A two-baseline elevation angle estimation method based on ionospheric sounding radar

Abstract

The application provides a double-baseline elevation angle estimation method based on an ionospheric sounding radar, the ionospheric sounding radar comprising an M-unit linear main array and an N-unit linear subarray; the method comprising: adding an antenna at a set position; canceling an antenna at the edge of the main array to form a new main array; the set position being located between the main array and the subarray and outside the field of view of the subarray; calibrating the amplitude and phase errors of the added antenna; and estimating the double-baseline elevation angle. The application has the advantages that: the antenna array pattern proposed in the application does not need to increase the floor area of the radar antenna array, and does not need to greatly improve the performance of other components of the radar. The antenna array layout has strong flexibility. The elevation angle estimation algorithm has strong applicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the cross field of space weather and electronic information engineering, and particularly relates to a double-baseline elevation angle estimation method based on an ionospheric sounding radar. BACKGROUND

[0002] Ionospheric plasma convection is an important representation of solar wind-magnetosphere-ionosphere coupling, and plays an important role in indicating space weather. Therefore, nearly 40 high-frequency coherent scatter radars have been deployed internationally to cover most of the mid-high latitude regions in the northern and southern hemispheres, forming the international Super Dual Auroral Radar Network (SuperDARN), which has been continuously detecting the convection for a long time. The second phase of the Meridian Project has deployed six high-frequency coherent scatter radars independently developed by China in the mid-latitude region of northern China, namely the Chinese Dual Auroral Radar Network (CN-DARN), which fills the detection blank of the SuperDARN radar in the eastern hemisphere at mid-high latitudes. In addition to the coverage of the radar, an accurate convection map also depends on the high-precision elevation angle estimation of the ionospheric backscatter signal. However, there is a significant problem in the existing SuperDARN radar elevation angle measurement, which is the 2π phase ambiguity problem caused by the distance between the main array and the sub-array being greater than the wavelength.

[0003] The existing SuperDARN system radar has a working frequency of 8-20MHz, and its antenna array is mainly composed of a 16-unit linear main array and a 4-unit linear sub-array, that is, a "16+4" antenna layout, wherein the distance between the main array and the sub-array is generally 60-180m. For high-frequency radio waves of 8-20MHz, the longest wavelength is 37.5m, that is, the distance between the two antenna arrays is greater than the maximum wavelength, which will cause the phase difference between the two arrays to be greater than 2π, and the actual measured phase is between -π and π, so the phase ambiguity problem will occur.

[0004] The existing solutions mainly have three aspects: one is to solve by empirical model, which considers that the actual phase difference Ψ t and the maximum phase difference Ψ max only exists a 2π phase folding (Ψ t ∈[Ψ max ,Ψ maxThe method is only applicable to ionospheric radar in high latitude and polar region, and is not applicable to ionospheric sounding radar in middle and low latitude. Second, a new antenna layout is adopted, which includes two interference arrays, one single antenna subarray in front of the main array, and one 3-unit subarray behind the main array, i.e. "1+16+3" layout. The layout needs to increase the area occupied by the radar antenna array by about one time, and needs to increase the power of the radar transmitter by one time to ensure the signal-to-noise ratio of the single antenna subarray and the 3-unit subarray. Third, a double-subarray array layout is also adopted, which adds a single antenna subarray between the main array and the subarray to solve the phase ambiguity problem. The antenna array of the scheme is a 16-unit main array, a single antenna subarray and a 3-unit subarray, i.e. "16+1+3" layout. The scheme solves the problem of area occupation, but due to the small number of atomic arrays, the number is reduced from 4 to 3, which greatly affects the signal-to-noise ratio of the subarray, resulting in low elevation angle estimation accuracy. Therefore, an antenna array with small area occupation, less modification of the radar system design and the ability to solve the 2π phase ambiguity and an elevation angle estimation method are needed. SUMMARY

[0005] The purpose of the present application is to overcome the defects of the existing 2π phase ambiguity solving elevation angle estimation method, such as large antenna array area occupation, high requirement for transmitter power and low elevation angle estimation accuracy.

[0006] In order to achieve the above-mentioned purpose, the present application provides a double-baseline elevation angle estimation method based on an ionospheric sounding radar, wherein the ionospheric sounding radar comprises an M-unit linear main array and an N-unit linear subarray; the method comprises:

[0007] Step 1: adding a new antenna at a set position; canceling an antenna at the edge of the main array to form a new main array; the set position is located between the main array and the subarray, and is outside the field of view of the subarray;

[0008] Step 2: calibrating the amplitude and phase errors of the new antenna;

[0009] Step 3: estimating the double-baseline elevation angle.

[0010] As an improvement of the above-mentioned method, the distance between the set position and the straight line where the main array is located is greater than 2 times the wavelength corresponding to the working frequency of the radar.

[0011] As an improvement of the above-mentioned method, the set position further comprises: the path difference between the first baseline and the second baseline is less than the wavelength corresponding to the working frequency of the radar;

[0012] The first baseline is the distance from the phase center of the new main array to the phase center of the subarray; and the second baseline is the distance from the phase center of the new main array to the new antenna.

[0013] As an improvement of the above-mentioned method, the step 2 comprises:

[0014] Step 2.1: Place the new antenna on the same line with the subarray; make the radar work in pure receiving mode, and the working frequency is consistent with the frequency of the calibration source carried by the UAV;

[0015] Step 2.2: Calibrate the original main array and the subarray by using the echo of the UAV calibration source and the method of linear least square fitting;

[0016] Step 2.3: Calibrate the new main array, the new antenna and the subarray by using the echo of the UAV calibration source and the method of linear least square fitting.

[0017] As an improvement of the above method, the step 2 further comprises:

[0018] Before calibration, calculate the phase difference caused by the height difference of the antennas

[0019]

[0020] wherein k is the wave number; is the measured target phase of the new antenna; and ΔZ is the height difference.

[0021] As an improvement of the above method, the step 3 comprises:

[0022] According to the relationship between the phase differences measured by the first baseline and the second baseline respectively and , the relationship formula of the 2π aliasing factors p and q is obtained, and the elevation angle is estimated according to the following formula:

[0023]

[0024] wherein the parameter ΔY=Y1-Y2, ΔZ=Z1-Z2; α is the elevation angle; X1, Y1 and Z1 respectively represent the coordinate values of the phase center of the new main array on the x-axis, y-axis and z-axis; X2, Y2 and Z2 respectively represent the coordinate values of the new antenna on the x-axis, y-axis and z-axis; ΔZ represents the height difference between the position of the new antenna and the main array antenna; and ΔY represents the offset of the new antenna and the phase center of the new main array on the y-axis; is the scanned azimuth angle; and are the phase differences measured by the first baseline and the second baseline respectively; and are the measured phases of the new main array, the subarray and the new antenna respectively;

[0025] The first baseline is the distance from the phase center of the main array to the phase center of the sub-array; and the second baseline is the distance from the phase center of the main array to the newly added antenna.

[0026] As an improvement of the above method, the relationship between the phase difference measured according to the first baseline and the second baseline respectively and is used to obtain the relationship between the 2π aliasing factors p and q, including:

[0027] The path difference ΔP of the first baseline and the second baseline is calculated as:

[0028]

[0029] Wherein, ,

[0030] The value range of |δP1-δP2| is obtained by an exhaustive method according to the azimuth angle of scanning and the position relationship of the array, and the value range of p and q is obtained according to the size relationship between δP1 and δP2. and The relationship between p and q is obtained according to the size relationship between p and q.

[0031] Compared with the prior art, the application has the following advantages:

[0032] 1. The 2π phase ambiguity problem of the SuperDARN system radar elevation angle estimation is solved in a low-cost and low-cost manner. The antenna array proposed in the application does not need to increase the area occupied by the radar antenna array, and does not need to greatly improve the performance of other components of the radar.

[0033] 2. The antenna array layout has strong flexibility. The antenna array and the elevation angle estimation algorithm can select the installation position according to the actual situation of each radar station site. At the same time, for the newly built SuperDARN system radar in the future, the site selection constraint of the radar site is lower.

[0034] 3. The elevation angle estimation algorithm has strong applicability. The elevation angle estimation algorithm is not only suitable for CN-DARN radar, but also can be widely used in other new type of digital distributed SuperDARN radar. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 Fig. 1 shows a SuperDARN system radar antenna array layout schematic diagram;

[0036] Figure 2 Fig. 2 shows a schematic diagram of the installation position of the newly added antenna;

[0037] ​Figure 3 The figure shows the antenna array layout when the external calibration of the new antenna is added;

[0038] Figure 4 The figure shows the echo received by the original unit antenna and the new antenna at an elevation angle of α0 (when there is a height difference);

[0039] Figure 5 The figure shows the antenna formation after the position of the newly added antenna is determined. DETAILED DESCRIPTION

[0040] The technical solution of this application is described in detail below with reference to the accompanying drawings.

[0041] The dual-baseline elevation angle estimation method based on ionospheric sounding radar provided by the present invention is applicable to the SuperDARN radar system for ionospheric sounding. The radar generally adopts an antenna configuration of 16-unit main array and 4-unit subarray, and both adopt a one-dimensional linear phased array. In this embodiment, the antenna configuration of 16-unit main array and 4-unit subarray is used as an example for explanation. The method of this application is also applicable to main array and subarray antenna configurations with other antenna numbers. The phase difference between the signals received by the two arrays consists of two parts: one is the geometric phase difference Ψ caused by the spatial distribution of the arrays GEO The second is the phase difference Ψ caused by the different electrical lengths of the signals received by the two arrays to the data processing point. ELE The geometric phase difference is determined by the positional relationship between the main array and the sub-array; the phase difference caused by the electrical length of the RF cable is determined by the time delay (t diff ) is determined. To accurately measure the elevation angle α, the above two phase differences must be accurately obtained. In this scheme, it is assumed that t diff is known and is 0.

[0042] The antenna array layout of the SuperDARN radar is as follows: Figure 1 Figure 1 shows a general model of an interferometer array. The origin of the coordinates is located at the center of the main array. The boresight direction (maximum gain direction) is the y-axis. The x-axis is perpendicular to and coplanar with the y-axis along the main array. The z-axis is perpendicular to the xy-axis plane. For the purpose of subsequent formula derivation, it is assumed that all antennas in the array are at the same vertical height and that the two arrays are parallel, that is, they have a common boresight direction.

[0043] Figure 1 middle is the wave vector of the incident plane wave, is the distance vector between the two parallel arrays, α is the elevation angle, is the azimuth angle, (X, Y, Z) is the center coordinate of the interference array. When the elevation angle α=0, the corresponding azimuth angle is (Azimuth in the XoY plane). The relationship between azimuth and elevation in the cone model is: Then

[0044]

[0045] In Figure 1 the model, the path difference between the two arrays can be expressed as

[0046]

[0047] Where X, Y, Z are the geometric offsets of the sub-array relative to the main array, is the azimuth angle, and is the elevation angle, which are known quantities; α is the elevation angle, which is the unknown to be solved.

[0048] The phase difference between the two arrays due to the different lengths of the transmission cable is:

[0049] Ψ ELE = -2πf TX ·t diff (3)

[0050] Where f TX is the transmission frequency of the radar, t diff is the time delay difference of the signal from the two arrays through the cable to the transceiver component, and the total phase difference between the two arrays is:

[0051] Ψ TOT = Ψ GEO + Ψ ELE = Ψ OBS + 2nπ (4)

[0052] Where Ψ OBS is the measured phase, ranging from [-π, π]; n is the 2π aliasing factor, and the value of n needs to be solved to obtain the elevation angle, and n is an integer.

[0053] Based on the general model of the radar antenna array, the elevation angle estimation scheme of the double baseline interference is as follows: a single antenna is added between the main array and the sub-array, and through double baseline interference, the 2π ambiguity problem in radar echo elevation angle estimation is eliminated. The double baseline elevation angle estimation method based on ionospheric detection radar includes three parts: first, the selection of the position of the new antenna; second, the calibration method of the amplitude and phase error of the new antenna sub-array; third, the algorithm of the elevation angle measurement under the new antenna array layout.

[0054] 1. Position selection scheme of the new antenna

[0055] Two factors need to be considered in the selection of the new antenna position of CN-DARN radar: one is to be far away from the main array to avoid being blocked by the main array, and the distance selected is more than 75m (2 times the wavelength corresponding to the working frequency of the radar) from the straight line where the main array is located; the other is to be outside the field of view of the subarray to avoid mutual coupling and blocking problems. The field of view of CN-DARN radar is -39°-39°, that is, more than 39° away from the phase center of the antennas on both sides of the subarray, and the selectable positions are shown in the shadow part in Figure 2 The principle in the process of selecting the position of the new antenna is: during the scanning of the field of view of the radar, for all scanning angles, the path difference corresponding to the two baselines d1 and d2 is as small as possible, and in this case, the solution of the 2π ambiguity in the elevation angle estimation is the simplest. In the simulation of position selection, it can be assumed that there is no offset between the height of the new antenna and the height of the original antenna. The baseline d1 is the distance from the phase center of the new main array to the phase center of the subarray; the baseline d2 is the distance from the phase center of the new main array to the new antenna.

[0056] 2. Calibration scheme of amplitude and phase error of the new antenna

[0057] Before implementing the double-baseline elevation angle estimation, the difference between the new antenna and the cable and the original antenna cable needs to be calibrated. The specific method is as follows:

[0058] First, place the new antenna and the subarray on a straight line, and according to the actual situation of the site, place the new antenna X m to the right of the subarray, as shown in Figure 3 At this time, the new single antenna and the original four-unit subarray form a new one-dimensional linear subarray, as shown by the dashed oval in Figure 3 Fix the new single-unit antenna, and use the unmanned aerial vehicle to carry the calibration source to calibrate the amplitude and phase error between the new antenna and the original single-unit antenna.

[0059] The specific scheme for calibrating the new antenna is as follows:

[0060] 1) The CN-DARN radar works in pure receiving mode, and the working frequency is consistent with the frequency of the signal emitted by the unmanned aerial vehicle carrying the calibration source;

[0061] 2) In order to improve the accuracy of calibration, first, use the echo received from the unmanned aerial vehicle calibration source and the linear least squares fitting method to calibrate the original 20 antennas.

[0062] 3) Then, use the echo of the unmanned aerial vehicle calibration source and the linear least squares fitting method to calibrate the main array and the new 5-unit subarray. If there is a height difference between the new antenna and the original antenna, the phase error caused by the height difference needs to be considered, as shown in Figure 4 ​As shown, ΔZ is the height difference, and a0 is the elevation angle of the UAV relative to the antenna array. The position (distance, height and azimuth) of the UAV is known. If there is no height difference, the phase compensation step can be omitted.

[0063] Before calibration, the phase difference caused by the antenna height difference (if any) needs to be calculated, which can be expressed as:

[0064]

[0065] where k is the wave number. Assuming that the target phase measured by the new antenna is After compensating for the phase difference caused by the height difference, it becomes:

[0066]

[0067] where is the phase of the new antenna used for calibration.

[0068] 3. Dual baseline elevation angle estimation algorithm

[0069] After calibration, the new antenna is fixed to the optimal position (X2, Y2) selected in the first step. If the height of the new antenna is different from that of the original antenna, the coordinates are (X2, Y2, Z2), as shown. The new antenna is connected to the indoor electronic equipment connected to the original No. 1 antenna. This change has minimal impact on the system because the number of main antenna array elements is large, and the impact of removing one element is small. The power of a single transmitter can be increased by a small amount to achieve the same power as the original 16-element antenna. This results in the original 16-element main array becoming a 15-element array, and the phase center will shift to (X1, Y1, Z1). At this time, the original 15-element main array and the 4-element sub-array and the new antenna sub-array form two baselines d1 and d2, and the multi-baseline elevation angle measurement algorithm will be based on the antenna array layout in Figure 5 . Figure 5 After the CN-DARN radar adds a new antenna, according to the mathematical model, the phase differences corresponding to the two baselines d1 and d2 are:

[0070]

[0071] where α is the elevation angle, is the azimuth angle of scanning,

[0072] and are the phase differences measured by the two baselines, both of which are in the range [-π, π). Among them are the measured phases of the main array, the sub-array and the new antenna. λ is the wavelength, and m and n are integers.

[0073] ​​

[0074] wherein, AY=Y1-Y2, AZ=Z1-Z2.

[0075] After the position of the new antenna is determined, according to formula (7), under different azimuth conditions, Ψ1-Ψ2 belongs to different ranges. According to the relationship between the phase difference and measured by the two baselines respectively, the relationship of m and n is obtained, and then the elevation angle can be solved according to formula (8). After formula (7) is divided by the wave number, the path difference of the two baselines is

[0076]

[0077] wherein

[0078]

[0079] (1) When δP1> δP2, and |δP1- δP2| < λ,

[0080] If m = n can be obtained; m = n + 1 can be obtained.

[0081] (2) When δP1< δP2, and |δP1- δP2| < λ,

[0082] If m = n - 1 can be obtained; m = n can be obtained.

[0083] (3) When δP1> δP2, and λ < |δP1- δP2| < 2λ,

[0084] If m = n + 1 can be obtained; m = n + 2 can be obtained.

[0085] (4) When δP1< δP2, and λ < |δP1- δP2| < 2λ,

[0086] If m = n - 2 can be obtained; m = n - 1 can be obtained.

[0087] When |δP1- δP2| is in a larger range, the relationship of m and n can be obtained by analogy. The value range of |δP1- δP2| can be determined according to the scanned azimuth The positional relationship of the array X1, Y1, Z1, X2, Y2, Z2 is obtained by an exhaustive method.

[0088] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the examples, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application do not deviate from the spirit and scope of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A dual-baseline elevation angle estimation method based on an ionospheric sounding radar, wherein the ionospheric sounding radar includes an M-unit linear main array and an N-unit linear subarray; The method includes: Step 1: Add an antenna at a set position; remove an antenna at the edge of the main array to form a new main array; the set position is between the main array and the sub-array and outside the field of view of the sub-array; Step 2: Calibrate the amplitude and phase errors of the newly added antenna; Step 3: Estimate the dual baseline elevation angle; The setting position also includes: the path difference between the first baseline and the second baseline is less than the wavelength corresponding to the radar operating frequency; The first baseline is the distance from the phase center of the new main array to the phase center of the sub-array; the second baseline is the distance from the phase center of the new main array to the newly added antenna; The step 2 includes: Step 2.1: Place the new antenna in line with the subarray. Set the radar to operate in pure receive mode, with the operating frequency consistent with the frequency of the calibration source transmitted by the drone. Step 2.2: Use the echo received from the UAV calibration source and the linear least squares fitting method to calibrate the original main array and sub-array; Step 2.3: Use the echo from the UAV calibration source and the linear least squares fitting method to calibrate the new main array, additional antennas, and subarrays; The step 3 comprises: The phase difference measured according to the first baseline and the second baseline and The relationship between the 2π aliasing factors p and q is obtained, and the elevation angle is estimated according to the following formula: Among them, the parameters ΔY=Y1-Y2, ΔZ=Z1-Z2; α is the elevation angle; X1, Y1, and Z1 represent the coordinates of the phase center of the new main array on the x-axis, y-axis, and z-axis, respectively; X2, Y2, and Z2 represent the coordinates of the added antenna on the x-axis, y-axis, and z-axis, respectively; ΔZ represents the height difference between the added antenna and the main array antenna; ΔY represents the offset between the phase center of the added antenna and the new main array on the y-axis; is the scanning azimuth; and Phase differences measured for the first and second baselines, respectively; and These are the measurement phases of the new main array, sub-array, and newly added antennas; The first baseline is the distance from the phase center of the new main array to the phase center of the sub-array; the second baseline is the distance from the phase center of the main array to the newly added antenna; The phase difference measured according to the first baseline and the second baseline and The relationship between the 2π aliasing factors p and q is obtained, including: The path difference ΔP between the first baseline and the second baseline is calculated as: in, , The value range of |δP1-δP2| depends on the scanning azimuth angle The position relationship of the array is obtained by exhaustive enumeration, and then the relationship between δP1 and δP2 is obtained. The range of values, finally according to and The relationship between p and q can be obtained by looking at the relationship between them.

2. The dual-baseline elevation angle estimation method based on ionospheric sounding radar according to claim 1, characterized in that: The distance between the set position and the straight line where the main array is located is greater than twice the wavelength corresponding to the radar operating frequency.

3. The dual-baseline elevation angle estimation method based on ionospheric sounding radar according to claim 1, characterized in that: The step 2 further comprises: Before calibration, calculate the phase difference caused by the antenna height difference Where k is the wave number; α0 is the elevation angle of the UAV relative to the antenna array; ΔZ is the height difference.

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