Lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology

Abstract

The present invention relates to the field of radar signal processing technology, and in particular to a lunar surface deformation measurement method based on radar differential interferometry technology. The present invention is another lunar surface deformation measurement method independent of LLR and LOLA, which is essentially different from the traditional method. It relies on the deformation phase change of the interference image to directly invert the lunar surface deformation, so the deformation measurement accuracy can reach the sub-wavelength level (mm level). In addition, since the measurement accuracy is much higher than the lunar surface tidal deformation, it does not require a large amount of data fitting and cross-validation processing to obtain accurate deformation measurement values, the model dependence is low, and the amount of calculation is smaller.

Description

Technical Field

[0001] The present invention relates to the field of radar signal processing technology, and in particular to a lunar surface deformation measurement method based on radar differential interferometry technology. Background Art

[0002] The measurement of lunar surface deformation is an important parameter for inverting the size of the lunar core, and it can also provide information for the health monitoring of future lunar scientific research stations. However, the deformation mainly caused by the tidal deformation of the lunar surface is only about 10 cm, and measuring such a small deformation is a very challenging task. At present, the measurement methods of tidal deformation of the lunar surface can be divided into two categories: the first is based on lunar laser ranging technology (LLR), which uses laser to illuminate the laser reflector placed on the lunar surface to obtain the distance between the earth and the moon, and inverts the lunar surface deformation by observing the change of the distance between the earth and the moon for a long time. At present, the accuracy of the Earth-Moon laser ranging can reach the centimeter level. Although the measurement accuracy is at the same order of magnitude as the lunar surface deformation, the size of the lunar surface deformation can be accurately inverted and fitted through a large amount of long-term observation data; the second method is based on the lunar orbiter laser altimeter (LOLA), which uses the time difference between different beams reflected by the lunar surface to determine the distance. Unlike the lunar laser ranging technology, it measures the distance from the lunar surface to the orbiter, so its measurement accuracy is seriously dependent on the orbit positioning accuracy of the orbiter. At present, the measurement accuracy of LOLA is 10 cm in height and 1 m in horizontal direction. Although the measurement accuracy is much greater than the deformation of the lunar surface, the deformation of the lunar surface can be effectively inverted through the cross-validation technology between a large number of data. At the same time, compared with LLR, the LOLA measurement method does not rely on laser transmitters on the lunar surface and has the potential for large-area measurement.

[0003] In summary, the measurement accuracy of the instruments of the above two methods cannot guarantee the direct measurement of the lunar surface deformation, and both methods are heavily dependent on the fitting and cross-validation of a large amount of data, are heavily model-dependent, and require a huge amount of data calculation. Summary of the invention

[0004] In view of this, the present invention provides a lunar surface deformation measurement method based on radar differential interferometry technology. The measurement method of the present invention is different from the traditional LLR and LOLA ranging methods. The method of the present invention can directly invert the deformation of the lunar surface through the phase change of the lunar surface at different times, and the measurement accuracy can reach sub-wavelength level (mm). The present invention has the advantages of high measurement accuracy, small amount of calculation, low model dependence, etc., and is another deformation measurement method independent of LLR and LOLA.

[0005] The technical solution of the present invention is:

[0006] A lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology, the method comprising the following steps:

[0007] The first step is to obtain a differential interferometric SAR image pair of the target. The differential interferometric SAR image pair includes a main image and an auxiliary image. The specific steps include:

[0008] S11, calculate the position on the lunar surface under the Earth's subsatellite point position and the radar's azimuth angle μ and incident angle θ to the target area according to the ephemeris file. The position on the lunar surface under the Earth's subsatellite point position includes the subsatellite point latitude and the subsatellite point longitude λ SEP ;

[0009] S12, according to the latitude of the sub-satellite point obtained in step S11 Subsatellite point longitude λ SEP , azimuth angle μ and incident angle θ, calculate the spatial decorrelation coefficient γ g ;

[0010]

[0011] Among them, Δμ is the azimuth difference, Δθ is the incident angle difference, Δμ c With Δθ c They are calculated by the following formulas:

[0012] Δμ c =λ / (2ρ x sinθ)

[0013] Δθ c =λ / (2ρ y cosθ)

[0014] Where λ is the signal wavelength, ρ x and ρ y is the azimuth resolution and range resolution of the SAR image;

[0015] S13, according to the spatial decorrelation coefficient γ obtained in step S12 g , selecting the acquisition time of the main image and auxiliary image data with a coherence coefficient greater than 0.6, collecting the corresponding ground-based radar data according to the acquisition time, and obtaining the differential interferometric SAR image pair through synthetic aperture radar signal processing;

[0016] The second step is to obtain the deformation phase of the target. The specific steps include:

[0017] S21, calculating the interference phase according to the differential interferometric SAR image pair obtained in step S13

[0018]

[0019] Among them, the differential interferometric SAR image pair is represented by S1 and S2;

[0020] Step S22, according to the interference phase obtained in step S21 Remove the terrain phase to obtain the target's deformation phase

[0021]

[0022] Among them, B ⊥ is the interferometric vertical baseline, h is the target area elevation, r1 is the main image slant distance, θ i is the incident angle of the target area;

[0023] The third step is to obtain the deformation information Δd of the lunar surface based on the deformation phase of the target obtained in the second step;

[0024]

[0025] Beneficial Effects

[0026] The present invention firstly uses the ground-based radar re-orbit differential interferometry technology to measure the deformation of the lunar surface, and compares it with the current measurement methods (LLR and LOLA) as follows:

[0027] (1) The LLR-based lunar surface deformation measurement method measures the transmission delay between the laser transmitter and the laser reflector deployed on the lunar surface, and inverts the lunar surface deformation through a large number of model parameter fitting. However, it has the problems of severe model dependence and low inversion accuracy. The present invention can directly invert the lunar surface deformation through spatial phase changes, has low model dependence, and high inversion accuracy.

[0028] (2) The lunar surface deformation measurement method based on LOLA measures the transmission delay from the lunar orbiter to the lunar surface and inverts the lunar surface deformation from a large amount of noise through cross-validation and other technologies. However, it has the problems of large data demand and can only measure radial tidal deformation. The present invention can directly invert the radial tidal deformation and horizontal tidal deformation of the lunar surface through spatial phase, and the measurement accuracy is sub-wavelength level (mm level). It does not need cross-validation to improve the accuracy to meet the tidal deformation measurement accuracy, so it is more efficient.

[0029] (3) The present invention is another lunar surface deformation measurement method independent of the LLR and LOLA methods. It is fundamentally different from the traditional method. It relies on the deformation phase change of the interference image to directly invert the lunar surface deformation. Therefore, the deformation measurement accuracy can reach the sub-wavelength level (mm level).

[0030] (4) Since the measurement accuracy of the method of the present invention is much higher than that of the tidal deformation of the lunar surface, it does not require the fitting and cross-validation of a large amount of data to obtain accurate deformation measurement values, has low model dependence, and requires less computation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the method flow of the present invention;

[0032] FIG2 is a schematic diagram of the change in lunar tidal deformation due to the change in the distance between the Earth and the Moon;

[0033] Figure 3 is a schematic diagram of the change in lunar tidal deformation due to SEP changes. DETAILED DESCRIPTION

[0034] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0035] The present invention discloses a method for measuring lunar surface deformation, which is essentially different from the traditional method, thanks to the innovative deformation measurement principle of the method, which directly inverts the lunar surface deformation by relying on the deformation phase change of the interference image. Therefore, it is first necessary to select the acquisition time of two SAR images that meet the baseline decorrelation according to the interferometric SAR baseline selection principle; then, the main and auxiliary SAR images are subjected to interference processing, and the reference phases such as the terrain phase are removed to obtain the differential interference phase; finally, according to the relationship between the differential interference phase and the deformation amount, the lunar tidal deformation value can be directly obtained.

[0036] Figure 1 The present invention is a flow chart, and the method specifically comprises the following steps:

[0037] Step 1: Select the differential interferometry heavy track baseline.

[0038] This step specifically includes implementing sub-steps S101 to S103 for baseline selection.

[0039] S101 differential interferometry baseline selection is the first and most important step of the present invention, which directly determines the performance of the method. According to the ephemeris file, the position of the lunar surface under the position of the Earth's subsatellite point can be calculated as and λ SEP ,in is the latitude of the subsatellite point, λ SEP is the longitude of the sub-satellite point; at the same time, the azimuth μ and the incident angle θ of the radar to the target area are calculated.

[0040] S102 calculates the spatial decorrelation coefficient according to the latitude and longitude of the sub-satellite point, the azimuth angle, and the incident angle calculated in the previous step, that is,

[0041]

[0042] Among them, Δμ is the azimuth difference, Δθ is the incident angle difference, Δμ c With Δθ c It can be calculated by the following formula:

[0043] Δμ c =λ / (2ρ x sinθ) (2)

[0044] Δθ c =λ / (2ρ y cosθ)

[0045] Where λ is the signal wavelength, ρ x and ρ y are the azimuth resolution and range resolution of SAR images.

[0046] S102 selects the primary and secondary image acquisition times greater than 0.6 according to the calculated spatial decorrelation coefficient, so as to obtain a differential interferometric SAR image pair.

[0047] Step 2: Calculate the differential interference phase.

[0048] This step specifically implements sub-steps S201 to S202 on the SAR image pair.

[0049] S201, obtaining interference phase.

[0050] Assuming that the SAR image acquired by the radar is divided into S1 and S2 according to the interference baseline obtained in the previous step, the interference phase is

[0051]

[0052] S202. Compensate for the terrain phase caused by the undulating terrain on the lunar surface.

[0053] The above interference phase includes not only the deformation phase caused by the lunar tidal deformation, but also the topographic phase caused by the terrain undulation. Therefore, by removing the terrain phase, the deformation phase can be obtained as

[0054]

[0055] Among them, B ⊥ is the interferometric vertical baseline, h is the target area elevation, r1 is the main image slant distance, θ i is the incident angle of the target area.

[0056] Step 3: According to the obtained differential interferometry phase, the deformation information of the lunar surface is obtained, that is,

[0057]

[0058] Table 1: Simulation parameters

[0059]

[0060]

[0061] Example

[0062] The simulation data are processed using the method of the present invention to further verify the feasibility and effectiveness of the proposed technology. The simulation parameters are shown in Table 1.

[0063] FIG2 shows the inversion process of the present invention for the change of the lunar tidal deformation caused by the change of the distance between the earth and the moon, wherein FIG2(a) is the tidal deformation of the lunar surface at time t1, and FIG2(b) is the tidal deformation of the lunar surface at time t2. Due to the change of the distance between the earth and the moon, there is a slight difference in the magnitude of the two tidal deformations. The purpose of the present invention is to obtain the change in tidal deformation between time t1 and time t2, that is, the lunar surface deformation value occurring at time t1 and time t2. FIG2(c) is the differential interference phase diagram obtained according to the method of the present invention, and FIG2(d) is the lunar surface deformation obtained according to the method of the present invention.

[0064] FIG3 shows the inversion process of the present invention for the change of the lunar tidal deformation caused by the change of SEP, wherein FIG3(a) is the tidal deformation of the lunar surface at time t1, and FIG3(b) is the tidal deformation of the lunar surface at time t2. Due to the change in the distance between the earth and the moon, there is a slight difference in the magnitude of the two tidal deformations. The purpose of the present invention is to obtain the change in the tidal deformation between time t1 and time t2, that is, the lunar surface deformation value occurring at time t1 and time t2. FIG3(c) is the differential interference phase diagram obtained by the method of the present invention, and FIG3(d) is the lunar surface deformation obtained by the method of the present invention.

[0065] It can be seen that the distribution of the deformation field of the lunar surface caused by the change in the distance between the Earth and the Moon does not change much, but only changes in magnitude. However, the distribution of the deformation field of the lunar surface caused by the change in SEP is very different. This method can correctly invert the corresponding lunar surface deformation.

[0066] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology, characterized in that The steps of the method include: The first step is to obtain a differential interferometric SAR image pair of the target; The second step is to obtain the deformation phase of the target based on the differential interferometric SAR image pair of the target obtained in the first step; The third step is to obtain the deformation information of the lunar surface according to the deformation phase of the target obtained in the second step; In the first step, the specific method of obtaining the differential interferometric SAR image pair of the target is: S11, calculate the position on the lunar surface under the Earth's subsatellite point position and the radar's azimuth angle μ and incident angle θ to the target area according to the ephemeris file. The position on the lunar surface under the Earth's subsatellite point position includes the subsatellite point latitude and the subsatellite point longitude λ SEP ; S12, according to the latitude of the sub-satellite point obtained in step S11 Subsatellite point longitude λ SEP , azimuth angle μ and incident angle θ, calculate the spatial decorrelation coefficient γ g ; Among them, Δμ is the azimuth difference, Δθ is the incident angle difference, Δμ c With Δθ c They are calculated by the following formulas: Dm c =λ / (2ρ x sinθ) Dth c =λ / (2ρ y cosθ) Where λ is the signal wavelength, ρ x and ρ y is the azimuth resolution and range resolution of the SAR image; S13, according to the spatial decorrelation coefficient γ obtained in step S12 g , the acquisition time of the main image and auxiliary image data with a coherence coefficient greater than a set value is selected, the corresponding ground-based radar data is collected according to the acquisition time, and the differential interferometric SAR image pair is obtained through synthetic aperture radar signal processing.

2. The lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology according to claim 1 is characterized in that: In the second step, the specific method of obtaining the deformation phase of the target is: S21, calculating the interference phase according to the differential interferometric SAR image pair obtained in step S13 Among them, the differential interferometric SAR image pair is represented by S1 and S2; S22, according to the interference phase obtained in step S21 Remove the terrain phase to obtain the target's deformation phase Among them, B ⊥ is the interferometric vertical baseline, h is the target area elevation, r1 is the main image slant distance, θ i is the incident angle of the target area.

3. The lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology according to claim 2 is characterized in that: In the third step, according to the deformation phase of the target, the deformation information Δd of the lunar surface is obtained as:

4. The lunar surface deformation measurement method based on ground-based radar re-orbit differential interferometry technology according to claim 3 is characterized in that: In the step S13, the acquisition time of the main image and auxiliary image data with a coherence coefficient greater than a set value of 0.6 is selected.

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