A method to improve the revisit coverage capability of high-orbit SAR

Abstract

The present invention belongs to the field of synthetic aperture radar technology and discloses a method for improving the revisit coverage capability of high-orbit SAR: step 1: reading satellite data; step 2: calculating the trajectory of the satellite's subsatellite point; step 3: when the geographical latitude of the subsatellite point decreases over time, proceed to step 4; otherwise, jump to step 6; step 4: calculating the time it takes for the satellite's subsatellite point trajectory to move to the bottom of an 8-shaped figure; step 5: the satellite's side view direction is switched from the right side view to the left side view through attitude adjustment, and jump to step 8; step 6: calculating the time it takes for the satellite's subsatellite point to move to the top of the 8-shaped figure; step 7: the satellite's side view direction is switched from the left side view to the right side view through attitude adjustment, and enter step 8; step 8: switching the side view direction every half of the satellite's operating cycle. The present invention adjusts the radar beam illumination left and right side views at a fixed position on the orbit so that the beam illumination range is always within the national coverage area, thereby significantly improving the revisit coverage capability of the high-orbit SAR.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthetic aperture radar and relates to a method for improving the revisit coverage capability of a high-orbit SAR. Background Art

[0002] Satellite ground coverage refers to the effective area of ​​the Earth's surface that can be observed by a spacecraft's instruments or antennas at a specific moment or over an extended period of time. Effective coverage of specific locations and areas is a key factor in mission design. Spaceborne SAR typically has two metrics: maximum revisit time and coverage time. The maximum revisit time refers to the longest interval during which any point on the simulation grid is not covered by the satellite. This statistical characteristic provides the worst-case scenario for the target point and is equal to the longest response time. The coverage time refers to the length of time that any point on the simulation grid is illuminated by the satellite beam. This statistical characteristic provides the best-case scenario for the target point and is equal to the observable duration. Typically, STK (Satellite Tool Kit) is used to simulate satellite revisit coverage capabilities.

[0003] In the revisit coverage improvement scheme, the orbital altitude, eccentricity, and orbital inclination are determined during the overall satellite design and cannot be changed. Traditional low-orbit SAR adjusts the local time of the descending node and the current satellite strip number to represent the satellite's ascending node right ascension and latitude argument, respectively. However, the beam pointing adjustment distance of this method is very limited and cannot achieve a span across the northern and southern hemispheres. The revisit coverage improvement method is also often used in satellite networking. It improves the constellation's ground coverage capability by correctly designing the total number of satellites in the network, the number of orbital planes, the inclination of each orbital plane, the number of satellites in each orbital plane, and the relative positions of each satellite. Since high-orbit SAR only has a single satellite, this method is also not applicable.

[0004] High-orbit SAR is the world's first satellite. Currently, the geosynchronous orbit satellites are mainly concentrated in the fields of navigation, communication, optical remote sensing, etc. They all have sub-satellite point illumination, the beam center points to the center of the earth, or the beam covers the entire territory of China. It is different from the SAR side-looking imaging mode beam illumination method, so it has no reference value.

[0005] Existing research on high-orbit SAR beam and satellite attitude adjustment primarily focuses on yaw correction and coordinated illumination from dual-stations in high and low orbits. Yaw correction technology adjusts the beam center direction to compensate the Doppler center to zero while ensuring that the beam illuminates the Earth's surface. This achieves full-orbit side-view imaging and reduces the difficulty of high-orbit SAR imaging processing. However, this method still points the beam toward the Northern Hemisphere half the time and toward the Southern Hemisphere the other half, which does not help improve the satellite's revisit coverage capability. Coordinated illumination from dual-stations in high and low orbits is applied to bistatic SAR based on high-orbit illumination sources. This technology uses high-orbit SAR satellites as illumination sources and airborne or LEO platforms as receiving stations. Beam adjustment is intended for coordination between the transmitting and receiving platforms and has nothing to do with improving satellite revisit coverage.

[0006] In summary, no research on methods to improve the revisit coverage capability of high-orbit SAR satellites has been found in the public literature. Summary of the Invention

[0007] The technical problem solved by the present invention is that during the research on high-orbit SAR satellites, it was found that due to the special nature of their orbits, one orbit illuminates the northern hemisphere half of the time and the southern hemisphere the other half of the time. As a result, the beam illumination time for the Chinese territorial coverage area, which is of key concern to high-orbit SAR, is halved, and the satellite payload's revisit coverage capability is greatly reduced.

[0008] In order to solve the above problems, the present invention provides the following technical solutions:

[0009] A method for improving the revisit coverage capability of a high-orbit SAR includes the following steps:

[0010] Step 1: Read satellite data and obtain six orbital numbers, including: orbital semi-major axis , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node and mean anomaly ;

[0011] Step 2: Calculate the current time in real time based on the six satellite orbit numbers Satellite subsatellite point trajectory;

[0012] Step 3: The geographical latitude of the sub-satellite point at each moment obtained in step 2 , determine the direction of the sub-satellite point trajectory, specifically: when the geographical latitude of the sub-satellite point As time decreases, the satellite subsatellite point trajectory moves from top to bottom and enters step 4; on the contrary, when the geographic latitude As time increases, the satellite subsatellite point trajectory moves from bottom to top, and jumps to step 6;

[0013] Step 4: Calculate the time it takes for the satellite's subsatellite point trajectory to reach the bottom of the "8" shape ;

[0014] Step 5: When the satellite reaches At this moment, the satellite side view direction is switched from right view to left view through attitude adjustment, and then jump to step eight;

[0015] Step 6: Calculate the time it takes for the satellite's subsatellite point to move to the top of the "8" shape ;

[0016] Step 7: When the satellite reaches At this moment, the satellite's side view direction is switched from the left side view to the right side view through attitude adjustment, and then enters step eight;

[0017] Step 8: After this, every At this moment, the satellite automatically switches the side view direction, is the satellite operation period.

[0018] The beneficial effects of the present invention compared with the prior art are:

[0019] In response to the existing problems in the revisit coverage capability of high-orbit SAR, the present invention provides a method for improving the revisit coverage capability of high-orbit SAR. By adjusting the radar beam illumination to the left and right sides at a fixed position on the orbit, the beam illumination range is always located in the area where the Chinese territory is located in the northern hemisphere, thereby improving the revisit coverage capability of high-orbit SAR and effectively solving the problem of insufficient revisit coverage capability of high-orbit SAR.

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The left and right side field of view of the high-orbit SAR;

[0022] Figure 2 Schematic diagram of the left and right side view switching position of the method of the present invention;

[0023] The present invention is further explained below with reference to the accompanying drawings and specific embodiments. DETAILED DESCRIPTION

[0024] The present invention provides a method for improving the revisit coverage capability of a high-orbit SAR, which specifically includes the following steps:

[0025] Step 1: Read satellite data and obtain six orbital element information.

[0026] Specifically, the orbit of each satellite can be described by the six basic orbital numbers, including: orbital semi-major axis , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node and mean anomaly Among them, the right ascension of the ascending node , orbital inclination , argument of perigee Defines the orientation of the satellite orbit relative to the equatorial plane, that is, clarifies the coordinates of the satellite orbit plane; orbital eccentricity , orbital semi-major axis , mean anomaly It represents the orbital geometry and satellite motion characteristics, and reflects the situation of the satellite in the orbital plane.

[0027] Step 2: Calculate the current value in real time based on the six satellite orbit numbers The sub-satellite point trajectory of the satellite at this moment includes the following sub-steps:

[0028] Step 21: The projection of the satellite on the ground is called the subsatellite point. Due to the movement of the satellite itself and the rotation of the earth, the subsatellite point also changes accordingly. This trajectory is called the subsatellite point trajectory. The geographical longitude of the subsatellite point at the time and geographic latitude , the calculation formula is as follows:

[0029]

[0030] Where:

[0031] t —Current time;

[0032] —Earth's average rotational angular velocity constant, ;

[0033] —The time when the satellite passes the ascending node, which is given by the satellite data read in step 1;

[0034] —The angle between the satellite and the ascending node, the solution formula is as follows:

[0035]

[0036]

[0037] in, For the true pericentric angle, is the near point angle.

[0038] ;

[0039] in, is the orbital eccentricity.

[0040] Step 22: calculate the geographical longitude of the sub-satellite point obtained in step 21 Update to get the new geographical longitude of the sub-satellite point , the specific operations are as follows:

[0041] (1) When When the calculated geographical longitude of the sub-satellite point is Add 180°;

[0042] (2) When When the calculated geographical longitude of the sub-satellite point is Subtract 180°;

[0043] (3) When The calculated geographical longitude of the sub-satellite point is constant.

[0044] Step 3: The geographical latitude of the sub-satellite point at each moment obtained in step 2 , determine the direction of the sub-satellite point trajectory, specifically: when the geographical latitude of the sub-satellite point As time decreases, the satellite subsatellite point trajectory moves from top to bottom and enters step 4; on the contrary, when the geographic latitude As time increases, the satellite subsatellite point trajectory moves from bottom to top, and jumps to step six.

[0045] Step 4: Calculate the time it takes for the satellite's subsatellite point trajectory to reach the bottom of the "8" shape .

[0046] Specifically, in orbital dynamics, the intersection line of the orbital plane and the equatorial plane is called the node line, and the intersection point between the node line and the orbit is defined as the ascending node, and the other intersection point is the descending node. It is the time it takes for the satellite's sub-satellite point trajectory to move from bottom to top to the center intersection of the "8" shape.

[0047] According to Kepler's third law, the satellite's orbital period It can be expressed as:

[0048]

[0049] Where, the gravitational constant , is the semi-major axis of the orbit;

[0050] After the satellite sub-satellite point trajectory moves from bottom to top to the center intersection of the "8" shape, it moves to the bottom of the "8" shape after three quarters of the satellite cycle. Therefore, the time it takes for the satellite sub-satellite point trajectory to move to the bottom of the "8" shape is It can be expressed as:

[0051] ;

[0052] Step 5: When the satellite reaches At this moment, the satellite's left and right side viewing fields are as follows Figure 1 As shown, the satellite side view direction is switched from right view to left view through attitude adjustment, as shown in Figure 2 as shown, then skip to step eight.

[0053] Step 6: Calculate the time it takes for the satellite's subsatellite point to move to the top of the "8" shape .

[0054] Specifically, referring to step 4, after the satellite sub-satellite point trajectory moves from bottom to top to the center intersection of the "8" shape, it moves to the top of the "8" shape after a quarter of the satellite cycle. Therefore, the time it takes for the satellite sub-satellite point trajectory to move to the top of the "8" shape is It can be expressed as:

[0055] ;

[0056] Step 7: When the satellite reaches At this moment, the satellite's side view direction switches from left view to right view through attitude adjustment, such as Figure 2 Then go to step eight.

[0057] Step 8: After this, every At this moment, the satellite automatically switches the side view direction.

[0058] The feasibility and effectiveness of the present invention are further illustrated below through simulation experiments.

[0059] Comparing the revisit and coverage times of high-orbit SAR satellites in right- and left-side viewing conditions shows that right-side viewing significantly outperforms left-side viewing. However, with single-side viewing, the beam scans the Northern Hemisphere only half the time, leaving the Southern Hemisphere the remaining half. Therefore, for the Northern Hemisphere, the area of ​​concern, we cannot observe for half the day.

[0060] In the case of dual-side-view observation, the satellite needs to be equipped with and powered on two radar systems at the same time, one on the left and one on the right. The payload equipment of high-orbit SAR satellites under dual-side-view observation is complex and consumes a lot of resources. The platform does not have this capability, so it is not feasible in actual situations.

[0061] In the left-right switching observation scenario, the radar payload remains the same as in the single-side view scenario, but the switching occurs at the upper and lower vertices of the beam's "8" shape. The satellite uses the left side view when flying in an upward-downward direction and the right side view when flying in an upward-downward direction, ensuring that the beam always illuminates the Northern Hemisphere. Regarding the revisit coverage of China's territory, a key concern for high-orbit SAR satellites, tests have shown that the left-right switching scenario significantly outperforms the single-side view scenario. Furthermore, the coverage time for the left-right switching scenario is comparable to that for the single-side view scenario, while the revisit time for the left-right switching scenario is slightly better than that for the dual-side view scenario. Therefore, with the dual-side view scenario, after a single beam sweeps across the territory, the other beam, at the diagonal of the "8," must first circumvent the upper and lower vertices before scanning China. In contrast, the left-right switching scenario allows the beam to turn directly at the upper and lower vertices, eliminating some of the path and shortening the revisit time.

[0062] Simulation results show that the present invention solves the problem of weak revisit coverage capability of high-orbit SAR without increasing any hardware cost, and is suitable for applications such as high-orbit SAR earth observation.

[0063] It should be noted that the contents not described in detail in the specification of the present invention are well-known technologies to those skilled in the art.

Claims

1. A method for improving the revisit coverage capability of high-orbit SAR, characterized in that: The specific steps include: Step 1: Read satellite data and obtain six orbital numbers, including: orbital semi-major axis , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node and mean anomaly ; Step 2: Calculate the current time in real time based on the six satellite orbit numbers Satellite sub-satellite point trajectory; specifically includes the following sub-steps: Step 21, solve the current The geographical longitude of the subsatellite point at the time and geographic latitude , the calculation formula is as follows: Where: t —Current time; —Earth's average rotational angular velocity constant, ; —Time when the satellite passes the ascending node; —The angle between the satellite and the ascending node, the solution formula is as follows: in, For the true pericentric angle, is the angle of approach; ； in, is the orbital eccentricity; Step 22: calculate the geographical longitude of the sub-satellite point obtained in step 21 Update to get the new geographical longitude of the sub-satellite point , the specific operations are as follows: (1) When When the calculated geographical longitude of the sub-satellite point is Add 180°; (2) When When the calculated geographical longitude of the sub-satellite point is Subtract 180°; (3) When The calculated geographical longitude of the sub-satellite point is constant; Step 3: The geographical latitude of the sub-satellite point at each moment obtained in step 2 , determine the direction of the sub-satellite point trajectory, specifically: when the geographical latitude of the sub-satellite point As time decreases, the satellite subsatellite point trajectory moves from top to bottom and enters step 4; on the contrary, when the geographic latitude As time increases, the satellite subsatellite point trajectory moves from bottom to top, and jumps to step 6; Step 4: Calculate the time it takes for the satellite's subsatellite point trajectory to reach the bottom of the "8" shape The calculation formula is as follows: Where: —Satellite operation cycle; —Gravitational constant, take ; — orbital semi-major axis; —The time it takes for the satellite's sub-satellite point trajectory to reach the bottom of the "8" shape; —The time it takes for the satellite to pass the ascending node, that is, the time it takes for the satellite's subsatellite trajectory to move from bottom to top to the center intersection of the "8" shape; — the current moment; Step 5: When the satellite reaches At this moment, the satellite side view direction is switched from right view to left view through attitude adjustment, and then jump to step eight; Step 6: Calculate the time it takes for the satellite's subsatellite point to move to the top of the "8" shape ; The calculation formula is as follows: Where: —The time it takes for the satellite's sub-satellite point trajectory to reach the top of the "8" shape; —Time when the satellite passes the ascending node; —Satellite operation cycle; — the current moment; Step 7: When the satellite reaches At this moment, the satellite's side view direction is switched from the left side view to the right side view through attitude adjustment, and then enters step eight; Step 8: After this, every At this moment, the satellite automatically switches the side view direction, is the satellite operation period.

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