Calibration method and system for spaceborne wide-swath radar altimeter based on aircraft-sea fusion measurement

Abstract

The present invention discloses a calibration method and system for a satellite-borne wide-swath radar altimeter for aircraft-sea fusion measurement, wherein the calibration method comprises: obtaining a buoy sea surface height sequence through a GNSS buoy array; obtaining an airborne sea surface height sequence through a one-dimensional airborne radar altimeter; obtaining sea surface height measurement values ​​through a satellite-borne wide-swath radar altimeter to be calibrated; using the GNSS buoy as a control point, associating the airborne sea surface height sequence with the buoy sea surface height sequence to obtain a fused sea surface height sequence; jointly estimating the fused sea surface height sequence and the sea surface height measurement value to obtain a rolling error and a baseline length error; correcting the rolling error and the baseline length error to obtain a calibration result of the satellite-borne wide-swath radar altimeter. The method of the present invention can take into account the requirements of wide swath, high precision and high resolution in calibration, and is more feasible and advanced than existing calibration schemes in the world.

Description

Technical Field

[0001] The present invention relates to the field of microwave remote sensing, and in particular to a calibration method and system for a satellite-borne wide-swath radar altimeter for machine-sea fusion measurement. Background Art

[0002] Wide Swath Radar Altimeter is a new microwave remote sensor that uses two antennas to achieve interferometric measurement at small angles of incidence, thereby extracting wide swath height field information. It can quickly achieve mesoscale elevation measurement of global waters, and plays an important role in the study of dynamic phenomena such as ocean mesoscale eddies and water volume changes in inland waters. Satellite plans with wide swath radar altimeter as the main payload have been proposed both at home and abroad.

[0003] For quantitative remote sensing, calibration and authenticity verification are one of the most critical links. The sea surface height measurement indicators of wide-swath radar altimeters are strict (centimeter level), the swath is very wide, and the error characteristics are complex. Therefore, calibration and authenticity verification have posed unprecedented challenges and have become one of the most difficult problems in microwave remote sensing technology. Among them, rolling error and baseline length error are the most difficult of all errors.

[0004] At present, the research on the calibration method of spaceborne wide-swath radar altimeters is in its infancy. Foreign calibration schemes generally use a single-point calibration at the calibration site (tide gauge station or GNSS buoy) combined with a cross-area and two-dimensional airborne test calibration method.

[0005] The various calibration methods proposed abroad so far have certain disadvantages:

[0006] 1) Calibration site The on-site calibration method is similar to that of the traditional sub-satellite altimeter, which is a sporadic calibration point and is not optimized for the characteristics of wide swaths.

[0007] 2) The cross-area calibration method is a relative calibration method that can only correct relative errors but not absolute errors, and cannot fully meet the calibration needs.

[0008] 3) The coverage required by the two-dimensional airborne test is too large and the feasibility is poor. In addition, the geometric relationship of the airborne altimeter under this scheme is different from that of the satellite (the incident angle enters the Bragg scattering region), resulting in large differences in the radar scattering mechanism and error characteristics. The test results conducted abroad for this method are poor. Summary of the invention

[0009] The purpose of the present invention is to overcome the defects of the prior art, and propose a calibration method for a satellite-borne wide-swath radar altimeter with machine-sea fusion measurement. The present invention also discloses a calibration system for a satellite-borne wide-swath radar altimeter with machine-sea fusion measurement. The method proposed in the present invention solves the shortcomings of these foreign calibration methods, can achieve high-precision calibration of satellite-borne wide-swath radar altimeters, and has better feasibility.

[0010] The present invention proposes a calibration method for a satellite-borne wide-swath radar altimeter with machine-sea fusion measurement, and the calibration method comprises:

[0011] In order to achieve the above object, the present invention proposes a calibration method for a satellite-borne wide-swath radar altimeter with machine-sea fusion measurement, the calibration method comprising:

[0012] The buoy sea surface height sequence is obtained through the GNSS buoy array;

[0013] The airborne sea surface height sequence is obtained by a one-dimensional airborne radar altimeter;

[0014] The sea surface height measurement is obtained by using a satellite-borne wide-swath radar altimeter to be calibrated;

[0015] Taking the GNSS buoy as the control point, the airborne sea surface height sequence is associated with the buoy sea surface height sequence to obtain the fused sea surface height sequence;

[0016] The rolling error and baseline length error are obtained by jointly estimating the fused sea surface height series and the sea surface height measurements;

[0017] The rolling error and baseline length error are corrected to obtain the calibration result of the spaceborne wide-swath radar altimeter.

[0018] As an improvement of the above method, the GNSS buoy array adopts multi-base station dynamic differential technology, and N GNSS buoys are evenly arranged in a straight line in the cross-track direction, N=2m+1, m is a non-negative integer, wherein the center of the GNSS buoy array is located at the sub-satellite point, and the distance between two adjacent GNSS buoys is d;

[0019] The buoy sea surface height sequence includes sea surface height observation values ​​of N GNSS buoys.

[0020] As an improvement of the above method, the one-dimensional airborne radar altimeter flies over the GNSS buoy via an aircraft platform to obtain an airborne sea surface height sequence.

[0021] As an improvement of the above method, the GNSS buoy is used as a control point, and the airborne sea surface height sequence is associated with the buoy sea surface height sequence to obtain a fused sea surface height sequence; specifically:

[0022] Taking the GNSS buoy as the control point, the buoy sea surface height sequence and the airborne sea surface height sequence are calibrated, and then the airborne sea surface height sequence is paired according to the calibration value to obtain the fused sea surface height sequence.

[0023] As an improvement of the above method, the rolling error and baseline length error are obtained by jointly estimating the fused sea surface height sequence and the sea surface height measurement value; specifically:

[0024] The ith sea surface height measurement H is estimated by combining the fused sea surface height sequence and the sea surface height measurement. obs,i for:

[0025]

[0026] Among them, H real,i is the sea surface height of the i-th GNSS buoy in the fused sea surface height sequence, ε i is the other error of the i-th GNSS buoy, D i =i*d, set ε i is a random noise with zero mean Gaussian distribution, δ roll D i is the i-th height measurement error caused by rolling, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, is the ith height measurement error caused by the change of baseline length, m = 0 represents the sub-satellite point;

[0027] According to the matrix representation:

[0028]

[0029] in, is the column vector of sea surface height measurement residuals, R i =H obs,i -H real,i , is an N×2 dimensional matrix, is the cross-track distance vector from the measurement point to the nadir point, is the parameter vector to be estimated, is a random noise vector;

[0030] Find the least squares solution for:

[0031]

[0032]

[0033] Get the rolling error of the satellite-borne wide-swath radar altimeter antenna for:

[0034]

[0035] Baseline length error for:

[0036]

[0037] As an improvement of the above method, the rolling error and the baseline length error are corrected to obtain the calibration result of the satellite-borne wide-swath radar altimeter; specifically:

[0038] In the area outside the calibration field, the rolling error and baseline length error are substituted into the following formula for correction: obs Inversely calculate H real , get the calibration result of the satellite-borne wide-swath radar altimeter;

[0039]

[0040] Among them, H obs is the measured value of sea level height, H real is the true sea level height, ε is other errors, δ roll is the rolling error, D is the cross-track distance from the measuring point to the nadir point, δ roll D is the height measurement error caused by rolling, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, It is the height measurement error caused by the change of baseline length.

[0041] A satellite-borne wide-swath radar altimeter calibration system for aircraft-sea fusion measurement, the calibration system comprising: a buoy sea surface height sequence acquisition module, an airborne sea surface height sequence acquisition module, a sea surface height measurement value acquisition module, a fusion sea surface height sequence generation module, an error estimation module and a calibration module, wherein:

[0042] The buoy sea surface height sequence acquisition module is used to obtain the buoy sea surface height sequence through the GNSS buoy array;

[0043] The airborne sea surface height sequence acquisition module is used to obtain the airborne sea surface height sequence through a one-dimensional airborne radar altimeter;

[0044] The sea surface height measurement value acquisition module is used to obtain the sea surface height measurement value through the satellite-borne wide-swath radar altimeter to be calibrated

[0045] The fused sea surface height sequence generation module is used to associate the airborne sea surface height sequence with the buoy sea surface height sequence using the GNSS buoy as a control point to obtain a fused sea surface height sequence;

[0046] The error estimation module is used to perform joint estimation by fusion of the sea surface height sequence and the sea surface height measurement value to obtain the rolling error and the baseline length error;

[0047] The calibration module is used to correct the rolling error and the baseline length error to obtain the calibration result of the satellite-borne wide-swath radar altimeter.

[0048] Compared with the prior art, the advantages of the present invention are:

[0049] 1. Aiming at the characteristics of the main error sources, the present invention designs a calibration scheme of joint calibration of GNSS buoy array and airborne radar altimeter, which can take into account the requirements of wide swath, high precision and high resolution in calibration, and is more feasible and advanced than the existing calibration schemes in the world;

[0050] 2. The method of the present invention adopts a one-dimensional airborne solution. Under the assumption that the sea surface height spectrum is approximately isotropic, the cross-track one-dimensional airborne calibration solution can achieve the same function as the two-dimensional airborne calibration test, while significantly reducing the cost of the airborne test and improving the feasibility;

[0051] 3. The method of the present invention adopts a symmetrical geometric configuration for the GNSS buoy array, which effectively solves the coupling problem of rolling error and baseline length error in the traditional calibration method, and can independently correct the two most important errors in the wide-swath radar altimeter;

[0052] 4. The calibration of satellite-borne wide-swath radar altimeters can also be achieved by simply using the GNSS buoy array proposed in the present invention. Its accuracy and resolution can be further improved by combining it with a one-dimensional airborne calibration solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a flow chart of a method for calibrating a satellite-borne wide-swath radar altimeter for machine-sea fusion measurement according to Embodiment 1 of the present invention;

[0054] Figure 2 It is a top view of the machine-sea integrated calibration geometric configuration diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0056] Example 1

[0057] like Figure 1 As shown, Embodiment 1 of the present invention proposes a calibration method for a satellite-borne wide-swath radar altimeter with machine-sea fusion measurement, and the specific steps are as follows:

[0058] Step 1: GNSS buoy array and 1D airborne measurement data acquisition

[0059] GNSS (Global Navigation Satellite System) buoy is a new type of ocean observation equipment that can realize real-time observation of sea surface height with high accuracy, and is used in ocean measurement and investigation, and calibration of satellite-borne radar altimeters. The GNSS buoy array proposed in the present invention is a combination of multiple GNSS buoys arranged in a row in a certain pattern. In order to ensure the measurement accuracy of GNSS buoys, this solution adopts the technology of multi-base station dynamic difference, and deploys multiple GNSS observation base stations at the coast to provide a stable elevation measurement benchmark. Assuming there are N buoys in the GNSS buoy array, the buoy array can provide N high-precision sea surface height observation values.

[0060] The configuration of the GNSS buoy array should maintain symmetry and uniform distribution. Figure 2 The figure shows a top view of the geometric configuration diagram of the machine-sea integrated calibration. The dotted line in the figure is the satellite sub-satellite point trajectory (vertical axis), and the solid line is the cross-track direction (horizontal axis, with the coordinate set as x). The GNSS buoy array is located in the cross-track direction, and one of them is located at the sub-satellite point.

[0061] If the swath width is 100 km, and 9 GNSS buoys are used to form an array, the distance between these buoys in the cross-track direction and the sub-satellite point (x coordinate) is:

[0062] Left side of swath: -88km, -66km, -44km, -22km;

[0063] Sub-satellite position: 0km;

[0064] Right side of the swath: 22km, 44km, 66km, 88km.

[0065] In the airborne measurement scheme, a one-dimensional airborne radar altimeter is used to fly back and forth once over the GNSS buoy. When the swath width of the altimeter is 100km, the overall calibration flight takes about 2 hours; according to calculations, an average sea surface height is obtained every 800m, which can achieve good relative accuracy. It should be noted that the data in this onboard measurement scheme is only an experimental example and is not limited to the above experimental parameters.

[0066] Step 2: Generate high-precision, high-resolution cross-track high-sequence data on the sea surface

[0067] Due to reasons such as the aircraft platform orbit error, the accuracy of the measured sea surface height is poor. Using the GNSS buoy as the control point, the airborne sea surface height measurement sequence is associated with the GNSS buoy sea surface height measurement sequence to improve its accuracy and obtain a high-resolution sea surface height measurement with strong consistency.

[0068] Step 3: Joint estimation of antenna roll error and baseline length error of the spaceborne wide-swath radar altimeter

[0069] In a wide-swath radar altimeter, one of the most important error terms is the antenna roll error, which is proportional to the distance from the nadir point in the cross-track direction. Another error term is the baseline length error, which is proportional to the square of the distance from the nadir point in the cross-track direction:

[0070]

[0071] Among them, H obs is the measured sea level height, H real is the true sea level height, ε is other errors, δ roll is the rolling error, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, D is the cross-track distance from the measurement point to the nadir point, δ roll D is the height measurement error caused by rolling, The error in height measurement is caused by the change in baseline length. In wide-swath radar altimeters, these two errors can reach meter level if they are not corrected, which is unacceptable under the centimeter level measurement accuracy requirement.

[0072] Therefore, the estimation and correction of the rolling error and baseline length error of the wide-swath radar altimeter becomes one of the central tasks of calibration, and is also the most important aspect that distinguishes the wide-swath interferometric imaging altimeter calibration from the traditional altimeter calibration.

[0073] Assuming that other errors ε have been corrected as much as possible (at least the systematic error has been corrected; the correction of the systematic error can be done by taking the average of the difference between the sea surface height measured by each GNSS buoy and the altimeter in the buoy array), the measurement of the GNSS buoy is used as H real , then for the high-precision sea surface height observation sequence (a total of 2m+1 measurements, where 0 represents the sub-satellite point), the following relationship exists:

[0074]

[0075] H real,i is the sea surface height of the i-th GNSS buoy in the fused sea surface height sequence, ε i is the other error of the i-th GNSS buoy, D i =i*d, set ε i is a random noise with zero mean Gaussian distribution, δ roll D i is the i-th height measurement error caused by rolling, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, is the ith height measurement error caused by the change of baseline length, m = 0 represents the sub-satellite point;

[0076] Assume ε i is a random noise with zero mean and Gaussian distribution, then the problem is transformed into a classic least squares adjustment problem. It can be expressed in matrix form as follows:

[0077]

[0078] in is the column vector R consisting of the sea surface height measurement residuals i =H obs,i -H real,i , is an N×2 dimensional matrix, is the cross-track distance vector from the measurement point to the nadir point, is the parameter vector to be estimated, is a random noise vector.

[0079] The least squares solution of the above equation is

[0080]

[0081] Easy to know:

[0082]

[0083] In the design of the present invention, the cross-track sea surface height sequence is symmetric about the subsatellite point, so the matrix A T A degenerates into a diagonal matrix, and the rolling error and baseline length error can be estimated independently of each other.

[0084] The estimated value can be expressed as:

[0085]

[0086]

[0087] Step 4: Correct the sea surface height measurement error caused by rolling error and baseline length error. The sea surface height measurement data product contains the sea surface height measurement error caused by rolling error and baseline length error, which is corrected using the following formula.

[0088] Δh roll =δ roll x (8)

[0089]

[0090] Step 5: Determination of calibration results of the satellite-borne wide-swath radar altimeter

[0091] In the area outside the calibration field, the rolling error and baseline length error Substitute into equation (1) for correction, according to the altimeter observation value H obs Inversely calculate H real , and obtain the calibration result of the satellite-borne wide-swath radar altimeter.

[0092] After the rolling error and baseline length error are corrected, the calibration result of the satellite-borne wide-swath radar altimeter system can be obtained.

[0093] It should be noted that the above steps can also realize the calibration of the satellite-borne wide-swath radar altimeter by only using the GNSS buoy array, which also belongs to the technical solution of the present invention, and its accuracy and resolution can be further improved by combining it with a one-dimensional airborne calibration solution.

[0094] Example 2

[0095] Embodiment 2 of the present invention provides a satellite-borne wide-swath radar altimeter calibration system for machine-sea fusion measurement, which is implemented based on the method of embodiment 1. Figure 2 The figure shows a top view of the geometric configuration diagram of the integrated machine-sea calibration. The calibration system includes: a buoy sea surface height sequence acquisition module, an airborne sea surface height sequence acquisition module, a sea surface height measurement value acquisition module, a fusion sea surface height sequence generation module, an error estimation module and a calibration module, wherein:

[0096] The buoy sea surface height sequence acquisition module is used to obtain the buoy sea surface height sequence through the GNSS buoy array;

[0097] The airborne sea surface height sequence acquisition module is used to obtain the airborne sea surface height sequence through a one-dimensional airborne radar altimeter;

[0098] The sea surface height measurement value acquisition module is used to obtain the sea surface height measurement value through the satellite-borne wide-swath radar altimeter to be calibrated

[0099] The fused sea surface height sequence generation module is used to associate the airborne sea surface height sequence with the buoy sea surface height sequence using the GNSS buoy as a control point to obtain a fused sea surface height sequence;

[0100] The error estimation module is used to perform joint estimation by fusion of the sea surface height sequence and the sea surface height measurement value to obtain the rolling error and the baseline length error;

[0101] The calibration module is used to correct the rolling error and the baseline length error to obtain the calibration result of the satellite-borne wide-swath radar altimeter.

[0102] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A calibration method for a spaceborne wide-swath radar altimeter for machine-sea fusion measurement, the calibration method comprising: The buoy sea surface height sequence is obtained through the GNSS buoy array; The airborne sea surface height sequence is obtained by a one-dimensional airborne radar altimeter; The sea surface height measurement is obtained by using a satellite-borne wide-swath radar altimeter to be calibrated; Taking the GNSS buoy as the control point, the airborne sea surface height sequence is associated with the buoy sea surface height sequence to obtain the fused sea surface height sequence; The rolling error and baseline length error are obtained by jointly estimating the fused sea surface height series and the sea surface height measurements; The rolling error and baseline length error are corrected to obtain the calibration result of the satellite-borne wide-swath radar altimeter. The rolling error and the baseline length error are corrected to obtain the calibration result of the satellite-borne wide-swath radar altimeter; specifically: In the area outside the calibration field, the rolling error and baseline length error are substituted into the following formula for correction: obs Inversely calculate H real , get the calibration result of the satellite-borne wide-swath radar altimeter; Among them, H obs is the measured value of sea level height, H real is the true sea level height, ε is other errors, δ roll is the rolling error, D is the cross-track distance from the measuring point to the nadir point, δ roll D is the height measurement error caused by rolling, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, It is the height measurement error caused by the change of baseline length.

2. The calibration method of the satellite-borne wide-swath radar altimeter for machine-sea fusion measurement according to claim 1 is characterized in that: The GNSS buoy array adopts multi-base station dynamic differential technology, and N GNSS buoys are evenly arranged in a straight line in the cross-track direction, N = 2m + 1, m is a non-negative integer, wherein the center of the GNSS buoy array is located at the sub-satellite point, and the distance between two adjacent GNSS buoys is d; The buoy sea surface height sequence includes sea surface height observation values ​​of N GNSS buoys.

3. The calibration method of the satellite-borne wide-swath radar altimeter for machine-sea fusion measurement according to claim 1 is characterized in that: The one-dimensional airborne radar altimeter flies over the GNSS buoy via an aircraft platform to obtain an airborne sea surface height sequence.

4. The calibration method of the spaceborne wide-swath radar altimeter for machine-sea fusion measurement according to claim 1 is characterized in that: The method uses the GNSS buoy as a control point, associates the airborne sea surface height sequence with the buoy sea surface height sequence, and obtains a fused sea surface height sequence; specifically: Taking the GNSS buoy as the control point, the buoy sea surface height sequence and the airborne sea surface height sequence are calibrated, and then the airborne sea surface height sequence is paired according to the calibration value to obtain the fused sea surface height sequence.

5. The calibration method of the satellite-borne wide-swath radar altimeter for machine-sea fusion measurement according to claim 4 is characterized in that: The rolling error and the baseline length error are obtained by performing a joint estimation by fusing the sea surface height sequence and the sea surface height measurement value; Specifically: The ith sea surface height measurement H is estimated by combining the fused sea surface height sequence and the sea surface height measurement. obs,i for: Among them, H real,i is the sea surface height of the i-th GNSS buoy in the fused sea surface height sequence, ε i is the other error of the i-th GNSS buoy, D i =i*d, d is the distance between two adjacent GNSS buoys, set ε i is a random noise with zero mean Gaussian distribution, δ roll D i is the i-th height measurement error caused by rolling, H is the satellite flight altitude, B is the baseline length, δ B is the baseline length error, is the ith height measurement error caused by the change of baseline length, and m = 0 represents the sub-satellite point; According to the matrix representation: in, is the column vector of sea surface height measurement residuals, R i =H obs,i -H real,i , is an N×2 dimensional matrix, is the cross-track distance vector from the measurement point to the nadir point, is the parameter vector to be estimated, is a random noise vector; Find the least squares solution for: Get the rolling error of the satellite-borne wide-swath radar altimeter antenna for: Baseline length error for:

6. A system for calibrating a satellite-borne wide-swath radar altimeter based on the machine-sea fusion measurement method of claim 1, characterized in that: The calibration system comprises: a buoy sea surface height sequence acquisition module, an airborne sea surface height sequence acquisition module, a sea surface height measurement value acquisition module, a fused sea surface height sequence generation module, an error estimation module and a calibration module, wherein: The buoy sea surface height sequence acquisition module is used to obtain the buoy sea surface height sequence through the GNSS buoy array; The airborne sea surface height sequence acquisition module is used to obtain the airborne sea surface height sequence through a one-dimensional airborne radar altimeter; The sea surface height measurement value acquisition module is used to obtain the sea surface height measurement value through the satellite-borne wide-swath radar altimeter to be calibrated The fused sea surface height sequence generation module is used to associate the airborne sea surface height sequence with the buoy sea surface height sequence using the GNSS buoy as a control point to obtain a fused sea surface height sequence; The error estimation module is used to perform joint estimation by fusion of the sea surface height sequence and the sea surface height measurement value to obtain the rolling error and the baseline length error; The calibration module is used to correct the rolling error and the baseline length error to obtain the calibration result of the satellite-borne wide-swath radar altimeter.

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