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This thesis presents a technique for the detection of spread spectrum signals, of arbitrary form, even when the signal power spectral density (PSD) is well below the surveillance receiver noise spectral density, using a pair of antennas with broadband (1 GHz or more) receivers. Cross correlating the outputs of two receivers, spatially separated by a distance of the order of one metre or more, produces a cross correlation function (ccf) in which the noise components are spread uniformly over the whole width while the signal component, the narrow autocorrelation function (acf) of the spread spectrum signal, is concentrated near to the centre. The acf is displaced from the centre of the ccf by a small time shift equal to the time difference of arrival of the signal at the two antennas. A simple time domain filter can select a narrow centre portion of the ccf, rejecting the remainder which contains only noise. Taking the Fourier transform of this windowed ccf produces the "time domain filtered cross spectral density" (TDFCSD), in which the signal to noise ratio is independent of receiver bandwidth. Spread spectrum signals can then be both detected and characterised in an extremely sensitive broadband system by threshold detection applied to the magnitude of this TDFCSD. High resolution direction fmding can then be achieved by estimating the time difference of arrival at the two antennas from the phase slope of the appropriate part of the TDFCSD. An analysis of the performance of this dual receiver system is presented. A computer simulation illustrates the signal processing involved and shows excellent agreement with the analysis. An analysis of the detection performance of this system acting in an electronic support measure (ESM) role and comparison with other systems shows that, in addition to being able to obtain more information, this system can offer significantly greater sensitivity than a crystal video receiver. Acousto-optic correlation may be used to perform the cross correlation and time domain filtering of wideband signals in real time, with final processing of the much reduced data set to obtain and analyse the TDFCSD being carried out digitally. A novel non-heterodyning space integrating architecture capable of forming the true correlation function using the zeroth diffraction orders from acousto-optic cells was invented, the operation of which is not explained by the commonly used methods of analysis. By looking again at the acousto-optic interaction, it is shown that there is considerable information in the zeroth diffraction order and a unified theory of one dimensional space integrating correlators is developed, in which many known architectures can be treated as special cases of a general all order correlator. Because of practical difficulties in using a space integrating correlator to obtain the TDFCSD for continuous inputs, later work concentrated on time integrating correlation. Theoretical analysis and practical results are presented for a time integrating acousto-optic correlator, demonstrating that it gives itself naturally to the signal processing operations required and could be used in a real surveillance system making use of the TDFCSD for detection and direction finding.

This thesis describes an Intelligent Differential GPS Navigation System developed for a PhD research project. The first part of the work was to apply differential technology to Global Positioning System to locate the current position of the user with an improved positioning accuracy. The essential part of this Differential GPS system is a Differential GPS Reference Station. This DGPS Reference Station includes a DGPS mathematical model and the corresponding algorithms, which calculates the differential correction messages. These messages are then transmitted to a mobile GPS receiver by a radio data link. By using these corrections, the mobile GPS receiver's positioning accuracy can be improved from about 100 m to 4 m. This DGPS Reference station has been used to implement system software for this research. Differential correction algorithms were modified, characteristics of system components were changed, and different digital filters were also applied at different locations to investigate the impact on system performance. Besides all these capabilities which are needed for the research purpose, this DGPS Reference Station has all the standard functions, and can be used as a standard DGPS Reference Station. The second part of the work was to combine this Differential GPS system with a suitable digital map to form a navigation system. A suitable digital map database was chosen and modified, and the content of the map was then reproduced on the mobile GPS receiver's host PC screen. This digital map, combined with the current location of the user, provides the basic navigational information for the user to reach a desired destination. To help the user further and demonstrate the potential use of the system, an intelligent route-planing algorithm that can produce the optimum route automatically was also designed. The system integration was achieved by the design of the mobile navigation unit and the combination of this mobile navigation unit with the constructed DGPS Reference Station. The final system consists of a DGPS Reference Station, a UHF radio data transmitter, a mobile GPS receiver, a digital map system, a route searching and planing algorithm and a UHF radio data receiver. Field trials were carried out to test the system static and dynamic performances. Repeated experiments showed that both the static and dynamic positioning accuracies were within the range of 4 meters. The constructed system is a prototype navigation system which incorporates the basic navigational functions. It is envisaged that this system can be directly used, or further developed to suit a special need, as required. A typical application of the system would be to guide a user to a desired destination. Other examples include: aircraft autolanding control system, car self-driving, taxi fleet control, criminal tracing and personal navigation systems.

Collisions and groundings at sea still occur, and can result in financial loss, loss of life, and damage to the environment. Due to the size and capacity of moden vessels, damage can be extensive. Statistics indicate that the primary cause of accidents at sea is human error, which is often attributed to misinterpretation of the information presented to the mariner. Until recently, data collected from sensors about the vessel were displayed on the bridge individually, leaving the mariner to assimilate the material, make decisions and alter the vessels controls as appropriate. With the advent of the microprocessor a small amount of integration has taken place, but not to the extent that it has in other industries, for example the aerospace industry. This thesis presents a practical method of integrating all the navigation sensors. Through the use of Kalman filtering, an estimate of the state of the vessel is obtained using all the data available. Previous research in this field has not been implemented due to the complexity of the ship modelling process required, this is overcome by incorporating a system identification proceedure into the filter. The system further reduces the demands on the mariner by applying optimal control theory to guide the vessel on a predetermined track. Hazards such as other vessels are not incorporated into this work but they are specified in further research. Further development work is also required to reduce computation time.

The drive to reduce costs and to ensure that ports can stay open to the maximum extent in all weathers has given impetus to the search for more flexible vessel traffic management; something that current advances in technology make it possible to contemplate. Various issues pose a significant challenge to implementation, including the inherent conservatism of the maritime community. Nonetheless, aspects of potential new services are already in regualr use. Remote pilotage is a detectable strand in EU research thinking and the port of Rotterdam is preparing for its all weather implementation. First the issues involved were established by means of a literature search followed by a series of structured interviews and a questionnaire. the results were analysed, leading to a body of results that can be used by those seeking to make decisions in this area of maritime operations. A comparison with Air Traffic Management concluded that there would be merit in adopting some of its philosophy and procedures. specific focus was then given to establishing, by means of a questionnaire, a generic set of indicators by which it can be decided whether specific vessels quality for a given new service. Throughout the research qualitative data was sought from highly qualified professionals. Two new services are proposed; remote pilotage and enhanced navigation assistance. In the hierarchy of services they fit between the provision of an on board pilot and those currently provided by a VTS. Working definitions of both services have been established. The required technology is either available or can be seen to be a realistic prospect. AIS has emerged as a key enabling technology and the exchange of passage plans will be important. However,technology alone will not suffice to make change practible. The non-technological issues are more intractable but capable of solution. Progress, if realised, can be expected to be slow; the current assessment for the spread of remote pilotage is 10-15 years. A case cannot be made for the implementation of remoter pilotage or enhanced navigation assistance on the grounds of improved safety; an acceptable level of safety must be assumed to exist. There is also a need for the appropriate legislative framework to be in place and the issue of liability to be resolved. Ultimately the prime requirement for successful implementation is the realisation of a commercial benefit to both a port and the ship owner.

Modem computerised navigation systems on board ships require an accurate log for optimal performance. Traditional electromechanica1logs on board merchant ships have generally been replaced by Doppler logs, which refer the velocity to the sea-bed. However, their operational depth is limited and in deep water the velocity is referred to the water-mass some depth below the hull, where it is influenced by ocean currents. The velocity accuracy ultimately depends on the estimated local sound velocity. The acoustic correlation log has acoustic beams similar to a conventional depth-sounder, with a vertically downward propagation direction and a broad beamwidth. Compared to the Doppler log it is affected less by ship pitch and roll due to its broad beamwidth, and the velocity is virtually independent of the velocity of sound and is referred to the sea-bed to a greater depth. Temporal correlation logs use a two-hydrophone array and the time delay between the two received signals is used to calculate the velocity. The full potential of these logs for world-wide ocean navigation has not been exploited. This thesis investigates the requirements and conditions for temporal correlation logs on board ships to navigate satisfactorily on and off the continental shelf. An experimental temporal correlation log working at high-frequencies with continuous wave propagation is described. It is used to measure the velocity of a tracked platform moving across a water tank and the results are compared to their theoretical values. When fully developed, the log would be suitable for shallow-water operation, and techniques learned can be applied to a low-frequency deep-water correlation log.

The Global Positioning System (GPS) is a satellite navigation system which, when fully operational, "will provide highly accurate position and velocity information in three dimensions, as well as precise time, to users around the globe 24 hours a day" (Anon, 1990). GPS can be operated under all weather conditions, with the only restriction being that the user must be able to receive radio signals from the satellites. Such a comprehensive positioning system has never been previously available, and thus GPS is currently being used for a diverse range of applications. This thesis is focused at the application of GPS for the offshore oil industry which is requiring increasingly higher instantaneous positioning accuracies. The GPS system and real-time positioning techniques are described, along with the main error sources that limit the available accuracy. The suitability of using GPS observations in a standard set of mathematical algorithms, the Kalman filter, in order to obtain position and velocity information has been examined. This is carried out by analysing the observations in order to determine some statistical properties that are usually ignored during the processing and data spanning a two year period has been analysed. The effect that these properties have on the resultant position and its precision was ascertained, finding that position discrepancies were insignificant but their associated precisions were highly dependent on the statistical properties were highly dependent of the data sets. Along similar lines., the ability of the Kalman filter to detect blunders, or gross errors, within GPS-type observations was analysed showing that the relevant test statistics performed sub-optimally and, again, this was dependent on the properties of the data.

No description available.