DESCRIPTION MOVABLE BODY POSITION MEASUREMENT SYSTEM, CENTRAL PROCESSING SECTION, AND QUESTION CONTROL METHOD USED 5 THEREIN Technical Field [0001] The present invention relates to a movable body position measurement 1 0 system, a central processing section, and a question control method used therein and, in particular, to an aircraft measurement system (a multilateration system) and a transmission control method of a transmission station used in the system. Background Art [0002] 1 5 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 a reception time of each reception station (for example, refer to Patent Literature 1). [0003] Here, the SSR mode A is the mode to acquire identification information of the aircraft, the SSR mode C is the mode to acquire pressure altitude information 2 5 of the aircraft, and the 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 all the aircraft or a particular aircraft can be individually asked a question and respond. 30 [0004] The multilateration system may include a transmission station in some cases. In this transmission 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 2 3 transmission time at the same time that it enables detection of an aircraft which responds only the SSR mode A/C response, it can detect a round-trip time from transmission to reception, and can be utilized for improving the accuracy of positioning as an active multilateration. 5 [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, the SSR mode A/C responses are superposed (become in a garbled and FRUIT (False Replies from Unsynchronized Interrogator Transmissions) state), and the 10 response signals cannot be decoded since a non-directional or a wide directional antenna is used as an antenna. [0006] In addition, in order to detect the SSR mode A/C responses, correlation processing is simultaneously performed for unifying into one a plural times of 15 response signals corresponding to each of mode A questions and mode C questions that the aircraft is separately asked plural times, and thus it is necessary to synchronize with transmission timing of an established SSR. [0007] However, as the cost for connecting the established SSR and a 20 transmission/reception station located at a remote place is expensive, realistically speaking it is difficult to do it. [0008] Acquisition of an aircraft mode S address (unique address information) needs to be initially detected in order that the multilateration system can ask the 2 5 target (aircraft) the mode S individual question (transmission) , and the multilateration system carries out the mode S individual question in a priority order starting from the initially detected target. [0009] Note that there is a regulation to suppress a transponder proportion to not 3 0 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 the established SSR etc. [0010] As a result of this regulation, in a case where a number of aircraft are 4 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 5 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 air traffic control operation. [0011] As technologies using the above-described multilateration, there are 1 0 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 discriminate whether there is only a mode A response or a mode C response by means of signals received by reception stations (for example, refer to Patent 15 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. Citation List 20 Patent Literature [0012] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-300146 Patent Literature 2: Japanese Unexamined Patent Application Publication 25 No. 2007-333427 Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2011-112465 Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2010-230448 30 Summary of Invention Technical Problem [0013] In the above-mentioned aircraft position measurement system relevant to 5 the present invention, in the case of the multilateration system that uses the non-directional or the wide directional antenna, many SSR mode A/C response signals from aircraft present in a surveillance airspace are received for the SSR mode A/C questions these aircraft are asked, and thus there is a problem that 5 superposition (the garbled and FRUIT state) occurs, and that the SSR mode A/C response signals cannot be decoded in the transmission/reception station or the reception station. [0014] In addition, in the aircraft position measurement system relevant to the 10 present invention, in the case of the multilateration system, both an SSR mode S aircraft and an SSR mode A/C aircraft are surveillance objects, and there is the regulation to suppress the transponder proportion to not more than 2% in ICAO (International Civil Aviation Organization) ANNEX 10 vol4amendment85 6. 6. 3 issued by ICAO. 15 [0015] As described above, when the multilateration system includes the transmission station, a transmission procedure is used in which transmission is sequentially-performed simply within a certain time (for example, one second interval). By this transmission procedure, in the SSR mode S question and the 20 SSR mode A/C question, a case may occur where the above-mentioned regulation in which the transponder proportion in the ICAO is not more than 2% cannot be satisfied. [0016] In addition, when transmission is just limited in order to satisfy the 2 5 regulation in which the transponder proportion is not more than 2%, there is a possibility of deteriorating the reliability and safety of the multilateration system in air traffic control. Note that since the above technologies described in Patent Literatures 1 to 4 are not the technologies concerning the case where the multilateration system includes the transmission station, they cannot solve these 3 0 problems. [0017] Consequently, an object of the present invention is to solve the above-described problems, and to provide a movable body position measurement system, a central processing section, and a question control method used therein 6 that can efficiently and reliably detect the SSR mode S response and the SSR mode A/C response, and can improve the reliability and safety of the multilateration system. Solution to Problem 5 [0018] A movable body position measurement system according to the present invention is the movable body position measurement system that includes: a plurality of reception stations that receive response signals from a movable body present in a surveillance region; and a central processing section that positions the 10 movable body based on reception times of the response signals 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 processing section, in which the movable body position measurement system includes at least one or 1 5 more transmission/reception stations that transmit a question signal to the movable body, the transmission/reception station limits a transmission coverage of the question signal to a coverage range of an SSR (Secondary Surveillance Radar) mode A/C question and an SSR mode A/C response of a scheme to ask all movable 20 bodies a common question, and asks each aircraft an SSR mode S individual question of a scheme to enable individual question and response, and in which the central processing section includes means for controlling the transmission/reception station to ask the movable bodies the SSR mode S individual question preferentially in an order starting from a movable body having 25 the highest importance in control. [0019] A central processing section according to the present invention is the central processing section used for a movable body position measurement system that includes: a plurality of reception stations that receive response signals from a 3 0 movable body present in a surveillance region; and a central processing section that positions the movable body based on reception times of the response signals 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 processing section, in which 7 in the movable body position measurement system, at least one or more transmission/reception stations that transmit a question signal to the movable body are arranged, in the transmission/reception station, a transmission coverage of the 5 question signal is limited to a coverage range of an SSR (Secondary Surveillance Radar) mode A/C question and an SSR mode A/C response of a scheme to ask all movable bodies a common question, and an SSR mode S individual question of a scheme to enable individual question and response, and in which the central processing section includes means for controlling the 10 transmission/reception station to ask each of the movable bodies the SSR mode S individual question preferentially in an order starting from a movable body having the highest importance in control. [0020] A question control method according to the present invention is the 1 5 question control method used for a movable body position measurement system that includes: a plurality of reception stations that receive response signals from a movable body present in a surveillance region; and a central processing section that positions the movable body based on reception times of the response signals in the plurality of reception stations, and measures a geometric position of the 20 movable body from the reception times of the plurality of reception stations in the central processing section, in which in the movable body position measurement system, at least one or more transmission/reception stations that transmit a question signal to the movable body are arranged, 2 5 in the transmission/reception station, a transmission coverage of the question signal is limited to a coverage range of an SSR (Secondary Surveillance Radar) mode A/C question and an SSR mode A/C response of a scheme to ask all movable bodies a common question, and ask each movable body an SSR mode S individual question of a scheme to enable individual question and response, and in 3 0 which the central processing section executes processing to control the transmission/reception station to ask the movable bodies the SSR mode S individual question preferentially in an order starting from a movable body having the highest importance in control. 8 Advantageous Effects of Invention [0021] Since the present invention can efficiently and reliably detect the SSR mode S response and the SSR mode A/C response by having the above-described 5 configuration and operation, an effect can be obtained that the reliability and safety of the multilateration system can be improved. Brief Description of the Drawings [0022] 10 Fig. 1 is a block diagram showing a configuration example of the aircraft position measurement system according to the present invention. Fig. 2 is a block diagram showing a configuration example of the transmission/reception stations 1-1 to 1-5 of Fig. 1. Fig. 3 is a block diagram showing a configuration example of the central 15 processing unit 2 shown in Fig. 1. Fig. 4A is an example of whisper-shout transmission scheme according to the present invention. Fig. 4B is an example of ring-shaped widths of transmission coverages (WS coverages) according to the present invention. 20 Fig. 5 is a means of transmission pattern according to the present invention. Fig. 6 shows one example of arrangement of the transmission/reception stations of the multilateration system according to the present invention. Fig. 7 is in the case where two or more transmission/reception stations are present according to the present invention. 25 Fig. 8 is scheme for selecting transmission/reception station that asks question according to the present invention. Description of Embodiments [0023] 3 0 Next, an embodiment 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 aircraft position measurement system [MLAT (Multilateration) system]. Although positions of movable bodies, such as an aircraft and a vehicle in an airport, can be measured by the aircraft position measurement system, the mobile body explained in the following explanation will be just an aircraft. [0024] 5 The present invention pertains to the aircraft position measurement system (multilateration system), a question control scheme, and a question control program of a transmission/reception station used in the system and, in particular, to a multilateration system in which reception processing of: a response signal sent from an aircraft which is asked by an SSR (Secondary Surveillance Radar) device,: 10 a response signal from an aircraft which is asked from the multilateration system,: or an SSR capture or extended squitter signal is performed in not less than four reception stations or transmission/reception stations, and in which position measurement of the aircraft is performed by a central processing section using this reception signal. 15 [0025] The present invention provides an SSR mode A/C aircraft detection scheme, a transmission control scheme, and a program for reliably detecting an SSR mode A/C aircraft of aircraft present in a surveillance airspace in the above-described multilateration system. Note that an SSR mode A/C and an SSR mode S are 20 similar to those in the explanation of the above-mentioned aircraft position measurement system relevant to the present invention. [0026] The present invention can easily detect the SSR mode A/C aircraft by using an SSR mode A/C question scheme utilizing a whisper-shout transmission scheme 2 5 in order to solve the above-described problem that the established SSR device and the transmission/reception station located at the remote place cannot be connected to each other. [0027] That is, in order to solve the above-described problems, the present 3 0 invention provides a question control scheme in which aircraft are preferentially asked an SSR mode S individual question in an order starting from an aircraft having the highest importance in air traffic control, and also includes a function to process an SSR mode A/C question and an SSR mode A/C response utilizing the whisper-shout transmission scheme with respect to an aircraft as a surveillance 10 object, and thereby the aircraft can be reliably detected, thus enabling to improve reliability and safety of the multilateration system. [0028] Fig. 1 is a block diagram showing a configuration example of the aircraft 5 position measurement system according to the present invention. In Fig. 1, the aircraft position measurement system according to the present invention includes transmission/reception stations 1-1 to 1-5 and a central processing section 2, and the transmission/reception stations 1-1 to 1-5 and the central processing section 2 are connected to each other through communication lines 4-1 to 4-5. 10 [0029] The present invention limits a question coverage to a coverage range of the SSR mode A/C questions and the SSR mode A/C responses by the transmission control scheme of the present invention in the transmission/reception stations 1-1 to 1-5, and reduces occurrence of superposition (a garbled and FRUIT state) of the 1 5 SSR mode A/C responses, and thereby provides a scheme in which the SSR mode A/C responses are reliably detected and decoded. In addition, the present invention also asks the SSR mode S individual question by means of a transmission pattern as in one example shown in Fig. 5. [0030] 20 The central processing section 2 sends a transmission control signal to the transmission/reception stations 1-1 to 1-5 via the communication lines 4-1 to 4-5, and provides means for selecting a transmission/reception station suitable for the question, instead of all the transmission/reception stations 1-1 to 1-5 simultaneously asking the question within a surveillance coverage. 25 [0031] In addition, the SSR mode A/C question provides question means with respect to the SSR mode A/C aircraft by means of a whisper-shout transmission scheme carried out in an existing ACAS (Airborne Collision Avoidance System). [0032] 30 The transmission/reception stations 1-1 to 1-5 perform SSR mode A/C questions 7-1 to 7-5 at specified transmission times in an order of the transmission/reception stations specified in transmission control by the central processing section 2, and thereby provides means for reliably detecting the SSR mode A/C aircraft. 11 [0033] In addition, as for the SSR mode A/C aircraft, means for performing transmission control so as to satisfy the above-mentioned regulation in ICAO in which the transponder proportion is suppressed to not more than 2% is provided. 5 [0034] Consequently, since the present invention can reliably detect an SSR mode S response and the SSR mode A/C response efficiently initiated by the multilateration system, the reliability and safety of the multilateration system can be improved. 10 [0035] Next, an embodiment of the present invention will be explained with reference to Fig. 1. The multilateration system according to the embodiment of the present invention, as shown in Fig. 1, includes: the plurality of transmission/reception stations 1-1 to 1-5; the central processing section 2, and the 15 communication lines 4-1 to 4-5. Note that not all the transmission/reception stations 1-1 to 1-5 are set to be transmission/reception stations, and that a combination of a transmission station and a reception station, a combination of the transmission/reception station, the transmission station, and the reception station, etc. can be considered. 20 [0036] The transmission/reception stations 1-1 to 1-5 perform synchronization using time synchronization from a GPS (Global Positioning System) satellite 6. In addition, the transmission/reception stations 1-1 to 1-5 receive the SSR mode A/C responses, the mode S responses, the capture or extended squitter signals 7-1 2 5 to 7-5 from an aircraft 5 through a non-directional or a wide directional antenna, decode the signals, subsequently give them time stamps of times when reception signals arrived, and transmit them to the central processing section 2 as response data using the communication lines 4-1 to 4-5. [0037] 30 Any one of the transmission/reception stations 1-1 to 1-5 that has been selected and collected asks SSR mode A/C questions 3-1 to 3-5 using the whisper-shout transmission scheme by a transmission control instruction by means of the transmission control signals from the central processing section 2 via the communication lines 4-1 to 4-5, and each of the transmission/reception stations 12 1-1 to 1-5 performs reception processing of the SSR mode A/C response signals 7-1 to 7-5 with respect to these SSR mode A/C questions 3-1 to 3-5. [0038] Fig. 2 is a block diagram showing a configuration example of the 5 transmission/reception stations 1-1 to 1-5 of Fig. 1, and Fig. 3 is a block diagram showing a configuration example of the central processing unit 2 shown in Fig. 1. In Fig. 2, the transmission/reception stations 1-1 to 1-5 are represented as a transmission/reception station 1, and each of the transmission/reception stations 1-1 to 1-5 has a configuration similar to that of the transmission/reception station 10 1. [0039] In Fig. 2, the transmission/reception station 1 includes: a GPS antenna 8; a GPS receiver 9; an antenna 10; a reception unit 11; a transmission unit 20; and a signal processing unit 21. The transmission unit 20 includes: a circulator 12; a 1 5 synthesizer 13; variable attenuators 14-1 to 14-2; an RF pulse selector switch 15; a transmission controller 16; a modulated pulse generator 17; a power amplifier 18; and an oscillator 19. [0040] In Fig. 3, the central processing section 2 includes: a 20 transmission/reception information collection unit 22; a target position positioning unit 23; a target information analysis unit 24; a target information generation unit 25; a transmission control information generation unit 26; a target tracking unit 27; and a target priority order determination unit 28. [0041] 25 The transmission/reception stations 1-1 to 1-5 each have the GPS antenna 8 and the GPS receiver 9 that receives a time synchronization signal from the GPS satellite 6, and perform time synchronization among the respective transmission/reception stations 1-1 to 1-5 located so as to be spaced apart from each other. 30 [0042] In addition, the non-directional or the wide directional antenna 10 receive the SSR mode A/C response, the SSR mode S response, and the capture or extended squitter signals 7-1 to 7-5 from the aircraft 5, and asks the SSR mode A/C questions 3-1 to 3-5. 13 [0043] The reception unit 11 performs reception processing of the SSR mode A/C response, the SSR mode S response, and the capture or extended squitter signals 7-1 to 7-5, converts them into reception video signals, and sends them to the signal 5 processing unit 21. Next, in the signal processing unit 21, the signals are decoded, subsequently, time stamps of arrival times of the reception signals are added together with the decoded data, and the data is sent out to the central processing section 2 as response data using the communication lines 4-1 to 4-5. [0044] 10 The central processing section 2 performs reception processing of the above-described response data in the communication unit 22, performs target positioning in the target position positioning unit 23 based on the received response data, and subsequently, analyzes information within the response data in the target information analysis unit 24 based on positioning data. The target 1 5 information generation unit 25 inputs the analysis data from the target information analysis unit 24, edits target position measurement information for an external output, and outputs it to an outside (for example, an air traffic control system etc.). [0045] The target priority order measurement unit 28 inputs the analysis data from 2 0 the target information analysis unit 24, and decides a target priority order based on preset parameters in order to decide on the transmission/reception station that should ask the SSR mode A/C question. [0046] The target tracking unit 27 performs tracking processing based on the 25 positioning data from the target position positioning unit 23, and order data from the target priority order determination unit 28. The question control information generating unit 26 decides a priority order of the transmission/reception stations that ask the SSR mode A/C question on the basis of the positioning data from the target position positioning unit 23, a positioning prediction value from the target 3 0 tracking unit 27, and the order data from the target priority order determination unit 28, also performs scheduling of transmission timing, and decides the transmission times. In addition, the question control information generation unit 26 performs scheduling of transmission timing of the SSR mode S individual question in parallel with the above-described processing. Note that the 14 above-described decision and scheduling in the question control information generation unit 26 can also be performed in the target tracking unit 27. [0047] The question control information generation unit 26 generates and edits 5 question control information from the above-described decision and a result of the scheduling, and sends out the question control information in which the transmission timing has been scheduled to the transmission/reception stations 1-1 to 1-5 via the communication unit 22 and the communication lines 4-1 to 4-5. [0048] 10 The signal processing unit 21 and the transmission unit 20 of the transmission/reception stations 1-1 to 1-5 ask the SSR mode A/C questions in accordance with the scheduling of the transmission timing by means of the whisper-shout transmission scheme based on the question control information from the central processing section 2. In addition, the signal processing unit 21 and 15 the transmission unit 20 of the transmission/reception stations 1-1 to 1-5 also ask the SSR mode S individual questions together with performing the above-described processing. Next, the transmission/reception stations 1-1 to 1-5 perform reception processing of the response signals with respect to these questions, and repeatedly perform the above-described processing. 20 [0049] The transmission unit 20 generates a high frequency excitation signal for transmission in the oscillator 19, performs pulse modulation and power amplification of an RF transmission signal in the power amplifier 18, and sends out the RF transmission signal to the RF pulse selector switch 15. In addition, the 25 transmission controller 16 generates various signals needed for the SSR mode A/C question by whisper-shout transmission at the transmission time based on the transmission control information data from the signal processing unit 21, also generates various signals needed for the SSR mode S individual question, and subsequently, controls the variable attenuators 14-1 to 14-2, the RF pulse selector 3 0 switch 15, and the modulated pulse generator 17. [0050] The transmission/reception stations 1-1 to 1-5 perform synchronization using the time synchronization signal from the GPS satellite 6. There is a technology described in Patent Literature 3 (Japanese Unexamined Patent 15 Application Publication No. 2010-230448) concerning the time synchronization using the GPS satellite 6, and it is possible to synchronize times with high accuracy by using this technology. [0051] 5 In addition, the transmission/reception stations 1-1 to 1-5 perform reception processing and signal decoding processing of the reception signals of the SSR mode A/C responses, the SSR mode S responses, and the capture or extended squitter signals 7-1 to 7-5 from the aircraft 5 via the non-directional or the wide directional antenna 10, subsequently, give them the time stamps of the arrival 10 times of the reception signals, and send them out to the central processing section 2 as the response data using the communication lines 4-1 to 4-5. [0052] The central processing section 2 performs reception processing of the above-mentioned response data in the communication unit 22, calculates a TDOA 15 (Time Difference Of Arrival) of each reception station from the time stamps of the arrival times given to the above-described response data in the target position positioning unit 23, and performs position positioning calculation of the aircraft 5. [0053] The TDOA between two antennas mathematically corresponds to a 20 three-dimensional hyperboloid, and a position of the aircraft is located on the hyperboloid. If signals 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. [0054] 25 The positioning data of the aircraft 5 whose positioning has been performed in the target position positioning unit 23 is sent to the target information analysis unit 24, the question control information generation unit 26 and the target tracking unit 27. [0055] 3 0 The target information analysis unit 24 analyzes information within the positioning data, and analyzes various target information (a mode S address, a mode A code, an altitude, aircraft dynamic state information, etc.). [0056] The target information generation unit 25 has: a message generation 16 function for inputting the analysis data from the target information analysis unit 24, editing the target position measurement information for the external output, and outputting it to the outside (for example, the air traffic control system etc.); and a communication protocol function for connecting to an external system. 5 [0057] The target priority order measuring portion 28 inputs the analysis data from the target information analysis unit 24 and the positioning prediction value from the target tracking unit 27, and decides target priority order and the transmission/reception station that asks the question (transmission) in order to 10 decide the transmission/reception station that should ask the SSR mode A/C question based on preset various parameters. [0058] The target tracking unit 27 performs tracking processing based on the positioning data from the target position positioning unit 23, and order data from 1 5 the target priority order determination unit 28 in which the priority order has been decided. The question control information generation unit 26 decides a priority order of the transmission/reception stations that ask the SSR mode A/C questions based on the order data from the targetpriority order determination unit 28 in which the priority order has been decided, also performs scheduling of the 20 transmission timing, and decides the transmission times. In addition, the question control information generation unit 26 performs scheduling of transmission timing of the SSR mode S individual questions in parallel with the above-described processing. [0059] 2 5 In addition, in order to suppress to not more than 2% the transponder proportion described in ICAO ANNEX 10 Vol4amendment85 6. 6. 3, the question control information generation unit 26 carries out a scheduling plan of the transmission timing (transmission times), decides the transmission times of the SSR mode A/C questions and the SSR mode S individual questions to the 3 0 respective aircraft, and sends out question control information generated based on the decision. [0060] Although one example of a transmission pattern is shown in Fig. 5, the SSR mode A/C question is asked with a repetition frequency at a constant period 17 decided in a maximum detection coverage distance, and scheduling of the transmission timing of the SSR mode S individual questions is performed during intervals of the SSR mode A/C questions of the constant period. [0061] 5 The question control information generation unit 26 sequentially sends out to the transmission/reception stations 1-1 to 1-5 the generated question control information in which the transmission timing has been scheduled via the communication unit 22 and the communication lines 4-1 to 4-5. [0062] 10 When the transmission/reception stations 1-1 to 1-5 receive the question control information from the transmission control information generation unit 26 via the transmission/reception information collection unit 22 and the communication lines 4-1 to 4-5, the transmission/reception stations 1-1 to 1-5 ask the SSR mode A/C questions and the SSR mode S individual questions in 1 5 accordance with the scheduling of the transmission timing by means of the whisper-shout transmission scheme based on the question control information. Next, the transmission/reception stations 1-1 to 1-5 perform reception processing of the response signals with respect to these questions in the reception unit 11 and the signal processing unit 21, and repeatedly perform the above-described 20 processing. [0063] Although in the whisper-shout transmission scheme, four pulses (S, PI, P3, P4) of transmission waveforms shown in Fig. 4A are transmitted, a transmission power level ratio (WS coverage) of the S pulse and the PI, P3, and P4 pulses (PI, 25 P3, and P4 are transmitted with the same power-level of transmission power, hereinafter, they are referred to as a P pulse) is controlled, and thereby ring-shaped widths of transmission coverages (WS coverages) of Fig. 4B (ranges where the aircraft can be detected) are decided. [0064] 3 0 In addition, although transmission 1 of the transmission waveform shows the transmission waveform of a shortest distance coverage, the S pulse is not transmitted since detection is performed over a short distance. In transmission 2, ..., transmission N-1, and transmission N of the transmission waveforms, the whisper-shout transmission scheme is performed by transmitting the S pulse and 18 the P pulse which are attached transmission power difference. [0065] Although the transmission N of the transmission waveform shows a case of a maximum detection coverage, a probability that a plurality of aircraft are present 5 in the coverage is high since an area of the ring width of the transmission coverage is the widest, and thus transmission of question signals is controlled, such as setting the transmission power level ratio (WS coverage) of the S pulse and the P pulse small, by control of the question control information data from the central processing section 2, whereby a superposition state of the SSR mode A/C 10 responses is avoided, and control to improve a target detection rate is also performed. [0066] Although Figs. 4A and 4B show one example of the priority order of the whisper-shout transmission, as a basic operation of the SSR mode A/C question, 1 5 the question is asked from an inner side in an order from the transmission 1 having the shortest distance coverage to the transmission 2, ...., and the transmission N-1, the order being opposite to the order from the transmission N having the maximum coverage. In addition, since five stations of the transmission/reception stations 1-1 to 1-5 need not ask the SSR mode A/C question simultaneously, the SSR mode 2 0 A/C question is asked by the transmission/reception station that is in a most suitable position relation with respect to the aircraft by control based on the transmission control information data from the central processing section 2. [0067]- For example, although Fig. 6 shows one example of arrangement of the 2 5 transmission/reception stations of the multilateration system, in this arrangement example, a transmission/reception station selection scheme can be considered in which the question is executed in a priority order from the transmission/reception station nearest the aircraft as shown in Figs. 7 and 8, and a scheme to select the transmission/reception station can be considered in which a position of the aircraft 3 0 is located within a circle of the WS coverage and in a.vertical direction in order to avoid the superposition (garbled and FRUIT) state of the SSR mode A/C responses, for example, in a case where the plurality of aircraft present within the WS coverage, etc. [0068] 19 In the case where two or more transmission/reception stations are present , in a case where as shown in Fig. 7, the aircraft present in a coverage in which transmission coverages of the transmission stations have been overlapped are asked the question, when the question is asked by the two stations of the 5 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 cannot be satisfied. In addition, it is unnecessary in the multilateration system for two stations having overlapping transmission coverages to ask the same aircraft the question. 10 [0069] For this reason, as described in "scheme for selecting transmission/reception station that asks question" shown in Fig. 8, there is a scheme for selecting the transmission/reception station that asks the question from a position relation of the transmission/reception stations A and B, and the aircraft 1 5 in the coverages. [0070] In this scheme, as shown in Fig. 8, as for aircraft Tl to T3 which are asked the question, a distance (yl to y3) from each of the aircraft Tl to T3 to the transmission/reception station A is compared with a distance (xl to x3) from each 20 of the aircraft Tl to T3 to the transmission/reception station B, the 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 25 in the case of the aircraft Tl, the transmission/reception station B is selected because of x2