METHOD FOR DETERMINING EFFECTIVE SOUND VELOCITY IN DEEP SEA

The present invention relates to the sound velocity propagation field, and in particular, to a method for determining an effective sound velocity in the deep sea.

An underwater acoustic positioning system measures a propagation time from a transmission point to a receiving point, then converts the propagation time into a slant distance between the transmission point and the receiving point, and finally implements underwater positioning. However, due to uneven distribution of a sound velocity in a vertical direction and resulting sound ray bending, the propagation distance of a sound signal from the transmission point to the receiving point is longer than the slant distance between the transmission point and the receiving point, which has great influence on the ranging accuracy and the positioning accuracy of the underwater acoustic positioning system. Therefore, sound ray bending needs to be corrected.

One idea is to use a sound ray tracing technology. In the sound ray tracing technology, a hierarchical approximation method and a layer-by-layer calculation method are used to compensate sound ray bending, so as to improve the positioning accuracy of the underwater acoustic positioning system. However, the sound ray tracing technology needs to balance between the positioning accuracy and the calculation amount, greatly restricting actual applications.

Another idea is to use an effective sound velocity technology. An effective sound velocity v_eis a ratio of a slant distance R between a transmission point and a receiving point to a propagation time T_tfrom the transmission point and the receiving point. Correspondingly, the slant distance R between the transmission point and the receiving point is equal to a product of the propagation time T_tand effective sound velocity v_e. The measurement accuracy of an underwater propagation time is very high, and therefore the accuracy of the slant distance R depends on the accuracy of effective sound velocity v_e.

Only a direct wave area (both an ultra short baseline positioning system and a long baseline positioning system work in this area) is considered. The sound velocity v_esatisfies a function between a transmission point depth z_A, a receiving point depth z_B, a pitch angle β between the transmission point depth z_Aand the receiving point depth z_B, and a sound velocity profile c(z) between the transmission point depth z_Aand the receiving point depth z_B, which is referred to a Taylor series analysis method:

v_e=v(1−C₂J₂−C₃J₃−C₄/J₄)   (1)

where v is an arithmetic mean sound velocity; w is a mean square-arithmetic mean sound velocity (used for rapid calculation); C₂, C₃, and C₄are a second-order sound velocity coefficient, a third-order sound velocity coefficient, and a fourth-order sound velocity coefficient; J₂, J₃, and J₄are a second-order J function, a third-order J function, and a fourth-order J function:

v_=1zB-zA∫zAzBc(z)dz(2a)w_=1zB-zA∫zAzBc2(z)dz(2b)C2=1-12cot2β(3a)C3=-C2-12cot4β(3b)C4=18cot4β(3-5cot2β)+C2(3c)Jn=1zB-zA∫zAzB(c(z)-v_v_)ndz,n=2,3,4(4)

Therefore, information required for calculating the effective sound velocity v_eby using the Taylor series analysis method is the transmission point depth z_A, the receiving point depth z_B, the pitch angle β between the transmission point depth z_Aand the receiving point depth z_B, and the sound velocity profile c(z) between the transmission point depth z_Aand the receiving point depth z_B.

A main calculation amount of calculating the effective sound velocity v_eby using the Taylor series analysis method is reflected in two integral operations, that is, Formula (2) and Formula (4). As long as the transmission point depth or the receiving point depth changes, it is necessary to recalculate corresponding v_e. In the deep sea, a difference between the transmission point depth and the receiving point depth is usually very large, and an actual sound velocity profile between the transmission point depth and the receiving point depth is very complex, leading to a very large main integral calculation amount of Formula (2) and Formula (4). As a result, it is difficult to satisfy an actual application requirement.

An objective of the present invention is to provide a method for determining an effective sound velocity in the deep sea, so as to rapidly and accurately determine an effective sound velocity.

To achieve the above purpose, the present invention provides the following technical solution.

The present invention provides a method for determining an effective sound velocity in the deep sea, where the method is applied to an apparatus for determining an effective sound velocity in the deep sea. The apparatus includes a transmission point, a receiving point, and an underwater mobile carrier. The transmission point is installed on the sea surface and the depth of the transmission point is unchanged. The receiving point is installed on the underwater mobile carrier, and the depth of the receiving point changes with movement of the underwater mobile carrier. The underwater mobile carrier can measure a sound velocity profile between the transmission point and the receiving point and a horizontal distance between the transmission point and the receiving point.

The method includes obtaining a transmission point depth at a moment k and a receiving point depth at the moment k and obtaining a receiving point depth at a moment k+1 and the horizontal distance between the transmission point and the receiving point. A pitch angle is determined between the transmission point and the receiving point according to the transmission point depth, the receiving point depth at the moment k+1, and the horizontal distance between the transmission point and the receiving point. A sound velocity profile is obtained between the transmission point and the receiving point at the moment k+1, and a difference between the receiving point depth at the moment k and the receiving point depth at the k+1 moment is also obtained. An arithmetic average sound velocity variable quantity and a mean square arithmetic average sound velocity variable quantity are determined according to the receiving point depth at the moment k+1, the receiving point depth at the moment k, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, and the sound velocity profile between the transmission point and the receiving point at the moment k+1.

A depth difference between the receiving point and the transmission point at the moment k is obtained and then an arithmetic average sound velocity at the moment k+1 and a mean square average sound velocity at the moment k+1 is determined according to a depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, an arithmetic average sound velocity at the moment k, a mean square average sound velocity at the moment k, the arithmetic average sound velocity variable quantity, and the mean square arithmetic average sound velocity variable quantity.

An n-order sound velocity function variable quantity (n=2,3,4) is determined according to the sound velocity profile between the transmission point and the receiving point at the moment k+1, the receiving point depth at the moment k, the receiving point depth at the moment k+1, and the arithmetic average sound velocity at the moment k+1.

An n-order sound velocity function (n=2,3,4) at the moment k+1 is then determined according to the n -order sound velocity function variable quantity, the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the sound velocity profile between the transmission point and the receiving point at the moment k+1, and the arithmetic average sound velocity;

An n-order sound velocity coefficient (n=2,3,4) at the moment k+1 is calculated according to a pitch angle at the moment k+1, and an effective sound velocity at the moment k+1 is determined according to the n-order sound velocity coefficient at the moment k+1, the n-order sound velocity function corresponding to the n-order sound velocity coefficient at the moment k+1, and the arithmetic average sound velocity at the moment k+1.

Optionally, the obtaining of the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1 includes determining the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1 according to the receiving point depth at the moment k and the receiving point depth at the moment k+1 by using a formula Δz_k=z_B,k+1−z_B,k, where

Δz_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; z_B,kis the receiving point depth at the moment k; and z_B+1is the receiving point depth at the moment k+1.

Optionally, the obtaining of the depth difference between the receiving point and the transmission point at the moment k includes determining the depth difference between the receiving point and the transmission point at the moment k according to the receiving point depth at the moment k and the transmission point depth at the moment k by using a formula z_ABk=z_B,k−z_A, where

z_ABkis the depth difference between the receiving point and the transmission point at the moment k; z_Ais the transmission point depth; and z_B,kis the receiving point depth at the moment k.

Optionally, the determining an arithmetic average sound velocity variable quantity and a mean square arithmetic average sound velocity variable quantity according to the receiving point depth at the moment k+1, the receiving point depth at the moment k, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, and the sound velocity profile between the transmission point and the receiving point at the moment k+1 includes respectively determining the arithmetic average sound velocity variable quantity and the mean square arithmetic average sound velocity variable quantity according to the receiving point depth at the moment k+1, the receiving point depth at the moment k, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, and the sound velocity profile between the transmission point and the receiving point at the moment k by using a formula ΔV_k=∫_zB,k^zB,k+Δzk c(z)dz and a formula ΔW_k=∫_zB,k^zB,k+Δzk c²(z)dz, where

ΔV_kis the arithmetic average sound velocity variable quantity; ΔW_kis the mean square arithmetic average sound velocity variable quantity; z_B,k+Δz_kis the receiving point depth at the moment k+1; z_B,kis the receiving point depth at the moment k; Δz_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; and c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1.

Optionally, the determining an arithmetic average sound velocity at the moment k+1 and a mean square average sound velocity at the moment k+1 according to a depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, an arithmetic average sound velocity at the moment k, a mean square average sound velocity at the moment k, the arithmetic average sound velocity variable quantity, and the mean square arithmetic average sound velocity variable quantity includes obtaining a sound velocity profile at the moment k, determining the arithmetic average sound velocity at the moment k and the mean square average sound velocity at the moment k according to the sound velocity profile c(z) at the moment k, the transmission point depth z_A, and the receiving point depth z_B,k; and

determining the arithmetic average sound velocity at the moment k+1 and the mean square average sound velocity at the moment k+1 according to the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the arithmetic average sound velocity at the moment k, the mean square average sound velocity at the moment k, the arithmetic average sound velocity variable quantity, and the mean square arithmetic average sound velocity variable quantity by using a formula

v_k+1=1zABk+Δzkz[ABk)vk+ΔVk

and a formula

w_k+1=1zABk+Δzk[zABkw_k+ΔWk],

where

v_k+1is the arithmetic average sound velocity at the moment k+1; w_k+1is the mean square average sound velocity at the moment k+1; z_ABkis the depth difference between the receiving point and the transmission point at the moment k+1; v_kis the arithmetic average sound velocity at the moment k; w_kis the mean square average sound velocity at the moment k; ΔV_kis the arithmetic average sound velocity variable quantity; and ΔW_kis the mean square arithmetic average sound velocity variable quantity.

Optionally, the determining an n-order sound velocity function variable quantity (n=2,3,4) according to the sound velocity profile between the transmission point and the receiving point at the moment k+1, the receiving point depth at the moment k, the receiving point depth at the moment k+1, and the arithmetic average sound velocity at the moment k+1 specifically includes determining the n-order sound velocity function variable quantity (n=2,3,4) according to the sound velocity profile between the transmission point and the receiving point at the moment k+1, the receiving point depth at the moment k, the receiving point depth at the moment k+1, and the arithmetic average sound velocity at the moment k+1 by using a formula

Qk=∫zB,kzB,k+1(c(z)-v_k+1v_k+1)ndz,

where

Q_kis the n-order sound velocity function variable quantity; c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1; z_B,kis the receiving point depth at the moment k; z_B,k+1is the receiving point depth at the moment k+1; and v_k+1is the arithmetic average sound velocity at the moment k+1.

Optionally, the determining an n -order sound velocity function (n=2,3,4) at the moment k+1 according to the n-order sound velocity function variable quantity, the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the sound velocity profile between the transmission point and the receiving point at the moment k+1, and the arithmetic average sound velocity specifically includes determining the n-order sound velocity function (n=2,3,4) at the moment k+1 according to the n -order sound velocity function variable quantity, the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the sound velocity profile between the transmission point and the receiving point at the moment k+1, and the arithmetic average sound velocity by using a formula

Jnk+1=1zABk+Δzk[∫zAzB,k(c(z)-v_k+1v_k+1)ndz+Qk],

where

J_n^k+1is the n-order sound velocity function corresponding to the n-order sound velocity coefficient at the moment k+1; Q_kis the n-order sound velocity function variable quantity; z_ABkis the depth difference between the receiving point and the transmission point at the moment k+1; Δz_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1; and v_k+1is the arithmetic average sound velocity at the moment k+1.

Optionally, the calculating an n-order sound velocity coefficient (n=2,3,4) at the moment k+1 according to a pitch angle at the moment k+1 includes calculating a second-order sound velocity coefficient, a third-order sound velocity coefficient, and a fourth-order sound velocity coefficient according to the pitch angle at the moment k+1 by using formulas: C₂^k+1=1−½cot²β_k+1; C₃^k+1=−C₂^k+1−1/2cot⁴β_k+1; and C₄^k+1=1/2cot⁴β_k+1(3−5cot²β_k+1)+C₂^k+1, where C₄^k+1J₄^k+1), where

β_k+1is the pitch angle at the moment k+1, and C₂^k+1, C₃^k+1, and C₄^k+1are respectively the second-order sound velocity coefficient, the third-order sound velocity coefficient, and the fourth-order sound velocity coefficient.

Optionally, the determining an effective sound velocity at the moment k+1 according to the n-order sound velocity coefficient at the moment k+1, the n-order sound velocity function corresponding to the n-order sound velocity coefficient at the moment k+1, and the arithmetic average sound velocity at the moment k+1 includes determining the effective sound velocity at the moment k+1 according to a formula v_e^k+1=v_k+1(1−C₂^k+1J₂^k+1−C₃^k+1J₃^k+1−C₄^k+1J₄^k+1), where

v_e^k+1is the effective sound velocity at the moment k+1, v_k+1is the arithmetic average sound velocity at the moment k+1, J₂^k+1, v₃^k+1, and J₄^k+1and respectively a second-order sound velocity function, a third-order sound velocity function, and a fourth-order sound velocity function, and C₂^k+1, C₃^k+1, and C₄^k+1are respectively the second-order sound velocity coefficient, the third-order sound velocity coefficient, and the fourth-order sound velocity coefficient.

According to specific embodiments provided in the present invention, the present invention discloses the following technical effects: The present invention provides a method for determining an effective sound velocity in the deep sea, which can convert an integral operation from original whole integral from the transmission point to the receiving point to local integral corresponding to a change of the receiving point depth, greatly reducing an integral calculation amount, so as to implement rapid calculation of t effective sound velocity in the deep sea.

The following clearly and completely describes the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are merely a part rather than all of the embodiments of the present invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

An objective of the present invention is to provide a method for determining an effective sound velocity in the deep sea, so as to rapidly and accurately determine an effective sound velocity.

To make the foregoing objective, features, and advantages of the present invention clearer and more comprehensible, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a flowchart of a method for determining an effective sound velocity in the deep sea according to an embodiment of the present invention. The method for determining an effective sound velocity in the deep sea is provided, where the method is applied to an apparatus for determining an effective sound velocity in the deep sea. The apparatus includes a transmission point, a receiving point, and an underwater mobile carrier. The transmission point is installed on the sea surface and the depth of the transmission point is unchanged. The receiving point is installed on the underwater mobile carrier and the depth of the receiving point changes with movement of the underwater mobile carrier. The underwater mobile carrier can measure a sound velocity profile between the transmission point and the receiving point and a horizontal distance between the transmission point and the receiving point. As shown in FIG. 1, the method includes the following steps:

Step 101. Obtain a transmission point depth at a moment k and a receiving point depth at the k moment.

Step 102. Obtain a receiving point depth at a moment k+1 and the horizontal distance between the transmission point and the receiving point.

Step 103. Determine a pitch angle between the transmission point and the receiving point according to the transmission point depth, the receiving point depth at the moment k+1, and the horizontal distance between the transmission point and the receiving point.

Step 104. Obtain a sound velocity profile between the transmission point and the receiving point at the moment k+1.

Step 105. Obtain a difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1.

Step 106. Determine an arithmetic average sound velocity variable quantity and a mean square arithmetic average sound velocity variable quantity according to the receiving point depth at the moment k+1, the receiving point depth at the k moment, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, and the sound velocity profile between the transmission point and the receiving point at the moment k+1.

Step 107. Obtain a depth difference between the receiving point and the transmission point at the moment k.

Step 108. Determine an arithmetic average sound velocity at the moment k+1 and a mean square average sound velocity at the moment k+1 according to a depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, an arithmetic average sound velocity at the moment k, a mean square average sound velocity at the moment k, the arithmetic average sound velocity variable quantity, and the mean square arithmetic average sound velocity variable quantity.

Step 109. Determine an n-order sound velocity function variable quantity (n=2,3,4) according to the sound velocity profile between the transmission point and the receiving point at the moment k+1, the receiving point depth at the moment k, the receiving point depth at the moment k+1, and the arithmetic average sound velocity at the moment k+1.

Step 110. Determine an n-order sound velocity function (n=2,3,4) at the moment k+1 according to the n -order sound velocity function variable quantity, the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the sound velocity profile between the transmission point and the receiving point at the moment k+1, and the arithmetic average sound velocity.

Step 111. Calculate an n-order sound velocity coefficient (n=2,3,4) at the moment k+1 according to a pitch angle at the moment k+1.

Step 112. Determine an effective sound velocity at the moment k+1 according to the n-order sound velocity coefficient at the moment k+1, the n-order sound velocity function corresponding to the n-order sound velocity coefficient at the moment k+1, and the arithmetic average sound velocity at the moment k+1.

Step 105 specifically includes:

determining the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1 according to the receiving point depth at the moment k and the receiving point depth at the moment k+1 by using a formula Δz_k=z_B,k+1−z_B,k, where

66_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; z_B,kis the receiving point depth at the moment k; and z_B,k+1is the receiving point depth at the moment k+1.

Step 107 specifically includes:

determining the depth difference between the receiving point and the transmission point at the moment k according to the receiving point depth at the moment k and the transmission point depth at the moment k by using a formula Z_ABk=z_B,k−Z_A, where

z_ABkis the depth difference between the receiving point and the transmission point at the moment k; z_Ais the transmission point depth; and z_B,kis the receiving point depth at the moment k.

Step 106 specifically includes:

respectively determining the arithmetic average sound velocity variable quantity and the mean square arithmetic average sound velocity variable quantity according to the receiving point depth at the moment k+1, the receiving point depth at the moment k, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, and the sound velocity profile between the transmission point and the receiving point at the moment k by using a formula ΔV_k=∫_zB,k^zB,k+Δkc(z)dz and a formula ΔW_k=∫_zB,k^zB,k+Δzkc²(z)dz, where

ΔV_kis the arithmetic average sound velocity variable quantity; ΔW_kis the mean square arithmetic average sound velocity variable quantity; z_B,k+Δz_kis the receiving point depth at the moment k+1; z_B,kis the receiving point depth at the moment k; Δz_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; and c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1.

Step 108 specifically includes:

obtaining a sound velocity profile at the moment k;

determining the arithmetic average sound velocity at the moment k and the mean square average sound velocity at the moment k according to the sound velocity profile c(z) at the moment k, the transmission point depth z_A, and the receiving point depth z_B,k; and

determining the arithmetic average sound velocity at the moment k+1 and the mean square average sound velocity at the moment k+1 according to the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the arithmetic average sound velocity at the moment k, the mean square average sound velocity at the moment k, the arithmetic average sound velocity variable quantity, and the mean square arithmetic average sound velocity variable quantity by using a formula

v_k+1=1zABk+Δzkz[ABk)vk+ΔVk

and a formula

w_k+1=1zABk+Δzk[zABkw_k+ΔWk],

where

v_k+1 is the arithmetic average sound velocity at the moment k+1; w_k+1is the mean square average sound velocity at the moment k+1; z_ABkis the depth difference between the receiving point and the transmission point at the moment k+1; v_kis the arithmetic average sound velocity at the moment k; w_kis the mean square average sound velocity at the moment k; ΔV_kis the arithmetic average sound velocity variable quantity; and ΔW_kis the mean square arithmetic average sound velocity variable quantity.

Step 109 specifically includes:

determining the n-order sound velocity function variable quantity (n=2,3,4) according to the sound velocity profile between the transmission point and the receiving point at the moment k+1, the receiving point depth at the moment k, the receiving point depth at the moment k+1, and the arithmetic average sound velocity at the moment k+1 by using a formula

Qk=∫zB,kzB,k+1(c(z)-v_k+1v_k+1)ndz,

where

Q_kis the n-order sound velocity function variable quantity; c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1; z_B,kis the receiving point depth at the moment k; z_B,k+1is the receiving point depth at the moment k+1; and v_k+1is the arithmetic average sound velocity at the moment k+1.

Step 110 specifically includes:

determining the n-order sound velocity function (n=2,3,4) at the moment k+1 according to the n-order sound velocity function variable quantity, the depth difference between the receiving point and the transmission point at the moment k+1, the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1, the sound velocity profile between the transmission point and the receiving point at the moment k+1, and the arithmetic average sound velocity by using a formula

Jnk+1=1zABk+Δzk[∫zAzB,k(c(z)-v_k+1v_k+1)ndz+Qk],

where

∫_n^k+1is the n-order sound velocity function corresponding to the n-order sound velocity coefficient at the moment k+1; Q_kis the n-order sound velocity function variable quantity; z_ΔBkis the depth difference between the receiving point and the transmission point at the moment k+1; Δz_kis the difference between the receiving point depth at the moment k and the receiving point depth at the moment k+1; c(z) is the sound velocity profile between the transmission point and the receiving point at the moment k+1; and v_k+1is the arithmetic average sound velocity at the moment k+1.

Step 111 specifically includes:

calculating a second-order velocity coefficient, a third-order sound velocity coefficient, and a fourth-order sound velocity coefficient according to the pitch angle at the moment k+1 by using formulas: C₂^k+1=1−½cot²β_k+1; C₃^k+1=−C₂^k+1=½cot⁴β_k+1; and C₄^k+1=⅛cot⁴β_k+1(3−5cot²β_k+1)+C₂^k+1, where

β_k+1is the pitch angle at the moment k+1, C₂^k+1, C₃^k+1, and C₄^k+1, are respectively the second-order sound velocity coefficient, the third-order sound velocity coefficient, and the fourth-order sound velocity coefficient.

Step 112 specifically includes:

determining the effective sound velocity at the moment k+1 according to a formula v_e^k+1=v_k+1(1−C₂^k+1J₂^k+1−C₃^k+1−C₄^k+1J₄^k+1), where

v_e^k+1is the effective sound velocity at the moment k+1; v_k+1, is arithmetic average sound velocity at the moment k+1; J₂^k+1, J₃^k+1, and J₄^k+1are respectively a second-order sound velocity function, a third-order sound velocity function, and a fourth-order sound velocity function, and C₂^k+1, C₃^k+1, and C₄^k+1are respectively the second-order sound velocity coefficient, the third-order sound velocity coefficient, and the fourth-order sound velocity coefficient.

Integral operations of Formula (2) and Formula (4) are converted from whole integral to local integral in a depth change area, to reduce an integral calculation amount of Formula (2) and Formula (4), so as to reduce a calculation amount of the effective sound velocity y_e. Only in this way, in an underwater acoustic positioning system, when a depth of the transmission point or the receiving point changes, a corresponding effective sound velocity v_ecan be calculated very rapidly, and then a measured propagation time is converted into a slant distance between the transmission point and the receiving point, and finally implements real-time underwater positioning.

In the present invention, it is assumed that a position of a sound source is near a sea surface and is stationary; a position of a receiving point is below the position of the sound source and the position can be moved; when both the sound source and the receiving point change, a specific method of the present invention is obtained through the following derivation method.

(1) Calculate an arithmetic average sound velocity v_k+1at the moment k+1 through the arithmetic average sound velocity v_kat the moment k.

From the moment k to the moment k+1, the corresponding depth difference Δz_kof the receiving point is as follows:

Δz_k=z_B,k+1−z_B,k  (5)

The corresponding depth difference z_ΔBkbetween the receiving point and the transmission point at the moment k is as follows:

z_ABk=z_B,k−z_A  (6)

The following formulas can be obtained through Formula (2a), Formula (2b), and Formula (6):

∫_zA^zB,kc(z)dz=z_ABkv_k  (7a):

∫_zA^zB,kC²(z)dz=z_ABkw_k  (7b)

At the moment k+1, the arithmetic average sound velocity v_k+1,and the mean square arithmetic average sound velocity Δw_k+1are respectively as follows:

v_k+1=1zB,k+1-zA∫zAzB,k+1c(z)dz=1zABk+Δzk[zABkv_k+∫zB,kzB,k+Δzkc(z)dz](8a)w_k+1=1zB,k+1-zA∫zAzB,k+1c2(z)dz=1zABk+Δzk[zABkw_k+∫zB,kzB,k+Δzkc2(z)dz](8b)

For the receiving point, from the moment k to the moment k+1, let the arithmetic average sound velocity variable quantity ΔV_kand the mean square arithmetic average sound velocity variable quantity ΔW_krespectively satisfy the following:

ΔV_k=∫_zB,k^zB,k+Δzkc(z)dz   (9a)

ΔW_k=∫_zB,k^zB,k+Δzkc²(z)dz   (9b)

In this way, Formula (8) changes to

v_k+1=1zABk+Δzk[zABkv_k+ΔVk](10a)w_k+1=1zABk+Δzk[zABkw_k+ΔWk](10b)

In this case, an integral operation of Formula (2) is converted from original whole integral of the transmission point to the receiving point to local integral corresponding to a change of the receiving point depth, greatly reducing an integral calculation amount.

(2) Calculate J_n^k(n=2,3,4) at the moment k+1 through J_n^k(n=2,3,4) at the moment k.

The following formula can be obtained through Formula (4) and Formula (7):

∫zAzB,k(c(z)-v_kv_k)ndz=(zB,k-zA)Jnk=zABkJnk,n=2,3,4(11)

At the moment k+1, an n-order J function J_n^k+1(n=2,3,4) is as follows:

Jnk+1=1zB,k+1-zA∫zAzB,k+1(c(z)-v_k+1v_k+1)ndz=1zABk+Δzk[∫zAzB,k(c(z)-v_k+1v_k+1)ndz+∫zB,kzB,k+1(c(z)-v_k+1v_k+1)ndz](12)

For the receiving point, from the moment k to the moment k+1, let an n-order J function variable quantity Q_ksatisfy the following:

Qk=∫zB,kzB,k+1(c(z)-v_k+1v_k+1)ndz,n=2,3,4(13)

In this case, Formula (12) changes to:

Jnk+1=1zABk+Δzk[∫zAzB,k(c(z)-v_k+1v_k+1)ndz+Qk],n=2,3,4(14)

A function ƒ(v_k) is defined as follows:

f(v_k)=(c(z)-v_kv_k)n,n=2,3,4(15)

A integral of the function ƒ(v_k) is as follows:

f′(v_k)=nv_k2c(z)-nv_k3c2(z)(16a)f″(v_k)=-2nv_k3c(z)+3nv_k4c2(z)(16b)f′′′(v_k)=6nv_k4c(z)-12nv_k5c2(z)(16c)f(4)(v_k)=-24nv_k5c(z)+60nv_k6c2(z)(16d)

According to the Taylor expansion, a function ƒ(v_k+1) at the moment k+1 is as follows:

ƒ(v_k+1)≈ƒ(v_k)+ƒ′(v_k)·Δv+½ƒ″(v_k)·Δv²+⅙ƒ′″(v_k)·Δv³+ 1/24ƒ⁽⁴⁾)v_k)·Δvv⁴  (17)

Δv_k=v_k+1−v_k  (18)

Substitute Formula (16a-d) is substituted into Formula (17) to conduct a series of algebraic operations, and the function ƒ(v_k+1) at the moment k+1 may be simplified to:

f(v_k+1)≈f(v_k)+(11·n·Δv_v_k2-12·2n-Δv_2v_k3+16·6n·Δv_3v_k4-124·24n·Δv_4v_k5)c(z)+(-11·n·Δv_v_k3+12·3n-Δv_2v_k4-16·12n·Δv_3v_k5+124·60n·Δv_4v_k6)c2(z)(19)

To further simplify Formula (19), let

Φk=11·n·Δv_v_k2-12·2n-Δv_2v_k3+16·6n·Δv_3v_k4-124·24n·Δv_4v_k5(20)Ψk=-11·n·Δv_v_k3+12·3n-Δv_2v_k4-16·12n·Δv_3v_k5+124·60n·Δv_4v_k6(21)

In this case, a function ƒ(v_k+1) at the moment k+1 is as follows:

ƒ(v_k+1)≈ƒ(v_k)+Φ_k·c(z)+Ψ_k·c²(z)   (22)

Formula (22) is substituted into Formula (12), and the n-order J function J_n^k+1(n=2,3,4) at the moment k+1 is as follows:

Jnk+1=1zABk+Δzk[∫zAzB,kf(v_k+1)dz+Qk]=1zABk+Δzk[∫zAzB,kf(v_k)dz+∫zAzB,kΦk·c(z)dz+∫zAzB,kΨk·c2(z)dz+Qk]=1zABk+Δzk∫zAzB,kf(v_k)dz+ΦkzABk+Δzk∫zAzB,kc(z)dz+ΨkzABk+Δzk∫zAzB,kc2(z)dz+1zABk+ΔzkQk(23)

J_n^k+1Formula (7) and Formula (11) are substituted into Formula (23), and the n-order J function J_n^k+1(n=2,3,4) at the moment k+1 is as follows:

Jnk+1=1zABk+Δzk[zABkJnk+ΦkZABkv_k+ΨkzABkw_k+Qk],n=2,3,4(24)

In this case, an integral operation of Formula (4) is converted from original whole integral of the transmission point to the receiving point to local integral of Formula (13) corresponding to a change of the receiving point depth, greatly reducing an integral calculation amount.

Each embodiment of the present specification is described in a progressive manner, each embodiment focuses on the difference from other embodiments, and the same and similar parts between the embodiments may refer to each other.

Several examples are used for illustration of the principles and implementation methods of the present invention. The description of the embodiments is used to help illustrate the method and its core principles of the present invention. In addition, those skilled in the art can make various modifications in terms of specific embodiments and scope of application in accordance with the teachings of the present invention. In conclusion, the content of this specification shall not be construed as a limitation to the invention.