Application of ultrashort baseline positioning system in deep water
Introduction
Ocean engineering survey is an important source of basic data required for the planning, design, construction and project environmental assessment (ecological protection, site disaster prevention, etc.) of various ocean engineering construction projects, and an indispensable link in ocean engineering construction [1]. Traditional ocean engineering investigation technology adopts shipborne and towed operation mode. With the increase of water depth, acoustic equipment such as sounder, geomorphic instrument and shallow formation profile increase the transmitting and receiving distance of sound wave, and the rapid attenuation of sound wave energy, resulting in greatly reduced resolution, accuracy and depth of detection, which cannot meet the needs of deep-water oil and gas structure engineering design and construction [2]. In the deep-water engineering investigation, the underwater carrier (such as deep-drag, AUV) is usually equipped with survey equipment such as depth sounder, geomorphic instrument and shallow formation profiler to carry out high-resolution investigation at a height of 40-80m from the seabed surface. Location information is the basis of ocean engineering survey results, all survey results can be used effectively only when given correct geographic location information. Therefore, accurate determination of underwater carrier location is a key step in deepwater engineering investigation. Although GNSS (Global Navigation Satellite System), represented by BDS, GPS, GLONASS and Galileo, can provide global users with full weather, all-day, high-precision navigation and positioning services, However, due to the strong absorption effect of seawater on electromagnetic wave energy, its propagation distance is limited, so that the radio navigation and satellite navigation technology means using electromagnetic wave as the propagation carrier can not be directly used for underwater target location. Acoustic wave is the main choice for underwater positioning due to its advantages of low attenuation of propagation energy in water and long propagation distance.
Underwater acoustic positioning technology is a high precision underwater positioning technology by measuring the propagation time and phase of sound waves in water. According to the different base length of receiving array, acoustic positioning system can be divided into long base line positioning system, short base line positioning system and ultra-short base line positioning system. The ultra-short baseline positioning system is widely used in deepwater engineering investigation for its advantages of simple structure, convenient operation, high ranging accuracy and low cost.
1. Positioning principle and composition of ultra-short baseline system
1.1 Positioning Principles
The ultra-short baseline transmits acoustic signals to the water through acoustic transducers installed on the ship. The acoustic transponder installed on the underwater target immediately returns a response signal energy exchanger different from the interrogation signal after receiving the interrogation signal [4], so as to determine the distance and Angle of the detected target relative to the transducer array. The measurement of distance and Angle is based on the following two principles: first, by accurately measuring the propagation time of sound wave between the transducer and the detected target, and then using the sound velocity profile to correct the beamline, to determine the distance between the target and the transducer; Second, the Angle of the target relative to the transducer is determined by measuring the phase difference of the echo signal received by different receiving units on the transducer array. According to the fixed relation between the transducer array coordinate system and the ship coordinate system, combined with the real-time ship attitude and heading information provided by the compass and attitude sensor and the ship geographical coordinates provided by the surface positioning system, the system can solve the geographical coordinates of the location of the underwater transponder in real time.
Conventional ultra-short baseline transducers contain one transmitting unit and more than three receiving units with identical performance, which are vertically orthogonal and distributed at equal intervals (less than half wavelength) to form a planar orthogonal receiving array [5]. The position between the receiving units is accurately measured to form the acoustic array coordinate system. As shown in Figure 1, H1, H, H3 and H4 are four orthogonal arranged transducer receiving units, and the distance between them and the origin O of the acoustic array coordinate system is D/2. The direction from origin O to H1 points to the ship's bow, which is the direction of X axis; the direction from origin O to H2 is perpendicular to the ship's bow and points to the starboard side of the ship, which is the direction of Y axis; and the Z axis points to the sea bottom.
As shown in FIG. 2, it is assumed that P is the position of the underwater transponder, the included angles between OP and each coordinate axis are respectively x, y and z, and the distance between P and the origin O of the acoustic array coordinate system is S. Fixed spot x and fixed spot y are the phase difference between the receiving unit H1 and H2, and between the receiving unit H3 and H4, which is the wavelength of sound wave. Because the distance between the receiving array baselined of the ultra-short baseline transducer is very small, generally in the order of centimeters, which is far less than the distance S from the transducer to the transponder P point, Therefore, it can be considered that the echo acoustic lines received by the receiving unit are parallel. Then, the included angles between OP and each coordinate axis are respectively z x, y and z, which can be expressed by the following formula:
FIG. 1 Schematic diagram of the acoustic array coordinate system of the ultra-short baseline transducer
FIG. 2 Schematic diagram of ultra-short baseline positioning
The coordinates (x, y, z) of point P in the transducer array coordinate system can be obtained directly according to the length of the space line OP and the Angle between it and each coordinate axis.
1.2 System Composition
The ultra-short baseline positioning system generally consists of three components: underwater positioning system, auxiliary sensor and surface positioning system, as shown in Figure 3. The underwater positioning system is composed of acoustic transducer installed in the hull and acoustic transponder underwater. The auxiliary sensors include high-precision optical fiber compass and attitude instrument, which can be used to accurately measure the heave, roll, pitch, heave/Heading of the ship; A surface positioning system usually consists of a Global Navigation Satellite System (GNSS) receiver to determine a ship's geographical position in real time.
2 Factors affecting the positioning performance of the ultra-short baseline system
The ultra-short baseline positioning system can directly measure the distance and azimuth of the underwater target relative to the transducer [6]. To further obtain the absolute position (geographical coordinates) of the underwater target, it is necessary to accurately measure the relative relationship between the acoustic array coordinate system of the transducer and the ship coordinate system. Including the positioning of the transducer in the ship's coordinate system and the installation Angle deviation of the array (roll, pitch and heading deviation) [7], the geographic coordinates provided by the ship's GNSS, the instantaneous pose data provided by the attitude sensor and the ship's heading data provided by the compass are integrated. Therefore, the main factors affecting the accuracy of ultra-short baseline positioning system can be summarized into three categories: measuring equipment error, installation Angle deviation, sound velocity error.
2.1 Measure the error of the device
The acoustic array error of ultra-short baseline transducer will affect the relative position of underwater target measurement, so it is necessary to calibrate the acoustic array of transducer before use. For commercial ultra-short baseline systems, manufacturers generally accurately calibrate the acoustic array error of the system before leaving the factory. The ultra-short baseline system is used to calculate the absolute alignment position of the underwater target, which needs to integrate geographic alignment data provided by GNSS installed on the ship, instantaneous attitude data provided by attitude sensor and ship heading data provided by compass. The inherent errors of these devices will also affect the calculation accuracy of absolute coordinates. Therefore, high-precision devices should be used as far as possible to reduce the errors that may be brought by the instruments themselves.
2.2 Installation Angle Deviation
In the ultra-short baseline positioning system, because the compass and pose sensor of auxiliary measuring equipment and the ultra-short baseline transducer are usually installed separately, there is rotation Angle deviation between the acoustic array coordinate system of the transducer and the hull coordinate system [8], namely, course deviation, roll deviation and pitch deviation. The course error will affect the horizontal positioning accuracy of the ultra-short baseline. However, roll deviation and pitch deviation have influence on horizontal and vertical positioning accuracy of ultrasheminar baseline. According to operational experience, under the 1 Yi Angle deviation, when the underwater target is 2km away from the transducer, the positioning error of 35m will be generated. When the Angle deviation is 0.1 Yi, the distance is also 2km, and the positioning error is only about 3m. Therefore, before use, it is necessary to calibrate the installation Angle deviation of the ultra-short baseline system. Especially in deep water operations, the installation Angle deviation calibration should be carried out in the maximum water depth within the operating area, so as to minimize the impact of installation Angle deviation on the positioning accuracy of the ultra-short baseline system.
2.3 Sound velocity error
The influence of sound velocity mainly comes from the error of sound velocity value and the bending of sound line. The distance between the underwater target and the transducer is determined by measuring the travel time of the acoustic signal between the transducer and the underwater target combined with the sound speed of the acoustic wave in the water. The error of sound velocity value will directly affect the accuracy of ultra short baseline ranging. The different temperature and salinity in different positions and depths of seawater result in the different density of seawater, and the value of sound velocity varies with the density. Due to the difference in sound speed, sound waves in the water body do not travel in a straight line, but curved. From the ultra-short baseline positioning formula, it can be found that the arrival Angle of acoustic wave return transducer array needs to be solved to determine the position of underwater target relative to the transducer array. The refraction caused by the change of sound velocity in water will change the path of sound wave, and the arrival Angle of sound wave back to the transducer array cannot truly reflect the actual direction of underwater target. The larger the incidence Angle of the sound wave, the more severe the bending of the sound line. When the underwater target is located directly under the transducer, the sound line does not bend due to the vertical incidence of the sound wave. Therefore, in the process of ultra-short baseline operation, the incidence Angle should be kept in a small range to reduce the influence of acoustic line bending on positioning accuracy.
3. Application of ultra-short baseline in deepwater engineering investigation
3.1 Applications in deepwater AUV survey operations
Autonomous Underwater Vehicle (AUV) is a kind of intelligent underwater vehicle. As an intelligent deep-water underwater carrier, it is similar to an unmanned diving survey ship. It can be equipped with various measuring equipment such as depth sounder, geomorphic instrument and shallow stratigraphic profiler to carry out seabed search, topographic and geomorphic detection, geological disaster investigation and other investigation work. In deep-water engineering exploration, AUV replaces conventional ship-borne and towed survey operations with high precision, high resolution and high efficiency. It is an important technical means for deep-sea oil and gas exploration and development at present. In 2015, COSL purchased Explorer 3000M AUV from abroad for deep-water engineering investigation. So far, it has completed survey operations of over 10,000 kilometers, providing a large number of high-precision and high-resolution survey data for the exploration and development of deep-water oil and gas fields. The Explorer 3000M AUV is equipped with Kongsberg EM2040 multi-beam sounding system, EdgeTech 2200M side scan sonar, and shallow profile system. It is capable of conducting deep water exploration up to 3000m in shallow waters. Obtain high precision topography and shallow strata profile data.
Like most AUVs, the Explorer 3000M AUV underwater navigation system uses a combination of inertial navigation (INS) and Doppler Log (DVL) navigation. When the altitude from the seabed is not more than 200m, although the high precision speed information provided by DVL can well suppress the cumulative error of INS [9], the positioning error of the underwater navigation system will still increase with the increase of working time and distance, making the AUV deviate from the preset survey line. Therefore, it is necessary to use underwater acoustic positioning technology to correct the cumulative error of AUV underwater navigation system and bring it back to the preset survey line. Take Explorer 3000M AUV as an example, IXblue MT9 underwater acoustic locator beacon is fixed near the upper part of the tail for ultra short baseline positioning. IXBlue GAPS is installed on the Explorer 3000M AUV to maintain IXblue Gaps directly above the AUV during underwater navigation to improve the positioning accuracy of the ultra-short baseline. The accurate absolute positions of the GAPS ultra-short baseline system tracked underwater beacons are transmitted in real time to the Explorer 3000M AUV internal inertial navigation system Phins through the acoustic communication system to eliminate the accuracy drift of INS+DVL integrated navigation. Phins is a set of high precision closed-loop navigation system of fiber optic gyroscope, which can provide true azimuth, motion attitude, velocity, heave and three-dimensional position information of the carrier. Through Kalman filtering, Phins can estimate the optimal underwater position of AUV in real time by integrating high precision velocity information, ultra-short baseline positioning information and AUV depth and height information provided by DVL.
3.2 Application of deep drag survey
Deep-Tow survey is a deep-sea engineering survey method in which a combination of one or several Marine survey instruments is installed on a deep-water towfish (body), and the influence of water on the instrument is reduced by sinking the body to a predetermined depth, so as to obtain high quality multi-wave beam, side scan sonar, shallow formation profile and other data. Taking EdgeTech DT-1 deep towing system as an example, the deep towing system is mainly composed of four parts: underwater towing body and carrying equipment, towing system (including ballast, towing cable, umbilical cable and winch), release recovery system, deck communication chain and system control processor. The deep-towing system is usually equipped with multi-beam sounding system, side-scan sonar system, shallow formation profiler, and auxiliary sensing equipment (fiber optic compass, motion sensor, real-time sound tachometer, pressure sensor, Doppler altimeter, and underwater acoustic positioning beacon, etc.). Unlike the AUV, the deep towing system is not powered and is towed entirely by the operating mother ship.
Like most AUVs, the Explorer 3000M AUV underwater navigation system uses a combination of inertial navigation (INS) and Doppler Log (DVL) navigation. When the altitude from the seabed is not more than 200m, although the high precision speed information provided by DVL can well suppress the cumulative error of INS [9], the positioning error of the underwater navigation system will still increase with the increase of working time and distance, making the AUV deviate from the preset survey line. Therefore, it is necessary to use underwater acoustic positioning technology to correct the cumulative error of AUV underwater navigation system and bring it back to the preset survey line. Take Explorer 3000M AUV as an example, IXblue MT9 underwater acoustic locator beacon is fixed near the upper part of the tail for ultra short baseline positioning. IXBlue GAPS is installed on the Explorer 3000M AUV to maintain IXblue Gaps directly above the AUV during underwater navigation to improve the positioning accuracy of the ultra-short baseline. The accurate absolute positions of the GAPS ultra-short baseline system tracked underwater beacons are transmitted in real time to the Explorer 3000M AUV internal inertial navigation system Phins through the acoustic communication system to eliminate the accuracy drift of INS+DVL integrated navigation. Phins is a set of high precision closed-loop navigation system of fiber optic gyroscope, which can provide true azimuth, motion attitude, velocity, heave and three-dimensional position information of the carrier. Through Kalman filtering, Phins can estimate the optimal underwater position of AUV in real time by integrating high precision velocity information, ultra-short baseline positioning information and AUV depth and height information provided by DVL.
3.2 Application of deep drag survey
Deep-Tow survey is a deep-sea engineering survey method in which a combination of one or several Marine survey instruments is installed on a deep-water towfish (body), and the influence of water on the instrument is reduced by sinking the body to a predetermined depth, so as to obtain high quality multi-wave beam, side scan sonar, shallow formation profile and other data. Taking EdgeTech DT-1 deep towing system as an example, the deep towing system is mainly composed of four parts: underwater towing body and carrying equipment, towing system (including ballast, towing cable, umbilical cable and winch), release recovery system, deck communication chain and system control processor. The deep-towing system is usually equipped with multi-beam sounding system, side-scan sonar system, shallow formation profiler, and auxiliary sensing equipment (fiber optic compass, motion sensor, real-time sound tachometer, pressure sensor, Doppler altimeter, and underwater acoustic positioning beacon, etc.). Unlike the AUV, the deep towing system is not powered and is towed entirely by the operating mother ship.
Deep towing system positioning using underwater acoustic navigation positioning, especially the ultra short baseline positioning system because of its low cost, simple operation, no need to lay the seabed transponder, flexible installation, high ranging accuracy, has become the mainstream technical means of deep towing system positioning. In deep towing operation, according to the horizontal distance behind the deep towing body, deep towing operation can be divided into two-ship positioning operation and single-ship positioning operation mode [10]. According to operation experience, single-ship positioning operation mode is generally adopted when 700m water depth is shallow, and double-ship positioning operation mode is adopted when 700m water depth is deep. In the single-ship positioning operation mode, the ultra short baseline transducer is installed on the mother ship, which is also responsible for the towing and positioning and tracking of the deep towing body. In the two-ship positioning operation mode, a tracking and positioning vessel and a deep towing vessel are usually equipped. The positioning of underwater towed body in deep towing system mainly depends on the ultra short baseline acoustic positioning system installed on the side of tracking and positioning ship and the DGNSS positioning system. The ship's navigation system transmits the position information received by the GNSS antenna to the USBL acoustic positioning system in real time, The USBL acoustic positioning system software calculates the position information of the underwater towing body by combining the relative position information of the USBL receiving and receiving probe and the offset between the GNSS antenna and the USBL receiving and sending probe. After tracking the positioning ship, the location information of the towing body is sent to the towing ship through radio data link, and the location information is integrated into the engineering survey data collection through time matching.
4 Conclusion
Ultra-short baseline positioning system is widely used in deep-water engineering investigation due to its advantages of simple structure, convenient operation, low cost and high ranging accuracy, and will still be the main way of underwater positioning for deep-water engineering investigation in the future. Improving positioning accuracy is the focus of the development and research of ultra-short baseline. For example, integrating high-precision attitude sensor and optical fiber compass into ultra-short baseline transducer can weaken the influence of Angle deviation. Using broadband digital signal instead of traditional audio signal can effectively reduce the influence of multipath effect and ambient noise, and improve ranging accuracy.
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