Multi-satellite cooperative target tracking method based on publishing and subscribing mechanism
By adopting publish and subscription mechanisms and different communication modes to manage message delivery in multi-star collaborative target tracking, the broadcast storm problem is solved, multi-star and multi-target tracking and precise information transmission are achieved, and network burden is reduced.
Patent Information
- Application Number
- CN202411184418.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-08-27
- Publication Date
- 2024-12-31
AI Technical Summary
In multi-star collaborative target tracking, traditional broadcast storm problems lead to increased network burden, which may cause interruption of equipment functions and security risks, especially when monitoring satellite data and tracking target data increases.
Using a multi-star collaborative target tracking method based on the publish subscription mechanism, the messaging is managed through different communication modes (first to fourth communication modes), ensuring that only satellites that meet specific observation conditions send and receive 'discover topics' and 'target topics' Message, avoid broadcast storms.
It effectively reduces the amount of data transmitted, avoids broadcast storms, ensures the tracking capabilities of multiple stars and multiple targets, and ensures the reasonable allocation of network burdens, ensuring that satellites in the constellation receive accurate monitoring target information.
Smart Images
Figure CN119232227A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the technical field of intersatellite links routing space networks, and in particular to a multi-satellite collaborative target tracking method based on a publish-subscribe mechanism. Background Art
[0002] The space network is a network built with space platforms as nodes based on intersatellite links. It is mainly aimed at solving information transmission problems of space platforms and serving satellite communication systems. The space network topology is constantly changing and node resources are limited. At present, the development of space information networks shows a trend of a sharp increase in the number of nodes, heterogeneous node types, and diversified functional services. The application scenarios are constantly enriched, among which multi-satellite collaborative target tracking is a relatively new and very important application scenario.
[0003] The most important point in multi-satellite coordinated target tracking is the transmission and exchange of observation information between satellites. According to traditional Ethernet applications, in order to ensure that multiple satellites receive observation information and data at the same time for the calculation required for tracking, it is often achieved through broadcasting. However, with the increase of monitoring satellite data and tracking target data, the number of broadcast messages in the network will surge, and there is a risk of forming a broadcast storm. In addition, if there are no relevant processing measures for the current on-board computer in the case of a broadcast storm, it will cause frequent interruptions in the device function, and there is even a risk of continuous reset, leading to the risk of not being able to work normally, which poses a serious hidden danger to the safety and smooth operation of the on-orbit spacecraft. Summary of the invention
[0004] The present invention proposes a multi-satellite cooperative target tracking method based on a publish-subscribe mechanism. In the application scenario of multi-satellite cooperative target tracking, the message transmission process of multi-satellite tracking observation, the content of specific message transmission and the processing method are elaborated in detail under the minimum space network burden, and the generation of broadcast storms can be effectively avoided. It not only ensures the realization of multi-satellite cooperative target tracking, but also does not cause a large burden on the space Ethernet, thereby making it possible to track multiple satellites and multiple targets.
[0005] In a first aspect, a multi-satellite cooperative target tracking method based on a publish-subscribe mechanism is provided, comprising:
[0006] When the satellite observes the monitoring target for the first time, the satellite is triggered to change the first communication mode to the second communication mode, wherein the first communication mode is used to obtain the discovery theme message; the second communication mode is used to obtain the discovery theme message and the target theme message;
[0007] When the communication mode of the satellite is the second communication mode, obtaining a target topic message in a first communication cycle;
[0008] According to all the target topic messages obtained in the first communication cycle, it is judged whether the satellite meets the first preset observation condition; if so, the communication mode of the satellite is updated to the third communication mode, and the third communication mode is used to obtain the discovery topic message and the target topic message, and to send the satellite's own target topic message;
[0009] When the communication mode of the satellite is the third communication mode, obtaining a discovery topic message and a target topic message in a second communication cycle, and sending the satellite's own target topic message;
[0010] Based on all the target topic messages obtained in the second communication cycle, determine whether the satellite meets the first preset observation condition and the second preset observation condition. If both are met, update the satellite's communication mode to the fourth communication mode. The fourth communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own discovery topic messages and target topic messages.
[0011] In combination with the first aspect, in certain implementations of the first aspect, it is found that the topic message includes: monitoring target identification, monitoring target coordinates, and a timestamp; the data content contained in the target topic message includes: satellite identification, monitoring target identification, and observation parameters, and the observation parameters include at least one of the following: observation angle, satellite coordinates, monitoring target coordinates, and remaining observation time.
[0012] In combination with the first aspect, in some implementations of the first aspect, all target topic messages acquired in the first communication cycle indicate a plurality of first values of the observation parameter, and if the observation parameter of the satellite itself is better than a suboptimal value among the plurality of first values, the satellite meets the first preset observation condition;
[0013] All target topic messages acquired in the second communication cycle indicate a plurality of second values of the observation parameter. If the satellite's own observation parameter is better than the optimal value among the plurality of first values, the satellite satisfies the second preset observation condition.
[0014] In combination with the first aspect, in certain implementations of the first aspect, the first preset observation condition is that the satellite's own observation angle is one of the two smallest observation angles of all visible stars, observation stars, and fused stars; the second preset observation condition is that the satellite's own observation angle is the smallest observation angle of all visible stars, observation stars, and fused stars.
[0015] In combination with the first aspect, in certain implementation methods of the first aspect, all target topic messages acquired within the second communication cycle are used to indicate the satellite coordinates of the remaining visible stars, observation stars, and fused stars other than the satellite itself, and then combined with the monitoring target coordinates indicated by the topic message discovered within the second communication cycle, the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars within the second communication cycle are determined to determine whether the satellite meets the second preset observation condition.
[0016] In combination with the first aspect, in certain implementation methods of the first aspect, all target topic messages acquired within the first communication cycle are used to indicate the satellite coordinates of the remaining visible stars, observation stars, and fused stars other than the satellite itself, and then combined with the monitoring target coordinates observed by the satellite itself, the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars within the first communication cycle are determined to determine whether the satellite meets the first preset observation condition.
[0017] In combination with the first aspect, in certain implementations of the first aspect, when the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the start of the next communication cycle; if so, it determines to adopt the fourth communication mode for communication in the next communication cycle; if not, it determines to adopt the third communication mode for communication in the next communication cycle.
[0018] In combination with the first aspect, in certain implementations of the first aspect, when the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the start of the next communication cycle; if so, it determines to adopt the fourth communication mode for communication in the next communication cycle; if not, it continues to determine whether it meets the first preset observation condition, and if it meets the first preset observation condition, it determines to adopt the third communication mode for communication in the next communication cycle; if it does not meet the first preset observation condition, it determines to adopt the second communication mode for communication in the next communication cycle.
[0019] In combination with the first aspect, in certain implementations of the first aspect, when the satellite determines that it cannot observe the monitoring target, the satellite directly switches to communicating in the first communication mode.
[0020] In the second aspect, a multi-satellite cooperative target tracking method based on a publish-subscribe mechanism is provided, comprising:
[0021] In the case where the communication mode of the satellite is used to obtain the discovery topic message and the target topic message, obtaining the target topic message in the third communication cycle;
[0022] According to all the target topic messages obtained in the third communication cycle, determine whether the satellite meets the first preset observation condition and the second preset observation condition; if both the first preset observation condition and the second preset observation condition are not met, the communication mode of the satellite in the next communication cycle is the second communication mode, and the second communication mode is used to obtain the discovery topic message and the target topic message; if the first preset observation condition is met but the second preset observation condition is not met, the communication mode of the satellite in the next communication cycle is the third communication mode, and the third communication mode is used to obtain the discovery topic message and the target topic message, and is used to send the satellite's own target topic message; if the first preset observation condition and the second preset observation condition are met, the communication mode of the satellite in the next communication cycle is the fourth communication mode, and the fourth communication mode is used to obtain the discovery topic message and the target topic message, and is used to send the satellite's own discovery topic message and the target topic message.
[0023] In combination with the second aspect, in certain implementation methods of the second aspect, it is found that the topic message includes: monitoring target identification, monitoring target coordinates, and timestamp; the data content contained in the target topic message includes: satellite identification, monitoring target identification and observation parameters, and the observation parameters include at least one of the following: observation angle, satellite coordinates, monitoring target coordinates, and remaining observation time.
[0024] In combination with the second aspect, in certain implementations of the second aspect, all target topic messages obtained within the third communication cycle indicate multiple third values of the observation parameter; if the satellite's own observation parameter is better than the optimal value among the multiple second values, the satellite satisfies the first preset observation condition and the second preset observation condition; if the satellite's own observation parameter is only better than the suboptimal value among the multiple second values, the satellite satisfies the first preset observation condition but does not satisfy the second preset observation condition.
[0025] In combination with the second aspect, in certain implementations of the second aspect, the first preset observation condition is that the satellite's own observation angle is one of the two smallest observation angles of all visible stars, observation stars, and fused stars; the second preset observation condition is that the satellite's own observation angle is the smallest observation angle of all visible stars, observation stars, and fused stars.
[0026] In combination with the second aspect, in certain implementation methods of the second aspect, all target topic messages obtained within the third communication cycle are used to indicate the satellite coordinates of the remaining visible stars, observation stars, and fused stars other than the satellite itself, and then combined with the monitoring target coordinates indicated by the discovery topic message within the third communication cycle, the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars within the third communication cycle are determined, and then it is determined whether the satellite meets the first preset observation condition and the second preset observation condition.
[0027] In combination with the second aspect, in certain implementations of the second aspect, when the satellite determines that it cannot observe the monitoring target, the satellite directly switches to communicating in the first communication mode.
[0028] In a third aspect, a satellite is provided, comprising a memory for storing a computer program, and a processor for executing a multi-satellite collaborative target tracking method based on a publish-subscribe mechanism as described in any one of the implementations of the first and second aspects above.
[0029] In a fourth aspect, a multi-satellite cooperative communication system based on a publish-subscribe mechanism is provided, wherein the multi-satellite cooperative communication system includes several satellites, and any satellite is used to execute the multi-satellite cooperative target tracking method based on the publish-subscribe mechanism as described in any one of the implementation methods in the first aspect or the second aspect above.
[0030] In a fifth aspect, a computer-readable storage medium is provided, wherein a computer program is stored on the computer-readable storage medium, and when the computer program is executed by a processor, the multi-satellite cooperative target tracking method based on the publish-subscribe mechanism as described in any one of the implementations of the first aspect and the second aspect is implemented.
[0031] In a sixth aspect, a computer program product is provided, comprising computer executable instructions, which, when executed, enable a device to implement a multi-satellite collaborative target tracking method based on a publish-subscribe mechanism as described in any one of the implementation methods in the first and second aspects above.
[0032] In a seventh aspect, a chip is provided, which is configured to execute the multi-satellite collaborative target tracking method based on the publish-subscribe mechanism as described in any one of the implementation methods in the first aspect and the second aspect above.
[0033] Compared with the prior art, the solution provided by this application includes at least the following beneficial technical effects:
[0034] (1) The publish-subscribe model is used to achieve multi-satellite collaborative target tracking. Compared with the traditional method of tracking targets through Ethernet broadcast, it effectively reduces the amount of transmitted data and avoids the occurrence of broadcast storms;
[0035] (2) A combination of “discovery theme” and “target theme” is adopted to ensure that all satellites in the constellation can receive monitoring target information and solve the problem of multi-satellite coordination;
[0036] (3) The target information processing and tracking method based on role classification is adopted to ensure that the most accurate monitoring target information is transmitted within the network, making multi-satellite and multi-target tracking possible. BRIEF DESCRIPTION OF THE DRAWINGS
[0037] Figure 1A schematic flow chart of a multi-satellite cooperative target tracking method based on a publish-subscribe mechanism is provided for the present invention.
[0038] Figure 2 A schematic flow chart of another multi-satellite cooperative target tracking method based on a publish-subscribe mechanism is provided for the present invention.
[0039] Figure 3 A multi-satellite collaborative target tracking flow chart is provided for the present invention.
[0040] Figure 4 This is a satellite role conversion diagram. DETAILED DESCRIPTION
[0041] The present application is further described in detail below with reference to the accompanying drawings and specific embodiments.
[0042] At present, space networks are booming, and application scenarios based on multi-satellite collaboration are becoming increasingly abundant. One of the most important application scenarios is multi-satellite collaborative relay for target tracking. In view of this new application scenario, the present invention considers implementing a multi-satellite collaborative target tracking method based on a publish-subscribe mechanism.
[0043] like Figure 1 As shown, the present invention provides a multi-satellite collaborative target tracking method based on a publish-subscribe mechanism, and the specific steps are as follows.
[0044] Step 101, when the satellite observes the monitoring target for the first time, the satellite is triggered to change the first communication mode to the second communication mode, wherein the first communication mode is used to obtain the discovery topic message; the second communication mode is used to obtain the discovery topic message and the target topic message.
[0045] As a possible description, the satellite that communicates using the first communication mode can be an ordinary satellite, and the satellite that communicates using the second communication mode can be a visible satellite. The satellite that cannot observe the monitoring target in the constellation is an ordinary satellite, and the first communication mode is applied. The satellite that can observe the monitoring target is one of a visible star, an observation star, or a fusion star. In this case, the satellite needs to subscribe to the target topic message, that is, the message with the message type of "target topic".
[0046] In this application, when the satellite is an ordinary satellite, the message types subscribed by the satellite include "discovery topic" but not "target topic". Ordinary satellites do not have the authority to send discovery topic messages, and do not have the authority to send target topic messages. When the satellite is a visible satellite, the message types subscribed by the satellite include "discovery topic" and "target topic". Visible satellites do not have the authority to send discovery topic messages, and do not have the authority to send target topic messages.
[0047] Optionally, the discovery topic message includes: monitoring target identification, monitoring target coordinates, and timestamp.
[0048] Optionally, the target subject message includes: a satellite identifier, a monitoring target identifier and observation parameters.
[0049] Furthermore, the observation parameters include at least one of the following: observation angle, satellite coordinates, monitoring target coordinates, and remaining observation time (for example, calculated based on the observation angle).
[0050] In addition to the satellite actually observing the monitoring target, there are many ways for the satellite to determine that it can observe the monitoring target. In one implementation, the satellite can obtain the coordinates of the monitoring target by subscribing to the discovery topic message (i.e., subscribing to the message type of "discovery topic"). In another implementation, the satellite can obtain the coordinates of the monitoring target through satellite-ground interaction. After obtaining the monitoring target, calculations can be performed based on the coordinates of the monitoring target and the satellite's own coordinates to determine whether the satellite can observe the monitoring target.
[0051] Step 102: When the communication mode of the satellite is the second communication mode, a target topic message is obtained within a first communication cycle.
[0052] Step 103, based on all target topic messages obtained in the first communication cycle, determine whether the satellite meets the first preset observation condition; if so, update the satellite's communication mode to the third communication mode, the third communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own target topic messages.
[0053] As a possible description, the satellite that communicates using the third communication mode may be an observation satellite. In the present application, when the satellite is an observation satellite, the message types subscribed by the satellite include "discovery topic" and "target topic". The observation satellite does not have the authority to send discovery topic messages, but has the authority to send target topic messages.
[0054] Step 104, when the communication mode of the satellite is the third communication mode, obtaining the discovery topic message and the target topic message in the second communication cycle, and sending the satellite's own target topic message.
[0055] Step 105, based on all target topic messages obtained in the second communication cycle, determine whether the satellite meets the first preset observation condition and the second preset observation condition. If both are satisfied, update the satellite's communication mode to the fourth communication mode. The fourth communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own discovery topic messages and target topic messages.
[0056] As a possible description, the satellite that communicates using the fourth communication mode may be a fusion satellite. In the present application, when the satellite is a fusion satellite, the message types subscribed by the satellite include "discovery topic" and "target topic". The fusion satellite has the authority to send discovery topic messages and target topic messages.
[0057] The second preset observation condition may be a condition that is more stringent than the first preset observation condition, or may be another condition added to the first preset observation condition.
[0058] For the above method, the upgrade process from a visible star to a fusion star must go through at least two communication cycles, that is, no cross-level jump is performed within the same communication cycle.
[0059] Optionally, all target topic messages acquired in the first communication cycle indicate a plurality of first values of the observation parameter, and if the observation parameter of the satellite itself is better than a suboptimal value among the plurality of first values, the satellite satisfies the first preset observation condition. The suboptimal value is a value among the plurality of first values that is second only to the optimal value.
[0060] All target topic messages acquired in the first communication cycle may not include target topic messages sent by the satellite itself. Taking the observation angle as an example, all target topic messages acquired in the first communication cycle indicate a total of multiple observation angles. If the satellite's own observation angle is less than the second smallest observation angle value among the multiple observation angles, that is, the satellite's own observation angle belongs to one of the two smallest observation angles of all visible stars, observation stars, and fusion stars, then the satellite meets the first preset observation condition.
[0061] In an optional embodiment, the first preset observation condition is that the satellite's own observation angle is one of the two smallest observation angles of all visible stars, observation stars, and fused stars. When calculating the observation angle, the satellite can determine the satellite coordinates of the remaining visible stars, observation stars, and fused stars except itself based on all target topic messages obtained in the first communication cycle; the satellite can determine the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars based on the monitoring target coordinates indicated by the discovery topic message, and then determine whether the satellite meets the first preset observation condition.
[0062] In another optional embodiment, the first preset observation condition is the satellite's own observation angle, which is one of the two smallest observation angles of all visible stars, observation stars, and fused stars. When calculating the observation angle, the satellite can determine the satellite coordinates of the remaining visible stars, observation stars, and fused stars except itself based on all target topic messages acquired during the first communication cycle; the satellite can determine the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars based on the monitoring target coordinates observed by itself, and then determine whether the satellite meets the first preset observation condition.
[0063] In one possible case, due to observation errors, for the same monitoring target, the monitoring target coordinates obtained by different satellites are slightly different. This may result in that there may be additional satellites that believe that they meet the first preset observation condition at the same time. The embodiment of the present application may not impose additional restrictions on this, that is, it may allow the existence of additional observation stars. However, the judgment of the second preset observation condition may be more stringent. Therefore, when judging the second preset observation condition, the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars are determined according to the monitoring target coordinates indicated by the discovery theme message, and then it is determined whether the satellite meets the second preset observation condition.
[0064] Optionally, all target topic messages acquired in the second communication cycle indicate a plurality of second values of the observation parameter. If the satellite's own observation parameter is better than the optimal value among the plurality of first values, the satellite satisfies the second preset observation condition.
[0065] All target topic messages acquired in the second communication cycle may not include target topic messages sent by the satellite itself. Taking the observation angle as an example, all target topic messages acquired in the second communication cycle indicate a total of multiple observation angles. If the satellite's own observation angle is less than the smallest observation angle value among the multiple observation angles, that is, the satellite's own observation angle is the smallest among all visible stars, observed stars, and fused stars, then the satellite meets the second preset observation condition and obviously meets the first preset observation condition.
[0066] Optionally, when the satellite determines that it cannot observe the monitoring target, the satellite can directly switch to communicate in the first communication mode.
[0067] Optionally, when the satellite adopts the third communication mode in the current communication cycle, the satellite determines whether it meets the first preset observation condition before the start of the next communication cycle; if the first preset observation condition is not met, it determines to adopt the second communication mode for communication in the next communication cycle; if the first preset observation condition is met, it continues to determine whether the second preset observation condition is met.
[0068] That is to say, the observation star first determines whether it can still retain the qualification of an observation star, and then confirms whether it can be upgraded to a fusion star on this basis. That is, the judgment of the second preset observation condition is based on the satisfaction of the first preset observation condition.
[0069] Optionally, when the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the start of the next communication cycle; if so, it determines to adopt the fourth communication mode for communication in the next communication cycle; if not, it determines to adopt the third communication mode for communication in the next communication cycle.
[0070] That is to say, the downgrading process from a fused star to a visible star must go through at least two communication cycles, that is, cross-level downgrading is not performed within the same communication cycle.
[0071] Optionally, when the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the start of the next communication cycle; if so, it determines to adopt the fourth communication mode for communication in the next communication cycle; if not, it continues to determine whether it meets the first preset observation condition, and if it meets the first preset observation condition, it determines to adopt the third communication mode for communication in the next communication cycle; if it does not meet the first preset observation condition, it determines to adopt the second communication mode for communication in the next communication cycle.
[0072] That is, a cross-level downshift can be performed from a fusion star to a visible star.
[0073] Example 1-1
[0074] In the first communication cycle, the N satellites in the space network confirm the coordinates of target A and start to perform the monitoring task of target A. In the first communication cycle, the N satellites can determine whether they can observe target A. Assuming that n of the N satellites can observe target A, and the remaining Nn satellites cannot observe target A, then in the first communication cycle, n satellites can confirm that they are visible satellites, and the corresponding communication mode is the second communication mode, that is, subscribing to messages with message types of "discovery topic" and "target topic"; the remaining Nn satellites can confirm that they are ordinary satellites, and the corresponding communication mode is the first communication mode, that is, subscribing to messages with message types of "discovery topic". Since the N satellites are started at the same time, no satellite publishes discovery topic messages and target topic messages in the first communication cycle.
[0075] In the second communication cycle, since all n satellites use the second communication mode for communication, at this time, none of the n satellites have the authority to send discovery theme messages and target theme messages, so in the second communication cycle, none of the n satellites can obtain discovery theme messages and target theme messages. Then, all n satellites believe that they are the best or second-best satellites, and all n satellites can confirm that they are observation satellites, and use the third communication mode for communication in the third communication cycle.
[0076] In the third communication cycle, all n satellites send target topic messages in the third communication cycle. At the same time, each of the n satellites can obtain target topic messages sent by the remaining n-1 satellites except itself. In addition, in the third communication cycle, none of the n satellites can obtain discovery topic messages. The n satellites can determine whether they meet the first preset observation condition and the second preset observation condition based on all target topic messages obtained in the third communication cycle. Thus, n of the n satellites can obtain target topic messages sent by the remaining n-1 satellites except itself. In addition, in the third communication cycle, none of the n satellites can obtain discovery topic messages. 1The n satellites can be confirmed to meet the first preset observation condition and the second preset observation condition. 1 Satellites are fusion satellites and can communicate using the fourth communication mode in the fourth communication cycle. 2 It can be confirmed that n satellites do not meet the second preset observation condition, but meet the first preset observation condition. 2 Satellites are observation satellites and can communicate using the third communication mode in the fourth communication cycle. 1 -n 2 Because a satellite does not meet the first preset observation condition and the second preset observation condition, the nn 1 -n 2 The satellite is a visible satellite and communicates in the second communication mode in the fourth communication cycle.
[0077] Example 1-2
[0078] In the first communication cycle, the N satellites in the space network include n satellites that can observe target B, and the remaining Nn satellites cannot observe target B. The n satellites include n 1 -n 2 visible stars, n 2 observed stars, and n 1 The n 1 Each fusion star can send discovery topic messages and target topic messages in the first communication cycle. 2 Each of the n observation satellites can send a target topic message in the first communication cycle. The n satellites also obtain a discovery topic message and a target topic message in the current cycle in the first communication cycle.
[0079] For this n 1 A fusion star, in the first communication cycle, first determines whether it can still observe target B. If it cannot observe target B, it adopts the first communication mode for communication in the second communication cycle. If it can still observe target B, it continues to determine whether it still meets the second preset observation condition. If it meets the second preset observation condition, it still adopts the fourth communication mode in the second communication cycle. If it does not meet the second preset observation condition, it adopts the third communication mode in the second communication cycle.
[0080] For this n 2An observation star, in the first communication cycle, first determines whether it can still observe target B. If target B cannot be observed, the first communication mode is used for communication in the second communication cycle. If target B can still be observed, it continues to determine whether it still meets the first preset observation condition and the second preset observation condition; if the first preset observation condition and the second preset observation condition are met, the fourth communication mode is used in the second communication cycle; if the first preset observation condition is met but the second preset observation condition is not met, the third communication mode is used in the second communication cycle; if both the first preset observation condition and the second preset observation condition are not met, the second communication mode is used in the second communication cycle.
[0081] For this nn 1 -n 2 In the first communication cycle, the satellite first determines whether it can still observe target B. If it cannot observe target B, it uses the first communication mode for communication in the second communication cycle. If it can still observe target B, it continues to determine whether it meets the first preset observation condition; if it meets the first preset observation condition, it uses the third communication mode in the second communication cycle; if it does not meet the first preset observation condition, it still uses the second communication mode in the second communication cycle.
[0082] In addition, the remaining Nn satellites obtain the discovery topic message in the first communication cycle in the first communication cycle. The remaining Nn satellites determine whether they can observe the target B. If the target B can be observed, the second communication mode is used for communication in the second communication cycle. If the target B cannot be observed, the first communication mode is still used for communication in the second communication cycle. It is assumed that the remaining Nn satellites cannot observe the target B for a period of time in the future.
[0083] In the first communication cycle, the n 1 In the first communication cycle, the fusion satellites send a total of n 1 Discovery topic messages, n 1 target topic message; the n 2 In the first communication cycle, the observing satellites send a total of n 2 Therefore, in the first communication cycle, a total of n 1 +n 2 Target topic messages can be sent.
[0084] Assume that in the first communication cycle, the n 1 Each of the fusion stars determines that it satisfies the first preset observation condition and the second preset observation condition. 2 Each observed star determines that it satisfies the first preset observation condition but does not satisfy the second preset observation condition. 1 -n 2There is a satellite a among the visible stars. According to n 1 +n 2 If a target topic message determines that it meets the first preset observation condition (for example, the observation angle of satellite a is smaller than the second smallest observation angle of all target topic messages in the first communication cycle, and for the sake of convenience, it is assumed that in the subsequent short period, the changes in the observation angles of each satellite are small and can be ignored), then satellite a can adopt the third communication mode to communicate in the next communication cycle.
[0085] In the second communication cycle, satellite a and the n 1 Fusion star, the n 2 Each observation satellite can send the target topic message. Therefore, in the second communication cycle, a total of n 1 +n 2 +1 target topic message can be sent. In addition, the n 1 Each fusion star can send discovery topic messages.
[0086] In the first case, the observation angle of satellite a not only meets the first preset observation condition, but also meets the second preset observation condition. Then in the second communication cycle, the n 1 Satellite b in the fusion star determines that it no longer meets the second preset observation condition, and the remaining n 1 -1 fusion star and satellite a communicate in the fourth communication mode in the third communication cycle. In addition, in the second communication cycle, the n 2 Satellite c among the n observation satellites determines that it no longer meets the first preset observation condition, and satellite c uses the second communication mode to communicate in the third communication cycle. 2 -1 fusion star and satellite b communicate using the third communication mode.
[0087] In the second case, the observation angle of satellite a only meets the first preset observation condition, but does not meet the second preset observation condition. Then in the third communication cycle, the n 1 Each fusion star uses the fourth communication mode for communication. In addition, in the second communication cycle, the n 2 Satellite d among the n observation satellites determines that it no longer meets the first preset observation condition, and satellite d adopts the second communication mode to communicate in the third communication cycle. 2 -1 fusion star and satellite a communicate using the third communication mode.
[0088] In summary, the advantages of the above scheme include that the dynamic process of discovering topic messages is relatively stable and the fault tolerance rate is improved.
[0089] like Figure 2 As shown, the present invention provides another multi-satellite collaborative target tracking method based on a publish-subscribe mechanism, and the specific steps are as follows.
[0090] Step 201, when the communication mode of the satellite is used to obtain the discovery topic message and the target topic message, obtain the target topic message in the third communication cycle.
[0091] Step 202, based on all the target topic messages obtained in the third communication cycle, determine whether the satellite meets the first preset observation condition and the second preset observation condition; if both the first preset observation condition and the second preset observation condition are not met, the communication mode of the satellite in the next communication cycle is the second communication mode, and the second communication mode is used to obtain the discovery topic message and the target topic message; if the first preset observation condition is met but the second preset observation condition is not met, the communication mode of the satellite in the next communication cycle is the third communication mode, and the third communication mode is used to obtain the discovery topic message and the target topic message, and is used to send the satellite's own target topic message; if the first preset observation condition and the second preset observation condition are met, the communication mode of the satellite in the next communication cycle is the fourth communication mode, and the fourth communication mode is used to obtain the discovery topic message and the target topic message, and is used to send the satellite's own discovery topic message and the target topic message.
[0092] For the above method, the upgrade process from visible star to fusion star can be a cross-level upshift or a cross-level downshift.
[0093] Optionally, all target topic messages acquired within the third communication cycle indicate multiple third values of the observation parameters; if the satellite's own observation parameter is better than the optimal value among the multiple second values, the satellite satisfies the first preset observation condition and the second preset observation condition; if the satellite's own observation parameter is only better than the suboptimal value among the multiple second values, the satellite satisfies the first preset observation condition but does not satisfy the second preset observation condition.
[0094] All target topic messages obtained within the third communication cycle may not include target topic messages sent by the satellite itself. Taking the observation angle as an example, all target topic messages obtained within the third communication cycle indicate multiple observation angles in total. If the satellite's own observation angle is less than the smallest observation angle value among multiple observation angles, and the satellite's own observation angle is the smallest of all visible stars, observation stars, and fusion stars, then the satellite meets the second preset observation condition and meets the first preset observation condition. If the satellite's own observation angle is less than the second smallest observation angle value among multiple observation angles, and the satellite's own observation angle is one of the two smallest observation angles among all visible stars, observation stars, and fusion stars, then the satellite meets the first preset observation condition but does not meet the second preset observation condition.
[0095] In an optional embodiment, when calculating the observation angle, the satellite can determine the satellite coordinates of the remaining visible stars, observation stars, and fused stars except itself based on all target topic messages obtained in the third communication cycle; the satellite can determine the observation angle of the satellite itself and the remaining visible stars, observation stars, and fused stars based on the monitoring target coordinates indicated by the discovery topic message, and then determine whether the satellite meets the first preset observation condition and the second preset observation condition.
[0096] The monitoring target coordinates may also be obtained from the monitoring target coordinates observed by the satellite itself. Due to observation errors, for the same monitoring target, the monitoring target coordinates obtained by different satellites are slightly different. This may result in additional satellites thinking that they meet the first preset observation condition or the second preset observation condition at the same time.
[0097] Optionally, before step 201, the method further includes: when the satellite observes the monitoring target for the first time, triggering the satellite to change the first communication mode to the second communication mode, wherein the first communication mode is used to obtain the discovery topic message.
[0098] Optionally, when the satellite determines that it cannot observe the monitoring target, the satellite can directly switch to communicate in the first communication mode.
[0099] Example 2-1
[0100] In the first communication cycle, the N satellites in the space network confirm the coordinates of target A and start to perform the monitoring task of target A. In the first communication cycle, the N satellites can determine whether they can observe target A. Assuming that n of the N satellites can observe target A, and the remaining Nn satellites cannot observe target A, then in the first communication cycle, n satellites can confirm that they are visible satellites, and the corresponding communication mode is the second communication mode, that is, subscribing to messages with message types of "discovery topic" and "target topic"; the remaining Nn satellites can confirm that they are ordinary satellites, and the corresponding communication mode is the first communication mode, that is, subscribing to messages with message types of "discovery topic". Since the N satellites are started at the same time, no satellite publishes discovery topic messages and target topic messages in the first communication cycle.
[0101] In the second communication cycle, since the n satellites all use the second communication mode for communication, at this time, the n satellites do not have the authority to send the discovery theme message and the target theme message, so in the second communication cycle, the n satellites cannot obtain the discovery theme message and the target theme message. Then the n satellites all believe that they are the best satellites, and the n satellites can confirm that they are fusion satellites, and use the fourth communication mode for communication in the third communication cycle.
[0102] In the third communication cycle, the n satellites all send target topic messages and discovery topic messages in the third communication cycle. At the same time, each of the n satellites can obtain target topic messages and discovery topic messages sent by the remaining n-1 satellites except itself. The n satellites can determine whether they meet the first preset observation condition and the second preset observation condition based on all the target topic messages obtained in the third communication cycle. 1 The n satellites can be confirmed to meet the first preset observation condition and the second preset observation condition. 1 Satellites are fusion satellites and can communicate using the fourth communication mode in the fourth communication cycle. 2 It can be confirmed that n satellites do not meet the second preset observation condition, but meet the first preset observation condition. 2 Satellites are observation satellites and can communicate using the third communication mode in the fourth communication cycle. 1 -n 2 Because a satellite does not meet the first preset observation condition and the second preset observation condition, the nn 1 -n 2 The satellite is a visible satellite and communicates in the second communication mode in the fourth communication cycle.
[0103] Example 2-2
[0104] In the first communication cycle, the N satellites in the space network include n satellites that can observe target B, and the remaining Nn satellites cannot observe target B. The n satellites include n 1 -n 2 visible stars, n 2 observed stars, and n 1 The n 1 Each fusion star can send discovery topic messages and target topic messages in the first communication cycle. 2 Each of the n observation satellites can send a target topic message in the first communication cycle. The n satellites also obtain a discovery topic message and a target topic message in the current cycle in the first communication cycle.
[0105] For the n satellites, first determine whether they can still observe target B. If target B cannot be observed, the first communication mode is used for communication in the second communication cycle. If target B can still be observed, continue to determine whether the first preset observation condition and the second preset observation condition are met. If the first preset observation condition and the second preset observation condition are met, the fourth communication mode is still used in the second communication cycle. If the first preset observation condition is met but the second preset observation condition is not met, the third communication mode is used in the second communication cycle. If both the first preset observation condition and the second preset observation condition are met, the second communication mode is used in the second communication cycle.
[0106] In addition, the remaining Nn satellites obtain the discovery topic message in the first communication cycle in the first communication cycle. The remaining Nn satellites determine whether they can observe the target B. If the target B can be observed, the second communication mode is used for communication in the second communication cycle. If the target B cannot be observed, the first communication mode is still used for communication in the second communication cycle. It is assumed that the remaining Nn satellites cannot observe the target B for a period of time in the future.
[0107] In the first communication cycle, the n 1 In the first communication cycle, the fusion satellites send a total of n 1 Discovery topic messages, n 1 target topic message; the n 2 In the first communication cycle, the observing satellites send a total of n 2 Therefore, in the first communication cycle, a total of n 1 +n 2 Target topic messages can be sent.
[0108] Assume that in the first communication cycle, the n 1 Each of the fusion stars determines that it satisfies the first preset observation condition and the second preset observation condition. 2 Each observed star determines that it satisfies the first preset observation condition but does not satisfy the second preset observation condition. 1 -n 2 There is a satellite a among the visible stars. According to n 1 +n 2 A target topic message determines that it meets the first preset observation condition (for example, the observation angle of satellite a is smaller than the second smallest observation angle of all target topic messages in the first communication cycle. In addition, for the sake of convenience, it is assumed that in the subsequent short period, the changes in the observation angles of each satellite are small and can be ignored), then the following situation exists in the second communication cycle.
[0109] In the first case, the observation angle of satellite a not only meets the first preset observation condition, but also meets the second preset observation condition. Then in the second communication cycle, satellite a and the n 1 Fusion star, the n 2 Each observation satellite can send a target topic message, and satellite a and the n 1 Each fusion star can send a discovery topic message. Therefore, in the second communication cycle, a total of n 1 +n 2 +1 target topic message can be sent, total n 1 +1 Discovery Topic message can be sent.
[0110] For the first scenario of this situation, in the second communication cycle, the n 1 Satellite b in the fusion star determines that it no longer meets the second preset observation condition, and the remaining n 1 -1 fusion star and satellite a communicate in the fourth communication mode in the third communication cycle. In addition, in the second communication cycle, the n 2 Satellite c among the n observation satellites determines that it no longer meets the first preset observation condition, and satellite c uses the second communication mode to communicate in the third communication cycle. 2 -1 fusion star and satellite b communicate using the third communication mode.
[0111] For the second scenario of this situation, in the second communication cycle, the n 1 Satellite b in the fusion star determines that it no longer meets the first preset observation condition and the second preset observation condition. 1 -1 fusion star and satellite a communicate in the fourth communication mode in the third communication cycle. In addition, in the third communication cycle, the n 2 The observation satellites still communicate using the third communication mode.
[0112] In the second case, the observation angle of satellite a only meets the first preset observation condition, but does not meet the second preset observation condition. Then in the second communication cycle, the n 1 Each fusion satellite uses the fourth communication mode to communicate. Satellite a and the n 2 Each observation satellite can send the target topic message. Therefore, in the second communication cycle, a total of n 1 +n 2 +1 target topic message can be sent, total n 1 A discovery topic message can be sent.
[0113] In this case, in the second communication cycle, the n 2 Satellite d among the n observation satellites determines that it no longer meets the first preset observation condition, and satellite d adopts the second communication mode to communicate in the third communication cycle.2 -1 fusion star and satellite a communicate using the third communication mode.
[0114] In summary, the advantages of the above scheme include more flexible dynamic process, which is conducive to promoting the sensitivity of inter-satellite interaction. Since the communication mode can be switched across levels under the guidance of the observation angle, more accurate target position information can be received by other satellites earlier.
[0115] Example 3
[0116] Figure 3 This is the flow chart of multi-satellite collaborative target tracking.
[0117] Step 1: Based on the publish-subscribe mechanism, set the "discovery topic" and "target topic".
[0118] Step 2, set four roles: ordinary star, visible star, observation star, and fusion star. And first set itself as an ordinary star. Ordinary star: Calculate according to the coordinates of the monitoring target in the "discovery topic" and the satellite's own coordinates to determine whether detection can be performed. Satellites that cannot observe the monitoring target in the constellation are ordinary stars, and ordinary stars must subscribe to the "discovery topic". Visible star: Calculate according to the coordinates of the monitoring target in the "discovery topic" and the satellite's own coordinates to determine whether detection can be performed. Satellites that can observe the monitoring target in the constellation are visible stars, and visible stars must subscribe to the "discovery topic" and "target topic". Observation star: Calculate the observation angle according to the coordinates of the monitoring target and the satellite's own coordinates, and receive the coordinates of other satellites and the coordinates of the monitoring target through the subscribed "target topic". The observation angle of other satellites can be calculated. If its own observation angle is the smallest of the two visible stars, it can become an observation star, otherwise it will exit the observation star. After becoming an observation star, it subscribes to the "discovery topic" and "target topic" and publishes messages under the "target topic". Fusion star: The fusion star is the observation star with the smallest observation angle. After becoming a fusion star, it subscribes to the "discovery topic" and "target topic" and publishes messages under the "discovery topic" and "target topic". This role classification is used for target information processing and tracking, which can realize the dynamic joining and exit of observation stars and fusion stars. The messages transmitted under the "discovery topic" and "target topic" are the most accurate monitoring target information at the current moment. The four role conversion relationships are shown in Figure 4 .
[0119] Step 3: Subscribe to the "Discovery Topic" message.
[0120] Step 4: In this step, the "discovery topic" message is received cyclically. If a new tracking target is found, a message based on the "discovery topic" is sent in this step.
[0121] Step 5: After receiving the message of the discovery topic, determine whether the satellite is visible to the monitoring target. Calculate based on the coordinates of the monitoring target in the "discovery topic" and the satellite's own coordinates. If the satellite can observe the monitoring target, it will set itself as a visible star.
[0122] Step 6: Subscribe to the message of the "target topic".
[0123] Step 7: Receive the message of the "target topic".
[0124] Step 8: Calculate the observation angle based on the coordinates of the monitored target and the satellite's own coordinates, and receive the coordinates of other satellites through the subscribed "target topic". Combined with the coordinates of the monitored target, the observation angles of other satellites can be calculated. If the satellite's own observation angle is the smallest of the two visible stars, it will be set as the observed star. Otherwise, exit the observed star and set it as a visible star (the visible star will no longer publish messages between the "target topic").
[0125] (The coordinates of the monitored target are observed by the satellite itself, for example)
[0126] Step nine, send a message based on the "target topic".
[0127] Step 10: Determine whether it is the observation star with the smallest observation angle. If so, set itself as the fusion star. Otherwise, exit the fusion star. If it is still the two satellites with the smallest observation angle, set it as the observation star (the visible star no longer publishes messages based on the "discovery topic"), otherwise set it as the visible star (the visible star no longer publishes messages based on the "target topic").
[0128] Step 11: Send a message based on the "discovery topic" and a message based on the "target topic".
[0129] The present invention adopts a publish-subscribe mechanism. Publish-subscribe is a message paradigm in which the sender of a message (called a publisher) does not send the message directly to a specific receiver (called a subscriber). Instead, the published messages are divided into different categories without knowing which subscribers (if any) may exist. Similarly, subscribers can express interest in one or more categories and only receive messages of interest without knowing which publishers (if any) exist. The connection between publishers and subscribers is achieved through topics. Topics achieve this goal by linking together names (unique in the system), data types, and characteristics related to the data itself. It is through topics that publishers and subscribers that are loosely related or even unrelated in space and time are associated. By using the publish-subscribe mechanism, it is possible to obtain data on demand without clarifying the sender and user of data during the transmission and use of intersatellite data. It also has the advantages of fast acquisition of data sources, autonomous joining and exiting of publishers and subscribers, and reduced bandwidth traffic.
[0130] After all satellites in the constellation are powered on, they subscribe to a "discovery topic". After any satellite in the constellation detects a new tracking target, it publishes a message based on the "discovery topic", and the message content is the discovered target information. After all satellites in the constellation receive the "discovery topic" message, they create a "target topic" contained in the message based on the message content and subscribe to the topic. When all satellites in the constellation detect the target, they send a message based on the "target topic", and the message contains the current monitoring information of the target. The data content contained in the "discovery topic" is designed to be: monitoring target identification, monitoring target coordinates, and timestamp. The data content contained in the "target topic" is designed to be: satellite identification, satellite coordinates, monitoring target identification, monitoring target coordinates, and the remaining observation time calculated based on the observation angle.
[0131] The present invention divides the satellites in the constellation into four categories: ordinary stars, visible stars, observation stars, and fusion stars. The information processing method and tracking scheme of each type of satellite are different. Ordinary stars: Calculate according to the coordinates of the monitoring target in the "discovery topic" and the satellite's own coordinates to determine whether detection can be performed. Satellites that cannot observe the monitoring target in the constellation are ordinary stars, and ordinary stars need to subscribe to the "discovery topic". Visible stars: Calculate according to the coordinates of the monitoring target in the "discovery topic" and the satellite's own coordinates to determine whether detection can be performed. Satellites that can observe the monitoring target in the constellation are visible stars, and visible stars need to subscribe to the "discovery topic" and "target topic". Observation stars: Calculate the observation angle according to the coordinates of the monitoring target and the satellite's own coordinates, and receive the coordinates of other satellites and the coordinates of the monitoring target through the subscribed "target topic", and calculate the observation angles of other satellites. If its own observation angle is the smallest of the two among all visible stars, it can become an observation star, otherwise it exits the observation star. After becoming an observation star, it subscribes to the "discovery topic" and "target topic", and publishes messages under the "target topic". Fusion star: The fusion star is the observation star with the smallest observation angle. After becoming a fusion star, it subscribes to the "discovery topic" and "target topic" and publishes messages under the "discovery topic" and "target topic". This role classification is used for target information processing and tracking, which can realize the dynamic joining and exit of observation stars and fusion stars. The messages transmitted under the "discovery topic" and "target topic" are the most accurate monitoring target information at the current moment.
[0132] Although the present invention is disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be based on the scope defined by the claims of the present invention.
Claims
1. A multi-satellite cooperative target tracking method based on publish-subscribe mechanism, characterized in that: include: When the satellite observes the monitoring target for the first time, the satellite is triggered to change the first communication mode to the second communication mode, wherein the first communication mode is used to obtain the discovery theme message; the second communication mode is used to obtain the discovery theme message and the target theme message; When the communication mode of the satellite is the second communication mode, obtaining a target topic message in a first communication cycle; According to all the target topic messages obtained in the first communication cycle, it is judged whether the satellite meets the first preset observation condition; if so, the communication mode of the satellite is updated to the third communication mode, and the third communication mode is used to obtain the discovery topic message and the target topic message, and to send the satellite's own target topic message; When the communication mode of the satellite is the third communication mode, obtaining a discovery topic message and a target topic message in a second communication cycle, and sending the satellite's own target topic message; Based on all the target topic messages obtained in the second communication cycle, determine whether the satellite meets the first preset observation condition and the second preset observation condition. If both are met, update the satellite's communication mode to the fourth communication mode. The fourth communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own discovery topic messages and target topic messages.
2. The method according to claim 1, characterized in that The discovery topic message includes: monitoring target identification, monitoring target coordinates, and timestamp; the target topic message contains data content including: satellite identification, monitoring target identification, and observation parameters. The observation parameters include at least one of the following: observation angle, satellite coordinates, monitoring target coordinates, and remaining observation time.
3. The method according to claim 1, characterized in that All target topic messages acquired in the first communication cycle indicate a plurality of first values of the observation parameter. If the observation parameter of the satellite itself is better than a suboptimal value among the plurality of first values, the satellite satisfies the first preset observation condition. All target topic messages acquired in the second communication cycle indicate a plurality of second values of the observation parameter. If the satellite's own observation parameter is better than the optimal value among the plurality of first values, the satellite satisfies the second preset observation condition.
4. The method according to claim 3, characterized in that The first preset observation condition is that the satellite's own observation angle is one of the two smallest observation angles of all visible stars, observation stars, and fused stars; the second preset observation condition is that the satellite's own observation angle is the smallest observation angle of all visible stars, observation stars, and fused stars.
5. The method according to claim 4, characterized in that All target topic messages obtained during the second communication cycle are used to indicate the satellite coordinates of the remaining visible stars, observation stars, and fused stars except the satellite itself, and then combined with the monitoring target coordinates indicated by the discovery topic message during the second communication cycle to determine the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars during the second communication cycle, and then determine whether the satellite meets the second preset observation condition.
6. The method according to claim 5, characterized in that All target topic messages obtained during the first communication cycle are used to indicate the satellite coordinates of the remaining visible stars, observation stars, and fused stars except the satellite itself, and then combined with the monitoring target coordinates observed by the satellite itself to determine the observation angles of the satellite itself and the remaining visible stars, observation stars, and fused stars during the first communication cycle, and then determine whether the satellite meets the first preset observation condition.
7. The method according to claim 1, characterized in that When the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the start of the next communication cycle; if so, it determines to adopt the fourth communication mode for communication in the next communication cycle; if not, it determines to adopt the third communication mode for communication in the next communication cycle.
8. The method according to claim 1, characterized in that When the satellite adopts the fourth communication mode in the current communication cycle, the satellite determines whether it meets the second preset observation condition before the next communication cycle starts; If yes, determine to use the fourth communication mode for communication in the next communication cycle; if no, continue to determine whether the first preset observation condition is met, and if the first preset observation condition is met, determine to use the third communication mode for communication in the next communication cycle; If the first preset observation condition is not met, it is determined to adopt the second communication mode for communication in the next communication cycle.
9. The method according to claim 1, characterized in that: When the satellite determines that it cannot observe the monitoring target, the satellite directly switches to the first communication mode for communication.
10. A multi-satellite cooperative target tracking method based on publish-subscribe mechanism, characterized in that: include: In the case where the communication mode of the satellite is used to obtain the discovery topic message and the target topic message, obtaining the target topic message in the third communication cycle; Determining whether the satellite meets the first preset observation condition and the second preset observation condition according to all target topic messages acquired in the third communication cycle; If both the first preset observation condition and the second preset observation condition are not met, the communication mode of the satellite in the next communication cycle is the second communication mode, and the second communication mode is used to obtain discovery topic messages and target topic messages; if the first preset observation condition is met but the second preset observation condition is not met, the communication mode of the satellite in the next communication cycle is the third communication mode, and the third communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own target topic messages; if the first preset observation condition and the second preset observation condition are met, the communication mode of the satellite in the next communication cycle is the fourth communication mode, and the fourth communication mode is used to obtain discovery topic messages and target topic messages, and to send the satellite's own discovery topic messages and target topic messages.