Frequency control array-based terrain matching method
阅读说明：本技术 一种基于频控阵的地形匹配方法 (Frequency control array-based terrain matching method ) 是由 李典 汪飞 周建江 于 2019-10-30 设计创作，主要内容包括：本发明公开了一种基于频控阵的地形匹配方法。频控阵雷达方向图呈“S”形,雷达各个阵元的载频不同,使得雷达方向图具有更大的自由度,并且具有普通雷达方向图不具有的距离依赖性和周期性,增大了空间分辨率。本发明的目的是针对飞行器地形匹配系统的需求,结合频控阵雷达“S”形方向图所具有的特性,提出了一种基于频控阵雷达的地形匹配方法,采用频控阵作为发射阵列,增大空间分辨力,处理目标回波时,采用非聚焦的多普勒锐化方式进行成像处理,能够缩短成像时间。该方法应用于地形跟踪/地形回避时能够得到理想的效果,具有较高的匹配精度和实时性且能够实现对待匹配场景的持续观测。(The invention discloses a terrain matching method based on a frequency control array. The frequency control array radar directional diagram is S-shaped, carrier frequencies of all array elements of the radar are different, so that the radar directional diagram has higher degree of freedom, distance dependence and periodicity which are not possessed by a common radar directional diagram are possessed, and spatial resolution is increased. The invention aims to provide a terrain matching method based on a frequency control array radar aiming at the requirements of an aircraft terrain matching system and combining the characteristics of an S-shaped directional diagram of the frequency control array radar. The method can obtain ideal effect when being applied to terrain tracking/terrain avoidance, has higher matching precision and real-time performance, and can realize continuous observation of the scene to be matched.)
1. A terrain matching method based on frequency control array. The method is characterized in that: a frequency control array is adopted as a transmitting array of a terrain matching system to replace a traditional radar array, and an observation scene distance model is constructed; then, reasonably designing the parameters of the transmitting array; then, low side lobe processing is carried out on the transmitted signals, the energy of the main beam is increased, and finally Doppler sharpening processing and fast matching imaging are carried out on the echo signals. The frequency control array radiation beam pattern has an angle-distance dependence characteristic, can overcome the defects that the traditional array cannot inhibit interference in a distance dimension and has low resolution, and can effectively improve the radar target detection performance.
2. The frequency-controlled array-based terrain matching method according to claim 1, characterized in that an observation scene distance model is constructed. The distance model is a key basis of radar target modeling and is related to the target detection and imaging effects. This variation of the distance with azimuth time results in phase modulation between different pulses, a requirement for high resolution. Setting the center time t of the point target beam to be 0 and the slant distance to be RsDistance, distanceThe model can be expressed as:
wherein, thetaeqIs an equivalent squint angle, v0Equivalent radar speed.
3. The method for matching the terrain based on the frequency control array as claimed in claim 1, wherein parameters of the frequency control array radar array and parameters of array element transmission signals are set according to a terrain matching application scene and requirements of a slant range model. Because the spatial accuracy of the frequency control array radar imaging is affected by the number of array elements, the spacing of the array elements and the frequency difference between the carrier frequencies of the array elements, a proper number M of array elements, an appropriate interval d of the array elements and a carrier frequency increment Δ f between the array elements need to be designed. Considering the influence of the parameters on the echo processing, the pulse repetition period f needs to be reasonably designedrAnd coherent accumulation time Ts. The frequency control array is specific to the transmitting array, so the frequency control array is selected as the transmitting array, and the phased array is selected as the receiving array to establish a transmitting-receiving framework.
4. The method for matching a frequency controlled array based terrain according to claim 3, wherein the waveform of the signal transmitted by each array element of the frequency controlled array radar is modeled according to the parameters. Because the carrier frequencies of the array elements of the frequency control array are different, the used frequency control array radar has M array elements, and the carrier frequency of the radiation signal of the first array element is f0The first array element radiated signal can be expressed as:
S1(t)＝a0(t)exp(j2πf0t)0＜t＜Tr
the frequency of the radiation signal of the m-th array element is as follows:
fm＝f0+m·Δf m＝1，2，...，M
the radiation signal of the m-th array element can be expressed as:
Sm(t)＝a0(t)exp(j2πfmt)＝a0(t)exp(j2π(f0+m·Δf)t) 0＜t＜Tr
wherein, a0(T) is a baseband signal, TrFor the pulse duration, Δ f is the frequency increment.
5. The method as claimed in claim 4, wherein the echo signal is Doppler sharpened (DBS) for satisfying the requirement of real-time imaging processing. And a proper sharpening ratio is set, non-focusing processing is carried out on the observation area, matching imaging of a large-range scene can be completed in a short time, the time required by imaging is shortened, and an imaging result is obtained quickly.
Technical Field
The invention belongs to the technical field of frequency control array radar imaging, and particularly relates to a terrain matching imaging method based on a frequency control array radar.
Background
The phased array radar changes the beam direction by changing the phase of a transmitting signal, and can freely realize the control of the beam direction to carry out space scanning by utilizing the beam forming mode, thus being widely applied to the field of radar target detection and imaging. Theoretically, when the direction of the beam is determined, the phased array radar can detect a target at any distance in that direction if the radar transmitter power can be large enough. Phased array radars transmit the same signal with an antenna array, thereby forming an array gain in space, which may increase the probability of detection of a target. However, the beam pointing direction is constant on all distance resolution units, the antenna pattern is only angle-dependent and distance-independent, and therefore the distance dimension resolution is poor, and in some cases, it is desirable that the antenna array beam can be pointed at different angles and at different distances, and therefore, the pointing direction of the antenna beam is required to be changed along with the change of the distance.
The concept of frequency controlled array (FDA) was first proposed and patented by Antonik and Wicks et al in the IEEE radar society in 2006. The frequency control array radar is different from the phased array radar in that different frequency offsets are applied to different array elements at the same time by the frequency control array radar, and small frequency offsets among the array elements can generate antenna directional patterns which are dependent on distance and angle and regularly change along with time. Therefore, compared with the traditional phased array antenna, the frequency control array has more system degrees of freedom, the array beam scanning is more flexible, and the distinguishing capability of distance dimensional signals is also realized. Due to the obvious advantages, the frequency control array radar is widely researched by domestic and foreign scholars once being put forward, so that the frequency control array radar quickly draws the wide attention of the domestic and foreign scholars.
The document "non Frequency direct Array for Range-Angle Imaging of targets" proposes a method of using a Frequency-controlled Array based on non-uniform Array element spacing as a transmitting Array and using a phased Array with the same number of Array elements as a receiving Array. According to the method, due to the array elements with non-uniform intervals, the radiation transmitting wave beams are not in an S shape any more, the transmitting wave beams are only related to distance and are not related to angle, the receiving wave beams are only related to angle and are not related to distance, and distance and azimuth two-dimensional imaging of a target can be realized, but the time variability of a frequency control array directional diagram is not considered. The document Forward-looking SAR Imaging With Frequency Array Antennas adopts different Array geometries and the number of antenna transmission carriers to form a point-like transmission beam. Conventional array antennas emit a single frequency signal and the array emits a multi-frequency signal that forms energy peaks at desired target points and nulls at non-target points. This approach concentrates the beam energy at the desired preset direction and distance, but does not take advantage of the original "S" shaped beam and does not take into account the time-shifting of the beam.
Disclosure of Invention
The invention aims to provide a high-spatial-precision terrain matching imaging method aiming at the requirements of a terrain matching system in the working process of an aircraft and combining the characteristics of a frequency control array radar directional diagram.
The invention adopts the following technical scheme in order to realize the aim, and the steps are as follows:
(1) firstly, a terrain matching imaging distance model is selected, in the traditional low-orbit synthetic aperture imaging process, the slant distance from a sensor to a target is the most important parameter, and the change of the distance along with the azimuth time can cause the phase modulation between different pulses, which is the necessary condition for obtaining high resolution. The traditional strabismus distance model has clear physical significance and definite brief introduction, and comprises two important parameters: radar speed and squint angle. Setting the center time t of the point target beam to be 0 and the slant distance to be RsThen the distance model can be expressed as:
in the formula, thetaeqIs an equivalent squint angle, v0Equivalent radar speed.
(2) And setting parameters of the frequency control array radar array element and each parameter of the transmitting signal according to the terrain matching application scene and the requirements of the slant range model. Because the spatial accuracy of the frequency control array radar imaging is related to the number of array elements, the spacing of the array elements and the frequency difference between the carrier frequencies of the array elements, an appropriate number M of array elements, an appropriate interval d of the array elements and an increment Δ f of the carrier frequency between the array elements are set. In addition, in consideration of echo processing, a suitable pulse repetition period f is selectedrAnd coherent accumulation time Ts。
The field intensity of the frequency control array radiation beam pattern at the target is as follows:
to satisfy the coherence between the transmitted signals
the main lobe width R of the antenna with 3dB and the main lobe width of 2c/N delta f on the slant distance can be calculated3dBShould be about c/N Δ f. Assuming that the range of observation requirements over the slant range is Δ R, then the coverage requirement is met, R3dBNeed to satisfy R3dBNot less than Δ R, then
In the coherent processing, the pulse repetition frequency f is set so that aliasing does not occur in the sampled spectrumrShould be greater than or equal to the Doppler bandwidth corresponding to the antenna main beam 3dB width, and need to satisfy according to the Nyquist sampling law, fr≥1.2Δfd. To eliminate the effect of beam time-variation of frequency diversity arrays, f is satisfiedrDelta f is less than or equal to delta f. In order to avoid the appearance of azimuth blur and distance blur during imaging, the requirement of satisfyingfr＜c/2r0. Wherein, Δ fdIs the main lobe doppler bandwidth. Coherent accumulation time if the object can realize coherent accumulation and no over-distance unit walkTsNeed to satisfy
The frequency control array is specific to the transmitting array, so the frequency control array is selected as the transmitting array, and the phased array is selected as the receiving array to establish a transmitting-receiving framework.
(3) And modeling the waveform of the signal transmitted by each array element of the frequency control array radar according to the parameters. The traditional phased array radar transmits signals as follows:
Sm(t)＝a0(t)exp(j2πft) 0＜t＜Tr
as the carrier frequencies of the array elements of the frequency control array are different, the used frequency control array radar has M array elements, and the carrier frequency of the radiation signal of the first array element is f0The first array element radiated signal can be expressed as:
S1(t)＝a0(t)exp(j2πf0t) 0＜t＜Tr
the frequency of the radiation signal of the m-th array element is:
fm＝f0+m·Δf m＝1，2，...，M
the radiation signal of the m-th array element can be expressed as:
Sm(t)＝a0(t)exp(j2πfmt)＝a0(t)exp(j2π(f0+m·Δf)t) 0＜t＜Tr
wherein, a0(T) is a baseband signal, TrFor the pulse duration, Δ f is the frequency increment.
(4) The low side lobe processing is carried out on the frequency control array transmitting signals which are modeled in a windowing function mode, so that the energy of the main lobe can be effectively increased, the energy of the low side lobe can be reduced, and the energy difference between the main lobe and the side lobe can be improved. The directional diagram of the frequency control array radar antenna has distance periodicity and time periodicity, the beam energy of a target area is increased, and the signal-to-interference-and-noise ratio of a received signal can be improved.
(5) Finally, in order to meet the requirements of real-time imaging processing, non-focused Doppler Sharpening (DBS) processing is performed on the obtained echo signals. And a proper sharpening ratio is set, non-focusing processing is carried out on the observation area, matching imaging of a large-range scene can be completed in a short time, the time required by imaging is shortened, and an imaging result is obtained quickly. And the imaging result is obtained quickly.
Compared with the prior art, the invention has the following beneficial effects:
the traditional phased array radar transmits the wave beam only related to the angle and not related to the distance, if the angle is fixed, only the target in the fixed direction can be detected, and the distance dimension resolution is low. The invention uses the frequency control array for terrain matching imaging, adopts the frequency diversity array as a transmitting array, adopts the traditional phased array as a receiving array, establishes the frequency control array-phased array transmitting and receiving array, utilizes the distance dependence characteristic provided by the unique S-shaped beam pattern of the frequency control array antenna beam to improve the distance and direction two-dimensional imaging precision, and adopts Doppler sharpening processing to the echo during the echo processing. Compared with the traditional focusing synthetic aperture imaging, the method has the advantages that the spatial precision is improved, and meanwhile, the imaging time is reduced. The method is applied to terrain matching imaging of the aircraft, and can improve imaging precision and real-time performance and the survival capability of the aircraft.
Drawings
Fig. 1 is a flow chart of a frequency control array-based terrain matching method according to the present invention.
Fig. 2(a) shows a frequency-controlled array radiation beam pattern, and fig. 2(b) shows a radiation beam pattern after low side lobe processing.
Fig. 3(a) shows a single target point matching imaging result, and fig. 3(b) shows a multi-target point matching imaging result.
Detailed Description
The invention is further explained below with reference to the drawings.
Firstly, constructing a distance model according to the requirements of terrain matching application scenes; secondly, setting parameters of array elements and parameters of transmitting signals of the frequency control array radar according to requirements of a terrain matching application scene and a slope model; then modeling each array element signal of the radar according to the set parameters; and then windowing low side lobe processing is carried out on the modeled array element transmitting signals so as to increase the signal to interference noise ratio of echoes, and the final array element transmitting signals are established. And finally, performing matched imaging on the target in the observation area in a non-focusing mode. Finally, the feasibility of using the frequency control array for terrain matching imaging is proved, and a better effect is achieved.
Fig. 1 is a flow chart of a frequency control array-based terrain matching method according to the present invention; the method comprises the following steps:
(1) selecting a matching scene distance model, selecting a terrain matching imaging distance model at first, wherein the traditional squint distance model is originated from squint airborne SAR set motion, the model has the advantages of clear physical significance, simple and clear analytic frequency spectrum, convenient algorithm realization and the like, and is widely used for airborne SAR imaging algorithms, and the squint distance model comprises two important parameters: radar speed and squint angle, the variation of sensor-to-target slant distance with azimuth time can result in phase modulation between different pulses, which is a necessary condition for obtaining high resolution. Setting the center time t of the point target beam to be 0 and the slant distance to be RsThen the distance model can be expressed as:
where θ is the equivalent squint angle, v0Equivalent radar speed.
The orthogonal demodulation is completed at the receiving end after the high-frequency transmitting signal at any azimuth moment t is delayed by a double-distance, and a broadband frequency modulation signal is formed in the azimuth direction, wherein the homeotropic phase of the signal is as follows:
the doppler history, i.e., instantaneous azimuth frequency, is:
the constant term of the Doppler history is defined as the Doppler center frequency fdThe linear coefficient being defined as the Doppler modulation frequency frUsing fd、krTo model parameterNumber v0θ inverse solution:
from the above equation, it can be seen that by performing power series expansion on the traditional squint distance model, the result can be accurately fitted to the linear term of the doppler course, i.e. the second-order component of the actual distance course, and the expansion terms above the second order depend on the hyperbolic root equation, so that the corresponding component of the distance course cannot be accurately fitted, and the fitting error is mainly expressed as the third-order fitting error. Because the synthetic aperture time for low-orbit imaging is relatively short in a suitable scene, the fitting error of the third-order component can be ignored. The distance model can still ensure the simplicity and the simplicity of the signal model, and the requirement of the distance model is met.
(2) The imaging space precision of the frequency control array comprises the distance resolution rhorAzimuthal resolution ρaRequire ρr≤2m，ρaLess than or equal to 2 m. They can be expressed as the following expressions, respectively:
ρr＝c/2(|KrTr|+MΔf)
ρa＝λ0/2(N-1)d
where c is the speed of light, M is the number of transmitting array elements, N is the number of receiving array elements, and lambda0The wavelength of the transmitted signal of the first array element of the array. If the radar emission signal is a narrow-band signal, the range resolution can be written as:
ρr＝c/2MΔf
the field intensity of the frequency control array radiation beam pattern at the target is as follows:
the frequency-steered array radiation beam pattern is shown in fig. 2 (a). Because the directional pattern has periodicity, the gain is maximized when the beam is formed at a certain point in space, that is, the above formula is maximized.
From the limit theorem, it can be seen that obtaining the maximum value according to the above formula needs to satisfy:
k is an arbitrary integer
Thus, it is possible to obtain:
Rswith a periodicity of c/Δ f, R for preventing blurring of the angular dimensionsNeed to satisfy
Namely:
thus, d ≦ c/2f0λ 0/2. The array element spacing d is typically taken to be half the wavelength of the first array element.
The most obvious difference between the frequency control array and the phased array lies in that the frequency increment delta f of the antenna array elements are different, and the range of the slant range of the emission main lobe is as follows according to the characteristics of a radiation antenna directional diagram and the periodic law of the slant range:
the main lobe width at 3dB of the antenna is about
Where R2 is the farthest distance from the radar array in the observation field and R1 is the closest distance from the radar array in the observation field.
To satisfy the coherence between the transmitted signals
According to the conditions required by the parameters, the frequency control array radar array in the embodiment works in an oblique band imaging mode with an oblique angle theta of 30 degrees and a pitch angle of
(3) The parameter calculated in (2) can obtain the transmission signal of the mth array element of the frequency control array transmission array, which can be expressed as:
Sm(t)＝rect(t/Tr-n/Trfr)exp(j2π(f0+(m-1)Δf))exp(jπkr(t-n/Trfr)2)0＜m＜M，n＝1，2，3…
wherein j is an imaginary unit, and the modulation frequency kr＝B/TrN is the number of the transmitted pulses, exp (-) is an exponential function with a base e.
(4) Suppressing strong interference signals in space and thus improving the signal-to-interference-and-noise ratio of received signals is an advantage embodied by low side lobe processing in a phased array, and in a frequency control array, windowing low side lobe processing of transmitted signals by using the phased array is still feasible. The traditional frequency control array radar adopts a guide vector of a target point as a weighting vector, namely:
the weight vector for the low side lobe can be expressed as:
w′＝w⊙wr
wherein w' is a low side lobe weight vector, wrWhen processing for windowingThe corresponding weight vector is the sharpening weight.
The directional diagram after hamming window weighting for the frequency controlled array can be expressed as:
in the formula, wmThe low side lobe weight value of the mth array element. The array element transmitting beam pattern after weighted low side lobe processing is shown in fig. 2(b), and it can be seen that the energy is concentrated in the main lobe area, and the side lobe energy is reduced to the minimum.
(5) Finally, Doppler sharpening processing (unfocused FFT) is carried out on the echo, the sharpening ratio is taken as 64, matched imaging is carried out on the observation region, and the final single-target and multi-target matched imaging results are respectively shown in the step (a) of FIG. 3 and the step (b) of FIG. 3. As can be seen, the final imaging result is clear and the effect is good. The effectiveness and the practicability of the invention are verified.