CN108007476B - Interference calibration method and system for space-based interference imaging radar altimeter - Google Patents

Interference calibration method and system for space-based interference imaging radar altimeter Download PDF

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CN108007476B
CN108007476B CN201711158570.3A CN201711158570A CN108007476B CN 108007476 B CN108007476 B CN 108007476B CN 201711158570 A CN201711158570 A CN 201711158570A CN 108007476 B CN108007476 B CN 108007476B
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sensitivity
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CN108007476A (en
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谭洪
李盛阳
刘志文
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Technology and Engineering Center for Space Utilization of CAS
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C25/00—Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/882—Radar or analogous systems specially adapted for specific applications for altimeters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/9021—SAR image post-processing techniques
    • G01S13/9023—SAR image post-processing techniques combined with interferometric techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/40—Means for monitoring or calibrating

Abstract

The invention relates to an interference calibration method and system for a space-based interference imaging radar altimeter, wherein the method comprises the following steps: determining a radar visual angle of each pixel point of the reference target according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interference imaging radar altimeter; then carrying out polynomial fitting on the spatial variation relation of the unwrapping phase along with the radar visual angle; then, determining the sensitivity of the unwrapping phase and the slant range to the radar viewing angle respectively; then determining the length and the inclination angle of the base line; and finally determining the interference phase offset. The system comprises: the device comprises a radar visual angle calculation module, a fitting coefficient calculation module, a sensitivity calculation module, an interference baseline determination module and an interference phase bias determination module. According to the method, the sensitivity of the unwrapping phase to the radar viewing angle is estimated through fitting, the estimation of the interference baseline and the estimation of the interference phase offset can be separated, and finally the complete estimation of three parameters of the baseline length, the baseline inclination angle and the interference phase offset is realized.

Description

Interference calibration method and system for space-based interference imaging radar altimeter
Technical Field
The invention belongs to the field of interference calibration of an interference imaging radar altimeter, and particularly relates to an interference calibration method and system of a space-based interference imaging radar altimeter.
Background
A novel radar altimeter developed in recent years, namely a space-based interference imaging radar altimeter, can obtain the sea surface average height value in a large-area range, the spatial resolution is in the level of 100 meters, the observation swath is in the level of 50 kilometers to 100 kilometers, and compared with the traditional radar altimeter for the satellite points, the space resolution and the time resolution are greatly improved. However, accurate interferometric scaling is an essential critical step to apply this technique. The interference calibration realizes accurate measurement of interference parameters of the space-based interference imaging radar altimeter on the rail, including baseline length, baseline inclination angle, interference phase offset and the like. As a space-based interference system which realizes global observation and is carried on a satellite or a space vehicle, the interference calibration based on a natural distribution target is a feasible method. Although the interferometric synthetic aperture radar technology is also adopted, the incidence angle of the space-based interferometric imaging radar altimeter is less than 8 degrees, land needs to be considered while observing the sea, and the observation mode of the observation condition with the extremely small incidence angle and the sea land is completely different from the existing space-based interferometric synthetic aperture radar which only observes the land. In the prior art, all the technical schemes refer to an airborne interference system with very limited flight altitude (generally about ten kilometers), are established under the assumption of flat ground, do not consider the influence of the curvature of the earth, and are not applicable to a space-based system with the flight altitude of hundreds of kilometers and the observation range of tens of kilometers;
in addition, the prior art scheme also needs to accurately estimate the frequency interval of the interference phase spectrum, which is applicable under the conditions that interference phase fringes are dense and the spatial distribution is uniform, such as the traditional interferometric synthetic aperture radar, but fails under the conditions that the interference phase is particularly sparse and the spatial variation is particularly large, such as an interferometric imaging radar altimeter;
secondly, although the estimation of the base line length and the base line inclination angle is realized, the estimation of the interference phase offset is not realized at the same time, in fact, the interference base line and the interference phase offset are coupled with each other, and the simultaneous estimation of the parameters is very easy to cause 'ill-conditioned solution' or the iterative algorithm cannot be converged.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: in the prior art, the influence of the curvature of the earth is not considered, and the estimation of the interference phase offset cannot be realized because a 'ill-conditioned solution' or an iterative algorithm cannot be converged easily when parameters are estimated.
In order to solve the above technical problem, the present invention provides an interferometric calibration method for a space-based interferometric imaging radar altimeter, the interferometric calibration method comprising:
s1, determining a radar view angle according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interferometric imaging radar altimeter;
s2, determining the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle by using a polynomial fitting model, and determining a polynomial fitting coefficient by using a least square algorithm according to the unwrapping phase of the reference target and the spatial variation relation;
s3, determining the sensitivity of the unwrapping phase to the radar perspective according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar perspective according to the height information of the reference target, the track and the slant range parameter;
s4, calculating a base line length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar visual angle, and determining an accurate base line length and an accurate base line inclination angle according to the base line length initial value, the inclination distance parameter and the sensitivity of the inclination distance to the radar visual angle;
and S5, determining the estimated value of the interference phase offset according to the accurate base line length and the base line inclination angle.
The invention has the beneficial effects that: by the method, a polynomial fitting coefficient describing the spatial change relation of the unwrapping phase of the reference target along with the radar visual angle is determined, the accurate base line length and the base line inclination angle can be obtained by a two-step method when the base line length is calculated subsequently according to the polynomial fitting coefficient, and finally the accurate estimated value of the interference phase offset can be obtained according to the accurate base line length and the base line inclination angle, so that the phase error is greatly reduced, meanwhile, the high-order micro component introduced by the sensitivity of the slant distance to the radar visual angle in the interference phase and the influence of the earth curvature are considered, so that the estimated values of the base line length, the base line inclination angle and the interference phase offset are more accurately measured, in addition, the sensitivity of the unwrapping phase to the radar visual angle is estimated by fitting, the estimation of the interference base line and the interference phase offset can be separated, and the estimation of the base line length, the, A complete estimate of baseline tilt angle and interferometric phase offset.
Further, the calculation formula for determining the radar view angle θ in S1 includes:
Figure RE-GDA0001571132990000031
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereH is the height information of the reference target, which is the radius of the reference ellipsoid at which the reference target is located.
Further, the S2 includes:
s21, describing the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta by using a polynomial fitting model, wherein the description formula is as follows:
Figure RE-GDA0001571132990000032
wherein phiunw(theta) is the unwrapping phase,
Figure RE-GDA0001571132990000033
fitting a polynomial model, p, of the spatial variation of the unwrapping phase with radar view angle θiN is (N +1) polynomial fitting coefficients of the model;
s22, selecting M different unwrapping phases of the radar view angle theta;
s23, calculating the polynomial fitting coefficient according to the unwrapping phase and the spatial variation relationship of the M different radar view angles theta, wherein the calculation formula of the polynomial fitting coefficient is as follows:
Figure RE-GDA0001571132990000041
wherein phiunw(θi) I 1, 2.. M is the unwrapping phase of M different radar views θ, and the matrix inversion calculation is performed based on a least squares algorithm.
Further, the S3 includes:
determining the sensitivity of the unwrapping phase to the radar view angle theta according to the polynomial fitting coefficient, wherein the specific calculation formula of the sensitivity of the radar view angle theta is as follows:
Figure RE-GDA0001571132990000042
wherein
Figure RE-GDA0001571132990000043
Is the partial derivative of the unwrapped phase to the radar perspective, i.e. the sensitivity, p i1,2, N being N polynomial fitting coefficients of the model;
determining the sensitivity of the slant range to the radar visual angle according to the height information of the reference target, the track and the slant range parameter, wherein the specific calculation formula of the sensitivity of the slant range to the radar visual angle is as follows:
Figure RE-GDA0001571132990000044
wherein the content of the first and second substances,
Figure RE-GDA0001571132990000045
computing symbols for partial derivatives, representing the computation
Figure RE-GDA0001571132990000046
Partial derivatives of the radar perspective theta, i.e. the slant range r1Sensitivity to the radar perspective θ.
Further, the step S4 specifically includes:
s41, according to the sensitivity of the unwrapping phase to the radar visual angle theta, removing the slant distance r1Under the condition of sensitivity to the radar visual angle theta, directly calculating a base line length initial value of the space-based interference imaging radar altimeter;
s42, according to the initial value of the length of the base line and the slope distance parameter, the slope distance r is not removed1And calculating the accurate base line length B and the base line inclination angle α under the condition of sensitivity to the radar visual angle theta.
Further, in the step S5, it includes:
s51, respectively calculating M interference phase offset estimated values corresponding to the unwrapping phases of M different radar visual angles theta according to the accurate base line length B and the base line inclination angle α;
s52, calculating the average value of the estimated values of M interference phase offsets to obtain the estimated value phi of the interference phase offset0Wherein the interference phase offset is estimated by0The calculation formula of (2) is as follows:
Figure RE-GDA0001571132990000051
wherein B isx、ByThe accurate base line length B is a horizontal component and a vertical component respectively, and the lambda is the carrier wave wavelength of the space-based interference imaging radar altimeter.
The invention also relates to an interference calibration system based on the space-based interference imaging radar altimeter, which comprises the following components: the device comprises a radar visual angle calculation module, a fitting coefficient calculation module, a sensitivity calculation module, an interference baseline determination module and an interference phase bias determination module;
the radar visual angle calculation module is used for determining a radar visual angle theta according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interference imaging radar altimeter;
the fitting coefficient calculation module is used for performing polynomial fitting on the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta to determine a polynomial fitting coefficient;
the sensitivity calculation module is used for determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar visual angle theta according to the height information of the reference target, the track and the slant range parameter;
the interference baseline determining module is used for calculating a baseline length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar viewing angle theta, and determining an accurate baseline length B and a baseline inclination angle α according to the baseline length initial value, the slope parameter and the sensitivity of the slope to the radar viewing angle;
the interference phase offset determining module is used for determining an estimated value phi of the interference phase offset according to the accurate baseline length B and the baseline inclination angle α0。
The invention has the beneficial effects that: through the system, a polynomial fitting coefficient describing the spatial change relationship of the unwrapping phase of the reference target along with the radar visual angle is determined, the accurate baseline length and baseline inclination angle can be obtained by a two-step method when the baseline length is calculated subsequently according to the polynomial fitting coefficient, and finally the accurate estimated value of interference phase offset can be obtained according to the accurate baseline length and baseline inclination angle, so that the phase error is greatly reduced, meanwhile, the high-order micro component introduced by the sensitivity of the slant distance to the radar visual angle in the interference phase and the influence of the earth curvature are considered, so that the estimated values of the baseline length, the baseline inclination angle and the interference phase offset are more accurately measured, in addition, the sensitivity of the unwrapping phase to the radar visual angle is estimated through fitting, the estimation of the interference baseline and the interference phase offset can be separated, and the estimation of the baseline length, the baseline inclination angle and the interference phase offset are finally realized, A complete estimate of baseline tilt angle and interferometric phase offset.
Further, the radar perspective calculation module is specifically configured to determine the radar perspective θ according to the following formula:
Figure RE-GDA0001571132990000061
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereThe parameter represents the influence of the curvature of the earth for the radius of the reference ellipsoid at the position of the reference target, and h is height information of the reference target.
Further, the fitting coefficient calculation module is specifically configured to determine the polynomial fitting coefficient according to the following formula:
Figure RE-GDA0001571132990000071
wherein phiunw(θi) I 1, 2.. M is the unwrapping phase of M different radar views θ, and the matrix inversion calculation is performed based on a least squares algorithm.
Further, the interference baseline determining module is specifically used for determining the sensitivity of the unwrapping phase to the radar view angle theta and removing the slant distance r1Under the condition of sensitivity to the radar visual angle theta, directly calculating a base line length initial value of the space-based interference imaging radar altimeter;
and the system is used for removing the slope distance r according to the initial value of the length of the base line and the slope distance parameter without removing the slope distance r1And calculating the accurate base line length B and the base line inclination angle α under the condition of sensitivity to the radar visual angle theta.
It also relates to a computer device comprising: a processor, a memory and a computer program stored on said memory and executable on said processor, said processor implementing the steps of the interferometric scaling method as described above when executing said program.
The invention has the beneficial effects that: the method comprises the steps of determining a polynomial fitting coefficient describing the spatial change relationship of the unwrapping phase of a reference target along with the radar visual angle through the computer equipment, acquiring accurate baseline length and baseline inclination angle by utilizing a two-step method when subsequently calculating the baseline length according to the polynomial fitting coefficient, and finally obtaining an accurate estimated value of interference phase offset according to the accurate baseline length and baseline inclination angle, thereby greatly reducing phase errors A complete estimate of baseline tilt angle and interferometric phase offset.
Drawings
FIG. 1 is a flow chart of an interferometric calibration method of a space-based interferometric imaging radar altimeter of the present invention;
FIG. 2 is a schematic diagram of an interferometric calibration system for a space-based interferometric imaging radar altimeter of the present invention;
FIG. 3 is a flow chart of the interferometric calibration method of the present invention;
FIG. 4 is a schematic diagram of the interferometric phase comparison of an interferometric imaging radar altimeter (left image) of the present invention with a conventional interferometric synthetic aperture radar (right image);
FIG. 5 is a schematic diagram of the interferometric phase spectrum comparison of an interferometric imaging radar altimeter (left image) of the present invention with a conventional interferometric synthetic aperture radar (right image);
FIG. 6 is a schematic diagram of the elevation error for interferometric calibration result inversion of the present invention.
Detailed Description
The principles and features of this invention are described below in conjunction with the following drawings, which are set forth by way of illustration only and are not intended to limit the scope of the invention.
As shown in fig. 1 and 3, embodiment 1 of the present invention provides an interference calibration method for a space-based interferometric imaging radar altimeter, S1, determining a radar view angle according to the altitude information of a reference target and the orbit and slope distance parameters obtained by the space-based interferometric imaging radar altimeter;
s2, determining the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle by using a polynomial fitting model, and determining a polynomial fitting coefficient by using a least square algorithm according to the unwrapping phase of the reference target and the spatial variation relation;
s3, determining the sensitivity of the unwrapping phase to the radar perspective according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar perspective according to the height information of the reference target, the track and the slant range parameter;
s4, calculating a base line length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar visual angle, and determining an accurate base line length and an accurate base line inclination angle according to the base line length initial value, the inclination distance parameter and the sensitivity of the inclination distance to the radar visual angle;
and S5, determining the estimated value of the interference phase offset according to the accurate base line length and the base line inclination angle.
It should be noted that, in this embodiment 1, the reference target is specifically an inland lake, the type of reference target has flatness, the height of the type of reference target can be regarded as a value, since the coherence of the water surface of the interferometric imaging radar altimeter is very high, which is very beneficial to interferometric phase unwrapping, the relationship between the sensitivity of the unwrapping phase to the radar view angle and the interferometric baseline is established based on the spatial variation relationship of the unwrapping phase of the flat lake surface with the radar view angle, the baseline length, the baseline tilt angle and the interferometric phase offset are estimated by using the least square algorithm0。
According to the method of the embodiment 1, a polynomial fitting coefficient describing a spatial change relation of the unwrapping phase of the reference target along with the radar view angle is determined, when the base line length is calculated subsequently according to the polynomial fitting coefficient, a two-step method is utilized to obtain the accurate base line length and the accurate base line inclination angle, and finally the accurate estimated value of interference phase offset can be obtained according to the accurate base line length and the accurate base line inclination angle, so that the phase error is greatly reduced, and meanwhile, the high-order micro component introduced by the sensitivity of the slant range to the radar view angle in the interference phase is considered, so that the estimated values of the measured base line length, the measured base line inclination angle and the interference phase offset are more accurate.
Optionally, in another embodiment 2, the calculation formula for determining the radar view angle θ in S1 includes:
Figure 1
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereH is the height information of the reference target, which is the radius of the reference ellipsoid at the position of the reference target (the parameter reflects the influence of the curvature of the earth).
It should be noted that, in this embodiment 2, the radar view angle θ mentioned in embodiment 1 is determined on the basis of the above embodiment 1, for a lake surface with a known position, a reference ellipsoid radius of the position of a reference target can be determined by a known earth curved surface reference model, and then the height information of the lake surface is known, so that the radar view angle θ can be calculated according to the above formula (1).
Optionally, in another embodiment 3, the S2 includes:
s21, describing the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta by using a polynomial fitting model, wherein the description formula is as follows:
Figure 2
wherein phiunw(theta) is the unwrapping phase,
Figure RE-GDA0001571132990000102
fitting a polynomial model, p, of the spatial variation of the unwrapping phase with radar view angle θiWherein, i is 0,1,2(N +1) polynomial fitting coefficients of the model;
s22, selecting M different unwrapping phases of the radar view angle theta;
s23, calculating the polynomial fitting coefficient according to the unwrapping phase and the spatial variation relationship of the M different radar view angles theta, wherein the calculation formula of the polynomial fitting coefficient is as follows:
Figure 3
wherein p isiN is (N +1) polynomial fitting coefficients of the polynomial fitting model, phiunw(θi) I 1, 2.. M is the unwrapping phase of M different radar views θ, and the matrix inversion calculation is performed based on a least squares algorithm.
It should be noted that, in this embodiment 3, which is explained based on the above embodiment 1 or embodiment 2, in this embodiment 3, in order to solve the polynomial fitting coefficient, the calculation formula for solving the polynomial fitting coefficient is shown as formula (8), and the formula (8) is derived from the following formula, first, the interference phase is known to have the following form according to the prior art
Figure RE-GDA0001571132990000111
The interference phase measured by the actual system is the phase after winding and is affected by the interference phase offset, so if the winding phase is unwrapped, the phase after unwrapping is
Figure RE-GDA0001571132990000112
Wherein phi is0Is the interferometric phase offset of the altimeter.
In equation (3), the left is a known quantity, the right baseline length B, the baseline tilt angle α, and the interference phase offset φ0The unknown quantity, the skew distance r, to be estimated for interferometric scaling1Can be expressed as radar view angle thetaFunction of (2)
Figure RE-GDA0001571132990000113
It can be seen from formula (3) that the unwrapping phase is a function of the radar perspective, and therefore, a polynomial can be used to fit the unwrapping phase with the continuous change of the radar perspective, and thus formula (7) can be obtained, and the least squares solution of formula (7) is formula (8).
Optionally, in another embodiment 4, the S3 includes:
determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the polynomial fitting coefficient, wherein the specific calculation formula of the sensitivity of the unwrapping phase to the radar visual angle theta is as follows:
Figure 4
wherein
Figure RE-GDA0001571132990000122
Is the partial derivative of the unwrapped phase to the radar perspective, i.e. the sensitivity, p i1,2, N being N polynomial fitting coefficients of the model;
determining the sensitivity of the slant range to the radar visual angle according to the height information of the reference target, the track and the slant range parameter, wherein the specific calculation formula of the sensitivity of the slant range to the radar visual angle is as follows:
Figure 5
wherein the content of the first and second substances,
Figure RE-GDA0001571132990000124
computing symbols for partial derivatives, representing the computation
Figure RE-GDA0001571132990000125
Partial derivatives of the radar perspective theta, i.e. the slant range r1For the radar view angle thetaThe sensitivity of (2).
Note that, in this embodiment 4, the calculation sensitivity can be obtained from the formulas (9) and (10).
Optionally, in another embodiment 5, the step S4 specifically includes:
s41, according to the sensitivity of the unwrapping phase to the radar visual angle theta, removing the slant distance r1Under the condition of sensitivity to the radar visual angle theta, directly calculating a base line length initial value of the space-based interference imaging radar altimeter;
s42, according to the initial value of the length of the base line and the slope distance parameter, the slope distance r is not removed1And calculating the accurate base line length B and the base line inclination angle α under the condition of sensitivity to the radar visual angle theta.
It should be noted that, in this embodiment 5, the exact baseline length B and baseline inclination α are obtained through the following process:
AX＝L-L0(11)
wherein the content of the first and second substances,
Figure RE-GDA0001571132990000131
Figure RE-GDA0001571132990000132
Figure RE-GDA0001571132990000133
l is the sensitivity of the unwrapping phase to the radar perspective, L0And M is the number of the unwrapping phases at different selected radar view angles, if M is more than or equal to 2, the equation set can be solved theoretically, and unwrapping phase data participating in calculation can be increased in order to reduce errors caused by phase noise. For such an overdetermined system of equations, the least squares solution is
X＝(ATA)-1AT(L-L0) (15)
From this, it can be obtained that the base line length and the base line inclination angle are respectively
Figure RE-GDA0001571132990000134
Figure RE-GDA0001571132990000141
Due to L in the formula (15)0The calculation of (a) requires a known base length, which is exactly the unknown quantity that needs to be estimated. Therefore, in this embodiment 5, a "two-step method" is adopted to optimize the calibration result. First, neglecting L0Under the condition of removing high-order micro components introduced by the sensitivity of the slant distance to the radar visual angle theta, directly solving to obtain an initial value of the length of a base line; second, using the estimated baseline length initial value to calculate L0The baseline length and baseline tilt angle are re-solved (i.e., without removing the high order minor components introduced by the sensitivity of the pitch to the radar perspective θ). In actual calculation, due to L0The components are small, the first step estimates already the very high accuracy of the baseline length initial value, and therefore the second step essentially updates the baseline tilt α.
It should be further explained that the equation represented by the formula (11) is derived by first obtaining derivatives of the radar view angle θ at both sides of the formula (3) respectively
Figure RE-GDA0001571132990000142
Wherein, BxAnd ByThe component sizes of the base length in the horizontal and vertical directions
Bx＝Bcosα
By＝Bsinα (6)
It can be found in equation (5) that, after derivation, the interference phase offset is eliminated because it is a constant, and only the interference baseline is still unknown, equation (5) is rewritten into a matrix form to obtain the equation represented by equation (11), and the least squares solution of equation (11) is equation (15).
Alternatively, in the step S5 described in another embodiment 6, it includes:
s51, respectively calculating M interference phase offset estimated values corresponding to the unwrapping phases of M different radar visual angles theta according to the accurate base line length B and the base line inclination angle α;
s52, calculating the average value of the estimated values of M interference phase offsets to obtain the estimated value phi of the interference phase offset0Wherein the interference phase offset is estimated by0The calculation formula of (2) is as follows:
Figure 6
wherein B isx、ByThe accurate base line length B is a horizontal component and a vertical component respectively, and the lambda is the carrier wave wavelength of the space-based interference imaging radar altimeter.
It should be noted that, in this embodiment 6, a further explanation is made on the basis of the above embodiment 5, where M different unwrapping phases of the radar view angle θ are selected, M estimated interference phase offsets corresponding to the unwrapping phases of the radar view angle θ are calculated according to the M different unwrapping phases, and an estimated interference phase offset value Φ is solved according to the formula (18)0。
In this embodiment 6, the averaging performed in the estimation of the interference phase offset mainly reduces the estimation error introduced by the phase noise and improves the estimation accuracy. Thus, estimates of baseline length, baseline tilt angle, and interferometric phase offset are solved.
As shown in fig. 2, embodiment 7 of the present invention further relates to an interferometric calibration system for a space-based interferometric imaging radar altimeter, where the interferometric calibration system includes: the device comprises a radar visual angle calculation module, a fitting coefficient calculation module, a sensitivity calculation module, an interference baseline determination module and an interference phase bias determination module;
the radar visual angle calculation module is used for determining a radar visual angle theta according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interference imaging radar altimeter;
the fitting coefficient calculation module is used for performing polynomial fitting on the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta to determine a polynomial fitting coefficient;
the sensitivity calculation module is used for determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar visual angle theta according to the height information of the reference target, the track and the slant range parameter;
the interference baseline determining module is used for calculating a baseline length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar viewing angle theta, and determining an accurate baseline length B and a baseline inclination angle α according to the baseline length initial value, the slope parameter and the sensitivity of the slope to the radar viewing angle;
the interference phase offset determining module is used for determining an estimated value phi of the interference phase offset according to the accurate baseline length B and the baseline inclination angle α0。
It should be noted that, in this embodiment 7, the reference target is specifically an inland lake, the type of reference target has flatness, and the height of the reference target can be regarded as a value, because the coherence of the interference imaging radar altimeter on the water surface is very high, which is very beneficial to the unwrapping of the interference phase, based on the spatial variation relationship of the unwrapping phase on the flat lake surface along with the radar viewing angle, the relationship between the sensitivity of the unwrapping phase on the radar viewing angle and the interference baseline is established, and the baseline length, the baseline inclination angle and the interference phase offset are estimated by using the least square algorithm. The specific process is that a radar visual angle theta is determined according to the height information of a reference target and the track and slope distance parameters obtained through a space-based interference imaging radar altimeter; after the radar visual angle theta is determined, the unwrapping phase of the reference target follows the radar visual angle thetaThe space change coefficient is subjected to polynomial fitting to determine a polynomial fitting coefficient, then the sensitivity of the unwrapping phase to the radar visual angle theta is determined according to the polynomial fitting coefficient, finally the initial value of the length of the baseline of the space-based interferometric imaging radar altimeter can be estimated according to the sensitivity, the accurate length B of the baseline and the accurate inclination angle α of the baseline are determined by utilizing the estimated initial value of the length of the baseline, and the estimated value phi of the interferometric phase offset is determined according to the accurate length B of the baseline and the accurate inclination angle α of the baseline0。
With the system of this embodiment 7, a polynomial fitting coefficient describing a spatial variation relationship of the unwrapping phase of the reference target with the radar view angle is determined first, and according to the polynomial fitting coefficient, when the baseline length is calculated in the subsequent process, a "two-step method" is used to obtain an accurate baseline length and baseline tilt angle, and finally, an accurate estimated value of interference phase offset can be obtained according to the accurate baseline length and baseline tilt angle, so that the phase error is greatly reduced.
Optionally, in another embodiment 8, the radar view angle θ calculating module is specifically configured to determine the radar view angle θ according to the following formula:
Figure RE-GDA0001571132990000171
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereH is the height information of the reference target, which is the radius of the reference ellipsoid at the position of the reference target (the parameter reflects the influence of the curvature of the earth).
Optionally, in another embodiment 9, the fitting coefficient calculating module is specifically configured to determine the polynomial fitting coefficient according to the following formula:
Figure RE-GDA0001571132990000172
wherein p isiN is (N +1) polynomial fitting coefficients of the polynomial fitting model, phiunw(θi) I 1, 2.. M is the unwrapping phase of M different radar views θ, and the matrix inversion calculation is performed based on a least squares algorithm.
Optionally, in another embodiment 10, the interference baseline determining module is specifically configured to remove the slant distance r according to the sensitivity of the unwrapping phase to the radar view angle θ1Under the condition of sensitivity to the radar visual angle theta, directly calculating a base line length initial value of the space-based interference imaging radar altimeter;
and the system is used for removing the slope distance r according to the initial value of the length of the base line and the slope distance parameter without removing the slope distance r1And calculating the accurate base line length B and the base line inclination angle α under the condition of sensitivity to the radar visual angle theta.
Embodiment 11 of the present invention also relates to a computer device, including: a processor, a memory and a computer program stored on said memory and executable on said processor, said processor implementing the steps of the interferometric scaling method as described above when executing said program.
It should be noted that, in this embodiment 11, the reference target specifically refers to an inland lake, and the type of reference target has flatness, and the height thereof can be regarded as a value, and since the coherence of the interference imaging radar altimeter on the water surface is very high, it is very beneficial to the interference phase unwrapping, a relationship between the sensitivity of the unwrapping phase to the radar viewing angle and the interference baseline is established based on the spatial variation relationship of the unwrapping phase on the flat lake surface along with the radar viewing angle, and the baseline length, the baseline inclination angle, and the interference phase offset are estimated by using the least square algorithm. The specific process is that a radar visual angle theta is determined according to the height information of a reference target and the track and slope distance parameters obtained through a space-based interference imaging radar altimeter; after the radar visual angle theta is determined, the unwrapping phase of the reference target follows the radar visual angle thetaPerforming polynomial fitting on the space change coefficient of the radar visual angle theta to determine a polynomial fitting coefficient, then determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the polynomial fitting coefficient, finally estimating the initial value of the base length of the space-based interferometric imaging radar altimeter according to the sensitivity, determining the accurate base length B and the base inclination angle α by utilizing the estimated initial value of the base length, and determining the estimated value phi of the interferometric phase offset according to the accurate base length B and the base inclination angle α0。
By the computer device in this embodiment 11, a polynomial fitting coefficient describing a spatial variation relationship of the unwrapping phase of the reference target with the radar view angle is determined first, and according to the polynomial fitting coefficient, when the baseline length is calculated in the subsequent process, a "two-step method" is used to obtain an accurate baseline length and baseline tilt angle, and finally, an accurate estimated value of interference phase offset can be obtained according to the accurate baseline length and baseline tilt angle, so that the phase error is greatly reduced, and meanwhile, because the influence of the earth curvature and a high-order micro component introduced by the sensitivity of the slant range to the radar view angle in the interference phase are considered, the estimated values of the measured baseline length, the baseline tilt angle and the interference phase offset are more accurate.
For the above examples 1-11, the following are experiments performed on these examples:
the effectiveness of the technical scheme of the invention is verified by using simulation test data. In the simulation, a typical Ku-band interferometric imaging Radar altimeter is compared with a C-band interferometric synthetic aperture Radar (SRTM) which was carried on a spacecraft Radar terrain mapping mission (SRTM) which is commonly completed in the united states, germany and italy in 2000, and simulation parameters are shown in table 1.
TABLE 1 Radar Primary simulation parameters
Figure RE-GDA0001571132990000191
Assuming that the ground height is zero and the observation bandwidth is 35KM, the interference phase obtained by the method is as shown in fig. 4, and it can be seen that the interference phase of the altimeter of the interferometric imaging radar is very sparse, and the interference phase fringes exhibit drastic changes in the whole observation bandwidth, i.e. dense at the near end and sparse at the far end, while the interference phase of the traditional interferometric synthetic aperture radar is dense, and the fringe density degree is uniformly distributed in the whole observation bandwidth.
Accordingly, as shown in fig. 5, it can be seen that the frequency spectrum of the conventional interferometric synthetic aperture radar has an amplitude similar to a rectangle, the maximum and minimum frequencies can be estimated more accurately in the prior art, while the frequency spectrum of the interferometric imaging radar altimeter is "fat", and the dynamic range of the corresponding non-zero frequency component is large, so that the upper and lower limits of the frequency are difficult to estimate accurately in the prior art. Therefore, the prior art scheme can not accurately finish interference calibration for the interference imaging radar altimeter.
The results of the interference calibration parameters obtained by the technical scheme of the invention are shown in table 2.
TABLE 2 interferometric calibration results of the inventive solution
Figure RE-GDA0001571132990000193
The elevation inversion is carried out by utilizing the interferometric calibration parameters, the elevation error is shown in fig. 6, the result on only one distance line (from the near distance to the far distance of a certain fixed observation time point of the altimeter) is displayed in the diagram for clarity, the results on different distance lines are consistent in practice, and all errors are within 1MM, so that the technical scheme of the invention is accurate in result and feasible.
In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (4)

1. An interference calibration method for a space-based interference imaging radar altimeter is characterized by comprising the following steps:
s1, determining a radar view angle according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interferometric imaging radar altimeter;
the calculation formula for determining the radar view angle θ in S1 includes:
Figure FDA0002314699500000011
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereThe radius of a reference ellipsoid at which the reference target is located, and h is height information of the reference target;
s2, determining the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle by using a polynomial fitting model, and determining a polynomial fitting coefficient by using a least square algorithm according to the unwrapping phase of the reference target and the spatial variation relation;
the S2 includes:
s21, describing the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta by using a polynomial fitting model, wherein the description formula is as follows:
Figure FDA0002314699500000012
wherein phiunw(theta) is the unwrapping phase,
Figure FDA0002314699500000013
fitting a polynomial model, p, of the spatial variation of the unwrapping phase with radar view angle θiN is (N +1) polynomial fitting coefficients of the model;
s22, selecting M different unwrapping phases of the radar view angle theta;
s23, calculating the polynomial fitting coefficient according to the unwrapping phase and the spatial variation relationship of the M different radar view angles theta, wherein the calculation formula of the polynomial fitting coefficient is as follows:
Figure FDA0002314699500000021
wherein phi isunw(θi) 1,2, wherein M is unwrapping phases of M different radar view angles θ, and matrix inversion calculation is performed based on a least square algorithm;
s3, determining the sensitivity of the unwrapping phase to the radar perspective according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar perspective according to the height information of the reference target, the track and the slant range parameter;
s4, calculating a base line length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar visual angle, and determining an accurate base line length and an accurate base line inclination angle according to the base line length initial value, the inclination distance parameter and the sensitivity of the inclination distance to the radar visual angle;
the step S4 specifically includes:
s41, according to the sensitivity of the unwrapping phase to the radar visual angle theta, removing the slant distance r1Under the condition of sensitivity to the radar visual angle theta, directly calculating a base line length initial value of the space-based interference imaging radar altimeter;
s42, according to the initial value of the length of the base line and the slope distance parameter, the slope distance r is not removed1Calculating the accurate baseline length B and the baseline inclination angle α under the condition of sensitivity to the radar viewing angle theta;
and S5, determining the estimated value of the interference phase offset according to the accurate base line length and the base line inclination angle.
2. The interferometric calibration method of claim 1, wherein S3 includes:
determining the sensitivity of the unwrapping phase to the radar view angle theta according to the polynomial fitting coefficient, wherein the specific calculation formula of the sensitivity of the radar view angle theta is as follows:
Figure FDA0002314699500000022
wherein
Figure FDA0002314699500000031
Is the partial derivative of the unwrapped phase to the radar perspective, i.e. the sensitivity, pi1,2, N being N polynomial fitting coefficients of the model;
determining the sensitivity of the slant range to the radar visual angle according to the height information of the reference target, the track and the slant range parameter, wherein the specific calculation formula of the sensitivity of the slant range to the radar visual angle is as follows:
Figure FDA0002314699500000032
wherein the content of the first and second substances,
Figure FDA0002314699500000033
computing symbols for partial derivatives, representing the computation
Figure FDA0002314699500000034
Partial derivatives of the radar perspective theta, i.e. the slant range r1Sensitivity to the radar perspective θ.
3. The interferometric calibration method of claim 1, wherein the step S5 includes:
s51, respectively calculating M interference phase offset estimated values corresponding to the unwrapping phases of M different radar visual angles theta according to the accurate base line length B and the base line inclination angle α;
s52, calculating the average value of the estimated values of M interference phase offsets to obtain the estimated value phi of the interference phase offset0Wherein the interference phase offset is estimated by0The calculation formula of (2) is as follows:
Figure FDA0002314699500000035
wherein B isx、ByThe accurate base line length B is a horizontal component and a vertical component respectively, and the lambda is the carrier wave wavelength of the space-based interference imaging radar altimeter.
4. An interferometric calibration system for a space-based interferometric imaging radar altimeter, the interferometric calibration system comprising: the device comprises a radar visual angle calculation module, a fitting coefficient calculation module, a sensitivity calculation module, an interference baseline determination module and an interference phase bias determination module;
the radar visual angle calculation module is used for determining a radar visual angle theta according to the height information of the reference target and the track and slope distance parameters obtained by the space-based interference imaging radar altimeter;
the radar perspective calculation module is specifically configured to determine the radar perspective θ according to the following formula:
Figure FDA0002314699500000041
wherein R issDistance parameter from main antenna to geocenter calculated for orbit parameter measured by space-based interferometric imaging radar altimeter, r1Is the calculated slope distance R according to the sampling delay of the space-based interference imaging radar altimetereThe radius of a reference ellipsoid at which the reference target is located, and h is height information of the reference target;
the fitting coefficient calculation module is used for performing polynomial fitting on the spatial variation relation of the unwrapping phase of the reference target along with the radar visual angle theta to determine a polynomial fitting coefficient;
the fitting coefficient calculation module is specifically configured to determine the polynomial fitting coefficient according to the following formula:
Figure FDA0002314699500000042
wherein phiunw(θi) 1,2, wherein M is unwrapping phases of M different radar view angles θ, and matrix inversion calculation is performed based on a least square algorithm;
the sensitivity calculation module is used for determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the polynomial fitting coefficient, and determining the sensitivity of the slant range to the radar visual angle theta according to the height information of the reference target, the track and the slant range parameter;
the interference baseline determining module is used for calculating a baseline length initial value of the space-based interference imaging radar altimeter according to the sensitivity of the unwrapping phase to the radar viewing angle theta, and determining an accurate baseline length B and a baseline inclination angle α according to the baseline length initial value, the slope parameter and the sensitivity of the slope to the radar viewing angle;
the interference baseline determining module is specifically used for determining the sensitivity of the unwrapping phase to the radar visual angle theta according to the sensitivity of the unwrapping phase and removing the slant distance r1Under the condition of sensitivity to the radar visual angle theta, directly estimating a base line length initial value of the space-based interference imaging radar altimeter;
and the system is used for removing the slope distance r according to the initial value of the length of the base line and the slope distance parameter without removing the slope distance r1Calculating the accurate baseline length B and the baseline inclination angle α under the condition of sensitivity to the radar viewing angle theta;
the interference phase offset determining module is used for determining an estimated value phi of the interference phase offset according to the accurate baseline length B and the baseline inclination angle α0。
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