Distance measuring system, positioning system, distance measuring method and positioning method

Abstract

To provide a distance measuring system, a positioning system, a distance measuring method and a positioning method in which increase of a circuit scale or cost can be suppressed without requiring time synchronization.SOLUTION: A distance measuring system 1 simultaneously transmits an electromagnetic wave Ew1 and an electromagnetic wave Ew2 respectively having different frequencies from a transmitting station Tx into sea-water, and a receiving station Rx receives the electromagnetic wave Ew1 and the electromagnetic wave Ew2 propagated in the sea-water, and calculates a distance L to the transmitting station Tx on the basis of a time difference of receipt time and propagation velocities V1 and V2 in the sea-water of the electromagnetic waves Ew1 and Ew2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a system and method for measuring distance in seawater, and a system and method for positioning in seawater.

Background Art

[0002] Conventionally, as a method for measuring the distance between two points in seawater, a method using electromagnetic waves is known. For example, in a first method using electromagnetic waves, the distance is calculated from the signal attenuation amount and attenuation constant of the transmitted electromagnetic wave. In a second method, the distance is calculated from the time (arrival time) from when the electromagnetic wave is transmitted until it is received and the propagation speed of the electromagnetic wave in seawater. In a third method, the distance is calculated from the amount of phase change and wavelength of the received electromagnetic wave with respect to the transmitted electromagnetic wave.

[0003] Further, as a fourth method, a method of measuring the distance by combining radio waves transmitted and received in air and sound waves transmitted and received in seawater is also known (see, for example, Patent Document 1). In this fourth method, the distance is calculated based on the transmission and reception times of radio waves in air and the transmission and reception times of sound waves in seawater.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the above-described first method, the signal attenuation amount may vary depending on the directivities of the transmitting antenna and the receiving antenna, and the reliability and accuracy may deteriorate. Further, in order to obtain a predetermined level of reliability and accuracy, correction processing of the signal attenuation amount considering the directivity is required, resulting in an increase in circuit scale and cost. Also, in the second method, since the transmission time and the reception time are measured, it is necessary to perform time synchronization between the transmission side and the reception side, leading to an increase in circuit scale and cost. In the third method, in order to measure the amount of phase change, it is necessary for the receiving side to have a reference signal synchronized with the transmission signal, resulting in an increase in circuit scale and cost. Furthermore, in the fourth method, since radio wave and sound wave transmission and reception circuits are required on both the transmission side and the reception side, and time synchronization is also required, the circuit scale and cost increase.

[0006] Therefore, an object of the present invention is to provide a distance measurement system, a positioning system, a distance measurement method, and a positioning method that do not require time synchronization and can suppress an increase in circuit scale and cost.

Means for Solving the Problems

[0007] In order to solve the above problems, the invention according to claim 1 includes a transmitting station that simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, and a receiving station that receives the first electromagnetic wave and the second electromagnetic wave propagated in the seawater. The receiving station calculates the distance to the transmitting station based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. A distance measurement system characterized by that.

[0008] The invention according to claim 2 comprises a first transmitting station that simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, a second transmitting station that is arranged at a position different from the first transmitting station and simultaneously transmits the first electromagnetic wave and the second electromagnetic wave having different frequencies into seawater, a third transmitting station that is arranged at a position different from the first transmitting station and the second transmitting station and simultaneously transmits the first electromagnetic wave and the second electromagnetic wave having different frequencies into seawater, and a receiving station that receives the first electromagnetic wave and the second electromagnetic wave propagated in seawater. The receiving station calculates the distance to the first transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the first transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. The receiving station calculates the distance to the second transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the second transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. The receiving station calculates the distance to the third transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the third transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. The receiving station calculates the relative position based on the distance to the first transmitting station, the distance to the second transmitting station, and the distance to the third transmitting station. A positioning system characterized by the above is provided.

[0009] The invention according to claim 3 is the positioning system according to claim 2, wherein the receiving station calculates the absolute position based on the position information of the first transmitting station, the position information of the second transmitting station, the position information of the third transmitting station, and the relative position.

[0010] The invention according to claim 4 is a distance measurement method characterized by simultaneously transmitting a first electromagnetic wave and a second electromagnetic wave having different frequencies from a transmitting station into seawater, receiving, by a receiving station, the first electromagnetic wave and the second electromagnetic wave propagated in the seawater, and calculating the distance to the transmitting station based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in the seawater, and the propagation speed of the second electromagnetic wave in the seawater.

[0011] The invention according to claim 5 is a positioning method characterized by simultaneously transmitting a first electromagnetic wave and a second electromagnetic wave having different frequencies from a first transmitting station into seawater, simultaneously transmitting a first electromagnetic wave and a second electromagnetic wave having different frequencies from a second transmitting station disposed at a position different from the first transmitting station into seawater, simultaneously transmitting a first electromagnetic wave and a second electromagnetic wave having different frequencies from a third transmitting station disposed at a position different from the first transmitting station and the second transmitting station into seawater, receiving, by a receiving station, the first electromagnetic wave and the second electromagnetic wave propagated in the seawater, calculating the distance to the first transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the first transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in the seawater, and the propagation speed of the second electromagnetic wave in the seawater, calculating the distance to the second transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the second transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in the seawater, and the propagation speed of the second electromagnetic wave in the seawater, calculating the distance to the third transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the third transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in the seawater, and the propagation speed of the second electromagnetic wave in the seawater, and calculating the relative position based on the distance to the first transmitting station, the distance to the second transmitting station, and the distance to the third transmitting station.

[0012] The invention according to claim 6 is the positioning method according to claim 5, characterized in that an absolute position is calculated based on the position information of the first transmitting station, the position information of the second transmitting station, the position information of the third transmitting station, and the relative position.

Advantages of the Invention

[0013] According to the inventions described in claim 1 and claim 4, since the distance to the transmitting station can be calculated based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, and the propagation speed of the first electromagnetic wave and the propagation speed of the second electromagnetic wave, it is possible to accurately calculate the distance without being affected by the signal attenuation amount of the received electromagnetic wave. In addition, since there is no need to perform time synchronization or reference signal synchronization between the transmitting and receiving stations, the circuit configuration can be simplified, contributing to miniaturization and cost reduction.

[0014] According to the inventions described in claim 2 and claim 5, since the distances to the first transmitting station, the second transmitting station, and the third transmitting station can be calculated based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, and the propagation speed of the first electromagnetic wave and the propagation speed of the second electromagnetic wave, and the relative positions with respect to the first transmitting station, the second transmitting station, and the third transmitting station can be calculated, it is possible to accurately measure the relative position with a receiving station having a simple configuration.

[0015] According to the inventions described in claim 3 and claim 6, since the absolute position of the receiving station can be calculated based on the relative positions with respect to the first transmitting station, the second transmitting station, and the third transmitting station, and the position information of the first transmitting station, the second transmitting station, and the third transmitting station, it is possible to accurately measure the absolute position with a receiving station having a simple configuration.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0017] Hereinafter, the present invention will be described based on the illustrated embodiments.

[0018] (Embodiment 1) FIG. 1 is a diagram showing the schematic configuration of a distance measurement system 1 according to an embodiment of the present invention. The distance measurement system 1 is used for implementing the distance measurement method according to the present invention. The distance measurement system 1 includes a transmitting station Tx that transmits electromagnetic waves into seawater, and a receiving station Rx that receives the electromagnetic waves propagated in the seawater.

[0019] The transmitting station Tx is, for example, a buoy, a ship, or marine equipment related to oil, gas, etc., floating on the sea or in the sea. The transmitting station Tx includes a circuit that outputs electromagnetic waves of a predetermined frequency, an antenna that transmits electromagnetic waves into seawater, and the like.

[0020] The receiving station Rx is, for example, a manned submarine, an unmanned submarine, an underwater drone, a diver, etc., navigating in the seawater. The receiving station Rx includes an antenna that receives the electromagnetic waves propagated in the seawater, a circuit that calculates the distance to the transmitting station Tx based on the received electromagnetic waves, and the like.

[0021] It is known that the propagation speed V of electromagnetic waves propagating in seawater changes according to the frequency f. Specifically, "propagation speed V ∝ √f". This is because in seawater, the conductivity is 5 s / m (zero in air), and the relative permittivity is 80 (1 in air), which is higher than in air.

[0022] Figure 2 shows the propagation speed V [km / s] in seawater according to the frequency f [Hz] of electromagnetic waves. The propagation speed V is obtained by "propagation speed V = frequency f × wavelength λg", and the wavelength λg is given by "2π / β" (β: phase constant). For example, when the frequency f of the electromagnetic wave is 1 Hz, the propagation speed V is 1.4 km / s, and when the frequency f of the electromagnetic wave is 10000 Hz, the propagation speed V is 141.4 km / s. Thus, the propagation speed V of electromagnetic waves propagating in seawater changes according to the frequency f.

[0023] The distance measurement system 1 according to the present embodiment measures the distance L between the transmitting station Tx and the receiving station Rx by utilizing the characteristic of electromagnetic waves that the propagation speed V in seawater changes according to the frequency f.

[0024] Specifically, the transmitting station Tx simultaneously transmits electromagnetic waves Ew1 and Ew2 with different frequencies. Here, the frequency f1 of the electromagnetic wave Ew1 is higher than the frequency f2 of the electromagnetic wave Ew2 (f1 > f2). Therefore, the propagation speed V1 of the electromagnetic wave Ew1 is faster than the propagation speed V2 of the electromagnetic wave Ew2 (V1 > V2).

[0025] The receiving station Rx receives the electromagnetic waves Ew1 and Ew2 transmitted from the transmitting station Tx and propagating in seawater. Since the propagation speed V1 of the electromagnetic wave Ew1 is faster than the propagation speed V2 of the electromagnetic wave Ew2, the electromagnetic wave Ew1 is received by the receiving station Rx first, and then the electromagnetic wave Ew2 is received. That is, there is a time difference of Δt between the reception time of the electromagnetic wave Ew1 by the receiving station Rx and the reception time of the electromagnetic wave Ew2.

[0026] Here, when the time from when the electromagnetic wave Ew1 is transmitted from the transmitting station Tx until it is received by the receiving station Rx is t1, it is "time t1 = distance L / propagation speed V1". Also, when the time from when the electromagnetic wave Ew2 is transmitted from the transmitting station Tx until it is received by the receiving station Rx is t2, it is "time t2 = distance L / propagation speed V2". Thus, the time difference Δt between the reception time of the electromagnetic wave Ew1 and the reception time of the electromagnetic wave Ew2 by the receiving station Rx is obtained as shown in the following formula (1). Also, by expanding formula (1) as shown in formula (2), the distance L can be obtained from the time difference Δt, the propagation speed V1, and the propagation speed V2. Δt = t2 - t1 = L / V2 - L / V1 ···(1) L = Δt×V1×V2 / (V1 - V2) ···(2)

[0027] Next, the operation of the above embodiment will be described based on the flowchart of FIG. 3. The transmitting station Tx simultaneously transmits the electromagnetic waves Ew1 and Ew2 into the seawater, for example, at predetermined time intervals (step S1).

[0028] The receiving station Rx receives the electromagnetic wave Ew1 propagated in the seawater (step S2). Also, the receiving station Rx similarly receives the electromagnetic wave Ew2 propagated in the seawater (step S3).

[0029] The receiving station Rx calculates the distance L to the transmitting station Tx (step S4). Specifically, the receiving station Rx calculates the time difference Δt between the reception time of the electromagnetic wave Ew1 and the reception time of the electromagnetic wave Ew2. Next, based on the time difference Δt, the propagation speed V1 of the electromagnetic wave Ew1, and the propagation speed V2 of the electromagnetic wave Ew2, the distance L is calculated by the above formula (2). Note that the receiving station Rx stores the propagation speed V1 of the electromagnetic wave Ew1 and the propagation speed V2 of the electromagnetic wave Ew2 in a memory or the like in advance, and uses the data read therefrom for the calculation of the distance L.

[0030] According to the distance measurement system 1 according to this embodiment, since the distance L from the receiving station Rx to the transmitting station Tx can be calculated based on the time difference Δt between the reception time of the electromagnetic wave Ew1 and the reception time of the electromagnetic wave Ew2, and the propagation speeds V1 and V2 of the electromagnetic waves Ew1 and Ew2, it is possible to accurately calculate the distance without being affected by the signal attenuation amount of the received electromagnetic wave. In addition, since it is not necessary to perform time synchronization or reference signal synchronization between the transmitting and receiving stations, the circuit configuration can be simplified, contributing to miniaturization and cost reduction.

[0031] (Embodiment 2) Next, the positioning system according to this embodiment will be described. FIG. 4 is a diagram showing a schematic configuration of a positioning system 2 according to an embodiment of the present invention. The positioning system 2 is used to implement the positioning method according to the present invention. Note that the positioning system 2 according to this embodiment measures the distances from the receiving station to three or more transmitting stations using the distance measurement system 1 according to Embodiment 1, and performs three-point positioning using the measured distances to detect the relative position of the receiving station with respect to a plurality of transmitting stations.

[0032] The positioning system 2 includes a first transmitting station Tx1 that transmits electromagnetic waves in seawater, a second transmitting station Tx2 installed at a position different from the first transmitting station Tx1, a third transmitting station Tx3 installed at a position different from the first transmitting station Tx1 and the second transmitting station Tx2, and a receiving station Rx that receives the electromagnetic waves propagated in seawater.

[0033] The first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 are, for example, buoys or ships floating on the sea or in the sea, or marine facilities related to oil, gas, etc. The first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 include a circuit that outputs electromagnetic waves of a predetermined frequency and an antenna that transmits electromagnetic waves in seawater.

[0034] The receiving station Rx is, for example, a manned submarine, an unmanned submarine, an underwater drone, a diver, etc. that navigate in seawater. The receiving station Rx includes an antenna that receives electromagnetic waves propagated in seawater, a circuit that calculates the relative positions with respect to the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 based on the received electromagnetic waves, and the like.

[0035] The first transmitting station Tx1 simultaneously transmits electromagnetic waves Ew11 and Ew12 having different frequencies into seawater. The second transmitting station Tx2 simultaneously transmits electromagnetic waves Ew21 and Ew22 having different frequencies into seawater. The third transmitting station Tx3 simultaneously transmits electromagnetic waves Ew31 and Ew32 having different frequencies into seawater.

[0036] Here, the frequency f11 of the electromagnetic wave Ew11, the frequency f21 of the electromagnetic wave Ew21, and the frequency f31 of the electromagnetic wave Ew31 are the same (f11 = f21 = f31), and the frequency f12 of the electromagnetic wave Ew12, the frequency f22 of the electromagnetic wave Ew22, and the frequency f32 of the electromagnetic wave Ew32 are the same (f12 = f22 = f32). Therefore, the propagation speed V11 of the electromagnetic wave Ew11 in seawater, the propagation speed V21 of the electromagnetic wave Ew21, and the propagation speed V31 of the electromagnetic wave Ew31 are faster than the propagation speed V12 of the electromagnetic wave Ew12 in seawater, the propagation speed V22 of the electromagnetic wave Ew22, and the propagation speed V32 of the electromagnetic wave Ew32 (V11 = V21 = V31 > V12 = V22 = V32).

[0037] Next, the operation of the above embodiment will be described based on the flowchart of FIG. 5. First, the first transmitting station Tx1 simultaneously transmits the electromagnetic waves Ew11 and Ew12 into seawater, for example, at predetermined time intervals (step S10).

[0038] The receiving station Rx sequentially receives the electromagnetic waves Ew11 and Ew12 propagated in seawater (step S11).

[0039] Next, the receiving station Rx calculates the distance L1 to the first transmitting station Tx1 (step S12). Specifically, the time difference Δt1 between the reception time of the electromagnetic wave Ew11 and the reception time of the electromagnetic wave Ew12 is calculated. Further, the receiving station Rx calculates the distance L1 to the first transmitting station Tx1 according to the above formula (2) based on the time difference Δt1, the propagation speed V11 of the electromagnetic wave Ew11, and the propagation speed V12 of the electromagnetic wave Ew12. Note that the receiving station Rx stores in advance in a memory or the like the propagation speed V11 of the electromagnetic wave Ew11 and the propagation speed V12 of the electromagnetic wave Ew12, and uses the data read therefrom for the calculation of the distance L1.

[0040] Next, for example, so as not to overlap with the transmission of the electromagnetic wave by the first transmitting station Tx1, the second transmitting station Tx2 simultaneously transmits the electromagnetic waves Ew11 and Ew12 into the seawater at predetermined intervals (step S13).

[0041] The receiving station Rx sequentially receives the electromagnetic waves Ew21 and Ew22 propagated in the seawater (step S14).

[0042] Next, the receiving station Rx calculates the distance L2 to the second transmitting station Tx2 (step S15). Specifically, the time difference Δt2 between the reception time of the electromagnetic wave Ew21 and the reception time of the electromagnetic wave Ew22 is calculated. Further, the receiving station Rx calculates the distance L2 to the second transmitting station Tx2 according to the above formula (2) based on the time difference Δt2, the propagation speed V21 of the electromagnetic wave Ew21, and the propagation speed V22 of the electromagnetic wave Ew22.

[0043] Next, for example, so as not to overlap with the transmission of the electromagnetic waves by the second transmitting station Tx1 and the second transmitting station Tx2, the third transmitting station Tx3 simultaneously transmits the electromagnetic waves Ew31 and Ew32 into the seawater at predetermined intervals (step S16).

[0044] The receiving station Rx sequentially receives the electromagnetic waves Ew31 and Ew32 propagated in the seawater (step S17).

[0045] Next, the receiving station Rx calculates the distance L3 to the third transmitting station Tx3 (step S18). Specifically, the time difference Δt3 between the reception time of the electromagnetic wave Ew31 and the reception time of the electromagnetic wave Ew32 is calculated. Also, the receiving station Rx calculates the distance L3 to the third transmitting station Tx3 by the above formula (2) based on the time difference Δt3, the propagation speed V31 of the electromagnetic wave Ew31, and the propagation speed V32 of the electromagnetic wave Ew32.

[0046] Based on the distances L1, L2, and L3 obtained previously, the receiving station Rx calculates the relative positions with respect to the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 by trilateration (step S19).

[0047] According to the positioning system 2 according to the present embodiment, based on the time difference between the reception times of electromagnetic waves having different frequencies and the propagation speeds of the electromagnetic waves, the distances L1, L2, and L3 to the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 are calculated, and the relative positions with respect to the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 can be calculated. Therefore, it is possible to accurately measure the relative position with a receiving station Rx having a simple configuration.

[0048] In the present embodiment, the frequency f11 of the electromagnetic wave Ew11, the frequency f21 of the electromagnetic wave Ew21, and the frequency f31 of the electromagnetic wave Ew31 are made the same (f11 = f21 = f31), and the frequency f12 of the electromagnetic wave Ew12, the frequency f22 of the electromagnetic wave Ew22, and the frequency f32 of the electromagnetic wave Ew32 are made the same (f12 = f22 = f32). However, the frequency f11 of the electromagnetic wave Ew11, the frequency f21 of the electromagnetic wave Ew21, the frequency f31 of the electromagnetic wave Ew31, the frequency f12 of the electromagnetic wave Ew12, the frequency f22 of the electromagnetic wave Ew22, and the frequency f32 of the electromagnetic wave Ew32 may be set to different frequencies.

[0049] According to this, as shown in the flowchart of FIG. 6, a set of electromagnetic waves (Ew11 to Ew32) are simultaneously transmitted from each of the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 (step S21). At the receiving station Rx, each electromagnetic wave (Ew11 to Ew32) is received in order (step S22). Based on the frequencies of the received electromagnetic waves, a set of electromagnetic waves (Ew11 and Ew12, Ew21 and Ew22, Ew31 and Ew32) are identified, and the distances L1, L2, and L3 to each transmitting station are simultaneously calculated (step S23). Based on the distances L1, L2, and L3, the relative position of the receiving station Rx can be calculated (step S24). According to this, it is possible to shorten the detection time of the relative position compared to the case where the distances to each transmitting station are calculated sequentially.

[0050] Also, in the positioning system 2 according to the above-described Embodiment 2, the relative position of the receiving station Rx with respect to the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3 is detected. However, the receiving station Rx acquires in advance the absolute position information (for example, latitude and longitude, etc.) of the first transmitting station Tx1, the second transmitting station Tx2, and the third transmitting station Tx3, and uses these position information and the relative position obtained from the distances L1, L2, and L3 to detect the absolute position of the receiving station Rx.

[0051] FIG. 7 shows an example in which a step S20 for obtaining the absolute position of the receiving station Rx is added to the procedure of obtaining the relative position after sequentially calculating the distances to each transmitting station. FIG. 8 shows an example in which a step S25 for obtaining the absolute position of the receiving station Rx is added to the procedure of simultaneously calculating the distances to each transmitting station and obtaining the relative position. According to this, it is possible to accurately measure the absolute position with a receiving station Rx having a simple configuration.

[0052] The embodiments of the present invention have been described above. However, the specific configuration is not limited to the above-described Embodiments 1 and 2, and even if there are design changes and the like within the scope not departing from the gist of the present invention, they are included in the present invention.

[0053] For example, in the positioning system 2 according to the above-described Embodiment 2, the first transmitter Tx1, the second transmitter Tx2, and the third transmitter Tx3 are used. However, the number of transmitters is not limited to three, and a plurality of three or more may be used. Further, although it has been described that the position information of each of the transmitters Tx1, Tx2, and Tx3 is acquired in advance by the receiver Rx, for example, the position information of the transmitter may be added to the electromagnetic wave transmitted from each of the transmitters Tx1, Tx2, and Tx3 and transmitted to the receiver Rx. According to this, even when the transmitter is moving, the latest position information can be transmitted to the receiver Rx, so that the absolute position of the receiver Rx can be accurately determined.

[0054] 1 Distance measurement system 2 Positioning system Tx Transmitter Tx1 First transmitter Tx2 Second transmitter Tx3 Third transmitter Rx Receiver

Claims

1. A transmitting station that simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, A receiving station that receives the first electromagnetic wave and the second electromagnetic wave propagated in seawater, and The receiving station calculates the distance to the transmitting station based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. A distance measurement system characterized by the above.

2. A first transmitting station that simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, A second transmitting station that is arranged at a position different from the first transmitting station and simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, A third transmitting station that is arranged at a position different from the first transmitting station and the second transmitting station and simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater, A receiving station that receives the first electromagnetic wave and the second electromagnetic wave propagated in seawater, and The receiving station Calculates the distance to the first transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the first transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, Calculates the distance to the second transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the second transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, Calculates the distance to the third transmitting station based on the time difference between the reception time of the first electromagnetic wave transmitted from the third transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, Calculates the relative position based on the distance to the first transmitting station, the distance to the second transmitting station, and the distance to the third transmitting station. A positioning system characterized by the above.

3. The receiving station calculates the absolute position based on the position information of the first transmitting station, the position information of the second transmitting station, the position information of the third transmitting station, and the relative position. The positioning system according to claim 2, characterized by the above.

4. A transmitting station simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater. A receiving station receives the first electromagnetic wave and the second electromagnetic wave propagated in seawater, and calculates the distance to the transmitting station based on the time difference between the reception time of the first electromagnetic wave and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater. A distance measurement method characterized by the above.

5. A first transmitting station simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater. A second transmitting station located at a position different from the first transmitting station simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater. A third transmitting station located at a position different from the first transmitting station and the second transmitting station simultaneously transmits a first electromagnetic wave and a second electromagnetic wave having different frequencies into seawater. A receiving station receives the first electromagnetic wave and the second electromagnetic wave propagated in seawater. Based on the time difference between the reception time of the first electromagnetic wave transmitted from the first transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, calculate the distance to the first transmitting station. Based on the time difference between the reception time of the first electromagnetic wave transmitted from the second transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, calculate the distance to the second transmitting station. Based on the time difference between the reception time of the first electromagnetic wave transmitted from the third transmitting station and the reception time of the second electromagnetic wave, the propagation speed of the first electromagnetic wave in seawater, and the propagation speed of the second electromagnetic wave in seawater, calculate the distance to the third transmitting station. Calculate the relative position based on the distance to the first transmitting station, the distance to the second transmitting station, and the distance to the third transmitting station. A positioning method characterized by the above.

6. Calculate the absolute position based on the position information of the first transmitting station, the position information of the second transmitting station, the position information of the third transmitting station, and the relative position. The positioning method according to claim 5, characterized by the above.

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