Research status and application prospect of single beacon underwater acoustic positioning technology
The application characteristics of traditional underwater acoustic positioning technology such as traditional long baseline and ultra-short baseline are analyzed, and the advantages of single beacon underwater acoustic positioning technology in global sea area positioning are pointed out. This paper introduces in detail two types of single beacon underwater acoustic positioning methods based on virtual long baseline and position tracking and their research progress, summarizes and analyzes the current status of single beacon underwater acoustic positioning technology, and prospects the application prospect of single beacon underwater acoustic positioning technology combined with the requirements of underwater positioning, navigation and timing system construction.
0 Introduction
The 21st century is a maritime century, and a world power must be a maritime power. With a coastline of more than 18,000 kilometers, China is rich in Marine resources such as oil, gas, minerals and biological resources. At the same time, it also faces severe challenges to its maritime rights and interests. All kinds of Marine survey, geological survey, engineering construction and support projects with the purpose of understanding and developing the ocean will inevitably require navigation and positioning [1], and military activities such as the deployment of submarines and surface ships and combat navigation are also inseparable from navigation and positioning. However, due to the strong absorption and shielding effect of seawater medium on electromagnetic wave, the underwater propagation distance of electromagnetic wave is very limited. global navigation satellite system (GNSS) is no longer suitable for underwater navigation, and sound wave has become the main way of underwater information transmission. Underwater acoustic positioning technology has also become an important means of underwater navigation and positioning [2].
The traditional underwater acoustic positioning system mainly includes ultra-short baseline (USBL) system and longbaseline (LBL) system. The ultra-short baseline positioning system generally has a baseline length of less than 1 m, and the system is simple in composition and small in volume, which is easy to lay and recover the baseline. However, a lot of calibration work is required, and the positioning accuracy is related to the slope distance [3]. The long baseline positioning system mostly lays more than 3 base point beacons on the seabed, and the baseline length is 100~6 000 m. Its positioning accuracy is independent of the depth, and it does not need to connect external equipment, and the positioning accuracy is relatively high. Because in the frequency range of 1~10 kHz, the environmental noise spectrum level of the shallow sea is basically between 40~70 dB, and with the reduction of frequency, the environmental noise increases, and the environmental noise below 1 kHz reaches more than 70 dB. At present, the operating frequency of the underwater acoustic positioning system is generally 10~30 kHz, the maximum ranging distance is about 10 km, and the positioning accuracy is not better than 0.15%×D (D is the operating distance). With the development of Marine research and development, in order to realize long-range covert underwater acoustic navigation and positioning above million meters in the future, the operating frequency of the system should be below 1 kHz [4]. Traditional long baseline and ultra-short baseline underwater acoustic positioning systems have the characteristics of relatively complex structure, large number of beacons, small coverage area, and low utilization efficiency. Unable to meet the needs of long distance positioning; The positioning technology based on single beacon ranging can simplify the positioning system structure, reduce the frequency, quantity and recovery cost of beacon calibration, save the synchronous tracking energy consumption of mother ship, and improve the utilization efficiency of beacon nodes, which is the development trend of underwater navigation and a new research direction for the development of acoustic positioning in the whole sea area in the future [5-6].
1 Overview of single beacon underwater acoustic positioning technology
The underwater acoustic positioning technology based on single beacon means that the distance information between the mobile node and the single beacon is measured by the rangefinder at regular intervals during the movement of the underwater mobile node, and the position estimation of the underwater mobile node is realized by combining the movement information of the mobile node. In 1995, Alexander, Far East Branch of the Institute of Marine Technical Problems of the Russian Academy of Sciences, first proposed the single beacon positioning method. In literature [7], when the location of single beacon and the depth of autonomous underwater vehicle (AUV) are known, the equations can be constructed by combining ranging information, AUV motion speed, attitude and ocean current speed, assuming that AUV travels in different directions in a straight line. The best position of AUV is calculated by least square method. The simulation results show that the maximum coordinate estimation error of AUV is less than 0.6m when the AUV travels 1 000 m in a straight line at different speeds. In reference [8], in view of the large position error after the completion of the AUV task, the extended Kalman filter algorithm is used to solve the AUV position with single beacon ranging and accurately guide the AUV to the recovery dock. In literature [9], the concept of synthetic long baseline (SLBL) was proposed, which was combined with the navigation dead estimation and underwater acoustic ranging methods, and modified by Kalman filtering to obtain the final position of the vehicle. The test results show that SLBL combined with the high-performance dead-reckoning navigation system can provide sub-meter positioning accuracy within the range of 1 000 m×1 000 m, and the positioning performance is almost independent of the water depth [9]. The virtual long baseline (VLBL) algorithm was proposed in the literature [10], and was applied to the test data processing of depth, course and Doppler speedometer of deep-sea exploration submersible of Woods Hole Oceanographic Institution. In the single beacon ranging navigation method analyzed in literature [11], when the AUV motion path is located in the vertical plane passing through the navigation beacon or in the horizontal plane with the same depth as the navigation beacon, the system cannot observe it. The theory and experiment fully verify the feasibility of the single beacon underwater acoustic location technology, and the effectiveness of applying VLBL and filter algorithm to solve and track the location.
According to different positioning principles, underwater acoustic positioning methods based on single beacons are mainly divided into two categories:
1) Based on the information of distance, motion speed and attitude of underwater vehicle, the positioning equation is established to solve the target position. By measuring the time for the spacecraft to receive the positioning message in at least three different positions, the distance information between the spacecraft and the beacon at different positions is obtained, and then the positioning equation is constructed by combining the motion and attitude data of the spacecraft to solve the position of the spacecraft. The single beacon virtual long baseline positioning method is based on the information such as slope distance, speed and heading Angle.
2) Tracking and positioning the position of underwater carrier based on filtering technology. The current state of the underwater carrier and the previous state are input into the designed filter to predict the current optimal estimator of the underwater carrier and ensure the minimum variance between the estimator and the real position of the carrier.
2 Single beacon virtual long baseline positioning method and research status
2.1 Virtual Long baseline Positioning Method
According to the principle of underwater acoustic positioning, a single distance measurement can only obtain the position of the sphere where the underwater carrier is located, and it must be combined with AUV and other navigation position or attitude information for single beacon positioning. In the single-beacon underwater acoustic positioning system, the acoustic beacon whose initial position has been calibrated is fixed underwater or on the seabed. The single beacon virtual long baseline positioning schematic is shown in Figure 1.
Figure 1 Schematic diagram of single beacon virtual long baseline positioning
Assume that the geographical position of the underwater single beacon Bs in geodetic coordinate system is X t =[xt, yt, zt] T when the underwater carrier moves from p1 position to p4 position, Their geographical positions in geodetic coordinate system are respectively [x1, y1, z1] T, [x2, y2, z2] T, [x3, y3, z3] T, [x4, y4, z4] T to solve the position of underwater carrier p4, the corresponding acoustic ranging observation equation is: d1, d2, d3, d4 are the distances between the underwater carrier moving to p1, p2, p3, p4 and acoustic beacon Bs; T is the ranging period of the underwater carrier, and the velocity of the underwater carrier in geodetic coordinate system at the i th ranging period is denoted as [Vx(i), Vy(i), Vz(i)] T
Equation (2) is equivalent to transferring the motion parameters of the underwater carrier to the actual acoustic beacon, so as to construct three virtual beacons B1, B2 and B3. At this time, the ranging equation is exactly the same as the traditional long baseline positioning physical model. Theoretically, if there is no error in the measurement of carrier motion, the positioning accuracy of single beacon virtual long baseline is the same as that of LBL.
If any location of the underwater carrier is to be located and solved, the information of any first i ranging period can be used, then the ranging equation corresponding to the i ranging period can be written as follows:
According to the above ranging equations, a set of quadratic nonlinear equations can be obtained, and the optimal solution of the current position Xn of the underwater carrier can be obtained by using the conventional least square method [3].
2.2 Research status of virtual long baseline positioning technology
In the single beacon virtual long baseline positioning technology, different measurement values can be used to establish the position solution model, the more common three types of measurement value combination modes are:
1) Based on the combination model of pure distance measurements, the complexity and nonlinearity of the motion model are generally low. Literature [12] proposes that when the AUV moves to different positions, hydrophones installed at the bottom of the AUV can receive signals periodically emitted from underwater sound sources, thus forming multiple virtual hydrophone matrices. In each virtual LBL window, the time difference from the sound source to each virtual hydrophone is obtained by the method of frequency domain weighted cross-correlation. By converting the time difference of more than 3 virtual hydrophones into the distance value, the motion equation similar to LBL can be established and the current position of the underwater robot can be calculated iteratively.
2) Based on azimuth-only measurement combination mode. There are few related studies on this kind of combination, and the requirements for AUV navigation trajectory are high. Literature [13] proposed that when the AUV was sailing in a straight line with a fixed heading Angle and a fixed speed, it received the acoustic signals emitted by the gateway nodes at different positions, solved the different azimuth angles between the gateway nodes and the AUV, and realized the AUV positioning by geometric solution, effectively avoiding the requirement of time synchronization.
3) Based on the combined range + azimuth measurement model, this type of combined model is widely used in underwater acoustic inertial integrated navigation system. In order to solve the cumulative error problem of inertial navigation system, literature [14] established a single beacon-assisted AUV navigation model based on indirect measurement by taking the range, speed and heading Angle information of single beacons as the observation objects, and analyzed the closed position expression.
Due to the measuring errors of the initial position, acoustic ranging, carrier attitude Angle, carrier motion speed, ranging period and virtual beacon distribution, the positioning accuracy and distribution of underwater carriers vary with different positions and motion states. In terms of positioning error model and influence, Reference [15] analyzes several typical errors in single-beacon navigation, such as sound velocity measurement, beacon position, inertial navigation system (INS), and time of arrival (TOA) of AUV received signals. The simulation results show that the robustness of beacon position and TOA measurement error is weak, which has great influence on navigation accuracy. Literature [16] systematically analyzed the effects of acoustic line bending, propagation delay, sailing speed and asynchronous transmission and receiving on the range error of a single beacon. The range error is proportional to the delay estimation error and carrier sailing speed, and the range error caused by the asynchronous transmission and reception of acoustic signals is relatively large. The simulation results show that the horizontal positioning error is several meters when the speed of underwater carrier is 1 m/s. Literature [17] studies the influence of carrier attitude Angle on SINS position error by establishing the error equation of strapdown inertial navigationsystem (SINS). The simulation results show that in the range of [0, 5°], the carrier heading Angle has the greatest influence on the longitude error drift, followed by the roll Angle, and the pitch Angle has the least influence. In order to improve the positioning accuracy, literature [3] established a single beacon VLBL positioning model to solve the divergence problem of position solution under the condition of unsatisfactory geometric distribution of virtual beacons, proposed an acoustical double precision difference optimization course Angle compensation method, and used robust Kalman filter (KF) to overcome the non-common point influence of transmitting and receiving. The sea test verifies the feasibility and effectiveness of the positioning method of the virtual ranging beacon. The deviation between the positioning results of the single beacon and the integrated navigation is (18.57 8.24) ± m, and the positioning accuracy is about 1.4%. The results of sea trials in literature [18] show that the equivalent average sound velocity method is used to calibrate the absolute position of a single underwater beacon for a survey ship positioned by the generalized differential global positioning system (GPS) (GPS positioning accuracy is about 2 m). The horizontal position accuracy can reach within 5 m. Literature [19] analyzed the error sources of AUV underwater single beacon positioning system. A densitybased spatial clustering of applications with noise (DBSCAN) algorithm is proposed to correct the location of the beacon. The simulation results show that the average horizontal positioning error of the beacon is reduced from 3.662 1 m to 2.101 9 m, which is reduced by 42.6%. Literature [20] compared and analyzed the performance differences of single beacon, double beacon and three beacon assisted inertial navigation system. The test results show that when the initial position error of the target horizontal square is [10 m,10 m], the positioning accuracy is corrected within 5 m by using the single beacon distance information to assist the inertial navigation system. Literature [21] proposed an integrated navigation method of SINS positioning aided by VLBL technology. Simulation results show that under pure inertial navigation, the average AUV positioning error is 13.344.5 m, and the average AUV positioning error is reduced to 1.881.4 m by using the hierarchical equal gradient sound velocity tracking algorithm, weighted cross correlation algorithm and periodic moving time window VLBL correction. The results show that beacon calibration error, TOA measurement error and asynchronous AUV signal transmission and reception have great influence on positioning error. Using VLBL positioning technology to assist inertial system navigation can effectively correct position error and improve AUV underwater integrated navigation positioning accuracy.
3. Single beacon position tracking and positioning methods and research status
3.1 Location tracking and positioning method
The single beacon virtual long baseline location adopts the conventional solution mode of spherical intersection, which has high positioning accuracy, but the system update rate is low and the stability is poor. The modern filtering algorithm and theory are mature, and selecting the appropriate filter to improve the positioning accuracy can be effectively improved. KF technology can analyze the motion state of AUV, and fuse the information and information of AUV's moving direction attitude data and acoustic distance measurement data to build a carrier motion tracking model, and update the positioning results more stably and at a higher rate, thus reducing the influence of target motion on positioning accuracy. The conventional Kalman filter is suitable for observing linear systems. extended Kalman filter (EKF) is expanded by Taylor series, while ignoring higher order nonlinear terms above the second order to achieve function linearization. Because the positioning observation equation between AUV and single beacon is non-linear mode, and AUV often moves at uniform low speed and has weak maneuverability, EKF algorithm is the most widely used.
In the single-beacon underwater acoustic positioning system, the acoustic beacon whose initial position has been calibrated is fixed underwater or at the bottom of the sea, and when the depth of underwater AUV can be accurately measured by the pressure sensor, the positioning problem can be simplified to a plane solving problem [22]. The AUV plane motion model is shown in Figure 2.
FIG. 2 AUV plane motion model
When using filter method to deal with information fusion problem, we must first establish mathematical model which can accurately reflect the law of system development, that is, accurate state equation and measurement equation of target system. Under the influence of ocean current velocity (assumed to be constant), the system equation of state [23] is
Equation (4) is obtained by discretization
The travel time of the AUV between k time and k+1 time; Wk is system excited Gaussian white noise. The measurement equation of discrete system is
3.2 Research status of position tracking and positioning
In the single beacon location tracking method, different measurement values can be combined to establish a motion model, which generally includes the distance between the underwater mobile node and the beacon and its own speed information to achieve position tracking. Literature [24] designed EKF to track and predict the current position and ocean current speed of the vehicle based on the distance value between the beacon and the vehicle, course Angle and relative water velocity. In literature [25], GPS was used to calibrate the position of the single beacon on the sea surface, and the slop distance and navigation information of the underwater mobile node were taken as the measurement quantity to establish the motion model of the mobile node, and the position estimation methods based on EKF and particle filter were analyzed and compared, and the feasibility of the method was verified by experiments. In literature [26], AUV needs to be equipped with an inertial measurement unit to obtain navigation information, take the AUV navigation dead estimation results and the propagation time of sound wave as input, and establish an EKF containing deviation to realize position tracking. In literature [27], the oblique distance and propagation delay were derived by combining the track of sound line, and a passive positioning model for underwater mobile nodes under asynchronous conditions was proposed. The relative distance to a single beacon, its own sailing speed, heading Angle and other parameter variables were calculated, and the EKF algorithm was used to reduce the accumulated position errors in the positioning process. Literature [28] analyzed the observability of the system under different auxiliary measurement values, and pointed out that the combination of yaw Angle and distance value is the simplest measurement combination that the system can observe.
Due to the complex underwater environment, many factors such as time asynchronism between single beacon and underwater mobile node, signal propagation delay, sound line bending and non-copoint ranging will affect the positioning accuracy, and it is difficult to accurately construct the measurement equation from the state quantity to the measured quantity. To solve the problem of positioning error correction, literature [29] proposed to take sound velocity as the state quantity of traditional KF, reconstruct the state transition equation and measurement equation, estimate the sound velocity in the current sea area, and realize sound velocity compensation. Based on the time-frequency relationship between cross-correlation function and cross-power spectrum, literature [30] adopts frequency-domain weighted cross-correlation method to reduce the pseudo-peak amplitude and obtain a higher precision delay difference. In literature [31], SINS nonlinear error model was analyzed by untraced Kalman filtering method, and the test results showed that: When the AUV initial position error range measured by a single beacon is less than 200 m, it has little effect on the attitude alignment results in SINS fine alignment, but constant errors will be introduced into the position estimation during SINS fine alignment. Literature [32] uses KF to correct the combined positioning error of INS/ Doppler velocity log (DVL)/depth sensor combined with the ranging information of underwater fixed single beacon. The simulation results show that, The latitude/longitude error of AUV converges from the initial value of 200 m/180 m to 1.72 m/1.47 m respectively. Literature [33] studies the convergence of SINS positioning errors assisted by single beacon ranging information. EKF is used for information fusion. Simulation results show that the longitude and latitude error of SINS output assisted by single beacon distance information is no more than 0.00004 °, and the accuracy is improved by nearly 2 orders of magnitude. The position tracking method based on single beacon generally has higher positioning accuracy and more complex algorithm calculation, and it is difficult to locate underwater mobile nodes in real time when the computing ability is weak.
4 Analysis and application prospect of single beacon positioning technology
4.1 Analysis of single beacon positioning technology
At present, China's terrestrial and spatiotemporal benchmark network has basically taken shape, but there is still a large gap in the construction of Marine spatiotemporal benchmark network, which provides positioning, navigation and timing (PNT) services on sea surface, underwater and seabed [34]. With the needs of underwater carrier navigation and positioning for long endurance, high precision and long distance, providing a unified spatiotemporal benchmark for key sea areas and even global sea areas has become a key problem to be solved by underwater PNT system at this stage. The underwater acoustic positioning technology based on single beacon greatly reduces the cost of large-scale long-distance underwater operations, expands the scope of operation, realizes passive positioning, and improves the concealment of underwater vehicles, which is the future development trend [35].
Whether it is single beacon virtual long baseline or location tracking and positioning method introduced above, most researchers have carried out a series of studies in the field of acoustics, focusing on system observability, positioning modeling, error analysis and positioning accuracy, data fusion in integrated navigation system, etc. Simulation has verified the feasibility of single beacon underwater acoustic positioning technology and the effectiveness of the algorithm. In terms of single-beacon range information assisted inertial navigation (INS) positioning, the extended Kalman filter algorithm is mainly used for position tracking, and certain achievements have been achieved. However, few experiments have been carried out, the experimental distance is short, and there is a lack of long-endurance and large-scale underwater vehicle experimental data support, and the positioning accuracy is about one order of magnitude lower than that of LBL positioning technology.
Some key technologies based on single beacon underwater acoustic positioning still need to be further studied in the following three aspects:
1) In terms of positioning methods, the current research on single beacon positioning technology is mostly based on single beacons fixed on the sea floor, which can be expanded to the direction of mobile single beacon underwater acoustic positioning or multi-moving carrier collaborative positioning in the future.
2) In terms of positioning accuracy, problems such as time synchronization between spacecraft and beacon, signal propagation delay, sound line bending, and non-simultaneous ranging are worthy of in-depth study; Beacon calibration position, TOA measurement error and asynchronous signal transmitting and receiving have great influence on positioning error. Multi-information joint tracking and positioning methods such as beacon position, TOA, sailing speed and motion attitude are also worthy of further study.
3) In terms of data fusion methods, motion models of underwater vehicles under different maneuvering states can be established to study and analyze the system's adaptive tracking performance under different motion states. When the degree of nonlinearity of the measurement equation is relatively serious, more filtering methods that are not affected by nonlinearity, such as unscented Kalman filter and particle filter, are tried to be used [29].
4.2 Application prospect of single beacon positioning technology
With reference to the development characteristics of foreign typical long-range long-baseline positioning systems, the future single beacon positioning technology will focus on improving its own positioning accuracy, and will pay more attention to underwater integrated navigation and integrated design research. As the main expansion space for future human activities, the construction of underwater PNT system is imperative [36]. In combination with the construction needs of China's underwater PNT system, the future development trend of single-signal calibration technology is prospected:
1) Coordinated development of single beacon positioning and underwater timing technology. In the single-beacon underwater acoustic positioning system, it is often assumed that the beacon and AUV time are synchronized, but the TOA measurement error is the main factor affecting the positioning accuracy. Underwater acoustic signal propagation speed is low, time delay is large, domestic underwater timing related technology can achieve about sub-millisecond accuracy, and satellite timing accuracy is about 6~7 orders of magnitude difference, underwater timing capability is seriously insufficient. In order to establish a unified space-time benchmark for key sea areas or even global sea areas, high-precision single beacon timing technology is bound to be an important support for underwater PNT system. In view of the close relationship between underwater ranging accuracy and time accuracy, joint research on underwater single beacon positioning and underwater timing technology is of practical significance for improving the positioning accuracy of single beacon and realizing the construction of underwater PNT system.
2) Integrated design of single beacon positioning and ultra-short baseline positioning system. With the development of deep-sea exploration tasks from manganese and cobalt nodules to hydrothermal search, more accurate positioning results are needed in deep-sea operations. Long baseline positioning system has high accuracy, but the cost of beacon array delivery and recovery is high, and the use of single beacon positioning can effectively reduce operating costs. Based on the combined positioning system of single beacon and ultra-short baseline, the positioning accuracy is independent of the working water depth, and the ultra-short baseline has the characteristics of mobility and flexibility, which can realize continuous high-precision navigation and positioning of underwater carriers in different regions near and far [37]. The AvTrak 6 Nano, a combined positioning system launched by the British Sonardyne company, combines LBL and USBL technology to provide a high-precision reference position for the AUV using the seabed transponder array, and uses the USBL technology for surface tracking, and the system works at a depth of 7,000 m. The ranging accuracy is better than 15 mm[38].
3) Single beacon combined with inertial navigation and Doppler technology. Inertial navigation system, with its advantages of good autonomy and strong anti-interference ability, has become the core navigation equipment of underwater carriers. However, positioning errors accumulate over time, and it needs to surface regularly for calibration, which is not conducive to long-term underwater concealment. As a new underwater navigation technology, single beacon positioning and ranging technology has initially possessed the ability of engineering navigation and positioning, and the positioning error does not diverge with time. Positioning based on single beacon ranging is the development trend of underwater assisted navigation and has become the research focus in navigation and positioning field at home and abroad [39-40]. The single beacon underwater acoustic positioning technology is integrated with inertial navigation, which can be used as an important means of remote guided measurement and improve the inertial navigation positioning performance. For example, the RAMSES long baseline positioning system launched by iXblue, a French company, can be used in combination with inertial navigation system and DVL. In the case of a transponder, the acoustic distance is integrated with the inertial navigation equation to obtain the sparse position of LBL, and the operating distance can reach 4 000 m. The positioning accuracy can reach decimeter level [41].
4) The single beacon positioning system is integrated with the Marine sensor module. Various Marine environmental elements are essential data sources for human understanding and development of the ocean. In the long-term observation of ocean sensors, sensor stability, drift, accuracy and collected data information need to be transmitted by underwater communication means. The integrated and integrated design of the single beacon underwater acoustic positioning system and sensor can effectively improve the convenience and efficiency of the system. The cNODE series acoustic transponder launched by Kongsberg Group in Norway has a transceiver transducer at the top, and can be flexibly assembled by selecting sensor modules such as pressure, sound velocity, temperature and inclination according to actual needs [42].
5 Closing remarks
In the 21st century, mankind has entered a period of large-scale development, utilization and protection of Marine resources. Ocean engineering operations such as polar exploration, deep-sea resource exploration and exploitation, seabed topography monitoring and maritime search and rescue have the needs of long-term, large-scale and long-distance high-precision positioning. Compared with the traditional underwater acoustic positioning system, the single beacon positioning system can effectively reduce the cost of equipment deployment and recovery, improve the ease of use and operational efficiency, and provide essential technical support for the realization of the whole sea area positioning and navigation under the premise of ensuring a certain positioning accuracy.
