CN111913179B - Method for improving offshore observation capability of satellite-borne radar altimeter - Google Patents

Method for improving offshore observation capability of satellite-borne radar altimeter Download PDF

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CN111913179B
CN111913179B CN202010843363.7A CN202010843363A CN111913179B CN 111913179 B CN111913179 B CN 111913179B CN 202010843363 A CN202010843363 A CN 202010843363A CN 111913179 B CN111913179 B CN 111913179B
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付朝伟
何静
王海涛
高亮
张金强
宫俊
卢护林
张湘斌
田明
张鹏
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Shanghai Radio Equipment Research Institute
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    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A10/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE at coastal zones; at river basins
    • Y02A10/40—Controlling or monitoring, e.g. of flood or hurricane; Forecasting, e.g. risk assessment or mapping
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    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
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Abstract

The invention discloses a method for improving offshore observation capability of a satellite-borne radar altimeter. The satellite-borne radar altimeter receives the update information of the position and the track direction of the sub-satellite point transmitted by the satellite platform in real time, and judges whether the altimeter passes through a sea-land switching point or not by combining a land-sea map, and the altimeter keeps the original working state when the altimeter does not reach the switching point; when entering the land from the sea surface, the altimeter is switched to a land working mode after confirming to enter the land through an echo tracking state; when entering the ocean from land, the altimeter enters a high-precision echo search mode in advance. By reasonably planning the working mode in the land-sea conversion area, the searching efficiency of the altimeter is greatly improved, the searching time is reduced, the interference of land echoes on ocean echoes is inhibited, and the offshore observation capability of the satellite-borne radar altimeter is greatly improved.

Description

Method for improving offshore observation capability of satellite-borne radar altimeter
Technical Field
The invention belongs to the technical field of satellite-borne radar altimeters, and particularly relates to a method for improving offshore observation capability of a satellite-borne radar altimeter.
Background
The spaceborne radar altimeter is an important active microwave remote sensing device, can acquire rich sea surface characteristic information by detecting and data inversion of the sea, and is widely applied to the research on aspects of marine geophysics, marine dynamics, marine climate, environmental monitoring and the like. The traditional spaceborne radar altimeter generally adopts a full-deskew technology and a bottom-view detection method. In an offshore conversion area (offshore shore area), the detection of the satellite-borne radar altimeter is provided with both marine echoes and land echoes, and under the condition, the pollution of the land echoes to the marine echoes exists, so that the detection data of the altimeter in the offshore shore area is invalid, and a detection blind area is formed. The detection footprint of the satellite-borne radar altimeter on the offshore land is shown in a schematic diagram in fig. 1, and fig. 2 is a schematic diagram of the echo power of the satellite-borne radar altimeter on the offshore land.
Two methods are generally adopted in the design of a satellite-borne radar altimeter system to improve the detection capability of land-sea conversion areas such as a near-coast area and an ice area. The first method reduces the size of a ground detection footprint by improving the detection frequency band of a satellite-borne radar altimeter; when the aperture of the antenna is 1.2m, the actual aperture footprints of the Ka-band and Ku-band spaceborne radar altimeters are shown in the table 1. From the table 1, the actual aperture footprint size of the Ka frequency band of the spaceborne radar altimeter is about 40% of that of the Ku frequency band, so that the offshore area detection capability of the Ka frequency band altimeter is superior to that of the Ku frequency band.
Table 1 Ka/Ku frequency band space-borne radar altimeter footprint comparison
Figure BDA0002642225930000011
The second method applies the synthetic aperture technology to the satellite-borne radar altimeter, independently observes a certain target for multiple times in the motion process of a satellite platform, and performs coherent processing, namely uses a small-size wide beam antenna, and is equivalent to a large-size narrow beam antenna after aperture synthesis, so that the azimuth resolution is improved, and the beam footprint of the satellite-borne radar altimeter is reduced. Comparing the detection footprints of the conventional spaceborne radar altimeter and the synthetic aperture altimeter through the figures 3 and 5 and the figures 4 and 6; wherein, fig. 3 is a schematic diagram of a detection footprint of a conventional space-borne radar altimeter, and fig. 4 is a corresponding top view thereof; fig. 5 is a schematic diagram of the detection footprint of the synthetic aperture altimeter, and fig. 6 is a corresponding top view thereof. In fig. 5 and fig. 6, the resolution of the synthetic aperture altimeter in the forward direction can reach hundreds of meters, but the resolution in the cross direction is still lower, and the resolution in the cross direction can be improved by adopting an interference method (namely an InSAR mode) on the synthetic aperture altimeter, so that the detection capability of the satellite-borne radar altimeter on land-sea conversion areas such as a near-coast area and an ice area can be effectively improved.
Although the two methods can effectively improve the detection capability of the satellite-borne radar altimeter on the land-sea transition area, the method is influenced by factors such as an on-orbit echo tracking strategy and search time of the altimeter, and the offshore observation capability is weak. At present, a satellite-borne radar altimeter running in an orbit adopts a real-aperture closed-loop tracking mode, so that the search efficiency is low, and the satellite-borne radar altimeter is not suitable for observation application in land and sea exchange areas. In an in-orbit real-aperture closed-loop tracking mode, a satellite-borne radar altimeter can cause pollution of land echoes and sea echoes even extremely-end conditions such as altimeter distance tracking unlocking and the like in a land-sea switching area because a measurement footprint is far larger than a synthetic aperture footprint.
Disclosure of Invention
In order to solve the problem that the satellite-borne radar altimeter is weak in observation capability in a land-sea transition area, the invention provides a method for improving the offshore observation capability of the satellite-borne radar altimeter.
The invention provides a method for improving offshore observation capability of a satellite-borne radar altimeter, which comprises the following steps of:
step 1, the satellite platform stores and updates land and sea map information in a double-area ping-pong operation mode;
step 2, the satellite-borne radar altimeter receives the updating information of the position and the track direction of the off-satellite point transmitted by the satellite platform in real time, and judges whether the altimeter passes through a sea-land switching point or not in advance by combining a land-sea map;
and 3, switching the satellite-borne radar altimeter to a corresponding working mode in advance according to the judged type of the switching point.
Preferably, the sea-land map information storage area comprises a first storage area and a second storage area, and when the satellite-borne radar altimeter performs switching point judgment by using the map information of the first storage area on the track, the map information of the second storage area is updated; and updating the sea-land map information according to a set time period or mission planning.
Preferably, the switching point from the sea to the land is taken as a first switching point, and the switching point from the land to the sea is taken as a second switching point, wherein the sea-land switching point comprises the first switching point and the second switching point.
Preferably, the intersatellite point is the center of the discrimination window W, and the track direction of the intersatellite point is consistent with the track direction of the discrimination window W; the specific method for judging whether the altimeter passes through the sea-land switching point is as follows:
s1, setting the equivalent narrow beam footprint size of a satellite-borne radar altimeter in a land-sea switching area to be L1 xL 2, wherein L1 is the dimension in the forward rail direction, and L2 is the dimension in the cross rail direction;
s2, setting a satellite trajectory direction, wherein a judgment window W sequentially comprises four sub-windows W _ L0, W _ M1, W _ M0 and W _ E0, and the size of each sub-window is the same as the equivalent narrow beam footprint;
s3, counting the proportions of land and sea areas in each sub-window, and respectively representing the proportions as P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0; and judging whether the altimeter passes through a sea-land switching point or not according to the switching condition met by the land and sea area proportion of each sub-window.
Preferably, when the land and sea area proportions P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0 of each sub-window meet the switching condition shown in the formula 1, the spaceborne radar altimeter enters the sea from the land and passes through a second switching point; the formula 1 is:
Figure BDA0002642225930000031
the high-precision searching mode is started before the altimeter enters a coastline, and when the altimeter is judged to be above the sea through echo data characteristics, the altimeter enters a tracking mode.
Preferably, when the land and sea area ratios P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0 of the sub-windows meet the switching condition shown in the formula 2, the spaceborne radar altimeter enters the land from the sea and passes through a second switching point; the formula 2 is:
Figure BDA0002642225930000032
the altimeter switches to a land mode of operation.
Preferably, when the land and sea area proportions P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0 of each sub-window satisfy the condition shown in formula 3, the spaceborne radar altimeter does not reach the change point; the formula 3 is:
Figure BDA0002642225930000041
the altimeter keeps the original working state.
Preferably, the method for improving the offshore observation capability of the satellite-borne radar altimeter is suitable for the satellite-borne radar altimeter with a synthetic aperture system, and the altimeter adopts a synthetic aperture mode in the processes of on-orbit searching and tracking.
Preferably, the detection pulses of the spaceborne radar altimeter adopt a cluster sending and receiving mode, the cluster period is 12.5ms, and 64 detection pulses are set in each cluster.
Preferably, the satellite-borne radar altimeter adopts a 50ms processing period to acquire a group of equivalent narrow beam echo data in a fine search mode, and enters a tracking mode when echoes of 4 continuous 50ms processing periods can confirm that the altimeter is above the sea.
The method for improving the offshore observation capability of the satellite-borne radar altimeter comprises the contents of land-sea map storage and updating, land-sea switching point judgment, planning of the working mode of the satellite-borne radar altimeter in a land-sea switching area and the like. Compared with the similar satellite-borne radar altimeter which runs in an orbit, the method for improving the offshore observation capability of the satellite-borne radar altimeter has the advantages that the land-sea switching point is judged in advance, the working mode of the altimeter in a land-sea switching area is reasonably planned, the searching efficiency of the altimeter is greatly improved, the searching time is reduced, the interference of land echoes on ocean echoes is inhibited, and the offshore observation capability of the satellite-borne radar altimeter is greatly improved.
Drawings
FIG. 1 is a schematic diagram of a detection footprint of a satellite-borne radar altimeter near an offshore land;
FIG. 2 is a schematic diagram of the echo power of a satellite-borne radar altimeter near the shore;
FIG. 3 is a schematic diagram of a detection footprint of a conventional satellite-borne radar altimeter, and FIG. 4 is a corresponding top view thereof;
FIG. 5 is a schematic diagram of a synthetic aperture altimeter detection footprint, and FIG. 6 is a corresponding top view thereof;
FIG. 7 is a flow chart of a method for improving the near-coast observation capability of a space-borne radar altimeter according to the invention;
FIG. 8 is a schematic diagram of a criterion of switching points in land-sea transition areas;
FIG. 9 is a flow chart of a fine search of a satellite-borne radar altimeter;
FIG. 10 is a schematic diagram of a satellite-borne radar altimeter transmitting and receiving pulse clusters;
FIG. 11 is a schematic diagram of the working timing sequence and equivalent wave beam of the altimeter of the satellite-borne radar;
FIG. 12 is a schematic diagram of a process of capturing sea surface echoes in a land-sea switching area of a satellite-borne radar altimeter;
FIG. 13 is a working timing sequence of a Ka-band spaceborne radar altimeter entering the sea from the land;
FIG. 14 is a schematic diagram of a fine search mode signal processing flow of a Ka-band spaceborne radar altimeter.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and examples.
The method for improving the offshore observation capability of the satellite-borne radar altimeter comprises the steps of land and sea map storage and updating, land and sea switching point judgment and working mode planning of the satellite-borne radar altimeter in a land and sea switching area. FIG. 7 is a flow chart of a method for improving the near-coast observation capability of a space-borne radar altimeter according to the invention. As shown in fig. 7, a satellite-borne radar altimeter receives update information of the position and the track direction of an infrastar point transmitted by a satellite platform in real time, and judges whether the altimeter passes through a sea-land switching point or not by combining a land-sea map, and the altimeter keeps an original working state (a synthetic aperture height measurement state) when the altimeter does not reach the switching point; when entering the land from the sea surface, the altimeter is switched to a land working mode after confirming to enter the land through an echo tracking state; when entering the ocean from land, the altimeter enters a high-precision echo search mode in advance.
1) Land and sea map storage and updating
The land and sea map track is 0.5', the corresponding track time is 0.1s, and the total time in the sub-period is about 0.15 multiplied by 10 9 And (4) point. The division of the earth by 0.5' grids can be divided into about 1.87 × 10 9 And (4) point. The map storage adopts a double-area (such as a first storage area and a second storage area) ping-pong operation mode. When the first storage area map is used for judging the switching point on the track, the second storage area map data can be updated, and the land and sea map can be updated according to a fixed time period or task planning.
2) Land-sea switching point judgment
The altimeter receives the position of the off-satellite point and the track direction updating information transmitted by the satellite platform in real time, and whether the altimeter passes through a sea-land switching point or not is obtained in advance by combining a land-sea map. Taking a switching point when the sea enters the land as a first switching point, and taking a switching point when the sea enters the sea as a second switching point, wherein the sea and land switching points comprise the first switching point and the second switching point. When the altimeter is in the land-sea transition area, the satellite flight trajectory and the altimeter detection footprint are as shown in fig. 8, and after the synthetic aperture processing, the equivalent narrow beam footprint size of the altimeter is L1 × L2 (along the rail direction × cross the rail direction).
A decision window W composed of W _ L0, W _ M1, W _ M0, and W _ E0 is set, as shown in FIG. 8. The size of each sub-window is the same as the equivalent beam footprint, the center of the window W is a subsatellite point, and the direction of the window is consistent with the moving direction of the subsatellite point. And counting the proportions of the land area and the sea area in each sub-window, namely P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0. And if the land-sea area proportion in each sub-window meets the switching condition shown in the formula 1, the altimeter is considered to enter the sea from the land and pass through a second switching point. The expression of the formula 1 is as follows:
Figure BDA0002642225930000061
and if the land-sea area proportion in each sub-window meets the switching condition shown in the formula 2, the altimeter is considered to enter the land from the sea and pass through the first switching point. The expression of the formula 2 is as follows:
Figure BDA0002642225930000062
and if the land and sea area proportion in each sub-window meets the condition shown in the formula 3, the altimeter is not considered to reach the switching point. The expression of formula 3 is:
Figure BDA0002642225930000063
3) Working mode of satellite-borne radar altimeter in land-sea switching area
The satellite-borne radar altimeter has three situations in land-sea transition areas: sea-to-land, land-to-sea, and no land-to-sea switch points. When the altimeter does not reach the land and sea switching point, the original working state is kept; when the altimeter enters the land from the sea and meets the corresponding switching condition, switching to a land working mode; when the altimeter enters the sea from the land, whether the altimeter can realize quick search and tracking is the key for improving the detection efficiency of the altimeter at the offshore place.
The altimeter closed loop search time determines the altimeter detection efficiency for land-sea transition areas when the satellite enters the sea from land. When the altimeter starts a high-precision searching mode before entering the sea from the land, and keeps a fine searching state until sea echo is captured, a tracking mode is entered, and a fine searching flow is shown as figure 9.
The method for improving the coastal observation capability of the satellite-borne radar altimeter is suitable for the satellite-borne radar altimeter with a synthetic aperture system, and the altimeter adopts a synthetic aperture mode in the processes of on-track searching and tracking. The detection pulse adopts a cluster sending and cluster receiving mode, the cluster period is 12.5ms, and 64 detection pulses are temporarily set in each cluster. The transmit and receive pulse bursts are shown in fig. 10.
In the fine search mode, the synthetic aperture working timing sequence and the equivalent beam are as shown in fig. 11, a group of equivalent narrow beam echo data is obtained by adopting a 50ms processing cycle (that is, 4 clusters are accumulated, and the number of accumulated pulses is 256), and whether the altimeter is above the ocean is judged according to the characteristics of the echo data.
The process of capturing sea surface echo in land-sea switching area of the satellite-borne radar altimeter is shown in fig. 12, and when 4 echo data within 50ms are confirmed to be above the sea, the tracking mode is entered, so that the tracking mode is entered. When the satellite speed is 7.4km/s, the altimeter base view detection footprint moves by 370m within 50ms, when the altimeter enters the sky above the sea from the land and the sea echo capture needs to be continuously realized within 4 detection periods of 50ms, the altimeter base view detection footprint moves by about 1.5km within 0.2 s. Compared with similar satellite-borne radar altimeters, such as the Altika altimeter closed-loop search time of 2.8s and the CryoSat altimeter search time of 0.4-0.7 s, the method disclosed by the invention can realize sea echo capture within 0.2s, and effectively improve the near-coast observation capability of the satellite-borne radar altimeter.
When the altimeter enters the sea from the land, the high-precision searching mode is started about 2km before the altimeter enters a coastline, the frequency bandwidth is adjusted to be 80MHz in a fine searching time sequence, and the corresponding searching distance range is 240m (the searching range meets the extreme value distribution of the highest +76m and the lowest-112 m of the global sea level). Taking a Ka frequency range altimeter as an example, the working timing sequence design from land to sea is shown in fig. 13, the synthetic aperture processing is adopted in the pulse cluster, 50ms is adopted as the processing period (namely, the processing period includes 4 cluster periods), and delay compensation is not needed among clusters. The fine search mode signal processing flow of the Ka-band space-borne radar altimeter is shown in fig. 14.
The search time is designed to be 0.2s, and when the altimeter enters the sea from the land, the distance of the corresponding sea footprint is 1.5km, namely, the altimeter has enough time to enter the tracking mode on the sea surface 2km away from the land. Compared with the similar satellite-borne radar altimeter running on the orbit, the method for detecting the offshore area of the altimeter can inhibit the interference of the terrestrial echo to the offshore echo, reduce the search time and greatly improve the offshore area observation capability of the satellite-borne radar altimeter.
While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims (9)

1. A method for improving offshore observation capability of a satellite-borne radar altimeter is characterized by comprising the following processes:
step 1, the satellite platform stores and updates land and sea map information in a double-area ping-pong operation mode;
step 2, the satellite-borne radar altimeter receives the update information of the position and the track direction of the sub-satellite point transmitted by the satellite platform in real time, and judges whether the altimeter passes through a sea-land switching point in advance by combining a land-sea map; the intersatellite point is the center of the discrimination window W, and the track direction of the intersatellite point is consistent with the track direction of the discrimination window W;
the specific method for judging whether the altimeter passes through the sea-land switching point is as follows:
s1, setting the equivalent narrow beam footprint size of a satellite-borne radar altimeter in a land-sea switching area to be L1 xL 2, wherein L1 is the dimension in the forward rail direction, and L2 is the dimension in the cross rail direction;
s2, setting a satellite trajectory direction, wherein a judgment window W sequentially comprises four sub-windows W _ L0, W _ M1, W _ M0 and W _ E0, and the size of each sub-window is the same as the equivalent narrow beam footprint;
s3, counting the proportion of land and sea areas in each sub-window, and respectively representing the proportions as P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0; judging whether the altimeter passes through a sea-land switching point or not according to the switching condition met by the land and sea area proportion of each sub-window;
and 3, switching the satellite-borne radar altimeter to a corresponding working mode in advance according to the judged type of the switching point.
2. The method for improving the offshore observation capability of the satellite-borne radar altimeter as claimed in claim 1, wherein the sea-land map information storage area comprises a first storage area and a second storage area, and when the satellite-borne radar altimeter uses the map information of the first storage area to perform switching point judgment in an on-track manner, the map information of the second storage area is updated; and updating the sea-land map information according to a set time period or mission planning.
3. The method of claim 1, wherein the first switching point is a switching point from sea to land and the second switching point is a switching point from land to sea, and the sea-land switching points comprise the first switching point and the second switching point.
4. The method for improving the offshore observation capability of the space-borne radar altimeter according to claim 1, wherein when the land-sea area ratios P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0 of the sub-windows satisfy the switching condition shown in formula 1, the space-borne radar altimeter enters the sea from the land and passes through the second switching point; the formula 1 is:
Figure FDA0004101342550000021
the altimeter starts a high-precision searching mode before entering a coastline, and enters a tracking mode when the altimeter is judged to be above the sea through echo data characteristics.
5. The method as claimed in claim 1, wherein when the land-sea area ratio P _ W _ L0, P _ W _ M1, P _ W _ M0 and P _ W _ E0 of each sub-window satisfies the switching condition shown in formula 2, the satellite-borne radar altimeter enters the land from the sea and passes through the first switching point; the formula 2 is:
Figure FDA0004101342550000022
the altimeter switches to a terrestrial mode of operation.
6. The method of claim 1, wherein when the land-sea area ratio P _ W _ L0, P _ W _ M1, P _ W _ M0, and P _ W _ E0 of each sub-window satisfy the condition shown in equation 3, the satellite-borne radar altimeter has not reached a change point; the formula 3 is:
Figure FDA0004101342550000023
the altimeter keeps the original working state.
7. The method for improving offshore observation capability of the satellite-borne radar altimeter as claimed in claim 4, wherein the method for improving offshore observation capability of the satellite-borne radar altimeter is applied to the satellite-borne radar altimeter with a synthetic aperture system, and the altimeter adopts the synthetic aperture mode in the processes of in-orbit searching and tracking.
8. The method for improving the offshore observation capability of the satellite-borne radar altimeter as recited in claim 4, wherein the detection pulses of the satellite-borne radar altimeter in the fine search mode are received in a cluster sending mode, the cluster period is 12.5ms, and 64 detection pulses are set in each cluster.
9. The method of claim 4, wherein the satellite-borne radar altimeter is configured to obtain a set of equivalent narrow-beam echo data in a fine search mode with a 50ms processing period, and enter a tracking mode when echoes of 4 consecutive 50ms processing periods confirm that the altimeter is above the sea.
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