DESCRIPTION MOVABLE BODY POSITION MEASUREMENT SYSTEM, CENTRAL STATION, QUESTION CONTROL METHOD USED THEREIN, AND STORAGE MEDIUM STORING PROGRAM THEREOF Technical Field [OOOl] The present invention relates to a movable body position measurement 10 system, a central station, a question control method used therein, and a program thereof and, in particular, to an aircraft measurement system (a multilateration system) and a question control method of a transmission station used therein. Background Art [0002] 15 The multilateration system is the system in which an SSR (Secondary Surveillance Radar) mode A/C response, an SSR mode S response, and a capture or extended squitter signal that are transmitted by an aircraft are received by not less than four reception stations on the ground, data of the signals are collected in a central processing section through a communication line, and in which a geometric 20 position of the aircraft is measured in the central processing section from differences of reception times among the respective reception stations (for example, refer to Patent Literature 1). [0003] Here, an SSR mode A is the mode to acquire identification information of 25 the aircraft, an SSR mode C is the mode to acquire pressure altitude information, and an SSR mode S is the mode for acquiring unique address information of the aircraft and individually asking each aircraft a question. In addition, the SSR mode A/C is a scheme in which all aircraft are asked a common question, and the SSR mode S is a scheme in which each aircraft can be individually asked a 30 question and responds to the aircraft. [0004] The multilateration system may include a transmission station or a transmission/reception station in some cases. In this transmission station or transmission/reception station, the same question as an SSR mode A/C question and an SSR mode S individual question that are asked by an SSR device can be transmitted. Consequently, since the system itself can recognize the transmission time at the same time that it enables detection of the aircraft of only the SSR mode 5 A/C response, a round-trip time from transmission to reception can be detected, and it can be utilized for improving accuracy of positioning. [0005] However, in a complete passive multilateration system, a number of problems occur. Namely, as a number of SSR mode A/C responses are received, 10 the SSR mode A/C responses are superposed (become in a garbled state), and the response signals cannot be decoded since a non-directional or wide directional antenna is used as an antenna. [0006] For this reason, in the complete passive multilateration system, it may not 15 be possible to acquire information and an amount of information necessary which are sufficient to pass information to be acquired (altitude information, a mode A code, etc.) to an air traffic control system of a subsequent stage. [0007] In order to enable the multilateration system to ask a target (an aircraft) the 20 mode S individual question (transmission), positioning of the target and acquisition of an aircraft mode S address (unique address information) are need for initially detected, and the kultilateration system asks the targets (aircraft) the mode S individual question in an order starting from the initially detected target. [0008] 2 5 Note that there is a regulation to suppress a transponder proportion to not more than 2% in ICAO (International Civil Aviation Organization) ANNEX 10 vol4amendment85 6. 6. 3 issued by ICAO in order to suppress the influence of radio wave interference etc. on an established SSR etc. [0009] As a result of this regulation, in a case where a number of aircraft are present in a surveillance airspace of the multilateration system, there is a possibility that not all the aircraft present in the surveillance system can be asked the SSR mode S individual question for obtaining necessary information, or that a question sufficient to acquire necessary information cannot be asked, since the transponder proportion is suppressed to not more than 2%. For this reason, there is a possibility of causing deterioration of reliability and safety of the multilateration system in an air traffic control operation. 5 [OOlO] As technologies using the above-described multilateration, there are included the following: a technology to complement airport surface surveillance by means of an airport surface detection radar by integrating information of multilateration (for example, refer to Patent Literature 2); a technology to 10 discriminate whether there is only a mode A response or only a mode C response by means of signals received by reception stations (for example, refer to Patent Literature 3); a technology to perform highly accurate time synchronization of the plurality of reception stations based on a signal arriving from a GPS (Global Positioning System) satellite (for example, refer to Patent Literature 4); etc. 15 Citation List Patent Literature [OOll] Patent Literature 1 : Japanese Unexamined Patent Application Publication 20 NO. 2009-300146 Patent Literature 2: Japanese Unexamined Patent Application Publication NO. 2007-333427 Patent Literature 3: Japanese Unexamined Patent Application Publication NO. 2011-112465 2 5 Patent Literature 4: Japanese Unexamined Patent Application Publication N0.2010-230448 Summary of Invention Technical Problem 30 [0012] In the above-mentioned aircraft position measurement system relevant to the present invention, in the case of the multilateration system that uses the non-directional or the wide directional antenna, a scheme is employed in which the aircraft existing in the surveillance airspace are asked the SSR mode S individual question in an order starting from an initially detected aircraft or an aircraft having the smallest azimuth angle from magnetic north. [00 131 5 In this case, since there is the regulation to suppress the transponder proportion to not more than 2% in ICAO ANNEX 10 Vol4amendment85 6. 6. 3) issued by ICAO in order to ask the aircraft present in a surveillance coverage the SSR mode s question in an order starting from an initially detected aircraft, a situation occurs in which an aircraft cannot be asked the SSR mode S individual 10 question within a certain time (for example, a one second interval). [00 141 Consequently, since there is a chance of it not being possible to ask a i highly important aircraft which is sending an emergency signal or which a flight status is changed, etc. in respect to the air traffic control, the reliability and safety 15 of the present system may deteriorate. Note that since the above technologies described in Patent Literatures 1 to 4 are not the technologies concerning a case where a transmission station or a transmission/reception station is included in the multilateration system, they cannot solve the above-described problem. [00 151 20 Consequently, an object of the present invention is to solve the above-described problem, and to provide a movable body position measurement system, a central station, a question control method used therein, and a program thereof that can ask moveable bodies as surveillance objects the SSR mode S individual question preferentially in an order starting from the movable body 25 having the highest importance in respect to air traffic control, and that can improve the reliability and safety of the multilateration system. Solution to Problem 100 161 A movable body position measurement system according to the present 30 invention is the movable body position measurement system that includes: a plurality of reception stations that receive a response signal from a movable body; and a central station that positions the movable body based on reception times of the response signal in the plurality of reception stations, and measures a geometric position of the movable body from the reception times of the plurality of reception stations in the central station, in which the movable body position measurement system further includes at least one or more transmission devices that transmit a question signal for obtaining the 5 response signal to the movable body, the central station includes: question signal decision means for deciding a question signal and a transmission time that the transmission device should transmit; generation means for generating the decided question signal and transmission time as question control information; and means for sending out the 10 question control information generated by the generation means to the transmission device, the transmission device includes: means for receiving the question control information; and means for transmitting the question signal specified in the question control information when the transmission time specified in the received 15 question control information and a time of the transmission device coincide with each other, and in which the question signal decision means decides the question signal and the transmission time that the transmission device should transmit based on states of a plurality of movable bodies. 20 [0017] A central station according to the present invention is the central station used for a movable body position measurement system that includes: a plurality of reception stations that receive a response signal from a movable body; and a central station that positions the movable body based on reception times of the 25 response signal in the plurality of receiving stations, and measures a geometric position of the movable body from the reception times of the plurality of reception stations in the central station, in which in the movable body position measurement system, at least one or more transmission devices that transmit a question signal for obtaining the response 3 0 signal to the movable body are disposed, the central station includes: question signal decision means for deciding a question signal and a transmission time that the transmission device should transmit; generation means for generating the decided question signal and transmission time as question control information; and means for sending out the question control information generated by the generation means to the transmission device, when the transmission time specified in the question control information 5 and a time held by the transmission device coincide with each other in the transmission device receiving the question control information, the transmission device transmits the question signal specified in the question control information, and in which the question signal decision means decides the question signal and the 10 transmission time that the transmission device should transmit based on states of a plurality of movable bodies. [00 181 A transmission control method according to the present invention is the transmission control method used for a movable body position measurement 15 system that includes: a plurality of reception stations that receive a response signal from a movable body; and a central station that positions the movable body based on reception times of the response signal in the plurality of reception stations, and measures a geometric position of the movable body from the reception times of the plurality of reception stations in the central station, in which 2 0 in the movable body position measurement system, at least one or more transmission devices that transmit a question signal for obtaining the response signal to the movable body are disposed, the central station executes: question signal decision processing that decides a question signal and a transmission time that the transmission device 25 should transmit; generation processing that generates the decided question signal and transmission time as question control information; and processing that sends out the question control information generated by the generation processing to the transmission device, when the transmission time specified in the question control information 30 and a time of the transmission device coincide with each other in the transmission device receiving the question control information, the transmission device transmits the question signal specified in the question control information, and in which in the question signal decision processing, the question signal and the transmission time that the transmission device should transmit are decided based on states of a plurality of movable bodies. [00 191 5 A program according to the present invention is the program that includes: a plurality of reception stations that receive a response signal from a movable body; and a central station that positions the movable body based on reception times of the response signal in the plurality of reception stations, and is made to be executed by a central processing unit in the central station used for a movable body 10 position measurement system that measures a geometric position of the movable body from reception times of the plurality of reception stations in the central station, characterized in that in the movable body position measurement system, at least one or more transmission devices that transmit a question signal for obtaining the response 15 signal to the movable body are disposed, the program includes: question signal decision processing that decides a question signal and a transmission time that the transmission device should transmit; generation processing that generates the decided question signal and transmission time as question control information; and processing that sends out 20 the question control information generated by the generation processing to the transmission device, the program controls the central station to transmit the question signal specified in the question control information, when the transmission time specified in the question control information and a time of the transmission device coincide 25 with each other in the transmission device receiving the question control information, and that the program makes the question signal decision processing decide the question signal and the transmission time that the transmission device should transmit based on states of a plurality of movable bodies. 3 0 Advantageous Effects of Invention [0020] According to the present invention, by employing a configuration and operation as described above, an effect can be obtained by which the movable bodies as the surveillance objects can be asked the SSR mode s individual question preferentially in an order starting from the movable body having the highest importance in regard to air traffic control, and by which the reliability and safety of the multilateration system can be improved. 5 Brief Description of the Drawings [002 11 Fig. 1 is a block diagram showing a configuration example of the aircraft measurement system (multilateration system) according to the embodiment of the 10 present invention. Fig. 2 is a block diagram showing a configuration example of a central processing section shown in Fig. 1. Fig. 3 is an example of the preset parameters of conditions for determining question priority right with respect to aircraft which are asked a question 15 according to the embodiment of the present invention. Fig. 4 is a transmission control scheme in a case where there are two or more transmission/reception stations according to another embodiment of the present invention. Fig. 5 is a selection scheme of a transmission/reception station that asks questions 20 according to another embodiment of the present invention. Fig. 6 is a diagram showing a transmission form of a question according to yet another embodiment of the present invention. Description of Embodiments 25 [0022] Next, embodiments of the present invention will be explained with reference to drawings. First, a summary of a movable body position measurement system according to the present invention will be explained. The movable body position measurement system according to the present invention relates to an 3 0 aircraft position measurement system [MLAT (Multilateration) system]. Although positions of movable bodies, such as an airplane and a vehicle in an airport, can be measured in the aircraft position measurement system, the mobile body explained in the following explalgation will be just an aircraft. [0023] The present invention pertains to the aircraft measurement system (multilateration system) and a question control scheme of a transmission station used therein and, in particular, provides a question control scheme in which 5 aircraft present in a surveillance airspace are asked an SSR mode s individual question in a prioritized order concerning the multilateration system in which reception processing of a response signal from an aircraft which a has been asked a question by an SSR (Secondary Surveillance Radar) device takes place, in which a response signal is transmitted from an aircraft which has been asked a question 10 by the multilateration system, and an SSR extended squitter signal is received in not less than four reception stations, and in which position measurement of the aircraft is performed in a central station (hereinafter referred to as a central processing section) using this reception signal. [0024] 15 In order to solve the above-described problem, the present invention provides the question control scheme in which aircraft as surveillance objects are asked the SSR mode S individual question in a preferential order starting from the aircraft having the highest importance in respect to air traffic control, and thereby can achieve an improvement in the reliability and safety of the multilateration 20 system. [0025] In the present invention, priority order in which the SSR mode S individual question is asked is determined by priority order conditions by means of parameters in the central processing section. The central processing section 25 determines the priority order in which the aircraft are asked the question, generates as question control information a question signal for obtaining a response signal and a transmission time, and sends the question control information to a transmission/reception station via a communication line. The transmission/reception station asks the SSR mode S individual question at the 30 transmission time in accordance with the priority order determined in the central processing section, and thereby asks the aircraft having the highest importance the question in regard to which an emergency signal is being sent or a flight status is changed, etc. in respect to the air traffic control. [0026] Consequently, the present invention can be improved reliability and safety of the multilateration system, since the SSR mode S individual question can be efficiently acquired at the initiative of the system. 5 [0027] Fig. 1 is a block diagram showing a configuration example of the aircraft measurement system (multilateration system) according to the embodiment of the present invention, and Fig. 2 is a block diagram showing a configuration example of a central processing section 3 shown in Fig. 1. 10 [0028] In Fig. 1, the aircraft measurement system according to the embodiment of the present invention includes: a plurality of reception stations 1-1 to 1-4; a transmission/reception station 2; a central processing section 3; and communication lines 4-1 to 4-4 and 9. Note that although a combination of the 15 reception stations and the transmission1 reception station is employed in the present embodiment, all stations can be set to be the transmission/reception stations, a combination of the transmission stations and the reception stations can be employed, a combination of the transmission/reception stations and the transmission stations can be employed, etc. 20 [0029] In Fig. 2, the central processing section 3 includes: a communication unit 10; a target positioning unit 11; a target information analysis unit 12; a target information generation unit 13; a question control information generation unit 14; a target tracking unit 15; and a target priority order determination unit 16. Note 25 that processing by each unit in the central processing section 3 can also be achieved by a CPU (central processing unit), which is not shown, executing a program. [003 01 The reception stations 1 - 1 to 1-4 and the transmission/reception station 2 3 0 perform synchronization using time synchronization from a GPS (Global Positioning System) satellite 6. In addition, the reception stations 1-1 to 1-4 and the transmissionlreception station 2 receive mode S responselsquitter signals 8-1 to 8-5 fro~na n aircraft 5 through a non-directional or a wide directional antenna, decode the signals, subsequently give them time stamps of times when the response signals arrived, and transmit them to the central processing section 3 as target data using the communication lines 4-1 to 4-4 and 9. [003 11 5 The central processing section 3 performs target positioning in the target positioning unit 11 from collection data (target data) collected in the communication unit 10, and subsequently, analyzes information in positioning data from the target positioning unit 11 in the target information analysis unit 12. [0032] 10 The central processing section 3 inputs analysis data from the target information analysis unit 12, edits target position measurement information for an external output in the target information generation unit 13, and outputs it to an outside (an air traffic control system etc.). The target priority order determination unit 16 inputs the analysis data from the target information analysis 15 unit 12, and decides a target priority order for asking the aircraft the question based on the preset parameters. [0033] The target tracking unit 15 performs tracking processing based on the positioning data from the target positioning unit 11. The question control 20 information generation unit 14 decides the priority order for asking the targets the SSR mode S individual question based on the positioning data from the target positioning unit 11, a positioning prediction value from the target tracking unit 15, and order data [priority right information] from the target priority order determination unit 16, and also performs scheduling of transmission timing, 25 decides a transmission time to generate question control information, and sequentially sends out the scheduled question control information to the transmission/reception station 2 via the communication unit 10 and the communication line 9. 100341 3 0 The transmission/reception station 2 asks the targets the SSR mode S individual question as scheduled based on the question control information from the transmission control information generation unit 14. Next, reception processing of a response signal to this question is performed by the reception stations 1 - 1 to 1-4 and the transmissionlreception station 2, and the above-described processing is repeated. [0035] The reception stations 1 - 1 to 1-4 and the transmissionlreception station 2 5 perform synchronization using time synchronization data from the GPS satellite 6. There is a technology concerning time synchronization using the GPS satellite 6, which is described in the above-described Patent Literature 4, and it is possible to establish highly accurate synchronization with by using this technology. 100361 10 In addition, the reception stations 1-1 to 1-4 and the transmission/reception station 2 receive the mode S responselsquitter signals 8-1 to 8-5 from the aircraft 5 via the non-directional or the wide directional antenna, perform reception processing and signal decoding processing, subsequently give these signals time stamps of arrival times of the response signals, and transmit the latter signals to 15 the central processing section 3 as target data using the communication lines 4-1 to 4-4 and 9. 100371 The central processing section 3 performs reception processing of the above-described target data in the communication unit 10, calculates a TDOA 20 (Time Difference Of Arrival) in each reception station from the time stamps of the arrival times given to the target data in the target positioning unit 11, and performs positioning calculation of the aircraft. [003 81 The TDOA between two antennas mathematically corresponds to a 25 three-dimensional hyperboloid, and a position of the aircraft is located on the hyperboloid. If a signal of the aircraft can be detected by not less than four antennas, the position of the aircraft can be calculated in three dimensions by calculating intersections of hyperbolas. [0039] 3 0 The positioning data of the aircraft whose positioning has been calculated in the target position positioning unit 11 is sent to the target information analysis unit 12 and the target tracking unit 15. [0040] The target information analysis unit 12 analyzes the information in the positioning data, and analyzes various target information (a mode S address, a mode A code, an altitude, aircraft dynamic state information, etc.). [004 11 5 The target information generation unit 13 has: a message generation function for inputting the analysis data from the target information analysis unit 12, editing the target position measurement information for external output, and outputting it to the outside (air traffic control system etc.); and a communication protocol function for connecting to the outside (air traffic control system etc.). 10 [0042] The target priority order determination unit 16 inputs the analysis data from the target information analysis unit 12, and decides the target priority order for asking the aircraft the question based on the preset parameters. Examples of the preset parameters are shown in "determining conditions for the priority right of 15 questions to the aircraft" of Fig. 3. [0043] In Fig. 3, as "determining conditions according to which the priority right for asking aircraft the question is set", set are: "question control conditions" (for example, "Target is sending emergency code", "Omissions have occurred in target 20 report N times in a row", "Mode A code has not been acquired", etc.) (question signal start conditions); "the number of times request questions have been asked"; "request question mode" [UF4 (pressure altitude information request signal), UF5 (identification code (mode A code) request signal), etc.] (type of question); "priority" (urgency); etc. 25 [0044] When the parameters shown in Fig. 3 are used, the target priority order determination unit 16 checks information of all the aircraft processed in its own processing against the conditions shown in Fig. 3, confirms the question control conditions for each aircraft, adds the priority values of the matching conditions 30 shown in Fig. 3, and thereby decides the target priority order of the aircraft to which the question is asked. In this case, it turns out that an aircraft having a higher total priority value than that of another aircraft has a higher priority right (priority) than that of the other aircraft. At this time, the nunlber of questions and a question mode (an inquiry content) are also added to the priority values as information. [0045] The target tracking unit 15 performs tracking processing based on the target 5 data from the target position positioning unit 11. The question control information generation unit 14 decides the priority order for asking the aircraft the SSR mode S individual question which starts from the aircraft having the highest priority right (priority) based on the priority right (priority) information determined by the target priority order determination unit 16. 10 [0046] When the values of the priority rights (priority) are the same, the question control information generation unit 14 shall ask a question in accordance with a priority order starting from an aircraft nearest the transmission/reception station 2. Simultaneously, the question control information generation unit 14 also decides 15 the number of questions the aircraft are to be asked and the content of the questions from the information of the number of questions and the question mode added in summing up the values of the priority rights. Note that although the priority order for asking the aircraft the question is decided by the question control information generation unit 14 in the present embodiment, the priority order for 20 asking the aircraft the question is not limited to being made by this unit, and may instead be decided by the target tracking unit 15. [0047] Next, in order to suppress to not more than 2% a transponder proportion described in ICAO (International Civil Aviation Organization) ANNEX 10 25 Vol4amendment85 6. 6. 3, the transmission control information generation unit 14 carries out scheduling of transmission timing (transmission times), decides the transmission times of the SSR mode S individual questions to the respective aircraft, and sends out question control information based on the decision. COO481 In scheduling, when the number of questions that the ratio of the transponder is greater than 2% was required, an aircraft having a low priority right is not asked a question. Note that it is necessary to adjust parameter setting of the priority shown in Fig. 3 so that an aircraft position and information of the aircraft having the low priority right as described above can be obtained by receiving a response signal or an SSR extended squitter signal from an aircraft which an SSR device in the reception station 1 has asked for, and performing reception processing even if the question is not transmitted from the 5 transmittinglreceiving station 2. [0049] The transmission control information generation unit 14 sequentially sends out the question control information generated and scheduled as described above to the transmissionlreception station 2 via the communication line 9. 10 [0050] The transmission/reception station 2 asks the aircraft the SSR mode S individual question as scheduled based on the question control information from the transmission control information generation unit 14. Next, reception processing of a response signal to this question is performed by the reception 15 stations 1 - 1 to 1-4 and the transmission/reception station 2, and the above-described processing is repeated. [005 11 Positioning of the target and acquisition of an aircraft mode S address need to be initially detected in order to allow the multilateration system to asks the 20 targets (aircraft) the mode S individual question (transmission), and the multilateration system asks the targets (aircraft) the mode S individual question in a priority order starting from the initially detected target. Note that there is a regulation to suppress the transponder proportion to not more than 2% in ICAO ANNEX 10 Vol4amendment85 6. 6. 3 issued by ICAO in order to suppress the 25 influence of radio wave interference etc. on the established SSR etc. I00521 As a result of this, in a case where a number of aircrafts are present in the surveillance airspace of the multilateration system, there is a possibility that not all the aircraft in the surveillance airspace can be asked the SSR mode S individual 3 0 question for obtaining necessary information , since the transponder proportion is suppressed to not more than 2%. For this reason, there is a possibility that a target detection rate may decrease or information necessary for an air traffic control operation may be lacking, which may cause deterioration of reliability and safety. [0053] A transmission function in the multilateration system can be considered to have the following four objects. 5 (1) When an established SSR response is received (in a case of an aircraft without a squitter function), a reception data rate is the same once (reception) per 4 seconds as a data rate of the established SSR (in a case of an airport surveillance radar), or once (reception) per 10 seconds (in a case of an air route surveillance radar) (in a case of a single SSR coverage). Meanwhile, a reception signal of 10 once (reception) per one second can be obtained by the transmission function, and the data rate can be improved. (2) A detection rate of a target having a poor detection rate depending on circumstances of radio wave propagation can be improved. (3) It becomes possible to acquire aircraft dynamic state information (a 15 selected altitude, a pressure correction altitude, a Mach number, an indicated air altitude, a true ground speed, a roll angle, a true track angle, etc.) registered within the aircraft in real time when needed. (4) Since a measurement function can be achieved by transmission, it becomes possible to improve positioning accuracy compared with positioning of a 20 complete passive multilateration system. [0054] In order to solve the above-mentioned problems, the present invention provides the question control scheme that asks aircraft as surveillance objects the SSR mode S individual question t preferentially in a priority order starting from 25 the aircraft having the highest importance in air traffic control, and thereby reliability and safety of the multilateration system can be improved. [0055] As another embodiment of the present invention, a transmission control scheme in a case where there are two or more transmission/reception stations 2 as 30 shown in Fig. 4 will be described. [0056] I11 the case where there are two or more transmission/reception stations, in a situation where as shown in Fig.4, questions are asked to the aircraft which is in a coverage in which transmission coverages of the transmission stations overlap, when the aircraft are asked the questions by the two stations of the transmission/reception stations A and B, it is assumed that the regulation to suppress the transponder proportion to not more than 2% in ICAO ANNEX 10 5 cannot be satisfied. In addition, it is unnecessary to for the two stations to ask the same aircraft questions except for particular cases, such as where stop of the transmission/reception station can be considered, or the aircraft as a transmission object straddles coverages from the transmission/reception stations. [0057] 10 For this reason, as described in " scheme for selecting transmission/reception station that asks a question" shown in Fig. 5, there is a scheme for selecting the transmission/reception station that asks the question from a positional relation of the transmission/reception stations A and B, and the aircraft in the coverages. 15 [0058] In this scheme, as shown in Fig. 5, as for aircrafts T1 to T3 which are asked the questions, a distance (y1 to y3) from each of the aircrafts T1 to T3 to the transmission/reception station A is compared with a distance (xl to x3) from each of the aircrafts T1 to T3 to the transmission/reception station B, the 20 transmission/reception station A being selected in a case of x>y, and the transmission/reception station B being selected in a case of xyl in the case of the aircraft T1, the transmission/reception station B is selected 25 because of x2