Double-baseline elevation angle estimation method based on ionospheric detection radar

Abstract

The invention provides a double-baseline elevation angle estimation method based on an ionized layer detection radar. The ionized layer detection radar comprises M units of linear main arrays and N units of linear sub-arrays. The method comprises the following steps: 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 is located between the main array and the sub-array and outside the view field of the sub-array; calibrating an amplitude-phase error of the newly-added antenna; and estimating a double-baseline elevation angle. The radar antenna array has the advantages that the occupied area of the radar antenna array does not need to be increased, and the performance of other components of the radar does not need to be greatly improved. And the antenna array layout has high flexibility. The elevation angle estimation algorithm has high applicability.

Description

Technical Field

[0001] This application belongs to the cross - field of space climatology and electronic information engineering, and specifically relates to a dual - baseline elevation angle estimation method based on an ionospheric sounding radar. Background Technique

[0002] Ionospheric plasma convection is an important manifestation of the solar wind - magnetosphere - ionosphere coupling and plays an important role in indicating space weather. Therefore, nearly 40 high - frequency coherent scatter radars have been specially deployed internationally, covering most mid - high - latitude regions of the Northern and Southern Hemispheres, forming the International Super Dual Auroral Radar Network (SuperDARN), which continuously detects this convection for a long time. The second phase of the Meridian Project has deployed 6 self - developed high - frequency coherent scatter radars in the mid - latitude region of northern China - the Chinese Dual Auroral Radar Network (CN - DARN), filling the detection gap of the SuperDARN radar in the mid - high - latitude region of the Eastern Hemisphere. An accurate convection map depends not only on the radar coverage but also 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 - type radar operates at a frequency of 8 - 20 MHz, and its antenna array is mainly composed of a 16 - element linear main array and a 4 - element linear sub - array, that is, a "16 + 4" antenna layout. The distance between the main array and the sub - array is generally 60 - 180 m. For high - frequency radio waves of 8 - 20 MHz, the longest wavelength is 37.5 m. 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, while the actually measured phase is between - π and π. Therefore, a phase ambiguity problem will occur.

[0004] The existing solutions mainly include three: One is to solve it using an empirical model. This model believes that the actual phase difference Ψ t between the main array and the sub - array and the maximum phase difference Ψ max only have a 2π phase folding (Ψ t ∈[Ψ max ,Ψ max(±2π]), this method is only applicable to ionospheric radars in high latitudes and polar regions and is not applicable to ionospheric sounding radars in mid - low latitudes. Second, a new antenna layout is adopted. This layout includes two interference arrays, a single - antenna sub - array in front of the main array and a 3 - element sub - array behind the main array, that is, the "1 + 16 + 3" layout. This layout requires doubling the footprint of the radar antenna array and doubling the power of the radar transmitter to ensure the signal - to - noise ratio of the single - antenna sub - array and the 3 - element sub - array. Third, a twin - array layout is also adopted, which is to add a single - antenna sub - array between the main array and the sub - array to solve the phase ambiguity problem. The antenna configuration of this scheme is a 16 - element main array, a single - antenna sub - array and a 3 - element sub - array, that is, the "16 + 1 + 3" layout. This scheme solves the problem of footprint, but due to the small number of original sub - arrays, the number is reduced from 4 to 3, which has a greater impact on the signal - to - noise ratio of the sub - array and results in a low elevation angle estimation accuracy. Therefore, an antenna configuration with a small footprint of the antenna array, few design changes to the radar system and the ability to solve the 2π phase ambiguity and an elevation angle estimation method are needed. Summary of the Invention

[0005] The purpose of this application is to overcome the defects of existing elevation angle estimation methods for solving 2π phase ambiguity, such as large footprint of the antenna configuration, high requirement for the transmitter power, and low elevation angle estimation accuracy.

[0006] To achieve the above purpose, this application proposes a dual - baseline elevation angle estimation method based on an ionospheric sounding radar. The ionospheric sounding radar includes an M - element linear main array and an N - element linear sub - array. The method includes:

[0007] 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.

[0008] Step 2: Calibrate the amplitude - phase error of the added antenna.

[0009] Step 3: Estimate the dual - baseline elevation angle.

[0010] As an improvement of the above method, the distance from the set position to the line where the main array is located is greater than 2 times the wavelength corresponding to the radar operating frequency.

[0011] As an improvement of the above method, the set 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.

[0012] 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 added antenna.

[0013] As an improvement of the above method, Step 2 includes:

[0014] Step 2.1: Place the newly added antenna on the same straight line as the sub-array; operate the radar in a pure reception mode with the operating frequency being the same as the frequency of the signal transmitted by the calibration source carried by the UAV.

[0015] Step 2.2: Calibrate the original main array and sub-array by using the echo received from the UAV calibration source and the method of linear least squares fitting.

[0016] Step 2.3: Calibrate the new main array, the newly added antenna and the sub-array by using the echo of the UAV calibration source and the method of linear least squares fitting.

[0017] As an improvement of the above method, Step 2 further includes:

[0018] Before calibration, first calculate the phase difference caused by the antenna height difference

[0019]

[0020] where k is the wave number; is the target phase measured by the newly added antenna; ΔZ is the height difference.

[0021] As an improvement of the above method, Step 3 includes:

[0022] According to the relationship between the phase differences and measured by the first baseline and the second baseline respectively, obtain the relational expressions of the 2π aliasing factors p and q, and then estimate the elevation angle according to the following formula:

[0023]

[0024] where 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 newly added antenna on the x-axis, y-axis and z-axis; ΔZ represents the height difference between the positions of the newly added antenna and the main array antenna; ΔY represents the offset of the newly added antenna from the phase center of the new main array on the y-axis; is the scanning 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, sub-array and newly added antenna respectively;

[0025] 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.

[0026] As an improvement of the above method, the phase differences measured according to the first baseline and the second baseline and to obtain the relational expressions of the 2π aliasing factors p and q, including:

[0027] Calculate the path difference ΔP between the first baseline and the second baseline as:

[0028]

[0029] where

[0030] The value range of |δP1 - δP2| is obtained by an exhaustive method according to the scanning azimuth angle and the positional relationship of the array, and then the value range of is obtained according to the magnitude relationship between δP1 and δP2. Finally, according to and the magnitude relationship, the relational expressions of p and q are obtained.

[0031] Compared with the prior art, the advantages of this application are:

[0032] 1. Solve the 2π phase ambiguity problem of elevation angle estimation of SuperDARN radar system in a low-cost and low-cost way. The antenna array proposed in this application does not need to increase the floor area of the radar antenna array, nor does it need to greatly improve the performance of other components of the radar.

[0033] 2. The layout of the antenna array has strong flexibility. For this antenna array and elevation angle estimation algorithm, the installation position of the newly added antenna can be selected according to the actual situation of each radar station site. At the same time, for newly built SuperDARN radars in the future, the site selection constraints of the radar site are lower.

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

[0035] Figure 1 The figure shows a schematic layout of a SuperDARN radar system antenna array;

[0036] Figure 2 The figure shows a schematic diagram of the installable positions of the newly added antennas;

[0037] Figure 3 The layout of the antenna array during the new antenna external calibration is shown as follows;

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

[0039] Figure 5 The following figure shows the antenna array configuration after the position of the new antenna is determined. Specific implementation manners

[0040] The technical solution of the present application will be described in detail below with reference to the accompanying drawings.

[0041] The dual-baseline elevation estimation method based on an ionospheric sounding radar provided by the present invention is applicable to the SuperDARN system radar for ionospheric sounding. This radar generally adopts an antenna configuration of a 16-element main array and a 4-element sub-array, and both use one-dimensional linear phased arrays. In this embodiment, the antenna configuration of a 16-element main array and a 4-element sub-array is taken as an example for description, and the method of the present application is also applicable to the main array and sub-array antenna configurations with other numbers of antennas. 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 , and the other is the phase difference Ψ caused by the different electrical lengths of the signals received by the two arrays when reaching 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 ). 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 system radar is as shown in Figure 1 the following figure, which is a general model of an interferometric array. The origin of the coordinates is located at the center of the main array. The boresight direction (the direction of maximum gain) is the y-axis direction. The x-axis direction is along the main array direction, perpendicular to and coplanar with the y-axis, and the z-axis is the direction perpendicular to the xy-plane. For subsequent formula derivation, it is assumed that all antennas in the array are at the same vertical height, and the two arrays are parallel, that is, they have a common boresight direction.

[0043] Figure 1 In , k is the wave vector of the incident plane wave, d is the distance vector between the two parallel arrays, α is the elevation angle, φ is the azimuth angle, and (X, Y, Z) are the central coordinates of the interferometric array. When the elevation angle α = 0, the corresponding azimuth angle is (the azimuth angle in the XoY plane). The relationship between the azimuth angle and the elevation angle in the conical 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, and Z are the geometric offsets of the sub-array relative to the main array, is the azimuth angle, all of which are known quantities; α is the elevation angle, which is the unknown quantity to be solved.

[0048] The phase difference caused by the different lengths of the transmission cables between the two arrays 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 transmitted from the two arrays through the cables to the transceiver module. Then 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, with a range of [-π, π]; n is the 2π aliasing factor. To solve for the elevation angle, the value of n needs to be obtained, and n is an integer.

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

[0054] 1. Scheme for the selection of the position of the new antenna

[0055] The selection of the new antenna position for the CN-DARN radar needs to consider two factors: one is that it needs to be far from the main array to avoid being blocked by the main array, and the selected distance is more than 75 m from the straight line where the main array is located (2 times the wavelength corresponding to the radar operating frequency); the other is that it needs to be outside the field of view of the subarray to avoid problems of mutual coupling and occlusion. The field of view of the CN-DARN radar is -39° - 39°, that is, more than 39° off the antenna phase centers on both sides of the subarray. The optional positions are as Figure 2 shown in the shaded part. The principle in the process of selecting the new antenna position: during the scanning of the radar field of view, for all scanning angles, make the path difference corresponding to the two baselines d1 and d2 less than one wavelength as much as possible. In this case, the solution of the 2π ambiguity in the elevation angle estimation is the simplest. In the position selection simulation, it can be assumed that the new antenna has the same height as the original antenna and has no directional offset. Baseline d1 is the distance from the phase center of the new main array to the phase center of the subarray; baseline d2 is the distance from the phase center of the new main array to the new antenna.

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

[0057] Before implementing the dual-baseline elevation angle estimation, it is necessary to calibrate the differences between the new antenna, the cables, and the cables of the original antenna first. The specific method is as follows:

[0058] First, place the new antenna and the subarray on a straight line. According to the actual situation of the site, place the new antenna X m to the right of the subarray, as Figure 3 shown. At this time, the new single antenna and the original four-element subarray form a new one-dimensional linear subarray, as Figure 3 shown by the dashed ellipse in. After fixing the new unit antenna, use a drone carrying a calibration source to calibrate the amplitude and phase errors between the new antenna and the original unit antenna.

[0059] The calibration scheme for the new antenna is specifically as follows:

[0060] 1). The CN-DARN radar operates in a pure receiving mode, and the operating frequency is the same as the frequency of the signal transmitted by the calibration source carried by the drone;

[0061] 2). To improve the calibration accuracy, first calibrate the original 20 antennas using the received echo of the drone calibration source and the method of linear least squares fitting.

[0062] 3). Then use the received echo of the drone calibration source and the method of linear least squares fitting to calibrate the main array and the new five-element subarray. If there is a height difference between the new antenna and the original antenna, the phase error caused by the height needs to be considered, as Figure 4As shown, ΔZ is the height difference, α0 is the elevation angle of the UAV relative to the antenna array, and the position (distance, height, and azimuth) of the UAV is known. If there is no height difference, this phase compensation step can be omitted.

[0063] Before calibration, it is necessary to first calculate the phase difference caused by the antenna height difference (if any), and this phase difference can be expressed as:

[0064]

[0065] where k is the wave number. Assume 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 when the new antenna is used for calibration.

[0068] 3. Dual-Baseline Elevation Estimation Algorithm

[0069] Fix the newly added calibrated antenna at the optimal position (X2, Y2) selected in step 1. If the heights of the newly added antenna and the original antenna are not the same, the coordinates are (X2, Y2, Z2), as Figure 5 shown. Connect the new antenna to the indoor electronic device connected to the original No. 1 antenna. Such a change has the least impact on the system because the number of main antenna array units is large, and the impact of removing one unit is small. The same power as that of the original 16-element antenna can be achieved by slightly increasing the power of a single transmitter. This causes the 16-element antenna of the original main array to become a 15-element antenna, and the phase center will shift in the direction, with coordinates (X1, Y1, Z1). At this time, the original 15-element main array, the 4-element sub-array, and the new antenna sub-array form two baselines d1 and d2. The algorithm for multi-baseline elevation measurement will be based on the Figure 5 antenna array layout in.

[0070] After adding a new antenna to the CN-DARN radar, according to the mathematical model, the phase differences corresponding to the two baselines d1 and d2 are respectively:

[0071]

[0072] In the formula, α is the elevation angle, is the azimuth angle of scanning, and are the phase differences measured by the two baselines, both within [-π, π). Among them, are the measured phases of the main array, sub-array, and new antenna. λ is the wavelength, and m and n are integers.

[0073]

[0074] Among them, ΔY = Y1 - Y2, ΔZ = Z1 - Z2.

[0075] After the position of the new antenna is determined, according to formula (7), it can be known that under different azimuth angle conditions, Ψ1 - Ψ2 belongs to different ranges. Then, based on the phase differences measured respectively by the two baselines and relationship, the relational expression of m and n is obtained, and then the elevation angle can be solved according to formula (8). After dividing formula (7) by the wave number and subtracting the two formulas, the path difference between the two baselines is

[0076]

[0077] Among them

[0078]

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

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

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

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

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

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

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

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

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

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit them. Although the present application has been described in detail with reference to the embodiments, those of ordinary skill in the art should understand that any modification or equivalent replacement of the technical solutions of the present application does not depart from the spirit and scope of the technical solutions of the present application, and they should all be covered within the scope of the claims of the present application.

Claims

1. A dual-baseline elevation estimation method based on an ionospheric sounding radar, where the ionospheric sounding radar includes an M-element linear main array and an N-element linear sub-array; The method includes: Step 1: Add an antenna at a set position; cancel 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 sub-array and outside the field of view of the sub-array; Step 2: Calibrate the amplitude-phase error of the newly added antenna; Step 3: Estimate the double-baseline elevation angle.

2. The dual-baseline elevation angle estimation method based on an ionospheric sounding radar according to claim 1, wherein The distance from the set position to the 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 an ionospheric sounding radar according to claim 1 or 2, characterized in that The set position further 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.

4. The dual-baseline elevation angle estimation method based on an ionospheric sounding radar according to claim 1, wherein Step 2 includes: Step 2.1: Place the newly added antenna on the same line as the sub-array; make the radar operate in the pure reception mode, and the operating frequency is the same as the frequency of the signal transmitted by the calibration source carried by the unmanned aerial vehicle; Step 2.2: Calibrate the original main array and the sub-array by using the received echo of the unmanned aerial vehicle calibration source and the method of linear least squares fitting; Step 2.3: Calibrate the new main array, the newly added antenna and the sub-array by using the received echo of the unmanned aerial vehicle calibration source and the method of linear least squares fitting.

5. The dual-baseline elevation angle estimation method based on an ionospheric sounding radar according to claim 4, wherein Step 2 further includes: Before calibration, first calculate the phase difference caused by the antenna height difference where k is the wave number; is the target phase measured by the newly added antenna; ΔZ is the height difference.

6. The dual-baseline elevation angle estimation method based on an ionospheric sounding radar according to claim 1, wherein Step 3 includes: Phase differences measured based on the first baseline and the second baseline respectively and According to the relationship, obtain the relational expressions of the 2π aliasing factors p and q, and then estimate the elevation angle according to the following formula: Among them, 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 newly added antenna on the x-axis, y-axis, and z-axis; ΔZ represents the height difference between the newly added antenna and the position of the main array antenna; ΔY represents the offset of the newly added antenna from the phase center of the new main array on the y-axis; is the azimuth angle for scanning; and are respectively the phase differences measured for the first baseline and the second baseline; and are respectively the measured phases of the new main array, the sub-array, and the newly added antenna; 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.

7. The dual-baseline elevation angle estimation method based on an ionospheric sounding radar according to claim 6, wherein The phase differences measured according to the first baseline and the second baseline respectively and Based on the relationship, the relational expressions of the 2π aliasing factors p and q are obtained, including: Calculate the path difference ΔP between the first baseline and the second baseline as: Among them, The value range of |δP1 - δP2| is obtained by the exhaustive method according to the azimuth angle of the scan and the positional relationship of the array, and then the value range of is obtained according to the magnitude relationship between δP1 and δP2. Finally, according to and the magnitude relationship, the relational expressions of p and q are obtained.

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