Investigations On Integration Of Underwater GPS Positioning System

Satellite Global Positioning System(GPS) works well in open land and outer space of earth, so does above the water. It will, however, be invalid in the huge space under the ocean surface due to the rapid attenuation of the radio signal propagating through water. To solve the problem of positioning under water, the acoustic wave can be used for it's good feature of propagating in water. Integration of GPS and sonar system into a GPS Intelligent Buoy(GIB) will extend GPS applications onto underwater domain and realize seamless positioning and navigation in whole space including land, underwater and outer space. This is the main idea of making use of GPS technology for under water positioning. To be convenient, such system is simply called under water GPS (UGPS).Under water GPS is a variable Long Baseline positioning System (LBS) on ocean surface based on Differential GPS. Multi-GIB, which forms a baseline network, will real-timely detect the acoustic signal and measure the arriving time. By calculating the time differences among GIBs, position of a underwater target can be resolved using hyperbola intersecting. The configuration of a UGPS includes four parts, i.e. GPS differential station, GIB, underwater pinger and ship-based control center. GPS differential station provides GPS differential corrections, which can improve GPS positioning accuracy to centimeter level. GIB, which is composed by GPS receiver, sonar system, attitude sensor, radio transmitter, detects sound signal, measures the arriving time and transmit all data to control center by radio wave. Underwater pinger is a user terminal which is installed on underwater carrier. The main task of underwater pinger is to produce and emit acoustic signal which is used to position and navigate. The Ship-based control center receives and records observation data from each GIB. Such data sets are then used to solve the underwater pinger's coordinates. The control center monitor whole system's state and send command to stop related actions in case of abnormity. Base on the configuration of the UGPS, several key technical issues are discussed as follows:1. UGPS Framework DesigningTraditional underwater positioning system includes Super Short BaseLine system (SSBL), Short BaseLine system (SBL) and Long BaseLine system (LBL). Among them, LBL has the best positioning accuracy. In this paper, the main principle of hyperbola intersection is designed based on LBL and GPS, a UGPS system framework. The main advantage is that the integrate GPS receiver with sonar system is quite easily.2. Virtual Long Baseline Formation Based on DGPSWith GPS-RTK, the real-time positioning accuracy can reach centimeter level, which is can be taken as a true point for meter-lever underwater positioning. However, the same acoustic signal will arrive different GIB at different time. The baseline between two GIB, which is used to resolve underwater target's location, is a asynchronous virtual baseline. In this paper, a new method, called Synchronous Acoustic Signal Matching Based on Two-step Buffer, is developed to find synchronous acoustic signal time from every GIB's observations, so as to form virtual baseline network. Additionally, the author proves the fact that the coordinate differential is embodied in hyperbola intersecting algorithm.3. Time measurement Based on GPS Time SystemThe basic observation for UGPS is the time that acoustic signal arrives at GIB. To measure the time difference, the time system of every GIB should be synchronized as all GIBs are working independently. In this paper, a method, which is based on GPS Time system, is applied to synchronize all GIBs. With GPS receiver's 1PPS output , GIB can compensate for the crystal oscillater's drift.4. Mathematic Model of UGPSBased on the principle of hyperbola intersection, an iterative algorithm and an analytic algorithm are deduced to resolve system equations with four GIBs observations. It can also be used to resolve the approx coordinate with more than five GIBs'observations. The reason to do so is that the hyperbola intersection model is not a global convergence model, and bad initial coordinate can lead to divergency for the Least Square solutions.In addition, a new algorithm is developed to reduce the sound speed error, which takes the mean velocity of sound in water as an unknown. Another algorithm, based on ray acoustic positioning , is investigated on condition that velocity grade observations are given.5. Error Analysis of UGPSError sources of UGPS include GPS observation error, attitude error, baseline network strength, sound velocity error, time measuring error. Based on polar coordinate system, an analytic method is investigated. It can be used to calculate positioning error related to baseline error and observation error. It can also be used to calculate convergent area and unstable area with given GIB network. Another method, based on hyperbola intersection LS solutions, is given to analyze PDOP of a given GIB network.6. Software DevelopmentWhole system is managed by a software working in ship-based control center. The functions of the software includes recording data, monitoring system state, resolving underwater target's coordinates, sending command. A object-oriented method is used to design the system data structure. Based on multi-thread and multi-layer frame, software framework is well constructed to realize data communication with multi-channel GPS Buoy receiver easily. In the end, three experiments are introduced, which are Lake Qiandaohu test, South-China sea test and Lake Fuxianhu test. The accuracy and stablity of the system are reviewed. Test results show algorithms and methods given in this paper are feasible and reasonable.