobility threshold.

[0025] Furthermore, the process of dynamically adjusting the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal includes:

[0026] Several ranging frequency differences are obtained and variance is calculated to obtain the frequency difference fluctuation value;

[0027] The stability of the ranging frequency difference is determined based on the frequency difference fluctuation value, and the alternating transmission period is adjusted based on the determination result.

[0028] Furthermore, the alternating transmission period can be extended or shortened based on the comparison result between the frequency difference fluctuation value and the fluctuation threshold.

[0029] Furthermore, the alternating transmission period is extended based on the comparison result between the first fluctuation difference and the preset first fluctuation difference, and the extension of the alternating transmission period is positively correlated with the first fluctuation difference.

[0030] Wherein, the first fluctuation difference is the difference between the lower limit of the fluctuation threshold and the frequency difference fluctuation value.

[0031] Furthermore, the alternating transmission cycle is shortened based on the comparison result between the second fluctuation difference and the preset second fluctuation difference, and the shortening of the alternating transmission cycle is positively correlated with the second fluctuation difference;

[0032] Wherein, the second fluctuation difference is the difference between the frequency difference fluctuation value and the upper limit value of the fluctuation threshold.

[0033] Furthermore, the process of generating a symmetrical frequency-modulated signal containing positive and negative slope segments includes:

[0034] The positive slope frequency modulation signal is calculated based on the radar center frequency, the frequency modulation bandwidth, and the first signal period.

[0035] The negative slope frequency-modulated signal is calculated based on the radar center frequency, the frequency modulation bandwidth, and the second signal period.

[0036] Wherein, the first signal period is the first half of the frequency modulation period, and the second signal period is the second half of the frequency modulation period.

[0037] Compared with existing technologies, the signal processing method for Doppler frequency offset interference of altimeter radar of the present invention has the following advantages: by monitoring the mobility of altimeter radar and dynamically adjusting the frequency modulation period and frequency modulation bandwidth of symmetrical frequency modulation signal based on the monitoring results, the real-time performance of symmetrical frequency modulation signal generation process is ensured, and the accuracy of target height measurement is improved; positive slope frequency modulation signal and negative slope frequency modulation signal are generated based on the adjusted frequency modulation period and frequency modulation bandwidth and periodically alternately transmitted; the first echo signal reflected by the positive slope frequency modulation signal and the second echo signal reflected by the negative slope frequency modulation signal are further collected, processed, and differentially calculated to obtain the ranging frequency difference after eliminating Doppler frequency offset interference, thereby avoiding the height measurement error caused by Doppler offset, and thus calculating the accurate target height based on the ranging frequency difference; wherein, according to the stability detection result of the ranging frequency difference, the alternating transmission period of positive slope frequency modulation signal and negative slope frequency modulation signal is dynamically adjusted, thereby improving the tracking speed of height changes, reducing dynamic lag error, and thus ensuring height measurement accuracy and reliability.

[0038] Furthermore, during the monitoring of the mobility of the altimeter radar, the mobility values ​​are acquired and compared with a set mobility threshold. Based on the comparison results, the corresponding coordinated adjustment method of the frequency modulation period and frequency modulation bandwidth is determined. Based on the comparison results of the first mobility difference and the preset first mobility difference, the extension of the frequency modulation period and the narrowing of the frequency modulation bandwidth are determined, or based on the comparison results of the second mobility difference and the preset second mobility difference, the shortening of the frequency modulation period and the widening of the frequency modulation bandwidth are determined. This allows for precise adjustment of the frequency modulation period and frequency modulation bandwidth during the generation of the symmetrical frequency modulation signal, thereby improving the generation accuracy of the symmetrical frequency modulation signal and thus improving the accuracy of the ranging frequency difference calculation.

[0039] Furthermore, when performing stability testing on the ranging frequency difference, the frequency difference fluctuation value is acquired and compared with a set fluctuation threshold. Based on the comparison result, the adjustment method of the alternating transmission cycle is determined. The extension range of the alternating transmission cycle is determined based on the comparison result of the first fluctuation difference value and the preset first fluctuation difference value, or the shortening range of the alternating transmission cycle is determined based on the comparison result of the second fluctuation difference value and the preset second fluctuation difference value. This enables precise adjustment of the transmission process of the positive slope frequency modulation signal and the negative slope frequency modulation signal, so as to better suppress random fluctuations and improve the measurement accuracy and reliability of the target altitude. Attached Figure Description

[0040] Figure 1 This is a flowchart illustrating the signal processing method for Doppler frequency offset interference of altimeter radar in an embodiment of the present invention.

[0041] Figure 2 This is a schematic diagram of the signal processing system used to implement Doppler frequency offset interference for altimeter radar in an embodiment of the present invention;

[0042] Figure 3 This is a logic decision diagram for dynamically adjusting the frequency modulation period and frequency modulation bandwidth based on the mobility of the altimeter radar in an embodiment of the present invention.

[0043] Figure 4 This is a logic diagram for dynamically adjusting the alternating transmission period based on the stability of the ranging frequency difference in an embodiment of the present invention.

[0044] Figure 5 This is a schematic diagram of symmetrical frequency modulation signal transmission and reception in an embodiment of the present invention. Detailed Implementation

[0045] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0046] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0047] It should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0048] In real-world high-speed flight scenarios, a severe Doppler interference problem arises: due to the high-speed relative motion between the aircraft and the ground, the Doppler frequency shift caused by the relative motion between the radar and the ground is calculated using the following formula: Where v is the flight speed, θ is the angle between the beam axis and the velocity direction, and λ is the wavelength. This offset will be directly superimposed on the ranging frequency difference determined by altitude, thus introducing a significant altitude measurement error.

[0049] For example, under typical conditions where an altimeter radar flies at a speed of 300 km / h (equivalent to 83.3 m / s), operates at a frequency of 4.3 GHz, has a wavelength of λ ≈ 0.07 m, and the beam axis forms an angle of θ = 90° with the velocity direction, the resulting Doppler frequency shift is fd = 2 × 83.3 × 1 / 0.07 ≈ 2.38 kHz. If the linear frequency modulated continuous wave radar uses a frequency modulation slope of 120 kHz / μs, this frequency shift is equivalent to a time deviation of approximately 0.02 μs, corresponding to a distance measurement error of approximately 3 meters. In the low-altitude flight phase at approximately 10 meters above the ground, this type of error can account for up to 30%, seriously threatening flight safety.

[0050] Please see Figure 1 The diagram shown is a flowchart illustrating the signal processing method for Doppler frequency shift interference of altimeter radar in an embodiment of the present invention; this embodiment includes at least the following steps:

[0051] S1: Based on the mobility of the altimeter radar, dynamically adjust the frequency modulation period and frequency modulation bandwidth used to generate the symmetrical frequency modulation signal, including extending the frequency modulation period and narrowing the frequency modulation bandwidth, or shortening the frequency modulation period and widening the frequency modulation bandwidth.

[0052] S2: Based on the dynamically adjusted frequency modulation period and frequency modulation bandwidth, generate a symmetrical frequency modulation signal containing positive slope segment and negative slope segment, and record the frequency modulation signal of the positive slope segment as the positive slope frequency modulation signal, and the frequency modulation signal of the negative slope segment as the negative slope frequency modulation signal.

[0053] S3: Periodically and alternately transmit positive slope frequency modulation signals and negative slope frequency modulation signals;

[0054] S4: After periodically transmitting the signal, acquire the first echo signal reflected by the positive slope frequency modulation signal and the second echo signal reflected by the negative slope frequency modulation signal;

[0055] S5: Preprocess the first echo signal and the second echo signal to obtain the corresponding positive slope echo signal and negative slope echo signal.

[0056] S6: Perform spectral analysis and feature extraction on the positive slope echo signal and the negative slope echo signal respectively to obtain the corresponding positive slope peak frequency and negative slope peak frequency;

[0057] S7: The ranging frequency difference is obtained by differential calculation based on the peak frequency of positive slope and the peak frequency of negative slope;

[0058] S8: Based on the stability of the ranging frequency difference, dynamically adjust the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal, including extending or shortening the alternating transmission period;

[0059] S9: Altitude is calculated based on the ranging frequency difference reacquired after adjusting the alternating transmission cycle to determine the target altitude.

[0060] Please see Figure 2 As shown, it is a schematic diagram of the signal processing system used to implement Doppler frequency offset interference of altimeter radar in an embodiment of the present invention.

[0061] This embodiment includes a signal conditioning module, a signal generation module, a signal transmission module, a receiving module, a signal processing module, a differential calculation module, a transmission cycle management module, and an altitude measurement module.

[0062] The signal conditioning module is used to monitor the mobility of the altimeter radar and dynamically adjust the frequency modulation period and frequency modulation bandwidth of the symmetrical frequency modulation signal based on the mobility monitoring results. The adjustment methods include simultaneously extending the frequency modulation period and narrowing the frequency modulation bandwidth, or simultaneously shortening the frequency modulation period and widening the frequency modulation bandwidth.

[0063] The signal generation module, which is connected to the signal conditioning module, is used to generate a symmetrical frequency modulation signal containing a positive slope segment and a negative slope segment based on the dynamically adjusted frequency modulation period and frequency modulation bandwidth, and to record the frequency modulation signal of the positive slope segment as a positive slope frequency modulation signal and the frequency modulation signal of the negative slope segment as a negative slope frequency modulation signal.

[0064] A signal transmission module, connected to a signal generation module, is used to acquire positive slope frequency modulation signals and negative slope frequency modulation signals and periodically alternately transmit positive slope frequency modulation signals and negative slope frequency modulation signals;

[0065] A receiving module, connected to a signal transmitting module, is used to monitor the transmission of positive slope frequency modulation (FM) signals and negative slope frequency modulation (FM) signals to synchronously acquire the first echo signal and the second echo signal reflected from the ground. The first echo signal corresponds to the positive slope FM signal, and the second echo signal corresponds to the negative slope FM signal. A signal processing module, connected to a receiving module, is used to preprocess the first echo signal and the second echo signal to obtain the positive slope echo signal corresponding to the positive slope FM signal and the negative slope echo signal corresponding to the negative slope FM signal.

[0066] The differential calculation module, which is connected to the signal processing module, is used to perform spectrum analysis and feature extraction on the positive slope echo signal and the negative slope echo signal respectively, so as to obtain the positive slope peak frequency corresponding to the positive slope frequency modulation signal and the negative slope peak frequency corresponding to the negative slope frequency modulation signal, and to perform differential calculation based on the positive slope peak frequency and the negative slope peak frequency to obtain the ranging frequency difference.

[0067] The transmission cycle management module is connected to the differential calculation module and the signal transmission module respectively. It is used to determine the stability based on the ranging frequency difference and dynamically adjust the alternating transmission cycle of the positive slope frequency modulation signal and the negative slope frequency modulation signal based on the determination result. The adjustment method includes extending or shortening the alternating transmission cycle.

[0068] The altitude calculation module is connected to the differential calculation module to obtain the ranging frequency difference in the differential calculation module in real time, and to calculate the altitude based on the latest ranging frequency difference to determine the target altitude.

[0069] By utilizing the opposite polarity of the Doppler shift in positive and negative slope frequency-modulated signals, a differential processing calculation process is constructed. The Doppler shift components are canceled out by frequency spectral line pairing and difference calculation. Finally, the target height is retrieved based on the corrected frequency information to achieve high-precision height measurement.

[0070] In this embodiment, the positive slope frequency modulation signal is calculated based on the radar center frequency, frequency modulation bandwidth, and the first signal period;

[0071] The formula for calculating a positive slope frequency modulated signal is: ;in, It is a positive slope frequency modulation signal. The center frequency of the radar. For frequency modulation bandwidth, For frequency modulation period, This is the first signal period. It is the single-sided bandwidth (i.e., the frequency change of the positive or negative slope segment).

[0072] The negative slope frequency-modulated signal is calculated based on the radar center frequency, frequency modulation bandwidth, and the second signal period.

[0073] The formula for calculating a negative slope frequency modulated signal is: ; It is a negative slope frequency modulation signal. This is the second signal period.

[0074] The first echo signal is The second echo signal is ;in, For signal round-trip time, The target altitude of the drone above the current ground level. At the speed of light, The angle deviating from the vertical direction.

[0075] Preprocessing includes mixing, filtering, and analog-to-digital conversion. In the mixing stage, the first and second echo signals are mixed with the local reference signal to obtain the positive deviation intermediate frequency (IF) signal and the negative deviation IF signal, respectively. The local reference signal and the symmetrical frequency modulation (FM) signal are from the same source. In the filtering stage, a filter with a cutoff frequency equal to the FM bandwidth B is used to remove noise components with frequencies higher than the cutoff frequency from the positive and negative deviation IF signals. In the FM conversion stage, an FM converter with a sampling frequency fs greater than or equal to twice the FM bandwidth B is used to perform analog-to-digital conversion on the filtered positive and negative deviation IF signals to obtain the positive slope echo signal and the negative slope echo signal, respectively.

[0076] The spectrum of the positive slope signal is obtained by performing a Fourier transform on the positive slope echo signal. And the spectrum of the positive slope signal Feature extraction was performed to obtain the peak frequency of the positive slope. ;

[0077] The spectrum of the negative slope signal is obtained by performing a Fourier transform on the negative slope echo signal. And the spectrum of negative slope signals Feature extraction was performed to obtain the peak frequency of negative slope. ;

[0078] Among them, the peak frequency of positive slope negative slope peak frequency , among them For the difference in ranging frequencies, Doppler offset;

[0079] The ranging frequency difference is obtained by differential calculation. This formula cancels out the Doppler shift. .

[0080] Obtaining the ranging frequency difference after eliminating Doppler frequency shift interference. Then, based on the formula Calculate the target height .

[0081] Please see Figure 3 As shown, it is a logic decision diagram for dynamically adjusting the frequency modulation period and frequency modulation bandwidth according to the mobility of the altimeter radar in an embodiment of the present invention.

[0082] Specifically, the frequency modulation period and bandwidth during the generation of the symmetrical frequency-modulated signal by the altimeter radar are dynamically correlated with the mobility of the platform on which the radar is mounted. This allows for optimization of altimeter accuracy and real-time performance under different dynamic scenarios. The platform can be a drone or other flying device, carrying the altimeter radar in the air. A signal conditioning module acquires and calculates the acceleration corresponding to several movement phases of the altimeter radar, resulting in the average normal acceleration, which characterizes the radar's mobility. This average normal acceleration is denoted as the mobility value M. Based on the mobility value M, the radar's mobility is determined, and the frequency modulation period and bandwidth are dynamically adjusted based on the determination result. This ensures the real-time performance of the symmetrical frequency-modulated signal generation process and improves the accuracy of target altitude measurement.

[0083] In this embodiment, combining comprehensive theoretical calculations and engineering design experience, a maneuverability threshold M0 is pre-set and compared with the maneuverability value M. Based on the comparison result, the corresponding coordinated adjustment method of the frequency modulation cycle and frequency modulation bandwidth is determined. Considering the platform's actual maneuverability and safety margin, the maneuverability threshold M0 can be set exemplarily to [0.35g, 0.5g], where g is the gravitational acceleration. The following comparison is made between the maneuverability value M and the maneuverability threshold M0:

[0084] If M is less than 0.35g, that is, M is less than the lower limit of M0, it is determined that the current platform is in a low-speed movement process. Therefore, in the process of generating symmetrical frequency modulation signals, the altimeter radar can extend the frequency modulation period and narrow the frequency modulation bandwidth. In the low-speed stable movement state, the corresponding extension of the frequency modulation period can reduce system power consumption and data processing load, and the corresponding narrowing of the frequency modulation bandwidth can obtain a higher signal-to-noise ratio and improve detection robustness while meeting the accuracy requirements.

[0085] If M is greater than or equal to 0.35g and less than or equal to 0.5g, i.e. M equals M0, it is determined that the platform maintains the initial frequency modulation period and frequency modulation bandwidth at the current moving speed, or maintains the frequency modulation period and frequency modulation bandwidth adjusted in the previous stage, to avoid switching the symmetrical frequency modulation signal too frequently.

[0086] If M is greater than 0.5g, that is, M is greater than the upper limit of M0, it is determined that the current platform is in a high-speed movement process. Therefore, in the process of generating symmetrical frequency modulation signals, the altimeter radar can shorten the frequency modulation period and widen the frequency modulation bandwidth. In the high-speed maneuvering state, the corresponding shortening of the frequency modulation period can improve the data rate, quickly track the altitude change, and reduce the accumulation of "speed-distance" coupling error caused by the platform movement. The corresponding widening of the frequency modulation bandwidth can ensure that there is still sufficient distance resolution and accuracy in the short period.

[0087] The adjusted frequency modulation period and bandwidth are synchronized to the subsequent signal processing module, differential calculation module, and altitude measurement module. This enables the altimeter radar to adapt to real-time responses in highly dynamic environments, improving the accuracy of target altitude measurement.

[0088] In a specific embodiment for determining the extension of the frequency modulation period and the narrowing of the frequency modulation bandwidth, the difference between the lower limit of the mobility threshold M0 and the mobility value M can be used to obtain a first mobility difference P. A preset first mobility difference P0 is set and compared with the first mobility difference P to determine the extension of the frequency modulation period and the narrowing of the frequency modulation bandwidth. Wherein, when the first mobility difference P is larger, the mobility value M is smaller, indicating that the movement of the platform is more stable, and the corresponding extension of the frequency modulation period is larger, and the narrowing of the frequency modulation bandwidth is larger. Therefore, the extension of the frequency modulation period and the narrowing of the frequency modulation bandwidth are both positively correlated with the first mobility difference P.

[0089] To more accurately determine the magnitude of the frequency modulation cycle extension and the magnitude of the frequency modulation bandwidth narrowing, the preset first maneuvering difference P0 can be divided into a first preset first maneuvering difference P1 and a second preset first maneuvering difference P2. Historical data statistical analysis exemplarily sets P1=0.05g and P2=0.08g. The comparison process between the first maneuvering difference P and P1 and P2 is as follows:

[0090] If P is less than or equal to P1, determine to extend the frequency modulation period of the current stage to twice the initial frequency modulation period, and determine to narrow the frequency modulation bandwidth of the current stage to 70% of the initial frequency modulation bandwidth; wherein, if the initial frequency modulation period is 10ms, then the extended frequency modulation period is 20ms; if the initial frequency modulation bandwidth is 200MHz, then the narrowed frequency modulation bandwidth is 140MHz.

[0091] If P is greater than P1 and less than or equal to P2, determine to extend the frequency modulation period of the current stage to 4 times the initial frequency modulation period, and determine to narrow the frequency modulation bandwidth of the current stage to 50% of the initial frequency modulation bandwidth.

[0092] If P is greater than P2, determine to extend the frequency modulation period of the current stage to 5 times the initial frequency modulation period, and determine to narrow the frequency modulation bandwidth of the current stage to 40% of the initial frequency modulation bandwidth.

[0093] It should be noted that extending the frequency modulation period, while meeting the system's maximum permissible height update delay, involves extending the period to an optimized value based on the current power budget or system workload. Narrowing the frequency modulation bandwidth, on the other hand, is done while meeting the system's minimum required distance resolution and altitude measurement accuracy requirements, by narrowing the bandwidth to an optimized value. After optimizing the frequency modulation period and bandwidth, the regenerated symmetrical frequency-modulated signal can result in more accurate target height calculations. It is understood that the magnitude of the frequency modulation period extension and bandwidth narrowing can also be set to other suitable values, not limited to the exemplary values ​​in this embodiment.

[0094] In a specific embodiment that determines the shortening of the frequency modulation period and the widening of the frequency modulation bandwidth, a second maneuvering difference J is obtained by the difference between the computer maneuvering value M and the upper limit of the maneuvering threshold M0. A preset second maneuvering difference J0 is set and compared with the second maneuvering difference J. Based on the comparison result, the shortening and widening values ​​are determined. Wherein, when the second maneuvering difference J is larger, the corresponding maneuvering value M is larger, which indicates that the platform is in a faster high-speed movement process, and thus requires a shorter frequency modulation period and a wider frequency modulation bandwidth to meet the real-time requirements of the symmetrical frequency modulation signal. Therefore, the shortening of the frequency modulation period and the widening of the frequency modulation bandwidth are both positively correlated with the second maneuvering difference J.

[0095] To more accurately determine the magnitude of the shortening of the frequency modulation cycle and the magnitude of the widening of the frequency modulation bandwidth, the preset second maneuvering difference J0 is divided into a first preset second maneuvering difference J1 and a second preset second maneuvering difference J2. Historical data statistical analysis exemplarily sets J1=0.07g and J2=0.13g. The comparison process between the second maneuvering difference J0 and J1 and J2 is as follows:

[0096] If J is less than or equal to J1, determine to shorten the frequency modulation period of the current stage to 60% of the initial frequency modulation period, and determine to widen the frequency modulation bandwidth of the current stage to twice the initial frequency modulation bandwidth; wherein, if the initial frequency modulation period is 50ms, then the shortened frequency modulation period is 30ms; if the initial frequency modulation bandwidth is 20MHz, then the widened frequency modulation bandwidth is 40MHz.

[0097] If J is greater than J1 and less than or equal to J2, determine to shorten the frequency modulation period of the current stage to 40% of the initial frequency modulation period, and determine to widen the frequency modulation bandwidth of the current stage to 3 times the initial frequency modulation bandwidth.

[0098] If J is greater than J2, determine to shorten the frequency modulation period of the current stage to 20% of the initial frequency modulation period, and determine to widen the frequency modulation bandwidth of the current stage to 5 times the initial frequency modulation bandwidth.

[0099] It should be noted that shortening the frequency modulation period, while meeting the system's requirements for high tracking speed and dynamic response, involves reducing the period to an optimized value based on the platform's maneuvering status or the rate of environmental change. Increasing the frequency modulation bandwidth, while meeting the system's requirements for improved target resolution and measurement accuracy, also considers signal processing capabilities and unambiguous distance constraints, resulting in an optimized bandwidth. After such optimizations to the frequency modulation period and bandwidth, the regenerated symmetrical frequency-modulated signal significantly enhances the system's tracking and resolution capabilities in dynamic scenarios. It is understood that the magnitude of the frequency modulation period reduction and bandwidth increase can also be set to other suitable values, not limited to the exemplary values ​​in this embodiment.

[0100] Please see Figure 4 As shown, this is a logic decision diagram for dynamically adjusting the alternating transmission period based on the stability of the ranging frequency difference in an embodiment of the present invention. Specifically, after calculating several ranging frequency differences, variance is calculated to obtain the frequency difference fluctuation value G. Since the frequency difference fluctuation value G can reflect the short-term fluctuation of the current altitude measurement result, the stability of the ranging frequency difference acquisition process is determined based on the short-term fluctuation. Based on the determination result, the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal is dynamically adjusted to ensure high measurement performance while optimizing system efficiency.

[0101] In this embodiment, a suitable fluctuation threshold G0 is determined based on the radar parameters used, the specific requirements of the mission for measurement accuracy, and the desired system response characteristics. This threshold is then compared with the frequency difference fluctuation value G, and the alternating transmission period is dynamically adjusted based on the comparison result. For example, the fluctuation threshold G0 is set to [3000Hz]. 2 5000Hz 2 The comparison process between the frequency difference fluctuation value G and the fluctuation threshold G0 is as follows:

[0102] If G is less than 3000Hz 2 If G is less than the lower limit of G0, then the altimeter radar is considered to be in good stability, and the alternating transmission cycle is extended to optimize computational resources.

[0103] If G is greater than or equal to 3000Hz 2 and less than or equal to 5000Hz 2 If G equals G0, then the stability of the altimeter radar at the current stage is determined to be in an intermediate state. Therefore, the current alternating transmission cycle is maintained to avoid frequent switching of the alternating transmission cycle around the fluctuation threshold G0, thus ensuring smooth system switching.

[0104] If G is greater than 5000Hz 2If G is greater than the upper limit of G0, it indicates that the measurement based on the current data rate can no longer effectively smooth the noise, or that the environmental dynamics have increased. Therefore, it is determined that the current stability of the altimeter radar is insufficient, and the alternating transmission cycle is shortened to improve the data rate and suppress fluctuations.

[0105] The transmission cycle management module transmits the adjusted alternating transmission cycle to the signal transmission module, thereby adjusting the transmission cycle of the positive slope FM signal and the negative slope FM signal in the next stage.

[0106] In a specific embodiment for determining the extension range of the alternating transmission cycle, a first fluctuation difference value D is obtained by calculating the difference between the lower limit of the fluctuation threshold G0 and the frequency difference fluctuation value G. A preset first fluctuation difference value D0 is set and compared with the first fluctuation difference value D. The extension range of the alternating transmission cycle is determined based on the comparison result. Wherein, when the first fluctuation difference value D is larger, the corresponding frequency difference fluctuation value G is smaller, indicating that the system stability is better, and thus the extension range of the alternating transmission cycle can be larger. Therefore, the extension range of the alternating transmission cycle is positively correlated with the magnitude of the first fluctuation difference value D.

[0107] To more accurately determine the extension range of the alternating transmission cycle, the preset first fluctuation difference D0 can be divided into a first preset first fluctuation difference D1 and a second preset first fluctuation difference D2. Based on historical data statistical analysis, D1 is exemplarily set to 100Hz. 2 D2=300Hz 2 Based on the first fluctuation difference D, the following comparison is made with D1 and D2:

[0108] If D is less than or equal to D1, the alternating transmission period is extended by a factor of 2; where the original alternating transmission period is 20ms, the extended alternating transmission period is 40ms. If D is greater than D1 and less than or equal to D2, the alternating transmission period is extended by a factor of 3. If D is greater than D2, the alternating transmission period is extended by a factor of 5.

[0109] It should be noted that extending the alternating transmission period is done under the premise of meeting the system's requirements for measurement stability and data validity. Based on the evaluation results of the current frequency difference fluctuation, the period is extended to an optimized value. The core idea is that when the frequency difference fluctuation value G is smaller (i.e., the corresponding first fluctuation difference value D is larger), it indicates higher consistency in short-term system measurements and a more stable environment. In this case, under the constraint of not exceeding the system's maximum allowable update delay, moderately extending the transmission period can effectively reduce the radar's transmission and processing power consumption and reduce system resource consumption without significantly affecting altitude measurement accuracy. It is understood that the extension range of the alternating transmission period can also be set to other suitable values ​​according to the specific design specifications of different radar systems, and is not limited to the exemplary values ​​in this embodiment.

[0110] In a specific embodiment for determining the shortening of the alternating transmission cycle, a second fluctuation difference value K is obtained by calculating the difference between the frequency difference fluctuation value G and the upper limit of the fluctuation threshold G0. A preset second fluctuation difference value K0 is set and compared with the second fluctuation difference value K. The shortening of the alternating transmission cycle is determined based on the comparison result. Wherein, when the second fluctuation difference value K is larger, the corresponding frequency difference fluctuation value G is larger, indicating that the system stability is worse, and therefore the shortening of the alternating transmission cycle needs to be larger. Therefore, the shortening of the alternating transmission cycle is positively correlated with the magnitude of the second fluctuation difference value K.

[0111] To more accurately determine the shortening of the alternating transmission period, the preset second fluctuation difference K0 can be divided into a first preset second fluctuation difference K1 and a second preset second fluctuation difference K2. Based on historical data statistical analysis, K1 is exemplarily set to 200Hz. 2 K2=450Hz 2 The comparison process based on the second fluctuation difference K with K1 and K2 is as follows:

[0112] If K is less than or equal to K1, the alternating transmission period is shortened by 20% based on the original alternating transmission period; where the original alternating transmission period is 20ms, the shortened alternating transmission period is 16ms.

[0113] If K is greater than K1 and less than or equal to K2, then the original alternating firing cycle will be shortened by 30%.

[0114] If K is greater than K2, then the original alternating firing cycle will be shortened by 40%.

[0115] It should be noted that shortening the alternating transmission cycle is done to address situations where system measurement stability decreases or environmental dynamism increases. Based on a quantitative assessment of the current frequency difference fluctuations, the cycle is shortened to an optimized value that ensures performance. The core principle is that a larger frequency difference fluctuation value G (i.e., a larger corresponding second fluctuation difference value K) indicates increased random error in a single measurement or a faster rate of altitude change. In such cases, shortening the transmission cycle is necessary to improve the data update rate. This not only improves the system's tracking speed of altitude changes and reduces dynamic lag errors, but also increases the number of effective samples per unit time, allowing for better suppression of random fluctuations through subsequent processing (such as averaging or filtering), thereby maintaining necessary measurement accuracy and reliability under different altimetry conditions. It is understood that the reduction in the alternating transmission cycle can be set to other suitable values ​​based on the specific design specifications of different radar systems, and is not limited to the exemplary values ​​in this embodiment.

[0116] To better illustrate the signal processing method for Doppler frequency offset interference of altimeter radar in this embodiment, a specific embodiment is given below to further describe the present invention.

[0117] Based on the specific application scenario of a certain type of aircraft primarily flying in sea-level and hilly areas, and considering factors such as adapting to terrain undulations and sea wave fluctuations, the radar's operating frequency was selected. Furthermore, the relevant radio management regulations regarding radio transmission frequencies were also referenced. The hardware operating parameters for this altimeter radar are as follows:

[0118] Radar parameters: center frequency f0 = 4.3 GHz, wavelength λ = 7 cm; exemplarily, the initial frequency modulation period T = 10 ms and the initial frequency modulation bandwidth B = 200 MHz are selected; to restore the signal waveform without distortion, the sampling frequency fs = 500 MHz is selected. To more clearly illustrate the effect of using the opposite polarity of the Doppler shift of the positive slope frequency modulation signal and the negative slope frequency modulation signal to cancel the Doppler shift component, the following embodiments determine that the platform maintains the initial frequency modulation period and frequency modulation bandwidth at the current moving speed; and determine that the stability of the current stage is in an intermediate state, therefore maintaining the current alternating transmission cycle.

[0119] Please see Figure 5 As shown, it is a schematic diagram of symmetrical frequency modulation signal transmission and reception in an embodiment of the present invention. Figure 5 The solid line segments corresponding to 0–1.64ms, 5ms–6.64ms, and after 10ms represent the transmitted symmetrical frequency-modulated signal; the dashed line segments corresponding to 0–1.64ms, 5ms–6.64ms, and after 10ms represent the received echo signal; and the straight line segments corresponding to 1.64ms–5ms and 6.64ms–10ms represent the rest period. Based on a modulation slope of 0.12MHz / µs and a frequency modulation bandwidth B = 200MHz, the calculated modulation time for a positive slope is 1.64ms. To maintain symmetry, the modulation time for a negative slope is also chosen to be 1.64ms. The rest period of 3.36ms is for data calculation and processing.

[0120] The process of calculating the target height h is as follows:

[0121] Step 1, Generation of positive and negative slope frequency modulation signals:

[0122] Determine the initial frequency modulation period and frequency modulation bandwidth at the current moving speed of the platform, and determine that the current stability of the altimeter radar is in an intermediate state. Then determine to directly generate an initial symmetrical frequency modulation signal with a frequency modulation period of T=10ms and a frequency modulation bandwidth of B=200MHz, including positive slope segment and negative slope segment.

[0123] The main operating parameters of this signal waveform are as follows:

[0124] The radar operates at a center frequency f0 = 4300MHz; the linear frequency modulated signal has a slope of ±0.12MHz / us; the triangular wave modulation alternates between positive and negative slopes for approximately 5ms each; the linear frequency modulated continuous wave radar modulation period is 1.64ms, and the rest period is 3.36ms; the complete cycle of a triangular wave is approximately 10ms.

[0125] Signal and data processing is completed within a 3.4ms pause period.

[0126] Step 2, Signal Transmission and Echo Acquisition:

[0127] The signal transmission module cycles through "positive slope transmission → rest period → negative slope transmission → rest period" to ensure that positive and negative slope signals are output alternately. The receiving module synchronously collects the echo signal reflected from the ground, where τ=2h / (c×cos(α)) is the round-trip time of the signal, h is the target height, c is the speed of light, and α is the angle of deviation from the vertical direction.

[0128] Step 3, echo signal preprocessing:

[0129] The acquired echo signals are processed as follows:

[0130] Mixing: Using an integrated mixer chip, the two echo signals are mixed with the local reference signal (which is from the same source as the transmitted signal and is allocated from the transmitting VCO circuit) to obtain two intermediate frequency signals, positive deviation and negative deviation, respectively.

[0131] Low-pass filtering: A low-pass filter with a cutoff frequency of B is used to filter out high-frequency noise;

[0132] Analog-to-digital conversion: Converting analog signals into digital signals at a sampling frequency fs≥2B.

[0133] Step 4, Doppler offset differential cancellation:

[0134] The echo signal, after being amplified by the intermediate frequency, is converted into a digital signal by an A / D converter.

[0135] A digital filter is used to filter out long-distance echoes with large frequency differences, ensuring the detection of echo signals within a 1.5km range, with a sampling frequency of fs=500MHz.

[0136] The filtered digital signal is dumped into 8192 data points, and 8192 points of fast Fourier processing are performed to calculate the power spectrum and obtain the echo power spectrum.

[0137] Assuming n is the nth point (0≤n≤8191) corresponding to the echo power spectrum after Fast Fourier Transform processing, the relevant calculation formulas for altimetry processing are as follows:

[0138] (1) Positive slope modulation: The frequency difference after mixing and deslopeing the echo signal and the transmitted signal is positive, and the height measurement process detects the echo with a positive frequency difference between 0 and 4095.

[0139] Frequency difference: Δf1=(n1+f(d))×5 / 8192(MHz), 0≤n1≤4095, f(d) is the Doppler bias;

[0140] Delay: Ts1 = Δf1 / 0.12 (µs);

[0141] Height: H1 = c × Ts1 / 2;

[0142] (2) Negative slope modulation: The frequency difference after mixing and deslopeing the echo signal and the transmitted signal is negative. The altimeter process detects the echo with negative frequency difference between 8191 and 4096.

[0143] Frequency difference: Δf2=(8191-(f(d)-n2))×5 / 8192(MHz), 4096≤n2≤8191, f(d) is the Doppler bias;

[0144] Delay: Ts2 = Δf2 / 0.12 (µs);

[0145] Height: H2 = c × Ts² / 2;

[0146] (3) Height calculation: Calculate the average height value H1 when the slope is positive and the height value H2 when the slope is negative;

[0147] H=(H1+H2) / 2=c×((n1+f(d))×5 / 8192 / 0.12) / 2+c×((8191-f(d)+n2)×5 / 8192 / 0.12) / 2=c×((8912+n1+n2)×5 / 8192 / 0.12) / 2.

[0148] Therefore, the Doppler deviation f(d) in the final calculation formula for the target height h is canceled out, thus eliminating the Doppler frequency offset interference.

[0149] Please refer to Tables 1 to 3, which are the actual test verification results of the prototype. The height measurement error of the algorithm of this invention is always controlled at about 2%, which is less than 3% of the initial target setting error value; the maximum height measurement error does not exceed 2.11%; the proportion of times the height measurement error exceeds 2% is less than 10%, and the Doppler suppression rate is stable at over 90%, indicating that the height measurement accuracy is relatively high.

[0150] Table 1. Prototype Measurement Results

[0151] ;

[0152] Table 2. Prototype Measurement Results (Part 2)

[0153] ;

[0154] Table 3. Prototype Measurement Results (Part 3)

[0155] ;

[0156] Any technologies not mentioned in the above embodiments are applicable to existing technologies.

[0157] It is understood that no specific limitation is made to any preset parameter or critical parameter in the embodiments of the present invention, and the above values ​​are not limited thereto. Those skilled in the art can make corresponding adjustments to the preset parameters or critical parameters according to actual needs, analysis of historical data, or equipment usage.

[0158] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A signal processing method for Doppler frequency shift interference in altimeter radar, characterized in that, include: Based on the mobility of the altimeter radar, the frequency modulation period and frequency modulation bandwidth used to generate the symmetrical frequency modulation signal are dynamically adjusted. Based on the dynamically adjusted frequency modulation period and frequency modulation bandwidth, a symmetrical frequency modulation signal containing a positive slope segment and a negative slope segment is generated, and the frequency modulation signal of the positive slope segment is recorded as the positive slope frequency modulation signal, and the frequency modulation signal of the negative slope segment is recorded as the negative slope frequency modulation signal. The positive slope frequency modulation signal and the negative slope frequency modulation signal are periodically and alternately transmitted; After periodically transmitting the signal, the first echo signal reflected by the positive slope frequency modulation signal and the second echo signal reflected by the negative slope frequency modulation signal are collected. The first echo signal and the second echo signal are preprocessed to obtain the corresponding positive slope echo signal and negative slope echo signal; Spectral analysis and feature extraction were performed on the positive slope echo signal and the negative slope echo signal respectively to obtain the corresponding positive slope peak frequency and negative slope peak frequency; The ranging frequency difference after eliminating the Doppler offset is obtained by performing differential calculation based on the positive slope peak frequency and the negative slope peak frequency; Based on the stability of the ranging frequency difference, the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal is dynamically adjusted, including extending or shortening the alternating transmission period; The altitude is determined by measuring the distance frequency difference reacquired after adjusting the alternating transmission cycle. The process of dynamically adjusting the frequency modulation period and frequency modulation bandwidth to generate a symmetrical frequency-modulated signal includes: The acceleration of the platform equipped with the altimeter radar is obtained in several stages of movement, and the maneuverability value is calculated. The mobility of the altimeter is determined based on the mobility value, and the frequency modulation period and the frequency modulation bandwidth are dynamically and collaboratively adjusted based on the determination result. Based on the comparison result between the mobility value and the mobility threshold, the frequency modulation period is extended and the frequency modulation bandwidth is narrowed, or the frequency modulation period is shortened and the frequency modulation bandwidth is widened. The process of dynamically adjusting the alternating transmission period of the positive slope frequency modulation signal and the negative slope frequency modulation signal includes: Several ranging frequency differences are obtained and variance is calculated to obtain the frequency difference fluctuation value; The stability of the ranging frequency difference is determined based on the frequency difference fluctuation value, and the alternating transmission period is adjusted based on the determination result. The alternating transmission period can be extended or shortened based on the comparison result between the frequency difference fluctuation value and the fluctuation threshold.

2. The signal processing method for Doppler frequency shift interference of altimeter radar according to claim 1, characterized in that, The frequency modulation period is extended and the frequency modulation bandwidth is narrowed based on the comparison result between the first maneuver difference and the preset first maneuver difference. Wherein, the first mobility difference is the difference between the lower limit of the mobility threshold and the mobility value.

3. The signal processing method for Doppler frequency shift interference of altimeter radar according to claim 1, characterized in that, The frequency modulation period is shortened and the frequency modulation bandwidth is widened based on the comparison result between the second maneuver difference and the preset second maneuver difference. The second mobility difference is the difference between the mobility value and the upper limit of the mobility threshold.

4. The signal processing method for Doppler frequency shift interference of altimeter radar according to claim 1, characterized in that, The alternating transmission period is extended based on the comparison result between the first fluctuation difference and the preset first fluctuation difference, and the extension of the alternating transmission period is positively correlated with the first fluctuation difference. Wherein, the first fluctuation difference is the difference between the lower limit of the fluctuation threshold and the frequency difference fluctuation value.

5. The signal processing method for Doppler frequency shift interference of altimeter radar according to claim 1, characterized in that, The alternating transmission cycle is shortened based on the comparison result between the second fluctuation difference and the preset second fluctuation difference, and the shortening of the alternating transmission cycle is positively correlated with the second fluctuation difference. Wherein, the second fluctuation difference is the difference between the frequency difference fluctuation value and the upper limit value of the fluctuation threshold.

6. The signal processing method for Doppler frequency shift interference of altimeter radar according to claim 1, characterized in that, The process of generating a symmetrical frequency-modulated signal containing both positive and negative slope segments includes: The positive slope frequency modulation signal is calculated based on the radar center frequency, the frequency modulation bandwidth, and the first signal period. The negative slope frequency-modulated signal is calculated based on the radar center frequency, the frequency modulation bandwidth, and the second signal period. Wherein, the first signal period is the first half of the frequency modulation period, and the second signal period is the second half of the frequency modulation period.

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