Satellite system and system control method

Abstract

In a system where multiple satellites autonomously take turns acting as servers, the servers will be rotated according to instructions from a ground station. [Solution] The server, which is a satellite 200 that has obtained server rights, receives a priority notification when a ground station 300 transmits the priority notification. If the server receives the priority notification, it selects the satellite 200 specified in the priority notification as the server for the next time interval. If the server does not receive the priority notification, it selects one of the multiple satellites as the server for the next time interval based on the status information of the server and the status information of each client.

Description

Technical Field

[0001] The present disclosure relates to a satellite system including a plurality of artificial satellites.

Background Art

[0002] With the development of mobile terminals, IoT, constellations by artificial satellites, etc., the configuration of computer systems in recent years has become more fluid, and the software configuration and system configuration may continuously change after the product is shipped.

[0003] In the operation of conventional artificial satellites, the operation plan for each satellite is considered on the ground, and the command content called a command is created based on the operation plan. Then, the command is transmitted from the operation facility called the ground system to the artificial satellite. The artificial satellite that has received the command implements missions such as ground observations and experiments in space according to the content of the command. The artificial satellite that has implemented the mission creates telemetry, which is information to be transmitted to the ground system, based on information such as the results of the mission, and transmits the telemetry at a determined timing. The mission is realized by repeating these processes. In mission operation, when the number of artificial satellites to be operated increases, the operation load of the ground system increases by the increased amount.

[0004] There is an operation concept called satellite constellation. In satellite constellation, a large number of satellites are operated as one system by cooperative operation. Since tens to thousands of satellites may be used in one satellite constellation, the load on the ground system is too large in the conventional operation, and there is a possibility that the operation of the satellite constellation may become impossible.

[0005] As a method for controlling a system including a plurality of entities, there is a method of centrally managing the system on a server by using one entity as a server and the remaining entities as clients. Applying this method to a satellite constellation presents a problem: if a server satellite fails, the entire system becomes dysfunctional.

[0006] Patent Document 1 proposes a method to reduce the risk of operational failure. In a system where multiple entities each consume resources and perform their respective functions, the method described in Patent Document 1 controls the performance of each entity while satisfying the constraints on the overall system's resource consumption. This method has the constraint that only one server can receive resources within a given time interval, and the time window for resource reception is constant. Although the amount of resources required should differ for each individual, this method alternates between servers and clients at the same time window for all of them. As a result, surplus resources are generated. In this method, servers are selected based solely on the resource requirements of each individual. Therefore, the state of each individual is not adequately considered, which can lead to poor processing efficiency. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2008-90607 [Overview of the project] [Problems that the invention aims to solve]

[0008] This disclosure aims to enable multiple satellites that autonomously take turns hosting each other to switch servers in accordance with instructions from a ground station. [Means for solving the problem]

[0009] The satellite system disclosed herein is In each time interval, one of the satellites acquires server rights, and the satellite that acquires said server rights performs resource consumption, and A ground station that transmits a priority notification specifying which satellite to perform resource consumption when one of the aforementioned multiple satellites is to perform the resource consumption, It is equipped with. The server among the aforementioned multiple artificial satellites that has obtained server rights is, When the ground station transmits the priority notification, the ground station receives the priority notification. In addition to carrying out the aforementioned resource consumption, status information is received from each client, which is a satellite other than the server. Upon receiving the aforementioned priority notification, the satellite specified in the priority notification is selected as the server for the next time interval. If the priority notification has not been received, one of the multiple satellites is selected as the server for the next time interval based on the status information of the server and the status information of each client. If the satellite selected as the server for the next time interval is the client, a server rights transfer notification will be sent to the selected satellite. [Effects of the Invention]

[0010] According to this disclosure, it becomes possible for multiple satellites that autonomously take turns acting as servers to switch servers in accordance with instructions from a ground station. [Brief explanation of the drawing]

[0011] [Figure 1] Configuration diagram of the satellite system 100 in Embodiment 1. [Figure 2] Hardware configuration diagrams for the artificial satellite 200 and the ground station 300 in Embodiment 1. [Figure 3] A diagram illustrating the functional configuration of the computer 210 of the artificial satellite 200 in Embodiment 1. [Figure 4] Flowchart of the satellite control method in Embodiment 1. [Figure 5] Flowchart of the server processing (S110) in Embodiment 1. [Figure 6]Flowchart of the client process (S120) in Embodiment 1. [Figure 7] Flowchart of the server selection process (S130) in Embodiment 1.

Mode for Carrying Out the Invention

[0012] In the embodiments and the drawings, the same elements or corresponding elements are denoted by the same reference numerals. The descriptions of the elements denoted by the same reference numerals as the described elements are omitted or simplified as appropriate. The arrows in the figures mainly indicate the flow of data or the flow of processing.

[0013] Embodiment 1. The satellite system 100 will be described based on FIGS. 1 to 7.

[0014] ***Description of the Configuration*** Based on FIG. 1, the configuration of the satellite system 100 will be described. The satellite system 100 is a satellite constellation system. The satellite system 100 includes a plurality of artificial satellites 200 and one or more ground stations 300. The number of artificial satellites 200 and the number of ground stations 300 are not limited to the numbers shown in FIG. 1.

[0015] The plurality of artificial satellites 200 fly in orbits around the Earth. The dashed circles represent the orbits in which the plurality of artificial satellites 200 fly. <00001​​​​​​​​

[0017] Each ground station 300 controls multiple satellites 200 via the satellite 200 with which it has communicated. Satellites 200 other than the one communicating with the ground station 300 are controlled via communication with the satellite 200 that has communicated with the ground station 300.

[0018] Based on Figure 2, the hardware configuration of satellite 200 and ground station 300 will be explained. Figure 2 shows a case where the satellite system 100 comprises four artificial satellites 200 and two ground stations 300. Dashed lines represent inter-satellite communication networks. Dotted lines represent inter-satellite and inter-ground communication networks. In artificial satellite 200, solid lines connecting elements represent intra-satellite communication networks or signal lines. In ground station 300, solid lines connecting elements represent intra-station communication networks or signal lines. The ground system 110 is a system consisting of one or more ground stations 300.

[0019] The artificial satellite 200 is equipped with a communication device 201, a measuring device 202, and a computer 210. The communication device 201 is a device for communicating between the ground system 110 and other satellites 200. Communication with satellites 200 is achieved using the communication device 201. The measuring device 202 is a device for calculating the distance (inter-satellite distance) to other artificial satellites 200. Examples of measuring device 202 include radar transmitters and receivers, and satellite positioning equipment. An example of satellite positioning is the Global Positioning System (GPS). Computer 210 is a computer equipped with hardware such as a processor and memory.

[0020] The ground station 300 includes a communication device 301, a movable device 302, and a computer 310. The communication device 301 is a device for communicating with the artificial satellite 200. Communication with the ground station 300 is achieved using the communication device 301. The movable device 302 is a device for moving the ground station 300. Computer 310 is a computer equipped with hardware such as a processor and memory.

[0021] Based on Figure 3, the configuration of the computer 210 will be explained. The computer 210 comprises elements such as an information gathering unit 211, a resource receiving unit 212, and an individual control unit 213. These elements are implemented in software. The individual control unit 213 includes a main control unit 220, a resource consumption unit 221, a completion notification unit 222, a bidding unit 223, a status diagnosis unit 224, a bidding unit 225, a server selection unit 226, a server replacement unit 227, a distance calculation unit 228, and a priority receiving unit 229.

[0022] The functions of each element of the computer 210 are primarily executed by the processor, and the data processed by each function and the data generated by each process are temporarily stored in memory.

[0023] ***Explanation of operation*** The operating procedure of satellite system 100 corresponds to the system control method. The operating procedure for artificial satellite 200 corresponds to the satellite control method. Furthermore, the operating procedure for computer 210 corresponds to the processing procedure of the satellite control program. The operating procedure of the ground station 300 corresponds to the ground control method. Furthermore, the operating procedure of the computer 310 corresponds to the processing procedure of the ground control program.

[0024] The system control method will be explained. In a system of multiple satellites 200, one satellite 200 acquires server rights at any given time interval, and that satellite 200 performs resource consumption. This resource consumption refers to the consumption (receipt) of resources from resource-providing equipment. General resource consumption (such as power consumption within the system) is always performed by each satellite 200. Resource supply facilities are artificial satellites that are located in the same position in space to provide resources. Each ground station 300 sends a priority notification when it wants to have one of the multiple satellites 200 perform resource consumption. The priority notification specifies which satellite 200 should perform resource consumption. For example, ground station 300 determines which satellite 200 will carry out the mission in space based on the operational plan, and sends a priority notification specifying the determined satellite 200. Alternatively, ground station 300 sends a priority notification specifying the satellite 200 selected by the operator.

[0025] If the distance between ground station 300 and satellite 200 is large, communication between ground station 300 and satellite 200 becomes difficult. Each ground station 300 moves by activating its movable device 302 if it is unable to communicate with any of the satellites 200. If the distance between the ground station 300 and the satellite 200 increases, the movable device 302 moves the ground station 300 to facilitate communication between the ground station 300 and the satellite 200.

[0026] The satellite control method will be explained based on Figure 4. The satellite control method processing is performed on each satellite 200 and is repeated each time the time interval in which server rights are exercised ends.

[0027] In step S101, the main control unit 220 checks whether its satellite 200 has server rights for the current time interval.

[0028] If the server replacement unit 227 has received a server rights transfer notification for the current time interval, then the server rights for the current time interval belong to its own satellite 200.

[0029] If the server rights for the current time interval belong to the satellite 200, the process proceeds to server processing (S110). If satellite 200 does not have server rights for the current time interval, the process proceeds to client processing (S120).

[0030] After server processing (S110) or client processing (S120), the process proceeds to step S101.

[0031] Based on Figure 5, the server processing (S110) will be explained. Server processing (S110) is performed by satellite 200, which has server rights. The satellite 200 that has server rights is referred to as the "server." Currently, the satellite 200 closest to the resource supply facility is the server. In the current segment, the server is receiving resources from the resource supply facility. Resource receipt means that each satellite 200 receives resources from the resource supply facility. Satellite-200, which does not have server rights, is referred to as a "client."

[0032] In step S111, the resource consumption unit 221 starts consuming resources.

[0033] In step S112, the bidding unit 223 sends an information request to each client.

[0034] An information request is sent to request the provision of status information. Status information includes information about resources and information indicating the distance between satellites. Resource information includes information about power, memory, and energy (such as propellant) within the system.

[0035] In step S113, the status diagnosis unit 224 calculates the status information of the server.

[0036] The status information includes the performance difference, segment difference, and inter-satellite distance.

[0037] The performance difference is the difference between the current performance value and the target performance value. The performance value could be, for example, memory usage. The performance difference is calculated as the performance lower limit difference and the performance upper limit difference. The lower performance limit difference is the difference between the current performance value and the target lower performance limit. If the current performance value is less than the target lower performance limit, the lower performance limit difference will be a negative value. A negative lower performance limit difference indicates a shortage of resources. The performance upper limit difference is the difference between the current performance value and the target performance upper limit. If the current performance value is greater than the target performance upper limit, the performance upper limit difference will be a positive value. A positive performance upper limit difference indicates that resources are sufficient.

[0038] The interval difference is the difference between the number of intervals and the number of elapsed intervals. The number of intervals is a predetermined number of time intervals. The elapsed interval is the number of time intervals from the previous time interval in which the server had rights to the current time interval. A large interval difference indicates a state where resources have not been supplied for some time.

[0039] The inter-satellite distance is the distance between satellite 200 (server) and another satellite 200 (client). The inter-satellite distance is calculated using the measuring device 202. If the measuring device 202 is a radar, the inter-satellite distance is calculated based on the time from when the radar transmits radio waves until the radar receives the radio waves reflected by the other satellite 200. If the measuring device 202 is a satellite positioning device, the distance is calculated based on the position information of each satellite 200. The position information is obtained using a satellite positioning system (e.g., GPS). The position information of the other satellite 200 is collected by the information collection unit 211. The distance between a client that is too far from the server and the server is calculated via inter-satellite communication.

[0040] In step S114, the bidding unit 225 receives information provision responses sent from each client. The information provided response indicates the client's status information.

[0041] In step S115, the priority receiving unit 229 receives a priority notification if any of the ground stations 300 transmit a priority notification.

[0042] In step S130, the server selection unit 226 selects a server for the next time interval based on the server status information, the status information of each client, and priority notifications. Details of step S130 will be described later.

[0043] In step S116, the resource consumption unit 221 stops consuming resources when the current time interval has passed. However, if resource consumption is completed before the current time interval expires, the resource consumption unit 221 will stop resource consumption when it is completed. The completion notification unit 222 will also notify the server replacement unit 227 of the completion of resource consumption.

[0044] In step S117, when the current time interval has passed or the completion of resource consumption has been notified, the server change unit 227 sends a server rights transfer notification to the satellite 200 selected as the server for the next time interval. However, if the satellite 200 selected as the server for the next time interval is the same satellite 200, it is not necessary to send a server rights transfer notification. The server transfer notification indicates, for example, the satellite 200 selected as the server for the following time intervals.

[0045] The client processing (S120) will be explained based on Figure 6. Client processing (S120) is performed by each individual client.

[0046] In step S121, the bidding unit 225 receives an information request sent from the server.

[0047] In step S122, the status diagnosis unit 224 calculates status information. The method for calculating the status information is the same as the method in step S113.

[0048] In step S123, the bidding unit 225 sends an information provision response to the server.

[0049] In step S124, the server replacement unit 227 receives a server rights transfer notification when the server sends a server rights transfer notification to its own satellite 200. This transfers server rights from the server to the client.

[0050] Step S130 will be explained based on Figure 7. In step S131, the server selection unit 226 classifies each of the artificial satellites 200.

[0051] Each of the 200 satellites is classified as either an On Card, a Coast Card, or a Skip Card. Satellite 200, whose performance lower limit difference is a negative value, is classified as "on-card". The current performance value of Satellite 200, which is classified as "on-card", satisfies the relationship "current performance value < target performance lower limit". Satellite 200, whose lower performance difference is a positive value and whose upper performance difference is a negative value, is classified as a Coast Card. The current performance value of Satellite 200 classified as a Coast Card satisfies the relationship "Target lower performance < Current performance value < Target upper performance". Satellite 200, whose performance upper limit difference is a positive value, is classified as a skip card. The current performance value of Satellite 200 classified as a skip card satisfies the relationship "current performance value > target performance lower limit".

[0052] The collection of 200 artificial satellites classified as "on-card" is called the "on-card group." The collection of 200 artificial satellites classified as Coast Cards is called the Coast Card group. The collection of 200 artificial satellites classified as skip cards is called the skip card group.

[0053] In step S132, the server selection unit 226 determines whether a priority notification has been received. If a priority notification has been received, the process proceeds to step S133. If no priority notification has been received, the process proceeds to step S134.

[0054] In step S133, the server selection unit 226 selects the satellite 200 specified in the priority notification as the server for the next time interval.

[0055] In step S134, the server selection unit 226 determines whether there is an artificial satellite 200 classified as on card. If there is a satellite 200 classified as on card, the process proceeds to step S135. If no satellite 200 is classified as on card, the process proceeds to step S136.

[0056] In step S135, the server selection unit 226 selects one or more satellites 200 included in the on-card group that are close to the resource supply facility as the server for the next time interval.

[0057] Satellite 200, which is closest to the resource supply facility, can be determined based on the inter-satellite distance between the server and each client.

[0058] For example, the server selection unit 226 sets the satellite 200 closest to the resource supply facility as the server for the next time interval.

[0059] In step S136, the server selection unit 226 determines whether there is a satellite 200 classified as a coast card. If there is a satellite 200 classified as a coast card, the process proceeds to step S137. If no satellite 200 is classified as a coast card, the process proceeds to step S138.

[0060] In step S137, the server selection unit 226 selects one or more satellites 200 included in the coast card group that are closest to the resource supply facility as the server for the next time interval. The selection method is the same as the method in step S135.

[0061] In step S138, the server selection unit 226 selects its own satellite 200 as the server for the next time interval.

[0062] ***Effects of Embodiment 1*** Embodiment 1 relates to an autonomous control method for a system (satellite constellation) consisting of multiple individuals (artificial satellites 200). By providing each individual with a resource consumption completion notification, the previously fixed interval width can be made variable, improving processing performance without generating excess resource consumption. By equipping each individual unit with a function to calculate inter-satellite distance, it becomes possible to consider not only the feasibility of resource consumption but also satellite operation and communication, thereby improving processing efficiency. By providing each unit with a priority notification receiving function, it can perform processing in response to changes in the surrounding environment. By increasing the number of ground stations (300) to match the number of satellites (200), the number of server rights will also increase, leading to more efficient resource supply.

[0063] ***Supplement to Embodiment 1*** A mission in space is accomplished by repeatedly transmitting commands from a ground station 300, having a satellite 200 receive the commands and execute the mission, and then having the satellite 200 transmit telemetry to the ground station 300. The telemetry is compiled based on the mission results and transmitted at predetermined intervals. Examples of missions include ground observations or experiments in space. Each satellite 200 is equipped with devices (observation equipment, experimental equipment, etc.) for carrying out the mission. Resource consumption is the act of using resources such as electricity to perform a specific action. For example, a mission in outer space can be carried out through resource consumption.

[0064] The information gathering unit 211 has the function of collecting information about the surroundings of the satellite 200 for operational purposes (information gathering). Information gathering includes calculating the distance between satellites and imaging the surroundings through mission operations. The resource receiving unit 212 has the function of receiving resources when its own satellite 200 is selected as a server. Processing performance can be improved by consuming the received resources. Resource consumption refers to the consumption of the received resources. The status diagnosis unit 224 has the function of diagnosing the resource consumption status of its own satellite 200.

[0065] Satellite system 100 functions as an autonomous distributed control system in which satellites cooperate with each other even without commands from the ground. Ground station 300 is also called the operational facility. Ground station 300 is the facility used to operate satellite 200 and observe telemetry from satellite 200. Conventional ground stations were fixed in one location. In contrast, ground station 300 can be moved by being mounted on a vehicle, ship, or aircraft.

[0066] The number of server rights may be the same as the number of ground stations (300).

[0067] The following clarifies the terminology related to Embodiment 1. "Resources" refer to the amount of control operations, such as electricity, that are input into a system within a unit of time. "Performance" refers to indicators such as memory usage. "Efficiency value" refers to the amount of performance variation per unit time. An "individual" is an element (such as a subsystem) that is placed in various locations and constitutes a system. Satellite 200 and ground station 300 are individual elements in satellite system 100. A "server" is an individual entity within a system that implements policies and makes decisions. A "client" is any individual entity other than a server. A "circuit network" is a network. In other words, a circuit network is a communication channel that allows information to be exchanged between the individual components that make up a system. "Bidding" means that the server sends a request for information to all client devices via the network, and all client devices report the information. "Bidding" means that each client unit reports its own performance and resources to the server unit in response to a bid from the server unit. "Evaluation" means deciding on a strategy for resource consumption based on the information gathered from bids. "Implementation" means carrying out control operations. "Replacement" means transferring server rights. A "segment" refers to the time period during which resources are allocated. Specifically, a segment corresponds to the interval from the start time of a function cycle (bidding, evaluation, implementation, and rotation) to the end time of that function cycle. "Priority" in a satellite constellation refers to the priority based on mission operations, such as wanting to image near a particular satellite. A priority notification is issued from the ground when it is necessary to supply resources to a satellite to improve its performance for that mission.

[0068] The functions of computer 210 and computer 310 may be implemented using software, hardware, firmware, or a combination thereof. The "part" in each element of the computer 210 may be read as "processing," "process," "circuit," or "circuit."

[0069] The various aspects of this disclosure are described below as appendices. (Note 1) In each time interval, one of the satellites acquires server rights, and the satellite that acquires said server rights performs resource consumption, and A ground station that transmits a priority notification specifying which satellite to perform resource consumption when one of the aforementioned multiple satellites is to perform the resource consumption, Equipped with, The server among the aforementioned multiple artificial satellites that has obtained server rights is, When the ground station transmits the priority notification, the ground station receives the priority notification. In addition to carrying out the aforementioned resource consumption, status information is received from each client, which is a satellite other than the server. Upon receiving the aforementioned priority notification, the satellite specified in the priority notification is selected as the server for the next time interval. If the priority notification has not been received, one of the multiple satellites is selected as the server for the next time interval based on the status information of the server and the status information of each client. If the satellite selected as the server for the next time interval is the client, a server rights transfer notification will be sent to the selected satellite. Satellite system.

[0070] (Note 2) If the resource consumption is completed before the time interval for which the server rights were obtained expires, the server will send the server rights transfer notification when the resource consumption is completed. The satellite system described in Appendix 1.

[0071] (Note 3) The ground station determines, based on the operational plan, which satellite will carry out the mission in space, and transmits the priority notification specifying the determined satellite. The satellite systems described in Appendix 1 or Appendix 2.

[0072] (Note 4) Each client calculates the distance to the server as the inter-satellite distance, The status information for each client includes information indicating the distance between satellites. A satellite system described in any one of the appendices 1 through 3.

[0073] (Note 5) Each client calculates the inter-satellite distance using radar or a satellite positioning system. The satellite system described in Appendix 4.

[0074] (Note 6) The ground station is equipped with a movable device for moving the ground station if it is unable to communicate with any of the multiple satellites. A satellite system described in any one of the appendices 1 through 5.

[0075] (Note 7) In each time interval, one of the satellites acquires server rights, and the satellite that acquires said server rights performs resource consumption, and A ground station that transmits a priority notification specifying which satellite to perform resource consumption when one of the aforementioned multiple satellites is to perform the resource consumption, A system control method for a satellite system comprising: The server among the aforementioned multiple artificial satellites that has obtained server rights is, When the ground station transmits the priority notification, the ground station receives the priority notification. In addition to carrying out the aforementioned resource consumption, status information is received from each client, which is a satellite other than the server. Upon receiving the aforementioned priority notification, the satellite specified in the priority notification is selected as the server for the next time interval. If the priority notification has not been received, one of the multiple satellites is selected as the server for the next time interval based on the status information of the server and the status information of each client. If the satellite selected as the server for the next time interval is the client, a server rights transfer notification will be sent to the selected satellite. System control method. [Explanation of Symbols]

[0076] 100 Satellite system, 110 Ground system, 200 Artificial satellite, 201 Communication equipment, 202 Measuring equipment, 210 Computer, 211 Information gathering unit, 212 Resource receiving unit, 213 Individual control unit, 220 Main control unit, 221 Resource consumption unit, 222 Completion notification unit, 223 Bidding unit, 224 Status diagnosis unit, 225 Bidding unit, 226 Server selection unit, 227 Server replacement unit, 228 Distance calculation unit, 229 Priority receiving unit, 300 Ground station, 301 Communication equipment, 302 Movable equipment, 310 Computer.

Claims

1. In each time interval, one of the satellites acquires server rights, and the satellite that acquires said server rights performs resource consumption, and multiple satellites, A ground station that transmits a priority notification specifying which satellite to perform resource consumption when one of the plurality of satellites is to perform resource consumption, Equipped with, The server among the aforementioned multiple artificial satellites that has obtained server rights is, When the ground station transmits the priority notification, the ground station receives the priority notification. In addition to carrying out the aforementioned resource consumption, status information is received from each client, which is a satellite other than the server. Upon receiving the aforementioned priority notification, the satellite specified in the priority notification is selected as the server for the next time interval. If the priority notification has not been received, one of the multiple satellites is selected as the server for the next time interval based on the status information of the server and the status information of each client. If the satellite selected as the server for the next time interval is a client, a server rights transfer notification will be sent to the selected satellite. Satellite system.

2. If the resource consumption is completed before the time interval for which the server rights were obtained expires, the server will send the server rights transfer notification when the resource consumption is completed. The satellite system according to claim 1.

3. The ground station determines, based on the operational plan, which satellite will carry out the mission in space, and transmits the priority notification specifying the determined satellite. The satellite system according to claim 1.

4. Each client calculates the distance to the server as the inter-satellite distance, The status information for each client includes information indicating the distance between satellites. The satellite system according to claim 1.

5. Each client calculates the inter-satellite distance using radar or a satellite positioning system. The satellite system according to claim 4.

6. The ground station is equipped with a movable device for moving the ground station if it is unable to communicate with any of the multiple satellites. The satellite system according to any one of claims 1 to 5.

7. In each time interval, one of the satellites acquires server rights, and the satellite that acquires said server rights performs resource consumption, and A ground station that transmits a priority notification specifying which satellite to perform resource consumption when one of the plurality of satellites is to perform resource consumption, A system control method for a satellite system comprising: The server among the aforementioned multiple artificial satellites that has obtained server rights is, When the ground station transmits the priority notification, the ground station receives the priority notification. In addition to carrying out the aforementioned resource consumption, status information is received from each client, which is a satellite other than the server. Upon receiving the aforementioned priority notification, the satellite specified in the priority notification is selected as the server for the next time interval. If the priority notification has not been received, one of the multiple satellites is selected as the server for the next time interval based on the status information of the server and the status information of each client. If the satellite selected as the server for the next time interval is a client, a server rights transfer notification will be sent to the selected satellite. System control method.

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