Method for improving revisit coverage capability of high orbit SAR (Synthetic Aperture Radar)

Abstract

The invention belongs to the technical field of synthetic aperture radars, and discloses a method for improving the revisit coverage capability of a high-orbit SAR. The method comprises the steps of 1, reading satellite data; 2, calculating a sub-satellite point trajectory of the satellite; step 3, when the geographic latitude of the sub-satellite point decreases progressively along with time, entering step 4; otherwise, skipping to step 6; 4, calculating the time when the sub-satellite point track of the satellite moves to the bottom of the 8 shape; 5, switching the satellite side-looking direction from the right side-looking direction to the left side-looking direction through attitude adjustment, and skipping to the step 8; 6, calculating the time when the sub-satellite point of the satellite moves to the top of the 8 shape; 7, switching the satellite side-looking direction from the left side-looking direction to the right side-looking direction through attitude adjustment, and entering the step 8; and step 8, switching the side view direction every more than half of the satellite operation cycle. According to the invention, radar beam irradiation left and right side views are adjusted at the fixed position of the track, so that the beam irradiation range is always located in the land coverage area, and the high-orbit SAR revisit coverage capability is remarkably improved.

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 ability of a high-orbit SAR. Background Art

[0002] The ground coverage of a satellite refers to the effective area of the Earth's surface that can be observed by the instruments or antennas of the spacecraft at a certain moment or over a relatively long period of time. In the design process of a flight mission, the effective coverage of specific positions and regions is a key factor. Spaceborne SAR usually has two indicators: the maximum revisit time and the coverage time. Among them, 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 gives the worst information situation of the target point, and it 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 gives the best information situation of the target point, and it is equal to the observable duration. Usually, we use STK (Satellite Tool Kit) to simulate the information of the satellite's revisit coverage ability.

[0003] In the revisit coverage improvement scheme, the altitude, eccentricity, and orbital inclination of the orbit have been determined during the overall design of the satellite and cannot be changed. Traditional low-orbit SAR adjusts the local time of the descending node and the current strip number of the satellite to represent the right ascension of the ascending node and the latitude argument of the satellite respectively. However, the adjustment distance of the beam pointing of this method is very limited and cannot achieve the crossing of the northern and southern hemispheres. The revisit coverage improvement method is also often used in satellite networking. It realizes the improvement of the constellation's ground coverage ability 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 the satellites. Since there is only a single high-orbit SAR satellite, this method is also not applicable.

[0004] The high-orbit SAR is the world's first satellite. Currently, the geosynchronous orbit on-orbit satellites mainly focus on the fields of navigation, communication, optical remote sensing, etc. They are all nadir-pointing irradiations, with the beam center pointing to the center of the Earth, or the beam covering the whole territory of China. Different from the beam irradiation method of the SAR side-looking imaging mode, they do not have reference significance.

[0005] Existing research on high-orbit SAR beam and satellite attitude adjustment mainly focuses on yaw correction and coordinated illumination between high and low orbits. The yaw correction technology is to adjust the beam center direction to compensate the Doppler center to zero on the premise of ensuring that the beam irradiates the Earth's surface, so as to achieve full-orbit squint imaging and reduce the difficulty of high-orbit SAR imaging processing. However, in this method, the beam still points to the Northern Hemisphere for half of the time and the Southern Hemisphere for the other half of the time, which is not helpful for improving the satellite revisit coverage ability. The coordinated illumination technology between high and low orbits is applied to bistatic SAR based on high-orbit illumination sources. Using a high-orbit SAR satellite as the illumination source and an airborne or LEO platform as the receiving station, the beam adjustment is for the cooperation between the transmitting platform and the receiving platform, and has nothing to do with improving the satellite revisit coverage ability.

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

[0007] The technical problem solved by the present invention is that in the process of researching high-orbit SAR satellites, due to the particularity of their orbits, for half of the time in one orbit, the satellite irradiates the Northern Hemisphere, and for the other half of the time, it irradiates the Southern Hemisphere. For the Chinese territory coverage area that high-orbit SAR focuses on, the beam irradiation time is halved, and the satellite payload revisit coverage ability is greatly reduced.

[0008] To solve the above problems, the present invention provides the following technical solutions: A method for improving the revisit coverage ability of high-orbit SAR, specifically including the following steps: Step 1: Read satellite data to obtain the six orbital elements information, and the six orbital elements include: semi-major axis of the orbit , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node and mean anomaly ; Step 2: According to the six orbital elements information of the satellite, calculate the satellite sub-satellite point trajectory in real time at the current moment ; Step 3: According to the geographical latitude of the sub-satellite point at each moment obtained in Step 2 , judge the movement direction of the sub-satellite point trajectory. Specifically: when the geographical latitude of the sub-satellite point decreases with time, the satellite sub-satellite point trajectory moves from top to bottom, and enter Step 4; otherwise, when the geographical latitude increases with time, the satellite sub-satellite point trajectory moves from bottom to top, and jump to Step 6; Step 4: Calculate the time when the satellite sub-satellite point trajectory moves to the bottom of the "8" shape ; Step Five: When the satellite reaches moment, the satellite's side-looking direction switches from right-looking to left-looking through attitude adjustment, and then jumps to Step Eight; Step Six: Calculate the time when the satellite's sub-satellite point moves to the top of the "8" shape ; Step Seven: When the satellite reaches moment, the satellite's side-looking direction switches from left-looking to right-looking through attitude adjustment, and then enters Step Eight; Step Eight: After that, every time moment passes, the satellite automatically switches the side-looking direction, which is the satellite's operation period.

[0009] The beneficial effects of the present invention compared with the prior art are as follows: The present invention provides a method for improving the revisit coverage ability of a high-orbit SAR, aiming at the problems existing in the revisit coverage ability of a high-orbit SAR. By adjusting the radar beam irradiation on the left and right side-looks at a fixed position in the orbit, the beam irradiation range is always located in the area of China's territory in the Northern Hemisphere, realizing the improvement of the revisit coverage ability of the high-orbit SAR and effectively solving the problem of insufficient revisit coverage ability of the high-orbit SAR.

[0010] The following will further elaborate on the present invention in conjunction with the drawings and embodiments. Description of the Drawings

[0011] Figure 1 is the left and right side-look field of view diagram of the high-orbit SAR; Figure 2 is the schematic diagram of the left and right side-look switching positions of the method of the present invention; The following further explains the present invention in conjunction with the drawings and specific embodiments. Detailed Embodiments

[0012] The present invention provides a method for improving the revisit coverage ability of a high-orbit SAR, which specifically includes the following steps: Step One: Read satellite data to obtain the information of six orbital elements.

[0013] Specifically, the orbit of each satellite can be described by six basic orbital elements, including: semi-major axis of the orbit , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node and mean anomaly . Among them, right ascension of the ascending node , orbital inclination , argument of perigee Defines the orientation of the satellite orbit with respect to the equatorial plane, that is, determines the coordinates of the satellite orbit plane; the orbital eccentricity , the semi-major axis of the orbit , the mean anomaly represent the orbital geometry and the characteristics of the satellite's motion, reflecting the situation of the satellite in the orbital plane.

[0014] Step 2: According to the six orbital elements of the satellite, calculate the ground track of the satellite at the current moment in real time. Specifically, it includes the following sub-steps: Step 21: The projection of the satellite on the ground is called the sub-satellite point. Due to the motion of the satellite itself and the rotation of the Earth, the sub-satellite point also changes accordingly. This trajectory is called the sub-satellite point track. Solve the geographical longitude and geographical latitude of the sub-satellite point at the current moment. The calculation formulas are as follows:

[0015] In the formula: t —The current time; —The constant of the mean angular velocity of the Earth, ; —The time when the satellite crosses the ascending node, given by the satellite data read in Step 1; —The angle between the satellite and the ascending node. The solution formula is as follows:

[0016]

[0017] Among them, is the true anomaly, is the eccentric anomaly.

[0018] ; Among them, is the orbital eccentricity.

[0019] Step 22: Update the geographical longitude of the sub-satellite point calculated in Step 21 to obtain the new geographical longitude of the sub-satellite point. The specific operation is as follows: (1) When , add 180° to the calculated geographical longitude of the sub-satellite point; (2) When , the calculated geographical longitude Subtract 180°; (3) When the calculated geolongitude of the sub-satellite point remains unchanged.

[0020] Step 3: Based on the geolatitude of the sub-satellite point at each moment obtained in Step 2 , determine the movement direction of the sub-satellite point trajectory, specifically: when the geolatitude of the sub-satellite point decreases with time, the sub-satellite point trajectory of the satellite moves from top to bottom, and enter Step 4; conversely, when the geolatitude increases with time, the sub-satellite point trajectory of the satellite moves from bottom to top, and jump to Step 6.

[0021] Step 4: Calculate the time when the sub-satellite point trajectory of the satellite moves to the bottom of the "8" shape .

[0022] Specifically, in orbital dynamics, the intersection line of the orbital plane and the equatorial plane is called the nodal line. Among the two intersections of the nodal line and the orbit, the one where the geolatitude changes from negative to positive is defined as the ascending node, and the other intersection is the descending node. Therefore, the time when the satellite passes through the ascending node is the time when the sub-satellite point trajectory of the satellite moves from bottom to top to the central intersection of the "8" shape.

[0023] According to Kepler's third law, the satellite's orbital period can be expressed as:

[0024] where the geocentric gravitational constant , is the semi-major axis of the orbit; After the sub-satellite point trajectory of the satellite moves from bottom to top to the central intersection of the "8" shape, it moves to the bottom of the "8" shape after three-quarters of the satellite's orbital period. Therefore, the time when the sub-satellite point trajectory of the satellite moves to the bottom of the "8" shape can be expressed as: ; Step 5: When the satellite runs to the moment, the left and right side-looking fields of view of the satellite are as Figure 1 shown. The side-looking direction of the satellite is switched from the right side-looking to the left side-looking through attitude adjustment, as Figure 2 shown, and then jump to Step 8.

[0025] Step 6: Calculate the time when the sub-satellite point of the satellite moves to the top of the "8" shape .

[0026] Specifically, referring to Step 4, after the satellite subsatellite point track moves upward from the bottom to the central intersection of the "8" shape, it moves to the top of the "8" shape after a quarter of the satellite period. Therefore, the time when the satellite subsatellite point track moves to the top of the "8" shape can be expressed as: ; Step 7: When the satellite runs to moment, the satellite's side-looking direction switches from left-looking to right-looking through attitude adjustment, as shown in Figure 2 , and then enters Step 8.

[0027] Step 8: After that, every time passes, the satellite automatically switches the side-looking direction.

[0028] The feasibility and effectiveness of the present invention are further illustrated through simulation experiments below.

[0029] In the case of single-side looking observation, by comparing the revisit and coverage times of the high-orbit SAR satellite under right-looking and left-looking, the right-looking effect is significantly better than the left-looking. However, in the case of single-side looking, the beam scans the Northern Hemisphere only half of the time, and the remaining half of the time is the Southern Hemisphere. Therefore, for the land coverage area of the Northern Hemisphere that we are concerned about, it cannot be observed for half of the day.

[0030] In the case of double-side looking observation, the satellite needs to be equipped with and turn on two sets of radar systems simultaneously, located on the left-looking and right-looking sides respectively. The payload equipment of the high-orbit SAR satellite under double-side looking is complex, consumes a large amount of resources, and the platform does not have this ability. Therefore, it is not feasible in actual situations.

[0031] In the case of left-right side-looking switching observation, like single-side looking, there is still one set of radar payload equipment. However, the left-right side-looking switching is performed at the upper and lower vertices of the beam "8" shape. When the satellite flies from top to bottom, it uses left-looking, and when it flies from bottom to top, it uses right-looking to ensure that the beam is always irradiating the Northern Hemisphere. For the revisit coverage ability of the high-orbit SAR satellite's key concern, the Chinese territory, through experiments, it can be known that the left-right side-looking switching effect is significantly better than the single-side looking situation. At the same time, the coverage time of the left-right side-looking switching is equivalent to that of the single-side looking, and the revisit time of the left-right side-looking is slightly better than that of the double-side looking. Therefore, in the case of double-side looking, after the single-side beam sweeps over the national territory area, the other beam is at the diagonal of the "8" shape and needs to bypass the upper and lower points of the "8" shape to scan the Chinese territory, while the left-right side-looking switching directly turns around at the upper and lower points of the "8" shape, saving part of the path and shortening the revisit time.

[0032] The simulation results show that the present invention well solves the problem of weak revisit coverage ability of high-orbit SAR without adding any hardware costs, and is applicable to applications such as high-orbit SAR earth observation.

[0033] It should be noted that the content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A method for improving the revisit coverage ability of a high-orbit SAR, characterized in that, Specifically, it includes the following steps: Step 1: Read satellite data to obtain the six orbital elements information, where the six orbital elements include: semi-major axis of the orbit , orbital inclination , orbital eccentricity , argument of perigee , right ascension of the ascending node , and mean anomaly ; Step 2: Calculate the satellite's sub-satellite point trajectory in real time based on the six orbital elements information of the satellite at the current moment of the satellite; Step 3: Based on the geographical latitude of the sub-satellite point at each moment obtained in Step 2 , determine the moving direction of the sub-satellite point trajectory. Specifically: when the geographical latitude of the sub-satellite point decreases with time, the sub-satellite point trajectory of the satellite moves from top to bottom, and proceed to Step 4; conversely, when the geographical latitude increases with time, the sub-satellite point trajectory of the satellite moves from bottom to top, and jump to Step 6; Step 4: Calculate the time when the subsatellite point trajectory of the satellite moves to the bottom of the "8" shape ; Step Five: When the satellite runs to moment, the satellite switches from right-side view to left-side view through attitude adjustment in the side-view direction, and then jumps to Step Eight; Step 6: Calculate the time for the satellite sub-satellite point to move to the top of the "8" shape ; Step 7: When the satellite runs to At this moment, the satellite switches from left-side view to right-side view through attitude adjustment in the side-looking direction, and then proceeds to Step 8; Step 8: Thereafter, every time after a certain moment, the satellite automatically switches its side-looking direction, where is the satellite's operating period.

2. The method for improving the revisit coverage ability of the high-orbit SAR according to claim 1, characterized in that Step 2 specifically includes the following sub-steps: Step 21: Solve for the geographical longitude and geographical latitude of the sub-satellite point at the current moment, and the calculation formula is as follows: Wherein: t — Current time; — Earth's mean angular velocity of rotation constant, ; — Satellite time of ascending node passage; — The angle between the satellite and the ascending node, and the solution formula is as follows: wherein, is the true anomaly, is the eccentric anomaly; ； Among them, is the orbital eccentricity; Step 22: Update the geographical longitude of the sub-satellite point calculated in Step 21 to obtain the updated geographical longitude of the sub-satellite point The specific operations are as follows: (1) When the calculated geographical longitude of the sub-satellite point is added with 180°; (2) When the geodetic longitude of the sub-satellite point calculated is subtracted by 180°; (3) When the geodetic longitude of the sub-satellite point calculated remains unchanged.

3. The method for improving the revisit coverage ability of the high-orbit SAR according to claim 2, characterized in that, The calculation formula of Step 4 is as follows: Wherein: — Satellite operating period; — the geocentric gravitational constant, taking ; — semi-major axis of the orbit; — the time when the subsatellite point track of the satellite moves to the bottom of the figure-eight; — The time when the satellite passes through the ascending node, that is, the time when the sub-satellite point track of the satellite moves from bottom to top to the central intersection point of the "8" shape; — The current moment.

4. The method for improving the revisit coverage ability of the high-orbit SAR according to claim 1, wherein The calculation formula of Step 6 is as follows: Wherein: — The time when the sub-satellite point track of the satellite moves to the top of the "8" shape; — Time of satellite passing the ascending node; — Satellite operation period; — The current moment.

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