On-orbit direction map calibration method for spaceborne microwave scatterometer based on active calibrator

Abstract

The present invention discloses an on-orbit directional pattern calibration method for a satellite-borne microwave scatterometer based on an active calibrator. The method comprises: obtaining the elevation angle and azimuth angle of the beam at different times within the scatterometer antenna system based on the position information of the active calibrator; obtaining the elevation angle and azimuth angle of the beam at different times within the active calibrator antenna system based on the pointing information and position information of the active calibrator; obtaining the data power received by the active calibrator at different times based on the power signal received by the active calibrator and the elevation angle and azimuth angle of the beam at different times within the active calibrator antenna system; and obtaining the antenna pattern of the scatterometer based on the data power received by the active calibrator at different times. The present invention solves the problem of directional pattern calibration under rapidly changing scatterometer antenna coordinate systems and compensates for the modulation effect of the active calibrator on the satellite-borne directional pattern, thereby improving the on-orbit test accuracy of the antenna pattern.

Description

Technical Field

[0001] The invention belongs to the technical field of space microwave remote sensing, and in particular relates to an on-orbit direction map calibration method of a spaceborne microwave scatterometer based on an active calibrator. Background Art

[0002] So far, there has been no report on the on-orbit directional map calibration method for a conical scanning microwave scatterometer based on an active calibrator. This method is mainly used for on-orbit directional map calibration of satellite-borne payloads such as SAR whose antennas do not rotate. The main implementation methods are as follows:

[0003] like Figure 1 As shown in the figure, a group of active calibrators are placed in a certain area of ​​the surveying strip along the range distribution. The active calibrators receive radar pulses, sample and quantize them through AD, and then store them. The pulse sampling time is also recorded. The data recorded by each receiver can be processed to obtain the corresponding azimuth direction pattern. The data recorded by multiple receivers distributed in the range can be processed to obtain the range direction pattern.

[0004] During operation, the antenna of a conical scanning microwave scatterometer rotates at a speed of 3 to 10 seconds per revolution. This presents the following difficulties compared to existing on-orbit radar directional pattern calibration methods: the scatterometer antenna's pitch and azimuth relative to the active calibrator are constantly changing; and the antenna rotates rapidly. Within a single scatterometer scan cycle, the pointing angle of the active calibrator relative to the scatterometer antenna varies significantly, significantly modulating the shape of the scatterometer antenna's directional pattern. Summary of the Invention

[0005] The technical problem solved by the present invention is to overcome the shortcomings of the existing technology and provide an on-orbit directional pattern calibration method for a satellite-borne microwave scatterometer based on an active calibrator, so as to solve the directional pattern calibration problem under the rapid change of the scatterometer antenna coordinate system, compensate for the modulation effect of the active calibrator on the satellite-borne directional pattern, and improve the on-orbit test accuracy of the antenna directional pattern.

[0006] The object of the present invention is achieved through the following technical solutions: A method for calibrating an on-orbit directional pattern of a spaceborne microwave scatterometer based on an active calibrator, comprising: obtaining the elevation angle and azimuth angle of a beam under the scatterometer antenna system at different times based on position information of the active calibrator; obtaining the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times based on pointing information and position information of the active calibrator; obtaining the data power received by the active calibrator at different times based on a power signal received by the active calibrator and the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times; and obtaining the antenna directional pattern of the scatterometer based on the data power received by the active calibrator at different times.

[0007] In the above-mentioned on-orbit direction map calibration method of a spaceborne microwave scatterometer based on an active calibrator, obtaining the elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times according to the position information of the active calibrator includes: obtaining the vector of the scatterometer pointing to the active calibrator at different times according to the position information of the active calibrator. According to the vector of the scatterometer pointing to the active calibrator at different times Get the vector of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times According to the vector of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times The elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained.

[0008] In the above-mentioned on-orbit direction map calibration method of the satellite-borne microwave scatterometer based on the active calibrator, the vector of the scatterometer pointing to the active calibrator at different times is It is obtained by the following formula:

[0009]

[0010] in, is the position vector of the active calibrator in the ECF coordinate system, is the position vector of the scatterometer in the ECF coordinate system at time t, where t is the time.

[0011] In the above-mentioned on-orbit direction map calibration method of the satellite-borne microwave scatterometer based on the active calibrator, the vector of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times is It is obtained by the following formula:

[0012]

[0013] Among them, M oe is the transformation matrix from the ECF coordinate system to the orbital coordinate system; M bo is the transformation matrix from the orbital coordinate system to the body coordinate system; M rb is the transformation matrix from the system to the scatterometer coordinate system; M ar is the transformation matrix from the scatterometer coordinate system to the antenna coordinate system, is the vector pointing from the scatterometer to the active calibrator at different times.

[0014] In the above-mentioned on-orbit directional diagram calibration method of the spaceborne microwave scatterometer based on the active calibrator, the elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained by the following formula:

[0015]

[0016] in, are vectors Projection on the x, y, and z axes, θ EL,s (t) is the elevation angle of the beam under the scatterometer antenna system at time t, θ AZ,s (t) is the azimuth of the beam under the scatterometer antenna system at time t.

[0017] In the above-mentioned on-orbit direction map calibration method of the satellite-borne microwave scatterometer based on the active calibrator, obtaining the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times according to the direction information and position information of the active calibrator includes: obtaining the vector of the active calibrator pointing to the scatterometer at different times according to the position information of the active calibrator. According to the vector of the active calibrator pointing to the scatterometer at different times Get the vector of the active calibrator pointing to the scatterometer in the antenna coordinate system at different times According to the vector of the active calibrator pointing to the scatterometer at different times in the antenna coordinate system The elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times are obtained.

[0018] In the above-mentioned on-orbit direction map calibration method of satellite-borne microwave scatterometer based on active calibrator, the vector of the active calibrator pointing to the scatterometer at different times is It is obtained by the following formula:

[0019]

[0020] in, is the position vector of the active calibrator in the ECF coordinate system, is the position vector of the scatterometer in the ECF coordinate system at time t.

[0021] In the above-mentioned on-orbit direction map calibration method of the satellite-borne microwave scatterometer based on the active calibrator, the vector of the active calibrator pointing to the scatterometer at different times in the antenna coordinate system is It is obtained by the following formula:

[0022]

[0023] Among them, M ge is the transformation matrix from the ECF coordinate system to the Northeast Sky coordinate system; M ag is the conversion matrix from the northeast sky coordinate system to the active calibrator antenna system; is the vector pointing from the active calibrator to the scatterometer at different times.

[0024] In the above-mentioned on-orbit directional diagram calibration method of the spaceborne microwave scatterometer based on the active calibrator, the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times are obtained by the following formula:

[0025]

[0026] in, are vectors Projection on the x, y, and z axes; θ EL,A (t) is the elevation angle of the beam at time t under the active calibrator antenna system; θ AZ,A (t) is the azimuth angle of the beam under the active calibrator antenna system at time t.

[0027] In the above-mentioned on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator, obtaining the data power received by the active calibrator at different times based on the power signal received by the active calibrator and the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times includes: performing down-conversion, matched filtering, and extraction processing on the power signal received by the active calibrator to obtain the received echo power at different times; performing a table lookup using the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times to obtain the antenna gain value of the active calibrator at different times; and obtaining the slant range of the source calibrator pointing to the scatterometer at different times.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention solves the problem of directional pattern calibration under the rapid change of the scatterometer antenna coordinate system, compensates for the modulation influence of the active calibrator on the satellite-borne directional pattern, and improves the on-orbit test accuracy of the antenna directional pattern. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. The same reference symbols are used throughout the drawings to represent the same components. In the drawings:

[0031] Figure 1 This is a schematic diagram of on-orbit direction map calibration based on an active calibrator in the prior art;

[0032] Figure 2 Schematic diagram of the relative position relationship between the calibrator and the microwave scatterometer provided in an embodiment of the present invention;

[0033] Figure 3 The present invention provides a flowchart of an on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator. DETAILED DESCRIPTION

[0034] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, unless there is a conflict, the embodiments of the present disclosure and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0035] The calibration method of the on-orbit antenna directional diagram involves different coordinate system conversions. Before introducing the calibration method, the coordinate system definitions involved in this method are given first.

[0036] Geocentric Fixed Coordinate System (ECF): Geocentric Fixed Coordinate System is a coordinate system that is fixed to the Earth and rotates with the Earth's rotation. Its origin is at the center of the Earth. e The axis lies in the equatorial plane and points to the intersection of the Greenwich meridian and the equatorial plane. e The axis points to the North Pole along the Earth's rotation axis, e The axis is perpendicular to the X axis in the equatorial plane. e Axis, X e 、Y e , Z e The three coordinate axes satisfy the right-hand coordinate rule.

[0037] Geodetic coordinate system: The geodetic coordinate system is a coordinate system that uses geodetic longitude L, geodetic latitude B, and geodetic altitude H to represent the position of a spatial point. The angle between the meridian plane passing through the ground or spatial point P and the prime meridian plane is the geodetic longitude L of point P. From the prime meridian plane, positive values ​​to the east are called east longitude (0° to 180°), and negative values ​​to the west are called west longitude (0° to -180°). The angle between the ellipsoid normal passing through point P and the equatorial plane is the geodetic latitude B of point P. From the equatorial plane, positive values ​​to the north are called north latitude (0° to 90°), and negative values ​​to the south are called south latitude (0° to -90°). The distance from point P along the ellipsoid normal to the ellipsoid surface is the geodetic altitude H.

[0038] Northeast celestial coordinate system: use ox g y g z g Indicates that the origin is the center of gravity of the carrier, x g The y axis points east. g The axis points north, z g The axis points to the sky. The orientation of the Northeast Celestial Coordinate System relative to the Earth Coordinate System is the geographic location information of the carrier (longitude B and latitude L).

[0039] Satellite orbit coordinate system: The origin of the satellite orbit coordinate system is the satellite center of mass, Z orb The Y axis points to the center of the Earth.orb The axis points in the negative normal direction of the orbital plane, X orb The axis is in the orbital plane and the Z orb The axis is vertical and along the direction of satellite flight, X orb 、Y orb , Z orb The axes follow the right-hand coordinate rule.

[0040] Satellite body coordinate system: The origin of the satellite body coordinate system is located at the center of the satellite-rocket separation plane. b The axis points from the origin to the direction of the satellite's movement, Z b Axis and X b The axis is vertical and points to the center of the earth during operation; b Axis and X b , Z b The axes form a right-hand coordinate system according to the right-hand rule. When the attitude data is 0, the satellite body coordinate system coincides with the satellite orbit coordinate system.

[0041] Scatterometer antenna coordinate system: antenna coordinate system X a Y a Z a When the installation matrix is ​​0, it coincides with the satellite body coordinate system. When the installation matrix is ​​not 0, the antenna coordinate system can be obtained after the installation matrix and the satellite body coordinate system are transformed. AZ is the azimuth of the antenna, defined as the beam angle at X a Z a The azimuth angle of the plane deviating from the Z axis is positive in the positive X direction and negative in the negative X direction. EL is the elevation angle of the antenna, defined as the beam angle at Y a Z a In-plane deviation Z a Azimuth of the axis, positive Y a Direction is positive, negative Y a The direction is negative.

[0042] Active calibrator antenna coordinate system: antenna coordinate system X aARC Y aARC Z aARC When the rotation matrix is ​​0, it coincides with the northeast sky coordinate system. When the rotation matrix is ​​0, the antenna coordinate system can be obtained after the rotation matrix and the satellite's own system are transformed into coordinates. The definitions of the pitch and azimuth angles in the coordinate system are the same as above.

[0043] The on-orbit test method of the satellite-borne microwave scatterometer antenna pattern based on the active calibrator is similar to the ground far-field test method of the antenna. The on-orbit measurement pattern method mainly relies on the change of the relative position relationship between the satellite movement and the microwave scatterometer conical scanning and the calibrator to obtain the antenna gain of multiple points of the microwave scatterometer antenna, such as Figure 2As shown in the figure, the scatterometer can generally receive data for about 6 to 10 scanning cycles when passing through the area where the calibrator is located. In different scanning cycles, the active calibrator can measure different sections of the scatterometer antenna pattern, and the microwave scatterometer antenna pattern can be obtained by long-term accumulation. The relative position relationship between the calibrator and the satellite is shown in the figure. Figure 2 shown.

[0044] This embodiment introduces a method for locating the on-orbit direction map of a scatterometer within a scanning cycle. From the one-way radar equation, we know that:

[0045]

[0046] In the above formula, P p_AD P is the signal power received by the active scaler; t is the scatterometer transmission power; G(θ EL,s ,θ AZ,s ) is the scatterometer at the pitch angle θ EL,s and azimuth angle θ AZ,s Antenna gain; G rc (θ EL,A ,θ AZ,A ) is the active scaler at the pitch angle θ EL,A and azimuth angle θ AZ,A The gain of the antenna; λ is the wavelength of the electromagnetic wave; G re is the receiver gain of the active calibrator; R is the distance from the scatterometer to the active calibrator; L F is the link loss.

[0047] From the above formula, we can see that the power changes received by the active scaler at different times are determined by the following aspects:

[0048] Scatterometer antenna gain (G(θEL,s,θ AZ,s ));

[0049] The gain of the active calibrator antenna (G rc (θ EL,A ,θ AZ,A ));

[0050] The distance from the scatterometer to the active calibrator (R);

[0051] The gain of the active scaler receiver (G re ).

[0052] To measure the scatterometer antenna pattern, it is necessary to correct the gain of the active calibrator receiving antenna, the distance from the scatterometer to the active calibrator, and the gain values ​​of different active calibrator receivers, that is:

[0053]

[0054] Where K is considered to be a constant within a scanning cycle:

[0055]

[0056] G rc (θ EL,A ,θ AZ,A ) is obtained through spatial geometric relationships and the pointing information of the active calibrator;

[0057] R is obtained through spatial geometric relationships.

[0058] Figure 3 FIG1 is a flow chart of an on-orbit direction map calibration method for a satellite-borne microwave scatterometer based on an active calibrator provided by an embodiment of the present invention. Figure 3 As shown, the method includes the following steps:

[0059] According to the position information of the active calibrator, the elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained;

[0060] According to the pointing information and position information of the active calibrator, the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times are obtained;

[0061] The data power received by the active calibrator at different moments is obtained according to the power signal received by the active calibrator and the elevation angle and azimuth angle of the beam under the antenna system of the active calibrator at different moments;

[0062] The antenna pattern of the scatterometer is obtained according to the data power received by the active calibrator at different times.

[0063] Specifically, the following processing steps are included:

[0064] (1) The elevation and azimuth angles of the beam under the scatterometer antenna system at different times

[0065] The vector of the scatterometer pointing to the active calibrator at different times can be obtained through remote sensing data and the position information of the active calibrator.

[0066]

[0067] In the above formula, is the position vector of the active calibrator in the ECF coordinate system, is the position vector of the scatterometer in the ECF coordinate system at time t.

[0068] The vector of the scatterometer pointing to the active calibrator in the antenna coordinate system for:

[0069]

[0070] Where: Moe is the transformation matrix from the ECF coordinate system to the orbital coordinate system; M bo is the transformation matrix from the orbital coordinate system to the body coordinate system; M rb is the transformation matrix from the system to the scatterometer coordinate system; M ar is the transformation matrix from the scatterometer coordinate system to the antenna coordinate system.

[0071] The elevation θ of the vector pointing from the scatterometer to the active calibrator in the scatterometer antenna coordinate system EL,s (t) and azimuth angle θ AZ,s (t) is:

[0072]

[0073] are vectors Projection on the x, y, and z axes.

[0074] (2) The elevation and azimuth angles of the beam at different times under the active calibrator antenna system

[0075] The vector of the active calibrator pointing to the scatterometer at different times can be obtained through remote sensing data and the position information of the active calibrator.

[0076]

[0077] The vector of the active calibrator pointing to the scatterometer in the antenna coordinate system for:

[0078]

[0079] Where: M ge is the transformation matrix from the ECF coordinate system to the Northeast Sky coordinate system; M ag is the conversion matrix from the northeast sky coordinate system to the active calibrator antenna system.

[0080] The elevation θ of the vector pointing from the active calibrator to the scatterometer in the active calibrator antenna coordinate system EL,A and azimuth angle θ AZ,A for:

[0081]

[0082] in, are vectors Projection on the x, y, and z axes.

[0083] (3) Calculate the data power received by the active scaler at different times

[0084] The power signal received by the active calibrator is processed by down-conversion, matched filtering, extraction, etc. to obtain the received echo power P at different times t. p_AD (t);

[0085] The antenna gain value G of the active calibrator at different times is obtained by looking up the table using the elevation azimuth angle calculated in step (2). rc (θ EL,A ,θ AZ,A );

[0086] Calculate the slant distance R of the source calibrator pointing to the scatterometer at different times AS (t).

[0087] (4) Obtaining the cross-sectional direction map

[0088] According to formula (1), the normalized echo power of the active calibrator is matched with the azimuth of the scatterometer antenna using UTC time to obtain the antenna pattern of any slice of the scatterometer. After accumulating multiple scanning cycles and calibration times, the antenna pattern of the scatterometer can be obtained.

[0089] This embodiment solves the problem of directional pattern calibration when the scatterometer antenna coordinate system changes rapidly, and compensates for the modulation effect of the active calibrator on the satellite-borne directional pattern, thereby improving the on-orbit test accuracy of the antenna directional pattern.

[0090] Although the present invention has been disclosed above in terms of preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solutions of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the scope of protection of the technical solutions of the present invention.

Claims

1. A method for calibrating on-orbit direction maps of a spaceborne microwave scatterometer based on an active calibrator, characterized in that include: According to the position information of the active calibrator, the elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained; According to the pointing information and position information of the active calibrator, the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times are obtained; The data power received by the active calibrator at different moments is obtained according to the power signal received by the active calibrator and the elevation angle and azimuth angle of the beam under the antenna system of the active calibrator at different moments; The antenna pattern of the scatterometer is obtained according to the data power received by the active calibrator at different times.

2. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 1, characterized in that: The elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained based on the position information of the active calibrator, including: According to the position information of the active calibrator, the vector of the scatterometer pointing to the active calibrator at different times is obtained; According to the vectors of the scatterometer pointing to the active calibrator at different times, the vectors of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times are obtained; According to the vector of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times, the elevation angle and azimuth angle of the beam in the scatterometer antenna system at different times are obtained.

3. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 2, characterized in that: The vector of the scatterometer pointing to the active calibrator at different times It is obtained by the following formula: in is the position vector of the active calibrator in the ECF coordinate system, is the position vector of the scatterometer in the ECF coordinate system at time t, where t is the time.

4. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 2, characterized in that: The vector of the scatterometer pointing to the active calibrator in the antenna coordinate system at different times It is obtained by the following formula: Among them, M oe is the transformation matrix from the ECF coordinate system to the orbital coordinate system; M bo is the transformation matrix from the orbital coordinate system to the body coordinate system; M rb is the transformation matrix from the system to the scatterometer coordinate system; M ar is the transformation matrix from the scatterometer coordinate system to the antenna coordinate system, is the vector pointing from the scatterometer to the active calibrator at different times.

5. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 2, characterized in that: The elevation angle and azimuth angle of the beam under the scatterometer antenna system at different times are obtained by the following formula: in, are vectors Projection on the x, y, and z axes, θ EL,s (t) is the elevation angle of the beam under the scatterometer antenna system at time t, θ AZ,s (t) is the azimuth of the beam under the scatterometer antenna system at time t.

6. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 1, characterized in that: According to the pointing information and position information of the active calibrator, the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times are obtained, including: According to the position information of the active calibrator, the vector pointing from the active calibrator to the scatterometer at different times is obtained; According to the vectors of the active calibrator pointing to the scatterometer at different times, the vectors of the active calibrator pointing to the scatterometer at different times in the antenna coordinate system are obtained; The elevation angle and azimuth angle of the beam in the active calibrator antenna system at different times are obtained according to the vector of the active calibrator pointing to the scatterometer in the antenna coordinate system at different times.

7. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 6, characterized in that: The vector of the active calibrator pointing to the scatterometer at different times It is obtained by the following formula: in, is the position vector of the active calibrator in the ECF coordinate system, is the position vector of the scatterometer in the ECF coordinate system at time t.

8. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 6, characterized in that: The vector of the active calibrator pointing to the scatterometer in the antenna coordinate system at different times It is obtained by the following formula: Among them, M ge is the transformation matrix from the ECF coordinate system to the Northeast Sky coordinate system; M ag is the conversion matrix from the northeast sky coordinate system to the active calibrator antenna system; is the vector pointing from the active calibrator to the scatterometer at different times.

9. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 6, characterized in that: The elevation angle and azimuth angle of the beam at different times in the active calibrator antenna system are obtained by the following formula: in, are vectors Projection on the x, y, and z axes; θ EL,A (t) is the elevation angle of the beam at time t under the active calibrator antenna system; θ AZ,A (t) is the azimuth angle of the beam under the active calibrator antenna system at time t.

10. The on-orbit direction map calibration method for a spaceborne microwave scatterometer based on an active calibrator according to claim 1, characterized in that: The data power received by the active calibrator at different times is obtained based on the power signal received by the active calibrator and the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times, including: The power signal received by the active calibrator is down-converted, matched filtered and extracted to obtain the received echo power at different times; The antenna gain value of the active calibrator at different times is obtained by looking up the table using the elevation angle and azimuth angle of the beam under the active calibrator antenna system at different times; Obtain the slant distance of the source calibrator pointing to the scatterometer at different times.

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