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CHC Bathymetric Survey Solution


Project Overview


Seabed levels need to be measured precisely to know if a ship can travel to berth area or if dredging is required to create enough depth to berth. Moreover, bed levels are required in Dam and Tank areas to calculate the capacity of the Dam on a yearly basis. Due to the constant movement of water into these dams, the silting causes increase in bed levels, and this decreases the overall capacity of the dam. De-silting of dams on time is essential to hold sufficient water to feed the command area. This solution template exploits the various functionalities of CHC Apache 5 for an efficient analysis of the seabed levels. CHC hydro survey software is also used for recording the data and analysis. It provides results in the form of 3D ground models, contours, and depth. Statistics are generated accurately for the required areas in WGS 84 coordinates.


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About CHC Apache 5 USV


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Apache 5 is a triple-hulled vessel autonomous boat made of macromolecule polyester carbon fiber. Carbon fibers are widely applied in the manufacture of advanced materials because of their excellent properties, such as high specific strength and modulus, low expansion coefficient, and relative flexibility. However, carbon fibers are difficult to be wetted by liquid (such as resin matrix) because of the low surface energy.


The Apache 5 has an echo sounder to find the depth and calculate the seabed levels. Multi-beam echosounder (MBES) is a popular device for oceanographic operations. It provides not only a good bathymetry map but also backscattering-strength (BS) profile of the seafloor. This information is used in various applications, such as seabed segmentation and characterization, geophysical parameters inversion.


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The Apache 5 also has a differential GPS to provide accurate positions at all times. The rover GPS is fitted on to the boat and base station is set up on the ground. Many outdoor robotics applications require positions that are more accurate than those obtained by a single GPS receiver. Existing RTK-GPS systems already provide accuracies in the sub-centimeter level. However, the geodetic grade double frequency receivers that are usually required for these systems are expensive. Important factors as such size, weight or power consumption limit the possible forms of applications and the acceptance in the field of mobile robotics.


Nowadays, inexpensive, small, light and power saving devices exist, but they usually don’t provide raw observation data, which is absolutely needed for any differential GPS (DGPS) solution. GPS-aided Inertial Navigation System (GPS-INS) is widely applied for navigation purpose on aircraft, land vehicles, marine surface vehicles. In situations where carrier phase measurements and differential sources are available, the centimeter level accuracy can be achieved by applying Real Time Kinematic (RTK) technique. However, conventional GPS-INS still has limitations, among which is that the performance of the EKF significantly depends on initial conditions and nonlinearities. This is due to the fact that previous improper EKF linearization points cannot be corrected at later times.


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The Apache 5 is fitted with an RF communication system for data transfer to a ground station. Future wireless communication systems dedicated to maritime applications are calling for increasing data rates. Establishing reliable communication oversea is a challenging task due to the extreme environmental conditions. A lack of knowledge in radio channel characteristics over sea limits the development of various wireless communication techniques. In fact, current maritime communications rely on VHF bands and high costly satellites links.


It is also fitted with a web video camera. The high dynamic range is critical for various imaging applications such as security cameras, mobile cameras, and automobile cameras. Non-HDR cameras take photographs with a limited exposure range, resulting in the loss of details in bright or dark areas. HDR imaging devices may compensate for this loss of details by capturing two or more images at different exposure levels and combining them to produce an image with a broader tonal range. Merging multiple exposures preserves both the saturated and the shadow regions and thus providing a higher dynamic range than a single exposure.


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Required Instruments and Techniques

                                     

● Apache 5 USV


APACHE 5 is integrated with intelligent water navigation robot technology, topographic survey technology, providing a smart, unmanned, integrated, motorized and networked measurement solution.


It is widely used for Bathymetric survey and mapping, channel survey, underwater geological exploration, etc. The system can reduce personal security risks and improve the flexibility and efficiency of underwater monitoring as well as hydrographic surveying. Digital radio and high definition web video camera are standard configurations. Meanwhile, APACHE 5 can carry the single or multi-beam, side-scan sonars, GNSS receiver for optional. APACHE 5 can collect data in local storage from multiple types of equipment in real time. The triple-hull adopts anti-corrosion sealing technology which can apply to different kinds of mission environments. It will navigate autonomously to finish the task in accordance with mission plans.


The bout is distinguished from its predecessors and other USVs due to its specifications and built. Apache 5 is unique with features such as the Triple-hulled design, Lightship body, less than 10 kg, Internal high sensitivity inertial navigation system, Adjustable speed, up to 5 m/s, Counter rotating dual propellers technology, High flexibility to carry different sensors, Equipped with high definition web video camera. These features make it suitable for the operations required for the project.

 

● Echo Sounder


Echo sounding is a type of sonar used to determine the depth of water by transmitting sound pulses into the water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes. Echo sounding can also refer to hydroacoustic "echo sounders" defined as active sound in water (sonar) used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish.


Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water, which is approximately 1.5 kilometers per second [T÷2×(4700 feet per second or 1.5 kilo per second )] For precise applications of echo sounding, such as hydrography, the speed of sound must also be measured typically by deploying a sound velocity probe into the water. Echo sounding is effectively a special purpose application of sonar used to locate the bottom.

 

● Differential GPS


Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System which provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.

DGPS uses a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudo ranges and actual (internally computed) pseudo-ranges, and receiver stations may correct their pseudo ranges by the same amount. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range.

 

● Digital Radio


Digital radio is the use of digital technology to transmit and/or receive across the radio spectrum. They may refer to digital transmission by radio waves, including digital broadcasting, and especially to digital audio radio services. In digital broadcasting systems, the analog audio signal is digitized, compressed using formats such as MP2, and transmitted using a digital modulation scheme. The aim is to increase the number of radio programs in a given spectrum, to improve the audio quality, to eliminate fading problems in mobile environments, to allow additional datacasting services, and to decrease the transmission power or the number of transmitters required to cover a region. However, analog radio (AM and FM) is still more popular and listening to radio over IP (Internet Protocol) is growing in popularity.

 

● Web Camera


A webcam is an input device that captures digital images and videos. These are transferred to the computer, which moves them to a server. From there, they can be transmitted to the hosting page. Laptops and desktops are often equipped with a webcam. The features of a webcam are largely dependent on the software operating system of the computer as well as the computer processor being used. Webcams can have additional features such as motion sensing, image archiving, automation or even custom coding.

 


Solution Methods


Apache 5 is dismantled for saving space and ease of transport. At the site of operation, it is to be assembled. The echo sounder, anti-drown support side, RF communication system are fixed, and the camera is fixed. The propellers, rover DGPS and RC communication system are connected.


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There are two separate batteries on the Apache 5. A large battery is provided to offer power to the propellers and to the RC communication system which is required for navigation. The remaining components are connected to a small battery. A ground communication system is a set-up which is connected to a computer for data transfer from the boat. A DGPS base station is set-up for a differential connection to provide accurate position of the boat. The limits of the survey are fixed. The flight plan is generated for the survey limits and is uploaded to the navigation system. The boat is then placed in water. A small test trial is run by moving it front and back and side to side. Once the test trial is successful, the RC is put into auto-pilot. This runs the flight plan which has been loaded. The boat follows the flight plan and collects the data at the predetermined points and transmits to the ground station and is recorded on a computer. This data can be then processed using the CHC hydro survey software and results are generated. The 3D ground models, contours and depth graphs are generated. From the results, in the port areas, the seabed levels are measured to know where a ship can travel to berth and where dredging is required. In dams, we measure the bed levels to calculate the capacity of the dam. The Dam bed levels are raised during monsoon days because of silting due to the inrush of heavy water to the dam area. This can cause serious changes in the capacity of the dam. De-silting needs to be carried out every time it exceeds an amount which would limit the function of the dam.



Applied Scenarios

 

● Seaport Area


In this scenario, the objective was to map the underground seabed of the existing port area. The area of interest was given by the client.

The same area of interest was used to create an autonomous mission using the Mission Planner in which the spacing between the lines was taken as 10m. The prepared mission is uploaded to the USV (APACHE 5) using a radio bridge. The data collection rate was set as one point every meter. The output of the GNSS receiver is set to NEMA output of 1Hz.

The Eco sounder and the GNSS Receiver were connected to the Hydro Sounder Software. The data capture procedure is started, and the autonomous mission is set. The USV moves to the start location and starts capturing the points along the pre-defined plan line. After Completion of the mission, the USV returns to the Home location after which the recording of the GNSS and Sounder data is stopped in Hydro Survey. The collected data is checked in the Hydro Survey software to find any anomalies in the obtained data.


The processed data is sampled and then exported as Comma Separated Value Format. A sample of the same is shown in Table 1.



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The data processed data is used to prepare 3D Model of the Seafloor and Generated contours. Sample 3D Data and contour are shown in Figure 1 and Figure 2.


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● River Cross Section


In this scenario, The Main Objective of this case study is the extraction of Cross Section of the river bed alone a flowing river. The places at which the cross-section is to be taken was provided by the client.


The Apache 5 was driven manually along the given cross section to obtain the depths at every one-meter interval. The data collection rate was set as one point every meter. The output of the GNSS receiver is set to NEMA output of 1Hz.

 

The Eco sounder and the GNSS Receiver were connected to the Hydro Sounder Software. The data capture procedure is started and the USV is driven manually along the cross-section line to obtain the depths.

 

The raw data collected was checked for any anomalies as there was a lot of fish movement during the survey. The checked depth data was samples and exported into CSV file Format. Sample raw data is shown in Table 2.


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The depth at different cross section was used to generate a cross-section. The depth data measured using the USV was compared to the depth data provided by the client at the same cross-section as shown in Figure 3. The data obtained from the USV matched with the depths collected by the client.


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● Lake Area

 

In this scenario, the objective is to map the bed levels of a lake.

 

The same area of interest was used to create an autonomous mission using the Mission Planner in which the spacing between the lines was taken as 10m. The prepared mission is uploaded to the USV (APACHE 5) using a radio bridge. The data collection rate was set as one point every meter. The output of the GNSS receiver is set to NEMA output of 1Hz.

 

The Eco sounder and the GNSS Receiver were connected to the Hydro Sounder Software. The data capture procedure is started, and the autonomous mission is set. The USV moves to the start location and starts capturing the points along the pre-defined plan line. After Completion of the mission, the USV returns to the Home location after which the recording of the GNSS and Sounder data is stopped in Hydro Survey. The collected data is checked in the Hydro Survey software to find any anomalies in the obtained data.

 

The processed data is taken out and is used to prepare a 3D Model (Shown in Figure 4) which will be used as a base reference to check the amount of quantity of silt removed. The area will be again mapped with USV (Apache 5) after completion of desilting the lake. Both the layers will be used to compare and estimate the quantity of silt which is removed.


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Conclusion

 

We have employed the methods used in this paper to efficiently map the seabed levels. The methods proposed have diverse applications such as Dam capacity calculations, desilting of river beds and reservoir. The results accurately depict the depth and seabed levels using 3d ground models, contours and depth charts. Due to continuous advancements in technology, Apache 5 will have a wide variety of applications in the near future.


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